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\begin{document}

\begin{frontmatter}

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%% \title{Performance of the $PWO$ calorimeter prototype for experiments at Jefferson Lab  \tnoteref{notice} }

% \title{PrimEx $\eta$  electromagnetic calorimeter, prototype for experiments at Jefferson Lab  \tnoteref{notice} }

\title{Electromagnetic calorimeters based on the scintillating lead tungstate crystals for experiments at Jefferson Lab  \tnoteref{notice} }

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\author[erphy]{A.Asaturyan}
\author[jlab]{F.Barbosa}
\author[cua]{V.Berdnikov}
\author[ncat]{J.Crafts}
\author[jlab]{H.Egiyan}
\author[uncw]{L.Gan}
\author[ncat]{A.Gasparian}
\author[cua]{T.Horn}
\author[erphy]{H.Mkrtchyan}
\author[regina]{Z.Papandreou}
\author[jlab]{V. Popov}
\author[jlab]{S.Taylor}
\author[jlab]{N.Sandoval}
\author[jlab]{A.Somov\corref{cor1}}
\ead{somov@jlab.org}
\author[mephi]{S.Somov}
\author[duke]{A. Smith}
\author[jlab]{C. Stanislav}
\author[erphy]{H. Voskanyan}
\author[jlab]{T. Whitlatch}
\author[jlab]{S. Worthington}
\address[erphy]{A. I. Alikhanian National Science Laboratory (Yerevan Physics Institute), 0036 Yerevan, Armenia}
\address[cua]{Catholic University of America, Washington, DC 20064, USA}
\address[jlab]{Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA}
\address[mephi]{National Research Nuclear University MEPhI, Moscow, Russia}
\address[regina]{University of Regina, Regina, Saskatchewan, Canada S4S 0A2}
\address[uncw]{University of North Carolina at Wilmington, Wilmington, NC 28403, USA}
\address[ncat]{North Carolina $A\&T$  State University, Greensboro, NC 27411, USA}
\address[duke]{Duke  University,  Durham,  North  Carolina  27708,  USA}
\tnotetext[notice]{Notice: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. 
The U.S. Government retains a non-exclusive,  paid-up, irrevocable, world-wide license to publish or reproduce this 
manuscript for U.S. Government purposes.}
\cortext[cor1]{Corresponding author. Tel.: +1 757 269 5553;  fax: +1 757 269 6331.}

\begin{abstract}


A new electromagnetic calorimeter consisting of 140  lead tungstate ($\rm PbWO_{\rm 4}$)  scintillating 
crystals was constructed for the  PrimEx $\eta$ experiment at Jefferson lab. The calorimeter was 
integrated to the DAQ and trigger system of the GlueX detector and used in the experiment 
to reconstruct  Compton events. The experiment started collecting data in the Spring of 2019 
and acquired about $30 \%$ of the required statistics. The calorimeter is a large-scale prototype 
of the two detectors, which are currently constructed in Jefferson Lab using similar type of 
crystals: the Neutral Particle Spectrometer (NPS) and the lead tungstate insert of
the forward calorimeter (FCAL) of the GlueX detector. The article presents the design and performance 
of the Compton calorimeter and describes the FCAL and NPS projects.



%The article presents the design and performance of the electromagnetic calorimeter 
%constructed for the PrimEx $\eta$ experiment at Jefferson lab.  The calorimeter was 
%integrated to the DAQ and trigger system of the GlueX detector and used in the experiment 
%to reconstruct  Compton events. The calorimeter 
%consists of 140 lead tungstate ($\rm PbWO_{\rm 4}$)  scintillating crystals produced 
%by Shanghai Institute of Ceramics. The calorimeter is a large-scale prototype of the two 
%detectors, which are being currently constructed in the laboratory using similar type of 
%crystals: the Neutral Particle Spectrometer (NPS) and the lead tungstate insert of
%the Forward Calorimeter (FCAL) of the GlueX detector. We will give an overview of the status 
%of these projects.


\end{abstract}

\begin{keyword}
Electromagnetic calorimeters, lead-tungstate crystals
\end{keyword}
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%% PACS codes here, in the form: \PACS code \sep code

%% MSC codes here, in the form: \MSC code \sep code
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\end{frontmatter}

\linenumbers

%% main text
\section{Introduction}

Electromagnetic calorimeters based on the  $\rm PbWO_{\rm 4}$ crystals have a widespread application in experiments 
in different accelerator facilities such as CERN, GSI, and Jefferson Lab. Electromagnetic showers produced in heavy 
lead tungstate $\rm PbWO_{\rm 4}$ scintillator crystals with the radiation length of $L_{\rm R} = 0.89\;\rm{cm}$ and 
Molier radius of $R_{\rm m} = 2.19\;\rm{cm}$ have a compact size and provide good separation of electromagnetic 
showers and resolution of reconstructed energies.

%Electromagnetic showers produced in heavy lead tungstate $\rm PbWO_{\rm 4}$ scintillator crystals with the radiation 
%length of $L_{\rm R} = 0.89\;\rm{cm}$ and Molier radius of $R_{\rm m} = 2.19\;\rm{cm}$ have a compact size, which provides 
%high presision in measurement of the energy resolution and good shower separation in electromagnetic calorimeters.
%The weak pont of this scintillators is the strong dependence of the light yield on the temperature, that requires 
%the temperature stabilization on the level of $\sim0.1^{\circ}$. Electromagnetic calorimeters based on the 
% $\rm PbWO_{\rm 4}$ crystals have a widespread application in experiments in different accelerator facilities 
%such as CERN, GSI, and Jefferson Lab.

Two electromagnetic calorimeters are currently built in the experimental Hall D and Hall C at Jefferson 
Lab using 2 cm x 2 cm x 20 cm  $\rm PbWO_{\rm 4}$  crystals. The inner part of the forward lead glass calorimeter 
of the GlueX detector in Hall D will be upgraded with the high-granularity, high-resolution crystals. This
upgrade is required by the physics program  with the GlueX detector, specifically the new experiment to study rare 
decays of $\eta$ mesons[]. The size of the insert will tentatively consist of 2496 lead tungstate modules. 
The neutral-particle spectrometer (NPS)[] in the Jefferson Lab experimental Hall C is the new calorimeter, which 
will allow to carry out experiments to study the transverse spatial and momentum structure of the nucleon. $\rm PbWO_{\rm 4}$ 
crystals will form an array of 30x36 modules.
% provided by two vendors: SICCAS and CRYTUR.



% One of the key detectors of the GlueX experiment at Jefferson Lab, which has started collecting data in 2016, 
% is the forward electromagnetic calorimeter (FCAL). The FCAL consists of 2800 lead glass modules and provides 
% the energy resolution of  $\sigma_{E}/E_{\gamma} = 6.2\%/ \sqrt E \oplus 4.7\%$. Physics program with the 
% GlueX detector, specifically the new experiment to study rare decays of $\eta$ mesons[] requires to improve 
% the energy resolution of the FCAL by about a factor of two and improve separation of electromagnetic showers 
% in the forward direction. The inner part of the forward calorimeter will be upgraded with the high-
% granularity, high-resolution $\rm PbWO_{\rm 4}$ crystals.

% The neutral-particle spectrometer (NPS) in the Jefferson Lab experimental Hall C offers unique scientific capabilities 
% to study the transverse spatial and momentum structure of the nucleon. The NPS consists of 1080  $\rm PbWO_{\rm 4}$ crystals, 
% which will form and an array of 30x36 modules. Crystals will be placed in the frame build from carbon plates and separated 
% from each other by a 0.5 mm-thick carbon layer to ensure good positioning. 

%Both calorimeters will be built using the same size crystals provided by two vendors: SICCAS (China) and CRYTUR (Czech republic). 
%Construction of these calorimeters depends on the performance of the small-size calorimeter prototype composed of 140 recently 
%produced scintillating $\rm PbWO_{\rm 4}$ crystals~\cite{pwo_crystals}, which was used in the PrimEx $\eta$ experiment. 

We built a  small-size calorimeter prototype composed of 140 crystals recently produced by Shanghai Institute of Ceramics (SICCAS).
This detector was used as the Compton calorimeter (CCAL) in the PrimEx $\eta$ experiment in the Spring of 2019. Fabrication and 
performance of the CCAL were critical to finalize  designs of electromagnetic calorimeters of the FCAL and NPS projects.

We will present the PrimEx $\eta$ experiment and the performance of the CCAL in Section 2 and Section 3. The brief description of 
the FCAL and NPS projects will be given in Section 4 and Section 5.  

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=0.95\linewidth,angle=0]{figures/Gluex_layout.png}
\end{center}
\caption{Schematic layout of the GlueX detector (not to scale). Numbers represent the following detector components: Solenoid magnet (1),  barrel calorimenter (2), 
central drift chambers (3), forward drift chambers (4), time-of-flight wall(5).}
\label{fig:gluex_det}
\end{figure}
%----------------------------------------------------------------------



% The Compton calorimeter is a prototype of the large-scale ($\rm PbWO_{\rm 4}$) calorimeter, which will be used to 
% upgrade the inner part of the GlueX FCAL, which is currently instrumented with lead glass modules. This upgrade is required by 
% the future physics program of Hall D, specifically the new experiment to study rare decays of $\eta$ mesons[]. Integrated to the GlueX 
% DAQ the CCAL performance was tested using the nominal GlueX running conditions. This allowed up to perform measurements of 
% realistic rates and PMT anode currents in the FCAL insert region. The measurements will be used to tune the design of the 
% front end electronics.


% The Compton calorimeter was constructed in cooperation with the Jefferson Lab group working on the Neutral Particle Spectrometer (NPS), 
% currently constructed in the experimental Hall C at Jefferson Lab. The NPS will use lead tungstate crystals with the same size provided by 
% two vendors SICCAS and CRYTUR from Czech republic.  The spectrometer will be equipped with the same photodetectors, Hamamatsu PMT4125, and 
% read out electronics. 

% In Section 2, we will present the design and performance of the Compton calorimeter during PrimEx $\eta$ run. Status of the 
% FCAL upgrade project will be described in Section 3. Specifications of the Neutral Particle Spectrometer will be discussed
% in Section 4.


% the GlueX detector was instrumented with a small (24 cm x  24 cm) 
% lead tungstate ($PbWO_4$) calorimeter, positioned downstream of the GlueX forward calorimeter 

% An electromagnetic calorimeter consisting of an array of 12 x 12 lead tungstate ($\rm PbWO_{\rm 4}$) 
% scintillating crystals, was fabricated and used in the PrimEx $\eta$  experiment in Hall D of Jefferson Lab. 
% electrons and photons at small angles 
% originating from Compton events, the GlueX detector was instrumented with a small (24 cm x  24 cm) 
% lead tungstate ($PbWO_4$) calorimeter, positioned downstream of the GlueX forward calorimeter (FCAL). 


% in order to detect the Compton scattering photonand recoiled electron in two calorimeters (FCAL and CCAL) simultaneously 
% to check thesetup stability, monitor the luminosity and FCAL detectionefficiency, and verify the overallsystematic errors in 
% absolute cross section measurement. 






%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_assembled.jpg}
\includegraphics[width=0.8\linewidth,angle=0]{figures/ccal_module.png}
\end{center}
\caption{Calorimeter module.}
\label{fig:ccal_module}
\end{figure}


%----------------------------------------------------------------------


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_assembled.jpg}
\end{center}
\caption{Schematic layout of the Compton calorimeter.}
\label{fig:ccal_design}
\end{figure}
%----------------------------------------------------------------------

\section{PrimEx $\eta$ experiment with the GlueX detector}

The GlueX detector in experimental Hall D was designed to perform experiments using a photon beam.
Beam photons are produced by electrons, provided by the JLab electron accelerator facility, incident on a 
thin radiator via the bremsstrahlung process. Energy of a beam photon is determined by detecting 
a scattered bremsstrahlung electron using tagging scintillator detectors with a typical precision 
of $0.3\%$. 

The PrimEx $\eta$ experiment started collecting data in the Spring of 2019 and has acquired about $30 \%$ of 
the required statistics. The physics goal of the experiment is  to perform a precision measurement of 
the $\eta \to \gamma \gamma$ decay width. The measurement will provide an important test of QCD symmetries 
and is essential for the determination of fundamental  properties such as the ratios of the light quark 
masses and the  $\eta$-$\eta^\prime$ mixing angle. The decay width will be extracted from the measurement 
of the production cross section of $\eta$ mesons in the Coulomb field of a nucleus by photons, which is 
known as the Primakoff effect. Photons originating from the  $\eta$ decays are detected in the forward 
calorimeter of the GlueX detector.

The cross section will be normalized using the Compton process, which will also be used to monitor the 
luminosity and control the detector stability during the run. Electrons and photon originating from Compton 
events in the target are produced at small angles outside the acceptance of the FCAL. In order to improve 
reconstruction of particles in the forward direction, we we built a small Compton calorimeter consisting of an array of 
12 x 12 lead tungstate scintillating crystals (24 cm x  24 cm) and  positioned it about 6 m downstream the FCAL.
The CCAL covers the angular range between $0.18^{\circ}$ and  $0.33^{\circ}$.

% calorimeter referred to as the Compton calorimeter (CCAL) 

\section{Compton calorimeter of the PrimEx $\eta$ experiment}
\label{sec_ccal}
% The PrimEx $\eta$ experiment in Hall D at Jefferson Lab started collecting data in the Spring of 2019 using a He target 
% and has acquired about $30 \%$ of the required statistics. The physics goal of the PrimEx $\eta$ experiment in Hall D 
% at Jefferson Lab is  to perform a precision measurement of the $\eta \to \gamma \gamma$ decay width. The measurement 
% will provide an important test of QCD symmetries and is essential for the determination of fundamental  properties 
% such as the ratios of the light quark masses and the  $\eta$-$\eta^\prime$ mixing angle. The decay width will be extracted 
% from the measurement of the production cross section of $\eta$ mesons in the Coulomb field of a nucleus by photons, 
% which is known as the Primakoff effect. $\eta$ mesons are produced in a liquid helium target by a tagged photon beam 
% with the energy between $10.5 - 11.7\;{\rm GeV}$ and is reconstructed by detecting two decay photons in the forward 
% calorimeter of the GlueX detector~\cite{gluex_det}. 

% The cross section will be normalized using the Compton process, which will 
% also be used to monitor the luminosity and control the detector stability during the run. In order to reconstruct the 
% Compton scattering photon and recoiled electron at small angles  we built a small  (24 cm x  24 cm) calorimeter referred 
% to as the Compton calorimeter (CCAL)  and  positioned it 6 m downstream the beam of the GlueX forward calorimeter. The 
% CCAL consists of an array of 12 x 12 lead tungstate ($\rm PbWO_{\rm 4}$) scintillating crystals, which have been recently 
% produced by  Shanghai Institute of Ceramics.

% The main purpose  of the Compton calorimeter is to provide a fast trigger and perform reconstruction of Compton events.
% which is required in the experiment 
% to normalize cross sections, monitor luminosity, and control detector stability 
% during the run. 
% The CCAL is positioned behind the GlueX forward calorimeter, about 12 m from the target, and covers the angular range 
% between $0.18^{\circ}$ and  $0.33^{\circ}$. Electrons and photon originating from Compton events in the target are produced 
% at small angles and are predominantly detected by the CCAL and FCAL, respectively. Schematic view of the GlueX detector 
% and the Compton calorimeter is illustrated in Fig.~\ref{fig:ccal_design}.
 
The detector was operated without the magnetic field at a relatively small luminosity.

\subsection{Calorimeter design}

The calorimeter design is shown in Fig.~\ref{fig:ccal_design}. The CCAL comprises an array of 12 x 12 
lead tungstate modules with a 2 x 2 hole in the middle for the photon beam, which are positioned inside 
the light tight box. A Tungsten absorber is placed in front of the innermost layer closest to the beamline, which 
is exposed to the high rate of particles originating from electromagnetic background.
 
The light yield from $\rm PbWO_{\rm 4}$ crystals depends on the temperature with the typical temperature 
coefficient of $2\% / ^\circ C$ at room temperature. Maintaining constant temperature is essential 
for the calorimeter operation. Calorimeter modules are surrounded by four copper plates with  built 
in pipes to circulate the cool liquid and provide temperature stabilization. An insulator was used 
around the detector box. The temperature was monitored and recorded during the experiment by four thermocouples 
attached to different points of the module assembly. During the experiment temperature was maintained at 
$17\pm 0.2^\circ C$. In order to prevent condensation, the nitrogen purge was applied. Two fans with 
the water-based cooling system were installed on the top of the crystal assembly to improve nitrogen 
circulation and heat  dissipation from PMT dividers. The detector was position on the movable 
platform, which provides motion in the vertical and horizontal directions perpendicular to the
beam. During detector calibration, each module was moved to the beam. 

\subsection{Module design}
Design of the ${\rm PbWO}_4$ module is based on the HyCal calorimeter, which was
used in several experiments in Hall B []. Assembled calorimeter module 
is presented in Fig.~\ref{fig:ccal_module}. The lead tungstate crystal is wrapped with a 
 $60\;{\rm \mu m}$ polymer Enhanced Specular Reflector film (ESR) manufactured by ${\rm 3M}^{TM}$, which 
allows to achieve $98.5\%$ reflectivity across the visible spectrum. In order to improve 
optical isolation of each module from its neighbors, each crystal was wrapped with a 
$25\;{\rm \mu m}$ thick Tedlar. 
The crystal is attached to the PMT housing which is made from G10 fiberglass. Two flanges are 
positioned at the crystal and housing ends and are
connected together using $25\;{\mu m}$ brass straps, which are brazed to the sides of the flanges. 
Four set screws are applied to the PMT housing flange to generate the tension in the straps 
and hold the assembly together. Light from the crystal is detected using a ten-stage Hamamatsu 
PMT 4125, which is inserted  to the housing and is coupled to the crystal using an optical grease. 
The PMT diameter is 19 mm. 
The PMT is pushed towards the crystal by using a G10 retaining plate attached to the back of 
the PMT and four tension screws applied to the PMT flange. The PMT is instrumented with the 
high voltage divider and amplifier positioned on the same printed circuit board, which 
is attached to the PMT socket.

%The PMT is instrumented with an active 
%base, which was designed for the Hall C lead tungstate calorimeter (NPS)~\cite{popov}. The base 
%combines a voltage divider and an amplifier powered by the current flowing through the divider. 
%Both the PMT divider and amplifier are positioned on the same printed circuit board, which 
%is attached to the PMT socket.





 

\subsection{Electronics}
The PMT of each calorimeter module is equipped with the active base prototype~\cite{popov},  
which was designed for the lead tungstate calorimeter of the Neutral-Particle Spectrometer 
(NPS) in the Jefferson Lab experimental Hall C. The base combines a voltage divider and an 
amplifier powered by the current flowing through the 
divider. The active base allows to operate the PMT at smaller voltage and consequently at lower 
anode current and improves the detector rate capability. Operation of the PMT at smaller anode 
current is also important for the extension of the photomultiplier tube life. The active base 
circuit contains 5 bipolar transistors, three in the amplifier circuit and two  on the last two dynodes 
of the voltage divider, which provide gain stabilization at high rate. Active bases from the 
NPS detector have a relatively large amplification of about a factor of 24 due to the large
PMT count rate predicted by Monte Carlo simulation. During PrimEx run, the CCAL was operated
at the HV of about 680 V and the divider current of $260\;\mu A$.


 Amplified PMT signals are digitized using a twelve-bit multi-channel  flash ADCs 
operated at a sampling rate of 250 MHz~\cite{fadc250}. The flash ADCs are positioned in the 
VXS crate. An example of 
the flash ADC signal pulse obtained from a calorimeter module is shown in Fig.~\ref{fig:pmt_pulse}. 
The calorimeter was integrated to the trigger system. 
The trigger is based on the energy deposition in the Compton and Forward calorimeters.



\subsection{Light Monitoring System}
To monitor performance of each calorimeter channel, we designed and installed an LED based light monitoring system (LMS). 
The LMS optics includes a blue LED, spherical lens to correct the conical dispersion of the LED, and a diffusion grating 
to homogenize the light. Light was incident on a bundle of plastic optical fibers (Edmund Optics)  with the core diameter 
of $250\;{\rm \mu m}$. Each fiber distributes light to the individual calorimeter module. On the crystal end, the fiber is 
attached to the module using a small acrylic cap glued to the crystal with a hole drilled through each cap to hold the fiber 
inside. 

To monitor stability of the LED, we use two reference Hamamatsu 4125 PMTs. Each PMT has a single fiber from the LED attached to their front face 
as well as one of the YAP:Ce scintillator sources. The PMTs were read out using flash ADC. HV on each PMT was adjusted in such a way to 
make signals from both the LED and the $\alpha$ source fit to the flash ADC range, as shown in Fig. 1. Each LED was driven by a CAEN 1495 module.

The LMS system was integrated to the GlueX trigger system and allowed to produce a special trigger type during data taking. 
The LMS system was extensively used during the detector commissioning and was running in parallel to the data production run injecting light to the 
detector with a typical frequency of 100 Hz. Stability of the LED system for the entire PrimEx run was measured to be better than 
$0.5\;\%$. The ratio of LED to $\alpha$-source signals for different run periods is presented in Fig.~\ref{fig:led_stability}. 
Typical LED amplitudes of calorimeter modules measured during the run are presented in Fig.~\ref{fig:led_amplitude}. The gain stability 
for most of crystals during 35 days of taking data is better than $5\%$.

%The only internal component that still needed to be installed at this point was the light monitoring system (LMS).  The LMS consisted of optics including an aspherical lens to correct the conical dispersion of the LED and a diffusion grating to homogenize the light (DG10-600 - Ø1").  These components along with the LED were housed in a one inch optical tube and then connected to a bundle of fiber optic cables. The fiber

%Figure 14.  LMS optical assembly with aspherical lens and diffusor installed in front of LED.
%bundle was hand built using one hundred and sixty one meter long fiber segments.  The fiber was a 250 µm plastic core design manufactured by Edmund Optics with stock #57-096.  Each was cut with a hot razor knife and had their cladding removed from the ending two inches.  They were then bundled together by hand, cemented, and polished on the bundle side end The other end of the fibers were attached to the individual modules of the detector using a small acrylic cap with a hole drilled through each to hold the fiber inside.  Each cap also had a small shroud of heat shrink material placed on each and tape patch was used to cover the front opening of the module to produce a light tight seal. Two reference PMTs were placed on a sheet of Delrin which was placed on the side of the detector away from the beamline when the detector was put into its retracted home position.  Each PMT had a single fiber from the LED attached to their front face as well as one of the YAP:Ce scintillator sources.  Both of the sources were cemented into place and their PMTs were wrapped in Tedlar to isolate them from any light leakage from outside.  The electronics for the LED were also placed on the same sheet along with the optics. 
       
%Figure 15.  Light monitoring system with fiber bundle installed and front end of detector with fiber terminations made and light sealed.


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=0.7\linewidth,angle=0]{figures/ccal_waveform.png}
\end{center}
\caption{Typical flash ADC signal waveform in the calorimeter module.}
\label{fig:pmt_pulse}
\end{figure}
%----------------------------------------------------------------------
%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/led_amplitudes.png}
\end{center}
\caption{Flash ADC signal amplitudes  induced by the LED and $\alpha$-source in the reference PMT.}
\label{fig:reference_amp}
\end{figure}
%----------------------------------------------------------------------


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/led_stability.jpg}
\end{center}
\caption{The ratio of LED to $\alpha$-source signals for different run periods.}
\label{fig:led_stability}
\end{figure}
%----------------------------------------------------------------------

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/led_module_stability.jpg}
\end{center}
\caption{Typical signal amplitudes in calorimeter modules induced by an LED for 
different PrimEx $\eta$ run periods. Amplitudes for each module are normalized to 
the beginning of the run.}
\label{fig:led_amplitude}
\end{figure}
%----------------------------------------------------------------------


\subsection{Calibration}

Energy calibration of the calorimeter was performed by moving each calorimeter module to 
the photon beam during special low-intensity calibration runs. The photon flux corresponded
to about AAA photons / sec in the energy range $E_{\gamma} > 1 {\rm GeV}$. Energy of each 
beam photons was determined using GlueX tagging detectors described in Section 1.  The typical 
energy resolution of the beam photon measured with tagger counters is about $0.2\%$. Flash ADC 
signal amplitudes in the  calorimeter module as a function of the beam energy is presented in 
Fig.~\ref{fig:ccal_calib}. We adjusted PMT high voltages on each module in order to set ADC amplitudes 
to about 3200 counts for 10 GeV photons and collected data sample for each calorimeter 
module positioned in the beam. Calibration was subsequently refined by constraining the reconstructed 
energy to the known beam energy determined by the tagger counter. CCAL energy in units of flash ADC counts 
induced by 10 GeV photons is shown in Fig.~\ref{fig:ccal_en_fit}. The distribution was fit 
to a Crystal Ball function.

We observed some non-linear performance on the level of of a few percents of the active 
base with the large amplification factor of 24. 
%Some non-linearity effects are 
%presented in Fig.~\ref{fig:ccal_en_res} for bases with different amplification factors. 
%After the PrimEx run, 
we studied the performance of the PMT active bases with different amplification factors.
We replaced the original front end electronics in the 3x3 cell calorimeter region with 
modified bases with the bypassed amplifier. Energy resolution measured in this region 
is shown in Fig.~\ref{fig:ccal_en_res}. 
The energy resolution was fit to the following function: 
%------------------------------------------------
\begin{equation}
\frac{\sigma_E}{E}  = \frac{S}{\sqrt E} \oplus \frac{N}{E} \oplus C,
\end{equation}
%------------------------------------------------
where $S$ represents the stochastic term, $N$ the noise and $C$ the constant term,  E is the energy in GeV, and  
the symbol $\oplus$ indicates a quadratic sum. The fit yields: $S = 2.63 \pm 0.06\%$,  $N = 1.95  \pm 0.2\%$, and 
$S = 0.41  \pm 0.03\%$. The resolution was found to be about $10\%$ better than that measured with the original base (gain 24).
The energy resolution is similar to that of the HyCal calorimeter~\cite{hycal_kubantsev}, which was instrumented 
with the same type of crystals (produced by SICCAS) and used in several experiments in 
the Jefferson Lab's experimental Hall B. 
 




%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_tagger.jpg}
\end{center}
\caption{CCAL signal pulse amplitude as a function of the beam energy.}
\label{fig:ccal_calib}
\end{figure}
%----------------------------------------------------------------------

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/crystal_fit_53.png}
\end{center}
\caption{Measured energy in units of flash ADC counts produced by 10 GeV beam photons. The spectrum is fit 
with a Crystal Ball function.}
\label{fig:ccal_en_fit}
\end{figure}
%----------------------------------------------------------------------


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_en_res_new.png}
\end{center}
\caption{Energy resolution measured in the 3x3 cell region as a function of the photon 
energy.}
\label{fig:ccal_en_res}
\end{figure}
%----------------------------------------------------------------------



  
%The hodoscope prototype was positioned in the tagger area of the experimental Hall B at Jefferson Lab~\cite{hallb_bl} 
%and was operated concurrently with the CLAS detector. The CLAS used a secondary beam of photons produced in 
%the tagger area by the Jefferson Lab electron beam in a thin radiator via the bremsstrahlung process. A beam 
%electron that radiates a photon is deflected in the dipole tagger magnet and sent to the floor of the experimental hall. 
%The prototype was placed inside a light-tight box and positioned on the floor of the tagger area. The hodoscope  was 
%oriented in such a way that it faced normal to the path of electrons, i.e.,  electrons travel 1 cm through the tile. 
%A schematic view of the tagger area and the location of the prototype is shown in Fig.~\ref{fig:hallb_tagger}. 
%Dashed curves on this plot denote trajectories of tagged electrons with the fractional energy $k/E_{\rm 0}$,
%where $k$ is the tagged electron energy and $E_0$ is the electron beam energy.
%Data was taken with an electron beam energy of 2.54 GeV, corresponding to a tagged electron energy of about 1.4 GeV

\subsection{Performance during PrimEx run}

The PrimEx $\eta$ experiment was taking data using about a factor of 
5 smaller flux of beam photons incident on the target compared to 
other GlueX experiment, which corresponds  to about 
$7\cdot 10^6\;\gamma/{\rm sec}$ in the energy range of interest 
between 9.5 GeV  and 11.6 GeV.    

The CCAL  was integrated to the GlueX DAQ and trigger systems. 
The physics trigger of the experiment was based on the total energy deposited 
in the forward and the Compton calorimeter. The trigger was implemented 
on a special-purpose programmable electronics modules with the Field-Programmable Gate Array (FPGA) chips. 
The trigger architecture is described in Ref.~\cite{l1_trigger}.
 

\subsubsection{Compton reconstruction}

\section{Upgrade of the GlueX forward calorimeter}

The forward calorimeter of the GlueX detector is positioned 6 m downstream the GlueX target, 
and consists of 2800 lead glass modules, with a size of 4 cm x 4 cm x 45 cm. The typical energy resolution 
of the FCAL is $\sigma_E/E = 6.2\%/ \sqrt E \oplus 4.7\%$. The calorimeter has been used in several 
GlueX experiments since 2016. Future physics program with the GlueX detector in experimental Hall D will require 
an upgrade of the inner part of the forward calorimeter with high-granularity, high-resolution ${\rm PbW0_{\rm4}}$ crystals.
The lead tungstate insert will improve the separation of clusters in the forward direction and the energy 
resolution  of reconstructed photons by about a factor of two. We propose to build a 1 m x 1 m insert, 
which will require about 2496 modules. Similar to the CCAL, there will be a 2 module x 2 module beam hole 
in the middle. The inner layer will be protected by a Tungsten absorber. Crystals are purchased from two vendors: 
SICCAS (China) and CRYTUR (Czech republic). The size of the FCAL insert may slightly vary depending on availability 
of funds.  A schematic view of the FCAL with the lead tungstate insert  is presented in Fig.~\ref{fig:fcal_frame}. 
The ${\rm PbW0_{\rm4}}$ module design will be essentially the same as for the CCAL, except for some small 
modifications needed to handle the magnetic field present in the FCAL region.


\subsubsection{Magnetic field measurement}

The longitudinal (directed along the beamline) and transverse (directed perpendicular to the axis of  of the beamline) 
components of the magnetic field produced by the GlueX solenoid magnet in the FCAL $\rm PbWO_{\rm 4}$ insert area  varies 
between 40 - 50 Gauss and 0 - 8 Gauss, respectively. The longitudinal filed is the largest on the beamline, where the 
transverse component is practically absent. We studied the PMT magnetic shielding using a prototype consisting of an array 
of 3x3 PMT soft iron (1020 steel) housings, which was positioned in the middle of Helmholtz coils. Each housing had a size of  
20.6 mm x 20.6 mm x 100 mm with a 19.9 mm round hole in the middle for the PMT. This corresponds to the realistic size of
the magnetic shield which will be used in the calorimeter module assembly.  Inside the housing 
we inserted two layers of $\mu$-metal Co-NETIC cylinders, with the thickness of $350\; {\rm \mu m}$ and $50\; {\rm \mu m}$, separated 
from each other by a Kapton film. The thickest cylinder was spot welded and annealed. 
%The prototype is shown in Fig. ~\ref{fig:field_prototype}.

The Helmholz coils had a diameter of about 1 m and can generate a uniform magnetic field with variable strength 
below 100 Gauss. A Hole probe was inserted to the central module of the prototype to measure magnetic field at 
different $Z$-positions along the PMT side. 
The field was measured for two  different orientations of the prototype with respect to the 
magnetic field: field oriented along the PMT (longitudinal, $B_{\rm z}$) and perpendicular to the PMT housing 
(transverse, $B_{\rm x}$). Field measurements are presented in Fig.~\ref{fig:field_distribution}. The PMT shield 
significantly reduce both the longitudinal and transverse fields to the level of $B_{\rm z}\sim 1$ Gauss and  
$B_{\rm x} \ll1\;{\rm Gauss}$. The transverse field, which is well shielded, is more critical for the PMT operation, 
as it is directed perpendicular to the electron trajectory inside  the photo tube  and deflects electrons resulting in the degradation of 
the photon detector efficiency and gain. The field reaches a plateau at Z = 3 cm from the face of the housing.
We will use a 3.5 cm long acrylic light guides, in order to place the PMT area between the photocathode and 
the last dynode (4.6 cm long) in the region with the  smallest magnetic field, as shown in Fig.~\ref{fig:field_distribution}.


We studied performance of the shielded PMT in the magnetic field using an LED pulser. The blue LED was placed about 
20 cm from the shield and the light diffuser in the middle. The PMT response was measured for different pulse amplitudes
and operational HVs. In order to study the contributions from longitudinal and transverse field components we rotated the 
prototype by different angles. Signal amplitudes as a function of the magnetic field are presented on the
left plot of Fig.~\ref{fig:field_led}. Amplitudes, normalized  to measurements without magnetic field are 
shown on the right plot. The relative degradation of the signal amplitude at B = 50 Gauss ($B_{\rm z}$ = 49 Gauss 
and $B_{\rm x}$ = 8.6 Gauss) was measured to be less than $1\%$.






\subsubsection{Light guide studies}

Studies of the magnetic shielding demonstrated that the PMT has to be positioned inside the $\mu$-metal 
cylinder about 3 cm from the face of the ${\rm PbW0_{\rm4}}$ crystal. Light from the crystal will be 
transmitted to the PMT using a 3.5 cm long  acrylic cylindrical lightguide  with a diameter of 18.5 cm. 
The light guide is wrapped with the reflective ESR foil. 
The light guide will be attached to the PMT with Dymax 3094 UV curing glue. Optical coupling to the crystal
will be provided using a 1 mm thick transparent rubber made of the room temperature vulcanized silicon 
compound, RTV615. This type of material has a widespread application in photodetectors and simplifies the 
module design. The silicon cookie is not glued to the light guide and the crystal so the module can 
be easily disassembles if PMT needs to be replaced.

We studied light losses induced by the light guide using a secondary beam of electrons provided by the 
Hall D pair spectrometer (PS)~\cite{ps}. The main goal of the PS is to monitor the flux of beam 
photons delivered to the expermental hall. This is done by reconstructing electromagnetic electron-positron pairs 
produced by the photons in a thin converter inserted to the beam. Leptons are deflected in a dipole 
magnet and detected using two scintillator detectors placed in the electron and positron arms 
of the spectrometer. Each detector consists of 145 tiles, which cover the energy range between 3 GeV and 6 GeV.
We positioned several fabricated ${\rm PbW0_{\rm4}}$ modules behind the PS detector of the electron arm  
around 4 GeV and compared light yields of two module configurations: (1) the PMT was directly attached 
to the crystal using an optical grease in the same way as it was done in the CCAL (2) the same PMT and crystal were connected to each other using an optical light 
guide as described above. Relative light collection of these two configurations were estimated by measuring 
flash ADC amplitudes induced by PS electrons. Coincidence  of hits between 
the PS tile and lead tungstate module was required. An example of signal pulse amplitudes obtained in the test 
module as a function of the PS tile is presented in Fig.~\ref{fig:lg_ps} for the configurations with and without light guide.
The light guide results in the typical losse of light of about $15\%$. We note, that wrapping light guide 
with the reflective material is important. Losses in unwrapped light guide constitute about $35\%$.



 

\subsubsection{Detector rates}

The GlueX detector was designed to carry out experiments using a continuous-energy secondary beam of photon 
produced by a 12 GeV beam of electrons via bremsstrahlung process. The maximum luminosity corresponds to a 
photon flux of  $5\cdot 10^7\;{\rm \gamma/sec}$ in the energy range between 8 GeV and 9 GeV incident on a 30 cm 
long liquid hydrogen target. The designed luminosity was achieved in the Fall run of 2019. This luminosity 
is about a factor of  2.5 larger than that in the PrimEx experiment, where the CCAL was originally utilized. We 
performed a study of the CCAL performance in GlueX runs at high luminosity. PMT anode current is one of the 
critical characteristics,  which has to be considered during the design of the PMT divider. Typically the anode
current should be on the level of a few micro amperes and significantly smaller than the divider current in order 
to provide stable performance of the PMT base and prevent from long-term 
degradation of the PMT[]. The anode current was measured with a special random trigger, which was used 
to read out flash ADC raw data for each CCAL channel in a time window of 400 ns. The window size corresponds to 
100 flash ADC samples. The average ADC voltage in the readout window was determined by summing up amplitudes and normalizing 
them to the window size. The voltage measured by the ADC is produced by the current going through the termination resistor 
of $\sim 50\;\Omega$. The anode current can be estimated as
%------------------------------------------------
\begin{equation}
 A = \frac{\bar{A}}{R}\cdot\frac{1}{G}, 
\end{equation}
%------------------------------------------------
where A is the average ADC amplitude in units of Volts,
%$V_{\rm ADC} = 0.5\;{\rm mV}$ is the ADC resolution, 
$R$ is the termination resistor, and $G$ is the amplifier gain equals to 24. The typical anode current measured in
CCAL modules in different detector layers situated at different distance from the beam line is 
presented in Fig.~\ref{fig:anode_current}. The rate in the detector is dominated by the forward-directed 
electromagnetic background. The anode current is the largest in the innermost layer of the detector closest 
to the beam line  and  constitutes to about $1.4\;{\rm \mu A}$. This current can be compared to the PMT divider 
current of $300\;{\rm \mu A}$. The CCAL measurements can be used to estimate anode current in the FCAL lead tungstate
insert. The largest PMT current in the $\rm PbWO_{\rm 4}$  module closest to the beam line is conservatively estimated 
to be about $20\;\mu{\rm A}$ if no  amplifier is used and the PMT base is operated at 1 kV. The detector rate drops rapidly 
with the increase of the radial distance from the beamline. We are considering to instrument PMTs in a few inner layers 
with an amplifier with the gain of 5 and do not user the amplifier on other modules.
 



\section{Neutral Particle Spectrometer}

The neutral-particle spectrometer (NPS) offers unique scientific capabilities to study the transverse spatial and 
momentum structure of the nucleon in the Jefferson Lab experimental Hall C. Five experiments have been currently 
approved using the NPS. The experiments and run conditions are listed Table 1. 


The Neutral Particle Spectrometer consists of 1080 $PbWO_{\rm 4}$ crystals, which will form and an array 
of 30x36 modules. Crystals with the same size as in the CCAL  purchased from two vendors: the CRYTUR and
SICCAS. Crystals will be placed in the frame build from carbon plates and separated from each other by a 0.5 mm-thick 
carbon layer to ensure good positioning. Hamamatsu R4125 PMTs will be attached to the back side of each module and 
be separated from each other with a  0.5 mm thick $\mu$-metal plates to reduce the 200 Gauss magnetic filed originating 
from the sweeping magnet. Blue LED will be used to calibrate modules and cure crystals degraded due to radiation. 
Light from the LED will be distributed through quartz optical fibers to each individual module.

The detector is positioned in a temperature controlled frame on the movable platforms, which will allow to place the 
detector at different angles. 


%The Neutral Particle Spectrometer is ~25 msr neutral particle detector consisting of ~1080 PbWO4 crystals (30x36 matrix) in a temperature 
% controlled frame including gain monitoring and curing systems.

%total of 160 PAC days approved: $\sim 20\%$ of all approved beam time in Hall C. E12-13-007: Measurement of Semi-inclusive 
%$\pi^0$ production as validation of factorization; E12-13-010: Exclusive DVCS and $\pi^0$ cross section measurements in Hall C; 
%E12-14-003: Wide-angle Compton Scattering at 8 and 10 GeV Photon Energies; E12-14-005: Wide Angle Exclusive Photoproduction of 
%π0 Mesons; E12-17-008: Polarization Observables in Wide-Angle CS at large s, t and u; C12-18-005: Timelike Compton Scattering 
%off a transversely polarized proton. 

%Motivation of NPS Experiments: Validation of Reaction mechanism, to extract the rich 
%information on nucleon structure encoded in GPD and TMDs one needs to show that the scattering process is understood. Combine 
%NPS with Compact Photon Source (CPS) enables a gain of a factor of 30 in figure-of-merit. Enables a new suite of high-energy photon 
%scattering experiments to image and understand the dynamical nucleon structure. Much progress in imaging nucleon structure can be 
%made with electron-scattering reactions, yet experiments with high-energy photons play a unique complementary role. Small 
%scattering probabilities of exclusive reactions demand high-intensity photon beams. Understanding strengthened by imaging 
%longitudinally-polarized and transversely-polarized nucleons.

%The Neutral Particle Spectrometer is ~25 msr neutral particle detector consisting of ~1080 PbWO4 crystals (30x36 matrix) in a temperature 
%controlled frame including gain monitoring and curing systems. HV distribution bases with built in amplifiers for operation in a high 
%rate environment. Essentially deadtime less digitizing electronics to independently sample the entire pulse form for each crystal, 
%JLab developed flash ADC. A vertical-bend sweeping magnet with integrated field strength of 0.3 Tm to suppress an eliminate charged 
%background. Cantilevered platforms off the Super High Momentum Spectrometer (SHMS) carriage to allow for remote rotation. For NPS 
%angles from 6 to 23 degrees, the platform will be on the left of the SHMS carriage for NPS angles 23-57.5 degrees it will be on the right. 
%A beam pipe with as large opening/critical angle for the beam exiting the target/scattering chamber region as possible to reduce 
%beamline-associated backgrounds. Calorimeter conceptual design 30x36 (1080) PbWO4 crystals of size: 2x2x20 cm3 coupled with Hamamatsu 
%R4125 PMTs with custom active HV bases. Design completed at IPN Orsay. Crystals will be placed in a 0.5 mm-thick carbon frame to ensure 
%good positioning. PMTs will be accessible from the back side to allow for maintenance. Calibration and radiation curing with blue LED 
%light though quartz optical fibers.


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=0.8\linewidth,angle=0]{figures/ccal_scalers.pdf}
\end{center}
\caption{Rates of the CCAL modules during PrimEx $\eta$ production run. The energy threshold corresponds to 30 MeV.}
\label{fig:ps_layout}
\end{figure}
%----------------------------------------------------------------------



%----------------------------------------------------------------------
%\begin{figure}[t]
%\begin{center}
%\includegraphics[width=0.8\linewidth,angle=0]{figures/ccal_scalers1.jpg}
%\end{center}
%\caption{Schematic view of the calorimeter module.}
%\label{fig:ps_layout}
%\end{figure}
%----------------------------------------------------------------------







%----------------------------------------------------------------------
%\begin{figure}[t]
%\begin{center}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/Mod_examp2.jpg}
%\end{center}
%\caption{Schematic view of the calorimeter module.}
%\label{fig:ps_layout}
%\end{figure}
%----------------------------------------------------------------------


%----------------------------------------------------------------------
%\begin{figure}[t]
%\begin{center}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/Latest_assembly.png}
%\end{center}
%\caption{Schematic view of the calorimeter module.}
%\label{fig:ps_layout}
%\end{figure}
%----------------------------------------------------------------------


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/compton_elast.jpg}
\end{center}
\caption{Elasticity distribution of reconstructed Compton candidates.}
\label{fig:ps_layout}
\end{figure}
%----------------------------------------------------------------------


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=0.8\linewidth,angle=0]{figures/fcal_frame.png}
\end{center}
\caption{FCAL frame with calorimeter modules installed: $PbWO_{\rm 4}$ 4 crystals (brown area),
lead glass blocks (green).}
\label{fig:fcal_frame}
\end{figure}
%----------------------------------------------------------------------


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.\linewidth,angle=0]{figures/nim_plot_field.pdf}
\end{center}
\caption{Magnetic field distribution inside the shield housing as a function of the distance from the housing face. Plot (a) corresponds to 
the longitudinal field and plot (b) corresponds to the transverse field. Markers denote different
fields produced by Helmholtz coils. }
\label{fig:field_distribution}
\end{figure}
%----------------------------------------------------------------------

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.\linewidth,angle=0]{figures/nim_plot_amp.pdf}
\end{center}
\caption{ Signal amplitudes of shielded PMT induced by an LED as a function of the magnetic field (a). Amplitudes, normalized  
to measurements without magnetic field (b). PMT response was measured for different intensities of light pulse and HV settings as 
shown by different polymarkers.}
\label{fig:field_led}
\end{figure}
%----------------------------------------------------------------------
%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=0.95\linewidth,angle=0]{figures/light_guide_ps.pdf}
\end{center}
\caption{ADC amplitudes of the calorimeter module with the light guide (boxes) and without light guide (circles) as a function of the pair spectrometer tile. }
\label{fig:lg_ps}
\end{figure}
%----------------------------------------------------------------------

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=0.95\linewidth,angle=0]{figures/anode_current.pdf}
\end{center}
\caption{PMT anode current of CCAL modules for different layers.}
\label{fig:anode_current}
\end{figure}
%----------------------------------------------------------------------


\section{Summary}
\label{sec_summary}

We have described the design and fabrication details of the pair spectrometer hodoscope, an array of 
thin scintillator tiles. Light from each tile is detected using a 3 mm x 3 mm Hamamatsu SiPM.
A detector prototype was built to perform  light collection studies using relativistic electrons 
produced in the experimental Hall B at Jefferson Lab. Two arms of the hodoscope detector 
were commissioned and installed in the experimental Hall D.

\section{Acknowledgments}

This work was supported by the Department of Energy. Jefferson Science 
Associates, LLC operated Thomas Jefferson National Accelerator Facility for the United States Department 
of Energy under contract DE-AC05-06OR23177. We would  like to thank ...


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\bibliographystyle{elsarticle-num}
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\begin{thebibliography}{00}

%% \bibitem{label}
%% Text of bibliographic item
\bibitem{popov} 
  V. Popov and H. Mkrtchyan {\it et al.}, Proceedings of the IEEE conference, California, 2012.

 \bibitem{gluex}
   JLab Experiment E12-06-102, (2006) \url{http://www.jlab.org/exp_prog/proposals/06/PR12-06-102.pdf.}


\bibitem{hallb_bl} 
   D.~I.~Sober {\it et al.},
% The bremsstrahlung tagged photon beam in Hall B at JLab, 
   Nucl.Inst.and Meth. A, 440 (2000),p.263.


 \bibitem{fadc250}
  F.~Barbosa {\it et al.}, Proceedings of IEEE Nuclear Science Symposium, Hawaii, USA (2007).

 \bibitem{f1tdc}
  F.~Barbosa {\it et al.}, Proceedings of IEEE Nuclear Science Symposium, Virginia, USA (2002).


\bibitem{ps}
F.~Barbosa, {\it et al.},
%``Pair spectrometer hodoscope for Hall D at Jefferson Lab,''
Nucl. Instrum. Meth. A \textbf{795}, 376-380 (2015).
%doi:10.1016/j.nima.2015.06.012

\bibitem{gluex_det}
S.~Adhikari, {\it et al.},
%``The GlueX beamline and detector,''
Nucl. Instrum. Meth. A \textbf{987}, 164807 (2021).
%doi:10.1016/j.nima.2020.164807
%[arXiv:2005.14272 [physics.ins-det]].
%10 citations counted in INSPIRE as of 07 Jan 2021

\bibitem{pwo_crystals}
T.~Horn, {\it et al.},
%``Scintillating crystals for the Neutral Particle Spectrometer in Hall C at JLab,''
Nucl. Instrum. Meth. A \textbf{956}, 163375 (2020).
%doi:10.1016/j.nima.2019.163375

\bibitem{l1_trigger}
A.~Somov,
%``Development of level-1 triggers for experiments at Jefferson Lab,''
AIP Conf. Proc. \textbf{1560}, no.1, 700-702 (2013).
%doi:10.1063/1.4826876

\bibitem{hycal_kubantsev}
M.~Kubantsev {\it et al.},
%``Performance of the PrimEx electromagnetic calorimeter,''
AIP Conf. Proc. \textbf{867}, no.1, 51-58 (2006). A.~Gasparian,  Proceedings of the 11th International Conference on Calorimetry in High-Energy Physics, 
109-115 (2004).
%doi:10.1063/1.2396938
%[arXiv:physics/0609201 [physics.ins-det]].

\end{thebibliography}

\end{document}
S.   Danagulian.  Proceedings  of   10th  International  Conference  on  Calorimetry  in   HighEnergyPhysics (CALOR 2002), Pasadena, California, 25-30 Mar 2002.Published in *Pasadena2002, Calorimetry in particle physics* 479-485
