\chapter{Conclusions}
\label{cha:conclusions}
The AlCap is an experiment proposed at PSI to study charged particles, neutrons
and photons emitting after nuclear muon capture on aluminium. These
measurements are important for backgrounds and hit rates estimation of the new
generation of \mueconv experiments, COMET and Mu2e. In the first stage of the
COMET experiment, hit rate on its main tracking detector is anticipated to be
dominated by low energy protons following muon capture on an aluminium target,
which has never been measured.

The first run of the AlCap which aims for proton measurement has been carried
out in 2013. Data analysis is in progress. Before finishing the complete AlCap
analysis, an initial analysis on partial data
was made. The main results are:
\begin{enumerate}
  \item demonstration of the analysis chain from trigger-less waveforms to
    correlated physics events;
  \item validation of the experimental method including: number of nuclear
    capture muons normalisation by muonic X-ray measurement, charged particle
    identification by specific energy loss, and unfolding of the proton energy
    spectrum using the iterative Bayesian method;
  \item obtaining preliminary results on proton emission rate and spectrum:
    the proton spectrum has a peak at \SI{3.7}{\MeV}, then reduces exponentially
    with a decay constant of \SI{2.6}{\MeV}. The partial emission rate in the
    energy range from \SIrange{4}{8}{\MeV} is $(1.7 \pm 0.1)\%$ per nuclear
    muon capture, and the total
    emission rate assuming the shape holds for the whole spectrum is
    $(3.5\pm0.2)\%$ per nuclear muon capture.
\end{enumerate}
The emission rate is consistent with the lower limit of 2.8\% set by
Wyttenbach et al.~\cite{WyttenbachBaertschi.etal.1978}. It is also compatible
with the theoretical calculation by Lifshitz and
Singer~\cite{LifshitzSinger.1980}. Compared with the existing result on
silicon~\cite{SobottkaWills.1968}, the emission rate from aluminium is
significantly smaller and the spectrum is softer.

The proton rate and spectrum have been used to optimise the planned proton
absorber for the drift chamber of the COMET Phase-I. The resulted proton hit
rate with the baseline configuration is very small compared with the current
figure.The recommendation to the COMET Phase-I is to remove the proton
absorber altogether. The momentum resolution of the drift chamber will be
slightly improved, and the level of integrated charge will still remain below
the safe level for the chamber.

The AlCap experiment is going to
submit a beam time request for the 2015 run to collect more data and other
measurements for neutrons and gamma rays.
