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%%% \journal{Nuclear Physics A}

\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 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[jlab]{E.Chudakov}
\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]{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[jlab]{S.Taylor}
\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]{The 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 115409, 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,  NC  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 (PbWO$_4$)  scintillating 
crystals was constructed for the  PrimEx-$\eta$ experiment at Jefferson lab. The calorimeter was 
integrated into the data acquisition 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 PbWO$_4$-based  
detectors: 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.

\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 PbWO$_4$ 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 PbWO$_4$  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.


Two electromagnetic calorimeters are currently under construction in experimental Hall D and Hall C at Jefferson Lab, 
both using rectangular $2.05\;{\rm cm}\:{\times}\:2.05\;{\rm cm}\:{\times}\:20\;{\rm cm}$  PbWO$_4$  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 JLab Eta Factory (JEF) experiment to 
perform precision measurements of various $\eta^{(\prime′)}$ decays with emphasis on rare neutral modes~\cite{jef}.
%the physics program  with the GlueX detector, specifically the new experiment to study rare decays of $\eta$ mesons~\cite{jef}. 
The Neutral Particle Spectrometer~\cite{nps} 
in experimental Hall C consists of a  PbWO$_4$ 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 
PbWO$_4$ crystals arranged in a $30\:{\times}\:36$ array. 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  PbWO$_4$ 
scintillators has been studied in detail by the NPS and EIC eRD1 collaborations and is described in Ref.~\cite{pwo_crystals}.  
PbWO$_4$ crystals are also being considered for an electromagnetic calorimeter of the future Electron-Ion Collider~\cite{eic}.


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. 

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]{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}
%----------------------------------------------------------------------



\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 ($E_{\gamma}$) is determined by detecting a scattered electron after radiating 
the photon as follows: $E_{\gamma} = E_{e} - E_{e}^\prime$, where $E_{e}$ is the primary electron beam energy and 
$E_{e}^\prime$ is the energy of the bremsstrahlung electron. The bremsstrahlung electron is deflected in a 6 m long 
dipole magnet operated at a field of $\sim1.5$ T and registered in the so-called tagging scintillator counters. Each counter 
corresponds to the specific energy of the reconstructed lepton. The tagging detectors span the beam photon energy 
range between $25\%$ and $98\%$ of the electron beam energy and covered the range between 2.8 GeV and 11.0 GeV during 
the PrimEx-$\eta$ experiment~\footnote{The electron beam energy during most production PrimEx-$\eta$ runs was 11.2 GeV.}. 
The typical energy resolution of the beam photon is about  $0.1\%$. The photon beam propagates 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 quantum chromodynamics 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.

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 
lead tungstate scintillating crystals and  positioned it about 6 m downstream from the FCAL as shown in Fig.~\ref{fig:gluex_det}.  
The CCAL covers the angular range between $0.19^{\circ}$ and  $0.47^{\circ}$. 

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 photon flux was about $5\cdot 10^6\;\gamma/{\rm sec}$ (about five times lower than the nominal GlueX flux) in the beam energy 
range of interest between 9.5 GeV and 11.6 GeV.

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{ccal_assembly.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}


\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 high rate of particles predominantly originating from electromagnetic 
interactions.
 
The light yield from PbWO$_4$ crystals depends on temperature with a typical 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 PbWO$_4$ 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 of the whole detector was equivalent to about 30 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 PbWO$_4$ module is based on the HyCal calorimeter, which was used in several experiments in Jefferson 
Lab Hall B~\cite{hycal_kubantsev,hycal_gasparian}. 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 (EJ-550). 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 (HV) divider and amplifier positioned on the same printed circuit board attached to the PMT socket.


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{ccal_module.pdf}
\end{center}
\caption{Calorimeter module showing main components: the PbWO$_4$ crystal, PMT housing, PMT divider, and signal and high-voltage 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. 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. 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 has a relatively large amplification of about a factor of 24 due to the large PMT count rate 
predicted by Monte Carlo simulation of the NPS detector. Large amplification was not needed for the planned 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 PMTs were 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]{ccal_waveform.pdf}
\end{center}
\caption{A typical flash ADC signal pulse obtained from a PbWO$_4$ 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 read out 
window size of 400 ns.  During the PrimEx-$\eta$ experiment, the ADC performed on-board integration of 
signal pulses, which amplitudes were above a threshold of 24 MeV. Amplitudes were summed in a time window 
of 64 ns and read out from the ADC module along with other parameters such as the pulse peak amplitude, 
pulse time, 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 data acquisition.  



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.

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{led_amplitudes.pdf}
\end{center}
\caption{Flash ADC signal amplitudes  induced by the LED and $\alpha$-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 range of a 12-bit flash ADC corresponding to 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 produced 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.


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{led_stability.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}
%----------------------------------------------------------------------


\subsection{Calibration}

The initial energy calibration of the CCAL was performed by moving the calorimeter platform and positioning 
each module into the photon beam during special low-intensity calibration runs. The maximum rate in the 
module exposed to the beam did not exceed 200 kHz at a  threshold of 15 MeV. The energy of each beam photon 
was determined by detecting a bremsstrahlung electron using the GlueX tagging detectors described in Section 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]{ccal_tagger.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]{ccal_mod_uniform.pdf}
\end{center}
\caption{Relative energy resolution of 140 PbWO$_4$ modules installed on the CCAL measured with 6 GeV beam photons.}
\label{fig:ccal_mod_uniform}
\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 11 GeV beam photons. The amplitude was set to 3500 ADC counts, which corresponds to  
$\sim1.7\;{\rm V}$. 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 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 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 $5\%$. 

During calibration, we observed some 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. An example of 
the energy deposited 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 is 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}. 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 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 amplifier 
gain of 24~\footnote{The linearity of the PMT active base is being currently improved; modified active bases will be installed before 
the new PrimEx-$\eta$ run in 2021.}. 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 PbWO$_4$ 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 after the PrimEx-$\eta$ run by using showers of reconstructed Compton scattering candidates and 
constraining the reconstructed energy in the event to the know beam energy. 

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{energy_fit.pdf}
\end{center}
\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]{energy_res.pdf}
\end{center}
\caption{The CCAL energy resolution as a function of the photon energy.}
\label{fig:ccal_en_res}
\end{figure}
%----------------------------------------------------------------------




\subsection{Performance during the PrimEx-$\eta$ run}

In the PrimEx-$\eta$ experiment, we reconstruct Compton events produced by beam photons with the energy larger than 
6 GeV. This energy range is covered by the GlueX pair spectrometer~\cite{ps}, which determines 
the photon flux needed for cross section measurements. An electron and photon produced in the Compton scattering 
process were detected by reconstructing two showers, one  in the FCAL and another one in the 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. 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 data acquisition and trigger systems. 
The trigger rate was reproduced  by a detailed Geant detector simulation. 


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 beamline. The maximum 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 up to 2 MHz~\cite{pmt_high_rate}.



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 
occurred 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 from accelerator bunches not associated with the interaction 
in the detector. 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-$\eta$ experiment the CCAL allowed a clear 
separation of beam photons originating from different beam bunches. 

%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{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=0.9\linewidth,angle=0]{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.}
\label{fig:ccal_rate}
\end{figure}
%----------------------------------------------------------------------

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,gams1}, 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 $130\;{\rm MeV}$.
In this plot, we subtracted background originating from accidental beam photons. This 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 the photon beam with the beamline material downstream the GlueX target was measured using empty-target runs and was also 
excluded from the elasticity distribution in  Fig.~\ref{fig:compton}. 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]{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]{compton_elast.pdf}
\end{center}
\caption{Elasticity distribution of reconstructed Compton candidates.}
\label{fig:compton}
\end{figure}
%----------------------------------------------------------------------


\section{Upgrade of the GlueX forward calorimeter}

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 
$\sigma_E/E = 6.2\%/ \sqrt E \oplus 4.7\%$~\cite{gluex_det}.


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 PbWO$_4$ 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. The size of the insert will tentatively 
comprise 1596 PbWO$_4$ crystals, which will form an array  of $40\:{\times}\: 40$ modules~\footnote{The insert 
size proposed for the JEF experiment~\cite{jef} is  $1\;{\rm m}\:{\times}\:1\;{\rm m}$; the actual size will depend on 
the availability of funds.}. Similar to the CCAL, the insert will have a beam hole of $2\:{\times}\: 2$ modules and a 
tungsten absorber used to shield the detector layer closest to the beamline. 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 PbWO$_4$  insert 
will form four regions with a relative offset between modules; those regions are shown in green color in this plot.

The PbWO$_4$ 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 
extend the magnetic shield beyond the PMT photocathode. 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 8.4 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 the 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]{fcal_frame.png}
\end{center}
\caption{FCAL frame with calorimeter modules installed: PbWO$_4$ 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]{plot_field.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]{plot_amp.pdf}
\end{center}
\caption{Signal amplitudes of shielded PMT induced by an LED as a function of the magnetic field (a). Amplitudes, normalized  
to measurements without magnetic field (b). 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{Magnetic shielding of PMTs}

The longitudinal (directed along the beamline) and transverse (directed perpendicular to the axis of the beamline) 
components of the magnetic field produced by the GlueX solenoid magnet in the FCAL PbWO$_4$ insert area  vary 
between 40 - 55 Gauss and 0 - 9 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  $20.6\;{\rm mm}\:{\times}\:20.6\;{\rm mm}\:{\times}\:104\;{\rm  mm}$ with a 20 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 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. 


%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=0.95\linewidth,angle=0]{light_guide_ps.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 PbWO$_4$ crystal (circles), and the PMT coupled to the module using 
optical light guide (boxes).}
\label{fig:lg_ps}
\end{figure}
%----------------------------------------------------------------------
The Helmholtz coils had a diameter of about 1.5 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}. The actual field inside the FCAL insert module is expected to be 
even smaller due to the collective shielding effect, i.e., the large amount of shielding material installed on surrounding 
modules~\cite{tosca}.


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 PMT response was measured 
for different pulse amplitudes and operational high voltages. 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 top 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 = 55 Gauss ($B_{\rm z} \sim54$ Gauss and $B_{\rm x} \sim9$ Gauss) 
was measured to be on the level of $1\%$. The proposed shielding configuration is sufficient to reduce the magnetic field 
to the level suitable for the PMT operation.







\subsubsection{Light guide studies}

Studies of the magnetic shielding demonstrated that the PMT has to be positioned inside the iron housing at the distance 
of at least 3 cm from the face of the PbW0$_4$ crystal. 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 PbWO$_4$ 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. The silicon cookie is not glued to the light guide or 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 registered 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. Each tile corresponds to the 
specific lepton energy. 

The relative light yield of the module with and without the light guide was estimated by positioning the module behind the 
PS and measuring signal amplitudes induced by the PS electrons. We first measured the ADC response in the CCAL module, which 
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  in 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 $15-20\%$ 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.



\subsubsection{Detector rate}
\label{sec:fcal_rates}

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. Some lifetime tests of 
the Hamamatsu 4125 PMT are described in Ref.~\cite{fnal_pmt}.

The anode current ($I$) was 
measured in the CCAL modules during  data production runs at the GlueX nominal luminosity. It was obtained by measuring the 
average voltage in the flash ADC induced by particles incident on the CCAL module as follows:
%------------------------------------------------
\begin{equation}
 I = \frac{\bar{U}}{R}\cdot\frac{1}{G}, 
\end{equation}
%------------------------------------------------
where  $\bar{U}$ is the average voltage in units of Volts,  $R$ is the input impedance of the amplifier ($\sim50\;\Omega$), and 
$G$ is the amplifier gain of 24. A periodic pulser not associated with an interaction in the detector was used as a trigger to read 
out flash ADC raw data for each CCAL module in a time window of 400 ns. The voltage  was determined by summing up ADC amplitudes 
in the readout window and normalizing the sum to the window size. The typical anode current measured in CCAL modules situated at 
different distances from the beamline 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 shielded  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 estimated anode current is the largest in the innermost layer of the detector closest to the beamline  and amounts to 
about $1.4\;{\rm \mu A}$. This current is significantly smaller than  the PMT divider current of about $300\;{\rm \mu A}$. 

We used the CCAL measurements to estimate the current in the FCAL insert. Taking the geometrical location of FCAL and CCAL modules 
into account, the largest PMT current in the FCAL insert modules closest to the beamline and not shielded by the absorber was 
conservatively estimated to be about $15\;{\rm \mu  A}$. We assume that the PMT base is operated at 1 kV and no amplifier is used. The detector rate drops rapidly with 
the increase of the radial distance from the beamline. The estimated anode current is relatively large and must be reduced by lowering the PMT 
high voltage. We are considering to instrument PMTs in a few inner FCAL insert layers with an amplifier with a gain of 5 and to omit 
the amplifier on other modules. We are planning to perform more beam tests of the FCAL insert active base using the CCAL in forthcoming 
GlueX runs in 2021 - 2022.
 
%----------------------------------------------------------------------
\begin{figure}[t]
\begin{center}
\includegraphics[width=1.0\linewidth,angle=0]{anode_current.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 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 
$\gamma p \to \pi^0 p$.  


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 3 m is 8.5 degrees; at 4 m 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 luminosity 
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 is anticipated to provide the spacial resolution of 2-3 mm and the energy resolution of 
about $2.5\%/\sqrt E$.
The NPS consists of 1080 PbWO$_4$ crystals that form an array of $30\:{\times}\:36$ modules. 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 type of PMTs and readout electronics.
The details of the mechanical assembly  and commissioning of the NPS are currently under development and will be described in a forthcoming 
publication. 


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 PbWO$_4$ 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.


\section{Summary}
\label{sec_summary}

We described the design and performance of the Compton calorimeter, which was constructed using 
140 lead tungstate PbWO$_4$ 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  PbWO$_4$ 
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  PbWO$_4$ 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 PHY-1714133, PHY-2012430 and 
PHY-1812396, and the  U.S. Department of Energy Grant DE-FG02-03ER41231. We thank the NPS collaboration/project 
for providing PbWO$_4$  crystals and PMTs used in the construction of the CCAL.


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