Samuel King Allison

PHYSICIST NAME: Samuel King Allison
STUDENT NAME: Angela Mari Peralta

BIOGRAPHY

Samuel K. Allison was born in Chicago, Illinois, and attended the University of Chicago for his undergraduate degree as well as for his PhD (in chemistry under the supvervision of William Draper Harkins, though his thesis was related to experimental physics). From 1923 until 1925 he was a research fellow at Harvard University and from 1925 until 1926 he was a research fellow at the Carnegie Institution. From 1926 until 1928 he taught physics at University of California, Berkeley, after which he returned to the University of Chicago, where he studied the Compton effect and the dynamical theory of x-ray diffraction. He developed a high resolution x-ray spectrometer with a graduate student, John H. Williams. In the late 1930s, he studied with John Cockcroft at the Cavendish Laboratory, learning about linear accelerators, and after returning to Chicago he built one. He authored a textbook on x-rays with Arthur Compton which became widely used.

During World War II, Allison was a consultant to the National Defense Research Committee and the S-1 Uranium Committee, the early investigations into the feasibility of an atomic bomb which would later become the Manhattan Project. He worked at the Chicago Metallurgical Laboratory and was its director from 1943 until 1944. He then went to work at the secret Los Alamos laboratory in New Mexico. Notably, he was the one who read the countdown over the loudspeakers for the "Trinity" test in 1945.

Samuel King Allison (November 13, 1900 – September 15, 1965) was an American physicist, most notable for his role in the Manhattan Project — where among other things he read the countdown for the detonation of the "Trinity" test — and his postwar work in the "scientists' movement".

CONTRIBUTIONS

NUCLEAR AND ATOMIC PHYSICS

Allison's contributions to nuclear physics began in the mid-1930s while he was visiting the Cavendish Laboratory as a Guggenheim fellow. In a paper presenting the results of his "Experiments on the Efficiencies of Production and the Half-Lives of Radio-Carbon and Radio-Nitrogen," he thanked "Dr. J. D. Cockroft for instruction in the use of the high-voltage apparatus at the Cavendish Laboratory [and] Lord Rutherford for permission to work in the laboratory."

When he returned to Chicago he built his own Cockroft-Walton accelerator in Eckhart Laboratory, home of the Physics Department. He soon had some five students measuring the energies of particles produced in lithium targets bombarded with protons and deuterons. Just as this work was achieving its initial success it was interrupted by war.

When he was free to return to the field, he reconstructed the accelerator in the new Research Institutes Building, which had just been built to house the Institute for Nuclear Studies. He called his accelerator the "kevatron" to emphasize its modest peak energy (400 KeV) at a time when his associates were building machines in the million- and then billion-volt range with names like "cosmotron" and "bevatron." The kevatron stood on the basement floor of the building, extended through a very large hole in the first floor, and reached almost to the level of the second floor. Access to the ion source was by way of a plank thrown across the gaping hole some 10 feet above the basement floor. His students tell of hair-raising adventures in coping with that feature of the laboratory. The high-voltage apparatus was operated from an adjacent room with a haywire but smoothly efficient rig of mirrors, pulleys, and strings culminating in an array of broomsticks--you turned the brooms that pulled the strings that worked the levers that made the beams.

The research had two objectives: the study of low energy nuclear reactions induced by light projectiles (protons, deuterons, helium ions, lithium ions) and the elucidation of the phenomena associated with the interaction of atomic and ionic beams with matter, in particular the energy loss and the capture and loss of electrons by the beam particles. A by-product of the research effort was the development of sophisticated apparatus for the production of monoenergetic beams of particles and for the precise measurement of their energy.

Allison's postwar studies of low-energy nuclear reactions in light nuclei were concerned at first with the energy release as determined by measurement of the kinetic energy of the reaction products. These studies included measurements of the energy levels of unstable reaction products, such as 7Be, 13B, 15C, and 17N. These light nuclei and the reactions leading to their formation later proved to be of great cosmological significance because of their role in the production of stellar energy and in nucleosynthetic processes.

In the kevatron, Allison's projectiles were protons or deuterons; the targets were lithium, beryllium, and boron. The reaction products were studied with his electrostatic or magnetic analyzers. Later, Allison acquired a 2-MeV Van de Graaff accelerator, which he equipped to accelerate lithium ions to energies sufficient to cause nuclear reactions in light nuclei. With his modest apparatus, first the kevatron and then the Van de Graaff, he was an early pioneer in a field of research that would later be known as "heavy ion physics." His projectiles were too light to qualify as heavy ions by modern standards, but they were heavier than could be found in other laboratories of that era.

Edwin Norbeck, then one of Allison's students, described the venture into lithium projectiles as follows:

By 1953 it was difficult to come up with good nuclear physics experiments that could be done with a low-energy accelerator. I remember a brainstorming session he had arranged to uncover promising projects. The conclusion of the meeting was that any new experiment would be difficult, either because it required high precision, had a low cross-section, or used exotic beams or targets. After this meeting Prof. Allison and I met in his office to discuss the situation. He recalled seeing an article, published many years earlier in Review of Scientific Instruments, that described a method for making a beam of lithium ions:

The authors, J. P. Blewett and E. J. Jones, had produced lithium ions by heating the lithium aluminum silicates, spodumene and beta-eucryptite, on a filament of platinum gauze. Eucryptite gave twice as much lithium current as spodumene. Allison contacted friends who were geologists and soon we had some spodumene, a semiprecious jewel, and then some alpha-eucryptite. These natural minerals gave good ion currents, but soon we were making our own beta-eucryptite using separated isotopes.

We put the source in a Van de Graaff accelerator and brought out a 1.2-MeV 7Li beam. This was more difficult than it sounds, but Allison had a good solution to every problem that arose. When the big day came to bring out the beam, we had a variety of detectors. If there were any nuclear reactions at such a low energy we wanted to be sure that we would not miss them. We had a gamma ray detector and a neutron survey meter. We used a thick target of LiF in a chamber with a thin window on one side. Outside the thin window we had a phototube coated on the end with a ZnS phosphor and covered with a thin aluminum foil.

When the beam hit the target I was pleased to see lots of gamma rays and neutrons, but what caught Prof. Allison's attention were the charged particles. He put a sheet of paper in front of the ZnS and found only a slight reduction in the counting rate. He commented that such a large number of high-energy protons could only come from the reaction 7Li(7Li,p)13B. He then noted that the only trouble with that explanation was that the nucleus 13B [was not supposed] to exist.
The discovery of this nucleus was only the beginning. It was soon followed by further studies of lithium-induced nuclear reactions. The study of reactions with lithium beams was a new branch of nuclear physics. Even with a maximum beam energy of only 2 MeV, the Van de Graaff accelerator could be used to study reactions of 6Li and 7Li with all of the stable isotopes of Li, Be, B, C, N, and O. The lithium ions produced nuclei far from stability, of which 13B was the first example. Reactions observed at energies near or below the Coulomb barrier included "fusion-like" processes such as 7Li(7Li,p)13B and 9Be(7Li,p)15C and "stripping or transfer" processes such as 9Be(7Li,8Li)8Be. Measurements of the products of various reactions made it possible to determine the masses of the ground and low-lying excited states of 12B, 13B, 15C, and 17N. The last of his nuclear studies involved elucidation of the mechanisms of complex reactions such as 6Li + 6Li yielding three alpha particles, and investigation of the role of intermediate nuclei (e.g., 8Be) in these reactions.

Using data on 9Be(7Li,8Li)8Be from an experiment by Norbeck et al. at the University of Minnesota, Allison calculated the neutron density out to 40 fm. The words "halo nuclei," now in common use, did not appear until much later.

Allison introduced the precision techniques he had developed for nuclear reaction spectroscopy to study the interaction of particles with matter. He commented that everyone wanted quantitative information about the passage of beams through matter, but no one wanted to make the measurements. Using the apparatus developed for precise determination of the energies and products of nuclear reactions he and his associates were able to measure the changes in energy, the "stopping power," and the charge-changing cross-sections as a function of energy, ionic species, and stopping material. The early work on the energy loss of slow protons, deuterons, alpha particles, and Li6 nuclei passing through thin aluminum and gold films was pioneering and established Allison and his collaborators as the leaders in this field. The work was extended to gaseous targets. The results of the measurements of cross-sections for electron capture and loss in hydrogen and air were outstanding. This work was followed by extensive studies of helium ions in gasses where neutral atoms and both the singly and doubly charged ions coexist. The work was then extended to 2-MeV lithium.

In this atomic beam work Allison was without peer. The review article "Passage of Heavy Particles Through Matter" by Allison and Warshaw (1956) was the definitive work on stopping powers for at least a decade. The measurements of atomic capture cross-sections became important in applications, such as neutral injection into plasma machines and production of H- ions in tandem Van de Graaff machines.

In the experiments on light nuclei it was often necessary to subtract a background due to a contamination of the targets by decomposed pump oil. Allison identified the unwelcome scattering nuclei by measuring the difference in energy between the incident and recoiling projectiles. That experience led him to suggest to his colleague Anthony Turkevich that this technique could be used to analyze surface materials where conventional chemical analysis was not feasible.

Turkevich and his colleague Anthony Tuzzolino built an instrument on this principle using the recently developed silicon detectors. Their scattering analysis instrument was carried to the moon on the last three Surveyor missions and made the first chemical analyses of the lunar surface. More recently, a successor to that instrument built by Tom Economu has analyzed the surface of Mars.

OBJECT OF INTEREST

I chose Samuel King Allison because I want to find out how he came up with the invention, “Nuclear and Atomic Physics”. I want to find out how he built this kind of experiment and how he made use of his low-energy accelerator.