%%%%%%%%%%%%%%%%%%%%%%% file template.tex %%%%%%%%%%%%%%%%%%%%%%%%%
%
% This is a template file for Web of Conferences Journal
%
% Copy it to a new file with a new name and use it as the basis for your article
%
%%%%%%%%%%%%%%%%%%%%%%%%%% EDP Science %%%%%%%%%%%%%%%%%%%%%%%%%%%%
%\documentclass{webofc}
% option "twocolumn" for typesetting an article in two columns format (default one column)

\documentclass[twocolumn]{webofc}
\usepackage[varg]{txfonts}   % Web of Conferences font
\usepackage{hyperref}
\usepackage{url}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\hypersetup{colorlinks=true,citecolor=blue,urlcolor=blue,linkcolor=blue}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%\usepackage{lineno} %%% use lineno package to generate line numbers
%\linenumbers
%
\begin{document}
%
\title{A Prototype of Lead Tungstate Electromagnetic Calorimeter Built in AANL
  for EIC Collider}
%

\author{\firstname{\normalfont Hamlet} \lastname{Mkrtchyan}\inst{1}\fnsep\thanks{\email{mkrtchyan@yerphi.am}} \and
  \firstname{\normalfont Hrachya} \lastname{Marukyan}\inst{1} %\fnsep\thanks
  %{\email{Mail address for second author if necessary}}
  \and \firstname{\normalfont Arthur} \lastname{Mkrtchyan}\inst{1} %\fnsep\thanks
  %{\email{Mail address for last author if necessary}}
  \and \firstname{\normalfont Albert} \lastname{Shahinyan}\inst{1} %\fnsep\thanks
  \and \firstname{\normalfont Vardan} \lastname{Tadevosyan}\inst{1} %\fnsep\thanks
  \and \firstname{\normalfont Hakob} \lastname{Voskanyan}\inst{1} %\fnsep\thanks
  \and \firstname{\normalfont Aram} \lastname{Movsisyan}\inst{1} %\fnsep\thanks
  \and \firstname{\normalfont Arthur} \lastname{Hoghmrtsyan}\inst{1} %\fnsep\thanks
  % etc.
}

\institute{Alikhanyan National Science Laboratory (Yerevan Physics Institute), Yerevan,
  Armenia 
%\and
%           the second here 
%\and
%           Last address
          }

\abstract{We present the results of studies of the characteristics of lead tungstate crystals (PbWO$_4$)
  and performance of prototype of electromagnetic calorimeter for the Electron-Ion Collider (EIC) which
  was designed and built in A.I. Alikhanyan National Science Laboratory (AANL).
  EIC, which is currently under construction at Brookhaven National Laboratory in the US, will be
  a unique particle accelerator where electrons will collide with protons or nuclei to scan the internal
  structure of nucleons.  Electromagnetic Calorimeter (EmCal) based on lead tungstate (PbWO$_4$) crystals
  is proposed to be a key element of particle identification system of EIC. 
  A quality assurance of crystals produced by CRYTUR, the light yield and transmission of crystals have
  been measured at AANL.
  The average light transmittance in the transverse direction of crystals is 62.8, 68.4, and 75.7\%
  at wavelengths $\lambda$ = 360, 420, and 620 nm.
  Measurements showed that the crystals have an average light output of $\sim$16 pe/MeV, and that within
  3−-5 \% is same at two different points located at a distance of $\sim$14 cm
  from each other.
  A prototype of EmCal was designed, constructed and tested with cosmic muons. It consists of 16 PbWO$_4$
  crystals arranged in a 4$\times$4 matrix.
  The results obtained confirm the conclusions of other groups in the Electron-Ion Collider collaboration
  that the quality of the crystals produced by CRYTUR meets the requirements for an electromagnetic
  calorimeter, and that they can be the basis for its creation.
}
%

\maketitle

%

\section{Introduction}
\label{intro}
The physics program of the Electron-Ion Collider (EIC \cite{Ref1}) includes the most important and key
questions that the
physics communities of the world need answers to: the origin of the spin of the nucleon, the gluon
structure of nuclei, origin of hadronic mass, etc. To implement such a physics program, the EIC will
provide an intense beam of 18 GeV electrons colliding with protons (41–275 GeV) and nuclei (up to 166
GeV/nucleon) at luminosities of the order of 10$^{34}$ cm$^{-2}$sec$^{-1}$. The EIC detector system must
provide (e/$\pi$) and (K/$\pi$) separation up to a momentum value of 60 GeV/c.

A detailed description of the particle identification (PID), trajectory reconstruction, electromagnetic
and hadron calorimetry systems of the EIC's main detector, called the Electron-Proton/Ion Collider
(ePIC), is presented in Ref.~\cite{Ref2,Ref3,Ref4,Ref5}. Options for selection, optimization, and
design of the Electromagnetic Calorimeter (EmCal) for ePIC are still in the development phase and
require additional research. To this end, many members of the ePIC collaboration create EmCal prototypes,
conduct independent tests, and compare results.

It is especially important to study the characteristics of EmCal crystals and prototypes, which is the main
goal of the work carried out at the AANL. A typical electromagnetic calorimeter is a transparent, homogeneous
radiator of crystals large enough to completely contain a shower of secondary particles.
In experiments over the past ten years, the preferred material has been the inorganic scintillator lead
tungstate, which is characterized by a small Moliere radius (R$_M$ = 2.0~cm), high density
($\rho\simeq 8.3~g/cm^3$), fast response (< 2 ns) and radiation resistance.
%
The EmCal design is based on a series of approximately 3000 PbWO$_4$ crystals
2.0$\times$2.0$\times20$~cm$^3$ in size.

The AANL group has extensive experience in the field of calorimetry. It led the design and construction
of electromagnetic calorimeters for the NPS and HMS/SHMS spectrometers in Experimental Hall C of the
Jefferson Laboratory \cite{Ref6,Ref7}.


\section{Selection of PbWO$_4$ crystals for the EmCal prototype}
\label{sec-1}
Currently, the only two companies in the world with mass-production capabilities for PbWO$_4$ crystals
are SICCAS in China and CRYTUR in the Czech Republic. CRYTUR and SICCAS produced about 1000 crystals
each for Jefferson Laboratory. The characteristics of these crystals are given in \cite{Ref6}. A small
batch of crystals produced by CRYTUR was provided to AANL for research on their quality and the
construction and testing of a prototype of electromagnetic calorimeter.

The crystal samples were rectangular, with nominal dimensions of 20.5$\times$20.5$\times$200~mm$^3$ .
Longitudinal (Z) and transverse (X, Y) dimensions of the crystals were measured with a high-precision
caliper (with accuracy better than 50 $\mu$m) and a Mituito Electric Digital Height Gage sensor
(with accuracy of 3–5 $\mu$m).
The average crystal dimensions are 20.47$\times$20.48$\times$200.13~mm$^3$, which
corresponds to the nominal requirements, and the divergences do not exceed $\pm50~\mu$m.
After the first visual inspection, all crystals were examined with a DeltaPix digital microscope.
It provides ultra-high resolution and all the features needed for precise measurements.
Research showed that despite some shortcomings in the form of scratches, all crystals were suitable for
further research and testing.

The transverse transmittance of the light of crystals was measured with a FLAME-S-XR1
optical spectrometer from Ocean Insight. The measurements used the OceanART software application, which
allows one to simultaneously obtain absorption, transmission, and reflection data.
Systematic errors and reproducibility of our results were determined by repeated measurements of the
transmittance of the same crystal, in the same position. The data shows that the accuracy of our
measurements is better than 10\%.

The light transmittance of PbWO$_4$ crystals in the transverse direction, measured at AANL, in comparison
with data from the manufacturer, is shown in Fig.~\ref{fig-1}.

\vspace{-0.5cm}
\begin{figure}[h]
\centering
%\includegraphics[width=5cm,clip]{pwo_trans.jpg}
\includegraphics[width=0.45\textwidth,clip]{pwo_trans.jpg}
\vspace{-1.0cm}
\caption{Transverse transmittance for the best and worst CRYTUR crystals (shown as dots). Arrows
  and squares indicate wavelengths and transmittance values provided by CRYTUR. The dotted lines indicate
  the acceptable transverse transmittance range for EIC ($\geq$60\% and $\geq$85\% at $\lambda$=420 and
  620 nm).}
\label{fig-1}
\end{figure}

To study the optical homogeneity of a crystal, transmittance was measured in the transverse direction,
at several distances, from 5 to 195 mm, from the end of the crystal. The average crystal transmittances
measured at AANL are 62.8, 68.4, and 75.7\% at wavelengths $\lambda$ = 360, 420, and 620 nm,
respectively, and pass the ePIC calorimeter specifications.

The spectrophotometer used in AANL measurements does not have capability to measure longitudinal
transmittance, which requires use of an integrating sphere.
The longitudinal and transverse transmittances of 20 cm long Crytur crystals was measured at JLab and CUA. 
Those crystals have been used in JLab Hall C NPS 1080 channel calorimeter which required 
similar to ePIC quality crystals. Based on the similarity of our and JLab/CUA data on transverse
transmittance, we can assume proximity of the longitudinal transmittance also.
The requirement for longitudinal transmittance  of NPS and EIC are $\geq$35\% at 360 nm, $\geq$60\% at 
420 nm, and $\geq$70\% at 620 nm (see Table 2 in Ref.~\cite{Ref6}).
The measured longitudinal transmittance of crystals selected for the NPS calorimeter are $\geq$40\% at
360 nm and $\geq$65\% at 420 nm (see Fig.12 in Ref.~{\cite{Ref6}), and light yield $\geq$15 pe/MeV.

Light yield (LY) is measured in the number of photoelectrons (pe) emitted per unit of energy loss
in the crystal (pe/MeV). Light output was measured using cosmic muons as they passed through a PbWO$_4$
crystal 20.5 mm thick. One of the end surfaces of the crystal was connected to the photomultiplier tube
(PMT) window using Bicron BC-630 optical grease. The remaining surfaces were covered with films of ESR
optical reflector and black Tedlar. Measurements of the light yield of crystals were performed using
two different photomultipliers: Hamamatsu R4125 and Hamamatsu H6533. Measuring LY with the R4125 is a
priority because we use these PMTs in our EmCal prototype. Measurements with a second multiplier (H6533)
were performed for cross-validation.

To form a trigger from cosmic muons, two identical scintillation counters were used, placed on both
sides of the crystal at a short distance. The active dimensions of the scintillator were
10.0$\times$10.0$\times$50.0~mm$^3$.
The signals of these counters, passing through the discriminator-shaper module with a threshold of
$\sim$30 mV, form pulses according to the Nuclear Instrument Module (NIM) standard with a duration of
40 ns.
Next, they enter the coincidence logic module (LU N455) and generate a 150 ns wide Gate signal for the
QDC charge-to-digital converter. The PMT anode signals are digitized directly using a charge-sensitive
12-bit integrating analog-to-digital converter of type CAEN V792N. More information about the electronics
and data acquisition system can be found in section \ref{sec-3}.

As an example, Fig.~\ref{fig-2} shows the distribution of recorded PMT QDC signals corresponding to the
energies released by cosmic muons in a PbWO$_4$ crystal.

\vspace{-0.1cm}
\begin{figure}[h]
  \centering
%\includegraphics[width=5cm,clip]{adc_signal.jpg}
\includegraphics[width=0.35\textwidth,angle=90,clip]{adc_signal.jpg}
\vspace{-1.0cm}
\caption{Distribution of PMT signals in QDC channels corresponding to the energy release of cosmic muons
  passing through a 20.5 mm thick PbWO$_4$ crystal.}
\label{fig-2} 
\end{figure}

To determine the light output of the crystal, this distribution is approximated by a Gaussian function,
the most probable value $\bar{A}$ is found, and the pedestal component (PED), corresponding to the
zero signal of the photomultiplier and electronics, is subtracted from it. If, at a given PMT voltage, on
the QDC scale the position of the single electron peak (SEP) is known, then the light yield LY can be
found as LY = $(\bar{A} - PED)$/SEP.
The light yield per unit energy lost by a particle in a crystal is
given by  LY/E$_{dep}$, where ${E_{dep} = dE/dx \times\rho\times d}$ is the released energy.
For 2.0 cm thick PbWO$_4$ it will be $\sim$25 MeV.

If the PMT gain G is known for a given high voltage value, then the position of the SEP in the QDC
spectrum can be calculated using the relation SEP = \( (q_e\times G)/q_0 \), where
%
q$_e$=1.6$\times10^{-19}$ C and q$_0$ = 100 ~fC = 10$^{-13}$ C 
%
are electron charge, and QDC scale step value.
For example, for the H6533 PMT at a high voltage of 2000 V, the PMT gain ${G = 2 \times 10^6}$,
  which gives SEP $\simeq$3 QCD scale units.

If the PMT gain is unknown, the SEP value can be determined by measuring the LED signal.
In this case, the amplitude distribution of PMT signals is measured at light intensity of the order of
several hundred photoelectrons, and the maximum of the distribution and its width is determined from the
approximation by the Gaussian function. Neglecting contributions from electronic noise and other possible
sources, the number of registered photoelectrons N$_{pe}$ at a given LED intensity can be estimated
simply as the inverse of the square of the normalized distribution width, ${ N_{pe} = 1/\sigma_{norm}^2}$,
  where ${\sigma_{norm} = \sigma/N_{ADC} }$, $\sigma$ is the width of the Gaussian approximation, and
  $N_{ADC}$ is the position of the maximum of the amplitude distribution minus the pedestal.
  
In our measurements, SEP was determined by the above two methods. The difference in the results
obtained was in the range of up to $\sim$20\%, and for the light output of CRYTUR crystals, an average
value of $\sim$16 pe/MeV was obtained.
Note that this number is scaled up by cover factor of photocathodes of the PMTs used in the measurements.

\section{EmCal Prototype}
\label{sec-3}
Several electromagnetic calorimeter prototypes for EIC have been constructed and tested by the ePIC
collaboration. We designed and built our prototype to conduct independent research into key aspects
of EIC calorimetry. Based on the results of visual and microscopic inspection and transmittance
measurements, we selected the best 16 PbWO$_4$ crystals to build the EmCal prototype.

The calorimeter prototype consists of 16 PbWO$_4$ crystals, 2.05$\times$2.05$\times$20~cm$^3$ in size,
arranged in a 4$\times$4 matrix. The modules are held in support frames printed using 3D printing
technology.
Thus, a compact, small calorimeter was obtained, with minimal gaps between the crystals (< 250 $\mu$m).

To provide light isolation, all crystals are individually wrapped in 65 $\mu$m ESR reflector and
30 $\mu$m Tedlar foil. Each crystal is connected to a Hamamatsu R4125-01 photomultiplier using optical
vaseline.
A matrix of crystals with photomultipliers and high-voltage dividers is placed in a dark, light-insulated
metal box.
On the rear end wall of the box, there are connectors for high-voltage power supply and output signals
of the photomultiplier. When testing the prototype with cosmic muons, we used the same trigger counters,
the same electronics, and the data acquisition system as in the measurements of the light output of the
crystals.

A picture of assembled EmCal prototype and the circuit diagram of the electronics and data acquisition
system used in cosmic ray studies are shown in Fig.~\ref{fig-3}.

\begin{figure}[h]
\centering
\includegraphics[width=0.45\textwidth,clip]{proto_pict.jpg} \\
\includegraphics[width=0.45\textwidth,clip]{pwo_proto_sketch.jpg}
\caption{(Top) Picture of assembled EmCal prototype. (Bottom) Circuit diagram of the electronics and
  data acquisition system used in cosmic ray research. C1 and C2 – scintillation counters, CFD –
  discriminator, Logic Unit – coincidence module, Gate – Gate
  signal, QDC –charge-to-digital converter, Delay – delay line.}
\label{fig-3} 
\end{figure}

In this case, it was necessary to provide high-voltage power to all 16 photomultipliers and analyze all
16 signals from QDC.

To obtain the total number of photoelectrons produced by muons in all four layers of the calorimeter,
it is necessary to know the SEP value of each photomultiplier.
To determine the position of the SEP signal of all photomultipliers in the assembled prototype, we used
the LED calibration method. In Fig.~\ref{fig-4} are shown the distributions of the total number of
photoelectrons produced by muons in the prototype, derived with such calibration.

%\vspace{-0.5cm}
\begin{figure}[h]
\centering
%\includegraphics[width=5cm,clip]{muons_4column.jpg}
\includegraphics[width=0.45\textwidth,clip]{muons_4column.jpg}
\caption{Distributions of the total number of photoelectrons produced by cosmic muons in 4 columns of
  the prototype.}
\label{fig-4}
\end{figure}

Taking into account that muons passing through all four crystals lose a total energy of
approximately 100 MeV ($\sim$25 MeV in each block), and partial coverage of the end face of crystal
by the photocathode of PMT, for the light output in the PbWO$_4$ crystal we obtain $\sim$16 pe/MeV,
which confirms the data obtained from measurements of the light output of the crystals.


\section{Conclusion}
\label{sec-4}

The quality of PbWO$_4$ crystals from CRYTUR was investigated at AANL. The crystals were visually
checked, and their sizes were measured. Light transmission through crystals was measured.
Measurements showed that the average transverse light transmittance of the crystals was 62.8, 68.4,
and 75.7\% at wavelengths $\lambda$ = 360, 420, and 620 nm.
The size variations, and light transmissions are within the EIC specifications.
An EmCal prototype was created, consisting of 16 PbWO$_4$ crystals arranged in a 4$\times$ 4 matrix.
The prototype was tested by cosmic muons. The measured light yield from straight through muons is in
agreement with previous measurements.
Our results confirm conclusions of other ePIC collaboration groups that the quality of crystals produced
by CRYTUR meets all the requirements for the ePIC electromagnetic calorimeter EmCal \cite{Ref5}.

As reported in Ref.~\cite{Ref6}, crystals have an average light output of $\sim$16 pe/MeV with a standard
deviation of 0.9 pe/MeV, which is within the measurement error. The light output of the crystal at two
different points located at a distance of $\sim$14 cm from each other is almost the same
(within 3–5\% accuracy), which indicates good optical homogeneity.

An EmCal prototype was created, consisting of 16 PbWO$_4$ crystals arranged in a 4$\times$4 matrix.
The prototype was tested by cosmic muons. Muons, passing through all four layers of the calorimeter,
lose a total energy of $\sim$100 MeV ($\sim$25 MeV in each block).
As a result, they generate $\sim$700\--800 photoelectrons, which is $\sim$7\--8 pe/MeV. This value is
almost two times less than the measured value of the light output of PbWO$_4$ \textcolor{red}{\cite{Ref6}}
because of the partial coverage of the end surface of the crystal with the PMT R4125 photocathode.

For the final selection of the calorimeter design, it is important to compare and jointly review the
research results of various EmCal groups of the ePIC collaboration and develop common requirements for
the characteristics of crystals and the calorimeter design. From this point of view, the data obtained
by AANL are an integral part of the basic data and are of general interest.

Our results confirm the conclusions of other ePIC collaboration groups that the quality of crystals
produced by CRYTUR meets all the requirements for an electromagnetic calorimeter and can be the basis
for the creation of EmCal \cite{Ref5}.

%\centerline{\bf Acknowledgments}
\section{Acknowledgments}
We thank the staff of Experimental Hall C of the Jefferson Laboratory and the team from the Catholic
University of America (CUA) for assistance and technical support. %at various stages of the work.

We are especially grateful to R. Lusinyants and the American charitable foundation PIERIS for the
assistance provided to AANL.
%
The research was carried out with the financial support of the Committee on Higher Education and Science
of the Republic of Armenia, within the framework of Scientific Project No 21AG-1C028.


%\vspace{1.0cm}
%
\begin{thebibliography}{}
%
\bibitem{Ref1} R. Abdul Khalek, A. Accardi, J. Adam, D. Adamiak, W. Akers, M. Albaladejo et al.,
  Science Requirements and Detector Concepts for the Electron-Ion Collider: EIC Yellow Report,
  Nucl. Phys. {\textbf {A~1026}}, 122447 (2022),
  https://doi.org/10.1016/j.nuclphysa.2022.122447. 
\bibitem{Ref2} Adam, J., Adamczyk, L., Agrawal, N., Aidala, C., Akers, W., et al.,
  ATHENA detector proposal — a totally hermetic electron nucleus apparatus proposed for IP6 at the
  Electron-Ion Collider, JINST {\textbf 17}, P10019 (2022),
  https://doi.org/10.1088/1748-0221/17/10/p10019.
\bibitem{Ref3} Adkins, J.K., Akiba, Y., Albataineh, A., Amaryan, M., Arsene, I.C., et al.,
  Design of the ECCE Detector for the Electron Ion Collider,  e-Print: 2209.02580 [physics.ins-det].
\bibitem{Ref4} Alarcon, R., Baker, M., Baturin, V., Brindza, P., Bueltmann, S., et al. ,
  CORE -- a COmpact detectoR for the EIC,  e-Print:  2209.00496 [physics.ins-det].
\bibitem{Ref5} Bock, F., Schmidt, N., Wang, P.K., Santiesteban, N., Horn, T., et al.,
  Design and simulated performance of calorimetry systems for the ECCE detector at the electron ion
  collider,  Nucl. Instrum. and Methods {\textbf {A~1013}}, 165683 (2021),
  https://doi.org/10.1016/j.nima.2023.168464.
\bibitem{Ref6} Horn, T., Berdnikov, V.V., Ali, S., Asaturyan, A., Carmignotto, M., et al.,
  Scintillating crystals for the Neutral Particle Spectrometer in Hall C at JLab,  Nucl. Instrum.
  and Methods {\textbf A~956}, 163375 (2020), vol. 956,
  https://doi.org/10.1016/j.nima.2019.163375.
\bibitem{Ref7} Mkrtchyan, H., Carlini, R., Tadevosyan, V., Arrington, J., Asaturyan, A., et al.,
  The lead-glass electromagnetic calorimeters for the magnetic spectrometers in Hall C at Jefferson Lab,
  Nucl. Instrum. and Methods {\textbf {A~719}}, 85 (2013), https://doi.org/10.1016/j.nima.2013.03.070.
\bibitem{Ref8} Zhu, R.Y., Ma, D.A., Newman, H.B., Woody, C.L., and Kierstead, J.A., et al.,
  A study on the properties of lead tungstate crystals,  Nucl. Instrum. and Methods
  {\textbf {A~376}}, 319 (1996), https://doi.org/10.1016/0168-9002(96)00286-0.
% 
\end{thebibliography}

\end{document}

