INTRODUCTION

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 solar neutrino physics at the Sudbury Neutrino Observatory in Canada and at SAGE in Russia, double beta decay and gravitation as well as user-mode research at large accelerator and reactor facilities around the world.

Our tandem and linac accelerators continue to work very well. In the past year we upgraded our tandem/linac bunching system with the installation of a new high energy buncher which operates at 13/12 of the linac fundamental. The new buncher, which is now being used in experiments, provides much cleaner time spectra by suppressing spurious 'satellite' beam bunches by an additional factor of 10 or more. We are also upgrading our capability for high energy gamma ray detection with the installation of 2 additional large NaI spectrometers (for a total of 3) in Cave 2. This project involves the construction of a new support platform, new detector carts which move on air pads, an upgraded vacuum system and detector electronics, and will be ready for experiments beginning in late Spring.

Some highlights of our research activities during the past year are given below.

Work continues at a rapid pace toward completion of the Sudbury Neutrino Observatory in January 1997. Following four years of planning and development, installation of the acrylic vessel began last July and is now 50% complete, with final completion scheduled for September. The 'miniSNO' array of 95 photomultipliers has been assembled in the old water-shield room of the former UW Cyclotron, and is providing realistic event signals for exercising SNO electronics and data acquisition software. The Neutral-Current Detector project has also made great strides in the last year, with successful production of prototype ultrapure nickel tubing by the CVD process. Completion of very low-noise preamplifier prototypes, logarithmic amplifiers, delay lines, cable design, anchor prototyping, and endcaps for proportional counters has been carried out in parallel with commissioning of a Class-1000 cleanroom and a complex array of assembly equipment for the production phase. Design of the remotely operated submersible vehicle to be used to deploy the neutral current detectors in the acrylic vessel has been completed.

The Russian-American Gallium Experiment (SAGE) has completed a successful ⁵¹Cr neutrino source experiment. The results indicate that the efficiency of the gallium detector for detection of neutrino induced events is in agreement with the efficiency determined from chemistry based methods.

The first data from ⁸B decay have been taken in the Mass-8 CVC/Second Class Current study. The ⁸B nuclei were produced with 15 microamps of beam from the new terminal ion source which was designed, constructed and installed this past year. The new terminal ion source will produce ³He beams of up to 30 microamps and energies of up to 7.5 MeV.

The analysis of our measured barrier distributions for Ca-induced fission of prolate ¹⁹²Os and oblate ¹⁹⁴Pt has been completed. In addition to the expected shape signatures, a splitting due to octupole excitation of the projectile has been identified. We have also determined sensitivities to the diffuseness of the nucleus-nucleus potential.

In a collaboration with a group from the Bhabha Atomic Research Centre we have shown that fission anisotropies at energies well above the barrier are not influenced by the mass asymmetry of the entrance channel relative to the Businaro-Gallone critical asymmetry. We also have preliminary evidence at higher bombarding energy that noncompound nucleus fission scales with the mean square angular momentum, in contrast to previous suggestions.

We have measured proton and alpha particle emission spectra from the decay of A ~ 200 compound nuclei at excitation energies of 50 - 100 MeV, and used these measurements to infer the nuclear temperature. Our temperatures are in disagreement with the results of recently published experiments, but are consistent with calculations which take into account the temperature dependence of the effective mass of nucleons inside hot nuclear matter.

We have developed a stable and reliable beam of C₆₀ and are starting to use it for fragmentation studies.

Our investigations of multiparticle Bose-Einstein interferometry have led to a new algorithm for putting Bose-Einstein and Coulomb correlations of up to 6th order into Monte Carlo simulations of ultra-relativistic collision events, and to a new fast algorithm for extracting event temperatures. We have shown that 3-particle correlations are not useful for single-event Hanbury-Brown, Twiss analysis, and we have found that high-order Bose-Einstein correlations amplify 'Fourier-ripple' effects arising from non-Gaussian asymmetric sources.

Preparation and construction of the detector to be used by the emiT Collaboration for a search for time reversal invariance in neutron beta-decay is nearly complete. Work at NPL has concentrated on development of the proton segment detectors and electronics as well as vacuum systems. Measurements are expected to commence in the coming year at the National Institute of Standards and Technology cold neutron facility in Gaithersburg, MD on this experiment and also on our parity-violating neutron spin rotation experiment.

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, Director, Nuclear Physics Laboratory, University of Washington, Seattle, WA 98195; (206) 543-4085 (e-mail; storm@npl.washington.edu).

We note the retirement of Professor William Weitkamp, after serving the Laboratory for over 27 years as Technical Director.

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 Karin M. Hendrickson for her help in producing this report.

Kurt Snover
Editor

Barbara Fulton
Assistant Editor