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progress14/Neutrons.tex

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+
+\subsubsection{Goals}
+
+	We propose to obtain the neutron emission spectrum due to muon
+	capture on Al, Ti, and H$_{2}$O.  The focus will be on
+	Al. Ti remains a secondary target and capture on O would
+	be interesting as existing neutron data is of
+	reasonable quality, and shows a prominent peak near 5 MeV,
+	purportedly due to a giant resonance excitation. The unfolded neutron
+	energies would lie between 1 to somewhat over 10 MeV.  
+
+\subsubsection{Geometry/Detectors}
+
+	The main emphasis of R2013 was on the detection of charged
+	particle emission after muon capture, however, some neutron
+	emission data were collected and used to develop analysis
+	codes, efficiencies, and rates.  In R2015, the basic
+	experimental setup will be
+	modified from that of R2013, by removing the vacuum chamber 
+        and hanging the stopping target in
+	air by thin wires from a large frame far from the
+	beam line. Thus while the associated beamline components remain, the
+	vacuum chamber, veto scintillators, and their readout electronics are
+	removed. The target is rotated approximately $45^\circ$ with
+	respect to the beam line to minimise the target material
+	through which emitted particles travel to reach the
+	detectors. 
+
+
+        We will borrow two neutron BC501 detectors ($5'' \times
+	2''$) from the Triangle Universities Nuclear Laboratory (TUNL)
+	for the proposed experiment. They are placed horizontally at
+	distances approximately 30 cm from the target.  If the
+	LYSO array is used, it can be placed beneath the target and the
+	target rotated around the horizontal as well as the vertical
+	axis. A beam momentum of approximately \SI{40}{MeV\per\cc} and rate of
+	40 kHz is anticipated.  The stopping thickness in an Al
+	target is approximately 0.2 cm (2000 $\mu$m), and with a
+	target rotation the Al target dimensions would be
+	$10\times10\times(0.1\textrm{\,--\,}0.2) \textrm{cm}^3$.  
+
+\subsubsection{Electronics}
+
+Analysis of the R2013 data demonstrated that a waveform digitizer of
+at least 250 MHz sampling rate is needed to optimise the pulse shape
+discrimination (PSD) allowing separation of neutron from gamma events.
+%Figure~\ref{psdplot_2013} shows the pulse shape discrimination
+%Figure~\ref{fig:neutron-psd} shows the pulse shape discrimination
+%achieved in a R2013 run. 
+Data will be acquired using the MIDAS
+framework and stored in MIDAS banks for offline analysis. Real time
+spectra will be available for online analysis and will be monitored
+from the counting house via the software tools developed during 2013
+run. 
+
+\subsubsection{Calibration}
+
+The neutron counters will be calibrated periodically using radioactive
+sources ($^{22}$Na, $^{137}$Cs, $^{22}$Na, AmBe). Data will be taken
+with $^{137}$Cs daily to monitor the gain of detector system. Also,
+data will be taken with beam-off to obtain a measure of
+backgrounds. The AmBe source is particularly useful to set the gain
+for higher energy neutrons as it has a Compton edge at 4.19 MeV, as
+well as neutron emission up to 10 MeV. The neutron detectors will be
+characterized at TUNL using $^7$Li(p,n)$^7$Be reaction and
+time-of-flight~\cite{gonzales2009}.
+
+In 2014 a similar detector was calibrated at TUNL, although the proton
+beam energy was restricted keeping neutron energies below 5 MeV.   
+The results of this characterization 
+are being used to calibrate a MC simulation of the response
+function, NRESP7~\cite{dietze1982}, in order to simulate the
+detector response function for an arbitrary incident neutron
+energy. The MC can then be used to obtain the folding matrix for the
+detector for any detected energy. 
+ The true neutron energy distribution must be obtained by
+spectrum unfolding techniques~\cite{matzke1994}, and this
+requires the knowledge of response of the detectors to
+neutrons in the relevant energy range of the experiment. Thus the
+detectors to be used in R2015 will be calibrated prior to their use at
+PSI.  As the neutron spectrum of AmBe is known, the measured spectrum will be unfolded after the unfolding function is determined and will be  compared to the known
+AmBe spectrum.
+
+\subsubsection{Rates}
+	
+%Analysis of error propagation in unfolding neutron spectra
+%indicate that a general error of 5\%  in the folded 
+%data can be amplified at least an order of magnitude in the
+%unfolding process~\cite{suman2014}. Thus in 2015, the intent  
+%is to hold statistical
+%error to better than 5\% per energy bin.
+
+In order to determine a neutron rate in the
+detector, a data run using a 50 \textmu m target was analyzed
+using timing cuts.  The beam rate corresponding to the 
+collected data was 4.5 kHz and after PSD cuts a neutron detector 
+observed 1.1 neutrons/s. If the beam intensity is increased to 40 kHz
+using an incident momentum of \SI{40}{MeV\per\cc} and a sufficiently thick target
+is used to stop the total beam, the rate in a similar neutron detector
+would be expected to increase by a factor of 100 to approximately 100
+per second. 
+However, the scintillator in the neutron detectors proposed for R2015 
+are thinner by approximately a factor of 2. This provides better
+resolution but decreases the detection efficiency.  
+The folded neutron spectrum in an energy bin between 5.5 to 6.0
+MeVee is approximately 0.2\% of the total spectrum. Thus not including the
+loss in detection efficiency due to the thinner scintillator, 
+approximately 19\,000 neutrons are expected to be obtained in 24 hours
+within the energy bin.  
+%However, this does not include  inefficiencies in data collection. 
+
+ \subsubsection{Anticipated results}
+	A two day run provides a 1\% statistical error in a 0.5 MeV
+	energy bin. Obviously there are
+	other errors which we  hope to control to a level
+	of 5\%. A data run for Al and perhaps a smaller
+	statistical run on Ti and H$_{2}$O is foreseen.