Neutron detection

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https://en.wikipedia.org/wiki/Neutron_detection

Neutron detection is the effective detection of neutrons entering a well-positioned detector. There are two key aspects to effective neutron detection: hardware and software. Detection hardware refers to the kind of neutron detector used (the most common today is the scintillation detector) and to the electronics used in the detection setup. Further, the hardware setup also defines key experimental parameters, such as source-detector distance, solid angle and detector shielding. Detection software consists of analysis tools that perform tasks such as graphical analysis to measure the number and energies of neutrons striking the detector.

Basic physics

Signatures by which a neutron may be detected

Atomic and subatomic particles are detected by the signature they produce through interaction with their surroundings. The interactions result from the particles' fundamental characteristics.

• Charge: Neutrons are neutral particles and do not ionize directly; hence they are harder than charged particles to detect directly. Further, their paths of motion are only weakly affected by electric and magnetic fields.
• Mass: The neutron mass of 1.0086649156(6) u is not directly detectable, but does influence reactions through which it can be detected.
• Reactions: Neutrons react with a number of materials through elastic scattering producing a recoiling nucleus, inelastic scattering producing an excited nucleus, or absorption with transmutation of the resulting nucleus. Most detection approaches rely on detecting the various reaction products.
• Magnetic moment: Although neutrons have a magnetic moment of −1.9130427(5) μ_N, techniques for detection of the magnetic moment are too insensitive to use for neutron detection.
• Electric dipole moment: The neutron is predicted to have only a tiny electric dipole moment, which has not yet been detected. Hence it is not a viable detection signature.
• Decay: Outside the nucleus, free neutrons are unstable and have a mean lifetime of 885.7±0.8 s (about 14 minutes, 46 seconds). Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:

n⁰
→
p⁺
+
e⁻
+
ν
_e.

Although the
p⁺
and
e⁻
produced by neutron decay are detectable, the decay rate is too low to serve as the basis for a practical detector system.

Classic neutron detection options

As a result of these properties, detection of neutrons fall into several major categories:

• Absorptive reactions with prompt reactions - low energy neutrons are typically detected indirectly through absorption reactions. Typical absorber materials used have high cross sections for absorption of neutrons and include helium-3, lithium-6, boron-10, and uranium-235. Each of these reacts by emission of high energy ionized particles, the ionization track of which can be detected by a number of means. Commonly used reactions include ³He(n,p) ³H, ⁶Li(n,t) ⁴He, ¹⁰B(n,α) ⁷Li and the fission of uranium.
• Activation processes - Neutrons may be detected by reacting with absorbers in a radiative capture, spallation or similar reaction, producing reaction products that then decay at some later time, releasing beta particles or gammas. Selected materials (e.g., indium, gold, rhodium, iron (⁵⁶Fe(n,p) ⁵⁶Mn), aluminum (²⁷Al(n,α)²⁴Na), niobium (⁹³Nb(n,2n) ^92mNb), & silicon (²⁸Si(n,p) ²⁸Al)) have extremely large cross sections for the capture of neutrons within a very narrow band of energy. Use of multiple absorber samples allows characterization of the neutron energy spectrum. Activation also enables the reconstruction of an historic neutron exposure (e.g., forensic reconstruction of neutron exposures during an accidental criticality).
• Elastic scattering reactions (also referred to as proton-recoil) - High energy neutrons are typically detected indirectly through elastic scattering reactions. Neutrons collide with the nuclei of atoms in the detector, transferring energy to those nuclei and creating ions, which are detected. Since the maximum transfer of energy occurs when the mass of the atom with which the neutron collides is comparable to the neutron mass, hydrogenous materials are often the preferred medium for such detectors.

Types of neutron detectors

Gas proportional detectors

Gas proportional detectors can be adapted to detect neutrons. While neutrons do not typically cause ionization, the addition of a nuclide with high neutron cross-section allows the detector to respond to neutrons. Nuclides commonly used for this purpose are helium-3, lithium-6, boron-10 and uranium-235. Since these materials are most likely to react with thermal neutrons (i.e., neutrons that have slowed to equilibrium with their surroundings), they are typically surrounded by moderating materials to reduce their energy and increase the likelihood of detection.

Further refinements are usually necessary to differentiate the neutron signal from the effects of other types of radiation. Since the energy of a thermal neutron is relatively low, charged particle reactions are discrete (i.e., essentially monoenergetic and lie within a narrow bandwidth of energies) while other reactions such as gamma reactions will span a broad energy range, it is possible to discriminate among the sources.

As a class, gas ionization detectors measure the number (count rate), and not the energy of neutrons.

³He gas-filled proportional detectors

Helium-3 is an effective neutron detector material because it reacts by absorbing thermal neutrons, producing a ¹H and ³H ion. Its sensitivity to gamma rays is negligible, providing a very useful neutron detector. Unfortunately the supply of ³He is limited to production as a byproduct from the decay of tritium (which has a 12.3 year half-life); tritium is produced either as part of weapons programs as a booster for nuclear weapons or as a byproduct of reactor operation.

BF₃ gas-filled proportional detectors

As elemental boron is not gaseous, neutron detectors containing boron may alternately use boron trifluoride (BF₃) enriched to 96% boron-10 (natural boron is 20% ¹⁰B, 80% ¹¹B). Boron trifluoride is highly toxic. The sensitivity of this detector is around 35-40 CPS/nv (counts per second per neutron flux) whereas that of Boron lined is about 4 CPS/nv. This is because in Boron lined, n reacts with Boron and hence produce ion pairs inside the layer. Hence charged particles produced (Alpha and Li) they lose some of their energy inside that layer. Low energy charged particles are unable to reach the Ionization chamber's gas environment. Hence, the number of ionizations produced in gas is also lower.

Whereas In BF3 gas filled, N reacts with B in gas. and fully energetic Alpha and Li are able to produce more ionizations and give more pulses.

Boron lined proportional detectors

Alternately, boron-lined gas-filled proportional counters react similarly to BF₃ gas-filled proportional detectors, with the exception that the walls are coated with ¹⁰B. In this design, since the reaction takes place on the surface, only one of the two particles will escape into the proportional counter.

Scintillation neutron detectors

Scintillation neutron detectors include liquid organic scintillators, crystals, plastics, glass and scintillation fibers.

Neutron-sensitive scintillating glass fiber detectors

Scintillating ⁶Li glass for neutron detection was first reported in the scientific literature in 1957 and key advances were made in the 1960s and 1970s. Scintillating fiber was demonstrated by Atkinson M. et al. in 1987 and major advances were made in the late 1980s and early 1990s at Pacific Northwest National Laboratory where it was developed as a classified technology. It was declassified in 1994 and first licensed by Oxford Instruments in 1997, followed by a transfer to Nucsafe in 1999. The fiber and fiber detectors are now manufactured and sold commercially by Nucsafe, Inc.

The scintillating glass fibers work by incorporating ⁶Li and Ce³⁺ into the glass bulk composition. The ⁶Li has a high cross-section for thermal neutron absorption through the ⁶Li(n,α) reaction. Neutron absorption produces a tritium ion, an alpha particle, and kinetic energy. The alpha particle and triton interact with the glass matrix to produce ionization, which transfers energy to Ce³⁺ ions and results in the emission of photons with wavelength 390 nm - 600 nm as the excited state Ce³⁺ ions return to the ground state. The event results in a flash of light of several thousand photons for each neutron absorbed. A portion of the scintillation light propagates through the glass fiber, which acts as a waveguide. The fibers ends are optically coupled to a pair of photomultiplier tubes (PMTs) to detect photon bursts. The detectors can be used to detect both neutrons and gamma rays, which are typically distinguished using pulse-height discrimination. Substantial effort and progress in reducing fiber detector sensitivity to gamma radiation has been made. Original detectors suffered from false neutrons in a 0.02 mR gamma field. Design, process, and algorithm improvements now enable operation in gamma fields up to 20 mR/h (⁶⁰Co).

The scintillating fiber detectors have excellent sensitivity, they are rugged, and have fast timing (~60 ns) so that a large dynamic range in counting rates is possible. The detectors have the advantage that they can be formed into any desired shape, and can be made very large or very small for use in a variety of applications. Further, they do not rely on ³He or any raw material that has limited availability, nor do they contain toxic or regulated materials. Their performance matches or exceeds that of ³He tubes for gross neutron counting due to the higher density of neutron absorbing species in the solid glass compared to high-pressure gaseous ³He. Even though the thermal neutron cross section of ⁶Li is low compared to ³He (940 barns vs. 5330 barns), the atom density of ⁶Li in the fiber is fifty times greater, resulting in an advantage in effective capture density ratio of approximately 10:1.

LiCaAlF₆

LiCaAlF₆ is a neutron sensitive inorganic scintillator crystal which like neutron-sensitive scintillating glass fiber detectors makes use of neutron capture by ⁶Li. Unlike scintillating glass fiber detectors however the ⁶Li is part of the crystalline structure of the scintillator giving it a naturally high ⁶Li density. A doping agent is added to provide the crystal with its scintillating properties, two common doping agents are trivalent cerium and divalent europium. Europium doped LiCaAlF₆ has the advantage over other materials that the number of optical photons produced per neutron capture is around 30.000 which is 5 times higher than for example in neutron-sensitive scintillating glass fibers. This property makes neutron photon discrimination easier. Due to its high ⁶Li density this material is suitable for producing light weight compact neutron detectors, as a result LiCaAlF₆ has been used for neutron detection at high altitudes on balloon missions. The long decay time of Eu²⁺ doped LiCaAlF₆ makes it less suitable for measurements in high radiation environments, the Ce³⁺ doped variant has a shorter decay time but suffers from a lower light-yield.

NaIL Dual Detection Neutron-Gamma Scintillator

Sodium Iodide crystal co-doped with Thallium and Lithium [NaI(Tl+Li)] a.k.a. NaIL has the ability to detect Gamma radiation and Thermal Neutrons in a single crystal with exceptional Pulse-shape Discrimination.The use of low ⁶Li concentrations in NaIL and large thicknesses can achieve the same neutron detection capabilities as 3He or CLYC or CLLB detectors at a lower cost.⁶Li (95% enriched) co-doping introduces efficient thermal neutron detection to the most established gamma-ray scintillator while retaining the favorable scintillation properties of standard NaI(Tl). NaIL can provide large volume, single material detectors for both gammas and neutrons at a low price per volume.

Semiconductor neutron detectors

There are two basic types of semiconductor neutron detectors, the first being electron devices coated with a neutron reactive material and the second being a semiconductor being partly composed of neutron reactive material. The most successful of these configurations is the coated device type, and an example would be a common planar Si diode coated with either ¹⁰B or ⁶LiF. This type of detector was first proposed by Babcock et al. The concept is straightforward. A neutron is absorbed in the reactive film and spontaneously emits energetic reaction products. A reaction product may reach the semiconductor surface, and upon entering the semiconductor produces electron-hole pairs. Under a reverse bias voltage, these electrons and holes are drifted through the diode to produce an induced current, usually integrated in pulse mode to form a voltage output. The maximum intrinsic efficiency for single-coated devices is approximately 5% for thermal neutrons (0.0259 eV), and the design and operation are thoroughly described in the literature. The neutron detection efficiency limitation is a consequence of reaction-product self-absorption. For instance, the range in a boron film of 1.47 MeV α particles from the ¹⁰B(n,α) ⁷Li reaction is approximately 4.5 microns, and the range in LiF of 2.7 MeV tritons from the ¹⁰B(n,α) ⁷Li reaction is approximately 28 microns. Reaction products originating at distances further from the film/semiconductor interface can not reach the semiconductor surface, and consequently will not contribute to neutron detection. Devices coated with natural Gd have also been explored, mainly because of its large thermal neutron microscopic cross section of 49,000 barns. However, the Gd(n,γ) reaction products of interest are mainly low energy conversion electrons, mostly grouped around 70 keV. Consequently, discrimination between neutron induced events and gamma-ray events (mainly producing Compton scattered electrons) is difficult for Gd-coated semiconductor diodes. A compensated pixel design sought to remedy the problem. Overall, devices coated with either ¹⁰B or ⁶LiF are preferred mainly because the energetic charged-particle reaction products are much easier to discriminate from background radiations.

The low efficiency of coated planar diodes led to the development of microstructured semiconductor neutron detectors (MSND). These detectors have microscopic structures etched into a semiconductor substrate, subsequently formed into a pin style diode. The microstructures are backfilled with neutron reactive material, usually ⁶LiF, although ¹⁰B has been used. The increased semiconductor surface area adjacent to the reactive material and the increased probability that a reaction product will enter the semiconductor greatly increase the intrinsic neutron detection efficiency.

Basic design of a microstructured semiconductor neutron detector (MSND). 

The MSND device configuration was first proposed by Muminov and Tsvang, and later by Schelten et al. It was years later when the first working example of a MSND was fabricated and demonstrated, then having only 3.3% thermal neutron detection efficiency. Since that initial work, MSNDs have achieved greater than 30% thermal neutron detection efficiency. Although MSNDs can operate on the built-in potential (zero applied voltage), they perform best when 2-3 volts are applied. There are several groups now working on MSND variations. The most successful types are the variety backfilled with ⁶LiF material. MSNDs are now manufactured and sold commercially by Radiation Detection Technologies, Inc. Advanced experimental versions of double-sided MSNDs with opposing microstructures on both sides of a semiconductor wafer have been reported with over 65% thermal neutron detection efficiency, and are theoretically capable of over 70% efficiency.

Semiconductor detectors in which one of more constituent atoms are neutron reactive are called bulk semiconductor neutron detectors. Bulk solid-state neutron detectors can be divided into two basic categories: those that rely on the detection of charged-particle reaction products and those that rely on the detection of prompt capture gamma rays. In general, this type of neutron detector is difficult to make reliably and presently are not commercially available.

The bulk materials that rely upon charged-particle emissions are based on boron and lithium containing semiconductors. In the search for bulk semiconductor neutron detectors, the boron-based materials, such as BP, BAs, BN, and B₄C, have been investigated more than other potential materials.

Boron-based semiconductors in cubic form are difficult to grow as bulk crystals, mainly because they require high temperatures and high pressure for synthesis. BP and Bas can decompose into undesirable crystal structures (cubic to icosahedral form) unless synthesized under high pressure. B₄C also forms icosahedral units in a rhombohedral crystal structure, an undesirable transformation because the icosahedral structure has relatively poor charge collection properties which make these icosahedral forms unsuitable for neutron detection.

BN can be formed as either simple hexagonal, cubic (zincblende) or wurtzite crystals, depending on the growth temperature, and it is usually grown by thin film methods. It is the simple hexagonal form of BN that has been most studied as a neutron detector. Thin film chemical vapor deposition methods are usually employed to produce BP, BAs, BN, or B₄C. These boron-based films are often grown upon n-type Si substrates, which can form a pn junction with the Si and, therefore, produce a coated Si diode as described at the beginning of this section. Consequently, the neutron response from the device can be easily mistaken as a bulk response when it is actually a coated diode response. To date, there is sparse evidence of boron-based semiconductors producing intrinsic neutron signals.

Li-containing semiconductors, categorized as Nowotny-Juza compounds, have also been investigated as bulk neutron detectors. The Nowotny-Juza compound LiZnAs has been demonstrated as a neutron detector; however, the material is difficult and expensive to synthesize, and only small semiconductor crystals have been reported. Finally, traditional semiconductor materials with neutron reactive dopants have been investigated, namely, Si(Li) detectors. Neutrons interact with the lithium dopant in the material and produce energetic reaction products. However, the dopant concentration is relatively low in Li drifted Si detectors (or other doped semiconductors), typically less than 10¹⁹ cm⁻³. For a degenerate concentration of Li on the order of 10¹⁹ cm⁻³, a 5-cm thick block of natural Si(Li) would have less than 1% thermal-neutron detection efficiency, while a 5-cm thick block of a Si(⁶Li) detector would have only 4.6% thermal-neutron detection efficiency.

Prompt gamma-ray emitting semiconductors, such as CdTe, and HgI₂ have been successfully used as neutron detectors. These detectors rely upon the prompt gamma-ray emissions from the ¹¹³Cd(n, γ)¹¹⁴Cd reaction (producing 558.6 keV and 651.3 keV gamma rays) and the ¹⁹⁹Hg(n, γ) ²⁰⁰Hg reaction (producing 368.1 keV and 661.1 keV gamma rays). However, these semiconductor materials are designed for use as gamma-ray spectrometers and, hence, are intrinsically sensitive to the gamma-ray background. With adequate energy resolution, pulse height discrimination can be used to separate the prompt gamma-ray emissions from neutron interactions. However, the effective neutron detection efficiency is compromised because of the relatively small Compton ratio. In other words, the majority of events add to the Compton continuum rather than to the full energy peak, thus, making discrimination between neutrons and background gamma rays difficult. Also, both natural Cd and Hg have relatively large thermal-neutron (n,γ) cross sections of 2444 b and 369.8 b, respectively. Consequently, most thermal neutrons are absorbed near the detector surface so that nearly half of the prompt gamma rays are emitted in directions away from the detector bulk and, thus, produce poor gamma-ray reabsorption or interaction efficiency.

Neutron activation detectors

Activation samples may be placed in a neutron field to characterize the energy spectrum and intensity of the neutrons. Activation reactions that have differing energy thresholds can be used including ⁵⁶Fe(n,p) ⁵⁶Mn, ²⁷Al(n,α)²⁴Na, ⁹³Nb(n,2n) ^92mNb, & ²⁸Si(n,p)²⁸Al.

Fast neutron detectors

Fast neutrons are often detected by first moderating (slowing) them to thermal energies. However, during that process the information on the original energy of the neutron, its direction of travel, and the time of emission, is lost. For many applications, the detection of “fast” neutrons that retain this information is highly desirable.

Typical fast neutron detectors are liquid scintillators, 4-He based noble gas detectors  and plastic detectors. Fast neutron detectors differentiate themselves from one another by their 1.) capability of neutron/gamma discrimination (through pulse shape discrimination) and 2.) sensitivity. The capability to distinguish between neutrons and gammas is excellent in noble gas based 4-He detectors due to their low electron density and excellent pulse shape discrimination property. In fact, inorganic scintillators such as zinc sulfide has been shown to exhibit large differences in their decay times for protons and electrons; a feature that has been exploited by combining the inorganic crystal with a neutron converter (such as polymethyl methacrylate) in the Micro-Layered Fast-Neutron Detector. Such detection systems are capable of selectively detecting only fast neutrons in a mixed neutron-gamma radiation field without requiring any additional discrimination techniques such as pulse shape discrimination.

Detection of fast neutrons poses a range of special problems. A directional fast-neutron detector has been developed using multiple proton recoils in separated planes of plastic scintillator material. The paths of the recoil nuclei created by neutron collision are recorded; determination of the energy and momentum of two recoil nuclei allow calculation of the direction of travel and energy of the neutron that underwent elastic scattering with them.

Applications

Neutron detection is used for varying purposes. Each application has different requirements for the detection system.

• Reactor instrumentation: Since reactor power is essentially linearly proportional to the neutron flux, neutron detectors provide an important measure of power in nuclear power and research reactors. Boiling water reactors may have dozens of neutron detectors, one per fuel assembly. Most neutron detectors used in thermal-spectrum nuclear reactors are optimized to detect thermal neutrons.
• Plasma physics: Neutron detection is used in fusion plasma physics experiments such as JET. For example, the detected neutron rate from a plasma can give information about the ion temperature.
• Particle physics: Neutron detection has been proposed as a method of enhancing neutrino detectors.
• Materials science: Elastic and inelastic neutron scattering enables experimentalists to characterize the morphology of materials from scales ranging from ångströms to about one micrometer.
• Radiation safety: Neutron radiation is a hazard associated with neutron sources, space travel, accelerators and nuclear reactors. Neutron detectors used for radiation safety must take into account the relative biological effectiveness (i.e., the way damage caused by neutrons varies with energy).
• Cosmic ray detection: Secondary neutrons are one component of particle showers produced in Earth's atmosphere by cosmic rays. Dedicated ground-level neutron detectors, namely neutron monitors, are employed to monitor variations in cosmic ray flux.
• Special nuclear material detection: Special nuclear materials (SNM) such as uranium-233 and plutonium-239 decay by spontaneous fission, yielding neutrons. Neutrons detectors can be used for monitor for SNM in commerce.

Experimental neutron detection

Experiments that make use of this science include scattering experiments in which neutrons directed and then scattered from a sample are to be detected. Facilities include the ISIS neutron source at the Rutherford Appleton Laboratory, the Spallation Neutron Source at the Oak Ridge National Laboratory, and the Spallation Neutron Source (SINQ) at the Paul Scherrer Institute, in which the neutrons are produced by spallation reaction, and the traditional research reactor facilities in which neutrons are produced during fission of uranium isotopes. Noteworthy among the various neutron detection experiments is the trademark experiment of the European Muon Collaboration, first performed at CERN and now termed the "EMC experiment." The same experiment is performed today with more sophisticated equipment to obtain more definite results related to the original EMC effect.

Challenges in neutron detection in an experimental environment

Neutron detection in an experimental environment is not an easy science. The major challenges faced by modern-day neutron detection include background noise, high detection rates, neutron neutrality, and low neutron energies.

Background noise

The main components of background noise in neutron detection are high-energy photons, which aren't easily eliminated by physical barriers. The other sources of noise, such as alpha and beta particles, can be eliminated by various shielding materials, such as lead, plastic, thermo-coal, etc. Thus, photons cause major interference in neutron detection, since it is uncertain if neutrons or photons are being detected by the neutron detector. Both register similar energies after scattering into the detector from the target or ambient light, and are thus hard to distinguish. Coincidence detection can also be used to discriminate real neutron events from photons and other radiation.

High detection rates

If the detector lies in a region of high beam activity, it is hit continuously by neutrons and background noise at overwhelmingly high rates. This obfuscates collected data, since there is extreme overlap in measurement, and separate events are not easily distinguished from each other. Thus, part of the challenge lies in keeping detection rates as low as possible and in designing a detector that can keep up with the high rates to yield coherent data.

Neutrality of neutrons

Neutrons are neutral and thus do not respond to electric fields. This makes it hard to direct their course towards a detector to facilitate detection. Neutrons also do not ionize atoms except by direct collision, so gaseous ionization detectors are ineffective.

Varying behavior with energy

Detectors relying on neutron absorption are generally more sensitive to low-energy thermal neutrons, and are orders of magnitude less sensitive to high-energy neutrons. Scintillation detectors, on the other hand, have trouble registering the impacts of low-energy neutrons.

Experimental setup and method

Figure 1: The experimental setup

Figure 1 shows the typical main components of the setup of a neutron detection unit. In principle, the diagram shows the setup as it would be in any modern particle physics lab, but the specifics describe the setup in Jefferson Lab (Newport News, Virginia).

In this setup, the incoming particles, comprising neutrons and photons, strike the neutron detector; this is typically a scintillation detector consisting of scintillating material, a waveguide, and a photomultiplier tube (PMT), and will be connected to a data acquisition (DAQ) system to register detection details.

The detection signal from the neutron detector is connected to the scaler unit, gated delay unit, trigger unit and the oscilloscope. The scaler unit is merely used to count the number of incoming particles or events. It does so by incrementing its tally of particles every time it detects a surge in the detector signal from the zero-point. There is very little dead time in this unit, implying that no matter how fast particles are coming in, it is very unlikely for this unit to fail to count an event (e.g. incoming particle). The low dead time is due to sophisticated electronics in this unit, which take little time to recover from the relatively easy task of registering a logical high every time an event occurs. The trigger unit coordinates all the electronics of the system and gives a logical high to these units when the whole setup is ready to record an event run.

The oscilloscope registers a current pulse with every event. The pulse is merely the ionization current in the detector caused by this event plotted against time. The total energy of the incident particle can be found by integrating this current pulse with respect to time to yield the total charge deposited at the end of the PMT. This integration is carried out in the analog-digital converter (ADC). The total deposited charge is a direct measure of the energy of the ionizing particle (neutron or photon) entering the neutron detector. This signal integration technique is an established method for measuring ionization in the detector in nuclear physics. The ADC has a higher dead time than the oscilloscope, which has limited memory and needs to transfer events quickly to the ADC. Thus, the ADC samples out approximately one in every 30 events from the oscilloscope for analysis. Since the typical event rate is around 10⁶ neutrons every second, this sampling will still accumulate thousands of events every second.

Separating neutrons from photons

The ADC sends its data to a DAQ unit that sorts the data in presentable form for analysis. The key to further analysis lies in the difference between the shape of the photon ionization-current pulse and that of the neutron. The photon pulse is longer at the ends (or "tails") whereas the neutron pulse is well-centered. This fact can be used to identify incoming neutrons and to count the total rate of incoming neutrons. The steps leading to this separation (those that are usually performed at leading national laboratories, Jefferson Lab specifically among them) are gated pulse extraction and plotting-the-difference.

Gated pulse extraction

Ionization current signals are all pulses with a local peak in between. Using a logical AND gate in continuous time (having a stream of "1" and "0" pulses as one input and the current signal as the other), the tail portion of every current pulse signal is extracted. This gated discrimination method is used on a regular basis on liquid scintillators. The gated delay unit is precisely to this end, and makes a delayed copy of the original signal in such a way that its tail section is seen alongside its main section on the oscilloscope screen.

After extracting the tail, the usual current integration is carried out on both the tail section and the complete signal. This yields two ionization values for each event, which are stored in the event table in the DAQ system.

Plotting the difference

Figure 2: Expected plot of tail energy against energy in the complete pulse plotted for all event energies. Dots represent number densities of events.

In this step lies the crucial point of the analysis: the extracted ionization values are plotted. Specifically, the graph plots energy deposition in the tail against energy deposition in the entire signal for a range of neutron energies. Typically, for a given energy, there are many events with the same tail-energy value. In this case, plotted points are simply made denser with more overlapping dots on the two-dimensional plot, and can thus be used to eyeball the number of events corresponding to each energy-deposition. A considerable random fraction (1/30) of all events is plotted on the graph.

If the tail size extracted is a fixed proportion of the total pulse, then there will be two lines on the plot, having different slopes. The line with the greater slope will correspond to photon events and the line with the lesser slope to neutron events. This is precisely because the photon energy deposition current, plotted against time, leaves a longer "tail" than does the neutron deposition plot, giving the photon tail more proportion of the total energy than neutron tails.

The effectiveness of any detection analysis can be seen by its ability to accurately count and separate the number of neutrons and photons striking the detector. Also, the effectiveness of the second and third steps reveals whether event rates in the experiment are manageable. If clear plots can be obtained in the above steps, allowing for easy neutron-photon separation, the detection can be termed effective and the rates manageable. On the other hand, smudging and indistinguishability of data points will not allow for easy separation of events.

Rate control

Detection rates can be kept low in many ways. Sampling of events can be used to choose only a few events for analysis. If the rates are so high that one event cannot be distinguished from another, physical experimental parameters (shielding, detector-target distance, solid-angle, etc.) can be manipulated to give the lowest rates possible and thus distinguishable events.

Finer detection points

It is important here to observe precisely those variables that matter, since there may be false indicators along the way. For example, ionization currents might get periodic high surges, which do not imply high rates but just high energy depositions for stray events. These surges will be tabulated and viewed with cynicism if unjustifiable, especially since there is so much background noise in the setup.

One might ask how experimenters can be sure that every current pulse in the oscilloscope corresponds to exactly one event. This is true because the pulse lasts about 50 ns, allowing for a maximum of 2×10⁷ events every second. This number is much higher than the actual typical rate, which is usually an order of magnitude less, as mentioned above. This means that is it highly unlikely for there to be two particles generating one current pulse. The current pulses last 50 ns each, and start to register the next event after a gap from the previous event.

Although sometimes facilitated by higher incoming neutron energies, neutron detection is generally a difficult task, for all the reasons stated earlier. Thus, better scintillator design is also in the foreground and has been the topic of pursuit ever since the invention of scintillation detectors. Scintillation detectors were invented in 1903 by Crookes but were not very efficient until the PMT (photomultiplier tube) was developed by Curran and Baker in 1944. The PMT gives a reliable and efficient method of detection since it can multiply the initial signal of a single scintillation photon hitting the PMT face millions of times into a measurable electrical pulse. Even so, scintillator detector design has room for improvement as do other options for neutron detection besides scintillation.

Women in STEM fields

From Wikipedia, the free encyclopedia

Biochemist Ainhoa Murua Ugarte (es) at work in her lab

Many scholars and policymakers have noted that the fields of science, technology, engineering, and mathematics (STEM) have remained predominantly male with historically low participation among women since the origins of these fields in the 18th century during the Age of Enlightenment.

Scholars are exploring the various reasons for the continued existence of this gender disparity in STEM fields. Those who view this disparity as resulting from discriminatory forces are also seeking ways to redress this disparity within STEM fields (these typically construed as well-compensated, high-status professions with universal career appeal).

History

Women's participation in science, technology and engineering has been limited and also under-reported throughout most of history. This has been the case, with exceptions, until large scale changes began around the 1970s. Scholars have discussed possible reasons and mechanisms behind the limitations such as ingrained gender roles, sexism, and sex differences in psychology. There has also been an effort among historians of science to uncover under-reported contributions of women. 

The term STEM was first used in 2001, primarily in connection with choice of education and career. Different STEM fields have different histories, but women's participation, although limited, has been seen throughout history. Science or protoscience and mathematics have been practiced since ancient times, and during this time women have contributed to such fields as medicine, botany, astronomy, algebra and geometry. In the Middle Ages in Europe and the Middle East, Christian monasteries and Islamic madrasas were places where women could work on such subjects as mathematics and the study of nature.

Universities in the Christian tradition began as places of education of a professional clergy that allowed no women, and the practice of barring women continued even after universities' mission broadened.  Because women were generally barred from formal higher education until late in the 19th century, it was very difficult for them to enter specialized disciplines.

The development of industrial technology was dominated by men, and early technical achievements, such as the invention of the steam engine, were mainly due to men. Nevertheless, there are many examples of women's contributions to engineering.

Initially a "computer" was a person doing computations, who was often a woman. Working as a computer required conscientiousness, accuracy and speed. Some women who initially worked as human computers later advanced from doing the simpler calculations to higher levels of work, where they specified tasks and algorithms and analyzed results.

Women's participation rates in the STEM fields started increasing noticeably in the 1970s and 1980s. Some fields, such as biotechnology, now have almost 50% participation of women.

Gender imbalance in STEM fields

According to PISA 2015 results, 4.8% of boys and 0.4% of girls expect an ICT career.

Studies suggest that many factors contribute to the attitudes towards the achievement of young men in mathematics and science, including encouragement from parents, interactions with mathematics and science teachers, curriculum content, hands-on laboratory experiences, high school achievement in mathematics and science, and resources available at home. In the United States, research findings are mixed concerning when boys' and girls' attitudes about mathematics and science diverge. Analyzing several nationally representative longitudinal studies, one researcher found few differences in girls' and boys' attitudes towards science in the early secondary school years. Students' aspirations to pursue careers in mathematics and science influence both the courses they choose to take in those areas and the level of effort they put forth in these courses.

A 1996 USA study suggested that girls begin to lose self-confidence in middle school because they believe that men possess more intelligence in technological fields. The fact that men outperform women in spatial analysis, a skillset many engineering professionals deem vital, generates this misconception. Feminist scholars postulate that boys are more likely to gain spatial skills outside the classroom because they are culturally and socially encouraged to build and work with their hands. Research shows that girls can develop these same skills with the same form of training.

A 1996 USA study of college freshmen by the Higher Education Research Institute shows that men and women differ greatly in their intended fields of study. Of first-time college freshmen in 1996, 20 percent of men and 4 percent of women planned to major in computer science and engineering, while similar percentages of men and women planned to major in biology or physical sciences. The differences in the intended majors between male and female first-time freshmen directly relate to the differences in the fields in which men and women earn their degree. At the post-secondary level, women are less likely than men to earn a degree in mathematics, physical sciences, or computer sciences and engineering. The exception to this gender imbalance is in the field of life science.

Effects of underrepresentation of women in STEM careers

In Scotland, a large number of women graduate in STEM subjects but fail to move onto a STEM career compared to men. The Royal Society of Edinburgh estimates that doubling women's high-skill contributions to Scotland's economy would benefit it by £170 million per annum.

A 2017 study found that closing the gender gap in STEM education would have a positive impact on economic growth in the EU, contributing to an increase in GDP per capita of 0.7–0.9% across the bloc by 2030 and of 2.2–3.0% by 2050.

Men's and women's earnings

See also: Gender pay gap

Female college graduates earned less on average than male college graduates, even though they shared the earnings growth of all college graduates in the 1980s. Some of the differences in salary are related to the differences in occupations entered by women and men. Among recent science and engineering bachelor's degree recipients, women were less likely than men to be employed in science and engineering occupations. There remains a wage gap between men and women in comparable scientific positions. Among more experienced scientists and engineers, the gender gap in salaries is greater than for recent graduates. Salaries are highest in mathematics, computer science, and engineering, which are fields in which women are not highly represented. In Australia, a study conducted by the Australian Bureau of Statistics has shown that the current gender wage gap between men and women in STEM fields in Australia stands at 30.1 percent as of 2013, which is an increase of 3 percent since 2012. In addition, according to a study done by Moss, when faculty members of top research institutions in America were asked to recruit student applicants for a laboratory manager position, both men and women faculty members rated the male applicants as more hireable and competent for the position, as opposed to the female applicants who shared an identical resume with the male applicants. In the Moss study, faculty members were willing to give the male applicants a higher starting salary and career mentoring opportunities.

Education and perception

The percentage of PhDs in STEM fields in the U.S. earned by women is about 42%, whereas the percentage of PhDs in all fields earned by women is about 52%. Stereotypes and educational differences can lead to the decline of women in STEM fields. These differences start as early as the third grade according to Thomas Dee, with boys advancing in math and science and girls advancing in reading.

Representation of women worldwide

Percentage of students who are female in (a) engineering, manufacturing and construction and (b) information and communication technology programmes in tertiary education, 2017 or latest year

UNESCO, among other agencies including the European Commission and The Association of Academies and Societies of Sciences in Asia (AASSA), have been outspoken about the underrepresentation of women in STEM fields globally.

Despite their efforts to compile and interpret comparative statistics, it is necessary to exercise caution. Ann Hibner Koblitz has commented on the obstacles regarding the making of meaningful statistical comparisons between countries:

For a variety of reasons, it is difficult to obtain reliable data on international comparisons of women in STEM fields. Aggregate figures do not tell us much, especially since terminology describing educational levels, content of majors, job categories, and other markers varies from country to country.

Even when different countries use the same definitions of terms, the social significance of the categories may differ considerably. Koblitz remarks:

It is not possible to use the same indicators to determine the situation in every country. The significant statistic might be the percentage of women teaching at the university level. But it might also be the proportion of women at research institutes and academies of sciences (and at what level), or the percentage of women who publish (or who publish in foreign as opposed to domestic journals), or the proportion of women who go abroad for conferences, post-graduate study, and so on, or the percentage of women awarded grants by national and international funding agencies. Indices can have different meanings in different countries, and the prestige of various positions and honors can vary considerably.

Africa

According to UNESCO statistics, 30% of the Sub-Saharan tech workforce are women; this share rose to 33.5 percent in 2018. South Africa features among the top 20 countries in the world for the share of professionals with skills in artificial intelligence and machine learning, with women representing 28 percent of these South African professionals.

Asia

Proportion of female graduates in science programmes in tertiary education in Asia

A fact sheet published by UNESCO in March 2015 presented worldwide statistics of women in the STEM fields, with a focus on Asia and the Pacific region. It reports that, worldwide, 30 percent of researchers are women; as of 2018, this share had increased to 33 percent. In these areas, East Asia, the Pacific, South Asia and West Asia had the most uneven balance, with 20 percent of researchers being women in each of those sub-regions. Meanwhile, Central Asia had the most equal balance in the region, with women comprising 46 percent of its researchers. The Central Asian countries Azerbaijan and Kazakhstan were the only countries in Asia with women as the majority of their researchers, though in both cases it was by a very small margin.

Countries	Percentage of researchers who are female
Central Asia	46%
World	30%
South and West Asia	20%
East Asia and the Pacific	20%

Cambodia

As at 2004, 13.9% of students enrolled in science programs in Cambodia were female and 21% of researchers in science, technology, and innovation fields were female as of 2002. These statistics are significantly lower than those of other Asian countries such as Malaysia, Mongolia, and South Korea. According to a UNESCO report on women in STEM in Asian countries, Cambodia's education system has a long history of male dominance stemming from its male-only Buddhist teaching practices. Starting in 1924, girls were allowed to enroll in school. Bias against women, not only in education but in other aspects of life as well, exists in the form of traditional views of men as more powerful and dignified than women, especially in the home and in the workplace, according to UNESCO's A Complex Formula.

Indonesia

UNESCO's A Complex Formula states that Indonesia's government has been working towards gender equality, especially through the Ministry of Education and Culture, but stereotypes about women's roles in the workplace persist. Due to traditional views and societal norms, women struggle to remain in their careers or to move up in the workplace. Substantially more women are enrolled in science-based fields such as pharmacy and biology than in mathematics and physics. Within engineering, statistics vary based on the specific engineering discipline; women make up 78% of chemical engineering students but only 5% of mechanical engineering students. As of 2005, out of 35,564 researchers in science, technology, and engineering, only 10,874 or 31% were female.

Japan

According to OECD data, about 25 percent of enrollment in STEM-related programs at the tertiary education level in Japan are women.

Kazakhstan

According to OECD data, about 66 percent of enrollment in STEM-related programs at the tertiary education level in Kazakhstan are women.

Malaysia

According to UNESCO, 48.19% of students enrolled in science programs in Malaysia were female as of 2011. This number has grown significantly in the past three decades, during which the country's employment of women has increased by 95%. In Malaysia, over 50% of employees in the computer industry, which is generally a male-dominated field within STEM, are women. Of students enrolled in pharmacy, more than 70% are female, while in engineering only 36% of students are female. Women held 49% of research positions in science, technology, and innovation as of 2011.

Mongolia

According to UNESCO's data from 2012 and 2018 respectively, 40.2% of students enrolled in science programs and 49% of researchers in science, technology, and innovation in Mongolia are female. Traditionally, nomadic Mongol culture was fairly egalitarian, with both women and men raising children, tending livestock, and fighting in battle, which mirrors the relative equality of women and men in Mongolia's modern-day workforce. More females than males pursue higher education and 65% of college graduates in Mongolia are women. However, women earn about 19–30% less than their male counterparts and are perceived by society to be less suited to engineering than men. Thirty percent or less of employees in computer science, construction architecture, and engineering are female while three in four biology students are female.

Nepal

As of 2011, 26.17% of Nepal's science students were women and 19% of their engineering students were also women. In research, women held 7.8% of positions in 2010. These low percentages correspond with Nepal's patriarchal societal values. In Nepal, women that enter STEM fields most often enter forestry or medicine, specifically nursing, which is perceived as a predominantly female occupation in most countries.

South Korea

In 2012, 30.63% of students who enrolled in science programs in South Korea were female, a number that has been increasing since the digital revolution. Numbers of male and female students enrolled at most levels of education are comparable as well, though the gender difference is larger in higher education. Confucian beliefs in the lower societal value of women as well as other cultural factors could influence South Korea's STEM gender gap. In South Korea, as in other countries, the percentage of women in medicine (61.6%) is much higher than the percentage of women in engineering (15.4%) and other more math-based stem fields. In research occupations in science, technology, and innovation, women made up 17% of the workforce as of 2011. In South Korea, most women working in STEM fields are classified as "non-regular" or temporary employees, indicating poor job stability. In a study conducted by the University of Glasgow which examined math anxiety and test performance of boys and girls from various countries, researchers found that South Korea had a high sex difference in mathematics scores, with female students scoring significantly lower than and experiencing more math anxiety on math tests than male students.

Thailand

According to OECD data, about 53 percent of enrollment in STEM-related programs at the tertiary education level in Thailand are women.

Gulf Cooperation Council States

Ann Hibner Koblitz reported on a series of interviews conducted in 2015 in Abu Dhabi with women engineers and computer scientists who had come to the United Arab Emirates and other Gulf states to find opportunities that were not available to them in their home country. The women spoke of a remarkably high level of job satisfaction and relatively little discrimination. Koblitz comments that

...most people in most countries outside of the Middle East have no idea that the region, in particular the UAE, is a magnet for young, dynamic Arab women making successful careers for themselves in a variety of high-tech and other scientific fields; "land of opportunity," "a tech-person's paradise," and yes, even "mecca" were among the terms used to describe the UAE by the women I met.

Central and South America

Nearly half of PhD degrees pursued in Central and South America are completed by women (2018). However, only a small minority is represented at decision-making levels.

A 2018 study gathered 6,849 articles published in Latin America and found that women researchers were 31% of published researchers in 2018, an increase from 27% in 2002. The same study also found that when women lead the research group, women contributors were published 60%, compared to when men are the leaders and the women contributors were published 20%.

When looking at over 1,500 articles related to Botany published in Latin America, a study found that participation from both women and men were equal, whether it be in publications or leading roles in scientific organizations. Also women had higher rates of publication in Argentina, Brazil, and Mexico when compared to other Latin American countries despite participation being nearly the same throughout the region. Although women have higher publications in Botany, men still out publish women and are often the ones cited in research papers and studies relating to the sciences.

Total Enrollment in STEM per Area of Study in Chile

	2015		2016		Change in Percent
Area of Study	Men	Women	Men	Women	Men	Women
Social Sciences	30.7%	69.3%	29.9%	70.1%	-0.8%	+0.8%
Education	30.2%	69.8%	27.4%	72.6%	-2.8%	+2.8%
Health	30.4%	69.6%	23.8%	76.2%	-6.6%	+6.6%
Technology	81.8%	18.2%	78.2%	21.8%	-3.6%	+3.6%

The study concluded that according to the data (shown in the table above), women in Chile that are enrolled in STEM have higher enrollment in the sciences closely related to Biology and Medicine than other sciences in the technological field. After graduation women made up 67.70% of the workers in Engineering in Health and 59.80% of workers in Biomedical Engineering. While in other fields, such as Mechanical Engineering or Electrical Engineering (the more technical fields), men dominated the workforce with over 90% of workers being male.

Europe

Percentage of women graduates in ICT tertiary education programmes

 

Share of women employed as ICT specialists

 

Share of women employed in the ICT sector, divided according to qualification level

In the European Union only 16.7% on average of ICT (Information and communication technology) specialists are women. Only in Romania and Bulgaria do women hold more than 25 percent of these roles. The gender distribution is more balanced, particularly in new member states when taking into account ICT technicians (middle and low-ranking positions).

In 2012, the percentage of women PhD graduates was 47.3% of the total, 51% of the social sciences, business and law, 42% of the science, mathematics and computing, and just the 28% of PhD graduates in engineering, manufacturing and construction. In the computing subfield only 21% of PhD graduates were women. In 2013 in the EU as an average men scientists and engineers made up 4.1% of total labour force, while women made up only 2.8%. In more than half of the countries women make up less than 45% of scientists and engineers. The situation has improved, as between 2008 and 2011 the number of women amongst employed scientists and engineers grew by an average of 11.1% per year, while the number of men grew only by 3.3% over the same period.

In 2015, in Slovenia, Portugal, France, Sweden, Norway, and Italy there were more boys than girls taking advanced courses in mathematics and physics in secondary education in Grade 12.

In 2018, European Commissioner for Digital Economy and Society Mariya Gabriel announced plans to increase the participation of women in the digital sector by challenging stereotypes; promoting digital skills and education and advocating for more women entrepreneurs. In 2018, Ireland took the step of linking research funding from the Higher Education Authority to an institution’s ability to reduce gender inequality.

North America

United States

According to the National Science Foundation, women comprise 43 percent of the U.S. workforce for scientists and engineers (S&E) under 75 years old. For those under 29 years old, women comprise 56% of the science and engineering workforce. Of scientists and engineers seeking employment, 50% under 75 are women, and 49% under 29 are women. About one in seven engineers are female. However, women comprise 28% of workers in S&E occupations - not all women who are trained as S&E are employed as scientists or engineers. Women hold 58% of S&E related occupations.

Women in STEM fields earn considerably less than men, even after controlling for a wide set of characteristics such as education and age. On average, men in STEM jobs earn $36.34 per hour while women in STEM jobs earn $31.11 per hour.

There are many reasons why gender pay gaps in STEM fields continue to exist which include women choosing STEM majors that pay less. However, even with the same degree, women still earned less. A research study on starting pay with an engineering degree found that women earned less than $61,000 while men earned more than $65,000.

Percentage distribution of total college graduates aged 25–34 in the U.S. (2019). Fields are defined by NCES.

Bachelor's degree field	Men (%)	Women (%)
Computer and information sciences	6.8	2.0
Engineering/ engineering technologies	14.8	3.7
Biology/ biomedical sciences	6.0	6.7
Mathematics/statistics	1.7	1.0
Physical sciences	3.5	2.3
STEM total	32.8	15.7
Business	21.7	15.5
Education	3.0	9.8
Health Studies	3.5	12.4
Humanities	9.6	12.4
Social sciences, psychology, and history	13.5	16.1
All other fields of study	15.9	18.0
Non-STEM total	67.2	84.3
Total graduates (%)	33.0	41.1

Women dominate the total number of persons with bachelor's degrees, as well as those in STEM fields defined by the National Center for Education Statistics. However, they are underrepresented in specific fields including Computer Sciences, Engineering, and Mathematics. Along with women, racial/ethnic minorities in the United States are also underrepresented in STEM.

Asian women are well represented in STEM fields in the U.S.(though not as much as males of the same ethnicity) compared to African American, Hispanic, Pacific Islander, and Native American women. Within academia, these minority women represent less than 1% of tenure-track positions in the top 100 U.S. universities despite constituting approximately 13% of total US population. A 2015 study suggested that attitudes towards hiring women in STEM tenure track positions has improved, with a 2:1 preference for women in STEM after adjusting for equal qualifications and lifestyles (e.g., single, married, divorced).

Ratio of number of actual to expected graduates if there were no imbalances due to gender/race ages 25–34 in the U.S. (2014). Fields defined by NCES.

	Total		STEM
Race/ethnicity	Men	Women	Men	Women
White	1.05	1.32	1.05	1.15
Black	0.49	0.73	0.44	0.68
Hispanic	0.37	0.54	0.37	0.48
Asian	1.85	1.94	3.12	2.61
Pacific Islander	0.32	0.44	0.38	0.52
American Indian/Alaska Native	0.32	0.46	0.27	0.44
Other race	1.00	1.35	1.22	1.33
Two or more races	0.97	1.15	1.11	1.19

African American women

According to Kimberly Jackson, prejudice and assumed stereotypes keep women of color, especially black women from studying in STEM fields. Psychologically, stereotypes on black women's intellect, cognitive abilities, and work ethic contribute to their lack of confidence in STEM. Some schools, such as Spelman College, have made attempts to change perceptions of African-American women and improve their rates of becoming involved and technically proficient in STEM. Students of color, especially Black students, face difficulty in STEM majors as they face hostile climates, microaggressions, and a lack of support and mentorship.

Latin American women

A 2015 NCWIT study estimated that Latin American women represented only 1% of the US tech workforce. A 2018 study on 50 Latin American women who founded a technology company indicated that 20% were Mexican, 14% bi-racial, 8% unknown, 4% Venezuelan.

Canada

A Statistics Canada study from 2019 found that first-year women make up 44% of STEM students, compared with 64% of non-STEM students. Those women who transfer out of STEM courses usually go to a related field, such as health care or finance. A study conducted by the University of British Columbia discovered that only 20–25% of computer science students from all Canadian colleges and universities are women. As well, only about 1 in 5 of that percentage will graduate from those programs.

Statistically, women are less likely to choose a STEM program, regardless of mathematical ability. Young men with lower marks in mathematics are more likely to pursue STEM fields than their women-identified peers with higher marks in mathematics.

Oceania

Australia

Australia has only recently made significant attempts to promote participation of women in STEMM disciplines, including the formation of Women in STEMM Australia in 2014, a non-profit organisation that aims at connecting women in STEMM disciplines in a coherent network. Similarly, the STEM Women directory has been established to promote gender equity by showcasing the diversity of talent in Australian women in STEM fields. In 2015, the SAGE (Science in Australia Gender Equity) was started as a joint venture of the Australian Academy of Science and the Australian Academy of Technology and Engineering. The program is tasked with implementing a pilot of the Athena SWAN accreditation framework within Australian higher education institutions.

Underrepresentation in STEM-related awards and competitions

In terms of the most prestigious awards in STEM fields, fewer have been awarded to women than to men. Between 1901 and 2017 the female:total ratio of Nobel Prizes were 2:207 for physics, 4:178 for chemistry, 12:214 for physiology/medicine, and 1:79 for economic sciences. The ratios for other fields were 14:114 in literature and 16:104 for peace. Maryam Mirzakhani was the first woman and first Iranian to receive the Fields Medal in 2014. The Fields Medal, is one of the most prestigious prize in mathematics, and has been awarded 56 times in total.

Fewer female students participate in prestigious STEM-related competitions such as the International Mathematical Olympiad. In 2017, only 10% of the IMO participants were female and there was one female on the South Korean winning team of six.

Recent advances in technology

Naomi Wu demonstrating how to configure a Raspberry Pi 2

Abbiss states that "the ubiquity of computers in everyday life has seen the breaking down of gender distinctions in preferences for and the use of different applications, particularly in the use of the internet and email." Both genders have acquired skills, competencies and confidence in using a variety of technological, mobile and application tools for personal, educational and professional use at high school level, but the gap still remains when it comes to enrollment of girls in computer science classes, which declines from grades 10 to 12. For higher education programs in information and communications technology, women make up only 3% of graduates globally.

A review of UK patent applications, in 2016, found that the proportion of new inventions registered by women was rising, but that most female inventors were active in stereotypically female fields such as "designing bras and make-up". 94% of inventions in the field of computing, 96% in automotive applications and mining, and 99% in explosives and munitions, were by men. In 2016 Russia had the highest percentage of patents filed by women, at about 16%. Then in 2019, the USPTO issued a report showing that the share of female inventors listed on US patents had recently risen to about 17%.

Explanations for low representation of women

There are a variety of proposed reasons for the relatively low numbers of women in STEM fields. These can be broadly classified into societal, psychological, and innate explanations. However, explanations are not necessarily restricted to just one of these categories.

Societal

Discrimination

This leakage may be due to discrimination, both overt and covert, faced by women in STEM fields. According to Schiebinger, women are twice as likely to leave jobs in science and engineering than men are. In the 1980s, researchers demonstrated a general evaluative bias against women.

In a 2012 study, email requests were sent to meet to professors in doctoral programs at the top 260 U.S. universities. It was impossible to determine whether any particular individual in this study was exhibiting discrimination, since each participant only viewed a request from one potential graduate student. However, researchers found evidence for discrimination against ethnic minorities and women relative to Caucasian men. In another study, science faculty were sent the materials of students who were applying for a lab manager position at their university. The materials were the same for each participant, but each application was randomly assigned either a male or a female name. The researchers found that faculty members rated the male candidates as both more competent and more hirable than the female candidates, despite applications being otherwise identical. If individuals are given information about a prospective student's gender, they may infer that he or she possesses traits consistent with stereotypes for that gender. A study in 2014 found that men are favored in some domains, such as tenure rates in biology, but that the majority of domains were gender-fair. The authors interpreted this to suggest that the underrepresentation of women in the professorial ranks was not solely caused by sexist hiring, promotion, and remuneration.

Audery Azoulay, UNESCO Chief, stated that even in, "21st century, women and girls are sidelined in science-related fields due to their gender." A 2017 survey showed that women working in the STEM fields are more likely to experience workplace discrimination than men. Around half of the women in the STEM profession have experienced gender-based discrimination, such as the man being paid more for the same job, being treated like they do not qualify for the job, or being mocked or insulted. Some women also stated that in a workplace where most employees were male, they felt that being a woman was a barrier to their success.

Stereotypes

Stereotypes about what someone in a STEM field should look and act like may cause established members of these fields to overlook individuals who are highly competent. The stereotypical scientist or individual in another STEM profession is usually thought to be male. Women in STEM fields may not fit individuals' conception of what a scientist, engineer, or mathematician "should" look like and may thus be overlooked or penalized. The Role Congruity Theory of Prejudice states that perceived incongruity between gender and a particular role or occupation can result in negative evaluations. In addition, negative stereotypes about women's quantitative abilities may lead people to devalue their work or discourage these women from continuing in STEM fields.

Both men and women who work in "nontraditional" occupations may encounter discrimination, but the forms and consequences of this discrimination are different. Individuals of a particular gender are often perceived to be better suited to particular careers or areas of study than those of the other gender. A study found that job advertisements for male-dominated careers tended to use more agentic words (or words denoting agency, such as "leader" and "goal-oriented") associated with male stereotypes. Social Role Theory, proposed in 1991, states that men are expected to display agentic qualities and women to display communal qualities. These expectations can influence hiring decisions. A 2009 study found that women tended to be described in more communal terms and men in more agentic terms in letters of recommendation. These researchers also found that communal characteristics were negatively related to hiring decisions in academia.

Although women entering traditionally male professions face negative stereotypes suggesting that they are not "real" women, these stereotypes do not seem to deter women to the same degree that similar stereotypes may deter men from pursuing nontraditional professions. There is historical evidence that women flock to male-identified occupations once opportunities are available. On the other hand, examples of occupations changing from predominantly female to predominantly male are very rare in human history. The few existing cases—such as medicine—suggest that redefinition of the occupations as appropriately masculine is necessary before men will consider joining them.

Although men in female-dominated occupations may contend with negative stereotypes about their masculinity, they may also experience certain benefits. In 1992 it was suggested that women in male-dominated occupations tended to hit a glass ceiling; while men in female-dominated occupations may hit a "glass escalator".

Black Sheep effect

Main article: Black sheep effect

The Black Sheep effect occurs when individuals are likely to evaluate members of their in-group more favorably than members of their out-group when those members are highly qualified. However, when an individual's in-group members have average or below average qualities, they are likely to evaluate them much lower than out-group members with equivalent qualifications. This suggests that established women in STEM fields will be more likely than established men to help early career women who display sufficient qualifications. However, established women will be less likely than men to help early career women who display insufficient qualifications.

Queen Bee effect

The Queen Bee effect is similar to the Black Sheep effect but applies only to women. It explains why higher-status women, particularly in male-dominated professions, may actually be far less likely to help other women than their male colleagues might be. A 2004 study found that while doctoral students in a number of different disciplines did not exhibit any gender differences in work commitment or work satisfaction, faculty members at the same university believed that female students were less committed to their work than male students. What was particularly surprising was that these beliefs by faculty members were most strongly endorsed by female faculty members, rather than male faculty members. One potential explanation for this finding is that individual mobility for a member of a negatively stereotyped group is often accompanied by a social and psychological distancing of oneself from the group. This implies that successful women in traditionally male-dominated careers do not see their success as evidence that negative stereotypes about women's quantitative and analytical abilities are wrong, but rather as proof that they personally are exceptions to the rule. Thus, such women may actually play a role in perpetuating, rather than abolishing, these negative stereotypes.

Mentorship

In STEM fields, the support and encouragement of a mentor can make a lot of difference in women's decisions of whether or not to continue pursuing a career in their discipline. This may be particularly true for younger individuals who may face many obstacles early on in their careers. Since these younger individuals often look to those who are more established in their discipline for help and guidance, the responsiveness and helpfulness of potential mentors is incredibly important. There are many emerging mentorship programs. However, many women experience harassment from their mentors which can cause them to be unable to finish the program among many other issues.

A 2020 study surveyed women who are working in STEM field and live in the U.S, Northeast, and Eastern Canada. Most women reported that finding a mentor at their workplace was complex, and only a third of the women had some sort of mentor, formal or informal. During their time in school , half of the participants were able find a professor to be their mentor. They added that mentorship helped them complete their degree and guided them from the educational sphere to the workplace. The majority of the women agreed that mentorship is a crucial resource, and many want to be involved in mentorship, but there are not enough resources or opportunities in their work environment.

Lack of support

Women in STEM may leave due to not being invited to professional meetings, the use of sexually discriminating standards against women, inflexible working conditions, the perceived need to hide pregnancies, and the struggle to balance family and work. Women in STEM fields that have children either need child care or to take a long leave of absence. When a nuclear family can not afford child care, typically it is the mother that gives up her career to stay at home with the children. This is due in part to women being paid statistically less in their careers. The man makes more money so the man goes to work and the woman gives up her career. Maternity leave is another issue women in STEM fields face. In the U.S., maternity leave is required by The Family and Medical Leave Act of 1993 (FMLA). The FMLA requires 12 weeks of unpaid leave annually for mothers of newborn or newly adopted children. This is one of the lowest levels of leave in the industrialized world. All developed countries except the United States guarantee mothers at least some paid time off. If a new mother does not have external financial support or savings, they may not be able to take their full maternity leave. Few companies allow men to take paternity leave and it may be shorter than women's maternity leave.

Harassment

In 1993, The New England Journal of Medicine indicated that three-quarters of women students and residents were harassed at least once during their medical training. The 2020 Tribeca Film Festival documentary, "Picture a Scientist", highlighted the severe sexual and physical harassment women in STEM fields can face, often without adequate recourse. In that film Jane Willenbring, a female scientist and associate professor at Scripps Institution of Oceanography, shared how she was harassed by her mentor David R. Marchant during her fieldwork. She was called many demeaning names, harassed when using the bathroom, and even had shards of volcanic sand blown into her eyes.

Lack of role models

In engineering and science education, women made up almost 50 percent of non-tenure track lecturer and instructor jobs, but only 10 percent of tenured or tenure-track professors in 1996. In addition, the number of female department chairs in medical schools did not change from 1976 to 1996. Moreover, women who do make it to tenured or tenure-track positions may face the difficulties associated with holding a token status. They may lack support from colleagues and may face antagonism from peers and supervisors. Research has suggested that women's lack of interest may in part stem from stereotypes about employees and workplaces in STEM fields, to which stereotypes women are disproportionately responsive.

Clustering and leaky pipeline

In the early 1980s, Rossiter put forth the concept of "territorial segregation" or occupational segregation, which is the idea that women "cluster" in certain fields of study. For example, "women are more likely to teach and do research in the humanities and social sciences than in the natural sciences and engineering", and the majority of college women tend to choose majors such as psychology, education, English, performing arts, and nursing.

Rossiter also used "hierarchical segregation" as an explanation for the low number of women in STEM fields. She describes "hierarchical segregation" as a decrease in the number of women as one "moves up the ladder of power and prestige." This is related to the leaky STEM pipeline concept. The metaphor of the leaky pipeline has been used to describe how women drop out of STEM fields at all stages of their careers. In the U.S., out of 2,000 high school aged persons, 1944 were enrolled in high school fall 2014. Assuming equal enrollment for boys and girls, 60 boys and 62 girls are considered "gifted." By comparing enrollment to the population of persons 20–24 years old, 880 of the 1000 original women, and 654 of the original 1000 men will enroll in college (2014). In freshman year 330 women and 320 men will express an intent to study science or engineering. Of these only 142 women and 135 men will actually obtain a bachelor's degree in science or engineering, and only 7 women and 10 men will obtain a PhD in science or engineering.

Psychological

Lack of interest

A meta-analysis concluded that men prefer working with things and women prefer working with people. When interests were classified by RIASEC type (Realistic, Investigative, Artistic, Social, Enterprising, Conventional), men showed stronger Realistic and Investigative interests, and women showed stronger Artistic, Social, and Conventional interests. Sex differences favoring men were also found for more specific measures of engineering, science, and mathematics interests.

In a 3-year interview study, Seymour and Hewitt (1997) found that perceptions that non-STEM academic majors offered better education options and better matched their interests was the most common (46%) reason provided by female students for switching majors from STEM areas to non-STEM areas. The second most frequently cited reason given for switching to non-STEM areas was a reported loss of interest in the women's chosen STEM majors. Additionally, 38% of female students who remained in STEM majors expressed concerns that there were other academic areas that might be a better fit for their interests. Preston's (2004) survey of 1,688 individuals who had left sciences also showed that 30 percent of the women endorsed "other fields more interesting" as their reason for leaving.

Advanced math skills do not often lead women to be interested in a STEM career. A Statistics Canada survey found that even young women of high mathematical ability are much less likely to enter a STEM field than young men of similar or even lesser ability.

A 2018 study originally claimed that countries with more gender equality had fewer women in science, technology, engineering and mathematics (STEM) fields. Some commentators argued that this was evidence of gender differences arising in more progressive countries, the so-called gender-equality paradox. However, a 2019 correction to the study outlined that the authors had created a previously undisclosed and unvalidated method to measure "propensity" of women and men to attain a higher degree in STEM, as opposed to the originally claimed measurement of "women’s share of STEM degrees". Harvard researchers were unable to independently recreate the data reported in the study. A follow-up paper by the researchers who discovered the discrepancy found conceptual and empirical problems with the gender-equality paradox in STEM hypothesis.

Lack of confidence

According to A. N. Pell, the pipeline has several major leaks spanning the time from elementary school to retirement. One of the most important periods is adolescence. One of the factors behind girls' lack of confidence might be unqualified or ineffective teachers. Teachers' gendered perceptions on their students' capabilities can create an unbalanced learning environment and deter girls from pursuing further STEM education. They can also pass these stereotyped beliefs onto their students. Studies have also shown that student-teacher interactions affect girls' engagement with STEM. Teachers often give boys more opportunity to figure out the solution to a problem by themselves while telling the girls to follow the rules. Teachers are also more likely to accept questions from boys while telling girls to wait for their turns. This is partly due to gender expectations that boys will be active but that girls will be quiet and obedient. Prior to 1985, girls were provided fewer laboratory opportunities than boys. In middle and high school, science, mathematics, mechanics and computers courses are mainly taken by male students and also tend to be taught by male teachers. A lack of opportunities in STEM fields could lead to a loss of self-esteem in math and science abilities, and low self-esteem could prevent people from entering science and math fields.

One study found that women steer away from STEM fields because they believe they are not qualified for them; the study suggested that this could be fixed by encouraging girls to participate in more mathematics classes. Out of STEM-intending students, 35% of women stated that their reason for leaving calculus was due to lack of understanding the material, while only 14% of men stated the same. The study reports that this difference in reason for leaving calculus is thought to develop from women's low level of confidence in their ability, and not actual skill. This study continues to establish that women and men have different levels of confidence in their ability and that confidence is related to how individual's performance in STEM fields. It was seen in another study that when men and women of equal math ability were asked to rate their own ability, women will rate their own ability at a much lower level. Programs with the purpose to reduce anxiety in math or increase confidence have a positive impact on women continuing their pursuit of a career in the STEM field. Not only can the issue of confidence keep women from even entering STEM fields, but even women in upper-level courses with higher skill are more strongly affected by the stereotype that they (by nature) do not possess innate ability to succeed. This can cause a negative effect on confidence for women despite making it through courses designed to filter students out of the field. Being chronically outnumbered and underestimated can fuel feelings of imposter syndrome reported by many women in the STEAM field.

Stereotype threat

Stereotype threat arises from the fear that one's actions will confirm a negative stereotype about one's in-group. This fear creates additional stress, consuming valuable cognitive resources and lowering task performance in the threatened domain. Individuals are susceptible to stereotype threat whenever they are assessed in a domain for which there is a perceived negative stereotype about a group to which they belong. Stereotype threat undermines the academic performance of women and girls in math and science, which leads to an underestimation of abilities in these subjects by standard measures of academic achievement. Individuals who identify strongly with a certain area (e.g., math) are more likely to have their performance in that area hampered by stereotype threat than those who identify less strongly with the area. This means that even highly motivated students from negatively stereotyped groups are likely to be adversely affected by stereotype threat and thus may come to disengage from the stereotyped domain. Negative stereotypes about girls’ capabilities in mathematics and science drastically lower their performance in mathematics and science courses as well as their interest in pursuing a STEM career. Studies have found that gender differences in performance disappear if students are told that there are no gender differences on a particular mathematics test. This indicates that the learning environment can greatly impact success in a course.

Stereotype threat has been criticized on a theoretical basis. Several attempts to replicate its experimental evidence have failed. The findings in support of the concept have been suggested to be the product of publication bias.

A study was done to determine how stereotype threat and math identification can affect women who were majoring in a STEM related field. There were three different situations, designed to test the impact of stereotype on performance in math. One group of women were informed that men had previously out-performed women on the same calculus test they were about to take. The next group was told men and women had performed at the same level. The last group was told nothing about how men had performed and there was no mention of gender before taking their test. Out of these situations, women performed at their best scores when there was no mention of gender. The worst scores were from the situation where women were told that men had performed better than women. For women to pursue the male-dominated field of STEM, previous research shows that they must have more confidence in math/science ability.

Innate versus learned skill

Some studies propose the explanation that STEM fields (and especially fields like physics, math and philosophy) are considered by both teachers and students to require more innate talent than skills that can be learned. Combined with a tendency to view women as having less of the required innate abilities, researchers proposed that this can result in assessing women as less qualified for STEM positions. In a study done by Ellis, Fosdick and Rasmussen, it was concluded that without strong skills in calculus, women cannot perform as well as their male counterparts in any field of STEM, which leads to the fewer women pursuing a career in these fields. A high percentage of women that do pursue a career in STEM do not continue on this pathway after taking Calculus I, which was found to be a class that weeds out students from the STEM pathway.

There have been several controversial statements about innate ability and success in STEM. A few notable examples include Lawrence Summers, former president of Harvard University who suggested cognitive ability at high end positions could cause a population difference. Summers later stepped down as president. Former Google engineer, James Damore, wrote a memo entitled Google's Ideological Echo Chamber suggesting that differences in trait distributions between men and women was a reason for gender imbalance in STEM. The memo stated that affirmative action to reduce the gap could discriminate against highly qualified male candidates. Damore was fired for sending out this memo.

Comparative advantage

A 2019 study by two Paris economists suggests that women's under-representation in STEM fields could be the result of comparative advantage, caused not by girls' 10% lower performance on math tests, but rather their far superior reading performance, which, when taken together with their math performance, results in almost one standard deviation better overall performance than boys, which is theorized to make women more likely to study humanities-related subjects than math-related ones.

The current gender gap, however, is widely considered to be economically inefficient overall.

Strategies for increasing representation of women

The CMS Girls Engineering Camp at Texas A&M University–Commerce in June 2015

There are a multitude of factors that may explain the low representation of women in STEM careers. Anne-Marie Slaughter, the first woman to hold the position of Director of Policy Planning for the United States Department of State, has recently suggested some strategies to the corporate and political environment to support women to fulfill to the best of their abilities the many roles and responsibilities that they undertake. The academic and research environment for women may benefit by applying some of the suggestions she has made to help women excel, while maintaining a work-life balance.

Social-psychological interventions

A number of researchers have tested interventions to alleviate stereotype threat for women in situations where their math and science skills are being evaluated. The hope is that by combating stereotype threat, these interventions will boost women's performance, encouraging a greater number of them to persist in STEM careers.

One simple intervention is simply educating individuals about the existence of stereotype threat. Researchers found that women who were taught about stereotype threat and how it could negatively impact women's performance in math performed as well as men on a math test, even when stereotype threat was induced. These women also performed better than women who were not taught about stereotype threat before they took the math test.

Role models

One of the proposed methods for alleviating stereotype threat is through introducing role models. One study found that women who took a math test that was administered by a female experimenter did not suffer a drop in performance when compared to women whose test was administered by a male experimenter. Additionally, these researchers found that it was not the physical presence of the female experimenter but rather learning about her apparent competence in math that buffered participants against stereotype threat. The findings of another study suggest that role models do not necessarily have to be individuals with authority or high status, but can also be drawn from peer groups. This study found that girls in same-gender groups performed better on a task that measured math skills than girls in mixed-gender groups. This was due to the fact that girls in the same-gender groups had greater access to positive role models, in the form of their female classmates who excelled in math, than girls in mixed-gender groups. Similarly, another experiment showed that making groups achievements salient helped buffer women against stereotype threat. Female participants who read about successful women, even though these successes were not directly related to performance in math, performed better on a subsequent math test than participants who read about successful corporations rather than successful women. A study investigating the role of textbook images on science performance found that women demonstrated better comprehension of a passage from a chemistry lesson when the text was accompanied by a counter-stereotypic image (i.e., of a female scientist) than when the text was accompanied by a stereotypic image (i.e., of a male scientist). Other scholars distinguish between the challenges of both recruitment and retention in increasing women's participation in STEM fields. These researchers suggest that although both female and male role models can be effective in recruiting women to STEM fields, female role models are more effective at promoting the retention of women in these fields. Female teachers can also act as role models for young girls. Reports have shown that the presence of female teachers positively influences girls' perceptions of STEM and increases their interest in STEM careers.

Self-affirmation

Researchers have investigated the usefulness of self-affirmation in alleviating stereotype threat. One study found that women who affirmed a personal value prior to experiencing stereotype threat performed as well on a math test as men and as women who did not experience stereotype threat. A subsequent study found that a short writing exercise in which college students, who were enrolled in an introductory physics course, wrote about their most important values substantially decreased the gender performance gap and boosted women's grades. Scholars believe that the effectiveness of such values-affirmation exercises is their ability to help individuals view themselves as complex individuals, rather than through the lens of a harmful stereotype. Supporting this hypothesis, another study found that women who were encouraged to draw self-concept maps with many nodes did not experience a performance decrease on a math test. However, women who did not draw self-concept maps or only drew maps with a few nodes did perform significantly worse than men on the math test. The effect of these maps with many nodes was to remind women of their "multiple roles and identities," that were unrelated to, and would thus not be harmed by, their performance on the math test.

Strategies to increase women's and girls' interest in STEM

Organized efforts

To increase women's enrollment in the STEM field, researchers believe that it should occur in elementary and middle schools. Gender differences are evident by kindergarten, and many children have developed an attitude towards math and their career. According to a study about high school and middle school students, there is evidence of a gender gap in science and math test scores. Another method to reduce the gender gap is to create communities and opportunities apart from school. For instance, creating a residential program, women's only college, and affiliation between high school and college for STEM programs will help eliminate the gender gap. The research has shown that gender gap in STEM might be because of unsupportive culture that hurts woman's advancement in their career. Therefore, women all over the United States are underrepresented in tenure faculty and leadership positions.

Organizations such as Girls Who Code, StemBox, and Stanford's Women in Data Science Initiative aim to encourage women and girls to explore male-dominated STEM fields. Many of these organizations offer summer programs and scholarships to girls interested in STEM fields.

The U.S. government has funded similar endeavors; the Department of State's Bureau of Educational and Cultural Affairs created TechGirls and TechWomen, exchange programs which teach Middle Eastern and North African girls and women skills valuable in STEM fields and encourage them to pursue STEM careers. There is also the TeachHer Initiative, spearheaded by UNESCO, Costa Rican First Lady, Mercedes Peñas Domingo, and Jill Biden which aims to close the gender gap in STEAM curricula and careers. The Initiative also emphasizes the importance of after school activities and clubs for girls. That’s why Dell Technologies teamed up with Microsoft and Intel in 2019 to create an after-school program for young girls and underserved K-12 students across the U.S. and Canada called Girls Who Game (GWG). The program uses Minecraft: Education Edition as a tool to teach the girls communication, collaboration, creativity, and critical thinking skills.

Current campaigns to increase women's participation within STEM fields include the UK's GlamSci, and Verizon's #InspireHerMind project. The US Office of Science and Technology Policy during the Obama administration collaborated with the White House Council on Women and Girls to increase the participation of women and girls within STEM fields along with the "Educate to Innovate" campaign.

In August 2019, the University of Technology Sydney announced that women, or anyone with a long term educational disadvantage, applying to the Faculty of Engineering and Information Technology, and for a construction project management degree in the Faculty of Design, Architecture and Building, will be required to have a minimum Australian Tertiary Admission Rank that is ten points lower than that required of other students.

Programs such as FIRST(For Inspiration and Recognition of Science and Technology) are constantly working to eliminate the gender gap in computer science. FIRST is a robotic and research platform for students from kindergarten through  high school. The activities and competitions  in the program are usually about current STEM problems. According to the report, around 13.7 percent of men and 2.6 percent of women entering college hope to major in engineering. In contrast, 67 percent of men and 47 percent of women who engaged in the FIRST program  tend to major in engineering.