Radioactive Source Insertion System for the Nab Experiment

The Nab experiment at the Spallation Neutron Source is designed to provide high statistics measurements of the electron-neutrino correlation coefficient and the Fierz interference term in free neutron beta-decay. Of critical importance to the success of Nab is the use of a Radioactive Source Insertion System (RSIS) designed to insert weak conversion-electron sources of known energy into the 70K Ultra High Vacuum (UHV) bore of the Nab magnet--spectrometer. The RSIS incorporates precise positioning of the sources throughout the neutron decay volume to scan individual pixels of two segmented Silicon detecters placed at 5 m and 1.5 m from the source. Beta response functions from individual pixels can then be evaluated for energy loss and calibration of detectors. I will discuss detailed aspects of the RSIS design features including the UHV system outside the magnet, the electro-mechanical system, and requirements for precise motion of the sources.