NUCLEAR PHYSICS AND ASTROPHYSICS
From Atoms to Exploding Stars

The likely shape of a deformed uranium-234 nucleus was determined in 1971 by ORNL physicists.

Nuclear physics research at ORNL took off in the late 1940s, largely because of the nuclear aircraft project's need for information about the behavior and effects of reactor-borne neutrons on shielding materials. In 1948 Arthur Snell initiated research using an upgraded 3-megavolt Van de Graaff accelerator, a high-voltage direct-current machine that produced a neutron stream by bombarding lithium with protons. In 1951 a 5-MV Van de Graaff accelerator was installed, the world's highest-energy machine of its kind.

At the Oak Ridge Y-12 Plant, Robert Livingston and his team built cyclotrons using discarded electromagnets from isotope-separating calutrons. The cyclotrons also obtained support from the aircraft project because they could be used for radiation damage research.

In 1951 Snell and Frances Pleasonton measured the half-life of the neutron, providing first experimental proof that a neutron decays into a proton, electron, and electron antineutrino. The next year Alex Zucker used ORNL's first heavy-ion cyclotron beam to lay to rest a nightmarish concern that a hydrogen bomb explosion could ignite the atmosphere.

Performing the first experiment on the Oak Ridge Research Reactor in 1958, Cleland, Johnson, Snell, and Pleasonton observed directions of neutron and electron emission in the decay of helium-6, confirming the electron-neutrino theory of nuclear beta decay, a key piece of today's Standard Model of particle physics.

In the 1960s at the ORR, Philip Miller, James Baird, and William Dress, in a collaboration with Harvard University's Norman Ramsey, set an upper limit for the electric dipole moment of the neutron, a sensitive way to test invariance of the laws of physics under time reversal. John Gibbons, Richard Macklin, and colleagues used the 3-MV Van de Graaff to confirm that atomic elements arose from nucleosynthesis in stars; Cal Tech's Willy Fowler won the Nobel Prize for the theory and credited the key influence of these data. Ray Satchler and colleagues developed computer codes to analyze nuclear scattering and reactions, and Joe McGrory and colleagues developed codes describing the nuclear shell model. All of these were used worldwide.

In 1964 the Oak Ridge Isochronous Cyclotron (ORIC) became one of the first cyclotrons to operate using the principle of strong focusing now commonplace in accelerator design. In 1971 the likely shape of a deformed uranium-234 nucleus was found using ORIC. In 1972 at ORIC, the giant quadrupole resonance was found by a team led by Fred Bertrand, sparking increased study of these vibrational modes of nuclei. Another team led by Curt Bemis found that the uranium-238 nucleus has a hexadecapole deformation resembling a rugby ball.

In 1980 the Holifield Heavy Ion Research Facility (HHIRF) began operation for nuclear physics studies and as a user facility. The 25-MV tandem accelerator, which was coupled to the ORIC, has the world's highest direct-current voltage. In 1995 using HHIRF, Cyrus Baktash discovered superdeformation extended to nuclei lighter than mass 100. In 1997 HHIRF morphed into the Holifield Radioactive Ion Beam Facility (HRIBF), to provide for nuclear structure and astrophysics research the first beams of ions that do not exist naturally on earth. Witek Nazarewicz and others predicted numerous new phenomena, such as steady loss of shell effects for nuclei far from stability, now a cornerstone of research using radioactive beams. In 2000 Jorge Gomez del Campo and Jim Beene led a group that discovered a new form of radioactivitysimultaneous emission of two protons from a decaying atomic nucleus.

Measurements at HRIBF and the Oak Ridge Electron Linear Accelerator help improve simulations of nova, supernova, and red giant stars. Michael Smith, Jeff Blackmon, Dan Bardayan, and others improved predictions of the abundances of 87 different isotopes in stellar explosions.

In 2001 Tony Mezzacappa formed a national collaboration to simulate core- collapse supernovae to pin down the supernova explosion mechanism. An ORNL group led by Glenn Young and Frank Plasil developed detectors for DOE's Relativistic Heavy Ion Collider at Brookhaven National Laboratory, to study the quark-gluon plasma, mimicking the beginning of the universe. In 2002 at HRIBF, a beam of tin-132 was produced for the first time, allowing "doubly magic" short-lived nuclei to be probed.

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