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Polarization of Helium-3 nuclei Axen, David Arnold

Abstract

An atomic beam type of apparatus designed to produce a polarized He³ beam with an intensity of approximately one microampere and 90% polarization is described. He³ is a monatomic gas consisting of atoms with zero electronic magnetic moment and nuclear spin of ½. As no initial molecular dissociation is required, an intense, supersonic neutral beam can be produced with a miniature Laval nozzle cooled to liquid He⁴ temperatures. The velocity distribution of the particles in the Laval beam and the trajectories of these particles in a radially symmetric hexapole magnet have been computed. Sufficient separation of the two beams, consisting of particles in the two possible nuclear spin states, is achieved with a magnet in which the diameter of the gap between the pole pieces increases from 3mms at the entrance to 6mm in 15 cms and is then constant for 35cms. After ionization the nuclear polarization of the singly ionized particles depends upon the magnetic field strength at the position of the ion. Theoretical calculations show that field strengths of 6000 gauss at both the ionizer and target (in reaction studies) are sufficient to give 90% nuclear polarization. For an input gas temperature and pressure of 2.2°K and 15 mm.Hg. the Laval nozzle (throat diameter, 0.2 mm) has been designed to produce a supersonic beam of Mach number 4 with an intensity of 6.5x10¹⁵ particles/sec at the magnet entrance. Assuming 40% transmission through the magnet (one half of the beam being removed by the polarization process) and an ionization efficiency of 0.25%, the resulting ion beam intensity is 6.5x10¹² ions per second or approximately one microampere. The low temperature atomic beam source has been tested at liquid nitrogen temperature with a He⁴ beam. The measured beam intensity of 9x10¹³ particles/cm²/sec at the magnet exit, 76 cms from the nozzle, agrees favourably with the calculated intensity of 1.2x10¹⁴ atoms/ cm² sec under these operating conditions. The measured field gradient of 70,000 gauss/cm. near the pole tips of the hexapole splitting magnet is more than required for separating the atoms in the two nuclear spin states. A pulsed nuclear magnetic resonance method for measuring the nuclear polarization of the neutral He^ beam prior to ionization is described. The angular distribution and polarization of the protons produced by the D(He³,p)He⁴ reaction with an incident, polarized He³ beam of 150 Kev bombarding energy has been calculated, This angular distribution is isotropic. In the plane of the reaction, the proton polarization, which may be measured by a second scattering experiment, is -2/3 the incident He³ polarization. The angular distribution and polarization of the protons from this reaction has been calculated for the case of polarized and unpolarized He³ beams incident on a polarized deuteron target.

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