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The design, fabrication, operation, and performance of a 3/4 He dilution refrigerator and superconducting magnet system for holding a frozen-spin polarized hydrogen deuteride target in the Jefferson Laboratory CLAS detector during photon beam running is reported. The device operates both vertically (for target loading) and horizontally (for target bombardment). The device proves capable of maintaining a base temperature of 50 mK and a holding field of 1 T for extended periods. These characteristics enabled multi-month polarization lifetimes for frozen spin HD targets having proton polarization of up to 50% and deuteron up to 27%. [ABSTRACT FROM AUTHOR]

A device has been developed with moveable liquid nitrogen and liquid helium volumes that is capable of reaching over 2m into the coldest regions of a cryostat or dilution refrigerator and reliably extracting or installing a target of solid, polarized hydrogen deuteride (HD). This Transfer Cryostat incorporates a cylindrical neodymium rare-earth magnet that is configured as a Halbach dipole, which is maintained at 77K and produces a 0.1T field around the HD target. Multiple layers provide a hermetic 77K-shield as the device is used to maintain a target at 2K during a transfer between cryostats. Tests with frozen-spin HD show very little polarization loss for either H (−1±2%, relative) or D (0±3%, relative) over typical transfer periods. Multiple target transfers with this apparatus have shown an overall reliability of about 95% per transfer, which is a significant improvement over earlier versions of the device. [ABSTRACT FROM AUTHOR]

Abstract: A liquid helium target system was designed and built to perform a precision measurement of the parity-violating neutron spin rotation in helium due to the nucleon–nucleon weak interaction. The measurement employed a beam of low energy neutrons that passed through a crossed neutron polarizer–analyzer pair with the liquid helium target system located between them. Changes between the target states generated differences in the beam transmission through the polarizer–analyzer pair. The amount of parity-violating spin rotation was determined from the measured beam transmission asymmetries. The expected parity-violating spin rotation of order placed severe constraints on the target design. In particular, isolation of the parity-odd component of the spin rotation from a much larger background rotation caused by magnetic fields required that a nonmagnetic cryostat and target system be supported inside the magnetic shielding, while allowing nonmagnetic motion of liquid helium between separated target chambers. This paper provides a detailed description of the design, function, and performance of the liquid helium target system. [Copyright &y& Elsevier]