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