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PERFORMANCE ANALYSIS AND RADIATION DAMAGE ESTIMATION OF THE STOPPING TARGET MONITOR DETECTORS IN MU2E
[Thesis]. Manchester, UK: The University of Manchester; 2020.
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Abstract
The Mu2e experiment aims to search for the charged lepton flavour violating (CLFV) process of a coherent, neutrinoless, conversion of a muon into an electron within the proximity of an aluminium nucleus. Mu2e seeks to measure the ratio ($R_{\mu e}$) of the rate of this conversion process, relative to that of ordinary muon capture. Mu2e will achieve world-leading sensitivity, improving the current limit of $R_{\mu e} < 7\times10^{-13}(90\%\,\mathrm{\mathrm{C.L.}})$, set by the SINDRUM-II experiment, by an order of $10^4$. This corresponds to a single-event sensitivity of $R_{\mu e} < 2.87\times10^{-17}(90\%\,\mathrm{C.L.})$. Many beyond Standard Model (BSM) theories require CLFV to occur at a rate accessible by Mu2e. Any observation of CLFV at Mu2e would have profound implications on particle physics.\\ The Stopping Target Monitor (STM) will be comprised of both a HPGe and a LaBr3 detector which will monitor the signals of photons produced in stopped-muon processes to a required accuracy of 10\%. To achieve the desired sensitivity, it is imperative that the detectors perform at their optimal level. The STM detectors are placed within a harsh radiation environment in the form of a high energy `flash' of gamma radiation as well as a flux of fast neutrons.\\ Estimations of radiation damage to HPGe in literature is limited. This thesis reports an approach which utilizes the Kinetic-Energy Released in Matter (KERMA) parameter that considers the energy dependence of the incident flux and the cross-section information for the incident particle. Resampling schemes have been adopted to give a realistic rate over the experiment run time. The results have then been normalized to the radiation damage caused by a ${}^{252}$Cf source. \\ The estimated time before annealing of the detector is necessary is found to be $75$ and $78$~months in the case of neutron damage and $\tau \sim $ 24~years, 11~months in the case of electron/positron induced damage. This shows significant improvement on the previous estimations.