NUCLEAR PHYSICS RESEARCH: LITTLE THINGS MEAN A LOT
Little things mean a lot, but in nuclear physics, big things are needed to discover them and find out what they mean. At the Laboratory, the chief goal of nuclear physics research has been to determine the structure of the nucleus of the atom and to understand the course of reactions between nuclei. Detailed information on the properties of tiny atomic nuclei can be obtained only by the use of huge accelerators. A beam of particles from an accelerator is propelled against a target; the effect of the bombardment on the projectile beam and the resulting emission of particles from the collision shed light on the structure and behavior of the target nuclei and on the reaction mechanisms.
Early nuclear physics research at the Laboratory made use of reactors. Later, the original Physics Division relied on a series of electrostatic, or Van de Graaff accelerators and the Oak Ridge Electron Linear Accelerator (which was operated by the Neutron Physics Division mainly to determine the neutron-absorbing abilities of nuclei of candidate breeder-reactor shielding materials). The Electronuclear Division depended on an increasingly sophisticated series of cyclotrons, which accelerated charged particles in circular orbits.
In the late 1940s a landmark experiment by Arthur Snell and Frances Pleasanton provided the first accurate measurement of the lifetime of the neutron. They also measured the gravitational force on the neutron.
Using newly built cyclotrons in the 1950s, laboratory physicists studied the reactions between heavy projectile ions and target nuclei that collided at high energies. Alex Zucker and Harry Reynolds pioneered in research on heavy-ion reactions using the 63-inch cyclotron, which produced the world's first multicharged heavy-ion beams.
At the Van de Graaff Laboratory, Paul Stelson and Francis McGowan carefully measured the Coulomb excitation-the energized state in a nucleus resulting from its interaction with the projectile particle's electric field-in a wide range of nuclei. This seminal work showed clearly that the classical interpretation of low-energy nuclear collisions was inadequate, setting the stage for the development of a quantum-mechanical model.
Using 22-MeV protons from the 86-inch cyclotron to generate particles of higher energies, Bernard Cohen and his associates showed that transfer reactions (in which a nucleon-neutron or proton-is transferred from the target to the projectile nucleus) at these energies did not result as expected from the decay of a compound nucleus formed during the collision between the incident and target nuclei. Instead, the transfer resulted from a direct nuclear reaction in which the projectile particle passing through the target nucleus interacts with only part of the nucleus. This "direct" process also was not well described in terms of a classical model.
Work by Cohen also resulted in the discovery of a new low-lying collective mode in nuclei. This collective mode is a low-energy state of the nucleus that causes it to vibrate as a single system, just as the tone from a ringing bell results from the vibration of the entire bell structure. Originally dubbed "anomalous inelastic scattering," these low-energy states appeared even stronger than the well-known low-lying "quadrupole vibrational states" in which the vibrating nuclei alternate between shapes resembling an egg and the earth. These newly discovered states proved to arise from "nuclear octupole vibration" in which the vibrating nuclei are alternately spherical and pear-shaped. This observation helped affirm the picture that the nucleus, as a system, could support many modes of collective resonant behavior.
In the early 1950s, the shell model was developed to explain many features of nuclei. In this model the nucleons are considered to occupy shells and subshells (like electrons in the atom) and act independently according to a preassigned set of shell energy levels. A model to explain direct nuclear reactions was also formulated. Neither of these models could be fully used until the advent of large, fast digital computers a decade later. The Laboratory was a pioneer in developing mathematical methods and using computing facilities to refine these models to interpret experimental measurements at ORNL and elsewhere.
In the 1960s ORNL theorists led by Ray Satchler pioneered the application of the distorted-wave Born approximation with which he and Bob Bassel and Dick Drisko made great advances in understanding nuclear reactions. They developed methods for extracting quantitative information from single-nucleon transfer reactions and inelastic scattering-scattering resulting from a collision in which the total kinetic energy of the colliding particles is not the same after the collision as before it. Results of their work include two computer codes to extract information from nuclear reactions in experiments-SALLY and JULIE. The latter became the world standard for extraction of nuclear structure data from direct nuclear reactions.
At the same time, Francis Perey and Brian Buck developed a computer program that was applied extensively to the understanding of neutron-scattering measurements. Perey developed the global optical model search code GENOA, which became the standard for use in calculations of the distorted-wave Born approximation, which was set down by Satchler (who is also known for his widely used college textbook on angular momentum). A long series of detailed measurements of scattering and transfer reactions done at the EN-tandem accelerator by Perey, Kirk Dickens, and Bob Silva served as critical benchmarks in the early development of these computer programs, validating their usefulness to a worldwide community.
In the late 1960s the interpretation of the shell model's low-lying nuclear levels (low-energy shells) was given a big boost with the development of the Oak Ridge-Rochester Multi-Shell Program. Completed under the direction of Edith Halbert, this was the most sophisticated program of its type for years and was used extensively for computing detailed nuclear properties and for understanding the general applicability of the nuclear shell model.
Also in the late 1960s at ORNL, measurements of the neutron-absorption crosssection in the energy region from 5,000 to 200,000 electron volts for nuclei from fluorine (mass 19) to uranium (mass 238) were made. These measurements by Jack Gibbons, Dick Macklin, and their colleagues proved useful not only to nuclear theorists and nuclear engineers but also to astrophysicists seeking to understand the process of nucleosynthesis in stars, which builds heavy elements from light ones and governs the relative amounts of elements in the universe.
By the early 1970s, the Laboratory nuclear physicists had available to them the higher-energy ions produced by the Oak Ridge Isochronous Cyclotron (ORIC) accelerator. One of the most fundamental discoveries to emerge from this program was the work of Fred Bertrand, Monte Lewis, and their collaborators, who made the first observation of the nuclear giant quadrupole resonances-types of a giant resonance in which an appreciable fraction of the nucleons move together in a collective mode when selectively excited by the appropriate nuclear reactions. This work opened up the new field of using charged particles and heavy ions to excite the multipole resonant modes of the nucleus, making it deform as it alternately expends and compresses in different directions.
As heavy ions became available from the ORIC, transfermium elements could be produced using its beams on transuranium targets prepared at the High Flux Isotope Reactor. Particularly notable was a series of complex experiments by Curt Bemis, Pete Dittner, Dick Hahn, and Bob Silva using coincident alpha and X-ray detection to provide the first unequivocal identification of elements 102, 103, 104 and 105.
The history of major contributions by ORNL researchers to the field of nuclear physics has been marked by the development of sophisticated instruments and by the use of large-scale computers and the development of long, complex computer codes to interpret and analyze experimental phenomena. This tradition of using big things to better understand little things continues to this day.
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