Argonne History

Understanding the Physical Universe

Much of the understanding of nuclear fission came from the work of physicists at the Metallurgical Laboratory and at Argonne. Argonne scientists led the country's basic research effort in support of reactors. They also developed and built two novel accelerators -- the Zero Gradient Synchrotron and the Argonne Tandem Linear Accelerator System, the world's first superconducting accelerator of heavy nuclei, which is now the country's premier research tool for studying the structure and dynamics of heavy nuclei.

Accelerating All Nuclei

Paul Markovich polishes a split-ring resonator, the technology that made ATLAS the world's first superconducting linear accelerator for heavy ions. (Click the image to see a larger photo.)

The Argonne Tandem Linear Accelerator System (ATLAS) was dedicated in 1985, one week after accelerating its first ion beam. The world's first superconducting linear accelerator for nuclear research was a major advance in accelerator technology. Acceleration is provided by superconducting resonators. This makes possible high-quality ion beams over a broad range of projectile energy and mass, spanning the energy domains in which nuclear structure effects are particularly important. Its beam intensity was increased 100-fold in 1988 which permitted acceleration of ions as heavy as uranium to energies as high as 1.9 billion electron volts.

Determining Nuclear Structure

In 1948, an Argonne scientist reasoned that the atomic nucleus consisted of protons and neutrons arranged in a shell. Her invention of the nuclear shell model provided a working theoretical infrastructure for studying the properties of atomic nuclei and earned her a share of the Nobel Prize in physics in 1963. Other Argonne scientists developed and extended the shell model to make it applicable to a wide range of nuclei. In the process they introduced modern electronic computing to nuclear physics.

Argonne scientists have also provided understanding of the nuclei through experiments in nuclear spectroscopy, in particular of the very heaviest of nuclei -- those beyond uranium.

University of Chicago and Argonne researchers challenged the conservation-of-parity principle for complex nuclei and mesons. (Click the image to see a larger photo.)

In 1956, the conservation of parity principle, posited by Eugene Wigner in 1936, was challenged. Wigner's original formula showed that atoms can fluctuate between two atomic states; that the laws of nature made no distinction between left and right. The challengers, who received a Nobel Prize for their efforts, demonstrated that the principle did not apply to all nuclear reactions and that parity was not preserved in weak interactions. In 1957, Argonne and University of Chicago physicists proved that the principle did not hold for complex nuclei or mesons. They later demonstrated, using a beam of slow neutrons, that parity conservation also does not apply in the radioactive decay of neutrons. These findings expanded knowledge in nuclear science significantly.

The Mössbauer effect, first demonstrated by a German physicist in 1958, presented the world with a new way of studying a wide variety of phenomena at a vastly enhanced level of precision by making use of the absorption X-rays at sharply defined energies in special nuclei and under special conditions. Argonne scientists almost immediately found a particular nucleus (iron-57) for which the Mössbauer effect could then be used as a practical tool under many conditions. Since then more than 90 percent of all research using Mössbauer effect has been done with iron-57. Argonne researchers were among the leaders in using Mössbauer effect for fundamental studies in nuclear physics, hot-atom chemistry, materials science and general relativity experiments.

Argonne, capitalizing on the synergy between the lab's basic research and its fission reactor development, has from the outset been a leader in the use of slow neutrons to study nuclear physics, materials science, chemistry and biology. Among the achievements in physics were: systematic investigation of the interactions of slow neutrons with nuclei; the invention of magnetic neutron mirrors that enabled neutrons to be used as a surrogate for X-rays in investigating properties of matter for which X-rays were not useful; and basic data on some properties of the universal weak interaction, one of the four fundamental forces of nature.

Accelerating Protons and Spallating Neutrons

In the early 1960s, Argonne scientists designed and built the 12.5 GeV Zero Gradient Synchrotron (ZGS), then the world's leading weak-focusing proton accelerator. Its program of polarized target development and exploitation led to greater understanding of the spin dependence of strongly interacting particles and, coupled with the later success in accelerating polarized protons in the ZGS, opened up a new area of high-energy physics.

Most synchrotrons are of the alternating gradient design -- the magnetic field keeps the circulating beams of particles focused. The ZGS had no field gradient, relying on edge-focusing to confine the particles to the beam orbit. By developing the highly efficient window-frame magnets used in the ZGS, the lab earned a reputation for working at the frontier in magnet technology.

The world's first neutrino observation in a hydrogen bubble chamber was found Nov. 13, 1970, on this historical photograph from the Zero Gradient Synchrotron's 12-foot bubble chamber. The invisible neutrino strikes a proton where three particle tracks originate (right). The neutrino turns into a mu-meson, the long center track. The short track is the proton. The third track is a pi-meson created by the collision. (Click the image to see a larger photo.)

Experimental facilities developed for use at the ZGS improved its scientific performance tremendously. Scientists built and incorporated the world's largest (12-foot) bubble chamber for high-energy physics research. Another important achievement was the use of a superconducting magnet in conjunction with this huge chamber. In November 1970, a neutrino interaction was observed in the bubble chamber -- the first time one had ever been seen in a liquid hydrogen chamber. The lab was a world leader in cryogenics and superconducting magnet development in the 1960s and 1970s.

On July 11, 1973, measurements taken at the ZGS proved it was possible to inject polarized protons and accelerate them to high energy while retaining their polarization. This has made possible the study of previously unobservable aspects of high-energy proton-proton interactions.

When the Zero Gradient Synchrotron shut down in 1979, its parts were recycled for inclusion in further state-of-the-art facilities. The 107-ton superconducting magnet from the 12-foot bubble chamber was incorporated into the High Resolution Spectrometer at the Stanford Linear Accelerator Center's PEP collider. The detector was built and operated by Argonne scientists in collaboration with several university groups. In 1983, the detector discovered a new decay mode of the tau lepton and made the most accurate measurement of the tau neutrino mass at that time.

The Intense Pulsed Neutron Source (IPNS), completed in 1981, also incorporated elements from the old ZGS. It is the country's most productive source of spallation neutrons. Based on a proton accelerator rather than on a reactor, it produces neutrons by firing a beam of accelerated protons at a uranium target; neutrons then boil off through a process known as spallation. Designed and built at Argonne, IPNS was enhanced by laboratory scientists in 1985 -- the year after it had fired its one-billionth pulse. The beam intensity was increased by an enriched uranium target -- from less than one percent uranium-235 to 77 percent. Scientists could now gather data faster, carry out difficult experiments more easily, use smaller samples, and heighten the precision of specialized instruments. In 1995, IPNS was approaching a world record 5 billionth pulse.

Wakes and Explosions

In 1987, the first ever demonstration of wakefield acceleration in structures and in plasmas was achieved at Argonne's pioneering accelerator test facility, which was based on the Chemistry Division's electron linear accelerator. The wakefield concept promises to accelerate subatomic particles to higher energies in substantially shorter distances than are possible by conventional techniques. Wakefield acceleration is accomplished by firing a relatively low-energy electron pulse through plasma or special dielectric loaded radio frequency waveguides to create an electromagnetic "wake." The strong electric fields in the wake can be used to accelerate a second, trailing pulse of particles. If this second pulse is properly timed relative to the first, it can ride the wake of the first pulse gaining energy as it travels like a surfer behind a motor boat.

Argonne built this 190-ton detector for the Collider Detector at Fermi National Accelerator Laboratory. (Click the image to see a larger photo.)

Argonne invented the Coulomb explosion technique now used at other nuclear accelerator laboratories worldwide. The technique, which is based on accelerating molecules and breaking them up in thin foils, provides greater sensitivity to measuring the geometric structures of molecular ions.

Chasing the Top Quark

Major discoveries during the 1980s included super deformation in heavy nuclei; new proton emitters; short-range correlations in nuclei through meson-exchange models for threshold meson production; and high-density laser, polarized hydrogen particles, including isotopes of deuterium and tritium. In 1993, Argonne scientists discovered the meson containing both bottom and strange quarks. Two years later, they topped that achievement by assisting in the discovery of the elusive top quark at Fermi National Accelerator Laboratory in nearby Batavia, Ill.