Stopping Target Monitor of the Mu2e Experiment

The Mu2e Experiment at Fermilab is looking for charged lepton flavour violating (CLFV) neutrinoless muon-to-electron conversion in the field of an atomic nucleus by measuring the ratio,

R_ue = Number of Direct conversions/ Number of all muon captures

The numerator is what the experiment seeks to detect and the denominator, which is proportional to the number of stopped muons will be measured by the Stopping Target Monitor (STM). A beam of pulsed negative muons is stopped at an Aluminium target to form a bound state with the Aluminium nucleus leading to emission of photons which the STM detects to determine the number of stopped muons. The STM uses two gamma detectors which complement each other: a high purity Germanium crystal calorimeter and a Lanthanum Bromide Scintillating crystal. These detectors are supposed to be placed far downstream along the beamline to achieve acceptable photon rates. Hence, the STM is split into the Upstream and the Downstream Infrastructure. The STM Downstream is designed to protect the detectors from radiation and achieve operable detector rates. The STM Upstream collimates the beam and ensures the detectors see only the Stopping target and little else to prevent photons from extraneous sources. The Upstream and the Downstream Infrastructure have alignment systems to ensure the detectors and the collimator is aligned with the STM axis. My presentation will explore the design & construction of the STM.