This has been a year of change at the Nuclear Physics Laboratory. We are pleased to have added several new faculty members to our group. R.G. Hamish Robertson and John Wilkerson have joined the Department of Physics as Professors. Peter Doe and Steve Elliott have joined the Research faculty as Professor and Assistant Professor, respectively. The primary interest of these new faculty is solar neutrino physics. They are presently playing a major role in the design and construction of detectors for the Sudbury Solar Neutrino Observatory (SNO). John Lestone has also recently joined the Research faculty as Assistant Professor. His primary interest is in nuclear reactions, particularly fission.
The Nuclear Physics Laboratory of the University of Washington has for over 40 years supported a broad program of experimental physics research. The current program includes "in-house" research using the local tandem Van de Graaff and superconducting linac accelerators and non-accelerator research in double beta decay and gravitation as well as user-mode research at large accelerator and reactor facilities around the world. We are also now actively using or developing solar neutrino observatories in Canada and Russia.
In June 1994 we completed an upgrade of the tandem accelerator. We now have a pelletron charging system, new resistors, a terminal control computer system and spiral inclined field beam tubes in the tandem. The position, energy and pulse transit time stability of the beam accelerated by this completed system as well as the transmission of the accelerator for heavy ions is significantly improved.
Some highlights of our research activities during the past year are given below.
Motivated by nuclear structure and astrophysical considerations, we have continued our studies of the distribution of Gamow-Teller strength. We focused on the high-energy-release beta decays of 37Ca and 36Ca, and find in both cases considerably more integrated Gamow-Teller strength than predicted by the shell model with effective operators. The GT matrix elements for 37Ca decay can be compared to the isospin-mirror GT matrix elements inferred from 37Cl(p,n) studies; to the extent that isospin symmetry is valid this comparison tests the procedure for extracting GT matrix elements from hadronic charge-exchange reactions. We find that the charge-exchange values can vary by up to a factor of two compared to the beta-decay values. Both of the discrepancies mentioned here provide challenges to nuclear theory.
We have considered the gravitational lensing properties of a plausible model for naturally-occurring wormholes, and find that they should be detectable by MACHO searches currently in progress. We find that the expected dipole anisotropy in the distribution of gamma ray bursts is not large enough to determine if they are cosmic in origin.
The Russian-American Gallium experiment (SAGE) continues to give a flux well below the Standard Solar Model. A calibration with 51Cr is in progress.
The University of Washington group continues to have major involvement in several aspects of the SNO project. The research and development effort for the acrylic vessel which holds the 1000 tons of heavy water has been completed. We are involved in the development of the data acquisition system for the readout of the SNO detector. In collaboration with groups at LASL and LBL we will provide an independent detector array for recording the neutral current signal of the SNO detector.
In our studies of sub-barrier fusion reactions we have measured the distributions of barriers for a prolate and an oblate target nucleus bombarded with 40Ca. The barrier distributions demonstrate important contributions from the hexadecapole as well as from the sign of the quadrupole moment.
A study of entrance channel effects on the energy spectra of light charged particles has revealed significant effects. The spectral shape for 12C + 144Sm is hotter than for 64Ni + 96Zr when the bombarding energies are chosen to match the excitation energy in the compound nucleus.
Recent results from the APEX collaboration on the positron spectra for uranium projectiles do not reveal peak structures and call into question previous GSI results.
The AMS group, with partial support from NSF, has continued its development and refinement of techniques for the isolation of essentially pure pollen fractions from lake sediment and peat samples for AMS radiocarbon (14C) dating. These techniques were successfully applied, for the first time, to a low-organic-carbon-content core taken from an Arctic lake. An apparent age-depth reversal in earlier "bulk carbon" results obtained by others for this core was rectified.
The ultra-relativistic heavy ion group participated in a very successful initial run of CERN experiment NA49, which used two large time-projection chambers to track charged reaction products from the new CERN 33 TeV lead beam on a fixed lead target. The University of Washington group played a major role in developing the tracking software used with the main time-projection chamber of the experiment. They also developed new general purpose slow control software used in the experiment.
We have devised an approximate procedure for analyzing Bose-Einstein correlation data without background generation which may be useful for single-event physics at CERN and RHIC. We have made progress in analyzing expected "wiggles" in multi-particle Bose-Einstein correlation data arising from non-Gaussian source shapes.
As always, we welcome applications from outsiders for the use of our
facilities. As a convenient reference for potential users, the table on the
following page lists the vital statistics of our accelerators.
For further information, please write or telephone:
We close this introduction with a reminder that the articles in this report describe work in progress and are not to be regarded as publications or to be quoted without permission of the authors. In each article, the names of the investigators have been listed alphabetically, with the primary author underlined, to whom inquiries should be addressed.