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\begin{frontmatter}

%% Title, authors and addresses

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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 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[cua,jlab]{J.Crafts}
\author[jlab]{H.Egiyan}
\author[uncw]{L.Gan}
\author[ncat]{A.Gasparian}
\author[jlab]{K.Harding}
\author[cua]{T.Horn}
\author[erphy]{V.Kakoyan}
\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 into the DAQ and trigger systems of the GlueX detector and used in the experiment 
to reconstruct  Compton scattering events. The experiment started collecting data in the spring of 2019 
and acquired about $30 \%$ of the required statistics. The calorimeter is a prototype 
for two $\rm PbWO_{\rm 4}$-based  detectors, which are currently under construction at Jefferson Lab: 
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 gives a brief overview of 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 calorimeter, Lead tungstate scintillator
\end{keyword}
%% keywords here, in the form: keyword \sep keyword

%% PACS codes here, in the form: \PACS code \sep code

%% MSC codes here, in the form: \MSC code \sep code
%% or \MSC[2008] code \sep code (2000 is the default)

\end{frontmatter}

\linenumbers

%% main text
\section{Introduction}

Electromagnetic calorimeters based on $\rm PbWO_{\rm 4}$ scintillating crystals have a widespread application in experiments 
at different accelerator facilities such as CERN, FNAL, GSI, and Jefferson Lab (JLab). The small radiation length  
($L_{\rm R} = 0.89\;\rm{cm}$) and Moli$\grave{\rm e}$re radius ($R_{\rm m} = 2.19\;\rm{cm}$) of $\rm PbWO_{\rm 4}$  allows to build  
high-granularity detectors with a good spatial separation and energy resolution of reconstructed electromagnetic showers, which  
makes these crystals the material of choice in many of these applications.

% 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 
% Moli$\grave{\rm e}$re 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.


Two electromagnetic calorimeters are currently under construction in experimental Hall D and Hall C at Jefferson 
Lab, both using rectangular $2.05\;{\rm cm}\:{\times}\: 2.05\;{\rm cm}\:{\times}\:20\;{\rm cm}$  $\rm PbWO_{\rm 4}$  scintillating modules. The inner 
part of the forward lead glass calorimeter 
of the GlueX detector~\cite{gluex_det} in Hall D will be upgraded with these 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~\cite{jef}. The size of the insert will tentatively consist of 2496 lead tungstate modules. 
The Neutral Particle Spectrometer~\cite{nps} in experimental Hall C consists of a  $\rm PbWO_{\rm 4}$ electromagnetic 
calorimeter preceded by a sweeping magnet. The NPS is required by Hall C's precision cross section measurement program 
with neutral final states~\cite{e12-13-010,e12-06-114,e12-13-007,e12-14-003,e12-17-008,e12-14-005}. 
Such precision measurements of small cross sections play a central role in studies 
of transverse spatial and momentum hadron structure. The NPS detector consists of 1080 $\rm PbWO_{\rm 4}$ crystals arranged 
in a $30\:{\times}\:36$ array.
%is a new calorimeter, that will allow to 
%carry out several experiments to study various physics topics such as the transverse spatial and momentum structure of the nucleon.
%$\rm PbWO_{\rm 4}$ crystals will form an array of 30x36 modules. 
Lead tungstate crystals for both detectors were procured from two vendors: Shanghai 
Institute of Ceramics (SICCAS) in China and CRYTUR in the Czech Republic. The quality of recently produced  $\rm PbWO_{\rm 4}$ scintillators 
has been studied in detail by the NPS and EIC eRD1 collaborations and is described in Ref.~\cite{pwo_crystals}.  
$\rm PbWO_{\rm 4}$ crystals are also being considered for an electromagnetic calorimeter of the future Electron-Ion Collider~\cite{eic}.


% 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 served as the Compton Calorimeter (CCAL) in the PrimEx $\eta$ experiment~\cite{primex} with the GlueX detector  in the 
%Spring of 2019. The CCAL was subsequently used during a few GlueX physics runs at high luminosity in order to study rates and operation 
%conditions corresponding to the FCAL lead tungstate insert. 

In this article we describe the design and construction of a calorimeter prototype composed of 140 SICCAS crystals, which served 
as the Compton Calorimeter (CCAL) in the PrimEx $\eta$ experiment~\cite{primex} with the GlueX detector in the spring of 2019. 
The CCAL was subsequently used during a few short  GlueX physics runs at high luminosity in order to study rates and operating 
conditions expected for the FCAL lead-tungstate insert. Experience gained during fabrication and operation of the CCAL was critical 
for finalizing the design of the FCAL insert and also helped further optimize the NPS calorimeter. 
%Crystals provided by SICCAS are considered to be used in 
%the electronagnetic calorimeter of the future Electron-Ion Collider.

This article is organized as follows: we will present the PrimEx $\eta$ experiment and performance of the CCAL in Section 2 and Section 3,  
and will briefly describe the FCAL and NPS projects in Sections 4 and 5.  

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/Gluex_layout.pdf}
\end{center}
\caption{Schematic layout of the GlueX detector (not to scale). Numbers represent the following detector components: solenoid magnet (1),  
barrel calorimeter (2), central drift chamber (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. 






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

The GlueX detector~\cite{gluex_det} was designed to perform experiments using a photon beam.
Beam photons are produced via the bremsstrahlung process by electrons, provided by the JLab electron 
accelerator facility,  incident on a thin radiator. The energy of a beam photon is determined by detecting 
a scattered electron after radiating the photon  with a typical precision of $0.2\%$. The electron is deflected 
in a 6 m long dipole magnet operated at a field of 1.8 T and registered in the so-called tagging scintillator 
counters. Each counter corresponds to the specific energy of the reconstructed lepton. The photon beam propogates 
toward the GlueX target. A schematic view of the GlueX detector is illustrated in Fig.~\ref{fig:gluex_det}~\footnote{Not 
shown on this plot is the DIRC detector, which was installed after the PrimEx $\eta$ experiment and is used for the particle 
identification in the forward direction.}.



The physics goal of the PrimEx $\eta$ 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 photoproduction cross section of $\eta$ mesons in the Coulomb field of a nucleus, which is 
known as the Primakoff effect. The $\eta$ mesons will be reconstructed by detecting two decay photons in the 
forward calorimeter of the GlueX detector.

% 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 scattering process, which will also be used to monitor the 
luminosity and control the detector stability during data taking. Electrons and photons originating from Compton 
events in the target are produced at small angles, typically outside the acceptance of the FCAL. In order to improve 
the reconstruction of particles in the forward direction, we built a small Compton calorimeter consisting of 140 
%an array of 12 x 12 
lead tungstate scintillating crystals and  positioned it about 6 m downstream from 
the FCAL as shown in in Fig.~\ref{fig:gluex_det}.  The CCAL covers the angular range between $0.18^{\circ}$ and  $0.33^{\circ}$. 
% A schematic view of the GlueX detector and the position of the Compton calorimeter are illustrated in Fig.~\ref{fig:gluex_det}.

The PrimEx $\eta$ experiment started collecting data in the spring of 2019 and has acquired $30 \%$ of 
the required statistics. During the experiment, the magnetic field of the solenoid magnet was switched off in order to allow reconstruction 
of Compton events. 
%The beam photon flux was about four times lower than the nominal GlueX beam conditions. 
The photon flux was about $5\cdot 10^6\;\gamma/{\rm sec}$ (four times lower than the nominal GlueX flux) in the beam energy range of 
interest between 9.5 GeV and 11.6 GeV.

% calorimeter referred to as the Compton calorimeter (CCAL) 

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_assembled.jpg}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_assembled1.png}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_assembled2.png}
\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_assembly10.pdf}
\end{center}
\caption{Schematic layout of the Compton calorimeter.}
\label{fig:ccal_design}
\end{figure}
%----------------------------------------------------------------------

\section{Compton calorimeter of the PrimEx $\eta$ experiment}
% \label{sec_ccal}


 
% 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 $12\:{\times}\:12$ 
lead tungstate modules with a $2\:{\times}\: 2$ hole in the middle for the passage of the photon beam. The modules are 
positioned inside a light tight box. A tungsten absorber is placed in front of the innermost layer closest 
to the beamline to provide protection from the 
%is exposed to a 
high rate of particles predominantly originating from electromagnetic interactions.
 
The light yield from $\rm PbWO_{\rm 4}$ crystals depends on temperature with a typical temperature 
coefficient of $2\% / ^\circ C$ at room temperature. Maintaining constant temperature is essential 
for the calorimeter operation. The calorimeter modules are surrounded by four copper plates with  built-in 
pipes to circulate a cooling liquid and provide temperature stabilization. Foam insulation surrounds 
the detector box. The temperature was monitored and recorded during the experiment by five thermocouples 
attached to different points of the $\rm PbWO_{\rm 4}$ module assembly. During the experiment the temperature 
was maintained at $17^\circ\pm 0.2^\circ C$. The typical heat released by the photomultiplier  tube (PMT) dividers 
was equivalent to 33 Watts. In order to prevent condensation, a nitrogen purge was applied. Two fans with a 
water-based cooling system were installed on the top of the crystal assembly to improve nitrogen 
circulation and heat  dissipation from the PMT dividers. The detector was positioned on a  
platform, which allowed to move it in the vertical and horizontal directions, perpendicular to the
beam. The platform was remotely controlled and provided a position accuracy of about $200\;{\rm \mu m}$. 
During detector calibration each module was moved into the beam. 

\subsection{Module design}
The design of the ${\rm PbWO}_4$ module is based on the HyCal calorimeter, which was
used in several experiments in Jefferson Lab Hall B~\cite{hycal_kubantsev}. An assembled calorimeter module 
is presented in Fig.~\ref{fig:ccal_module}. Each lead tungstate crystal is wrapped with a 
 $60\;{\rm \mu m}$ polymer Enhanced Specular Reflector film (ESR) manufactured by ${\rm 3M}^{\rm TM}$, which 
allows $98.5\%$ reflectivity across the visible spectrum. In order to improve 
optical isolation of each module from its neighbors, each crystal is wrapped with a layer 
of $25\;{\rm \mu m}$ thick Tedlar. The PMT is located inside a G-10 fiberglass housing at the rear end 
of the crystal. Two flanges are positioned at the crystal and housing ends and are
connected together using $25\;{\rm \mu m}$ brass straps, which are brazed to the sides of the flanges. 
Four set screws are pressed to the PMT housing flange to generate  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  into the housing and is coupled to the crystal using optical grease (BC-631). 
The PMT diameter is 19 mm. 
The PMT is pushed towards the crystal by using a G-10 retaining plate attached to the back of 
the PMT and four tension screws applied to the PMT flange. The PMT is instrumented with a 
high-voltage divider and amplifier positioned on the same printed circuit board 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.


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
% \includegraphics[width=0.9\linewidth,angle=0]{figures/ccal_module.png}
% \includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_module1.pdf}
\includegraphics[width=1.0\linewidth,angle=0]{figures/test1.pdf}
\end{center}
\caption{Calorimeter module showing main components: the $\rm PbWO_{\rm 4}$ crystal, PMT housing, PMT divider, and signal and HV cables.}
\label{fig:ccal_module}
\end{figure}


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



 

\subsection{Electronics}
The PMT of each calorimeter module was equipped with an active base prototype~\cite{popov},  
which was designed for the Neutral Particle Spectrometer in experimental Hall C.
% lead tungstate calorimeter of the Neutral-Particle Spectrometer 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 the operation of the PMT at lower voltage and consequently at lower 
anode current, which improves the detector rate capability and 
prolongs the PMT's life.
%Operation of the PMT at smaller 
%anode current is also important for the extension of the photomultiplier tube life. 
The original Hamamatsu divider for this type of PMT was modified by adding two  bipolar transistors 
on the last two dynodes,  which provides gain stabilization at high rate. The active base from the 
NPS detector has a relatively large amplification of about a factor of 24 due to the large
PMT count rate predicted by Monte Carlo simulation. Large amplification was not needed for the planned run conditions for
the PrimEx $\eta$ experiment. However, we subsequently used CCAL in GlueX runs at significantly 
larger luminosity in order to study run conditions of the FCAL lead tungstate insert, where the amplifier will 
be required. This will be discussed in Section~\ref{sec:fcal_rates}. During the PrimEx run, the CCAL was 
operated at about ${\rm 680\; V}$, which produced a divider current of $260\;{\rm \mu A}$. The high voltage for 
each PMT was supplied by a 24-channel CAEN A7236SN module positioned in a SY4527 mainframe. 

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_waveform.pdf}
\end{center}
\caption{A typical flash ADC signal pulse obtained from a $\rm PbWO_{\rm 4}$ module.}
\label{fig:pmt_pulse}
\end{figure}
%----------------------------------------------------------------------

Amplified PMT signals were digitized using a twelve-bit 16-channel  flash ADCs electronics module 
operated at a sampling rate of 250 MHz. The ADC was designed at Jefferson Lab~\cite{fadc250} and 
is used for the readout of several sub-detectors of the GlueX detector. 
The Field-Programmable Gate Array (FPGA) chip inside the ADC module allows the implementation of various  programmable 
data processing algorithms for the trigger and readout.  An example of a flash ADC signal pulse obtained 
from a calorimeter module is shown in Fig.~\ref{fig:pmt_pulse}. In this example, the ADC is operated in 
the raw readout mode, where digitized amplitudes are read out for 100 samples, corresponding to 
the 400 ns read out window.  During the PrimEx $\eta$ experiment, the ADC performed on-board integration of 
signal pulses, which amplitudes were above a threshold of 24 MeV. ADC amplitudes are summed in a time window 
of 64 ns and read out from the ADC along with other parameters such as the pulse amplitude, pulse 
time, amplitude of the ADC pedestal, and data processing quality factors. This readout mode allowed to 
significantly reduce the data size and ADC readout time, and therefore did not induce any 
dead time in the DAQ.  



CCAL flash ADCs are positioned in a VXS (ANSI/VITA 41.0 standard) crate. VXS crates are used to host all readout electronics of
the GlueX experiment. In addition to the VME-bus used to read out data from electronics modules, the 
VXS is instrumented with a high-speed  serial bus in order to increase the bandwidth to several Gb/sec 
and  provide an interconnected network between modules. The bus is used to transmit amplitudes digitized 
by the ADC to trigger electronics modules to include the CCAL in the Level 1 trigger system of the GlueX 
detector.
%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 Level 1 trigger system of the GlueX detector. 
% The trigger is based on the energy deposition in the Compton and Forward calorimeters.
% 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.

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
% \includegraphics[width=1.0\linewidth,angle=0]{figures/led_amplitudes.png}
\includegraphics[width=1.0\linewidth,angle=0]{figures/led_amplitudes1.pdf}
\end{center}
\caption{Flash ADC signal amplitudes  induced by the LED and the $\alpha$-source in the reference PMT.}
\label{fig:led_amp}
\end{figure}
%----------------------------------------------------------------------


\subsection{Light Monitoring System}
To monitor performance of each calorimeter channel, we designed an LED-based light monitoring system (LMS). 
The LMS optics includes a blue LED, a spherical lens to correct the conical dispersion of the LED, and a diffusion grating 
to homogeneously mix the light. Light produced by the LED  is incident on a bundle of plastic optical fibers (Edmund Optics)  with a 
core diameter of $250\;{\rm \mu m}$. Each fiber distributes light to an 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 used two reference Hamamatsu 4125 PMTs, the same type as in the CCAL detector. 
Each PMT receives light from two sources: a single fiber from the LED and a YAP:Ce pulser unit, both glued to the PMT face.
The pulser unit consists of a 0.15 mm thick YAP:Ce scintillation crystal with a diameter of 3 mm spot activated by an  
${}^{241}{\rm Am}$ $\alpha$ source. The $\alpha$ source is used to monitor stability of the LED. 
The PMT was read out using a flash ADC. The high voltage on each reference  PMT was adjusted to have the signals 
from both the LED and  $\alpha$ source fit within the flash ADC range of 4096 counts, as shown in Fig.~\ref{fig:led_amp}. Each LED 
was driven by a CAEN 1495 module, which allowed to generate LED pulses with a programmable rate. The LMS was integrated into the 
GlueX trigger system and provided  a special trigger type during data taking. The LMS was extensively used during the detector 
commissioning and injected light to the CCAL detector with a typical frequency of 100 Hz continuously during the PrimEx $\eta$ 
experiment. This LED rate is similar to the trigger rate of events generated by the reference $\alpha$ source.

Most LMS components were positioned inside the temperature-stabilized detector box. The stability of the LED system measured using 
the reference PMTs during the entire PrimEx run was on the level of $1\%$. The ratio of signal ADC amplitudes from the LED pulser to 
the $\alpha$ source obtained during different run periods of the 48-day long PrimEx $\eta$ experiment is presented in  
Fig.~\ref{fig:led_stability}. The ratio is normalized to the data in the beginning of the experiment. 
Stability of most CCAL modules observed using the LMS during the  experiment was better than  $6\%$. We did not
apply any PMT gain adjustments during the experiment.

%Typical LED amplitudes of calorimeter modules measured during the run are presented in Fig.~\ref{fig:led_stability}. 
%Stability of LED amplitudes for most CCAL modules during the experiment was better than  $6\%$.
%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=1.0\linewidth,angle=0]{figures/led_stability.jpg}
% \includegraphics[width=1.0\linewidth,angle=0]{figures/led_stability1.pdf}
\includegraphics[width=1.0\linewidth,angle=0]{figures/led_stability2.pdf}
\end{center}
\caption{Ratio of signal ADC amplitudes from the LED pulser to the $\alpha$-source 
measured by the reference PMT during different run periods of the 48-day long 
PrimEx $\eta$ experiment. The ratio is normalized to data in the beginning 
of the run.}
\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}

The initial energy calibration of the CCAL was performed by moving the calorimeter frame and positioning each 
module into the photon beam during special low-intensity calibration runs. The maximum rate in the module, for 
a  threshold of 15 MeV, did not exceed 200 kHz. The energy of 
each beam photon was determined by detecting a bremsstrahlung electron using the GlueX tagging 
detectors as described in Section 2.  The tagging detectors cover the photon energy range 
between 2.9 GeV and 11.4 GeV and provide a relative energy resolution of about $0.2\%$. 
The spot size of the collimated photon beam had a diameter of about 6 mm.

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
% \includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_tagger.pdf}
\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_tagger1.pdf}
\end{center}
\caption{ADC signal pulse amplitude in the CCAL module as a function of the beam energy.}
\label{fig:ccal_calib}
\end{figure}
%----------------------------------------------------------------------

In the beginning of the calibration run, we adjusted the PMT high voltage for 
each module in order to equalize signal pulse amplitudes induced by 10 GeV beam photons. 
The amplitude was set to 3200 ADC counts. An example of  flash ADC signal amplitude in the  calorimeter module as a 
function of the beam energy is presented in Fig.~\ref{fig:ccal_calib}.
The calibration of each module was 
%subsequently 
refined by reconstructing showers in the 
calorimeter and constraining the reconstructed energy to the known beam energy. 

During the calibration runs, we estimated the non-uniformity of the 140 CCAL modules by measuring the relative energy resolution for
each individual module exposed to the beam.
%The non-uniformity of 140  CCAL modules  was estimated by measuring the relative energy resolution for
%each individual module exposed to the beam. 
The energy resolution obtained for 6 GeV photons is presented in Fig.~\ref{fig:ccal_mod_uniform}. 
The distribution is fit to a Gaussian function. The non-uniformity of the modules, i.e., the spread 
of the distribution is found to be smaller than $5\%$. 
% The non-uniformity can be mostly accounted for the quality of crystals used for the CCAL fabrication.

During calibration, we observed some a non-linearity of the PMT active base with the large amplification 
factor of 24, on the level of a few percent, which impacted both the pulse peak and pulse 
integral. The base performance became linear when the amplifier gain was reduced. 
In order to study the impact of the non-linearity on the detector energy resolution, we replaced the original 
PMT active bases  for 9 CCAL modules (in the array of $3\:{\times}\: 3$ modules) with modified bases where 
the amplifier was bypassed. After adjusting high voltages and recalibrating PMT gains we  measured the energy 
resolution for different beam energies. The beam was incident on the center of the middle module in the array.
%We subsequently replaced the original PMT active bases with the gain of 24 for 9 CCAL modules (in the array of $3\:{\times}\: 3$ modules) 
%with modified bases where the amplifier was bypassed and measured the energy resolution for different beam energies.
An example of the energy deposited by  10 GeV photons is shown in Fig.~\ref{fig:ccal_en_fit}.
%CCAL energy in units of flash ADC counts induced by 10 GeV photons is shown in Fig.~\ref{fig:ccal_en_fit}. 
The energy resolution was obtained from a fit of the energy distribution to a Crystal Ball function~\footnote{The 
function was named after the Crystal Ball collaboration.} implemented in the ROOT data analysis framework~\cite{root}. 
The energy resolution as a function of the beam energy is shown in Fig.~\ref{fig:ccal_en_res}. 
% Energy resolution measured in this region is shown in Fig.~\ref{fig:ccal_en_res}. 
The distribution 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 
%$S = 0.41  \pm 0.03\%$. 
where $S$ represents the stochastic term, $N$ the electronic noise and $C$ the constant term, $E$  is the beam energy in GeV, and  
the symbol $\oplus$ indicates a quadratic sum. The fit yields: $S = 2.63 \pm 0.01\%$,  $N = 1.07  \pm 0.09\%$, and 
$C = 0.53  \pm 0.01\%$. The resolution was found to be about $10\%$ better than that measured with the original base 
with the gain of 24. The energy resolution is consistent with that of the HyCal calorimeter~\cite{hycal_kubantsev}, 
which was instrumented with crystals produced by SICCAS in 2001 and was used in several experiments in Jefferson Lab's 
experimental Hall B. The HyCal $\rm PbWO_{\rm 4}$ crystals have the same transverse size of $2.05\, {\rm cm}\: {\times}\: 2.05\; {\rm cm}$, but a smaller 
length of $18\; {\rm cm}$. The initial CCAL calibration performed with the beam scan was fine-tuned during the PrimEx run by 
using showers of reconstructed Compton scattering candidates and constraining the reconstructed enegy in the event to 
the know beam energy. The linearity of the CCAL electronics is being currently improved; modified active bases will be 
installed before the new PrimEx $\eta$ run in 2021.

%The resolution was found to be about $10\%$ better than that measured with the original base 
%with the gain of 24. The energy resolution is consistent with 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_mod_uniform.pdf}
\end{center}
\caption{Relative energy resolution of 140 $\rm PbWO_{\rm 4}$ modules installed on the CCAL measured with 6 GeV beam photons.}
\label{fig:ccal_mod_uniform}
\end{figure}
%----------------------------------------------------------------------


\subsection{Performance during the PrimEx $\eta$ 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.    
In the PrimEx $\eta$ experiment, we reconstruct Compton events produced by beam photons with 
$E_{\rm beam} > 6\;{\rm GeV}$. This energy range is covered by the pair spectrometer~\cite{ps}, which determines 
the photon flux needed for cross section measurements. In order to accept Compton events during data 
taking and to reduce background originating from low-energy electromagnetic and  hadronic interactions, 
the CCAL was integrated to the Level 1 trigger system of the GlueX detector. The physics trigger was 
based on the total energy deposited in the forward and Compton calorimeters. The GlueX trigger is 
implemented on special-purpose programmable electronics modules with FPGA chips. The trigger architecture 
is described in Ref.~\cite{l1_trigger}. The trigger rate as a function of the energy threshold is 
presented in Fig.~\ref{fig:trig_rate}. We collected data using a relatively small energy threshold 
of 3 GeV at a trigger rate of about 18 kHz. This rate did not produce any dead time 
in the DAQ and trigger systems. The trigger rate was reproduced  by a detailed Geant detector 
simulation. 

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
% \includegraphics[width=1.0\linewidth,angle=0]{figures/crystal_fit_53.pdf}
\includegraphics[width=1.0\linewidth,angle=0]{figures/crystal_fit_53_new.pdf}
\end{center}
%\caption{Energy in units of flash ADC counts measured in CCAL modules for 10 GeV beam photons. The spectrum is fit 
%with a Crystal Ball function.}
\caption{Energy distribution deposited by 10 GeV beam photons. The spectrum is fit to  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}
% \includegraphics[width=1.0\linewidth,angle=0]{figures/Fit_v5.png}
% \includegraphics[width=1.0\linewidth,angle=0]{figures/Fit_v7_2.png}
% \includegraphics[width=1.0\linewidth,angle=0]{figures/Fit_v9.pdf}
\includegraphics[width=1.0\linewidth,angle=0]{figures/Fit_v10.pdf}
\end{center}
\caption{Energy resolution as a function of the photon energy.}
\label{fig:ccal_en_res}
\end{figure}
%----------------------------------------------------------------------

The rate in the CCAL modules during the experiment is presented in Fig.~\ref{fig:ccal_rate}. 
In this plot, the photon beam goes through the center of the hole of $2\:{\times}\:2$ modules in the middle of the detector. 
The rate is the largest in innermost detector layers closest to the beam line. The maximum trigger rate
in the detector module was about 200 kHz for an energy threshold of 30 MeV, which is equivalent to a signal 
pulse amplitude of 5 mV. Before the experiment, we performed a high-rate performance study of the PMT and electronics 
using a laser and an LED pulser and did not find any degradation of the PMT gain in run conditions similar to the 
PrimEx experiment up to 3-4 MHz~\cite{pmt_high_rate}.

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/primex_trig_rate.pdf}
\end{center}
\caption{Trigger rate as a function of the total energy deposited in the FCAL and CCAL. The arrow indicates the energy threshold 
used in PrimEx $\eta$ production runs. }
\label{fig:trig_rate}
\end{figure}
%----------------------------------------------------------------------

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
%\includegraphics[width=0.8\linewidth,angle=0]{figures/ccal_scalers.pdf}
\includegraphics[width=0.9\linewidth,angle=0]{figures/ccal_scalers1.pdf}
\end{center}
\caption{Rates in the CCAL modules during PrimEx $\eta$ production run. The energy threshold corresponds to 30 MeV. 
The beam goes through the center of the hole in the middle of the plot.}
\label{fig:ccal_rate}
\end{figure}
%----------------------------------------------------------------------

Timing resolution of reconstructed showers is an important characteristic of the detector performance. In the experiment 
we used timing information provided by the calorimeters to identify the accelerator beam bunch for which the interaction occured 
in the detector and therefore relate showers in the calorimeters with hits in the tagging detector, from the same event. 
A hit in the tagging detector defines the energy of the beam photon. The time in the calorimeter module is provided 
by an algorithm implemented on the programmable FPGA chip of the flash ADC. The algorithm performs a search of the peak of 
the signal pulse and determines the time from the shape of the leading edge of the pulse. The times of all hits constituting the CCAL 
shower are combined to form the shower time by using an energy-weighted sum. The time difference between beam photon candidates and CCAL showers 
originating from Compton events is presented in Fig.~\ref{fig:ccal_time}. The main peak on this plot corresponds to beam photons 
and CCAL clusters produced in the same accelerator bunch. Satellite peaks, separated by the beam bunch period of about 4 ns,  
represent accidental beam photons not associated with the detector time. The time resolution of CCAL showers is improved with the 
increase of the shower energy and was measured to be about 330 ps and 140 ps for 1 GeV and 9 GeV showers, respectively. In the PrimEx 
experiment, CCAL allowed a clear separation of beam photons originating from different beam bunches. 



An electron and photon produced in the Compton scattering process were detected by reconstructing two showers, one  
in the FCAL and another one in CCAL. The event topology of the reaction is such that the more energetic electron 
predominantly goes into the Compton calorimeter, while the photon strikes the FCAL. Reconstruction of electromagnetic 
showers in the FCAL is performed using an algorithm described in Ref.~\cite{fcal_clust}, which is a part of the standard 
GlueX reconstruction software. For the CCAL, we implemented an algorithm originally developed for the GAMS spectrometer~\cite{gams}, 
which was subsequently adopted for the HyCal ~\cite{hycal_kubantsev} in JLab's experimental Hall B. The algorithm provides a good separation 
of overlapping showers in the calorimeter by using profiles of electromagnetic showers.  
The elasticity distribution, defined as the 
reconstructed energy in the event minus the beam energy, is presented in Fig.~\ref{fig:compton} for Compton candidates produced by beam photons in the energy 
range between 6 GeV and 7 GeV. The solid line shows the fit of this distribution 
to the sum of a Gaussian and a second order polynomial function. The energy resolution 
of reconstructed Compton candidates in this energy range is about $130\;{\rm MeV}$.
% Compton eventswere required to be produced by beam photons with $E_{\rm beam} > 6\;{\rm GeV}$. 
In this plot, we subtracted background 
originating from multiple beam photon candidates in the event due to accidental hits in the GlueX tagging detectors. 
The background was measured using off-time interactions and amounted to about $15\%$. The relatively small 
background, on the level of $10\%$,  produced by interactions of  beam photons with the beamline material 
downstream the GlueX target was measured using empty-target runs and was excluded from the elasticity distribution 
in  Fig.~\ref{fig:compton}. % The typical energy resolution of reconstructed Compton candidates is about $120\;{\rm MeV}$. 
The CCAL allowed to clearly reconstruct Compton events in the PrimEx $\eta$ experiment. 

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{figures/ccal_time.pdf}
\end{center}
\caption{Time difference between beam photons and reconstructed CCAL showers for Compton candidates. Peaks are 
separated by the beam bunch period of 4 ns.}
\label{fig:ccal_time}
\end{figure}
%----------------------------------------------------------------------




%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/compton_elast.jpg}
\includegraphics[width=1.0\linewidth,angle=0]{figures/compton_elast1.pdf}
\end{center}
\caption{Elasticity distribution of reconstructed Compton candidates.}
\label{fig:compton}
\end{figure}
%----------------------------------------------------------------------


\section{Upgrade of the GlueX forward calorimeter}

% The GlueX detector in Hall D started collecting data in 2016. The flux of beam photons was gradually increased 
% and reached the designed value of $5\cdot 10^7\;{\rm \gamma/sec}$ in the energy range between 8 GeV and 9 GeV in the 
% Fall of 2019 in the experiment, which used a 30 cm long liquid hydrogen target. 

The forward calorimeter of the GlueX detector consists of 2800 lead glass modules, each with a size of 
$4\;{\rm cm}\:{\times}\:4\;{\rm cm}\:{\times}\:45\;{\rm cm}$, and 
is positioned about 6 m downstream of the target, as shown in Fig.~\ref{fig:gluex_det}. The FCAL covers a polar angle 
of photons produced from the target between $1^\circ$ and  $11^\circ$ and detects showers with energies in the range of 
0.1 - 8 GeV. The Cherenkov light produced in the module is detected by FEU-84-3 photomultiplier tubes, instrumented 
with Cockcroft-Walton bases~\cite{fcal_base}. 
The typical energy resolution of the FCAL is $\sigma_E/E = 6.2\%/ \sqrt E \oplus 4.7\%$.
%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 forward calorimeter of the GlueX detector is positioned 6 m downstream the target (as shown in Fig.~\ref{fig:gluex_det}), 
%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. 

The future physics program with the GlueX detector in Hall D will require 
an upgrade of the inner part of the forward calorimeter with high-granularity, high-resolution ${\rm PbWO_{\rm4}}$ 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. Lead tungstate crystals possess better radiation hardness compared
to lead glass, which is important for the long term operation of the detector at high luminosity.
We propose to build a $1\;{\rm m}\:{\times}\:1\;{\rm m}$ insert, which will require about 2496 modules. Similar to the CCAL, the insert will have 
a beam hole of $2\:{\times}\: 2$ modules and a tungsten absorber used to cover the detector layer closest to the beamline. 
%The size of the FCAL insert may slightly vary depending on availability of funds.  
A schematic view of the FCAL frame with the installed lead tungstate insert  is presented in Fig.~\ref{fig:fcal_frame}. 
Due to the different size of the lead glass bars and lead tungstate crystals, the lead glass modules stacked around 
the ${\rm PbWO_{\rm4}}$  insert will form four regions with a relative offset between modules; those regions are shown 
in green color in this plot.

The ${\rm PbWO_{\rm4}}$ module design of the FCAL insert will essentially be the same as for the CCAL, except for some small 
modifications needed to handle the magnetic field present in the FCAL region. The PMT housing made of the 
G-10 fiberglass material will be replaced by iron housing in order to reduce the magnetic field. The housing length will be increased 
to extended the magnetic shield beyond the PMT photo cathode. An acrylic optical light guide will be inserted inside the PMT housing 
to couple the crystal and PMT. 

The upgraded FCAL  will be operated in GlueX experiments using a 30 cm long liquid hydrogen target at the designed photon flux 
of  $5\cdot 10^7\;{\rm \gamma/sec}$ in the energy range between 8 GeV and 9 GeV. The designed luminosity is significantly larger than that used 
in the PrimEx $\eta$ experiment and was achieved  after the PrimEx run in the fall of 2019. In order to finalize the design of the PMT electronics, 
it is important to understand detector rates in the FCAL insert, especially in layers close to the beamline. We used CCAL during high-intensity GlueX runs to study run  conditions for the FCAL insert.  


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=0.9\linewidth,angle=0]{figures/fcal_frame.png}
\end{center}
\caption{FCAL frame with calorimeter modules installed: ${\rm PbWO_{\rm 4}}$ crystals  (brown area),
lead glass blocks (green). The photon beam passes through the hole in the middle of the calorimeter.}
\label{fig:fcal_frame}
\end{figure}
%----------------------------------------------------------------------

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.\linewidth,angle=0]{figures/nim_plot_field1.pdf}
\end{center}
\caption{Magnetic field distribution inside the PMT 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 produced by the Helmholtz 
coils. Markers denote different field values. }
\label{fig:field_distribution}
\end{figure}
%----------------------------------------------------------------------

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.\linewidth,angle=0]{figures/nim_plot_amp1.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). The PMT response was measured for different intensities of light pulse and HV settings as 
shown by different polymarkers.}
\label{fig:field_led}
\end{figure}
%----------------------------------------------------------------------

\subsubsection{PMT magnetic shield}

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  vary 
between 40 - 50 Gauss and 0 - 8 Gauss, respectively. The longitudinal field 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 $3\:{\times}\:3$ PMT iron housings made of AISI 1020 steel, which was positioned in the middle of Helmholtz coils. Each housing had a size of  
$20.6\;{\rm mm}\:{\times}\:20.6\;{\rm mm}\:{\times}\:100\;{\rm  mm}$ with a 19.9 mm round hole in the middle for the PMT. 
This corresponds to the realistic size of
the magnetic shield that will be used in the calorimeter module assembly.  Inside the housing 
we inserted two layers of $\mu$-metal Co-Netic cylinders, with thicknesses 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}.


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
% \includegraphics[width=0.95\linewidth,angle=0]{figures/light_guide_ps.pdf}
\includegraphics[width=0.95\linewidth,angle=0]{figures/light_guide_ps1.pdf}
\end{center}
\caption{ADC amplitudes of the calorimeter module as a function of the pair spectrometer tile for two configurations: 
the PMT directly coupled to the $\rm PbWO_{\rm 4}$ crystal (circles), and the PMT coupled to the module using an optical light guide (boxes).}
\label{fig:lg_ps}
\end{figure}
%----------------------------------------------------------------------
The Helmholtz coils had a diameter of about 1 m and can generate a uniform magnetic field with variable strength 
below 100 Gauss. A Hall probe was inserted into the central module of the prototype to measure the magnetic field at 
different $Z$-positions along the length of the cylinder. 
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 reduces 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 3.5 cm long acrylic light guides, in order to place the most sensitive to the magnetic 
field area of the PMT 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 the performance of the shielded PMT in the magnetic field using an LED pulser. A blue LED with a light diffuser 
was placed about 20 cm from the PMT housing prototype and was aligned with the middle module.  
%The light diffuser was placed between the LED and the prototype. in the middle. 
The PMT response was measured for different pulse amplitudes
and operational high voltage. 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 measured in the prototype tilted by 
about 10 degrees are presented on the left plot of Fig.~\ref{fig:field_led}. Amplitudes, normalized  to measurements 
without magnetic field, are shown on the bottom plot. The relative degradation of the signal amplitude for the maximum 
field in the FCAL insert region of 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 iron housing 
%and Co-Netic $\mu$-metal cylinder 
at the distance of at least 3 cm from the face of the ${\rm PbW0_{\rm4}}$ crystal, where the magnetic field is 
reduced and reaches the plateau. In order to do this, in the FCAL insert module we decided to use a 3.5 cm long  
acrylic cylindrical light guide with a diameter of 
18.5 mm between the PMT and the crystal. The light guide is wrapped with reflective ESR foil and  attached to the PMT with 
Dymax 3094 UV curing glue. Optical coupling to the crystal is provided by a ``silicon cookie'':  a 1 mm thick transparent 
rubber cylinder  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 disassembled if its PMT needs to be replaced.

We compared light losses of the FCAL insert module instrumented with the light guide with the CCAL module, 
where the PMT was coupled directly to the crystal using an optical grease.
Light collection was measured using electrons provided by the Hall D pair spectrometer (PS)~\cite{ps}.
The PS is used to measure the flux of beam photons delivered to the experimental hall by detecting 
electromagnetic electron-positron pairs produced by the photons in a thin converter inserted to the beam. 
Leptons from the pair are deflected in a dipole magnet and detected using scintillator detectors placed in the electron 
and positron arms of the spectrometer.  The energy of a lepton is detected using a high-granularity PS hodoscope, 
which consists of 145 scintillating tiles and covers the energy range between 3 GeV and 6 GeV.
The relative light yield of these two ${\rm PbW0_{\rm4}}$ module configurations was estimated by comparing ADC amplitudes of signal 
pulses in the module induced by electrons with know energies.

%Each detector consists of 145 tiles, which cover the energy range of leptons 
%between 3 GeV and 6 GeV.
%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}. 
 
We first measured the ADC response in the CCAL module, which was positioned behind the PS.
%We first positioned the CCAL module behind the PS and measured ADC amplitudes of signal pulses induced by 
%electrons with the energy of about 4 GeV. 
The module was subsequently modified by adding the light guide to 
the same PMT and crystal and was placed to the same spot of the PS test setup. Results of the measurements 
are presented  Fig.~\ref{fig:lg_ps}. The ADC amplitude of the calorimeter module is presented
as a function of the PS tile for the two module configurations with and without the light guide.
The light guide results in a relatively small loss of light  of about $15\%$ compared with the CCAL module.
We note that wrapping the light guide with the reflective material is important. Losses in unwrapped light guide 
constitute about $35\%$. We repeated light collection measurements using two more modules and obtained 
consistent results.




%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 rate}
\label{sec:fcal_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. 
%The luminosity in the  PrimEx $\eta$ experiment was  about a factor of  2.5 smaller than the  designed luminosity 
%of the GlueX, at which the FCAL lead tungstate insert will be operated.
%We performed a study of the CCAL performance in GlueX runs at high luminosity. 
The PMT anode current is one of the critical characteristics that have 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 the long-term degradation of the PMT. The anode current was 
estimated in CCAL modules during  data production runs at the GlueX nominal luminosity. The  special random trigger was used to 
read out flash ADC raw data for each CCAL channel in a time window of 400 ns, which corresponds to 100 flash ADC samples.
%The anode current was measured 
%using 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, $\bar{A}$,  in the readout window was determined by summing up 
amplitudes in the readout window and normalizing them to the window size. 
%The voltage measured by the ADC is produced by the current going through a $\sim50\;\Omega$  termination resistor. 
The anode current can be related to the average ADC voltage as
%------------------------------------------------
\begin{equation}
 I = \frac{\bar{A}}{R}\cdot\frac{1}{G}, 
\end{equation}
%------------------------------------------------
where $\bar{A}$ is the voltage in units of Volts,
%where $\bar{A}$ is the average ADC amplitude in the readout window in units of Volts, 
%$V_{\rm ADC} = 0.5\;{\rm mV}$ is the ADC resolution, 
$R$ is the input impedance of the amplifier ($\sim50\;\Omega$), and $G$ is the amplifier gain of 24. The typical anode current measured 
in CCAL modules situated at different distances from the beam line is presented in Fig.~\ref{fig:anode_current}.
Modules from the first CCAL layer closest to the beamline and the outer most layer were not used in the analysis.  
The inner modules were covered by a Tungsten absorber and the outer modules were obscured by the FCAL. 
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 amounts 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 anode current in the FCAL lead tungstate insert can be estimated by using the CCAL measurements and geometrical 
location of the CCAL and FCAL modules.
The largest PMT current in the insert $\rm PbWO_{\rm 4}$  module closest to the beam line is conservatively 
estimated to be about $20\;{\rm \mu  A}$  for a PMT base operated at 1 kV, assuming that no amplifier is used. 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 a gain of 5 and to omit the amplifier on other modules.
 
%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/anode_current1.pdf}
%\includegraphics[width=1.0\linewidth,angle=0]{figures/anode_current2.pdf}
\includegraphics[width=1.0\linewidth,angle=0]{figures/anode_current3.pdf}
\end{center}
\caption{Typical PMT anode current of CCAL modules positioned at different distances from the beamline. 
Circles correspond to the nominal GlueX luminosity, boxes correspond to $60\%$ of the nominal luminosity.}
\label{fig:anode_current}
\end{figure}
%----------------------------------------------------------------------


\section{Neutral Particle Spectrometer}

The NPS is a new facility in Hall C that will allow access to precision  measurements of small cross sections of reactions  with neutral final states.
%The NPS is a new facility in Hall C that will allow access to precision small cross section measurements with neutral final states.
 The NPS consists of an electromagnetic calorimeter preceded by a sweeping magnet. As operated in Hall C, it replaces one of the 
focusing spectrometers. 

The NPS science program currently features six fully approved experiments. E12-13-010~\cite{e12-13-010} and E12-06-114~\cite{e12-06-114} experiments 
will measure the Exclusive Deeply Virtual Compton Scattering and $\pi^0$ cross sections to the highest $Q^2$ accessible 
at Jefferson Lab. Both experiments will provide important information for understanding Generalized Parton Distributions (GPDs). 
The E12-13-007~\cite{e12-13-007} 
experiment will study semi-inclusive $\pi^0$ electroproduction process and seeks to validate the factorization framework that is needed by the entire 
12 GeV Jefferson Lab semi-inclusive deep-inelastic scattering program. Measurements of Wide-Angle and Timelike Compton Scattering reactions will 
be performed by the E12-14-003~\cite{e12-14-003} and E12-17-008~\cite{e12-17-008} experiments. These measurements will allow to test universality of 
GPDs using high-energy photon beams. The NPS will also be used in the E12-14-005~\cite{e12-14-005} experiment to study exclusive production of 
$\pi^0$ at large momentum transfers in the process $\gamma p \to \pi^0 p$.  Hard exclusive reactions provide a testing ground for quantum 
chromodynamics at intermediate energies.




%The NPS science program currently features six fully approved experiments: E12-13-010, E12-06-114 (DVCS and exclusive $\pi^0$)~\cite{E12-13-010}, E12-13-007 ($\pi^0$ SIDIS)~\cite{E12-13-007}, E12-14-003 (Wide-Angle Compton Scattering)~\cite{E12-14-003}, E12-14-005 (Wide Angle exclusive $\pi^0$ production)~\cite{E12-14-005}, E12-18-008 ($A_{LL}$ \& $A_{LS}$ polarization observables in WACS at large $s$, $t$ and $u$)~\cite{E12-17-008}, and one conditionally approved experiment C12-18-005 (Timelike Compton Scattering off a Transversely Polarized Proton)~\cite{C12-18-005}. E12-13-010 will provide precision DVCS and $\pi^0$ cross sections to the highest $Q^2$ accessible at Jefferson Lab. For the DVCS experiments, this can further validate the handbag-diagram assumed reaction mechanism required for access to the Generalized Parton Distributions (GPDs). The $\pi^0$ data may validate access to a novel class of transversity GPDs accessible at Jefferson Lab. The E12-13-007 experiment seeks to validate the factorization framework that is needed by the entire 12 GeV Jefferson Lab SIDIS program. The NPS will also be used for measurements of Wide-Angle and Timelike Compton Scattering reactions that will allow universality tests of GPDs using high-energy photon beams. 



The NPS science program requires neutral particle detection over an angular range between 6 and 57.3 degrees at distances of between 3 and 11 meters~\footnote{The minimum NPS angle at 3m is 8.5 degrees; at 4m it is 6 degrees.} from the experimental target.  
The experiments will use a high-intensity beam of electrons with the energies of  6.6, 8.8, and 11 GeV, and a typical luminisity 
of $\sim10^{38}\;{\rm cm^{-2} s^{-1}}$ as well as a secondary beam of 
photons incident on a liquid hydrogen target. A vertical-bend sweeping magnet with integrated field strength of 0.3 Tm will be 
installed in front of the spectrometer in order to suppress and eliminate  background of charged particle tracks originating from the target. 
The photon detection is the limiting factor of the experiments. Exclusivity of the reaction is ensured by the missing 
mass technique and the missing-mass resolution is dominated by the energy resolution of the calorimeter. The calorimeter should provide the spacial 
resolution of 2-3 mm and the energy resolution of about $2\%/\sqrt E$.

The NPS consists of 1080 $\rm PbWO_{\rm 4}$ crystals that form an array of $30\:{\times}\:36$ modules.
% where the outer layers only have to catch the showers. 
Similarly to the FCAL insert in Hall D, the NPS will be built from the crystals of the same size, and instrumented with the same PMTs and readout electronics.
Each crystal will be wrapped with the reflective ESR foil and positioned inside the support structure, where the modules will be separated from each other 
by thin carbon fiber plates. The detector will be positioned inside a temperature-controlled box on a movable platform. The details of the mechanical assembly  
and commissioning of the NPS are currently under development and will be described in a forthcoming publication. 

The maximum rate in the calorimeter of the NPS experiments will be maintained on the level below 1 MHz per module.
%at high luminosity and forward angles ranges up to  1 MHz per module.
 Based on Monte Carlo 
simulation, the integrated doses for the  E12-13-010 experiment are 1.7 MRad at the center and 3.4 MRad at the edges of the calorimeter and the maximum 
anticipated dose rate is on the level of a kRad/hour. The integrated doses for the other experiments do not exceed $500\;{\rm kRad}$~\footnote{The radiation 
doses are larger than that for the FCAL lead tungstate insert, where the integrated dose for modules positioned closest to the beamline is smaller than 200 kRad.}.
The detector will be instrumented with a light monitoring system. Light from a blue LED will be distributed to each calorimeter module through a quartz 
optical fiber, attached to the crystal from the PMT side. The LED system will be used for calibration and allow to cure crystals whose performance is degraded 
due to radiation. Signal pulses from the PMT will be digitized  using flash ADCs hosted in VXS crates. 
%High voltage for PMTs is supplied by a 36-channel 
%CAEN 7030N module installed in a  SY4527 crate. 
Energy deposition  in the calorimeter, will be used in the trigger system of the experiments. Integration of the detector to the trigger will be performed by 
means of the trigger electronics modules designed at Jefferson Lab.

The radiation hardness and good optical quality of lead tungstate crystals are critical for the NPS calorimeter.  The NPS collaboration, in a synergistic 
effort with the EIC eRD1 consortium, has characterized to date over 1200 $\rm PbWO_{\rm 4}$ crystals produced by CRYTUR and SICCAS from 2014 to the present. The results 
of these studies have been published in Ref.~\cite{pwo_crystals}.  CRYTUR crystal samples were found to have greater overall uniformity in transmittance and 
light yield, and better radiation hardness. Of the samples characterized by the NPS collaboration 140 SICCAS crystals have been used in the CCAL detector.

%The expected rates of the NPS experiments in the high luminosity Hall C range up to 1 MHz per module. Given the high luminosity and very forward 
%angles required in the experiments, radiation hardness is an essential factor when choosing the detector material. Based on Monte Carlo simulation, 
%the integrated doses for the  E12-13-010 experiment are 1.7 MRad at the center and 3.4 MRad at the edges of the calorimeter. The integrated doses for 
%the other experiments do not exceed $< 500\;{\rm kRad}$


%The anticipated doses depend on the experimental 
%kinematics and are highest at the small forward angles. Based on background simulations dose rates of 1-5 kRad/hour are anticipated at the most forward angles. 
%The integrated doses for E12-13-010 are 1.7 MRad at the center and 3.4 MRad at the edges of the calorimeter. The integrated doses for the other experiments 
%are $< 500\;{\rm kRad}$~\footnote{The radiation doses are significantly larger than for Hall D lead tungstate FCAL insert, where the integrated dose in the modules 
%close to the beamline will not exceed 100 kRad/year.}.



% and with 2-3 mm spatial and about $2\%$ energy resolution. Electron beam energies of 6.6, 8.8, and 11 GeV will be used. The photon detection is the limiting factor of the experiments. Exclusivity of the reaction is ensured by the missing mass technique and the missing-mass resolution is dominated by the energy resolution of the calorimeter. The scintillator material should thus have properties to allow for an energy resolution of $1-2\%/\sqrt(E)$. The expected rates of the NPS experiments in the high luminosity Hall C range up to 1 MHz per module. The scintillator material response should thus be fast, and respond on the tens of nanosecond level. Given the high luminosity and very forward angles required in the experiments, radiation hardness is also an essential factor when choosing the detector material. The anticipated doses depend on the experimental kinematics and are highest at the small forward angles. Based on background simulations dose rates of 1-5 kRad/hour are anticipated at the most forward angles. The integrated doses for E12-13-010 are 1.7 MRad at the center and 3.4 MRad at the edges of the calorimeter. The integrated doses for the other experiments are $<$ 500 kRad. The ideal scintillator material would be radiation hard up to these doses. The ideal material would also be independent of environmental factors like temperature.



%To address the experimental requirements the NPS has the following components:

%\begin{itemize}

%\item{A 25 msr neutral particle detector consisting of 1080 PbWO4 crystals in a temperature-controlled frame including gain monitoring and curing systems}

%\item{HV distribution bases with built-in amplifiers for operation in a high-rate environment}

%\item{Essentially deadtime-less digitizing electronics to independently sample the entire pulse form for each crystal}

%\item{A vertical-bend sweeping magnet with integrated field strength of 0.3 Tm to suppress and eliminate charged background.}

%\item{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 (see Fig.~\ref{fig:NPS-right}); for NPS angles 23-57.5 degrees it will be on the right.}

%\item{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}

%\end{itemize}

%Here, the focus is on the crystalline scintillator used in the NPS. The details of the mechanical assembly and commissioning of the NPS is subject of a forthcoming publication.



%The material of choice for the NPS calorimeter is rectangular PbWO$_4$ crystals of 2.05 by 2.05 cm$^2$ (each 20.0 cm long). The crystals are arranged in a 30 x 36 matrix, where the outer layers only have to catch the showers. This amounts to a total of 1080 PbWO$_4$ crystals. For NPS standard configurations, each crystal covers 5 mrad and the expected angular resolution is 0.5-0.75 mrad, which is comparable with the resolution of the High Momentum Spectrometer (HMS), one of the well established Hall C spectrometers. 



%Good optical quality and radiation hard PbWO$_4$ crystals are essential for the NPS calorimeter. Such crystals or more cost-effective alternatives are 
%also of great interest for calorimeters at the Electron-Ion Collider (EIC). The NPS collaboration, in a synergistic effort with the EIC eRD1 consortium, 
%has characterized to date over 1200 PbWO$_4$ crystals produced by CRYTUR and SICCAS from 2014 to present. The results of these studies have been published 
%in Ref.~\cite{pwo_crystals}. CRYTUR crystal samples were found to have the overall better uniformity in ransmittance, light yield, and better radiation hardness.
%Of the samples characterized by the NPS collaboration 144 SICCAS crystals have been used in the CCAL detector, which is discussed in this article.



%Based on NPS specifications, the overall quality of CRYTUR crystal samples was found to be better than that of SICCAS samples. Categories in which CRYTUR samples performed better include uniformity of samples, e.g. in transmittance and light yield, and considerably better radiation hardness. CRYTUR samples also showed fewer mechanical defects, both macroscopic and microscopic. Of the samples characterized by the NPS collaboration 144 SICCAS crystals have been used in the CCAL detector, which is discussed in this article.




%The neutral-particle spectrometer offers unique scientific capabilities to study the transverse spatial and momentum 
%structure of the nucleon in the Jefferson Lab experimental Hall C. Much progress in imaging nucleon structure can be 
%made with electron-scattering reactions, yet experiments with high-energy photons play a unique complementary role. 
%The small scattering probabilities of exclusive reactions demand high-intensity photon beams. Understanding strengthened 
%by imaging longitudinally-polarized and transversely-polarized nucleons. Five experiments have been currently approved 
%using the NPS. The experiments will use a high-intensity beam of electrons, with a typical lumonisity 
%of $\sim 10^{38}\;{\rm cm^{-2} s^{-1}}$ as well as a secondary beam of photons incident on a liquid hydrogen target.


%Similar to the FCAL insert in experimental Hall D, the NPS will be built from the same type of crystals, and instrumented 
%with the same PMTs and readout electronics. The NPS consists of 1080 $PbWO_{\rm 4}$ crystals, which form an array of 
%30x36 modules. The detector is positioned on a movable platforms in a temperature controlled frame designed by INPN Orsay. 
%In different experiments, the detector will be placed at different angles between 5.5 and 60 degrees with respect to the 
%Hall C beamline. A sweep magnet will be installed in front of the spectrometer in order to reduce background of charged 
%particle tracks originating from the target. The integrated field of the magnet varies between 0.3 Tm and 0.6 Tm depending 
%on the distance of the NPS detector from the beamline. The maximum fringe field originating from the magnet in the detector 
%region is 200 Gauss.


%$PbWO_{\rm 4}$ crystals produced by CRYTUR are expected to have slightly better radiation hardness compared to the SICCAS 
%crystals~\cite{pwo_crystals}. The CRYTUR crystals will be installed in the region close to the beamline, where the radiation 
%level is the highest, whereas the SICCAS crystals will be placed at the edges of the detector further away from the beam. Each 
%crystal will be wrapped with the reflective ESR foil. The crystal's support structure consists of horizontal and vertical layers 
%of 0.5 mm thick carbon plates. Each crystal is positioned inside a cell surrounded by the carbon plates. Hamamatsu R4125 PMT 
%is situated inside a round $\mu$-metal housing with an out diameter of 19.6 mm  and is attached to the back side of each module.
%In order to  provide additional magnetic field shielding, 0.5 mm thick $\mu$-metal plates are used around the PMT housing. 
%The initial magnetic field is reduced by surrounding the NPS frame, except the face and the back of the detector, by a 10 mm 
%thick soft iron shield box.

%The design of the PMT base will be adjusted according to run conditions of experiments with the NPS. The gain of the amplifier 
%will be determined according to rates expected in the detector, which are currently sumulated using Geant detector simulation. 
%Signal pulses from the PMT are digitized  using flash ADCs hosted in VXS crates. High voltage for PMTs is supplied by a 36-channel 
%CAEN 7030N module installed in a  SY4527 crate. Energy deposition  in the calorimeter, will be used in the trigger system of 
%the experiments. Integration of the detector to the trigger will be performed by means of the trigger electronics modules designed 
%at Jefferson Lab. A blue LED is distributed to each calorimeter module through a quartz optical fiber, glued to the crystal from 
%the PMT side. The LED system will be used for calibration and allow to cure crystals whose performance is degraded due to radiation.  
%The cluster reconstruction software of the NPS is currently under development. It will adopt the already existing clustering 
%algorithm of the DVCS experiment in experimental Hall A [reference needed].  




%----------------------------------------------------------------------
%\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}
%----------------------------------------------------------------------









\section{Summary}
\label{sec_summary}

We described the design and performance of the Compton calorimeter, which was constructed using 
140 lead tungstate $\rm PbWO_{\rm 4}$ crystals recently produced by SICCAS. The calorimeter was successfully 
used in the PrimEx $\eta$ experiment in spring of 2019 for reconstruction of Compton scattering events.
The CCAL served as a prototype for two large-scale electromagnetic calorimeters based on the  $\rm PbWO_{\rm 4}$ 
crystals: the lead tungstate insert of the forward calorimeter of the GlueX detector and the 
neutral particle spectrometer. Experience gained during construction and operation of the CCAL provided  
important information for finalizing the design of FCAL  $\rm PbWO_{\rm 4}$ modules and PMT dividers and also 
served to further optimize the NPS calorimeter. We presented the design of the FCAL lead tungstate insert 
and gave an overview of the NPS project.

\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. This work was supported in part by NSF grants PHY1714133 and PHY2012430. 
We thank the NPS collaboration/project for providing $\rm PbWO_{\rm 4}$  crystals and PMTs used in the construction of the CCAL.


%% The Appendices part is started with the command \appendix;
%% appendix sections are then done as normal sections
%% \appendix

%% \section{}
%% \label{}

%% If you have bibdatabase file and want bibtex to generate the
%% bibitems, please use
%%
%%  \bibliographystyle{elsarticle-num} 
%%  \bibliography{<your bibdatabase>}

%% else use the following coding to input the bibitems directly in the
%% TeX file.

\bibliographystyle{elsarticle-num}
%% \bibliography{pair_spect}

\begin{thebibliography}{00}

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%``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{label}
%% Text of bibliographic item

\bibitem{jef}
JLab Experiment {\bf E12-12-002A}, Eta Decays with Emphasis on Rare Neutral Modes:The JLab Eta Factory (JEF) Experiment, 
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\bibitem{gams}
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%doi:10.1063/1.4826876


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\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
