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.’ These neutron decay correlation parameters 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 V_ud and test the unitarity of the CKM matrix with neutron decay that is free of nuclear corrections. The protons and electrons from neutron beta decay are measured 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. Under stable conditions, temperatures are maintained to +/- 0.5 K. This talk will focus on the implementation of the final design and performance of the detector cooling system which includes completely independent operation of designated loops for the upper and lower detector mounts in the magnetic spectrometer.