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Friday, October 9, 2015

Deuterium


From Wikipedia, the free encyclopedia
Deuterium
Hydrogen-2.svg
Deuterium
General
Name, symbol Hydrogen-2,2H or D
Neutrons 1
Protons 1
Nuclide data
Natural abundance 0.0115% (Earth)
Isotope mass 2.01410178 u
Spin 1+
Excess energy 13135.720± 0.001 keV
Binding energy 2224.52± 0.20 keV
Deuterium (symbol D or 2H, also known as heavy hydrogen) is one of two stable isotopes of hydrogen. The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the far more common hydrogen isotope, protium, has no neutron in the nucleus. Deuterium has a natural abundance in Earth's oceans of about one atom in 6420 of hydrogen. Thus deuterium accounts for approximately 0.0156% (or on a mass basis 0.0312%) of all the naturally occurring hydrogen in the oceans, while the most common isotope (hydrogen-1 or protium) accounts for more than 99.98%. The abundance of deuterium changes slightly from one kind of natural water to another (see Vienna Standard Mean Ocean Water).
The deuterium isotope's name is formed from the Greek deuteros meaning "second", to denote the two particles composing the nucleus.[1] Deuterium was discovered and named in 1931 by Harold Urey, earning him a Nobel Prize in 1934. This was followed by the discovery of the neutron in 1932, which made the nuclear structure of deuterium obvious. Soon after deuterium's discovery, Urey and others produced samples of "heavy water" in which the deuterium had been highly concentrated.

Deuterium is destroyed in the interiors of stars faster than it is produced. Other natural processes are thought to produce only an insignificant amount of deuterium. Theoretically nearly all deuterium found in nature was produced in the Big Bang 13.8 billion years ago, as the basic or primordial ratio of hydrogen-1 (protium) to deuterium (about 26 atoms of deuterium per million hydrogen atoms) has its origin from that time. This is the ratio found in the gas giant planets, such as Jupiter (see references 2,3 and 4). However, other astronomical bodies are found to have different ratios of deuterium to hydrogen-1. This is thought to be as a result of natural isotope separation processes that occur from solar heating of ices in comets. Like the water-cycle in Earth's weather, such heating processes may enrich deuterium with respect to protium. The analysis of deuterium/protium ratios in comets found results very similar to the mean ratio in Earth's oceans (156 atoms of deuterium per million hydrogens). This reinforces theories that much of Earth's ocean water is of cometary origin.[2][3] The deuterium/protium ratio of the comet 67P/Churyumov-Gerasimenko, as measured by the Rosetta space probe, is about three times that of earth water. This figure is the highest yet measured in a comet.[4]

Deuterium/protium ratios thus continue to be an active topic of research in both astronomy and climatology.

Differences between deuterium and common hydrogen (protium)

Chemical symbol


Deuterium discharge tube

Deuterium is frequently represented by the chemical symbol D. Since it is an isotope of hydrogen with mass number 2, it is also represented by 2H. IUPAC allows both D and 2H, although 2H is preferred.[5] A distinct chemical symbol is used for convenience because of the isotope's common use in various scientific processes. Also, its large mass difference with protium (1H) (deuterium has a mass of 2.014102 u, compared to the mean hydrogen atomic weight of 1.007947 u, and protium's mass of 1.007825 u) confers non-negligible chemical dissimilarities with protium-containing compounds, whereas the isotope weight ratios within other chemical elements are largely insignificant in this regard.

Spectroscopy

In quantum mechanics the energy levels of electrons in atoms depend on the reduced mass of the system of electron and nucleus. For the hydrogen atom, the role of reduced mass is most simply seen in the Bohr model of the atom, where the reduced mass appears in a simple calculation of the Rydberg constant and Rydberg equation, but the reduced mass also appears in the Schrödinger equation, and the Dirac equation for calculating atomic energy levels.

The reduced mass of the system in these equations is close to the mass of a single electron, but differs from it by a small amount about equal to the ratio of mass of the electron to the atomic nucleus. For hydrogen, this amount is about 1837/1836, or 1.000545, and for deuterium it is even smaller: 3671/3670, or 1.0002725. The energies of spectroscopic lines for deuterium and light-hydrogen (hydrogen-1) therefore differ by the ratios of these two numbers, which is 1.000272. The wavelengths of all deuterium spectroscopic lines are shorter than the corresponding lines of light hydrogen, by a factor of 1.000272. In astronomical observation, this corresponds to a blue Doppler shift of 0.000272 times the speed of light, or 81.6 km/s.[6]

The differences are much more pronounced in vibrational spectroscopy such as infrared spectroscopy and Raman spectroscopy,[1] and in rotational spectra such as microwave spectroscopy because the reduced mass of the deuterium is markedly higher than that of protium.

Deuterium and Big Bang nucleosynthesis

Deuterium is thought to have played an important role in setting the number and ratios of the elements that were formed in the Big Bang. Combining thermodynamics and the changes brought about by cosmic expansion, one can calculate the fraction of protons and neutrons based on the temperature at the point that the universe cooled enough to allow formation of nuclei. This calculation indicates seven protons for every neutron at the beginning of nucleogenesis, a ratio that would remain stable even after nucleogenesis was over. This fraction was in favor of protons initially, primarily because the lower mass of the proton favored their production. As the universe expanded, it cooled. Free neutrons and protons are less stable than helium nuclei, and the protons and neutrons had a strong energetic reason to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium.
Through much of the few minutes after the big bang during which nucleosynthesis could have occurred, the temperature was high enough that the mean energy per particle was greater than the binding energy of weakly bound deuterium; therefore any deuterium that was formed was immediately destroyed. This situation is known as the deuterium bottleneck. The bottleneck delayed formation of any helium-4 until the universe became cool enough to form deuterium (at about a temperature equivalent to 100 keV). At this point, there was a sudden burst of element formation (first deuterium, which immediately fused to helium). However, very shortly thereafter, at twenty minutes after the Big Bang, the universe became too cool for any further nuclear fusion and nucleosynthesis to occur. At this point, the elemental abundances were nearly fixed, with the only change as some of the radioactive products of big bang nucleosynthesis (such as tritium) decay.[7] The deuterium bottleneck in the formation of helium, together with the lack of stable ways for helium to combine with hydrogen or with itself (there are no stable nuclei with mass numbers of five or eight) meant that insignificant carbon, or any elements heavier than carbon, formed in the Big Bang. These elements thus required formation in stars. At the same time, the failure of much nucleogenesis during the Big Bang ensured that there would be plenty of hydrogen in the later universe available to form long-lived stars, such as our Sun.

Abundance

Deuterium occurs in trace amounts naturally as deuterium gas, written 2H2 or D2, but most natural occurrence in the universe is bonded with a typical 1H atom, a gas called hydrogen deuteride (HD or 1H2H).[8]

The existence of deuterium on Earth, elsewhere in the solar system (as confirmed by planetary probes), and in the spectra of stars, is also an important datum in cosmology. Gamma radiation from ordinary nuclear fusion dissociates deuterium into protons and neutrons, and there are no known natural processes other than the Big Bang nucleosynthesis, which might have produced deuterium at anything close to the observed natural abundance of deuterium (deuterium is produced by the rare cluster decay, and occasional absorption of naturally occurring neutrons by light hydrogen, but these are trivial sources). There is thought to be little deuterium in the interior of the Sun and other stars, as at temperatures there nuclear fusion reactions that consume deuterium happen much faster than the proton-proton reaction that creates deuterium. However, deuterium persists in the outer solar atmosphere at roughly the same concentration as in Jupiter, and this has probably been unchanged since the origin of the Solar System. The natural abundance of deuterium seems to be a very similar fraction of hydrogen, wherever hydrogen is found, unless there are obvious processes at work that concentrate it.

The existence of deuterium at a low but constant primordial fraction in all hydrogen is another one of the arguments in favor of the Big Bang theory over the Steady State theory of the universe. The observed ratios of hydrogen to helium to deuterium in the universe are difficult to explain except with a Big Bang model. It is estimated that the abundances of deuterium have not evolved significantly since their production about 13.8 bya.[9] Measurements of Milky Way galactic deuterium from ultraviolet spectral analysis show a ratio of as much as 23 atoms of deuterium per million hydrogen atoms in undisturbed gas clouds, which is only 15% below the WMAP estimated primordial ratio of about 27 atoms per million from the Big Bang. This has been interpreted to mean that less deuterium has been destroyed in star formation in our galaxy than expected, or perhaps deuterium has been replenished by a large in-fall of primordial hydrogen from outside the galaxy.[10] In space a few hundred light years from the Sun, deuterium abundance is only 15 atoms per million, but this value is presumably influenced by differential adsorption of deuterium onto carbon dust grains in interstellar space.[11]

The abundance of deuterium in the atmosphere of Jupiter has been directly measured by the Galileo space probe as 26 atoms per million hydrogen atoms. ISO-SWS observations find 22 atoms per million hydrogen atoms in Jupiter.[12] and this abundance is thought to represent close to the primordial solar system ratio.[3] This is about 17% of the terrestrial deuterium-to-hydrogen ratio of 156 deuterium atoms per million hydrogen atoms.

Cometary bodies such as Comet Hale Bopp and Halley's Comet have been measured to contain relatively more deuterium (about 200 atoms D per million hydrogens), ratios which are enriched with respect to the presumed protosolar nebula ratio, probably due to heating, and which are similar to the ratios found in Earth seawater. The recent measurement of deuterium amounts of 161 atoms D per million hydrogen in Comet 103P/Hartley (a former Kuiper belt object), a ratio almost exactly that in Earth's oceans, emphasizes the theory that Earth's surface water may be largely comet-derived.[2][3] Most recently the deuterium/protium (D/H) ratio of 67P/Churyumov-Gerasimenko as measured by Rosetta is about three times that of earth water, a figure that is high.[4] This has caused renewed interest in suggestions that Earth's water may be partly of asteroidal origin.

Deuterium has also observed to be concentrated over the mean solar abundance in other terrestrial planets, in particular Mars and Venus.

Production

Deuterium is produced for industrial, scientific and military purposes, by starting with ordinary water—a small fraction of which is naturally-occurring heavy water—and then separating out the heavy water by the Girdler sulfide process, distillation, or other methods.
In theory, deuterium for heavy water could be created in a nuclear reactor, but separation from ordinary water is the cheapest bulk production process.

The world's leading supplier of deuterium was Atomic Energy of Canada Limited, in Canada, until 1997, when the last heavy water plant was shut down. Canada uses heavy water as a neutron moderator for the operation of the CANDU reactor design.

Properties

Physical properties

The physical properties of deuterium compounds can exhibit significant kinetic isotope effects and other physical and chemical property differences from the hydrogen analogs. D2O, for example, is more viscous than H2O.[13] Chemically, there are differences in bond energy and length for compounds of heavy hydrogen isotopes compared to normal hydrogen, which are larger than the isotopic differences in any other element. Bonds involving deuterium and tritium are somewhat stronger than the corresponding bonds in hydrogen, and these differences are enough to cause significant changes in biological reactions.

Deuterium can replace the normal hydrogen in water molecules to form heavy water (D2O), which is about 10.6% denser than normal water (so that ice made from it sinks in ordinary water). Heavy water is slightly toxic in eukaryotic animals, with 25% substitution of the body water causing cell division problems and sterility, and 50% substitution causing death by cytotoxic syndrome (bone marrow failure and gastrointestinal lining failure). Prokaryotic organisms, however, can survive and grow in pure heavy water, though they develop slowly.[14] Despite this toxicity, consumption of heavy water under normal circumstances does not pose a health threat to humans. It is estimated that a 70 kg person might drink 4.8 liters of heavy water without serious consequences.[15] Small doses of heavy water (a few grams in humans, containing an amount of deuterium comparable to that normally present in the body) are routinely used as harmless metabolic tracers in humans and animals.

Quantum properties

The deuteron has spin +1 ("triplet") and is thus a boson. The NMR frequency of deuterium is significantly different from common light hydrogen. Infrared spectroscopy also easily differentiates many deuterated compounds, due to the large difference in IR absorption frequency seen in the vibration of a chemical bond containing deuterium, versus light hydrogen. The two stable isotopes of hydrogen can also be distinguished by using mass spectrometry.

The triplet deuteron nucleon is barely bound at EB = 2.23 MeV, so all the higher energy states are not bound. The singlet deuteron is a virtual state, with a negative binding energy of ~60 keV. There is no such stable particle, but this virtual particle transiently exists during neutron-proton inelastic scattering, accounting for the unusually large neutron scattering cross-section of the proton.[16]

Nuclear properties (the deuteron)

Deuteron mass and radius

The nucleus of deuterium is called a deuteron. It has a mass of 2.013553212724(78) u[17] The charge radius of the deuteron is 2.1402(28) fm[18]

Spin and energy

Deuterium is one of only five stable nuclides with an odd number of protons and an odd number of neutrons. (2H, 6Li, 10B, 14N, 180mTa; also, the long-lived radioactive nuclides 40K, 50V, 138La, 176Lu occur naturally.) Most odd-odd nuclei are unstable with respect to beta decay, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects. Deuterium, however, benefits from having its proton and neutron coupled to a spin-1 state, which gives a stronger nuclear attraction; the corresponding spin-1 state does not exist in the two-neutron or two-proton system, due to the Pauli exclusion principle which would require one or the other identical particle with the same spin to have some other different quantum number, such as orbital angular momentum. But orbital angular momentum of either particle gives a lower binding energy for the system, primarily due to increasing distance of the particles in the steep gradient of the nuclear force. In both cases, this causes the diproton and dineutron nucleus to be unstable.

The proton and neutron making up deuterium can be dissociated through neutral current interactions with neutrinos. The cross section for this interaction is comparatively large, and deuterium was successfully used as a neutrino target in the Sudbury Neutrino Observatory experiment.

Isospin singlet state of the deuteron

Due to the similarity in mass and nuclear properties between the proton and neutron, they are sometimes considered as two symmetric types of the same object, a nucleon. While only the proton has an electric charge, this is often negligible due to the weakness of the electromagnetic interaction relative to the strong nuclear interaction. The symmetry relating the proton and neutron is known as isospin and denoted I (or sometimes T).

Isospin is an SU(2) symmetry, like ordinary spin, so is completely analogous to it. The proton and neutron form an isospin doublet, with a "down" state (↓) being a neutron, and an "up" state (↑) being a proton.

A pair of nucleons can either be in an antisymmetric state of isospin called singlet, or in a symmetric state called triplet. In terms of the "down" state and "up" state, the singlet is
\frac{1}{\sqrt{2}}\Big( |\uparrow \downarrow \rangle - |\downarrow \uparrow \rangle\Big).
This is a nucleus with one proton and one neutron, i.e. a deuterium nucleus. The triplet is

\left(
\begin{array}{ll}
|\uparrow\uparrow\rangle\\
\frac{1}{\sqrt{2}}( |\uparrow\downarrow\rangle + |\downarrow\uparrow\rangle )\\
|\downarrow\downarrow\rangle
\end{array}
\right)
and thus consists of three types of nuclei, which are supposed to be symmetric: a deuterium nucleus (actually a highly excited state of it), a nucleus with two protons, and a nucleus with two neutrons. The latter two nuclei are not stable or nearly stable, and therefore so is this type of deuterium (meaning that it is indeed a highly excited state of deuterium).

Approximated wavefunction of the deuteron

The deuteron wavefunction must be antisymmetric if the isospin representation is used (since a proton and a neutron are not identical particles, the wavefunction need not be antisymmetric in general). Apart from their isospin, the two nucleons also have spin and spatial distributions of their wavefunction. The latter is symmetric if the deuteron is symmetric under parity (i.e. have an "even" or "positive" parity), and antisymmetric if the deuteron is antisymmetric under parity (i.e. have an "odd" or "negative" parity). The parity is fully determined by the total orbital angular momentum of the two nucleons: if it is even then the parity is even (positive), and if it is odd then the parity is odd (negative).

The deuteron, being an isospin singlet, is antisymmetric under nucleons exchange due to isospin, and therefore must be symmetric under the double exchange of their spin and location. Therefore it can be in either of the following two different states:
  • Symmetric spin and symmetric under parity. In this case, the exchange of the two nucleons will multiply the deuterium wavefunction by (−1) from isospin exchange, (+1) from spin exchange and (+1) from parity (location exchange), for a total of (−1) as needed for antisymmetry.
  • Antisymmetric spin and antisymmetric under parity. In this case, the exchange of the two nucleons will multiply the deuterium wavefunction by (−1) from isospin exchange, (−1) from spin exchange and (−1) from parity (location exchange), again for a total of (−1) as needed for antisymmetry.
In the first case the deuteron is a spin triplet, so that its total spin s is 1. It also has an even parity and therefore even orbital angular momentum l ; The lower its orbital angular momentum, the lower its energy. Therefore the lowest possible energy state has s = 1, l = 0.

In the second case the deuteron is a spin singlet, so that its total spin s is 0. It also has an odd parity and therefore odd orbital angular momentum l. Therefore the lowest possible energy state has s = 0, l = 1.

Since s = 1 gives a stronger nuclear attraction, the deuterium ground state is in the s =1, l = 0 state.

The same considerations lead to the possible states of an isospin triplet having s = 0, l = even or s = 1, l = odd. Thus the state of lowest energy has s = 1, l = 1, higher than that of the isospin singlet.

The analysis just given is in fact only approximate, both because isospin is not an exact symmetry, and more importantly because the strong nuclear interaction between the two nucleons is related to angular momentum in spin-orbit interaction that mixes different s and l states. That is, s and l are not constant in time (they do not commute with the Hamiltonian), and over time a state such as s = 1, l = 0 may become a state of s = 1, l = 2. Parity is still constant in time so these do not mix with odd l states (such as s = 0, l = 1). Therefore the quantum state of the deuterium is a superposition (a linear combination) of the s = 1, l = 0 state and the s = 1, l = 2 state, even though the first component is much bigger. Since the total angular momentum j is also a good quantum number (it is a constant in time), both components must have the same j, and therefore j = 1. This is the total spin of the deuterium nucleus.

To summarize, the deuterium nucleus is antisymmetric in terms of isospin, and has spin 1 and even (+1) parity. The relative angular momentum of its nucleons l is not well defined, and the deuteron is a superposition of mostly l = 0 with some l = 2.

Magnetic and electric multipoles

In order to find theoretically the deuterium magnetic dipole moment µ, one uses the formula for a nuclear magnetic moment
\mu = 
{1\over (j+1)}\langle(l,s),j,m_j=j|\overrightarrow{\mu}\cdot \overrightarrow{j}|(l,s),j,m_j=j\rangle
with
\overrightarrow{\mu} = g^{(l)}\overrightarrow{l} + g^{(s)}\overrightarrow{s}
g(l) and g(s) are g-factors of the nucleons.

Since the proton and neutron have different values for g(l) and g(s), one must separate their contributions. Each gets half of the deuterium orbital angular momentum \overrightarrow{l} and spin \overrightarrow{s}. One arrives at
\mu = 
{1\over (j+1)}\langle(l,s),j,m_j=j|\left({1\over 2}\overrightarrow{l} {g^{(l)}}_p + {1\over 2}\overrightarrow{s} ({g^{(s)}}_p + {g^{(s)}}_n)\right)\cdot \overrightarrow{j}|(l,s),j,m_j=j\rangle
where subscripts p and n stand for the proton and neutron, and g(l)n = 0.

By using the same identities as here and using the value g(l)p = µ
N
, we arrive at the following result, in nuclear magneton units
\mu = 
{1\over 4 (j+1)}\left[({g^{(s)}}_p + {g^{(s)}}_n)\big(j(j+1) - l(l+1) + s(s+1)\big) + \big(j(j+1) + l(l+1) - s(s+1)\big)\right]
For the s = 1, l = 0 state (j = 1), we obtain
\mu = {1\over 2}({g^{(s)}}_p + {g^{(s)}}_n) = 0.879
For the s = 1, l = 2 state (j = 1), we obtain
\mu = -{1\over 4}({g^{(s)}}_p + {g^{(s)}}_n) + {3\over 4} = 0.310
The measured value of the deuterium magnetic dipole moment, is 0.857 µ
N
, which is 97.5% of the 0.879 µ
N
value obtained by simply adding moments of the proton and neutron. This suggests that the state of the deuterium is indeed to a good approximation s = 1, l = 0 state, which occurs with both nucleons spinning in the same direction, but their magnetic moments subtracting because of the neutron's negative moment.
But the slightly lower experimental number than that which results from simple addition of proton and (negative) neutron moments shows that deuterium is actually a linear combination of mostly s = 1, l = 0 state with a slight admixture of s = 1, l = 2 state.

The electric dipole is zero as usual.

The measured electric quadrupole of the deuterium is 0.2859 e·fm2. While the order of magnitude is reasonable, since the deuterium radius is of order of 1 femtometer (see below) and its electric charge is e, the above model does not suffice for its computation. More specifically, the electric quadrupole does not get a contribution from the l =0 state (which is the dominant one) and does get a contribution from a term mixing the l =0 and the l =2 states, because the electric quadrupole operator does not commute with angular momentum.

The latter contribution is dominant in the absence of a pure l = 0 contribution, but cannot be calculated without knowing the exact spatial form of the nucleons wavefunction inside the deuterium.
Higher magnetic and electric multipole moments cannot be calculated by the above model, for similar reasons.

Applications


Ionized deuterium in a fusor reactor giving off its characteristic pinkish-red glow

Emission spectrum of an ultraviolet deuterium arc lamp

Deuterium has a number of commercial and scientific uses. These include:

Nuclear reactors

Deuterium is used in heavy water moderated fission reactors, usually as liquid D2O, to slow neutrons without the high neutron absorption of ordinary hydrogen.[19] This is a common commercial use for larger amounts of deuterium.

In research reactors, liquid D2 is used in cold sources to moderate neutrons to very low energies and wavelengths appropriate for scattering experiments.

Experimentally, deuterium is the most common nuclide used in nuclear fusion reactor designs, especially in combination with tritium, because of the large reaction rate (or nuclear cross section) and high energy yield of the D–T reaction. There is an even higher-yield D–3He fusion reaction, though the breakeven point of D–3He is higher than that of most other fusion reactions; together with the scarcity of 3He, this makes it implausible as a practical power source until at least D–T and D–D fusion reactions have been performed on a commercial scale. However, commercial nuclear fusion is not yet an accomplished technology.

NMR spectroscopy

Deuterium is most commonly used in hydrogen nuclear magnetic resonance spectroscopy (proton NMR) in the following way. NMR ordinarily requires compounds of interest to be analyzed as dissolved in solution. Because of deuterium's nuclear spin properties which differ from the light hydrogen usually present in organic molecules, NMR spectra of hydrogen/protium are highly differentiable from that of deuterium, and in practice deuterium is not "seen" by an NMR instrument tuned for light-hydrogen. Deuterated solvents (including heavy water, but also compounds like deuterated chloroform, CDCl3) are therefore routinely used in NMR spectroscopy, in order to allow only the light-hydrogen spectra of the compound of interest to be measured, without solvent-signal interference.
Nuclear magnetic resonance spectroscopy can also be used to obtain information about the deteron's environment in isotopically labelled samples (Deuterium NMR). For example, the flexibility in the tail, which is a long hydrocarbon chains, in deuterium-labelled lipid molecules can be quantified using solid state deuterium NMR.[20]

Deuterium NMR spectra are especially informative in the solid state because of its relatively small quadrupole moment in comparison with those of bigger quadrupolar nuclei such as chlorine-35, for example.

Tracing

In chemistry, biochemistry and environmental sciences, deuterium is used as a non-radioactive, stable isotopic tracer, for example, in the doubly labeled water test. In chemical reactions and metabolic pathways, deuterium behaves somewhat similarly to ordinary hydrogen (with a few chemical differences, as noted). It can be distinguished from ordinary hydrogen most easily by its mass, using mass spectrometry or infrared spectrometry. Deuterium can be detected by femtosecond infrared spectroscopy, since the mass difference drastically affects the frequency of molecular vibrations; deuterium-carbon bond vibrations are found in locations free of other signals.

Measurements of small variations in the natural abundances of deuterium, along with those of the stable heavy oxygen isotopes 17O and 18O, are of importance in hydrology, to trace the geographic origin of Earth's waters. The heavy isotopes of hydrogen and oxygen in rainwater (so-called meteoric water) are enriched as a function of the environmental temperature of the region in which the precipitation falls (and thus enrichment is related to mean latitude). The relative enrichment of the heavy isotopes in rainwater (as referenced to mean ocean water), when plotted against temperature falls predictably along a line called the global meteoric water line (GMWL). This plot allows samples of precipitation-originated water to be identified along with general information about the climate in which it originated. Evaporative and other processes in bodies of water, and also ground water processes, also differentially alter the ratios of heavy hydrogen and oxygen isotopes in fresh and salt waters, in characteristic and often regionally distinctive ways.[21] The ratio of concentration of 2H to 1H is usually indicated with a delta as δ2H and the geographic patterns of these values are plotted in maps termed as isoscapes. Stable isotope are incorporated into plants and animals and an analysis of the ratios in a migrant bird or insect can help suggest a rough guide to their origins.[22][23]

Contrast properties

Neutron scattering techniques particularly profit from availability of deuterated samples: The H and D cross sections are very distinct and different in sign, which allows contrast variation in such experiments. Further, a nuisance problem of ordinary hydrogen is its large incoherent neutron cross section, which is nil for D. The substitution of deuterium atoms for hydrogen atoms thus reduces scattering noise.

Hydrogen is an important and major component in all materials of organic chemistry and life science, but it barely interacts with X-rays. As hydrogen (and deuterium) interact strongly with neutrons, neutron scattering techniques, together with a modern deuteration facility,[24] fills a niche in many studies of macromolecules in biology and many other areas.

Nuclear weapons

This is discussed below. It is notable that although most stars, including the Sun, generate energy over most of their lives by fusing hydrogen into heavier elements, such fusion of light hydrogen (protium) has never been successful in the conditions attainable on Earth. Thus, all artificial fusion, including the hydrogen fusion that occurs in so-called hydrogen bombs, requires heavy hydrogen (either tritium or deuterium, or both) in order for the process to work.

Drugs

Suggested neurological effects of natural abundance variation

The natural deuterium content of water has been suggested from preliminary correlative epidemiology to influence the incidence of affective disorder-related pathophysiology and major depression, which might be mediated by the serotonergic mechanisms.[25]

History

Suspicion of lighter element isotopes

The existence of nonradioactive isotopes of lighter elements had been suspected in studies of neon as early as 1913, and proven by mass spectrometry of light elements in 1920. The prevailing theory at the time, however, was that the isotopes were due to the existence of differing numbers of "nuclear electrons" in different atoms of an element. It was expected that hydrogen, with a measured average atomic mass very close to 1 u, the known mass of the proton, always had a nucleus composed of a single proton (a known particle), and therefore could not contain any nuclear electrons without losing its charge entirely. Thus, hydrogen could have no heavy isotopes.

Deuterium detected


Harold Urey

It was first detected spectroscopically in late 1931 by Harold Urey, a chemist at Columbia University. Urey's collaborator, Ferdinand Brickwedde, distilled five liters of cryogenically produced liquid hydrogen to mL of liquid, using the low-temperature physics laboratory that had recently been established at the National Bureau of Standards in Washington, D.C. (now the National Institute of Standards and Technology). The technique had previously been used to isolate heavy isotopes of neon. The cryogenic boiloff technique concentrated the fraction of the mass-2 isotope of hydrogen to a degree that made its spectroscopic identification unambiguous.[26][27]

Naming of the isotope and Nobel Prize

Urey created the names protium, deuterium, and tritium in an article published in 1934. The name is based in part on advice from G. N. Lewis who had proposed the name "deutium". The name is derived from the Greek deuteros (second), and the nucleus to be called "deuteron" or "deuton". Isotopes and new elements were traditionally given the name that their discoverer decided. Some British chemists, like Ernest Rutherford, wanted the isotope to be called "diplogen", from the Greek diploos (double), and the nucleus to be called diplon.[1]

The amount inferred for normal abundance of this heavy isotope of hydrogen was so small (only about 1 atom in 6400 hydrogen atoms in ocean water (156 deuteriums per million hydrogens)) that it had not noticeably affected previous measurements of (average) hydrogen atomic mass. This explained why it hadn't been experimentally suspected before. Urey was able to concentrate water to show partial enrichment of deuterium. Lewis had prepared the first samples of pure heavy water in 1933. The discovery of deuterium, coming before the discovery of the neutron in 1932, was an experimental shock to theory, but when the neutron was reported, making deuterium's existence more explainable, deuterium won Urey the Nobel Prize in chemistry in 1934. Lewis was embittered by being passed over for this recognition given to his former student.[1]

"Heavy water" experiments in World War II

Shortly before the war, Hans von Halban and Lew Kowarski moved their research on neutron moderation from France to England, smuggling the entire global supply of heavy water (which had been made in Norway) across in twenty-six steel drums.[28][29]
During World War II, Nazi Germany was known to be conducting experiments using heavy water as moderator for a nuclear reactor design. Such experiments were a source of concern because they might allow them to produce plutonium for an atomic bomb. Ultimately it led to the Allied operation called the "Norwegian heavy water sabotage", the purpose of which was to destroy the Vemork deuterium production/enrichment facility in Norway. At the time this was considered important to the potential progress of the war.

After World War II ended, the Allies discovered that Germany was not putting as much serious effort into the program as had been previously thought. They had been unable to sustain a chain reaction. The Germans had completed only a small, partly built experimental reactor (which had been hidden away). By the end of the war, the Germans did not even have a fifth of the amount of heavy water needed to run the reactor[clarification needed], partially due to the Norwegian heavy water sabotage operation. However, even had the Germans succeeded in getting a reactor operational (as the U.S. did with a graphite reactor in late 1942), they would still have been at least several years away from development of an atomic bomb with maximal effort. The engineering process, even with maximal effort and funding, required about two and a half years (from first critical reactor to bomb) in both the U.S. and U.S.S.R, for example.

Deuterium in thermonuclear weapons

A view of the Sausage device casing of the Ivy Mike hydrogen bomb, with its instrumentation and cryogenic equipment attached. This bomb held a cryogenic Dewar flask containing room for as much as 160 kilograms of liquid deuterium. The bomb was 20 feet tall. Note the seated man at the right of the photo for the scale.

The 62-ton Ivy Mike device built by the United States and exploded on 1 November 1952, was the first fully successful "hydrogen bomb" or thermonuclear bomb. In this context, it was the first bomb in which most of the energy released came from nuclear reaction stages that followed the primary nuclear fission stage of the atomic bomb. The Ivy Mike bomb was a factory-like building, rather than a deliverable weapon. At its center, a very large cylindrical, insulated vacuum flask or cryostat, held cryogenic liquid deuterium in a volume of about 1000 liters (160 kilograms in mass, if this volume had been completely filled). Then, a conventional atomic bomb (the "primary") at one end of the bomb was used to create the conditions of extreme temperature and pressure that were needed to set off the thermonuclear reaction.

Within a few years, so-called "dry" hydrogen bombs were developed that did not need cryogenic hydrogen. Released information suggests that all thermonuclear weapons built since then contain chemical compounds of deuterium and lithium in their secondary stages. The material that contains the deuterium is mostly lithium deuteride, with the lithium consisting of the isotope lithium-6. When the lithium-6 is bombarded with fast neutrons from the atomic bomb, tritium (hydrogen-3) is produced, and then the deuterium and the tritium quickly engage in thermonuclear fusion, releasing abundant energy, helium-4, and even more free neutrons.

Data for elemental deuterium

Formula: D2 or 2
1
H
2
  • Density: 0.180 kg/m3 at STP (0 °C, 101.325 kPa).
  • Atomic weight: 2.0141017926 u.
  • Mean abundance in ocean water (from VSMOW) 155.76 ± 0.1 ppm (a ratio of 1 part per approximately 6420 parts), that is, about 0.015% of the atoms in a sample (by number, not weight)
Data at approximately 18 K for D2 (triple point):
  • Density:
    • Liquid: 162.4 kg/m3
    • Gas: 0.452 kg/m3
  • Viscosity: 12.6 µPa·s at 300 K (gas phase)
  • Specific heat capacity at constant pressure cp:
    • Solid: 2950 J/(kg·K)
    • Gas: 5200 J/(kg·K)

Anti-deuterium

An antideuteron is the antimatter counterpart of the nucleus of deuterium, consisting of an antiproton and an antineutron. The antideuteron was first produced in 1965 at the Proton Synchrotron at CERN[30] and the Alternating Gradient Synchrotron at Brookhaven National Laboratory.[31] A complete atom, with a positron orbiting the nucleus, would be called antideuterium, but as of 2005 antideuterium has not yet been created. The proposed symbol for antideuterium is D, that is, D with an overbar.[32]

Helium-3


From Wikipedia, the free encyclopedia

Helium-3
He-3 atom.png
Helium-3
General
Name, symbol Helium-3, He-3,3He
Neutrons 1
Protons 2
Nuclide data
Natural abundance 0.000137% (% He on Earth)
Half-life stable
Parent isotopes 3H (beta decay of tritium)
Isotope mass 3.0160293 u
Spin 12
Helium-3 (He-3) is a light, non-radioactive isotope of helium with two protons and one neutron, in contrast with two neutrons in common helium. Its hypothetical existence was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while he was working at the University of Cambridge Cavendish Laboratory. Oliphant had performed experiments in which fast deuterons collided with deuteron targets (incidentally, the first demonstration of nuclear fusion).[1] Helium-3 was thought to be a radioactive isotope until helions were also found in samples of natural helium, which is mostly helium-4, taken both from the terrestrial atmosphere and from natural gas wells.[2]

Helium-3 occurs as a primordial nuclide, escaping from the Earth's crust into the atmosphere and into outer space over millions of years. Helium-3 is also thought to be a natural nucleogenic and cosmogenic nuclide, one produced when lithium is bombarded by natural neutrons. Those are released by spontaneous fission and by nuclear reactions with cosmic rays. Some of the helium-3 found in the terrestrial atmosphere is also a relic of atmospheric and underwater nuclear weapons testing, conducted by the three big nuclear powers before 1963. Most of this comes from the decay of tritium (hydrogen-3), which decays into helium-3 with a half life of 12.3 years. Furthermore, some nuclear reactors (landbound or shipbound) periodically release some helium-3 and tritium into the atmosphere. The nuclear reactor disaster at Chernobyl released a huge amount of radioactive tritium into the atmosphere, and smaller accidents have caused smaller releases. Furthermore, significant amounts of tritium and helium-3 have been deliberately produced in national arsenal nuclear reactors by the irradiation of lithium-6. The tritium is used to "boost" nuclear weapons, and some of this inevitably escapes during its production, transportation, and storage. Hence, helium-3 enters the atmosphere both through its direct release and through the radioactive decay of tritium.

The abundance of helium-3 is thought to be greater on the Moon than on Earth, having been embedded in the upper layer of regolith by the solar wind over billions of years,[3] though still lower in quantity than in the solar system's gas giants.[4][5]

Physical properties

Because of its lower atomic mass of 3.02 atomic mass units, helium-3 has some physical properties different from those of helium-4, with a mass of 4.00 atomic mass units. Because of the weak, induced dipole–dipole interaction between helium atoms, their macroscopic physical properties are mainly determined by their zero-point energy. Also, the microscopic properties of helium-3 cause it to have a higher zero-point energy than helium-4. This implies that helium-3 can overcome dipole–dipole interactions with less thermal energy than helium-4 can.

The quantum mechanical effects on helium-3 and helium-4 are significantly different because with two protons, two neutrons, and two electrons, helium-4 has an overall spin of zero, making it a boson, but with one fewer neutron, helium-3 has an overall spin of one half, making it a fermion.

Helium-3 boils at 3.19 K compared with helium-4 at 4.23 K, and its critical point is also lower at 3.35 K, compared with helium-4 at 5.2 K. Helium-3 has less than one-half of the density when it is at its boiling point: 59 gram per liter compared to the 125 gram per liter of helium-4—at a pressure of one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026 kilojoule per mole compared with the 0.0829 kilojoule per mole of helium-4.[6]

Fusion reactions

Comparison of neutronicity of reactions[7][8][9][10][11]
Reactants
Products Q n/MeV
First-generation fusion fuels
21H + 21H (D-D) 32He + 10n 3.268 MeV 0.306
21H + 21H (D-D) 31H + 11p 4.032 MeV 0
21H + 31H (D-T) 42He + 10n 17.571 MeV 0.057
Second-generation fusion fuel
21H + 32He (D-3He) 42He + 11p 18.354 MeV 0
Third-generation fusion fuels
32He + 32He 42He+ 211p 12.86 MeV 0
115B + 11p 3 42He 8.68 MeV 0
Net result of D burning (sum of first 4 rows)
6D 2(4He + n + p) 43.225 MeV 0.046
Current nuclear fuel
235U + n 2 FP+ 2.5n ~200 MeV 0.001

The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The DT rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.

3He can be used in fusion reactions by either of the reactions 2D + 3He →   4He +  1p + 18.3 MeV, or 3He + 3He → 4He   + 2 1p+ 12.86 MeV

The conventional deuterium + tritium ("D-T") fusion process produces energetic neutrons which render reactor components radioactive with activation products. The appeal of helium-3 fusion stems from the aneutronic nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy by-product, the proton, can be contained using electric and magnetic fields. The momentum energy of this proton (created in the fusion process) will interact with the containing electromagnetic field, resulting in direct net electricity generation.[12]

Because of the higher Coulomb barrier, the temperatures required for 21H + 32He fusion are much higher than those of conventional D-T fusion. Moreover, since both reactants need to be mixed together to fuse, reactions between nuclei of the same reactant will occur, and the D-D reaction (21H + 21H) does produce a neutron. Reaction rates vary with temperature, but the D-3He reaction rate is never greater than 3.56 times the D-D reaction rate (see graph). Therefore fusion using D-3He fuel may produce a somewhat lower neutron flux than D-T fusion, but is by no means clean, negating some of its main attraction.

The second possibility, fusing 32He with itself (32He + 32He), requires even higher temperatures (since now both reactants have a +2 charge), and thus is even more difficult than the D-3He reaction.
However, it does offer a possible reaction that produces no neutrons; the protons it produces possess charges and can be contained using electric and magnetic fields, which in turn results in direct electricity generation. 32He + 32He fusion has been demonstrated in the laboratory and is thus theoretically feasible and would have immense advantages, but commercial viability is many years in the future.[13]

The amounts of helium-3 needed as a replacement for conventional fuels are substantial by comparison to amounts currently available. The total amount of energy produced in the 21H + 32He reaction is 18.4 MeV, which corresponds to some 493 megawatt-hours (4.93×108 W·h) per three grams (one mole) of ³He. If the total amount of energy could be converted to electrical power with 100% efficiency (a physical impossibility), it would correspond to about 30 minutes of output of a gigawatt electrical plant per mole of 3He. Thus, a year's production (at 6 grams for each operation hour) would require 52.5 kilograms of helium-3.[citation needed] The amount of fuel needed for large-scale applications can also be put in terms of total consumption: electricity consumption by 107 million U.S. households in 2001[14] totaled 1,140 billion kW·h (1.14×1015 W·h). Again assuming 100% conversion efficiency, 6.7 tonnes per year of helium-3 would be required for that segment of the energy demand of the United States, 15 to 20 tonnes per year given a more realistic end-to-end conversion efficiency.[citation needed]

Neutron detection

Helium-3 is a most important isotope in instrumentation for neutron detection. It has a high absorption cross section for thermal neutron beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction
n + 3He → 3H + 1H + 0.764 MeV
into charged particles tritium (T, 3H) and protium (p, 1H) which then are detected by creating a charge cloud in the stopping gas of a proportional counter or a Geiger-Müller tube.[15]

Furthermore, the absorption process is strongly spin-dependent, which allows a spin-polarized helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in neutron polarization analysis, a technique which probes for magnetic properties of matter.[16][17][18][19]

The United States Department of Homeland Security had hoped to deploy detectors to spot smuggled plutonium in shipping containers by their neutron emissions, but the worldwide shortage of helium-3 following the drawdown in nuclear weapons production since the Cold War has to some extent prevented this.[20] As of 2012, DHS determined the commercial supply of boron-10 would support converting its neutron detection infrastructure to that technology.[21]

Cryogenics

A helium-3 refrigerator uses helium-3 to achieve temperatures of 0.2 to 0.3 kelvin. A dilution refrigerator uses a mixture of helium-3 and helium-4 to reach cryogenic temperatures as low as a few thousandths of a kelvin.[22]

An important property of helium-3, which distinguishes it from the more common helium-4, is that its nucleus is a fermion since it contains an odd number of spin 12 particles. Helium-4 nuclei are bosons, containing an even number of spin 12 particles. This is a direct result of the addition rules for quantized angular momentum. At low temperatures (about 2.17 K), helium-4 undergoes a phase transition: A fraction of it enters a superfluid phase that can be roughly understood as a type of Bose–Einstein condensate. Such a mechanism is not available for helium-3 atoms, which are fermions. However, it was widely speculated that helium-3 could also become a superfluid at much lower temperatures, if the atoms formed into pairs analogous to Cooper pairs in the BCS theory of superconductivity. Each Cooper pair, having integer spin, can be thought of as a boson. During the 1970s, David Lee, Douglas Osheroff and Robert Coleman Richardson discovered two phase transitions along the melting curve, which were soon realized to be the two superfluid phases of helium-3.[23][24] The transition to a superfluid occurs at 2.491 millikelvins (i.e., 0.002491 K) on the melting curve. They were awarded the 1996 Nobel Prize in Physics for their discovery. Tony Leggett won the 2003 Nobel Prize in Physics for his work on refining understanding of the superfluid phase of helium-3.[25]

In zero magnetic field, there are two distinct superfluid phases of 3He, the A-phase and the B-phase. The B-phase is the low-temperature, low-pressure phase which has an isotropic energy gap. The A-phase is the higher temperature, higher pressure phase that is further stabilized by a magnetic field and has two point nodes in its gap. The presence of two phases is a clear indication that 3He is an unconventional superfluid (superconductor), since the presence of two phases requires an additional symmetry, other than gauge symmetry, to be broken. In fact, it is a p-wave superfluid, with spin one, S=1, and angular momentum one, L=1. The ground state corresponds to total angular momentum zero, J=S+L=0 (vector addition). Excited states are possible with non-zero total angular momentum, J>0, which are excited pair collective modes. Because of the extreme purity of superfluid 3He (since all materials except 4He have solidified and sunk to the bottom of the liquid 3He and any 4He has phase separated entirely, this is the most pure condensed matter state), these collective modes have been studied with much greater precision than in any other unconventional pairing system.

Medical lung imaging

Helium-3 nuclei have an intrinsic nuclear spin of 12, and a relatively high magnetogyric ratio. Helium-3 can be hyperpolarized using non-equilibrium means such as spin-exchange optical pumping.[26] During this process, circularly polarized infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in an alkali metal, such as caesium or rubidium inside a sealed glass vessel. The angular momentum is transferred from the alkali metal electrons to the noble gas nuclei through collisions. In essence, this process effectively aligns the nuclear spins with the magnetic field in order to enhance the NMR signal. The hyperpolarized gas may then be stored at pressures of 10 atm, for up to 100 hours. Following inhalation, gas mixtures containing the hyperpolarized helium-3 gas can be imaged with an MRI scanner to produce anatomical and functional images of lung ventilation. This technique is also able to produce images of the airway tree, locate unventilated defects, measure the alveolar oxygen partial pressure, and measure the ventilation/perfusion ratio. This technique may be critical for the diagnosis and treatment management of chronic respiratory diseases such as chronic obstructive pulmonary disease (COPD), emphysema, cystic fibrosis, and asthma.[27]

Production

Current US industrial consumption of helium-3 is approximately 60,000 liters (approximately 8 kg) per year;[28] cost at auction has typically been approximately $100/liter although increasing demand has raised prices to as much as $2,000/liter in recent years.[29] Helium-3 is naturally present in small quantities due to radioactive decay, but virtually all helium-3 used in industry is manufactured. Helium-3 is a product of tritium decay, and tritium can be produced through neutron bombardment of deuterium, lithium, boron, or nitrogen targets. Production of tritium in significant quantities requires the high neutron flux of a nuclear reactor; breeding tritium with lithium-6 consumes the neutron, while breeding with lithium-7 produces a low energy neutron as a replacement for the consumed fast neutron.

Current supplies of helium-3 come, in part, from the dismantling of nuclear weapons where it accumulates,[30][31] however the need for warhead disassembly is diminishing. Consequently tritium itself is in short supply, and the US Department of Energy recently began producing it by the lithium irradiation method at the Tennessee Valley Authority's Watts Bar reactor.[28] Substantial quantities of tritium could also be extracted from the heavy water moderator in CANDU nuclear reactors.

Production of helium-3 from tritium at a rate sufficient to meet world demand will require significant investment, as tritium must be produced at the same rate as helium-3, and approximately eighteen times as much tritium must be maintained in storage as the amount of helium-3 produced annually by decay (production rate dNdt from number of moles or other unit mass of tritium N, is N γ = N (ln 2)t1/2 where the value of t1/2(ln 2) is about 18 years; see radioactive decay). If commercial fusion reactors were to use helium-3 as a fuel, they would require tens of tonnes of helium-3 each year to produce a fraction of the world's power, requiring substantial expansion of facilities for tritium production and storage.[32]

Abundance

Solar nebula (primordial) abundance

One early estimate of the primordial ratio of 3He to 4He in the solar nebula has been the measurement of their ratio in the atmosphere of Jupiter, measured by the mass spectrometer of the Galileo atmospheric entry probe. This ratio is about 1:10,000,[33] or 100 parts of 3He per million parts of 4He. This is roughly the same ratio of the isotopes in lunar regolith, when it contains 28 ppm helium-4 and 2.8 ppb helium-3 (which is at the lower end of actual sample measurements, which vary from about 1.4 to 15 ppb). However, terrestrial ratios of the isotopes are lower by a factor of 100, mainly due to enrichment of helium-4 stocks in the mantle by billions of years of alpha decay from uranium and thorium.

Terrestrial abundance

3He is a primordial substance in the Earth's mantle, considered to have become entrapped within the Earth during planetary formation. The ratio of 3He to 4He within the Earth's crust and mantle is less than that for assumptions of solar disk composition as obtained from meteorite and lunar samples, with terrestrial materials generally containing lower 3He/4He ratios due to ingrowth of 4He from radioactive decay.
In the space, 3He has a ratio of 300 atoms per million atoms of 4He (at. ppm),[34] the original ratio of these primodal gases in mantle was around 200-300 ppm when Earth was formed. A lot of 4He was generated by alpha-particle decay of uranium and thorium, and now mantle has only around 7% primodal helium,[35] lowering the total 3He/4He ratio to around 20 at ppm. Ratios of 3He/4He in excess of atmospheric are indicative of a contribution of 3He from the mantle. Crustal sources are dominated by the 4He which is produced by the decay of radioactive elements in the crust and mantle.

The ratio of helium-3 to helium-4 in natural Earth-bound sources varies greatly.[36][37] Samples of the lithium ore spodumene from Edison Mine, South Dakota were found to contain 12 parts of helium-3 to a million parts of helium-4. Samples from other mines showed 2 parts per million.[36]

Helium is also present as up to 7% of some natural gas sources,[38] and large sources have over 0.5% (above 0.2% makes it viable to extract).[39] Algeria's annual gas production is assumed to contain 100 million normal cubic metres[39] and this would contain between 5 and 50 m3 of helium-3 (about 1 to 10 kilograms) using the normal abundance range of 0.5 to 5 ppm. Similarly the US 2002 stockpile of 1 billion normal m3[39] would have contained about 10 to 100 kilograms of helium-3.

3He is also present in the Earth's atmosphere. The natural abundance of 3He in naturally occurring helium gas is 1.38×106 (1.38 parts per million). The partial pressure of helium in the Earth's atmosphere is about 0.52 Pa, and thus helium accounts for 5.2 parts per million of the total pressure (101325 Pa) in the Earth's atmosphere, and 3He thus accounts for 7.2 parts per trillion of the atmosphere. Since the atmosphere of the Earth has a mass of about 5.14×1015 tonnes,[40] the mass of 3He in the Earth's atmosphere is the product of these numbers, or about 37,000 tonnes of 3He.

3He is produced on Earth from three sources: lithium spallation, cosmic rays, and beta decay of tritium (3H). The contribution from cosmic rays is negligible within all except the oldest regolith materials, and lithium spallation reactions are a lesser contributor than the production of 4He by alpha particle emissions.

The total amount of helium-3 in the mantle may be in the range of 0.1–1 million tonnes. However, most of the mantle is not directly accessible. Some helium-3 leaks up through deep-sourced hotspot volcanoes such as those of the Hawaiian Islands, but only 300 grams per year is emitted to the atmosphere. Mid-ocean ridges emit another 3 kilogram per year. Around subduction zones, various sources produce helium-3 in natural gas deposits which possibly contain a thousand tonnes of helium-3 (although there may be 25 thousand tonnes if all ancient subduction zones have such deposits). Wittenberg estimated that United States crustal natural gas sources may have only half a tonne total.[41] Wittenberg cited Anderson's estimate of another 1200 metric tonnes in interplanetary dust particles on the ocean floors.[42] In the 1994 study, extracting helium-3 from these sources consumes more energy than fusion would release.[43] Wittenberg also writes that extraction from US crustal natural gas, consumes ten times the energy available from fusion reactions.[44][clarification needed]

Extraterrestrial abundance

Materials on the Moon's surface contain helium-3 at concentrations on the order of between 1.4 and 15 ppb in sunlit areas,[45][46] and may contain concentrations as much as 50 ppb in permanently shadowed regions.[5] A number of people, starting with Gerald Kulcinski in 1986,[47] have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion. Because of the low concentrations of helium-3, any mining equipment would need to process extremely large amounts of regolith (over 150 million tonnes of regolith to obtain one ton of helium 3),[48] and some proposals have suggested that helium-3 extraction be piggybacked onto a larger mining and development operation.[citation needed]

The primary objective of Indian Space Research Organization's first lunar probe called Chandrayaan-I, launched on October 22, 2008, was reported in some sources to be mapping the Moon's surface for helium-3-containing minerals.[49] However, this is debatable; no such objective is mentioned in the project's official list of goals, while at the same time, many of its scientific payloads have noted helium-3-related applications.[50][51]

Cosmochemist and geochemist Ouyang Ziyuan from the Chinese Academy of Sciences who is now in charge of the Chinese Lunar Exploration Program has already stated on many occasions that one of the main goals of the program would be the mining of helium-3, from which operation "each year three space shuttle missions could bring enough fuel for all human beings across the world."[52] To "bring enough fuel for all human beings across the world",[32] more than one Space Shuttle load (and the processing of 4 million tonnes of regolith) per week, at least 52 per year, would be necessary.[citation needed][dubious ]

In January 2006, the Russian space company RKK Energiya announced that it considers lunar helium-3 a potential economic resource to be mined by 2020,[53] if funding can be found.[54][55]

Mining gas giants for helium-3 has also been proposed.[56] The British Interplanetary Society's hypothetical Project Daedalus interstellar probe design was fueled by helium-3 mines in the atmosphere of Jupiter, for example. Jupiter's high gravity makes this a less energetically favorable operation than extracting helium-3 from the other gas giants of the solar system, however.

Power generation

A second-generation approach to controlled fusion power involves combining helium-3 (32He) and deuterium (21H). This reaction produces a helium-4 ion (42He) (like an alpha particle, but of different origin) and a high-energy proton (positively charged hydrogen ion) (11p). The most important potential advantage of this fusion reaction for power production as well as other applications lies in its compatibility with the use of electrostatic fields to control fuel ions and the fusion protons. Protons, as positively charged particles, can be converted directly into electricity, through use of solid-state conversion materials as well as other techniques. Potential conversion efficiencies of 70% may be possible, as there is no need to convert proton energy to heat in order to drive a turbine-powered electrical generator[citation needed].

There have been many claims about the capabilities of helium-3 power plants. According to proponents, fusion power plants operating on deuterium and helium-3 would offer lower capital and operating costs than their competitors due to less technical complexity, higher conversion efficiency, smaller size, the absence of radioactive fuel, no air or water pollution, and only low-level radioactive waste disposal requirements. Recent estimates suggest that about $6 billion in investment capital will be required to develop and construct the first helium-3 fusion power plant. Financial breakeven at today's wholesale electricity prices (5 US cents per kilowatt-hour) would occur after five 1-gigawatt plants were on line, replacing old conventional plants or meeting new demand.[57]

The reality is not so clear-cut. The most advanced fusion programs in the world are inertial confinement fusion (such as National Ignition Facility) and magnetic confinement fusion (such as ITER and other tokamaks). In the case of the former, there is no solid roadmap to power generation. In the case of the latter, commercial power generation is not expected until around 2050.[58] In both cases, the type of fusion discussed is the simplest: D-T fusion. The reason for this is the very low Coulomb barrier for this reaction; for D+3He, the barrier is much higher, and it is even higher for 3He–3He. The immense cost of reactors like ITER and National Ignition Facility are largely due to their immense size, yet to scale up to higher plasma temperatures would require reactors far larger still. The 14.7 MeV proton and 3.6 MeV alpha particle from D–3He fusion, plus the higher conversion efficiency, means that more electricity is obtained per kilogram than with D-T fusion (17.6 MeV), but not that much more. As a further downside, the rates of reaction for helium-3 fusion reactions are not particularly high, requiring a reactor that is larger still or more reactors to produce the same amount of electricity.

To attempt to work around this problem of massively large power plants that may not even be economical with D-T fusion, let alone the far more challenging D–3He fusion, a number of other reactors have been proposed – the Fusor, Polywell, Focus fusion, and many more, though many of these concepts have fundamental problems with achieving a net energy gain, and generally attempt to achieve fusion in thermal disequilibrium, something that could potentially prove impossible,[59] and consequently, these long-shot programs tend to have trouble garnering funding despite their low budgets. Unlike the "big", "hot" fusion systems, however, if such systems were to work, they could scale to the higher barrier "aneutronic" fuels, and therefore their proponents tend to promote p-B fusion, which requires no exotic fuels like helium-3.