Relaxation of gaseous spin-polarizedHe3targets due to ionizing radiation

The theory of spin-polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ relaxation due to hyperfine coupling in ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ and due to spin-rotation coupling in ${}_{}{}^3{}_{}{}^{}\mathrm{He}_2^{+}{}_{}{}^{}$ is presented. Comparison is made between the theory and recent experiments which utilized spin-polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ as a charged-particle beam target. The theory predicts that at low He pressures applying a magnetic field significantly reduces the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ depolarization rate due to formation of ${}_{}{}^3{}_{}{}^{}\mathrm{He}_2^{+}{}_{}{}^{}$. At high He pressures the relaxation rate decreases with pressure. In addition, charge-transfer processes with foreign gases suppress ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ depolarization by reducing the ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}_2^{+}{}_{}{}^{}$ concentrations.