Polarization and Relaxation Processes inHe3Gas

Abstract

Polarization of He 3 when used as a buffer gas for optically pumped rubidium vapor has been investigated experimentally and theoretically. Experiment shows that the coupling between a rubidium and He 3 atom is scalar in form (i.e., I* S) and is about three orders of magnitude greater than would be expected from the direct magnetic interaction of the dipoles associated with each atom. A theoretical explanation of this effect, which depends upon the overlap of the Rb valence electron with the electrons of the He 3 atom is presented and is in agreement with experiment. Experiments concerning the optical pumping of metastable He 3 atoms in order to produce ground-state polarization are described. Polarizations of 60% in 1-Torr (1 STP mm) samples at room temperature and 56% in 3-Torr samples at 77°K have been achieved. The process of metastable pumping works only at these lower pressures. Mercury liquid is shown experimentally, however, to present a poor surface for He 3 relaxation. This result opens the possibility of achieving high polarization at high pressure using metastable pumping and gas compression. Various wall coatings of He 3 samples are found to have only small effects upon relaxation rates. A 3.1-amagat (1 STP atm) sample in Pyrex but of high purity is found to have a relaxation time of 2X10 4 sec. This value sets present limits upon the experimental He 3 -He 3 relaxation cross section. A new effect which involves relaxation processes of the He 3 polarization in a magnetic field with a gradient is investigated both experimentally and theoretically. The results of both theory and experiment are in close agreement with each other. An effect involving enhanced polarization of either sign of spin temperature of a high-pressure sample of He 3 in which a gas discharge is struck, and which is in a 10-kG field, is described experimentally.

Deep inelastic scattering of polarized electrons by polarizedHe3and the study of the neutron spin structure

“…Table 4 presents a breakdown of the material in the three cells used in the experiment. The dilution factor is dependent on x and Q 2 and takes the form, f He (x; Q 2 ) = n He He (x; Q 2 ) n He He (x; Q 2 ) + n N 2 N 2 (x; Q 2 ) + n g g (x; Q 2 ) (48) where n i is the total number of nucleons found in species i ( (50) where RC(x; Q 2 ) is a radiative correction factor which relates the Born cross section to the experimental cross section for different species. The quantity R np = F n 2 =F p 2 is the ratio of unpolarized structure functions for neutrons and protons.…”

A Precision Measurement of the Neutron Spin Structure Functions Using a Polarized HE-3 Target

“…The cross section for this interaction is small, on the order of cm2 [61], and the electronic spin of the rubidium is quickly realigned by optical pumping after a spin exchanging interaction because the optical pumping process is approximately six orders of magnitude faster. The result is that the 3He nuclear polarization is driven towards the average rubidium polarization because each 3He atom diffuses through the cell, thus averaging the rubidium polarization.…”

The SLAC high-density gaseous polarized 3He target

“…The cell is connected to a high-vacuum system capable of 10 −8 Torr, and baked under vacuum for up to 7 days at 450˚C. A small amount of rubidium is then "chased" into the pumping chamber with a torch, and the target chamber is inclosed in a vacuum-walled enclosure through which liquid 4 He is blown. A small amount of 99.9995% pure nitrogen is introduced into the cell (this suppresses "radiation trapping" in the optical pumping process [7]) and the desired amount of high-purity 3 He s flowed through a liquid 4 He trap into a calibrated volume, then introduced into the cell.…”

“…A small amount of 99.9995% pure nitrogen is introduced into the cell (this suppresses "radiation trapping" in the optical pumping process [7]) and the desired amount of high-purity 3 He s flowed through a liquid 4 He trap into a calibrated volume, then introduced into the cell. The temperature of the target chamber is lowered enough (by the liquid 4 He blowing by) to reach a pressure below atmospheric, and the small entrance tube at the top of the pumping chamber is then sealed with a torch.…”

“…Thus the spin of the nucleus is mainly provided by the neutron, and so polarized 3 He 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 3 He polarized targets have been developed [3][4][5][6][7][8][9][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 3 He pressures (more than 10 atm [8]) and in the presence of intense electron beams [10], as required by the SLAC experiment.…”