132 lines
6.8 KiB
TeX
132 lines
6.8 KiB
TeX
%The first run of the AlCap experiment was performed at the $\pi$E1 beam line
|
|
%area, PSI from November 26 to December 23, 2013. The goal of the run was to
|
|
%measure protons rate and their spectrum following muon capture on aluminium.
|
|
|
|
The low energy muons from the $\pi$E1 beam line were stopped in thin aluminium
|
|
and silicon targets, and charged particles emitted were measured by two pairs
|
|
of silicon detectors inside of a vacuum vessel
|
|
(\cref{fig:alcap_setup_detailed}). A stopped muon event is defined by
|
|
a group of upstream detectors (wire chamber, plastic scintillator) and a downstream muon veto (scintillator).
|
|
The number of stopped muons is monitored by a germanium detector placed outside
|
|
of the vacuum chamber. In addition, several plastic scintillators were used to
|
|
provide veto signals for the silicon and germanium detectors.
|
|
\begin{figure}[btp]
|
|
\centering
|
|
\subfigure{
|
|
\includegraphics[width=0.55\textwidth]{figs/Chamber_layout}
|
|
}
|
|
\subfigure{
|
|
\includegraphics[width=0.4\textwidth,trim= 0 -20 0 0]{figs/Chamber_dimensions}
|
|
}
|
|
\caption{Layout of the AlCap experiment for R2013. Left: Photograph final setup. Muons entered from the right of the image and after passing through the muon trigger system and collimator stopped in the target in the centre. Charged particles could then be detected in the Silicon detector packages(top and bottom), X-rays in the germanium (lower-right corner, out of image) and neutrons in the liquid scintillator (lower-left corner, out of image). Right: The dimensions of the R2013 setup. All measurements are in \si{mm}.
|
|
%The two silicon packages inside the vacuum vessel (SiL and SiR) measure the charged particles emitted from the target, the upstream muon counters consist of plastic scintillators and a wire chamber for defining the input muon beam ($\mu$PC),
|
|
%germanium detector measures the emitted X-rays and gamma rays and the veto plastic scintillator vetos muons that don't stop in target ($\mu$Ve).
|
|
}
|
|
\label{fig:alcap_setup_detailed}
|
|
\end{figure}
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
|
|
\subsubsection{Muon beam}
|
|
\label{ssub:muon_beam}
|
|
%\Cref{fig:alcap_setup_detailed} shows the experimental setup. The muon
|
|
%beam enters from the right of the image and hits the target, which is
|
|
%placed at the center of the vacuum chamber and orientated at 45
|
|
%degrees to the beam axis.
|
|
|
|
%In order to define
|
|
%stopped muon events, four muon counters are used: a 500~\rmmu m thick
|
|
%scintillator muon trigger counter (\rmmu SC); a muon anti-coincidence
|
|
%counter (\rmmu SCA) surrounding the trigger counter with a hole of 35
|
|
%mm diameter to define the beam radius; and a multiwire proportional
|
|
%chamber (\rmmu PC) that uses 24 X wires and 24 Y wires with at 2 mm
|
|
%intervals. This detector system belongs to the MuSun experiment and
|
|
%was well tuned in advance. A muon veto counter (\rmmu Ve) is placed
|
|
%at the downstream end of the chamber and is used to reject muons that
|
|
%pass through the stopping target.
|
|
One of the main requirements of the AlCap experiment was a muon beam
|
|
with narrow momentum bite in order to achieve a high fraction of
|
|
stopping muons in the very thin targets. The actual set up used in the Run
|
|
2013 was: muon momentum from \SIrange{28}{45}{\MeV\per c} and momentum spread of
|
|
3\% FWHM.
|
|
|
|
\subsubsection{Silicon detectors}
|
|
\label{ssub:silicon_detectors}
|
|
The main detectors for charged particles measurement are four large area
|
|
silicon detectors. The silicon detectors were grouped into two detector
|
|
packages located symmetrically at 90 degrees relative to
|
|
the nominal muon beam path, SiL
|
|
and SiR in \cref{fig:alcap_setup_detailed}. Each arm consists of: one
|
|
$\Delta$E counter, a \SI{65}{\micro\meter}-thick silicon detector, divided into
|
|
4 quadrants; one E counter made from \SI{1500}{\micro\meter}-thick silicon; and
|
|
one plastic scintillator to identify electrons or high energy protons that
|
|
pass through both silicon detectors.
|
|
The area of each of these silicon detectors and the
|
|
scintillators is $50\times50 \textrm{mm}^2$.
|
|
|
|
Due to the large capacitance of the thin detectors, noise and
|
|
pickup suppression had to be carefully optimised in the real PSI accelerator
|
|
environment. The achievable electronic resolution was between 55 and 76
|
|
keV FWHM in the thin silicon detectors, and between 35 and 40 keV for the
|
|
thick detectors.
|
|
|
|
\subsubsection{Germanium detector}
|
|
\label{ssub:germanium_detectors}
|
|
We used a germanium detector to normalise the number of stopped muons by
|
|
measuring characteristic muonic X-rays from the target material. The primary
|
|
X-rays of interest are the \atrn{2p}{1s} transitions,
|
|
the 346.828~keV line for aluminium targets, and the
|
|
400.177~keV line for silicon targets.
|
|
|
|
The germanium detector is
|
|
a model GMX20P4-70-RB-B-PL, n-type, coaxial high purity germanium detector produced
|
|
by ORTEC. The detector was optimised for low energy gamma and X-ray
|
|
measurements, with an ultra-thin entrance window of 0.5-mm-thick beryllium and
|
|
a 0.3-\si{\micro\meter}-thick ion implanted contact. The germanium crystal is
|
|
\SI{52.5}{\mm} in diameter and \SI{55.3}{\mm} in length. The axial well has
|
|
a diameter of \SI{9.9}{\mm} and is \SI{47.8}{\mm} deep.
|
|
|
|
The detector was energy and acceptance calibrated with the many lines
|
|
from a Eu-152 source of known activity.
|
|
Energy resolutions are better than 2 keV for all calibrated peaks. Absolute
|
|
efficiencies at the energies of interest are listed in \cref{tab:xray_ref}.
|
|
\begin{table}[btp]
|
|
\centering
|
|
\caption{Reference values of major muonic X-rays from aluminium and silicon. Energy and intensity values are taken from~\cite{MeasdayAl}.}
|
|
\begin{tabular}{c l l l l c}
|
|
\addlinespace
|
|
\toprule
|
|
\textbf{Elements} & \textbf{Transition}
|
|
& \textbf{Energy (keV)} & \textbf{Intensity (\%)} & \textbf{Calibrated eff.}\\
|
|
\midrule
|
|
$^{27}\textrm{Al}$ & \atrn{2p}{1s} & $346.828 \pm 0.002$ & $79.8\pm 0.8$ &$4.95\times10^{-4}$\\
|
|
& \atrn{3p}{1s} & $412.87 \pm 0.05$ & $7.62\pm 0.15$ & $4.41\times10^{-4}$\\
|
|
\addlinespace
|
|
$^{28}\textrm{Si}$ & \atrn{2p}{1s} & $400.177 \pm 0.005$ & $80.3\pm 0.8$
|
|
&$4.40\times10^{-4}$\\
|
|
& \atrn{3p}{1s} & $476.80 \pm 0.05$ & $7.40 \pm 0.20$ &$3.81\times10^{-4}$\\
|
|
\bottomrule
|
|
\end{tabular}
|
|
\label{tab:xray_ref}
|
|
\end{table}
|
|
|
|
|
|
\subsubsection{Neutron detectors}
|
|
\label{ssub:neutron_detectors}
|
|
|
|
|
|
The main emphasis of R2013 was on the measurement of charged
|
|
particle emission after muon capture, however, some neutron
|
|
emission data were collected and used to develop analysis
|
|
tools, and to study efficiencies and rates. Two BC501 neutron
|
|
counters ($5''$ diameter and $5''$ depth) were borrowed from the
|
|
MuSun experiment, and placed outside the vacuum chamber on
|
|
both sides of the muon beam and centred on the stopping
|
|
target. Data collection used a 12-bit, 170-MHz FADC and readout
|
|
was by means of the MIDAS framework.
|
|
|
|
%346.828,4.95E-04,1.216E-05
|
|
%412.87,4.41E-04,9.780E-06
|
|
%400.177,4.40E-04,9.747E-06
|
|
%476.800,3.81E-04,7.680E-06
|
|
|