Heavy Atoms and High Energy

Abstract

High energy accelerators, usually viewed as tools to create sub-nuclear elementary particles, are also useful in the study of large atomic nuclei. Much basic research is being carried out in nuclear chemistry, but unanswered questions are being raised by each experiment. The dynamic state of nuclear chemistry is the main reason for the dismay of many chemists and lower energy physicists over the shutting down of the Brookhaven Cosmotron, which shot its last proton in December. Dr. Gerhart Friedlander, who received the American Chemistry Society Award for Nuclear Applications in Chemistry this year, says it is ironic that this area of research should be deemed worthy of an award by the when the machine on which the work is being done was closed down by the Atomic Energy Commission for financial reasons. The Cosmotron is the smallest of several accelerators in the billion-electron-volt range where experiments in nuclear chemistry can be done. In all of these machines, such as the Berkeley Bevatron at the University of California and the Zero Gradient Synchrotron at Argonne, Ill., the large share of the experiments are in the field of high energy particle physics. When the high energy experimenters lost interest in the Cosmotron, which was overshadowed by more powerful machines, the AEC's high energy division decided not to foot the bill. The chemistry section, which had been using from 10 to 15 percent of the machine's time, could not afford the whole cost of operating the accelerator. Contrary to general belief, nuclear chemists have been working with high energy machines continually. In 1952, when the three-billion-electron-volt Cosmotron was the most powerful atom smasher in existence, Dr. Friedlander analyzed a sample of lead that had been used to tune up the Cosmotron. He found that the 3-Bev beam had done unexpected things to the lead-the elements produced were different from those resulting from exposure to lower energy beams. Experiments carried out on the same machine and others built later continued to show up unexpected results. The complicated things that happen when an atom is struck by a high energy proton have been divided by nuclear chemists into three basic categories: * Spallation, or chipping off a few protons or neutrons from the nucleus; * Fission, or splitting the nucleus into two more-or-less equal pieces; * Fragmentation, where larger chunks of nuclear particles, containing from 10 to 40 protons and neutrons, are blasted from the target nucleus. The first two types of reactions are fairly well understood, although the catalogue of fission and spallation products for all the elements at various energies is nowhere near complete. But fragmentation is another matter. There is no agreement on exactly what happens when larger chunks are broken off. The controversy centers on the amount of time it takes for nuclear reactions to take place. Fission and spallation are generally considered to take place in two stages. In the first, the incoming proton, with very high energy, thrusts into the nucleus and sets all the nuclear particles into motion, just as a shotgun pellet speeding into a soup plate filled with other pellets would set them in motion. Some particles shoot out of the nucleus immediately-just as some pellets would be driven out of the soup plate. In the next stage, the energy of any one particle isn't enough to carry it out of the nucleus, but occasionally the motion of two particles will accidently reinforce each other and a single particle will escape. In fragmentation, the two-stage process doesn't seem to fit. To fill the gap it was theorized that local hotspots in the nucleus are created, and larger nuclear fragments spill off from the highly energized sections of the nucleus. The need for the hotspot concept was challenged at the ACS meeting in Miami Beach, Fla., recently by Dr. J. N. Miller of Columbia University. It is part of the hotspot idea that some of the protons and neutrons have very high energies and the rest low energies. In the two-stage process that is typical of fission and spallation, the energy of the nuclear particles is pretty evenly distributed. Dr. Miller calculated the amount of time necessary for excited, hotspot particles to distribute their energy to the rest of the nucleus-and he found