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\begin{document}
\title{Hit rate estimation for STM detectors}
\author{Nam H. Tran \\ Boston University}
\date{\today}
\maketitle

\begin{abstract}
  This report presents estimated hit rate on STM detectors with the updated
  geometry in Offline version TODO.
\end{abstract}
\section{Overview}
\label{sec:overview}
In order to measure the eponymous rate of the Mu2e experiment, the number of
stopped muons needs to be known to about \SI{10}{\percent}. The most promising
schemes have involved photon detectors far downstream of the Muon Beam Stop
measuring emissions from the Stopping Target (ST) at different times with
respect to each pulse of muons stopping in the target. There are three
categories we explore here for this stopping-target monitor (STM):
\begin{itemize}
  \item Prompt: X-rays emitted when the muon comes to rest in the ST and is
    captured into atomic orbit (atomic capture, sometimes referred to as “muon
    stop” in this text). These X-rays are emitted essentially O(ps) with the
    atomic capture time.
  \item Semiprompt: $\gamma$-rays emitted upon nuclear capture (sometimes
    referred to simply as ``capture'' in this text) of the muon after stop.
    These exhibit timing characteristics of the muonic lifetime ($\tau
    = \SI{864}{\ns}$).
  \item Delayed: $\gamma$-rays from activated daughters resulting from muon
    nuclear capture in the ST.
\end{itemize}
The baseline design of the STM has been described by Miller et
al~\cite{Miller2016}, the estimated hit rate on STM detector was about
\SI{1.1}{\kHz} in the time window \SIrange{200}{1695}{\ns} from arrival time of
a proton bunch. But this design would cause a high hit rate on the Cosmic
Ray Veto (CRV) system, therefore changes have been made in the Offline version
v6\textunderscore 0\textunderscore 2 by the CRV team to reduce the CRV hit
rate. The goal of this study is to re-estimate hit rates on STM detectors in
this new geometry.

The actual changes relevant to the STM are:
\begin{itemize}
  \item \SI{10}{\mm} polyethylene liner added to CRV shielding,
  \item field-of-view (FOV) collimator absorber thickness reduced to
    \SI{10}{\mm} (from \SI{20}{\mm}).
\end{itemize}

\section{Simulation details}
\label{sec:simulation_details}
The study was done using Mu2e Offline version v6\textunderscore
0\textunderscore 2 (released on Oct 16,
2016), hashtag \texttt{3d1e9154d7}. The simulation starts from the entrance of
TS5 (see \cref{fig:stm_geo_all}), taking \texttt{cd3-beam-g4s2-mubeam.0728a}
dataset as input. The dataset contains 5098 files, each corresponds to
\num{1e6} proton-on-target (POT). The dataset were reused 16 times with
different random seeds, where \SI{97}{\percent} of runs succeeded, equivalent
to \num{8e11} POTs.

\begin{figure}[htbp]
  \centering
  \includegraphics[width=1.0\textwidth]{figs/stm_geo_all}
  \caption{Simulation geometry showing the DS region on the left, sweeper magnet,
    FOV collimator, spot-size collimator, and the STM detectors on the right.
    Particles saved in the input files are shoot from the TS5 (orange circle),
    and transported to the STM region.}
  \label{fig:stm_geo_all}
\end{figure}

There were 8 virtual detectors (VD) in STM region enabled in this study, their
identification numbers (\texttt{vdid}), locations, and abbreviation names
(appear in the simulation output) are listed in \cref{tab:vds_list}.
Information recorded by the VDs includes: particle type (\texttt{pdgid}),
global and local coordinates, time, kinetic energy, and parent particle type.
Only particles considered important to the STM, namely electrons,
positrons, negative and positive muons, neutrons and photons, were written to
the output file.

\begin{table}[htbp]
\centering
\caption{List of virtual detectors read out in this study}
\label{tab:vds_list}
\begin{tabular}{@{}ccll@{}}
\toprule
  &VDID & Location               & Abbreviation              \\
\midrule
1 & 81  & Exit of neutron shield of the DS      & DSNeutronShieldExit       \\
2 & 86  & Upstream of the STM system            & STM\textunderscore UpStr                \\
3 & 87  & Downstream of the sweeper magnet      & STM\textunderscore MagDnStr             \\
4 & 101 & Upstream of the spot-size collimator  & STM\textunderscore SpotSizeCollUpStr    \\
5 & 88  & Downstream of the spot-size collimator& STM\textunderscore CollDnStr            \\
6 & 89  & Upstream of the STM detector 1        & STM\textunderscore Det1UpStr            \\
7 & 90  & Upstream of the STM detector 2        & STM\textunderscore Det2UpStr            \\
8 & 100 & Downstream of the FOV collimator      & STM\textunderscore FieldOfViewCollDnStr \\
\bottomrule
\end{tabular}
\end{table}

\section{Simulation and analysis code}
\label{sec:simulation_and_analysis_code}

The simulation and analysis code are located at:
\url{/mu2e/app/users/namtran/STM_study_201611}.

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\texttt{step00} contains configuration files for this simulation and a script to
submit all 5098 jobs (correspond to number of input files) to the FermiGrid.
It took about 14 hours to complete a job in average.

The \texttt{analysis} folder contains a script 
(\texttt{run\textunderscore statistics.sh}) which checks if a job has finished
successfully, and makes a list of such runs.
There is a simple analysis code (\texttt{main.cc}) to read the VD records and
make plots.

\section{Results}
\label{sec:results}
\subsection{STM detector spectra}
\label{sub:stm_detector_spectra}

Energy spectrum of particles hitting STM detectors are presented in
\cref{fig:stm_det_ke}. There were not many hits, and only the annihilation
peak stands out. Most of the particles are photons as shown in
\cref{fig:stm_det_ptype}.
\begin{figure}[htbp]
  \centering
  \includegraphics[width=0.7\textwidth]{figs/ke_det1UpStr}
  \includegraphics[width=0.7\textwidth]{figs/ke_det2UpStr}
  \caption{Kinetic energy of particles hitting STM detectors 1 (top), and
    2 (bottom).}
  \label{fig:stm_det_ke}
\end{figure}
\begin{figure}[htbp]
  \centering
  \includegraphics[width=0.7\textwidth]{figs/ke_pdg_det1UpStr}
  \includegraphics[width=0.7\textwidth]{figs/ke_pdg_det2UpStr}
  \caption{Kinetic energy and type of particles hitting STM detectors 1 (top),
    and 2 (bottom).}
  \label{fig:stm_det_ptype}
\end{figure}

\subsection{STM detector hit rate estimation}
\label{sub:stm_detector_hit_rate_estimation}
The average number of hits on a STM detector per POT is:
\begin{equation}
  \frac{888 + 888}{2 \times 8 \times 10^{11}} = 8.7 \times 10^{-9}. 
\end{equation}
There are 3.1 POTs per proton bunch, so the number of hits per bunch is:
\begin{equation}
  8.7 \times 10^{-9} \times 3.1 \times 10^7 = 0.27
\end{equation}
The instantaneous hit rate, assuming an interval of \SI{1695}{\ns} between
bunches, is:
\begin{equation}
  \frac{0.27}{1695\times 10^{-9}} = \SI{159e3}{\Hz}
\end{equation}

\appendix
\section{How to run the simulation and analyze data}
\label{sec:how_to_run_the_simulation_and_analyze_data}

\section{Muonic X-rays in Geant4}
\label{sec:muonic_x_rays_in_geant4}
The muonic energy levels and transition probabilities were calculated using
a simple model described by Mukhopadhyay~\cite{Mukhopadhyay.1977}.

\begin{itemize}
  \item Energies of K-shell electrons were precisely corrected based on
    that of hydrogen atom, taking finite size of the nucleus into account
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  \item Energies of 
\end{itemize}

\bibliographystyle{h-physrev}
\bibliography{stm_study}
\end{document}
