Closed Loop Helium Gas Cooling System for the Nab Experiment

The Nab experiment at the Spallation Neutron Source will use an unpolarized neutron beam to measure the electron-neutrino correlation coefficient, 'a', and the Fierz interference term, 'b.' The electron-neutrino correlation coefficient will be used to extract λ, the ratio of the axial-vector to vector coupling constants, to provide a measurement of the CKM matrix element Vud and test the unitarity of the CKM matrix with neutron decay that is free of nuclear corrections. The momentum and energy of the proton and electron from neutron beta decay are measured in the Nab magnetic spectrometer by two highly pixelated silicon detectors. To minimize noise and optimize energy and timing resolution, the detectors are actively cooled to below 150 K. Temperature control is achieved by two closed loop helium gas systems which utilize cryocoolers. For stable conditions, temperatures are maintained to +/- 0.5 K by PID controllers. I focus on the implementation of the final design and the performance of the detector cooling system which includes independent operation of designated loops for the upper and lower detector systems.