The Nuclear Physics Laboratory at the University of Washington in Seattle pursues a broad program of nuclear physics. These activities are conducted locally and at remote sites. The current programs include ‘in-house’ research using the local tandem Van de Graaff and superconducting linac accelerators and non-accelerator research in solar neutrino physics at the Sudbury Neutrino Observatory in Canada and at SAGE in Russia, and gravitation as well as user-mode research at large accelerators and reactor facilities around the world.

Significant progress has been made in the test of CVC and Second Class currents in the Mass-8 system. Data taking for the Mass-8 b -decay experiment has been finished, after accumulating almost as much data from ⁸B beta decay as from ⁸Li. The final data analysis is in progress. In the ⁴He(a ,g )⁸Be radiative capture measurement, we have made a precise g -ray spectrum shape measurement using the long gas cell. In a shakedown run we have obtained (preliminary) high quality excitation function data with all 3 NaI spectrometers using a new short gas cell. In addition the response of the three spectrometers in the new high-energy g -ray setup has been measured to +3% at Eg = 15.1 MeV.

Our experimental measurements of Giant Dipole Resonance decay in hot Sn compound nuclei formed in ¹⁸O + ¹⁰⁰Mo collisions have been completed, and data analysis is underway including effects of pre-equilibrium losses and bremsstrahlung.

The important role of K-state equilibration in the calculation of the statistical model fission width has been noted. Results of sample calculations are compared to experimental pre-scission neutron multiplicities. Both fusion-fission and fusion-evaporation have been explored for the ¹⁹F + ¹⁸¹Ta -> ²⁰⁰Pb system from E_lab = 121 to 195 MeV. The results for light charged particles measured in coincidence with evaporation residues provide insight into the Fermi-gas level density parameter. The residue and fission cross sections will be used to explore the importance of K-states in the competition between residue formation and fission.

Preliminary investigations into the possible dependence of fission fragment anisotropies on the shape of target nuclei have been performed.

An experiment to test the suggestion that anomalous fission fragment anisotropies for high-Z compound nuclei near the fusion barrier are due to quasifission has been performed. The evaporation residue and fission cross sections have been measured at energies near the fusion barrier. No evidence for suppression of the evaporation residue yields, as would be expected for a quasifission interpretation, has been found. We have previously suggested an alternative explanation for the anomalous anisotropies in terms of slow equilibration of the projection of the angular momentum on the symmetry axis of deformed nuclei.

Small but statistically significant differences have been found in the stopping power of small carbon clusters as compared to single carbon atoms at the same velocity.

The design and construction of the target chamber and related apparatus for our planned ⁷Be(p,g )⁸B experiment is roughly 80% complete. In collaboration with TRIUMF, our target development project has made ⁹Be targets which have achieved 60% metallic Be purity. This fabrication process is the same as will be used to make the necessary ⁷Be targets.

The Russian-American Gallium Experiment (SAGE) has submitted an archive paper on the ⁵¹Cr neutrino source experiment. A large amount of solar neutrino data has been analyzed and is being prepared for publication.

Construction of the Sudbury Neutrino Observatory (SNO) is complete and commissioning of the detector is underway. Water fill has commenced. The UW SNO group is currently heavily involved with the installation of the data acquisition and monitoring systems and with the overall detector commissioning process. Work continues, both at UW and at site, on refinements to the UW produced SNO data acquisition and monitoring systems. The Neutral-Current Detector project (NCD) has begun production of the counters which comprise the array. All parts and electronics are being assembled and counters are being shipped to site. The remotely-operated vehicle to be used during installation of the counters into the acrylic vessel is being tested.

With the steadily improving precision of data from currently operating solar neutrino experiments, we have updated our model-independent analysis in which we originally showed that, if the experimental uncertainties are correct, a solar neutrino problem exists at the 95% confidence level that cannot be resolved even by scaling the individual neutrino fluxes arbitrarily. New SuperKamiokande data have reduced uncertainties, but also a reduced central value, and our current analysis gives approximately the previous conclusion.

Our precision measurement of the electron-neutrino correlation in the 0⁺ ® 0⁺ beta decay of ³²Ar ran at ISOLDE in late summer l997. Our instrument gave the highest resolution delayed proton data ever achieved. The results set tight constraints on exotic beta decay processes that could arise from multipole Higgs doublets or leptoquarks. Final analysis of our results is nearly completed.

A measurement of the parity-violating rotation of neutron spins in liquid helium was run at the NIST reactor. Although the statistics were not good enough to resolve the effect, our device produced the most precise measurement of a neutron spin rotation and demonstrated the power of the experimental design. A second-generation experiment will be mounted that should provide, in conjunction with existing parity-violating p+alpha data, the dominant weak isovector and isosclar meson-nucleon-nucleon coupling constants.

The emiT experiment had its first data run at the NIST Center for Neutron Research. Data were collected during five six-week reactor cycles starting in January and continuing into September of 1997. In general, the emiT detector performed well and a total of roughly 15 million coincidence events were recorded. Analysis of these data is ongoing. The detector is undergoing hardware upgrades at NPL in preparation for a second run.

The new Monte Carlo algorithm for simulating ultrarelativistic heavy ion collisions including high-order Bose-Einstein and Coulomb correlations is now being used to produce a library of STAR-type events with various source sizes, multiplicities, and temperatures. A new HBT analysis program written in C++ is being developed for STAR. Analysis of NA49 interferometer results show a surprisingly high phase-space occupancy in Pb+Pb systems.

During the past year the URHI group has completed a major upgrade of an event-by-event analysis system based on scaled topological measures which searches for deviations from equilibrium behavior in heavy ion collisions possibly due to formation of a quark-gluon plasma. This system has been applied in a pilot program to 400k events of NA49 data and has resulted in the identification of one or more anomalous event classes at the few permil level. Work is underway to understand the nature of the observed anomalous behavior in the transverse-mass spectrum, and a full-scale analysis of more than 1.5M central Pb-Pb events is about to begin.

With RHIC turn-on a little over a year away efforts are intensifying to prepare the STAR solenoidal tracker experiment for first data. The URHI group has contributed to final cosmic-ray testing of the STAR TPC prior to its shipment to RHIC last November, and are playing a leading role in design and implementation of the STAR offline physics analysis system within the RHIC computing facility.

As always, we encourage outside applications 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 Professor Derek W. Storm, Executive Director, Nuclear Physics Laboratory, the University of Washington, Seattle, Washington, USA, 98195; (206)543-4085 (e-mail; STORM@NPL.WASHINGTON.EDU).

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 to whom inquiries should be addressed underlined.

We thank Richard J. Seymour and Karin M. Hendrickson for their help in producing this report.

Steve Elliott, Editorsre@u.washington.edu, (206)543-9522

Barbara Fulton, Assistant Editor