writeup/AlCapPSI/Goals.tex

As outlined in the introduction, the Mu2e and COMET collaborations
propose to study background reactions to the 
\muec process for the candidate target materials (Al, Ti).  These studies
are needed to optimise the designs of the two experiments, as existing
information on these background processes is limited and what is
known, is insufficiently precise.
The new piE1 beamline at PSI offers a unique opportunity for these
experiments, as emphasized in section~\ref{PSI}. Our experimental
program is organised in three distinct work packages (WP), directed
by different team leaders, given in parentheses.

\begin{itemize}
\item[WP1:] (Kammel (Seattle), Kuno(Osaka)) \textbf{Charged Particle
Emission after Muon Capture.}\\
Protons emitted after nuclear muon capture in the stopping target
 dominate the single-hit rates in the tracking chambers for both the
 Mu2e and COMET Phase-I experiments. We plan to measure both the total
 rate and the energy spectrum to a precision of 5\% down to proton
 energies of 2.5 MeV.

\item[WP2:] (Lynn(PNNL), Miller(BU)) \textbf{Gamma and X-ray Emission
after Muon Capture.}\\
A Ge detector will be used to measure X-rays from the muonic atomic
cascade, in order to provide the muon-capture normalization for WP1, and is
essential for very thin stopping targets. It is also the primary
method proposed for calibrating the number of muon stops in the
Mu2e and COMET experiments. Two
additional calibration techniques  will also be explored; (1)
detection of delayed gamma rays from nuclei activated during nuclear
muon capture, and (2) measurement of the rate of photons
produced in radiative muon decay. The first of these would use a Ge
detector and the second a NaI detector. 

The NaI
calorimeter will measure the rate of high energy  photons from
radiative muon capture (RMC), electrons from muon decays in orbit
(DIO), and photons from radiative muon decay (RMD), as potential
background sources for the conversion measurement. As these rates are
expected to be extremely low near the conversion electron energy, only
data at energies well below 100 MeV will be obtained. 

\item[WP3:] (Hungerford(UH), Winter(ANL)) \textbf{Neutron Emission
after Muon Capture.}\\ 
Neutron rates and spectra after capture in Al and Ti are not well
known. In particular, the low energy region below 10 MeV is
important for determining backgrounds in the Mu2e/COMET detectors and 
veto counters as
well as evaluating the radiation damage to electronic
components. Carefully calibrated  liquid scintillation detectors, employing
neutron-gamma discrimination and spectrum unfolding techniques, will
 measure these spectra. The measurement will attempt to obtain spectra
 as low or lower than 1 MeV up to 10 MeV. \\
\end{itemize} 

WP1 is the most developed project in this program. Most of the
associated apparatus has been built and optimized. We are ready to
start this experiment in 2013, while preparing and completing test
measurements and simulations to undertake WP2 and WP3. 

\subsubsection{WP1: Charged Particle Emission after Muon Capture}

\subsubsection*{Present knowledge}
 
The yield, energy spectrum and composition of the charged particles
emitted in muon capture on Al and Ti have not been measured directly
in the relevant energy range for COMET Phase-I and Mu2e. Only high
 energy spectra are available for Al (Fig.~\ref{fg:AlHE}), while low
 energy spectra are only measured for Si (
 Figure~\ref{fg:silicon-proton}), where  muons can be stopped and
 captured in an active silicon detector \cite{sobo68}. The peak below
 1.4~MeV is presumed to be due to recoiling heavy ions, mainly $^{27}$Al, when no
 charged particles were emitted. Hungerford~\cite{hung34} fitted the
 silicon spectrum in Fig.~\ref{fg:silicon-proton} with an empirical
 function given by;
%
\begin{equation}
p(T) = A(1-{T_{th} \over T})^{\alpha} e^{-(T/T_0)}
\label{eq:protons}
\end{equation}
%
where $T$ is the kinetic energy and the fitted parameters are
$A=0.105$ MeV$^{-1}$, $T_{th}$ = 1.4 MeV, $\alpha$=1.328 and $T_0$ =
3.1 MeV. The spectrum is normalized to 0.1 per muon capture. Some
other results in the past experiments are summarized in
Table~\ref{tb:proton}.

\begin{table}[b!]
\centering
\caption{Probabilities in units of $10^{-3}$ per muon capture for
inclusive proton emission from~\cite{Lifshitz}. The numbers in
crescent parenthesis are estimates for the total inclusive rate
derived from the measured exclusive channels by the use of the
approximate regularity, such as $(\mu, \nu p):(\mu, \nu p n):(\mu, \nu
p 2n):(\mu. \nu p 3n) = 1:6:4:4$.}\label{tb:proton} 
\vskip 3mm
\begin{tabular}{|c|c|c|c|c|}\hline
Target nucleus & Calculation & Experiment & Estimate & Comments \\ \hline
$_{10}$Ne & & $200\pm 40$ & & \\
$^{27}_{13}$Al & 40 & $>28 \pm 4$ & (70) & 7.5 for $T>40$ MeV \\
$^{28}_{14}$Si & 144 & $150\pm30$ & & 3.1 and 0.34 $d$ for $T>18$ MeV \\
$^{31}_{15}$P & 35 & $>61\pm6$ & (91) & \\
$^{46}_{22}$Ti & & & & \\
$^{51}_{23}$V & 25 & $>20\pm1.8$ & (32) & \\ \hline
\end{tabular}
\end{table}

\begin{figure}[htb!]
\begin{minipage}{0.35\textwidth}
\centering
\includegraphics[width=0.9\textwidth]{figs/AlHE.png}
\caption{Energetic charged particle spectrum from muon capture in Al
and other targets~\cite{Krane:1979}.} 
\label{fg:AlHE}
\end{minipage}
\hspace{3mm}
\begin{minipage}{0.6\textwidth}
\centering
\includegraphics[width=0.9\textwidth]{figs/si-proton.eps}
\caption{Charged particle spectrum from muons stopping and being
captured in a silicon detector~\cite{sobo68}.} 
\label{fg:silicon-proton}
\end{minipage}
\end{figure}

\subsubsection*{Relevance for $\mu-e$ Conversion Experiments}

The tracking detectors of COMET Phase-I and Mu2e are designed to
measure the helical trajectories of ~105 MeV conversion electrons in a 
uniform, cylindrical magnetic field. The detector geometry coupled
with the field strength accepts charge particle momenta between ~54 to
~200 MeV/c. In this momentum range ``hits'' in a tracker plane are
dominated by proton emission after nuclear muon capture. Such events
are particularly troublesome due to their large energy deposition. In
addition to protons, electrons emitted during muon decays in
orbit (DIO) are also a source of background. Background events in the
tracking detector can produce ambiguous track reconstructions, which
may lead to mis-identified events in the signal region. Both
experiments plan to introduce thin, low-Z proton
absorbers in front of the tracking chambers to reduce proton hit
rates. These are effective at removing protons due to their very low energies
(~5 MeV for 100 MeV/c protons). However, such absorbers degrade the
momentum resolution of conversion electron 
candidates and their thickness and geometry must be carefully
designed. For similar reasons, the design of the stopping target is
also important. 
The limited information available at present makes it difficult to
arrive at a conclusive detector design. From Table~\ref{tb:proton},
the relative experimental yield for proton emission from Al for 
energies above 40 MeV is 3\%.  From theory the expectation is 4\%, 
if estimated from the ratio of
exclusive channels from other nuclei it is 7\%, or it may be as
high as that from Si or Ne, 15-20\%. The energy spectrum can only be
inferred from the Si data or from Ref.~\cite{Bala67}. At this moment,
~\cite{Bala67} has been
used to estimate proton emission for both experiments. The emission of
deuterium and alpha particles is also not known. Charged particle
yields from Ti can only be estimated
from V to be around 3\%. Thus a measurement of the rate and spectrum
of proton emission after 
muon capture  is required in order to estimate background and optimize
the detector design.
% Typical background rates in a tracker are shown in Table
%~\ref{bk_hits} as a function of the time interval relative to the peak
%of the proton pulse.\\ 

In COMET Phase-I,
singles rates in the tracking chamber (cylindrical drift chamber) have
been estimated based on the spectrum given in
Eq.(\ref{eq:protons}). To reduce the proton flux entering the
tracking chamber, in addition to the inner wall of the drift chamber
(of 400 $\mu$m) a cylindrical proton absorber is located in front of
the tracking chamber. Monte Carlo simulations were run for three
different thicknesses of proton degrader, 0~mm, 5~mm, and
7.5~mm and are
are summarized in Table~\ref{tb:protonhits}, where the
proton emission rate of 0.15 per muon capture is assumed. For a
typical number of stopped muons and for a 5 mm degrader, the total
number of hits in the  
first plane is estimated to be 530 kHz (1.3 MHz). According to simulations,
rates are similar
for Mu2e.  In simulation studies of reconstructed conversion electron
tracks mixed with a nominal proton background, a decease of
approximately 17\% in energy resolution in the conversion electron
peak and a down shift of 0.7 MeV was found when a standard proton
absorber was inserted. A downshifted conversion peak is an issue as it
pushes the signal up the DIO background curve. \\
%
\begin{table}[ht!]
	\begin{center}
	\caption{Total numbers of hits in the first layer by protons
	emitted from muon capture for different trigger counter
	thickness. 100 k proton events were generated for COMET
	Phase-I. 15 \% protons per muon capture is assumed.}
	\label{tb:protonhits} 
	\vspace{5mm}
		\begin{tabular}{|l|c|c|c|} \hline
		proton degrader thickness & 0 mm & 5 mm& 7.5
	mm\cr\hline\hline 
		hits & 2644 & 103 & 30 \cr\hline
		hits per proton emission & 2.6 \% & 0.1 \% & 0.03 \% \cr\hline
		hits per muon capture$^{*}$ & $3.9\times10^{-3}$  &
		$1.5\times10^{-4}$ & $4.5\times10^{-5}$ \cr\hline 
		\end{tabular}
	\end{center}
\end{table}

\subsubsection{WP2: Gamma and X-ray Emission after Muon Capture.}

\subsubsection*{Present knowledge}
When a negative muon is captured in an atomic orbit, it
cascades down to the 1s level within $10^{-13}$s.  Initially the
cascade occurs with Auger emission, but near the $n =5$ atomic level,
muonic X-rays start to dominate the process. 
Theoretical determination of the energy levels is complex, due to
effects such as screening of the nucleus by inner electrons, shift of
low-level states due to the finite charge distribution of the nucleus,
relativistic corrections, and fine structure splitting of levels.
Nevertheless, muonic X-rays have been used to identify the capturing
element, typically from the $2p \rightarrow 1s$ transition X-ray,
which occurs $\sim$80\% of the time.  An X-ray spectrum for phosphorus
is shown in figure \ref{phos_spectrum}. 

The spectra of prompt gammas from muon capture have been measured for
Al~\cite{MeasdayAl} but not for Ti, using time coincidences with the
incoming muon. To our knowledge there are no data for singles mode or
for delayed gammas from decays of unstable nuclei produced in the muon
capture process. 

The RMD photon spectra have been measured in previous experiments
(including MEG) and is theoretically well reproduced for a free
muon.  We propose to evaluate the feasibility of measuring the spectrum
using a NaI detector for energies between 20 and 54 MeV.  In this
region the spectrum
is only slightly distorted by the fact that the muon is bound in
an atomic orbit. 

We also propose to confirm with existing data, the high energy tail of
the electron and
photons from RMD, DIO, and RMC. We note that the high energy tails for
RMD and DIO are the result of
atomic binding, which is absent in free muon decay.  There is a recent
theoretical calculation~\cite{czarnecki} of the DIO 
electrons which should be tested. Due to rates at PSI, we would only be able to
observe spectra at energies below 80 MeV.  The spectra near the
end-point energy can only be observed with the extremely
large number of stopped muons envisioned in the Mu2e and COMET
experiments.  

\begin{figure}[htb] 
	\begin{center}
		\includegraphics[scale=0.50]{figs/phos_spectrum.png}
		\caption{A typical muonic X-ray spectrum \cite{shera80}.} 
		\label{phos_spectrum} 
	\end{center} 
\end{figure}


\begin{figure}[htb] 
	\begin{center}
		\includegraphics[scale=0.50]{figs/rmd-spect.png}
		\caption{A typical radiative muon decay spectrum.} 
		\label{rmd-spect} 
	\end{center} 
\end{figure}


\subsubsection*{Relevance for $\mu-e$ Conversion Experiments}
The rate of muon capture in the stopping target is critical
for proper normalization of data in the \muec experiments, and the
proposed method of observing this rate will count  muonic
X-rays as the $\mu \to e$ data are collected. 
A germanium detector will be placed far from the stopping
target to reduce both rate and damage to the detector.  A dipole magnetic
field will be used to remove charged particles moving along the field
of view from the detector to the target.
However, the environment will be challenging, as high photon
rates and a significant neutron background will be present.  Pacific
Northwest
National Laboratory is currently developing high-rate germanium
detectors utilizing such methods as custom fast preamplifiers and
segmented detectors. Measurements using these designs in muon
beam tests at PSI will be invaluable proofs of principle for the Mu2e
experiment. Also, identifying 
how the gamma and neutron backgrounds affect the gamma and X-ray lines
as a function of dose is of critical importance. 

In the past, experiments that have studied muonic X-ray spectra used time
coincidence between the observed X-ray and the
incident beam muon. In the 
Mu2e experiment, the muon beam will be pulsed, with an average muon
stopping rate in excess of $10^{10}$ Hz.  This is too large for a beam
gate to tag muon arrival. Thus, it is necessary
to operate the Ge detector in singles mode, and the PSI measurements
are essential to determine whether the X-rays can be detected above
background in this situation. 

In case background in singles mode are too severe, two
alternate means to monitor the Mu2e stopping rate will be
evaluated. In one of the alternatives,
simultaneously with the collection of muonic
X-ray spectra, we will search for delayed gammas arising from the decay of
nuclei activated by nuclear muon capture.
For example, in the case of muon capture on $^{27}$Al, the reaction
$^{27}$Al($\mu^-,\nu)^{27}$Mg occurs in 16\% of the captures. The
lifetime of the $^{27}$Mg is 9.458~minutes. It decays to excited
states in $^{27}$Al, leading to a 1014.45~keV gamma 21\% of the time,
and an 843.76~MeV gamma 100\% of the time. In normal operation, Mu2e
will have a steady stream of 8~GeV proton pulses on the production
target (spaced at 1.5~microsecond intervals) for about 0.4~s, followed
by about 0.9~s period of beam-off. It may be possible to observe the
$^{27}$Mg decays 
with a Ge detector in the reduced background environment when the beam
is off. At PSI, we would test whether these gammas can be cleanly
separated from background. 
Another normalization alternative would be the measurement
of energetic photons from the radiative decays of the muons bound in
atomic orbits in the stopping target (RMD). Similar to free muon decay,
the branching ratio for photons above 10~MeV relative to regular decay
is about 1.4\% and the energy distribution peaks at low energy, uniformly
decreasing up to about 54~MeV (see
Fig.~\ref{rmd-spect}). Above 54~MeV, the free muon probability is
zero, but the energy endpoint actually goes above 100~MeV for bound
$\mu^-$, albeit with very small probability. In addition to this inner
bremsstrahlung process, there will be a calculable contribution from
the bremsstrahlung of electrons from the dominant decay mode of the
muon, DIO. At energies above ~10 MeV, the backgrounds
are much less than at lower energies, possibly allowing a good signal
to background ratio for a normalization measurement. Using tagged electrons from DIO, we would measure RMD as
well as DIO rates. Photons from RMD (and hence electrons from pair
production in the target surrounding materials) and electrons
from DIO have endpoint energies equal to the conversion electron
energy, with probabilities decreasing rapidly as the energy
approaches the endpoint. These potential backgrounds are controlled
with sufficiently good conversion electron energy resolution. We note that using RMD photons may not be as clean as using muonic X-rays or activation gammas, depending on the quality of photon collimation to the detector, since the RMD spectrum is not unique to the target species, and the energy distribution is not ideal, dropping rapidly with increasing energy.

A NaI calorimeter will be used to measure the higher energy photons
and electrons from RMD and DIO. We have access to a large NaI
detector, however we are searching for a more easily managed small
array of crystals for this task which will be easier to transport, mount, and
shield.
With the implementation of an NaI calorimeter, we have the additional 
possibility to affirm old measurements of the rate of radiative muon
capture (RMC) at energies from 55~MeV (just above the bulk of the muon
decay flux) to about 75-85~MeV (where the rate becomes too small to
measure at the integrated fluxes envisioned at PSI). The branching
ratio is on the order of $10^{-5}$. Pair production from the photons
produce electrons detectable above background in approximately the
same energy range, with the precise endpoint depending on the rest energies of the daughter nuclei



\subsubsection{WP3: Neutron Emission after Muon Capture}

\subsubsection*{Present knowledge}

\noindent Nucleon emission after muon capture, particularly the nuclear
dynamics, is not well understood. Neutron emission is described by direct
and evaporative processes with energies ranging from thermal up to some
50 MeV. However, most neutrons, at least for heavy nuclei, are emitted by 
evaporation after an excited nucleus is formed. Theoretical 
studies indicate that giant resonance levels, ~\cite{Lifshitz, Uberall} are
important doorways leading to neutron emission. If this is the case, the
reaction occurs through a two-step process as described by; 
\\

\begin{eqnarray}
\mu^{-} +  A(N,p) &\rightarrow& \nu_{\mu} \, + A(N+1,p-1)^{*}     \\
\nonumber
 A(N+1,p-1)^*    & \rightarrow & xN\,+ \, x^{\prime} p \, + \,
 x^{\prime\prime}\gamma
\end{eqnarray} 

\noindent In the above reactions, the $x$'s represent emission of 
any number of particles including photons as the nucleus de-excites.
If neutrons are emitted from giant 
resonance excitation, broad peaks at lower energies would be
expected and are observed, Figure ~\ref{expCO} ~\cite{Plett}.  Also,
one would expect multi-particle  
emission, as has indeed been 
 observed for various targets ~\cite{Macdonald, Wyttenbach}.  
Multiplicity measurements for targets of relevance ( close to Atomic
Numbers of Al and
Ti) are shown in  Table ~\ref{multi_neut}. 

\begin{table}[h!]
\caption{Neutron multiplicities for various targets.  The distribution
  is adjusted to 0.545~\cite{Macdonald}.}
\label{multi_neut}
\begin{center}
\begin{tabular}{lccccc}
\hline
\multicolumn{2}{c}{   } & \multicolumn{3}{c}{Multiplicity} \\ 
Target & Avg. Mult. & 0 & 1 & 2 & 3 \\
\hline
Al& $1.262 \pm 0.059$ & $0.449 \pm 0.027$ & $0.464 \pm 0.028$
& $0.052 \pm 0.0013 $& $ 0.036 \pm 0.007$\\
Si & $0.864 \pm 0.072$ & $0.611 \pm 0.042$ & $0.338 \pm 0.042$
& $0.045 \pm 0.0018 $& $ 0.000 \pm 0.008$\\
Ca & $0.746 \pm 0.032$ & $0.633 \pm 0.021$ & $0.335 \pm 0.022$ & $0.025 \pm
0.0009 $& $ 0.004 \pm 0.006$\\
Fe & $1.125 \pm 0.041$ & $0.495 \pm 0.018$ & $0.416 \pm 0.019$ & $0.074 \pm
0.0011 $& $ 0.014 \pm 0.005$\\
\hline
\end{tabular}
\end{center}
\end{table}

At higher energies, direct emission 
involves photo-production on a proton in the nucleus, with the emission of a
neutron.  The end-point energy of this process is
approximately 6 MeV for at rest protons, but protons move with Fermi
motion in the nucleus so  
an energy spectrum is produced. Thus correlated proton-neutron
emission and proton-photon emission would be expected. Examples of
this are described in radio-chemical experiments ~\cite{Wyttenbach}.

\begin{figure}[htb!]
\begin{center}
\begin{minipage}{8.cm}
\includegraphics[width=7.5cm] {figs/C_O_exp_spectrum.pdf}
  \caption{\label{expCO} 
Spectra of neutron energies after muon capture showing emission from
giant resonance excitations in C (above) and O (below).  Note the
rise in evaporative emission at low energies ~\cite{Plett}.}
\end{minipage}
\parbox{0.3 cm}{ }
\begin{minipage}{7. cm}
\includegraphics[width=6.3 cm, angle=180]{figs/exp_neutron_spectra.pdf}
\caption{\label{exp_neutron_spectra} Higher energy neutron
  spectrum for various light nuclei.  Note the exponential decrease
  with energy ~\cite{Sundelin}.}
\end{minipage}
\end{center}
\end{figure}

\noindent  A number of
experiments studied high energy neutron emission from targets as light as Si
in order to observe the neutron asymmetry. In the process of such measurements neutron
spectra were extracted ~\cite{Sundelin} and
examples are shown in Figure
~\ref{exp_neutron_spectra}.  
These spectra have a low energy cut-off ranging from 4 to 10 MeV as only
direct emission is expected to preserve asymmetric emission.
They are consistent with an exponential decrease as a function of
increasing energy, and show no
indication of an evaporative increase or resonance emission at lower
energies. However, the break in the slope of the spectrum in 
heavier nuclei occurs around 10 MeV, and evidence of a spectrum break
may have been missed due to the energy cut off . From these
data on Si, the
measured number of emitted neutrons per muon capture above 
approximately 4 MeV is approximately 0.43.  
This experimental result is 
corrected for multiple neutron emission which is small, at least at
the measured energies.\\

\noindent  In summary, the neutron energy spectrum seems
reasonably determined for neutron energies above 10 MeV.  Low energy
emission depends on nuclear structure and is less well defined.  At
energies of less than a few MeV there is an evaporative increase as
well as emission from giant resonant states. \\

\subsubsection*{Relevance for $\mu-e$ Conversion Experiments}

The Mu2e and COMET experiments stop negative muons in an $^{27}$Al
target.  The  
signal of interest is an 
electron, emitted with approximately 100 MeV/c momentum. 
The Mu2e tracking detector, which is constructed of straw-tubes or a
drift chamber, is  
relatively insensitive to neutrons, but this is not the case for
the calorimeter, the cosmic ray veto scintillators, and the
readout electronics inside the detector solenoid. There is a large
flux of neutrons from the production target which is reduced by
shielding 
between the production and detection solenoids. With this shielding in
place, simulation shows that neutrons from muon captures in the
stopping target then dominate the neutron background in the tracker,
Table ~\ref{neut_background}. Due to their proximities, neutron backgrounds in
the cosmic ray veto, and 
particularly the calorimeter, are more sensitive to neutrons emitted
after muon capture in the 
beam-stop. 
 
\begin{table}[h!]
\caption{Neutron Background Sources on the Tracker as a function of
Neutron Kinetic Energy, $T$}
\label{neut_background}
\begin{center}
\begin{tabular}{cccc}
\hline
\multicolumn{1}{c}{ } & \multicolumn{3}{c}{Neutrons/$cm^{2}$ ($\times
10^{10}$) }\\ 
Source & Thermal($ T  <  1$ eV) & Epithermal ($1 \mbox{eV}  <
T  <  1$ MeV) &  Fast ($T  >  1$ eV)\\ 
\hline
Stopping Target & 16 & 77 & 100 \\
Muon Beam Stop & 0.2 & 2 & 0.8 \\ 
Beam Flash & 0.2 & 1 & 2 \\ 
Production Solenoid & 0.6 & 0.09 & 0 \\ 
\hline
\end{tabular}
\end{center}
\end{table}

Aside from radiation damage to the electronics, fast neutrons cause single
event upsets (SEU) during dynamic operations in electronic systems.
Depending on the upset, redundancy, and software verification, these
either could be ignored, result in data contamination, or in
electronic failure. A dose of $5 \times10^{4}\,
n/s/cm^{2}$ was found using a MARS estimate of the neutron spectrum from the
stopping target 
(see Fig.~\ref{fg:mars}).  Thus, for a
nominal IC area of 1cm2, each IC must be tolerant to about $ 10^{12}$
neutrons which would be expected in $4 \times 10^{7}$ beam-on-target
seconds. Better limits on these estimations should be obtained. 

\begin{figure}[htb!]
\begin{center}
\includegraphics[scale=0.70]{figs/mars_spectra.pdf}
\caption{\label{fg:mars} 
The neutron energy spectrum after emission
from an Al Target as obtained from a MARS Simulation
and used as input to neutron background calculations}
\end{center} 
\end{figure}

Many muons, protons, and photons produced in the stopping target reach the
calorimeter. The estimated rates of neutrons and gammas in the
calorimeter are approximately 300 kHz and 85 kHz, respectively.
Simulation shows this introduces pileup probabilities of 40\% and
20\%, but with average 
energy depositions of 0.5 MeV and 0.7 MeV, respectively. 

Background (e.g. false cosmic ray vetoes) in the cosmic ray veto, CRV,
must not affect live time by 
more than 1\%.  Most neutrons reaching the CRV have kinetic energies
below 10 MeV, with the most probable energy about 1 MeV. The CRV is
shielded by concrete and most likely Boron loaded polyethylene, the
thickness to be determined by shielding simulations.
The CRV scintillator (C8H8) is sensitive to neutrons
which capture and scatter on protons with a
cross section increasing with decreasing energy
(~1/velocity for the captures). Thus thermal and epithermal neutron
energies are most important.  These neutrons would be 
produced by attenuation of MeV neutrons as they penetrate
the shielding between the CRV and the stopping target
or the beam dump. Simulation studies for the neutron spectrum
at the CRV use Geant3 GCALOR with an input spectrum from
MARS as shown in Fig.~\ref{fg:mars}.  This
spectrum is certainly not correct at low energies as seen
when compared to the C and O experimental data in Figure
~\ref{expCO} above.  
Thus since detector design and simulation are based on input of
estimated neutron emission, as discussed above, it is important to
obtain reliable experimental measurements for calibration. As 
examples, we must know the impact of radiation effects on the
electronics, and must design the detectors to reduce the effects of
neutron backgrounds.