48 lines
2.6 KiB
TeX
48 lines
2.6 KiB
TeX
\chapter{Conclusions}
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\label{cha:conclusions}
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The AlCap is an experiment proposed at PSI to study charged particles, neutrons
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and photons emitting after nuclear muon capture on aluminium. These
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measurements are important for backgrounds and hit rates estimation of the new
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generation of \mueconv experiments, COMET and Mu2e. In the first stage of the
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COMET experiment, hit rate on its main tracking detector is anticipated to be
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dominated by low energy protons following muon capture on an aluminium target,
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which has never been measured.
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The first run of the AlCap which aims for proton measurement has been carried
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out in 2013. Data analysis is in progress. Before finishing the complete AlCap
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analysis, an initial analysis on partial data
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was made. The main results are:
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\begin{enumerate}
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\item demonstration of the analysis chain from trigger-less waveforms to
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correlated physics events;
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\item validation of the experimental method including: number of nuclear
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capture muons normalisation by muonic X-ray measurement, charged particle
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identification by specific energy loss, and unfolding of the proton energy
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spectrum using the iterative Bayesian method;
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\item obtaining preliminary results on proton emission rate and spectrum:
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the proton spectrum has a peak at \SI{3.7}{\MeV}, then reduces exponentially
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with a decay constant of \SI{2.6}{\MeV}. The partial emission rate in the
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energy range from \SIrange{4}{8}{\MeV} is $(1.7 \pm 0.1)\%$ per nuclear
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muon capture, and the total
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emission rate assuming the shape holds for the whole spectrum is
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$(3.5\pm0.2)\%$ per nuclear muon capture.
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\end{enumerate}
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The emission rate is consistent with the lower limit of 2.8\% set by
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Wyttenbach et al.~\cite{WyttenbachBaertschi.etal.1978}. It is also compatible
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with the theoretical calculation by Lifshitz and
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Singer~\cite{LifshitzSinger.1980}. Compared with the existing result on
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silicon~\cite{SobottkaWills.1968}, the emission rate from aluminium is
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significantly smaller and the spectrum is softer.
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The proton rate and spectrum have been used to optimise the planned proton
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absorber for the drift chamber of the COMET Phase-I. The resulted proton hit
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rate with the baseline configuration is very small compared with the current
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figure.The recommendation to the COMET Phase-I is to remove the proton
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absorber altogether. The momentum resolution of the drift chamber will be
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slightly improved, and the level of integrated charge will still remain below
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the safe level for the chamber.
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The AlCap experiment is going to
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submit a beam time request for the 2015 run to collect more data and other
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measurements for neutrons and gamma rays.
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