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\begin{document}

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%Title of paper
\title{Simulation of Shielding Options for an Active Collimator System}

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\author{Christopher Gauthier}
%\email[]{Your e-mail address}
%\homepage[]{Your web page}
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\affiliation{University of Connecticut}

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\date{\today}

\begin{abstract}
% insert abstract here
This paper covers the details of a simulation of three shielding options for
an active collimator system to be used in the upcoming GlueX experiment. 
The simulations were run with GEANT, a high-energy particle physics
simulation program developed at CERN. Three collimator shielding options,
heavy, moderate and light shielding, were simulated. The simulations
revealed that the light shielding option was the least effective in
blocking secondary particles from penetrating into the experimental hall while 
the heavy and moderate shielding options both provided adequate shielding for 
the collimator. The heavy and moderate shielding options lead to acceptable 
background levels in the detectors, however, cost and ease of construction make 
the moderate shielding option the more desirable of the three.
\end{abstract}

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\section{Introduction}
The GlueX Experiment will test our current picture of the subatomic world. The 
force between quarks is thought to be caused by a nuclear field called "glue". 
Although the properties of the glue are well understood where the field is 
weak, inside the nucleus where this field is quite strong its properties are 
largely unknown. In the GlueX experiment, a beam of high energy gamma rays 
will excite this glue, thereby revealing its physical properties at high 
energies. The GlueX experiment will rely on the use of high energy gamma rays 
to probe the internal structure of the bond between quarks. Unpolarized gamma 
rays can scatter into many more final states than a linearly polarized beam, 
which would needlessly complicate a partial wave analysis of the scattered 
beam. An initial state of linearly polarized gamma rays limits the number of 
possible final states, which makes a partial wave analysis easier and reduces 
the number of statistics needed. Using a beam of linearly polarized gamma rays 
in the GlueX experiment would improve the precision of the results.

The Coherent Bremsstrahlung process creates a beam of gamma rays with a degree
of linear polarization that is dependent on the angle of emission. This angular 
dependence can be used to select and remove the unpolarized component of the 
gamma ray beam. A collimator can absorb a significant amount of the 
unpolarized gamma rays, while leaving the polarized unaffected. Increasing the 
polarization of the gamma rays through collimation has the adverse effect of 
decreasing the total flux reaching the target. It has been determined that 
collimating the beam to 40\% polarization will reduce the flux to 20\%, with 
80\% of the beam's total power absorbed by the collimator. The process of 
collimation has the disadvantage of creating copious amounts of unwanted 
secondary particles.

This experiment will use a charged particle track 
detector that is sensitive to low energy particles, therefore, effective 
shielding for the collimator is necessary to keep any background from 
interfering with the performance of this detector. Three different shielding 
scenarios for the collimator system have been designed. For each design, a 
simulated beam of coherent bremsstrahlung photons passed through the collimator 
and the flux of secondary particles reaching the tracker was measured. The 
results for each design were compared with each other and with the cosmic ray 
background levels to determine which option worked the best at shielding the 
tracker from secondary particels.   

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%\section{\label{}}

\subsection{}
\subsubsection{}
\section{Software}
The simulation was carried out using {\it Geant}, a physics simulation program
 developed at CERN. Geant simulates the passage of high-energy particles
 through matter using the Monte Carlo technique. The program uses a random
 number sequence to assign each particle a unique track history based on known
 interaction probabilities.  For example, as a photon traverses a given
 material, for each type of physical process that photons can undergo in that
 medium, a "length until interaction" is calculated, with a probability of
 occurrence given by the exponential distribution whose average is determined by the cross section.  The process with the shortest "length until interaction"
 will be the process that takes place. A similar procedure determines the
 types and four momenta of the final particles produced by the interaction.
The original {\it Geant} release includes many of the common processes that
 occur in high-energy particle physics. Some interactions that are important 
in this analysis are not included in the standard Geant release these include 
photo-hadronic interactions and muon pair production. Art Snyder from SLAC has
 created a {\it Geant} plugin package called {\it Gelhad} that simulates the 
photoproduction of neutrons, protons, alpha particles, and pions. To simulate 
the production of muon pairs by photons, the electron-positron pair production package {\it Gpairg} was modified to convert a fraction of the electron-positron
 pairs into muon pairs. The fraction of electron positron pairs lost by this 
algorithm is on the order of (m$_{e}$/m$_{\mu}$)$^{2}$ = 2.34$\times$10$^{-5}$
, therefore the decrease in the electron positron background level is 
negligible. Seven virtual detector volumes named DET1 through DET7 are placed 
in the virtual GlueX environment in areas that are of the most interest. Each 
time a particle passes through any one of these virtual detectors the 
particle's position, momentum, energy and species are recorded in one of seven 
ntuples, one ntuple for each virtual detector. This system cuts down on the 
amount of disk space that a completed simulation uses, and allows for quicker 
access of data. The placement of the virtual detectors in the simulated GlueX 
environment is shown in Fig.~\ref{DetPlace}.
\begin{figure}
\includegraphics[width=10cm]{DetPlacement.eps}
\caption{\label{DetPlace}Figure showing the placement of the virtual
detectors in the simulated GlueX environment. Virtual detector placement
is the same for all three shielding options }
\end{figure}
The static properties of the GlueX 
experimental volume, which includes the design
parameters for the collimator system 
and the detector assembly, are stored in HDDS, an XML-based
geometry specification. 
In HDDS, the geometry of a virtual object is declared by
the user in an XML file by 
the use of XML tags. For example, the tag
\begin{verbatim}
         <box name="BOX1" X_Y_Z="10.0  10.0  10.0" material="Air"/>
\end{verbatim}
creates a box in Geant named BOX1 with dimensions 10.0 cm by 10.0 cm by 10.0 cm 
and is filled with air. The tag
\begin{verbatim}
         <composition name="CompBox" envelope="MBOX">
            <posXYZ volume="BOX1" X_Y_Z="0.0  0.0  0.0" />
         </composition>
\end{verbatim}
positions BOX1 at the origin of a mother volume MBOX and then declares the 
composite volume as CompBox. Each subsystem of GlueX has its own XML file 
description, and these are linked and converted into a FORTRAN file using a 
program, hdds-geant.

\section{Three Scenarios}
The basic collimator system itself is composed of two separate stages. When 
the gamma rays reach the first (primary) collimator, rays that left the source 
at large enough angles are absorbed on the collimator face while allowing 
gamma rays with low emission angles to pass through unscathed. The 
polarization of gamma rays created through Coherent Bremsstrahlung depend 
sensitively on the emission angle; therefore highly polarized gamma rays will 
pass through the collimator, while unpolarized gamma rays will be absorbed on 
the collimator face. To reduce the background further, a sweeping magnet with 
a strength of 0.4 T$\cdot$m is placed in front of the collimator along with a 
large iron absorber to stop charged particles. This process is repeated by a 
second stage in the collimator assembly. Simulations of three shielding 
scenarios were run; each incorporating the same basic two-collimator design. 
The shielding designs have been designated as heavy, moderate and light, to 
distinguish between their levels of shielding. The dimensions of the hall are 
the same as given in the most recent blueprints of HallD.

\subsection{Heavy Shielding Scenario}
The Heavy shielding option shown in Fig.~\ref{hspict}, utilizes sufficient 
shielding to stop essentially all particles produced by collimation. The 
collimator assembly is encased by a thick concrete shield that is, at its 
thinnest, 50 cm and at its thickest, 100 cm. The iron absorbers in front of 
the first and second sweeping magnets are 5.0 m and 3.0 m long, respectively. 
A lead wall with dimensions 5.5$\times$4.2$\times$.15m is placed in front of 
the entire collimator assembly for additional shielding. The dimensions of the 
heavy shielded collimator components and their placement relative to the 
detector assembly are shown in Table~\ref{HeavyP}.  

\subsection{Moderate Shielding Scenario}
The moderate shielding option shown in Fig.~\ref{mspict} is a variation of 
the heavy shielding scenario. The front of the concrete shield has been reduced 
to a thickness of 35 cm and the lead plate has been reduced in size to the 
dimensions 4.7$\times$2.9$\times$.15 m. To compensate for this loss in shielding
, two large concrete blocks are placed in front of the first and second 
absorbers. Both the absorbers themselves are shorter in this shielding 
version. The first absorber is now 2.0 m in length and the second absorber is 
now 1.0 m in length. The design parameters for the moderate shielding option 
are shown in Table~\ref{ModerateP}. 

\subsection{Light Shielding Scenario}
The light shielding option shown in Fig.~\ref{lspict} is a further 
reduction of the heavy shielding 
option. The lead plate is now 4.5$\times$2.7$\times$.15 m and is inside the 
concrete shield right in front of the second absorber. The first and second 
iron absorbers are 2.0 m and 1.0 m in length, respectively. Similar to the 
moderate shielding design, the thickness of the concrete shield is thinner 
than in the heavy shielding option, here the concrete in the front of the 
collimator cave is 35 cm. The design parameters for this scenario are shown in 
Table~\ref{LightP}.

\section{Results}
Simulations of each of these designs were run starting with 30 million 
original photons produced through a coherent bremsstrahlung generator, this 
corresponds to 7.0 ms of run time at 1.0 $\mu$A. The simulation was used to 
measure the flux as a function of the radial distance from the beam line for 
photons, electron-positron pairs, neutrons, and muon pairs at DET7. The 
charged particle tracker, shown in Fig.~\ref{GlueDet}, is sensitive to low 
energy particles, therefore, an analysis of the background levels of electrons 
and muons will be an important factor in determining the best shielding option. 
Only particles with a radial distance greater than 5 cm were included in this 
analysis since photons at large angles are the ones that the barrel 
calorimeter must be shielded from and because it is important to know how much 
background is reaching the particle tracker, which has an inner radius of 5 cm 
centered on the beamline. The fraction of counts made by the tracker due to 
background is given by (1-e$^{-B\cdot \delta t}$), where B is the average 
number of background particles entering the tracker per unit time and 
$\delta$t is the shutter time of the tracker, which is roughly equal to 
150 ns. After several simulations, it was discovered that the background could 
be significantly reduced with the placement of a vacuum pipe that extends from 
the face of the second sweeping magnet to the face of the GlueX detector 
assembly. The vacuum pipe in each simulation was made of an iron tube 3 mm 
thick and was closed at both ends by a plastic Mylar cap that was 0.5 mm 
thick. Study of different arrangements of the collimator components showed 
that the background flux depends on the separation between the primary and 
secondary collimators. This is caused by the decrease in the distance between 
the secondary collimator and the detector assembly, which increases the 
intensity of the particle shower since the intensity decreases with increasing 
distance from its source. Analysis of the background flux lead to the 
conclusion that much of the background is emitted at small angles, therefore 
much emphasis was put on shielding the front of the collimator cave.  

\subsection{Heavy Shielding}    
Figure~\ref{HeavyG} and Table~\ref{HBGRates} show the results of the heavy shielding option. The heavy shielding scenario had the lowest total flux of all 
the shielding options. The rate of secondary particles entering the 
tracker was approximately ten thousand times greater than the rate of cosmic 
rays entering the tracker. The muon flux was very small relative to the 
electron-positron flux and was not a significant component of the total 
background reaching the tracker. The flux as a function of radius decreases 
quickly as the radial distance increases. This indicates that there is 
sufficient shielding in the front of the collimator cave.   

\subsection{Moderate Shielding} 
The background flux in the moderate shielding option, as shown in 
Fig.~\ref{ModerateG} and Table~\ref{MBGRates}, was very close to the 
levels in the heavy shielding option. There were only slight 
increases from the heavy shielding option in the background levels of the 
electrons, photons and muons and a decrease in the neutron flux. The decrease 
in neutron flux is a result of the two concrete blocks in front of the iron 
absorbers, which absorb excess neutrons. From the background rates at the 
tracker given in Tables 4 and 5, it is clear that the moderate shielding 
option is better at shielding the tracker than the heavy shielding option, 
which is again a result of the two concrete blocks.   

\subsection{Light Shielding}    
As indicated in Fig.~\ref{LightG} and Table~\ref{LBGRates}, there was a big 
increase in the flux levels for each particle type. Some particle fluxes 
increased by two orders of magnitude. This significant increase in flux is not 
only the result of a reduction in the shielding but also the increased 
distance between the primary and secondary collimators. The decrease in the 
flux as a function of distance is considerably slower in this scenario, which 
indicates a lack of shielding in the front of the collimator cave.        

\section{Conclusion}
These simulated collimators do not take into account all aspects of a real 
active collimator system. The simulations do not take into account an access 
tunnel into the collimator housing, which could change the background levels 
upstream. However, an access tunnel would most likely be placed in the center 
of the collimator housing where only particles with large emission angles 
would escape, therefore no noticeable increase in the background flux would be 
seen at the detectors. Other components of the collimator system that were 
left out of the simulation were fixtures to hold the collimator components in 
place, and a flux monitor inside the primary collimator to help position the 
collimator on the beamline. These components were seen as irrelevant to this 
study since little of the initial gamma ray beam flux would be absorbed by 
these components. The photohadronic interaction generator {\it Gelhad}, which 
was used in this simulation, should be checked against other hadronic 
generators to confirm {\it Gelhad's} reliability. Muon-nuclear interactions 
were not included in this study, since it is a negligible second order 
effect. The electromagnetic generator in {\it Geant} is well tested and has 
proven itself reliable. From this study, the light shielding option does not 
have enough shielding to stop a large amount of the background produced in the 
collimator. Therefore, the light shielding scenario would not be the most 
desirable option. The differences in the flux between the heavy and moderate 
shielding scenarios were negligible. The heavy shielding option, although 
having the lowest total flux, did not shield the tracker as effectivly as the 
moderate shielding option could. The moderate shielding scenario would be more 
cost effective and easier to manufacture than the heavy shielding option and 
this coupled with low background levels at the detector assembly make the 
moderate shielding option the better of the three scenarios.




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\begin{figure}
\includegraphics[width=10cm]{hspicture.eps}
\caption{\label{hspict}Diagram of the heavy shielded collimator}
\end{figure}
%\end{turnpage}

\begin{table}%[H] add [H] placement to break table across pages
\caption{\label{HeavyP}Dimensions of the heavy shielded collimator components.}
\begin{ruledtabular}
\begin{tabular}{cccccc}
Component & Material & Width & Height & Length & Distance from detector \\
(shape: cubic) &     & (cm)  & (cm)   & (cm)   & assembly (cm)          \\ \hline
concrete housing       & concrete & 550 & 420 & 1200 & na    \\
inner collimator cave  & air      & 450 & 270 & 1050 & na    \\
first sweeping magnet  & iron     & 20  & 30  & 50   & 2070  \\
first absorber         & iron     & 50  & 50  & 500  & 1560  \\
second sweeping magnet & iron     & 20  & 30  & 50   & 1440  \\
second absorber        & iron     & 100 & 100 & 300  & 1130  \\
lead wall              & lead     & 550 & 420 & 15   & 980   \\
                       &          &          &           &      &     \\ \hline
Component  & Material  & r$_{in}$ & r$_{out}$ & Length & Distance from detector \\
(shape: cylinderical)  &          & (cm) & (cm) & (cm) & assembly (cm) \\ \hline
primary collimator     & tungsten & 0.25 & 10  & 30   & 2130 \\
secondary collimator   & nickel   & 0.50 & 10  & 50   & 1500 \\
vacuum pipe            & iron     & 2.2  & 2.5 & 1488 & na   \\      

\end{tabular}
\end{ruledtabular}
\end{table}

\begin{figure}
\includegraphics[width=10cm]{mspicture.eps}
\caption{\label{mspict}Diagram of the moderate shielded collimator}
\end{figure}

\begin{table}%[H] add [H] placement to break table across pages
\caption{\label{ModerateP}Dimensions of the moderate shielded collimator 
components.}
\begin{ruledtabular}
\begin{tabular}{ccccccccccc}
Component & Material & Width & Height & Length & Distance from detector \\ 
(shape: cubic)&      & (cm)  & (cm)   & (cm)   & assembly (cm) \\ \hline

concrete housing       & concrete & 550 & 420 & 1200 & na    \\
inner collimator cave  & air      & 450 & 270 & 1150 & na    \\
first sweeping magnet  & iron     & 20  & 30  & 50   & 2020  \\ 
first absorber         & iron     & 50  & 50  & 200  & 1810   \\
first concrete block   & concrete & 200 & 200 & 100  & 1670   \\
second sweeping magnet & iron     & 20  & 30  & 50   & 1390   \\
second absorber        & iron     & 100 & 100 & 100  & 1280   \\
second concrete block  & concrete & 430 & 250 & 100  & 1140   \\
lead wall              & lead     & 550 & 420 & 15   & 980    \\
                       &          &     &     &      &       \\ \hline
Component  & Material  & r$_{in}$ & r$_{out}$ & Length & Distance from detector \\
(shape: cylinderical)  &          & (cm) & (cm) & (cm) & assembly (cm) \\ 
\hline
primary collimator     & tungsten & 0.25 & 10  & 30   & 2080 \\
secondary collimator   & nickel   & 0.50 & 10  & 50   & 1500 \\
vacuum pipe            & iron     & 2.2  & 2.5 & 1438 & na   \\     

\end{tabular}
\end{ruledtabular}
\end{table}

\begin{figure}
\includegraphics[width=10cm]{lspicture.eps}
\caption{\label{lspict}Diagram of the light shielded collimator}
\end{figure}

\begin{table}%[H] add [H] placement to break table across pages
\caption{\label{LightP}Dimensions of the light shielded collimator components.}
\begin{ruledtabular}
\begin{tabular}{ccccccccccc}
Component & Material & Width & Height & Length & Distance from detector \\ 
(shape: cubic)&      & (cm)  & (cm)   & (cm)   & assembly (cm) \\ \hline
concrete housing       & concrete & 550 & 420 & 1200 & na   \\   
inner collimator cave  & air      & 450 & 270 & 1150 & na   \\ 
first sweeping magnet  & iron     & 20  & 30  & 50   & 2020 \\    
first absorber         & iron     & 50  & 50  & 200  & 1810  \\ 
second sweeping magnet & iron     & 20  & 30  & 50   & 1155  \\ 
second absorber        & iron     & 100 & 100 & 100  & 1045   \\ 
lead wall              & lead     & 450 & 270 & 15   & 1030   \\ 
                       &          &     &     &      &       \\ \hline
Component  & Material  & r$_{in}$ & r$_{out}$ & Length & Distance from detector \\
(shape: cylinderical)  &          & (cm) & (cm) & (cm) & assembly (cm) \\ 
\hline
primary collimator     & tungsten & 0.25 & 10  & 30   & 2080 \\ 
secondary collimator   & nickel   & 0.50 & 10  & 50   & 1215  \\
vacuum pipe            & iron     & 2.2  & 2.5 & 1203 & na   \\    

\end{tabular}
\end{ruledtabular}
\end{table}

\begin{figure}
\includegraphics[width=10cm]{detpicture1.eps}
\caption{\label{GlueDet}Diagram of the GlueX detector assembly}
\end{figure}

\begin{figure}
\includegraphics[width=10cm]{HeavyGraphs.eps}
\caption{\label{HeavyG}Graphs of the flux near the detector assembly for 
various particles with the heavy shielding option}
\end{figure}

\begin{table}%[H] add [H] placement to break table across pages
\caption{\label{HBGRates}Background rates for the heavy shielding option.}
\begin{ruledtabular}
\begin{tabular}{cccc}
particle type & background rate & background occupation & total 
background \\
    & in barrel DC (s$^{-1}$) & fraction in barrel DC & through DET7 (s$^{-1}$) \\ \hline 
photons & (9.52$\pm$0.12)$\times$10$^{5}$ & 0.133$\pm$0.002 & (1.05$\pm$0.003)$\times$10$^{7}$ \\ 
e$^{-}$ e$^{+}$ & (8.03$\pm$0.34)$\times$10$^{4}$ & 
(1.20$\pm$0.05)$\times$10$^{-2}$ & (5.02$\pm$0.09)$\times$10$^{5}$ \\
neutrons & (1.19$\pm$0.13)$\times$10$^{4}$ & (2.0$\pm$0.2)$\times$10$^{-3}$ & 
(8.60$\pm$0.35)$\times$10$^{4}$ \\
$\mu^{-}$ $\mu^{+}$ & (2.60$\pm$0.61)$\times$10$^{3}$ & 
(4.0$\pm$0.9)$\times$10$^{-4}$ & (1.33$\pm$0.14)$\times$10$^{4}$ \\

total & (1.05$\pm$0.01)$\times$10$^{6}$ & (0.145$\pm$0.002) & (1.11$\pm$0.004)$\times$10$^{7}$ \\  

\end{tabular}
\end{ruledtabular}
\end{table}

\begin{figure}
\includegraphics[width=10cm]{ModerateGraphs.eps}
\caption{\label{ModerateG}Graphs of flux near the detector assembly for 
various particles with the moderate shielding option}
\end{figure}

\begin{table}%[H] add [H] placement to break table across pages
\caption{\label{MBGRates}Background rates for the moderate shielding option.}
\begin{ruledtabular}
\begin{tabular}{cccc}
particle type & background rate & background occupation & total 
background \\
    & in barrel DC (s$^{-1}$) & fraction in barrel DC & through DET7 (s$^{-1}$) \\ \hline 
photons & (9.07$\pm$0.11)$\times$10$^{5}$ & 0.127$\pm$0.002 & 
(1.09$\pm$0.004)$\times$10$^{7}$ \\
e$^{-}$ e$^{+}$ & (7.30$\pm$0.32)$\times$10$^{4}$ & 
(1.1$\pm$0.05)$\times$10$^{-2}$ & (5.91$\pm$0.09)$\times$10$^{5}$ \\
neutrons & (9.00$\pm$1.13)$\times$10$^{3}$ & (1.35$\pm$0.17)$\times$10$^{-3}$ & 
(6.70$\pm$0.31)$\times$10$^{4}$ \\
$\mu^{-}$ $\mu^{+}$ & (4.43$\pm$0.80)$\times$10$^{3}$ & (6.64$\pm$1.19)$\times$10$^{-4}$ & 
(1.53$\pm$0.15)$\times$10$^{4}$ \\
total & (9.93$\pm$0.12)$\times$10$^{5}$ & 0.138$\pm$0.002 & (1.16$\pm$0.004)$\times$10$^{7}$ \\  

\end{tabular}
\end{ruledtabular}
\end{table}

\begin{figure}
\includegraphics[width=10cm]{LightGraphs.eps}
\caption{\label{LightG}Graphs of flux near the detector assembly for various particles with the light shielding option}
\end{figure}

\begin{table}%[H] add [H] placement to break table across pages
\caption{\label{LBGRates}Background rates for the light shielding option.}
\begin{ruledtabular}
\begin{tabular}{cccc}
particle type & background rate & background occupation & total 
background \\
    & in barrel DC (s$^{-1}$) & fraction in barrel DC & through DET7 (s$^{-1}$) \\ \hline
photons & (1.47$\pm$0.005)$\times$10$^{7}$ & 0.890$\pm$0.003 & 
(3.15$\pm$0.002)$\times$10$^{8}$ \\
e$^{-}$ e$^{+}$ & (8.45$\pm$0.11)$\times$10$^{5}$ & 0.119$\pm$0.002 & 
(1.93$\pm$0.005)$\times$10$^{7}$ \\ 
neutrons & (1.14$\pm$0.13)$\times$10$^{4}$ & (1.71$\pm$0.19)$\times$10$^{-3}$ &
 (1.01$\pm$0.04)$\times$10$^{5}$ \\
$\mu^{-}$ $\mu^{+}$ & (4.57$\pm$0.81)$\times$10$^{3}$ & 
(7.00$\pm$1.24)$\times$10$^{-4}$ & (2.60$\pm$0.19)$\times$10$^{4}$ \\
total & (1.56$\pm$0.005)$\times$10$^{7}$ & (0.903$\pm$0.003) & 
(3.34$\pm$0.002)$\times$10$^{8}$ \\  

\end{tabular}
\end{ruledtabular}
\end{table}

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% \end{turnpage}

% tables should appear as floats within the text
%
% Here is an example of the general form of a table:
% Fill in the caption in the braces of the \caption{} command. Put the label
% that you will use with \ref{} command in the braces of the \label{} command.
% Insert the column specifiers (l, r, c, d, etc.) in the empty braces of the
% \begin{tabular}{} command.
% The ruledtabular enviroment adds doubled rules to table and sets a
% reasonable default table settings.
% Use the table* environment to get a full-width table in two-column
% Add \usepackage{longtable} and the longtable (or longtable*}
% environment for nicely formatted long tables. Or use the the [H]
% placement option to break a long table (with less control than 
% in longtable).


% Surround table environment with turnpage environment for landscape
% table
% \begin{turnpage}
% \begin{table}
% \caption{\label{}}
% \begin{ruledtabular}
% \begin{tabular}{}
% \end{tabular}
% \end{ruledtabular}
% \end{table}
% \end{turnpage}

% Specify following sections are appendices. Use \appendix* if there
% only one appendix.
%\appendix
%\section{}

% If you have acknowledgments, this puts in the proper section head.
%\begin{acknowledgments}
% put your acknowledgments here.
%\end{acknowledgments}

% Create the reference section using BibTeX:
\bibliography{basename of .bib file}

\end{document}
%
% ****** End of file template.aps ******


