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Saturday, February 7, 2015

Glenn T. Seaborg


From Wikipedia, the free encyclopedia

Glenn T. Seaborg
Glenn Seaborg - 1964.jpg
Born Glenn Theodore Seaborg
(1912-04-19)April 19, 1912
Ishpeming, Michigan
Died February 25, 1999(1999-02-25) (aged 86)
Lafayette, California
Nationality United States
Fields Nuclear chemistry
Institutions
Alma mater
  • UCLA
  • University of California, Berkeley
Doctoral advisor
Doctoral students
Known for his contributions and he was part of a team to the synthesis, discovery and investigation of ten transuranium elements
Notable awards
Signature

Glenn Theodore Seaborg (/ˈsiːbɔrɡ/; April 19, 1912 – February 25, 1999) was an American chemist whose involvement in the synthesis, discovery and investigation of ten transuranium elements earned him a share of the 1951 Nobel Prize in Chemistry.[1] His work in this area also led to his development of the actinide concept and the arrangement of the actinide series in the periodic table of the elements.

Seaborg spent most of his career as an educator and research scientist at the University of California, Berkeley, serving as a professor, and, between 1958 and 1961, as the university's second chancellor.[2] He advised ten US Presidents – from Harry S. Truman to Bill Clinton – on nuclear policy and was Chairman of the United States Atomic Energy Commission from 1961 to 1971, where he pushed for commercial nuclear energy and the peaceful applications of nuclear science. Throughout his career, Seaborg worked for arms control. He was a signatory to the Franck Report and contributed to the Limited Test Ban Treaty, the Nuclear Non-Proliferation Treaty and the Comprehensive Test Ban Treaty. He was a well-known advocate of science education and federal funding for pure research. Toward the end of the Eisenhower administration, he was the principal author of the Seaborg Report on academic science, and, as a member of President Ronald Reagan's National Commission on Excellence in Education, he was a key contributor to its 1983 report "A Nation at Risk".

Seaborg was the principal or co-discoverer of ten elements: plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium and element 106, which, while he was still living, was named seaborgium in his honor. He also discovered more than 100 atomic isotopes and is credited with important contributions to the chemistry of plutonium, originally as part of the Manhattan Project where he developed the extraction process used to isolate the plutonium fuel for the second atomic bomb. Early in his career, he was a pioneer in nuclear medicine and discovered isotopes of elements with important applications in the diagnosis and treatment of diseases, most notably iodine-131, which is used in the treatment of thyroid disease. In addition to his theoretical work in the development of the actinide concept, which placed the actinide series beneath the lanthanide series on the periodic table, he postulated the existence of super-heavy elements in the transactinide and superactinide series.

After sharing the 1951 Nobel Prize in Chemistry with Edwin McMillan, he received approximately 50 honorary doctorates and numerous other awards and honors. The list of things named after Seaborg ranges from his atomic element to an asteroid. He was a prolific author, penning numerous books and 500 journal articles, often in collaboration with others. He was once listed in the Guinness Book of World Records as the person with the longest entry in Who's Who in America.

Early life

Glenn Theodore Seaborg was born in Ishpeming, Michigan, on April 19, 1912, the son of Herman Theodore (Ted) and Selma Olivia Erickson Seaborg. He had one sister, Jeanette, who was two years younger. His family spoke Swedish at home. When Glenn Seaborg was a boy, the family moved to Los Angeles County, California, settling in a subdivision called Home Gardens, later annexed to the City of South Gate, California. About this time he changed the spelling of his first name from 'Glen' to "Glenn".[3]

Seaborg kept a daily journal from 1927 until he suffered a stroke in 1998.[4] As a youth, Seaborg was both a devoted sports fan and an avid movie buff. His mother encouraged him to become a bookkeeper as she felt his literary interests were impractical. He did not take an interest in science until his junior year when he was inspired by Dwight Logan Reid, a chemistry and physics teacher at David Starr Jordan High School in Watts.[5]

Seaborg graduated from Jordan in 1929 at the top of his class and received a bachelor of arts (AB) degree in chemistry at the University of California, Los Angeles, in 1933.[3] he worked his way through school as a stevedore and a laboratory assistant at Firestone.[6] Seaborg took his PhD in chemistry at the University of California, Berkeley, in 1937 with a doctoral thesis on the "Interaction of Fast Neutrons with Lead",[7] in which he coined the term "nuclear spallation".[8]

Seaborg was a member of the professional chemistry fraternity Alpha Chi Sigma. As a graduate student in the 1930s Seaborg performed wet chemistry research for his advisor Gilbert Newton Lewis,[8] and published three papers with him on the theory of acids and bases.[9][10][11] Seaborg studied the text Applied Radiochemistry by Otto Hahn, of the Kaiser Wilhelm Institute for Chemistry in Berlin, and it had a major impact on his developing interests as a research scientist. For several years, Seaborg conducted important research in artificial radioactivity using the Lawrence cyclotron at UC Berkeley. He was excited to learn from others that nuclear fission was possible—but also chagrined, as his own research might have led him to the same discovery.[12]

Seaborg also became an expert in dealing with noted Berkeley physicist Robert Oppenheimer. Oppenheimer had a daunting reputation, and often answered a junior man's question before it had even been stated. Often the question answered was more profound than the one asked, but of little practical help. Seaborg learned to state his questions to Oppenheimer quickly and succinctly.[13]

Pioneering work in nuclear chemistry


Seaborg in his lab

Seaborg remained at the University of California, Berkeley, for post-doctoral research. He followed Frederick Soddy's work investigating isotopes and contributed to the discovery of more than 100 isotopes of elements. Using one of Lawrence's advanced cyclotrons, John Livingood, Fred Fairbrother, and Seaborg created a new isotope of iron, iron-59 (Fe-59) in 1937. Iron-59 was useful in the studies of the hemoglobin in human blood. In 1938, Livingood and Seaborg collaborated (as they did for five years) to create an important isotope of iodine, iodine-131 (I-131), which is still used to treat thyroid disease.[14] (Many years later, it was credited with prolonging the life of Seaborg's mother.) As a result of these and other contributions, Seaborg is regarded as a pioneer in nuclear medicine and is one of its most prolific discoverers of isotopes.[15]

In 1939 he became an instructor in chemistry at Berkeley, was promoted to assistant professor in 1941 and professor in 1945.[16] University of California, Berkeley, physicist Edwin McMillan led a team that discovered element 93, which he named neptunium in 1940. In November, he was persuaded to leave Berkeley temporarily to assist with urgent research in radar technology. Since Seaborg and his colleagues had perfected McMillan's oxidation-reduction technique for isolating neptunium, he asked McMillan for permission to continue the research and search for element 94. McMillan agreed to the collaboration.[17] Seaborg first reported alpha decay proportionate to only a fraction of the element 93 under observation. The first hypothesis for this alpha particle accumulation was contamination by uranium, which produces alpha-decay particles; analysis of alpha-decay particles ruled this out. Seaborg then postulated that a distinct alpha-producing element was being formed from element 93.[18]

In February 1941, Seaborg and his collaborators produced plutonium-239 through the bombardment of uranium. In their experiments bombarding uranium with deuterons, they observed the creation of neptunium, element 93. But it then underwent beta-decay, forming a new element, plutonium, with 94 protons. Plutonium is fairly stable, but undergoes alpha-decay, which explained the presence of alpha particles coming from neptunium.[18] Thus, on March 28, 1941, Seaborg, physicist Emilio Segrè and Berkeley chemist Joseph W. Kennedy were able to show that plutonium (then known only as element 94) was fissile, an important distinction that was crucial to the decisions made in directing Manhattan Project research.[19] In 1966, Room 307 of Gilman Hall on the campus at the Berkeley, where Seaborg did his work, was declared a U.S. National Historic Landmark.[20]

In addition to plutonium, he is credited as a lead discoverer of americium, curium, and berkelium, and as a co-discoverer of californium, einsteinium, fermium, mendelevium, nobelium and seaborgium. He shared the Nobel Prize in Chemistry in 1951 with Edwin McMillan for "their discoveries in the chemistry of the first transuranium elements."[1]

Scientific contributions during the Manhattan Project

On April 19, 1942, Seaborg reached Chicago and joined the chemistry group at the Metallurgical Laboratory of the Manhattan Project at the University of Chicago, where Enrico Fermi and his group would later convert uranium-238 to plutonium-239 in a controlled nuclear chain reaction. Seaborg's role was to figure out how to extract the tiny bit of plutonium from the mass of uranium.

Plutonium-239 was isolated in visible amounts using a transmutation reaction on August 20, 1942, and weighed on September 10, 1942, in Seaborg's Chicago laboratory. He was responsible for the multi-stage chemical process that separated, concentrated and isolated plutonium. This process was further developed at the Clinton Engineering Works in Oak Ridge, Tennessee, and then entered full-scale production at the Hanford Engineer Works, in Richland, Washington.[21]

Seaborg's theoretical development of the actinide concept resulted in a redrawing of the Periodic Table of the Elements into its current configuration with the actinide series appearing below the lanthanide series. Seaborg developed the chemical elements americium and curium while in Chicago. He managed to secure patents for both elements. His patent on curium never proved commercially viable because of the element's short half-life, but americium is commonly used in household smoke detectors and thus provided a good source of royalty income to Seaborg in later years. Prior to the test of the first nuclear weapon, Seaborg joined with several other leading scientists in a written statement known as the Franck Report (secret at the time but since published) unsuccessfully calling on President Truman to conduct a public demonstration of the atomic bomb witnessed by the Japanese.[22]

Professor and Chancellor at the University of California, Berkeley


Seaborg (second from left) during Operation Plumbob

After the conclusion of World War II and the Manhattan Project, Seaborg was eager to return to academic life and university research free from the restrictions of wartime secrecy. In 1946, he added to his responsibilities as a professor by heading the nuclear chemistry research at the Lawrence Radiation Laboratory operated by the University of California on behalf of the United States Atomic Energy Commission. Seaborg was named one of the "Ten Outstanding Young Men in America" by the U.S. Junior Chamber of Commerce in 1947 (along with Richard Nixon and others). Seaborg was elected to the National Academy of Sciences in 1948. From 1954 to 1961 he served as associate director of the radiation laboratory. He was appointed by President Truman to serve as a member of the General Advisory Committee of the Atomic Energy Commission, an assignment he retained until 1960.[23]

Seaborg served as chancellor at the University of California, Berkeley, from 1958 to 1961. His term coincided with a relaxation of McCarthy-era restrictions on students' freedom of expression that had begun under his predecessor, Clark Kerr.[24] In October 1958, Seaborg announced that the University had relaxed its prior prohibitions on political activity on a trial basis,[25] and the ban on communists speaking on campus was lifted. This paved the way for the Free Speech Movement of 1964-65.[24]

Seaborg was an enthusiastic supporter of Cal's sports teams. San Francisco columnist Herb Caen was fond of pointing out that Seaborg's surname is an anagram of "Go Bears", a popular cheer at UC Berkeley.[26] Seaborg was proud of the fact that the Cal Bears won their first and only National Collegiate Athletic Association (NCAA) basketball championship in 1959, while he was chancellor. The football team also won the conference title and played in the Rose Bowl that year.[27] He served on the Faculty Athletic Committee for several years and was the co-author of a book, Roses from the Ashes: Breakup and Rebirth in Pacific Coast Intercollegiate Athletics (2000), concerning the Pacific Coast Conference recruiting scandal, and the founding of what is now the Pac-12, in which he played a role in restoring confidence in the integrity of collegiate sports.[27][28]

Seaborg served on the President's Science Advisory Committee (PSAC) during the Eisenhower administration. PSAC produced a report on "Scientific Progress, the Universities, and the Federal Government", also known as the "Seaborg Report", in November 1960, that urged greater federal funding of science.[29] In 1959, he helped found the Berkeley Space Sciences Laboratory with Clark Kerr.[30]

Chairman of the Atomic Energy Commission

After appointment by President John F. Kennedy and confirmation by the United States Senate, Seaborg was chairman of the Atomic Energy Commission (AEC) from 1961 to 1971. His pending appointment by President-elect Kennedy was nearly derailed in late 1960 when members of the Kennedy transition team learned that Seaborg had been listed in a U.S. News & World Report article as a member of "Nixon's Idea Men". Seaborg said that as a lifetime Democrat he was baffled when the article appeared associating him with outgoing Vice President Richard Nixon, a Republican whom Seaborg considered a casual acquaintance.[31]

President Kennedy and his Atomic Energy Commission Chairman, Glenn Seaborg

While chairman of the AEC, Seaborg participated on the negotiating team for the Limited Test Ban Treaty (LTBT), in which the US, UK, and USSR agreed to ban all above-ground test detonations of nuclear weapons. Seaborg considered his contributions to the achievement of the LTBT as one of his greatest accomplishments. Despite strict rules from the Soviets about photography at the signing ceremony, Seaborg sneaked a tiny camera past the Soviet guards to take a close-up photograph of Soviet Premier Nikita Khrushchev as he signed the treaty.[32]

Seaborg enjoyed a close relationship with President Lyndon Johnson and influenced the administration to pursue the Nuclear Non-Proliferation Treaty.[33] Seaborg was called to the White House in the first week of the Nixon Administration in January 1969 to advise President Richard Nixon on his first diplomatic crisis involving the Soviets and nuclear testing. He clashed with Nixon presidential adviser John Ehrlichman over the treatment of a Jewish scientist, Zalman Shapiro, whom the Nixon administration suspected of leaking nuclear secrets to Israel.[34]

Seaborg published several books and journal articles during his tenure at the Atomic Energy Commission. He predicted the existence of elements beyond those on the period table,[35] the transactinide series and the superactinide series of undiscovered synthetic elements. While most of these theoretical future elements have extremely short half-lives and thus no expected practical applications, he also hypothesized the existence of stable super-heavy isotopes of certain elements in an island of stability.[36] Seaborg served as chairman of the Atomic Energy Commission until 1971.[37]

Return to California


Seaborg (right) with marine biologist Dixy Lee Ray on September 17, 1968

Following his service as Chairman of the Atomic Energy Commission, Seaborg returned to UC Berkeley where he was awarded the position of University Professor. At the time, there had been fewer University Professors at UC Berkeley than Nobel Prize winners. He also served as Chairman of the Lawrence Hall of Science where he became the principal investigator for Great Explorations in Math and Science (GEMS)[38] working with director Jacqueline Barber. Seaborg served as at the University of California, Berkeley, from 1958 to 1961, and served as President of the American Association for the Advancement of Science in 1972 and as President of the American Chemical Society in 1976.[39]

In 1980, he transmuted several thousand atoms of bismuth into gold at the Lawrence Berkeley Laboratory. His experimental technique, using nuclear physics, was able to remove protons and neutrons from the bismuth atoms. Seaborg's technique would have been far too expensive to enable routine manufacturing of gold, but his work was close to the mythical Philosopher's Stone.[40][41]

In 1983, President Ronald Reagan appointed Seaborg to serve on the National Commission on Excellence in Education. The commission produced a report "A Nation at Risk: The Imperative for Educational Reform",[42] which focused national attention on education as a national issue germane to the federal government.[43] In 2008, Margaret Spellings wrote that
A Nation at Risk delivered a wake up call for our education system. It described stark realities like a significant number of functionally illiterate high schoolers, plummeting student performance, and international competitors breathing down our necks. It was a warning, a reproach, and a call to arms.[44]

Seaborg with Vice President Al Gore in the White House during a visit of the 1993 Science Talent Search (STS) finalists on March 4, 1993

Seaborg lived most of his later life in Lafayette, California, where he devoted himself to editing and publishing the journals that documented both his early life and later career. He rallied a group of scientists who criticized the science curriculum in the state of California, which he viewed as far too socially oriented and not nearly focused enough on hard science. California Governor Pete Wilson appointed Seaborg to head a committee that proposed changes to California's science curriculum despite outcries from labor organizations and others.[45]

Personal life

In 1942, Seaborg married Helen Griggs, the secretary of physicist Ernest Lawrence. Under wartime pressure, Seaborg had moved to Chicago while engaged to Griggs. When Seaborg returned to accompany Griggs for the journey back to Chicago, friends expected them to marry in Chicago. But, eager to be married, Seaborg and Griggs impulsively got off the train in the town of Caliente, Nevada, for what they thought would be a quick wedding. When they asked for City Hall, they found Caliente had none—they would have to travel 25 miles (40 km) north to Pioche, the county seat.
With no car, this was no easy feat, but one of Caliente's newest deputy sheriffs turned out to be a recent graduate of the Cal Berkeley chemistry department and was more than happy to do a favor for Seaborg. The deputy sheriff arranged for the wedding couple to ride up and back to Pioche in a mail truck. The witnesses at the Seaborg wedding were a clerk and a janitor.[46] Glenn Seaborg and Helen Griggs Seaborg had six children, of whom the first, Peter Glenn Seaborg, died in 1997. The others were Lynne Seaborg Cobb, David Seaborg, Steve Seaborg, Eric Seaborg, and Dianne Seaborg.[47]

Seaborg was an avid hiker. Upon becoming Chairman of the Atomic Energy Commission in 1961, he commenced taking daily hikes through a trail that he blazed at the headquarters site in Germantown, Maryland. He frequently invited colleagues and visitors to accompany him, and the trail became known as the "Glenn Seaborg Trail." He and his wife Helen are credited with blazing a 12 miles (19 km) trail in the East Bay area near their home in Lafayette, California. This trail has since become a part of the American Hiking Association's cross-country network of trails. Seaborg and his wife walked the trail network from Contra Costa County all the way to the California-Nevada border.[48][49]
There is a beauty in discovery. There is mathematics in music, a kinship of science and poetry in the description of nature, and exquisite form in a molecule. Attempts to place different disciplines in different camps are revealed as artificial in the face of the unity of knowledge. All literate men are sustained by the philosopher, the historian, the political analyst, the economist, the scientist, the poet, the artisan and the musician.
Glenn Seaborg [50]

Seaborg was elected a foreign member of the Royal Swedish Academy of Sciences in 1972 and the Royal Society of London.[51] He was honored as Swedish-American of the Year in 1962 by the Vasa Order of America. In 1991, the organization named "Local Lodge Glenn T. Seaborg No. 719" in his honor during the Seaborg Honors ceremony at which he appeared. This lodge maintains a scholarship fund in his name, as does the unrelated Swedish-American Club of Los Angeles.[52]

Seaborg kept a close bond to his Swedish origin. He visited Sweden every so often, and his family were members of the Swedish Pemer Genealogical Society, a family association open for every descendant of the Pemer family, a Swedish family with German origin, from which Seaborg was descended on his mother's side.[53]

On August 24, 1998, while in Boston to attend a meeting by the American Chemical Society, Seaborg suffered a stroke, which led to his death six months later on February 25, 1999, at his home in Lafayette.[54]

Honors and awards

During his lifetime, Seaborg is said to have been the author or co-author of numerous books and 500 scientific journal articles, many of them brief reports on fast-breaking discoveries in nuclear science while other subjects, most notably the actinide concept, represented major theoretical contributions in the history of science. He held more than 40 patents – among them the only patents ever issued for chemical elements, americium and curium, and received more than 50 doctorates and honorary degrees in his lifetime.[55] At one time, he was listed in the Guinness Book of World Records as having the longest entry in Marquis Who's Who in America. In February 2005, he was posthumously inducted into the National Inventors Hall of Fame.[37] His papers are in the Library of Congress.[56]The element seaborgium was named after Seaborg by Albert Ghiorso, E. Kenneth Hulet, and others, who also credited Seaborg as a co-discoverer.[55] It was named while Seaborg was still alive, which proved controversial. He influenced the naming of so many elements that with the announcement of seaborgium, it was noted in Discover magazine's review of the year in science that he could receive a letter addressed in chemical elements: seaborgium, lawrencium (for the Lawrence Berkeley Laboratory where he worked), berkelium, californium, americium.[57] Seaborgium is the only element to have been named after a living person.[55][58]

Selected Bibliography


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Particle accelerator


From Wikipedia, the free encyclopedia


Sketch of an electrostatic Van de Graaff accelerator

Sketch of the Ising/Widerøe linear accelerator concept, employing oscillating fields (1928)

A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams.[1] Large accelerators are best known for their use in particle physics as colliders (e.g. the LHC at CERN, RHIC at Brookhaven National Laboratory, and Tevatron at Fermilab). Other kinds of particle accelerators are used in a large variety of applications, including particle therapy for oncological purposes, and as synchrotron light sources for the study of condensed matter physics. There are currently more than 30,000 accelerators in operation around the world.[2]

There are two basic classes of accelerators: electrostatic and oscillating field accelerators.  
Electrostatic accelerators use static electric fields to accelerate particles. A small-scale example of this class is the cathode ray tube in an ordinary old television set. Other examples are the Cockcroft–Walton generator and the Van de Graaff generator. The achievable kinetic energy for particles in these devices is limited by electrical breakdown. Oscillating field accelerators, on the other hand, use radio frequency electromagnetic fields to accelerate particles, and circumvent the breakdown problem. This class, which was first developed in the 1920s, is the basis for all modern accelerator concepts and large-scale facilities.

Rolf Widerøe, Gustav Ising, Leó Szilárd, Donald Kerst, and Ernest Lawrence are considered pioneers of this field, conceiving and building the first operational linear particle accelerator,[3] the betatron, and the cyclotron.

Because colliders can give evidence of the structure of the subatomic world, accelerators were commonly referred to as atom smashers in the 20th century.[4] Despite the fact that most accelerators (but not ion facilities) actually propel subatomic particles, the term persists in popular usage when referring to particle accelerators in general.[5][6][7]

Uses


Beamlines leading from the Van de Graaff accelerator to various experiments, in the basement of the Jussieu Campus in Paris.

Breakdown of the cumulative number of industrial particle accelerators according to their applications.

The now disused Koffler particle accelerator at the Weizmann Institute, Rehovot, Israel.

Beams of high-energy particles are useful for both fundamental and applied research in the sciences, and also in many technical and industrial fields unrelated to fundamental research. It has been estimated that there are approximately 26,000 accelerators worldwide. Of these, only about 1% are research machines with energies above 1 GeV, while about 44% are for radiotherapy, 41% for ion implantation, 9% for industrial processing and research, and 4% for biomedical and other low-energy research.[8] The bar graph shows the breakdown of the number of industrial accelerators according to their applications. The numbers are based on 2012 statistics available from various sources, including production and sales data published in presentations or market surveys, and data provided by a number of manufacturers.[9]

High-energy physics

The largest particle accelerators with the highest particle energies are the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and the Large Hadron Collider (LHC) at CERN (which came on-line in mid-November 2009[10][11][12]) These accelerators are used for experimental particle physics.

For the most basic inquiries into the dynamics and structure of matter, space, and time, physicists seek the simplest kinds of interactions at the highest possible energies. These typically entail particle energies of many GeV, and the interactions of the simplest kinds of particles: leptons (e.g. electrons and positrons) and quarks for the matter, or photons and gluons for the field quanta. Since isolated quarks are experimentally unavailable due to color confinement, the simplest available experiments involve the interactions of, first, leptons with each other, and second, of leptons with nucleons, which are composed of quarks and gluons. To study the collisions of quarks with each other, scientists resort to collisions of nucleons, which at high energy may be usefully considered as essentially 2-body interactions of the quarks and gluons of which they are composed. Thus elementary particle physicists tend to use machines creating beams of electrons, positrons, protons, and antiprotons, interacting with each other or with the simplest nuclei (e.g., hydrogen or deuterium) at the highest possible energies, generally hundreds of GeV or more. Nuclear physicists and cosmologists may use beams of bare atomic nuclei, stripped of electrons, to investigate the structure, interactions, and properties of the nuclei themselves, and of condensed matter at extremely high temperatures and densities, such as might have occurred in the first moments of the Big Bang. These investigations often involve collisions of heavy nuclei – of atoms like iron or gold – at energies of several GeV per nucleon.

Particle accelerators can also produce proton beams, which can produce proton-rich medical or research isotopes as opposed to the neutron-rich ones made in fission reactors; however, recent work has shown how to make 99Mo, usually made in reactors, by accelerating isotopes of hydrogen,[13] although this method still requires a reactor to produce tritium. An example of this type of machine is LANSCE at Los Alamos.

Synchrotron radiation

Besides being of fundamental interest, high energy electrons may be coaxed into emitting extremely bright and coherent beams of high energy photons via synchrotron radiation, which have numerous uses in the study of atomic structure, chemistry, condensed matter physics, biology, and technology.
Examples include the ESRF in Grenoble, France, which has recently been used to extract detailed 3-dimensional images of insects trapped in amber.[14] Thus there is a great demand for electron accelerators of moderate (GeV) energy and high intensity.

Low-energy machines and particle therapy

Everyday examples of particle accelerators are cathode ray tubes found in television sets and X-ray generators. These low-energy accelerators use a single pair of electrodes with a DC voltage of a few thousand volts between them. In an X-ray generator, the target itself is one of the electrodes. A low-energy particle accelerator called an ion implanter is used in the manufacture of integrated circuits.

At lower energies, beams of accelerated nuclei are also used in medicine as particle therapy, for the treatment of cancer.

DC accelerator types capable of accelerating particles to speeds sufficient to cause nuclear reactions are Cockcroft-Walton generators or voltage multipliers, which convert AC to high voltage DC, or Van de Graaff generators that use static electricity carried by belts.

Electrostatic particle accelerators


A Cockcroft-Walton generator (Philips, 1937), residing in Science Museum (London).
A 1960s single stage 2 MeV linear Van de Graaff accelerator, here opened for maintenance

 Historically, the first accelerators used simple technology of a single static high voltage to accelerate charged particles. While this method is still extremely popular today, with the electrostatic accelerators greatly out-numbering any other type, they are more suited to lower energy studies owing to the practical voltage limit of about 30 MV (when the accelerator is placed in a gas with high dielectric strength, such as sulfur hexafluoride, allowing the high voltage). The same high voltage can be used twice in a tandem accelerator if the charge of the particles can be reversed while they are inside the terminal; this is possible with the acceleration of atomic nuclei by first adding an extra electron or forming an anionic (negatively charged) chemical compound, and then putting the beam through a thin foil to strip off electrons inside the high voltage conducting terminal, making a beam of positive charge.

Although electrostatic accelerators accelerate particles along a straight line, the term linear accelerator is more often associated with accelerators that use oscillating rather than static electric fields. Thus, many accelerators arranged in a straight line are not termed "linear accelerators" but rather "electrostatic accelerators" to differentiate the two cases.

Oscillating field particle accelerators

Due to the high voltage ceiling imposed by electrical discharge, in order to accelerate particles to higher energies, techniques involving more than one lower, but oscillating, high voltage sources are used. The electrodes can either be arranged to accelerate particles in a line or circle, depending on whether the particles are subject to a magnetic field while they are accelerated, causing their trajectories to arc.

Linear particle accelerators[edit]

Main article: Linear particle accelerator

Modern superconducting, multicell linear accelerator component.

In a linear particle accelerator (linac), particles are accelerated in a straight line with a target of interest at one end. They are often used to provide an initial low-energy kick to particles before they are injected into circular accelerators. The longest linac in the world is the Stanford Linear Accelerator, SLAC, which is 3 km (1.9 mi) long. SLAC is an electron-positron collider.

Linear high-energy accelerators use a linear array of plates (or drift tubes) to which an alternating high-energy field is applied. As the particles approach a plate they are accelerated towards it by an opposite polarity charge applied to the plate. As they pass through a hole in the plate, the polarity is switched so that the plate now repels them and they are now accelerated by it towards the next plate. Normally a stream of "bunches" of particles are accelerated, so a carefully controlled AC voltage is applied to each plate to continuously repeat this process for each bunch.

As the particles approach the speed of light the switching rate of the electric fields becomes so high that they operate at radio frequencies, and so microwave cavities are used in higher energy machines instead of simple plates.

Linear accelerators are also widely used in medicine, for radiotherapy and radiosurgery. Medical grade linacs accelerate electrons using a klystron and a complex bending magnet arrangement which produces a beam of 6-30 MeV energy. The electrons can be used directly or they can be collided with a target to produce a beam of X-rays. The reliability, flexibility and accuracy of the radiation beam produced has largely supplanted the older use of Cobalt-60 therapy as a treatment tool.

Circular or cyclic accelerators

In the circular accelerator, particles move in a circle until they reach sufficient energy. The particle track is typically bent into a circle using electromagnets. The advantage of circular accelerators over linear accelerators (linacs) is that the ring topology allows continuous acceleration, as the particle can transit indefinitely. Another advantage is that a circular accelerator is smaller than a linear accelerator of comparable power (i.e. a linac would have to be extremely long to have the equivalent power of a circular accelerator).

Depending on the energy and the particle being accelerated, circular accelerators suffer a disadvantage in that the particles emit synchrotron radiation. When any charged particle is accelerated, it emits electromagnetic radiation and secondary emissions. As a particle traveling in a circle is always accelerating towards the center of the circle, it continuously radiates towards the tangent of the circle. This radiation is called synchrotron light and depends highly on the mass of the accelerating particle. For this reason, many high energy electron accelerators are linacs. Certain accelerators (synchrotrons) are however built specially for producing synchrotron light (X-rays).

Since the special theory of relativity requires that matter always travels slower than the speed of light in a vacuum, in high-energy accelerators, as the energy increases the particle speed approaches the speed of light as a limit, but never attains it. Therefore particle physicists do not generally think in terms of speed, but rather in terms of a particle's energy or momentum, usually measured in electron volts (eV). An important principle for circular accelerators, and particle beams in general, is that the curvature of the particle trajectory is proportional to the particle charge and to the magnetic field, but inversely proportional to the (typically relativistic) momentum.

Cyclotrons

The earliest operational circular accelerators were cyclotrons, invented in 1929 by Ernest O. Lawrence at the University of California, Berkeley. Cyclotrons have a single pair of hollow 'D'-shaped plates to accelerate the particles and a single large dipole magnet to bend their path into a circular orbit. It is a characteristic property of charged particles in a uniform and constant magnetic field B that they orbit with a constant period, at a frequency called the cyclotron frequency, so long as their speed is small compared to the speed of light c. This means that the accelerating D's of a cyclotron can be driven at a constant frequency by a radio frequency (RF) accelerating power source, as the beam spirals outwards continuously. The particles are injected in the centre of the magnet and are extracted at the outer edge at their maximum energy.
Cyclotrons reach an energy limit because of relativistic effects whereby the particles effectively become more massive, so that their cyclotron frequency drops out of synch with the accelerating RF. Therefore simple cyclotrons can accelerate protons only to an energy of around 15 million electron volts (15 MeV, corresponding to a speed of roughly 10% of c), because the protons get out of phase with the driving electric field. If accelerated further, the beam would continue to spiral outward to a larger radius but the particles would no longer gain enough speed to complete the larger circle in step with the accelerating RF. To accommodate relativistic effects the magnetic field needs to be increased to higher radii like it is done in isochronous cyclotrons. An example for an isochronous cyclotron is the PSI Ring cyclotron in Switzerland, which provides protons at the energy of 590 MeV which corresponds to roughly 80% of the speed of light. The advantage of such a cyclotron is the maximum achievable extracted proton current which is currently 2.2 mA. The energy and current correspond to 1.3 MW beam power which is the highest of any accelerator currently existing.

Synchrocyclotrons and isochronous cyclotrons

A magnet in the synchrocyclotron at the Orsay proton therapy center

A classic cyclotron can be modified to increase its energy limit. The historically first approach was the synchrocyclotron, which accelerates the particles in bunches. It uses a constant magnetic field B, but reduces the accelerating field's frequency so as to keep the particles in step as they spiral outward, matching their mass-dependent cyclotron resonance frequency. This approach suffers from low average beam intensity due to the bunching, and again from the need for a huge magnet of large radius and constant field over the larger orbit demanded by high energy.

The second approach to the problem of accelerating relativistic particles is the isochronous cyclotron. In such a structure, the accelerating field's frequency (and the cyclotron resonance frequency) is kept constant for all energies by shaping the magnet poles so to increase magnetic field with radius. Thus, all particles get accelerated in isochronous time intervals. Higher energy particles travel a shorter distance in each orbit than they would in a classical cyclotron, thus remaining in phase with the accelerating field. The advantage of the isochronous cyclotron is that it can deliver continuous beams of higher average intensity, which is useful for some applications. The main disadvantages are the size and cost of the large magnet needed, and the difficulty in achieving the high magnetic field values required at the outer edge of the structure.

Synchrocyclotrons have not been built since the isochronous cyclotron was developed.

Betatrons

Another type of circular accelerator, invented in 1940 for accelerating electrons, is the Betatron, a concept which originates ultimately from Norwegian-German scientist Rolf Widerøe. These machines, like synchrotrons, use a donut-shaped ring magnet (see below) with a cyclically increasing B field, but accelerate the particles by induction from the increasing magnetic field, as if they were the secondary winding in a transformer, due to the changing magnetic flux through the orbit.[15]

Achieving constant orbital radius while supplying the proper accelerating electric field requires that the magnetic flux linking the orbit be somewhat independent of the magnetic field on the orbit, bending the particles into a constant radius curve. These machines have in practice been limited by the large radiative losses suffered by the electrons moving at nearly the speed of light in a relatively small radius orbit.

Synchrotrons

Aerial photo of the Tevatron at Fermilab, which resembles a figure eight. The main accelerator is the ring above; the one below (about half the diameter, despite appearances) is for preliminary acceleration, beam cooling and storage, etc.

To reach still higher energies, with relativistic mass approaching or exceeding the rest mass of the particles (for protons, billions of electron volts or GeV), it is necessary to use a synchrotron. This is an accelerator in which the particles are accelerated in a ring of constant radius. An immediate advantage over cyclotrons is that the magnetic field need only be present over the actual region of the particle orbits, which is much narrower than that of the ring. (The largest cyclotron built in the US had a 184-inch-diameter (4.7 m) magnet pole, whereas the diameter of synchrotrons such as the LEP and LHC is nearly 10 km. The aperture of the two beams of the LHC is of the order of a millimeter.)

However, since the particle momentum increases during acceleration, it is necessary to turn up the magnetic field B in proportion to maintain constant curvature of the orbit. In consequence, synchrotrons cannot accelerate particles continuously, as cyclotrons can, but must operate cyclically, supplying particles in bunches, which are delivered to a target or an external beam in beam "spills" typically every few seconds.

Since high energy synchrotrons do most of their work on particles that are already traveling at nearly the speed of light c, the time to complete one orbit of the ring is nearly constant, as is the frequency of the RF cavity resonators used to drive the acceleration.

Note also a further point about modern synchrotrons: because the beam aperture is small and the magnetic field does not cover the entire area of the particle orbit as it does for a cyclotron, several necessary functions can be separated. Instead of one huge magnet, one has a line of hundreds of bending magnets, enclosing (or enclosed by) vacuum connecting pipes. The design of synchrotrons was revolutionized in the early 1950s with the discovery of the strong focusing concept.[16][17][18] The focusing of the beam is handled independently by specialized quadrupole magnets, while the acceleration itself is accomplished in separate RF sections, rather similar to short linear accelerators. Also, there is no necessity that cyclic machines be circular, but rather the beam pipe may have straight sections between magnets where beams may collide, be cooled, etc. This has developed into an entire separate subject, called "beam physics" or "beam optics".[19]

More complex modern synchrotrons such as the Tevatron, LEP, and LHC may deliver the particle bunches into storage rings of magnets with constant B, where they can continue to orbit for long periods for experimentation or further acceleration. The highest-energy machines such as the Tevatron and LHC are actually accelerator complexes, with a cascade of specialized elements in series, including linear accelerators for initial beam creation, one or more low energy synchrotrons to reach intermediate energy, storage rings where beams can be accumulated or "cooled" (reducing the magnet aperture required and permitting tighter focusing; see beam cooling), and a last large ring for final acceleration and experimentation.

Segment of an electron synchrotron at DESY
Electron synchrotrons
Circular electron accelerators fell somewhat out of favor for particle physics around the time that SLAC's linear particle accelerator was constructed, because their synchrotron losses were considered economically prohibitive and because their beam intensity was lower than for the unpulsed linear machines. The Cornell Electron Synchrotron, built at low cost in the late 1970s, was the first in a series of high-energy circular electron accelerators built for fundamental particle physics, the last being LEP, built at CERN, which was used from 1989 until 2000.
A large number of electron synchrotrons have been built in the past two decades, as part of synchrotron light sources that emit ultraviolet light and X rays; see below.

Storage rings

For some applications, it is useful to store beams of high energy particles for some time (with modern high vacuum technology, up to many hours) without further acceleration. This is especially true for colliding beam accelerators, in which two beams moving in opposite directions are made to collide with each other, with a large gain in effective collision energy. Because relatively few collisions occur at each pass through the intersection point of the two beams, it is customary to first accelerate the beams to the desired energy, and then store them in storage rings, which are essentially synchrotron rings of magnets, with no significant RF power for acceleration.

Synchrotron radiation sources

Some circular accelerators have been built to deliberately generate radiation (called synchrotron light) as X-rays also called synchrotron radiation, for example the Diamond Light Source which has been built at the Rutherford Appleton Laboratory in England or the Advanced Photon Source at Argonne National Laboratory in Illinois, USA. High-energy X-rays are useful for X-ray spectroscopy of proteins or X-ray absorption fine structure (XAFS), for example.
Synchrotron radiation is more powerfully emitted by lighter particles, so these accelerators are invariably electron accelerators. Synchrotron radiation allows for better imaging as researched and developed at SLAC's SPEAR.

FFAG accelerators

Fixed-Field Alternating Gradient accelerators (FFAG)s, in which a very strong radial field gradient, combined with strong focusing, allows the beam to be confined to a narrow ring, are an extension of the isochronous cyclotron idea that is lately under development.[20] They use RF accelerating sections between the magnets, and so are isochronous for relativistic particles like electrons (which achieve essentially the speed of light at only a few MeV), but only over a limited energy range for protons and heavier particles at sub-relativistic energies. Like the isochronous cyclotrons, they achieve continuous beam operation, but without the need for a huge dipole bending magnet covering the entire radius of the orbits.

History

Ernest Lawrence's first cyclotron was a mere 4 inches (100 mm) in diameter. Later, in 1939, he built a machine with a 60 in diameter pole face, and planned one with a 184-inch diameter in 1942, which was, however, taken over for World War II-related work connected with uranium isotope separation; after the war it continued in service for research and medicine over many years.
The first large proton synchrotron was the Cosmotron at Brookhaven National Laboratory, which accelerated protons to about 3 GeV (1953–1968). The Bevatron at Berkeley, completed in 1954, was specifically designed to accelerate protons to sufficient energy to create antiprotons, and verify the particle-antiparticle symmetry of nature, then only theorized. The Alternating Gradient Synchrotron (AGS) at Brookhaven (1960–) was the first large synchrotron with alternating gradient, "strong focusing" magnets, which greatly reduced the required aperture of the beam, and correspondingly the size and cost of the bending magnets. The Proton Synchrotron, built at CERN (1959–), was the first major European particle accelerator and generally similar to the AGS.

The Stanford Linear Accelerator, SLAC, became operational in 1966, accelerating electrons to 30 GeV in a 3 km long waveguide, buried in a tunnel and powered by hundreds of large klystrons. It is still the largest linear accelerator in existence, and has been upgraded with the addition of storage rings and an electron-positron collider facility. It is also an X-ray and UV synchrotron photon source.

The Fermilab Tevatron has a ring with a beam path of 4 miles (6.4 km). It has received several upgrades, and has functioned as a proton-antiproton collider until it was shut down due to budget cuts on September 30, 2011. The largest circular accelerator ever built was the LEP synchrotron at CERN with a circumference 26.6 kilometers, which was an electron/positron collider. It achieved an energy of 209 GeV before it was dismantled in 2000 so that the underground tunnel could be used for the Large Hadron Collider (LHC). The LHC is a proton collider, and currently the world's largest and highest-energy accelerator, expected to achieve 14 TeV energy per beam, and currently operating at half that.

The aborted Superconducting Super Collider (SSC) in Texas would have had a circumference of 87 km. Construction was started in 1991, but abandoned in 1993. Very large circular accelerators are invariably built in underground tunnels a few metres wide to minimize the disruption and cost of building such a structure on the surface, and to provide shielding against intense secondary radiations that occur, which are extremely penetrating at high energies.

Current accelerators such as the Spallation Neutron Source, incorporate superconducting cryomodules. The Relativistic Heavy Ion Collider, and Large Hadron Collider also make use of superconducting magnets and RF cavity resonators to accelerate particles.

Targets and detectors

The output of a particle accelerator can generally be directed towards multiple lines of experiments, one at a given time, by means of a deviating electromagnet. This makes it possible to operate multiple experiments without needing to move things around or shutting down the entire accelerator beam. Except for synchrotron radiation sources, the purpose of an accelerator is to generate high-energy particles for interaction with matter.

This is usually a fixed target, such as the phosphor coating on the back of the screen in the case of a television tube; a piece of uranium in an accelerator designed as a neutron source; or a tungsten target for an X-ray generator. In a linac, the target is simply fitted to the end of the accelerator. The particle track in a cyclotron is a spiral outwards from the centre of the circular machine, so the accelerated particles emerge from a fixed point as for a linear accelerator.

For synchrotrons, the situation is more complex. Particles are accelerated to the desired energy. Then, a fast acting dipole magnet is used to switch the particles out of the circular synchrotron tube and towards the target.

A variation commonly used for particle physics research is a collider, also called a storage ring collider. Two circular synchrotrons are built in close proximity – usually on top of each other and using the same magnets (which are then of more complicated design to accommodate both beam tubes). Bunches of particles travel in opposite directions around the two accelerators and collide at intersections between them. This can increase the energy enormously; whereas in a fixed-target experiment the energy available to produce new particles is proportional to the square root of the beam energy, in a collider the available energy is linear.

Higher energies


A Livingston chart depicting progress in collision energy through 2010. The LHC is the largest collision energy to date, but also represents the first break in the log-linear trend.

At present the highest energy accelerators are all circular colliders, but both hadron accelerators and electron accelerators are running into limits. Higher energy hadron and ion cyclic accelerators will require accelerator tunnels of larger physical size due to the increased beam rigidity.

For cyclic electron accelerators, a limit on practical bend radius is placed by synchrotron radiation losses and the next generation will probably be linear accelerators 10 times the current length. An example of such a next generation electron accelerator is the 40 km long International Linear Collider, due to be constructed between 2015–2020.

It is believed that plasma wakefield acceleration in the form of electron-beam 'afterburners' and standalone laser pulsers might be able to provide dramatic increases in efficiency over RF accelerators within two to three decades. In plasma wakefield accelerators, the beam cavity is filled with a plasma (rather than vacuum). A short pulse of electrons or laser light either constitutes or immediately trails the particles that are being accelerated. The pulse disrupts the plasma, causing the charged particles in the plasma to integrate into and move toward the rear of the bunch of particles that are being accelerated. This process transfers energy to the particle bunch, accelerating it further, and continues as long as the pulse is coherent.[21]

Energy gradients as steep as 200 GeV/m have been achieved over millimeter-scale distances using laser pulsers[22] and gradients approaching 1 GeV/m are being produced on the multi-centimeter-scale with electron-beam systems, in contrast to a limit of about 0.1 GeV/m for radio-frequency acceleration alone. Existing electron accelerators such as SLAC could use electron-beam afterburners to greatly increase the energy of their particle beams, at the cost of beam intensity. Electron systems in general can provide tightly collimated, reliable beams; laser systems may offer more power and compactness. Thus, plasma wakefield accelerators could be used – if technical issues can be resolved – to both increase the maximum energy of the largest accelerators and to bring high energies into university laboratories and medical centres.

Higher than 0.25 GeV/m gradients have been achieved by a dielectric laser accelerator, which may present another viable approach to building compact high-energy accelerators.[23]

Black hole production and public safety concerns

In the future, the possibility of black hole production at the highest energy accelerators may arise if certain predictions of superstring theory are accurate.[24][25] This and other exotic possibilities have led to public safety concerns that have been widely reported in connection with the LHC, which began operation in 2008. The various possible dangerous scenarios have been assessed as presenting "no conceivable danger" in the latest risk assessment produced by the LHC Safety Assessment Group.[26] If black holes are produced, it is theoretically predicted that such small black holes should evaporate extremely quickly via Bekenstein-Hawking radiation, but which is as yet experimentally unconfirmed. If colliders can produce black holes, cosmic rays (and particularly ultra-high-energy cosmic rays, UHECRs) must have been producing them for eons, but they have yet to harm anybody.[27] It has been argued that to conserve energy and momentum, any black holes created in a collision between an UHECR and local matter would necessarily be produced moving at relativistic speed with respect to the Earth, and should escape into space, as their accretion and growth rate should be very slow, while black holes produced in colliders (with components of equal mass) would have some chance of having a velocity less than Earth escape velocity, 11.2 km per sec, and would be liable to capture and subsequent growth. Yet even on such scenarios the collisions of UHECRs with white dwarfs and neutron stars would lead to their rapid destruction, but these bodies are observed to be common astronomical objects. Thus if stable micro black holes should be produced, they must grow far too slowly to cause any noticeable macroscopic effects within the natural lifetime of the solar system.[26]

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