A Nuclear Frozen Spin Target for Deuteron Photodisintegration Experiments at the High Intensity Gamma Source

The HIGS Frozen Spin Polarized Target, or Hifrost, is a nuclear polarized target apparatus consisting of a dilution refrigerator, internal magnetic coil, microwave guide channel and NMR coils. It is named because it was designed and commissioned to run at HIGS, the High Intensity Gamma Source located on the campus of Duke University. External components of Hifrost include a polarizing magnet, microwave generator, pump and vacuum system to run the dilution refrigerator, and the Q-Meter/Yale Card setup for running the NMR.

Polarized nuclear targets allow measurement of spin observables, the subset of observables from scattering experiments that do not average out the spin component of the scattered and/or scattering particles' wave functions. Polarization of nuclear targets unlock a new degree of freedom to probe and learn about the nuclear system. Experimental data from double polarized spin observables, where both the target and beam probe have some degree of polarization, are rare due to the high technical hurdles of polarizing both target and beam.

To polarize solid targets, chemically doped or irradiated beads are refrigerated to a fraction of a kelvin. The refrigerator is assembled around the beads and swung to align with the HIGS gamma beam. A large external magnet polarizes the target, first to a fraction of percent in thermal equilibrium, then orders of magnitude higher with a technique called dynamic nuclear polarization (DNP).

Frozen spin mode is a configuration where the internal magnet of the apparatus is ramped up as the external magnet is ramped down, so eventually the external magnet can be disconnected and removed from the scattering area while keeping the beads polarized. Frozen spin allows a detector array to be placed around the polarized target without having to consider the effects of the heavy polarizing magnet on the products of the scattering experiment.

Target material in frozen spin mode loses its polarization with time because the DNP process is halted before transitioning between the polarizing and holding magnets. The gamma beam at HIGS does not meaningfully degrade the polarization like ionizing beams used at most other facilities. Therefore, with the holding field held constant, the target temperature is the sole parameter that determines $T_1$, the characteristic time describing target polarization loss. In Hifrost, a deuteron target has a characteristic time ranging from hours to days, depending on the run time temperature achieved. Given the beam schedule at HIGS, this is adequate for any polarized target experiment proposed so far.