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AlCapPSI/Xray.tex

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+The conversion experiment proposes to monitor the number of stopped muons
+by measuring the various muonic X-rays generated by the captured muons
+as they cascade down to the muonic atom ground state.  We propose to
+test this scheme at PSI, as well as two alternate normalization
+methods. 
+
+At PSI, we propose to install a high-purity germanium detector at a
+port in the stopping target vacuum vessel to measure the rate of
+muonic X-ray production and hence the number of muons stopped in the
+target. Table \ref{tb:xray_energies} lists the most prevalent gamma
+rays from the Si, Al, and Ti targets.   
+Typical energy resolution of the HPGe detector will be of order 2~keV
+for 1~MeV photons. Normalization of the other measurements in this
+proposal will rely on the HPGe detector, although use of an active Si
+target will provide a useful cross-check. 
+
+A mechanical cooling system will be provided for the HPGe detector,
+eliminating the need for a liquid nitrogen supply, although cooling
+with cryogens is still a possibility if the mechanical system is
+deemed insufficient.  A high-speed, 14-16 bit data acquisition system
+will be used to record and store high-resolution raw waveforms for
+offline analysis, although real-time spectra will also be available
+for monitoring.  The raw data stream will be provided to the
+DAQ systems of the other detectors. 
+
+Measurements of particular interest using the Mu2e germanium detector
+include the muonic X-ray and gamma spectra as well as careful analysis
+of both immediate and long-term effects of neutron on the detector.
+Fast neutrons can create spurious peaks due to excitation of Ge nuclei
+through inelastic neutron scattering, background signals from activation
+products in both the detector and surrounding cryostat, and long-term
+resolution degradation due to creation of hole-trapping defects in the
+germanium lattice. The distance between the target and HPGe detector
+in the PSI experiment will be selected such that long-term damage to
+the HPGe is minimized, although the actual neutron flux is expected to
+be small. 
+
+A large NaI detector will be used to measure the high energy photons
+from the muon stopping target, with the primary goal of evaluating alternative
+means of monitoring the stopped muon rate. Photons from radiative muon
+decay have a probability distribution that decreases to nearly zero at
+about 54~MeV. The NaI detector has 9 PMTs viewing a large single
+crystal, whose signals will be digitized using waveform digitizers and
+then fed into the regular DAQ data stream. The rate of photons from
+radiative muon capture, ranging up to about 80-90~MeV, will also be
+measured. 
+
+\begin{table}[ht!]
+\begin{center}
+	\caption{Energies of muonic X-rays in selected target elements.}
+	\label{tb:xray_energies}
+	\vspace{5mm}
+	\begin{tabular}{|l|rrr|}
+		\hline
+		Transition		& Si (keV)	& Al (keV)	& Ti (keV)	\\
+		\hline
+		$2p \rightarrow 1s$	& 400		& 347		& 1021		\\
+		$3p \rightarrow 1s$	& 477		& 413		& 1210		\\
+		$4p \rightarrow 1s$	& 504		& 436		& 1277		\\
+		$3d \rightarrow 2p$	&  77		&  66		&  189		\\
+		\hline
+	\end{tabular}
+\end{center}
+\end{table}