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 decay correlation parameters will be used to extract λ, the ratio of the axial-vector to vector coupling constants, to provide a measurement of V_ud and test CKM matrix unitarity free of nuclear corrections. The protons and electrons from neutron beta decay will be measured by pixelated silicon detectors. To minimize noise and optimize energy and timing resolution, the detector will be actively cooled to below 200K. Temperature regulation will be achieved by a closed loop helium gas cooling system. This talk will focus on the performance of an upgrade to the detector cooling system that incorporates a counter-flow heat exchanger into the loop and outline future improvements.