THE SLAC HIGH–DENSITY GASEOUS POLARIZED 3He TARGET*

A large-scale high-pressure gaseous 3He polarized target has been developed for use with a high-intensity polarized electron beam at the Stanford Linear Accelerator Center. This target was used successfully in an experiment to study the spin structure of the neutron. The target provided an areal density of about 7×1021 nuclei/cm2 and operated at 3He polarizations between about 30% and 40% for the six-week duration of the experiment. *Work supported in part by Department of Energy contracts DE–AC03–76SF00515 (SLAC), DE–FG02–90ER40557 (Princeton), and DE–AC02–76ER00881 (Wisconsin); by National Science Foundation grants 8914353 and 9200621 (Michigan); and by the Bundesministerium für Forschung und Technologie (W. Meyer). *Permanent address: Universität Bonn, Bonn, Germany. INTRODUCTION Experiment E-142 at SLAC [1] was proposed to measure the spin-dependent structure function of the neutron by studying deep-inelastic scattering of high-energy polarized electrons from polarized neutrons. Polarized 3He was chosen as the target material to provide the polarized neutrons for two main reasons. First, 3He nuclear wave-function calculations [2] indicate that the nucleus is primarily in a spatially symmetric S-state where the spins of the two protons must be anti-aligned. Thus the spin of the nucleus is mainly provided by the neutron, and so polarized 3He is a good approximation to a polarized neutron target, diluted by the presence of the protons but with only small corrections needed due to the polarization of the protons. Second, practical 3He polarized targets have been developed [3–10] using the technique of collisional spin-exchange with optically pumped alkali-metal vapor, typically rubidium. Using modern high-power Ti:sapphire lasers for the depopulation optical pumping of rubidium, such targets have been shown to be capable of operating effectively at high 3He pressures (more than 10 atm [8]) and in the presence of intense electron beams [10], as required by the SLAC experiment. A disadvantage of this technique is the weakness of the hyperfine interaction between the polarized rubidium valence electron and the 3He nucleus, leading to typical time constants for the build-up of 3He polarization of the order of 10 hours. Thus, long 3He spin relaxation times are required to achieve high polarization.