Design and Performance of the Stopping Target Monitor for the Mu2e experiment

The Stopping Target Monitor (STM) system is a crucial component of the Mu2e experiment at Fermilab, designed to normalize the number of stopped muons in the target. The STM measures X-rays and gamma rays emitted after muon capture to accurately determine the number of stopped muons. In this talk, I will explain the principles behind the STM's measurements, highlighting how results from the AlCap collaboration—particularly gamma emission rates following muon capture in aluminum—inform our approach. I will then describe the STM's detector systems, including high-purity germanium (HPGe) detectors for precise energy measurements and lanthanum bromide (LaBr3) detectors for handling high-rate environments. Performance tests at facilities like ELBE and Fermilab have demonstrated their capability to meet Mu2e's requirements under realistic conditions.