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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[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 to the DAQ and trigger systems 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 
for the two detectors, which are currently under construction at Jefferson Lab using similar 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 calorimeter, Lead tungstate scintillator
\end{keyword}
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\end{frontmatter}

\linenumbers

%% main text
\section{Introduction}

Electromagnetic calorimeters based on the  $\rm PbWO_{\rm 4}$ scintillating crystals have a widespread application in experiments 
in different accelerator facilities such as CERN, FNAL, GSI, and Jefferson Lab (JLab). 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 the experimental Hall D and Hall C at Jefferson 
Lab using 2.05 cm x 2.05 cm x 20 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 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~\cite{jef}. The size of the insert will tentatively consist of 2496 lead tungstate modules. 
The neutral-particle spectrometer (NPS)~\cite{nps} in experimental Hall C 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 were provided by 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 and is described in Ref.~\cite{pwo_crystals}.  Crystals  of the same type are considered to be used in 
the electromagnetic calorimeter of the future Electron-Ion Collider.


% 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. 

We built a  small-size 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 operation conditions expected for the FCAL lead tungstate insert.
Experience gained during fabrication and operation of the CCAL was critical for 
finalizing the designs of electromagnetic calorimeters of the FCAL and NPS projects. 
%Crystals provided by SICCAS are considered to be used in 
%the electronagnetic calorimeter of the future Electron-Ion Collider.

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

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\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 calorimeter (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. 










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

The GlueX detector~\cite{gluex_det} in experimental Hall D was designed to perform experiments using photon beams.
Beam photons are produced by electrons provided by the JLab electron accelerator facility, incident on a 
thin radiator via the bremsstrahlung process. The energy of a beam photon is determined by detecting 
a scattered bremsstrahlung electron using tagging scintillator detectors with a typical precision 
of $0.2\%$. 

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. $\eta$ 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 process, which will also be used to monitor the 
luminosity and control the detector stability during taking data. Electrons and photon originating from Compton 
events in the target are produced at small angles typically outside the acceptance of the FCAL. In order to improve 
reconstruction of particles in the forward direction, 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 from 
the FCAL. The CCAL covers the angular range between $0.18^{\circ}$ and  $0.33^{\circ}$. Schematic view of the 
GlueX detector and the position of the Compton calorimeter is illustrated in Fig.~\ref{fig:gluex_det}.

The PrimEx $\eta$ experiment started collecting data in the spring of 2019 and has acquired about $30 \%$ 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 $5\cdot 10^6\;\gamma/{\rm sec}$ 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 is 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 a 
light tight box. A Tungsten absorber is placed in front of the innermost layer closest to the beamline, which 
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 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. Foam insulation surrounded 
the detector box. The temperature was monitored and recorded during the experiment by four thermocouples 
attached to different points of the $\rm PbWO_{\rm 4}$ module assembly. During the experiment the temperature was maintained at 
$17\pm 0.2^\circ C$. The typical heat released by the divider is equivalent to 33 Watts. 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 the PMT dividers. The detector was positioned on a 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}
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}. 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 $98.5\%$ reflectivity across the visible spectrum. In order to improve 
optical isolation of each module from its neighbors, each crystal was wrapped with a layer 
of $25\;{\rm \mu m}$ thick Tedlar. The PMT is locates 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 $25\;{\mu m}$ brass straps, which are brazed to the sides of the flanges. 
Four set screws are applied to the PMT housing flange to generate  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 optical grease. 
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, 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.


%----------------------------------------------------------------------
\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.}
\label{fig:ccal_module}
\end{figure}


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



 

\subsection{Electronics}
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 to operate the PMT at smaller voltage and consequently at lower 
anode current and therefore 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 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 run conditions of 
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 680 V, which produced a divider current of $260\;\mu A$. High voltage for 
each PMT was supplied by a 24-channel CAEN A7236SN module positioned in a SY4527 mainframe. 


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 instrumentation of several sub-detectors of the GlueX detector. 
The Field-Programmable Gate Array (FPGA) chip inside the ADC module allows to implement 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, for amplitudes above a threshold of 24 MeV. ADC amplitudes are summed in a time window 
of 64 ns and reported by 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 read out 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 and include the CCAL to 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/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}
%----------------------------------------------------------------------

\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 homogenize 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 in such a way to make 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 to 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 better than $1\%$. The ratio of signal ADC amplitudes from the LED pulser to 
the $\alpha$ 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_amplitudes.png}
\includegraphics[width=1.0\linewidth,angle=0]{figures/led_amplitudes1.png}
\end{center}
\caption{Flash ADC signal amplitudes  induced by the LED and $\alpha$-source in the reference PMT.}
\label{fig:led_amp}
\end{figure}
%----------------------------------------------------------------------


%----------------------------------------------------------------------
\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 $\alpha$-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 each calorimeter module to 
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.


In the beginning of the calibration run, we performed adjustment of the PMT high voltage for 
each module in order to equalize signal pulse amplitudes induced by 10 GeV beam photons. 
The amplitude was set 3200 ADC counts. An example of  flash ADC signal amplitudes 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. 

In calibration runs, we estimated the non-uniformity of 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 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 non-linear performance of the PMT active base, with the large amplification 
factor of 24, on the level of a few percents, which impacted both the pulse peak and pulse 
integral. The base performance became linear when the amplifier gain was reduced. The CCAL electronics 
is being currently adjusted; modified active bases will be installed before the new PrimEx $\eta$ run in 2021.
We subsequently replaced the original front end electronics for an array of 3x3 CCAL 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 incident on the center of the middle module 
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 distribution was fit to a Crystal Ball function, which was named after the Crystal Ball collaboration and 
was 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 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 
%$S = 0.41  \pm 0.03\%$. 
where $S$ represents the stochastic term, $N$ the 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 cm x 2.05 cm, but a smaller 
length of 18 cm. The CCAL calibration was fine-tuned during the PrimEx run by using showers of reconstructed Compton 
candidates.


%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_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}
%----------------------------------------------------------------------

%----------------------------------------------------------------------
\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 CCAL modules for 6 GeV beam photons.}
\label{fig:ccal_mod_uniform}
\end{figure}
%----------------------------------------------------------------------


\subsection{Performance during the 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.    
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 the 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}
\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 2x2 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 $\eta$ 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. 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}
\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 (plot will be updated).}
\label{fig:ccal_rate}
\end{figure}
%----------------------------------------------------------------------

Time resolution of reconstructed showers is an important characteristic of the detector performance. In the experiment 
we used timing information provided by calorimeters to identify the accelerator beam bunch where the interaction occurs 
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 of a hit 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 time from the shape of the leading edge of the pulse. The times of all hits constituting the CCAL 
shower are combined to form 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. Satellites 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 to clearly separate 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 is sent to 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 later 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, for Compton candidates produced by beam photons in the energy 
range between 6 GeV and 7 GeV is presented in Fig.~\ref{fig:compton}. The solid line shows the fit of this distribution 
to the sum of a Gaussian and a second order polynomial functions. The energy resolution 
of reconstructed Compton candidates 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  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, with a size of 4 cm x 4 cm x 45 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 produced Russia, 
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 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. 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 m x 1 m insert, which will require about 2496 modules. Similar to the CCAL, the insert will have 
a beam hole of 2 x 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 ${\rm PbW0_{\rm4}}$  insert will form four regions with a relative offset between modules; those regions are shown 
in green color in this plot.

The ${\rm PbW0_{\rm4}}$ 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: $PbWO_{\rm 4}$ crystals  (brown area),
lead glass blocks (green). Photon beam goes in 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. Markers denote different
field values produced by the Helmholtz coils. }
\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). 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 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 iron housings made of AISI 1020 steel, 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 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: 
PMT directly coupled to the $\rm PbWO_{\rm 4}$ crystal (circles), 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 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 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 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. 
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. 
The light guide is 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 disassembles if PMT needs to be replaced.

We compared light losses of the FCAL insert module instrumented with the light guide with the CCAL module, 
where PMT is 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 two scintillator detectors placed in the electron 
and positron arms of the spectrometer.  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 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}. On this plot 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 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 $35\%$.


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


 

\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 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 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 a 
$\sim 50\;\Omega$  termination resistor. The anode current can be estimated as
%------------------------------------------------
\begin{equation}
 I = \frac{\bar{A}}{R}\cdot\frac{1}{G}, 
\end{equation}
%------------------------------------------------
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 termination resistor, 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.  
These modules were covered by a Tungsten absorber and 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 largest PMT current in the $\rm PbWO_{\rm 4}$  module closest to the beam line is conservatively 
estimated to be about $20\;\mu{\rm A}$  for a PMT base operated at 1 kV, and 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.
 



\section{Neutral Particle Spectrometer}

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 events.
The CCAL served as a prototype for two large-scale electromagnetic calorimeters, designed to use $\rm PbWO_{\rm 4}$ 
crystals of the same size: 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 these electromagnetic calorimeters. The design of the NPS and  
FCAL lead tungstate insert was presented.

\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 ...


%% The Appendices part is started with the command \appendix;
%% appendix sections are then done as normal sections
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%% \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,''
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%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}
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%``A PbWO$_4$-based Neutral Particle Spectrometer in Hall C at 12 GeV JLab,''
J. Phys. Conf. Ser. \textbf{587} (2015) no.1, 012048.
% doi:10.1088/1742-6596/587/1/012048
%3 citations counted in INSPIRE as of 14 Jan 2021

\bibitem{pwo_crystals}
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%``Scintillating crystals for the Neutral Particle Spectrometer in Hall C at JLab,''
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%doi:10.1016/j.nima.2019.163375

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http://www.wiener-d.com/sc/powered-crates/vxs/


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%doi:10.1063/1.2396938
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% \bibitem{gluex}
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% \bibitem{f1tdc}
%  F.~Barbosa {\it et al.}, Proceedings of IEEE Nuclear Science Symposium, Virginia, USA (2002).

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%doi:10.1016/S0168-9002(97)00048-X
%2670 citations counted in INSPIRE as of 31 Jan 2021


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%doi:10.1016/j.nima.2015.06.012

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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
