An Electromagnetic Calorimeter for the JLab Real Compton Scattering Experiment
D. J. Hamiltona,∗, A. Shahinyanb, B. Wojtsekhowskic, J. R. M. Annanda, T.-H. Changd, E. Chudakovc, A. Danagouliand,
P. Degtyarenkoc, K. Egiyanb,1, R. Gilmane, V. Gorbenkof, J. Hinesg,2, E. Hovhannisyanb,1, C. E. Hyde-Wrighth, C.W. de Jagerc,
A. Ketikyanb, V. H. Mamyanb,c,3, R. Michaelsc, A. M. Nathand, V. Nelyubini, I. Rachekj, M. Roedelbromd, A. Petrosyanb,1,
R. Pomatsalyukf, V. Popovc, J. Segalc, Y. Shestakovj, J. Templong,4, H. Voskanyanb
aUniversity of Glasgow, Glasgow G12 8QQ, Scotland, UK
bYerevan Physics Institute, Yerevan 375036, Armenia
cThomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA
dUniversity of Illinois, Urbana-Champaign, IL 61801, USA
eRutgers University, Piscataway, NJ 08855, USA
fKharkov Institute of Physics and Technology, Kharkov 61108, Ukraine
gThe University of Georgia, Athens, GA 30602, USA
hOld Dominion University, Norfolk, VA 23529, USA
i St. Petersburg Nuclear Physics Institute, Gatchina, 188350, Russia
jBudker Institute for Nuclear Physics, Novosibirsk 630090, Russia
Abstract
A lead-glass hodoscope calorimeter that was constructed for use in the Jefferson Lab Real Compton Scattering experiment is
described. The detector provides a measurement of the coordinates and the energy of scattered photons in the GeV energy range
with resolutions of 5 mm and 6%/
Eγ [GeV]. Features of both the detector design and its performance in the high luminosity
environment during the experiment are presented.
Keywords: Calorimeters, Čerenkov detectors
PACS: 29.40Vj, 29.40.Ka ‘
1. Introduction
A calorimeter was constructed as part of the instrumentation
of the Jefferson Lab (JLab) Hall A experiment E99-114, “Ex-
clusive Compton Scattering on the Proton” [1], the schematic
layout for which is shown in Fig. 1. The study of elastic pho-
ton scattering provides important information about nucleon
structure, which is complementary to that obtained from elastic
electron scattering [2]. Experimental data on the Real Comp-
ton Scattering (RCS) process at large photon energies and large
scattering angles are rather scarce, due mainly to the absence of
high luminosity facilities with suitable high-resolution photon
detectors. Such data are however crucial, as the basic mecha-
nism of the RCS reaction is the subject of active debate [3, 4, 5].
The only data available before the JLab E99-114 experiment
were obtained at Cornell about 30 years ago [6].
The construction of the CEBAF (Continuous Electron Beam
Accelerator Facility) accelerator has led to an extension of
many experiments with electron and photon beams in the GeV
energy range and much improved precision. This is the result of
a number of fundamental improvements to the electron beam,
∗Tel.: +44-141-330-5898; Fax: +44-141-330-5889
Email address: d.hamilton@physics.gla.ac.uk (D. J. Hamilton)
1deceased
2present address: Applied Biosystems/MDS, USA
3present address: University of Virginia, Charlottesville, VA 22901, USA
4present address: NIKHEF, 1009 DB Amsterdam, The Netherlands
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Beam dump
Cable lines
Calorimeter
Front−end electronics
Beam line
Magnet
Radiator and target
Figure 1: Layout of the RCS experiment in Hall A. An electron beam incident
on a radiator produces an intense flux of high energy photons.
including a 100% duty cycle, low emittance and high polariza-
tion, in addition to new dedicated target and detector systems.
The CEBAF duty factor provides an improvement of a factor of
15 compared to the best duty factor of a beam extracted from a
synchrotron, at a similar instantaneous rate in the detectors.
In 1994 work began on the development of a technique for an
RCS experiment at JLab, leading in 1997 to the instigation of
a large-scale prototyping effort. The results of the subsequent
test runs in 1998 and 1999 [7] provided sufficient information
Preprint submitted to NIM A August 16, 2018
http://arxiv.org/abs/0704.1830v3
Figure 2: A photograph of the experimental set-up for E99-114, showing the calorimeter (center) and part of the proton spectrometer (rear).
for the final design of the apparatus presented in the present
article. The fully realized physics experiment took place in
2002 (see Fig. 1) at a photon-nucleon luminosity which was
a factor of 1300 higher than in the previous Cornell experi-
ment. The experimental technique involves utilizing a mixed
electron-photon beam which is incident on a liquid hydrogen
target and passes to a beam dump. The scattered photons are
detected in the calorimeter, while the recoiling protons are de-
tected in a high resolution magnetic spectrometer (HRS-L). A
magnet between the hydrogen target and the calorimeter de-
flects the scattered electrons, which then allows for clean sep-
aration between Compton scattering and elastic e-p scattering
events. The Data Acquisition Electronics (DAQ) is shielded by
a 4 inch thick concrete wall from the beam dump and the target.
Figure 2 shows a photograph of the experimental set-up with
the calorimeter in the center.
The experiment relied on a proton-photon time coincidence
and an accurate measurement of the proton-photon kinematic
correlation for event selection. The improvement in the event
rate over the previous measurement was achieved through the
use of a mixed electron-photon beam, which in turn required
a veto detector in front of the calorimeter or the magnetic de-
flection of the scattered electron [1]. In order to ensure redun-
dancy and cross-checking, both a veto and deflection magnet
were designed and built. The fact that a clean photon beam was
not required meant that the photon radiator could be situated
very close to the hydrogen target, leading to a much reduced
background near the beam line and a dramatic reduction of the
photon beam size. This small beam size in combination with
the large dispersion in the HRS-L proton detector system [8]
resulted in very good momentum and angle resolution for the
recoiling proton without the need for a tracking detector near
the target, where the background rate is high.
Good energy and coordinate resolutions were key features
of the photon detector design goals, both of which were sig-
nificantly improved in the JLab experiment as compared to the
Cornell one. An energy resolution of at least 10% is required to
separate cleanly RCS events from electron bremsstrahlung and
neutral pion events. In order to separate further the background
from neural pion photo-production, which is the dominant com-
ponent of the high-energy background in this measurement, a
high angular resolution between proton and photon detectors is
crucial. This was achieved on the photon side by constructing a
highly segmented calorimeter of 704 channels. The RCS exper-
iment was the first instance of a calorimeter being operated at
an effective electron-nucleon luminosity of 1039 cm2/s [9, 10]
(a 40 µA electron beam on a 6% Cu radiator upstream of a
15 cm long liquid hydrogen target). It was observed in the test
runs that the counting rate in the calorimeter fell rapidly as the
threshold level was increased, which presented an opportunity
to maintain a relatively low trigger rate even at high luminosity.
However, on-line use of the calorimeter signal required a set of
summing electronics and careful equalizing and monitoring of
the individual channel outputs during the experiment.
As the RCS experiment represented the first use of such a
calorimeter at very high luminosity, a detailed study of the
calorimeter performance throughout the course of the experi-
ment has been conducted. This includes a study of the rela-
tionship between luminosity, trigger rate, energy resolution and
ADC pedestal widths. An observed fall-off in energy resolu-
tion as the experiment progressed allowed for characterization
of radiation damage sustained by the lead-glass blocks. It was
possible to mitigate this radiation damage after the experiment
by annealing, with both UV curing and heating proving effec-
tive.
We begin by discussing the various components which make
up the calorimeter and the methods used in their construction.
This is followed by a description of veto hodoscopes which
were used for particle identification purposes. An overview
of the high-voltage and data acquisition systems is then pre-
sented, followed, finally, by a discussion on the performance
of the calorimeter in the unique high-luminosity environment
during the RCS experiment.
2. Calorimeter
The concepts and technology associated with a fine-
granularity lead-glass Čerenkov electromagnetic calorimeter
(GAMS) were developed by Yu. Prokoshkin and collaborators
at the Institute of High Energy Physics (IHEP) in Serpukhov,
Russia [11]. The GAMS type concept has since been employed
for detection of high-energy electrons and photons in several
experiments at JLab, IHEP, CERN, FNAL and DESY (see for
example [12]). Many of the design features of the calorimeter
presented in this article are similar to those of Serpukhov. A
schematic showing the overall design of the RCS calorimeter
can be seen in Fig. 3. The main components are:
• the lead-glass blocks;
• a light-tight box containing the PhotoMultiplier Tubes
(PMTs);
• a gain-monitoring system;
• a doubly-segmented veto hodoscopes;
• the front-end electronics;
• an elevated platform;
• a lifting frame.
The calorimeter frame hosts a matrix of 22×32 lead-glass
blocks together with their associated PMTs and High Voltage
(HV) dividers. Immediately in front of the lead-glass blocks
is a sheet of UltraViolet-Transmitting (UVT) Lucite, which is
used to distribute calibration light pulses for gain-monitoring
purposes uniformly among all 704 blocks. The light-tight box
provides protection of the PMTs from ambient light and con-
tains an air-cooling system as well as the HV and signal cable
systems. Two veto hodoscopes, operating as Čerenkov coun-
ters with UVT Lucite as a radiator, are located in front of the
calorimeter. The front-end electronics located a few feet be-
hind the detector were assembled in three relay racks. They are
comprised of 38 analog summers, trigger logic and patch pan-
els. The elevated platform was needed to bring the calorimeter
to the level of the beam line, while the lifting frame was used
to re-position the calorimeter in the experimental hall by means
of an overhead crane. This procedure, which took on average
around two hours, was performed more than 25 times during
the course of the experiment.
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Figure 3: Schematic side view of the RCS calorimeter detector system.
2.1. Calorimeter Design
The main frame of the calorimeter is made of 10 inch wide
steel C-channels. A thick flat aluminum plate was bolted to the
bottom of the frame, with a second plate installed vertically and
aligned to 90◦ with respect to the first one by means of align-
ment screws (see Fig. 4). Another set of screws, mounted inside
and at the top of the main frame on the opposite side of the ver-
tical alignment plate, was used to compress all gaps between the
lead-glass modules and to fix their positions. The load was ap-
plied to the lead-glass blocks through 1 inch× 1 inch × 0.5 inch
plastic plates and a 0.125 inch rubber pad. In order to further as-
sist block alignment, 1 inch wide stainless steel strips of 0.004
inch thickness running from top to bottom of the frame were
inserted between every two columns of the lead-glass modules.
2.1.1. Air Cooling
All PMTs and HV dividers are located inside a light-tight
box, as shown in Fig. 5. As the current on each HV divider
is 1 mA, simultaneous operation of all PMTs would, without
cooling, lead to a temperature rise inside the box of around
50-70◦C. An air-cooling system was developed to prevent the
PMTs from overheating, and to aid the stable operation of the
calorimeter. The air supply was provided by two parallel oil-
less regenerative blowers of R4110-2 type5, which are capable
5Manufactured by S&F Supplies, Brooklyn, NY 11205, USA.
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Figure 4: Front cross-section of the calorimeter, showing the mechanical com-
ponents.
of supplying air at a maximum pressure of 52 inches water and
a maximum flow of 92 CFM. The air is directed toward the HV
divider via vertical collector tubes and numerous outlets. When
the value on any one of the temperature sensors installed in sev-
eral positions inside the box exceeds a preset limit, the HV on
the PMTs is turned off by an interlock system. The air line is
equipped with a flow switch of type FST-321-SPDT which was
included in the interlock system. The average temperature in-
side the box during the entire experimental run did not exceed
the preset limit of 55◦C.
2.1.2. Cabling System
A simple and reliable cabling system is one of the key fea-
tures of multichannel detectors, with easy access to the PMTs
and HV dividers for installation and repair being one of the key
features. The cabling system includes:
• 1 foot long HV and signal pig-tails soldered to the HV
divider;
• patch panels for Lemo and HV connectors;
• 10 feet long cables from those patch panels to the front-
end electronics and the HV distribution boxes;
• the HV distribution boxes themselves;
• BNC-BNC patch panels for the outputs of the front-end
modules;
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cables
Figure 5: A schematic showing the calorimeter air cooling and cabling systems.
• BNC-BNC patch panels on the DAQ side for the analog
signals;
• BNC-Lemo patch panels on the DAQ side for the veto-
counter lines.
Figure 6 shows the cabling arrangement inside the PMT box.
The patch panels, which are custom-built and mounted on the
air supply tubes, have the ability to swing to the side in order to
allow access to the PMTs and the HV dividers. The box has two
moving doors, the opening of which leads to activation of an in-
terlock system connected to the HV supply. In order to reduce
the diameter of the cable bundle from the PMT box, RG-174
cable (diameter 0.1 inch) was used for the PMT signals, and a
twisted pair for the HV connection (two individually insulated
inner 26 AWG conductors with an overall flame-retardant PVC
jacket, part number 001-21803 from the General Wire Product
company). The box patch panels used for the HV lines each
convert 24 of the above twisted pairs (single HV line) to the
multi-wire HV cables (the part 001-21798 made by General
Wire Product), which run to the HV power supply units located
in the shielded area near DAQ racks.
2.2. Lead-Glass Counter
The basic components of the segmented calorimeter are the
TF-1 lead-glass blocks and the FEU 84-3 PMTs. In the 1980s
the Yerevan Physics Institute (YerPhI) purchased a consignment
of TF-1 lead-glass blocks of 4 cm × 4 cm ×40 cm and FEU 84-3
PMTs of 34 mm diameter (with an active photo-cathode diam-
eter of 25 mm) for the construction of a calorimeter to be used
Lemo connector
from HV divider
HV connector
Patch panel
Al T−channel
HV cables
Signal cables
HV&Sig. cables
Figure 6: A photograph of the cabling inside the PMT box.
in several experiments at the YerPhI synchrotron. In January
of 1998 the RCS experiment at JLab was approved and soon
after these calorimeter components were shipped from Yerevan
to JLab. This represented the YerPhi contribution to the exper-
iment, as the properties of the TF-1 lead-glass met the require-
ments of the experiment in terms of photon/electron detection
with reasonable energy and position resolution and radiation
hardness. The properties of TF-1 lead-glass [12, 13] are given
in Table 1.
Table 1: Important properties of TF-1 lead-glass.
Density 3.86 gcm−3
Refractive Index 1.65
Radiation Length 2.5 cm
Moliére Radius 3.50 cm
Critical Energy 15 MeV
All PMTs had to pass a performance test with the follow-
ing selection criteria: a dark current less than 30 nA, a gain of
106 with stable operation over the course of the experiment (2
months), a linear dependence of the PMT response (within 2 %)
on an incident optical pulse of 300 to 30000 photons. 704 PMTs
out of the 900 available were selected as a result of these per-
formance tests. Furthermore, the dimensional tolerances were
checked for all lead-glass blocks, with strict requirements de-
manded on the length (400±2 mm) and transverse dimensions
(40±0.2 mm).
2.2.1. Design of the Counter
In designing the individual counters for the RCS calorime-
ter, much attention was paid to reliability, simplicity and the
possibility to quickly replace a PMT and/or HV divider. The
individual counter design is shown in Fig. 7. A titanium flange
is glued to one end of the lead-glass block by means of EPOXY-
190. Titanium was selected because its thermal expansion coef-
ficient is very close to that of the lead glass. The PMT housing,
which is bolted to the Ti flange, is made of an anodized Al
flange and an Al tube. The housing contains the PMT and a µ-
metal shield, the HV divider, a spring, a smaller Al tube which
transfers a force from the spring to the PMT, and a ring-shaped
spring holder. The optical contact between the PMT and the
lead-glass block is achieved by use of optical grease, type BC-
630 (Bicron), which was found to increase the amount of light
detected by the PMT by 30-40% compared to the case without
grease. The PMT is pressed to the lead-glass block by means
of a spring, which pushes the HV base with a force of 0.5-1 lbs.
Such a large force is essential for the stability of the optical
contact over time at the elevated temperature of the PMTs. The
glue-joint between the lead glass and the Ti flange, which holds
that force, failed after several months in a significant fraction
(up to 5%) of the counters. An alternative scheme of force
compensation was realized in which the force was applied to
the PMT housing from the external bars placed horizontally be-
tween the PMT housing and the patch-panel assembly. Each
individual lead-glass block was wrapped in aluminized Mylar
film and black Tedlar (a polyvinyl fluoride film from DuPont)
for optimal light collection and inter-block isolation. Single-
side aluminized Mylar film was used with the Al layer on the
opposite side of the glass. Such an orientation of the film limits
the diffusion of Al atoms into the glass and the non-oxidized
surface of aluminum, which is protected by Mylar, provides a
better reflectivity. The wrapping covers the side surface of the
lead-glass block, leaving the front face open for the gain mon-
itoring. The signal and the HV cables are each one foot long.
They are soldered to the HV divider on one end and terminated
with Lemo c©00 and circular plastic connectors (cable mount re-
ceptacle from Hypertronics) on the other end. The cables leave
the PMT housing through the open center of the spring holder.
2.2.2. HV Divider
At the full luminosity of the RCS experiment (0.5 ×
1039 cm2/s) and at a distance of 6 m from the target the back-
ground energy load per lead-glass block reaches a level of
108 MeVee (electron equivalent) per second, which was found
from the average value of anode current in the PMTs and the
shift of the ADC pedestals for a 150 ns gate width. At least
30% of this energy flux is due to high energy particles which
define the counting rate. The average energy of the signals for
that component, according to the observed rate distribution, is
in the range of 100-300 MeVee, depending on the beam en-
ergy and the detector angle. The corresponding charge in the
PMT pulse is around 5-15 pC collected in 10-20 ns. The elec-
tronic scheme and the selected scale of 1 MeV per ADC chan-
nel (50 fC) resulted in an average anode current of 5 µA due to
background load. A high-current HV base (1 mA) was there-
fore chosen to reduce the effect of the beam intensity variation
on the PMT amplitude and the corresponding energy resolu-
tion to the level of 1%. The scheme of the HV base is shown
in Fig. 8. According to the specification data for the FEU 84-
3 PMTs the maximum operation voltage is 1900 V. Therefore
a nominal voltage value of 1800 V and a current value in the
voltage divider of 1 mA were chosen.
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7.5ns
VME  Scalers
Fastbus  TDC
Fastbus  ADC
Fastbus  ADC
Fastbus  ADC
DAQ  Platform100m cable lines
Calorimeter Platform
LG  PMT  signals
600  central  channels
LG  PMT  signals
104  rim  channels
SUM−32  analog  signals
56  channels
SUM−32  logic signals
56  channels
LRS428F PS706D
LRS3412
PHOTON  TRIGGER
To Trigger Superviser2ns6ns
Sum−8
PS755
PS757
Figure 9: A block diagram of the calorimeter electronics.
R1 R2
R9 R10
D9 D10
R11 R12
Figure 8: Schematic of the high-voltage divider for the FEU 84-3 PMT. The
values of the resistors are R(1 − 10) = 100 kΩ, R11 = 130 kΩ,R12 =
150 kΩ, R13 = 200 kΩ, R14 = 150 kΩ, R15 = 10 kΩ,R16 = 10 MΩ, R17 =
4 kΩ. The capacitance C is 10 nF.
2.3. Electronics
The calorimeter electronics were distributed over two loca-
tions; see the block diagram in Fig. 9. The first group of
modules (front-end) is located in three racks mounted on the
calorimeter platform in close vicinity to the lead-glass blocks.
These are the trigger electronics modules which included a mix
of custom-built and commercially available NIM units:
• 38 custom-built analog summing modules used for level-
one signal summing 6;
• 14 linear fan-in/fan-out modules (LeCroy model 428F) for
a second-level signal summation;
• 4 discriminator units (Phillips Scientific model 706);
• a master OR circuit, realized with Phillips Scientific logic
units (four model 755 and one model 757 modules);
• several additional NIM modules used to provide auxiliary
trigger signals for the calorimeter calibration with cosmics
and for the PMT gain-monitoring system.
The second group of electronic modules, which include
charge and time digitizers as well as equipment for the Data
Acquisition, High Voltage supply and slow-control systems, is
6This module was designed by S. Sherman, Rutgers University.
placed behind a radiation-protecting concrete wall. All 704
lead-glass PMT signals and 56 SUM-32 signals are digitized by
LeCroy 1881M FastBus ADC modules. In addition, 56 SUM-
32 discriminator pulses are directed to scalers and to LeCroy
1877 FastBus TDCs. Further detailed information about the
electronics is presented in Section 5.
The signals between these locations are transmitted via
patch-panels and coaxial cables, consisting of a total number
of 1040 signal and 920 HV lines. The length of the signal ca-
bles is about 100 m, which serve as delay lines allowing the
timing of the signals at the ADC inputs to be properly set with
respect to the ADC gate, formed by the experiment trigger. The
width of the ADC gate (150 ns) was made much wider than
the duration of PMT pulse in order to accommodate the wider
pulses caused by propagation in the 500 ns delay RG-58 signal
cables. The cables are placed on a chain of bogies, which per-
mits the calorimeter platform to be moved in the experimental
hall without disconnecting the cables. This helped allow for a
quick change of kinematics.
2.3.1. Trigger Scheme
The fast on-line photon trigger is based on PMT signals from
the calorimeter counters. The principle of its operation is a sim-
ple constant-threshold method, in which a logic pulse is pro-
duced if the energy deposition in the calorimeter is above a
given magnitude. Since the Molière radius of the calorimeter
material is RM ≈ 3.5 cm, the transverse size of the electro-
magnetic shower in the calorimeter exceeds the size of a single
lead-glass block. This enables a good position sensitivity of
the device, while at the same time making it mandatory for the
trigger scheme to sum up signals from several adjacent coun-
ters to get a signal proportional to the energy deposited in the
calorimeter.
From an electronics point of view, the simplest realization of
such a trigger would be a summation of all blocks followed by
a single discriminator. However, such a design is inappropri-
ate for a high-luminosity experiment due to the very high back-
ground level. The opposing extreme approach would be to form
a summing signal for a small group including a single counter
hit and its 8 adjacent counters, thus forming a 3 × 3 block struc-
ture. This would have to be done for every lead-glass block, ex-
cept for those at the calorimeter’s edges, leading to an optimal
signal-to-background ratio, but an impractical 600 channels of
analog splitter→analog summer→discriminator circuitry fol-
lowed by a 600-input fan-in module. The trigger scheme that
was adopted and is shown in Fig. 10 is a trade-off between the
above extreme cases. This scheme contains two levels of ana-
log summation followed by appropriate discriminators and an
OR-circuit. It involved the following functions:
• the signals from each PMT in the 75 2×4 sub-arrays of ad-
jacent lead-glass blocks, excluding the outer-most blocks,
are summed in a custom-made analog summing module
to give a SUM-8 signal (this module duplicates the signals
from the PMTs with less then 1% integral nonlinearity);
• these signals, in turn, are further summed in overlapping
groups of four in LeCroy LRS428F NIM modules to pro-
duce 56 SUM-32 signals. Thus, each SUM-32 signal is
proportional to the energy deposition in a subsection of the
calorimeter of 4 blocks high and 8 blocks wide, i.e. 16×32
cm2. Although this amounts to only 5% of the calorime-
ter acceptance, for any photon hit (except for those at the
edges) there will be at least one segment which contains
the whole electromagnetic shower.
• the SUM-32 signals are sent to constant-threshold discrim-
inators, from which the logical pulses are OR-ed to form
the photon singles trigger T1 (see Section 5). The discrim-
inator threshold is remotely adjustable, and was typically
set to around half of the RCS photon energy for a given
kinematic setting.
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141312
56 57 58 59 60
73 75747271
66 67
07 08 09
01 0403
S06 S07 S08
S50 S51
S11S09 S10
S03S01
Figure 10: The principle of two-level summation of signals for the hardware
trigger: 75 eight-block sub-arrays and 56 overlapping groups of four sub-arrays
forming SUM-32 signals labeled as S01-S56. In the highlighted example the
sums 02,03,07, and 08 form a S02 signal.
2.4. Gain Monitoring System
The detector is equipped with a system that distributes light
pulses to each calorimeter module. The main purpose of this
system is to provide a quick way to check the detector opera-
tion and to calibrate the dependence of the signal amplitudes on
the applied HV. The detector response to photons of a given en-
ergy may drift with time, due to drifts in the PMT gains and to
changes in the glass transparency caused by radiation damage.
For this reason, the gain monitoring system also allowed mea-
surements of the relative gains of all detector channels during
the experiment. In designing the gain-monitoring system ideas
developed for a large lead-glass calorimeter at BNL[14] were
used.
The system includes two components: a stable light source
and a system to distribute the light to all calorimeter modules.
The light source consists of an LN300 nitrogen laser7, which
provides 5 ns long, 300 µJ ultraviolet light pulses of 337 nm
7 Manufactured by Laser Photonics, Inc, FL 32826, USA.
wavelength. The light pulse coming out of the laser is atten-
uated, typically by two orders of magnitude, and monitored
using a silicon photo-diode S1226-18BQ8 mounted at 150◦ to
the laser beam. The light passes through an optical filter, sev-
eral of which of varying densities are mounted on a remotely
controlled wheel with lenses, before arriving at a wavelength
shifter. The wavelength shifter used is a 1 inch diameter semi-
spherical piece of plastic scintillator, in which the ultraviolet
light is fully absorbed and converted to a blue (∼ 425 nm) light
pulse, radiated isotropically. Surrounding the scintillator about
40 plastic fibers (2 mm thick and 4 m long) are arranged, in or-
der to transport the light to the sides of a Lucite plate. This plate
is mounted adjacent to the front face of the lead-glass calorime-
ter and covers its full aperture (see Fig.11). The light passes
through the length of the plate, causing it to glow due to light
scattering in the Lucite. Finally, in order to eliminate the cross-
talk between adjacent counters a mask is inserted between the
Lucite plate and the detector face. This mask, which reduces
the cross-talk by at least a factor of 100, is built of 12.7 mm
thick black plastic and contains a 2 cm × 2 cm hole in front of
each module.
Light  distributor
Optic fibers
Lead−glass modules
Light  source
Figure 11: Schematic of the Gain-monitoring system.
Such a system was found to provide a rather uniform light
collection for all modules, and proved useful for detector test-
ing and tuning, as well as for troubleshooting during the exper-
iment. However, it was found that monitoring over extended
periods of time proved to be less informative than first thought.
The reason for this is due to the fact that the main radiation dam-
age to the lead-glass blocks occurred at a depth of about 2-4 cm
8Manufactured by Hamamatsu Photonics, Hamamatsu, Japan.
from the front face. The monitoring light passes through the
damaged area, while an electromagnetic shower has its maxi-
mum at a depth of about 10 cm. Therefore, as a result of this
radiation damage the magnitude of the monitoring signals drops
relatively quicker than the real signals. Consequently, the re-
sulting change in light-output during the experiment was char-
acterized primarily through online analysis of dedicated elastic
e-p scattering runs. This data was then used for periodic re-
calibration of the individual calorimeter gains.
3. Veto Hodoscopes
In order to ensure clean identification of the scattered pho-
tons through rejection of high-energy electrons in the compli-
cated environment created by the mixed electron-photon beam,
a veto detector which utilizes UVT Lucite as a Čerenkov radi-
ator was developed. This veto detector proved particularly use-
ful for low luminosity runs, where its use made it possible to
take data without relying on the deflection magnet (see Fig. 1).
The veto detector consists of two separate hodoscopes located
in front of the calorimeter’s gain monitoring system. The first
hodoscope has 80 counters oriented vertically, while the second
has 110 counters oriented horizontally as shown in Fig. 12. The
segmentation scheme for the veto detector was chosen so that
it was consistent with the position resolution of the lead-glass
calorimeter. An effective dead time of an individual counter is
about 100 ns due to combined double-pulse resolution of the
PMT, the front-end electronics, the TDC, and the ADC gate-
width.
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PMT XP2971
Honeycomb plate
Lucite radiator
Figure 12: Cut-off view of the “horizontal” veto hodoscope.
Each counter is made of a UVT Lucite bar with a PMT glued
directly to one of its end, which can be seen in Fig. 13. The Lu-
cite bar of 2×2 cm2 cross section was glued to a XP2971 PMT
and wrapped in aluminized Mylar and black Tedlar. Counters
are mounted on a light honeycomb plate via an alignment
groove and fixed by tape. The counters are staggered in such
a way so as to allow for the PMTs and the counters to overlap.
The average PMT pulse generated by a high-energy electron
corresponds to 20 photo-electrons. An amplifier, powered by
the HV line current, was added to the standard HV divider, in
order that the PMT gain could be reduced by a factor of 10
XP2971
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Lucite radiator
Figure 13: Schematic of the veto counter.
[15, 16]. After gain-matching by using cosmic ray data a good
rate uniformity was achieved, as can be seen in the experimental
rate distribution of the counters shown in Fig. 14. The regular
variation in this distribution reflects the shielding effect result-
ing from the staggered arrangement of the counters. A signif-
0 25 50 75 100 125
0 20 40 60 80
Horizontal Veto Detector
Vertical Veto Detector
Counter  number
Figure 14: The counting rate in the veto counters observed at luminosity of
1.5 · 1038 cm−2/s.
icant reduction of the rate (by a factor of 5) was achieved by
adding a 2 inch polyethylene plate in front of the hodoscopes.
Such a reduction as a result of this additional shielding is con-
sistent with the observed variation of the rate (see Fig. 14) and
indicates that the typical energy of the dominant background
is around a few MeV. The veto plane efficiency measured for
different beam intensities is shown in Table 2. It drops signif-
icantly at high rate due to electronic dead-time, which limited
the beam intensity to 3-5 µA in data-taking runs with the veto.
An analysis of the experimental data with and without veto de-
tectors showed that the deflection of the electrons by the magnet
Table 2: The efficiency of the veto hodoscopes and the rate of a single counter
at different beam currents. The detector was installed at 30◦ with respect to the
beam at a distance 13 m from the target. The radiator had been removed from
the beam path, the deflection magnet was off and the 2 inch thick polyethylene
protection plate was installed.
Run Beam Rate of the Efficiency Efficiency
current counter V12 horizontal vertical
[µA] [MHz] hodoscope hodoscope
1811 2.5 0.5 96.5% 96.8%
1813 5.0 1.0 95.9% 95.0%
1814 7.5 1.5 95.0% 94.0%
1815 10. 1.9 94.4% 93.0%
1816 14. 2.5 93.4% 91.0%
1817 19 3.2 92.2% 89.3%
provided a sufficiently clean photon event sample. As a result
the veto hodoscopes were switched off during most high lu-
minosity data-taking runs, although they proved important in
analysis of low luminosity runs and in understanding various
aspects of the experiment.
4. High Voltage System
Each PMT high-voltage supply was individually monitored
and controlled by the High Voltage System (HVS). The HVS
consists of six power supply crates of LeCroy type 1458 with
high-voltage modules of type 1461N, a cable system, and a set
of software programs. The latter allows to control, monitor,
download and save the high-voltage settings and is described
below in more detail. Automatic HV monitoring provides an
alarm feature with a verbal announcement and a flashing signal
on the terminal. The controls are implemented over an Ethernet
network using TCP/IP protocol. A Graphical User Interface
(GUI) running on a Linux PC provides access to all features
of the LeCroy system, loading the settings and saving them in
a file. A sample distribution of the HV settings is shown in
Fig. 15.
The connections between the outputs of the high-voltage
modules and the PMT dividers were arranged using 100 m long
multi-wire cables. The transition from the individual HV sup-
ply outputs to a multi-wire cable and back to the individual
PMT was arranged via high-voltage distribution boxes that are
located inside the DAQ area and front-end patch panels outside
the PMT box. These boxes have input connectors for individual
channels on one side and two high-voltage multi-pin connec-
tors (27 pins from FISCHER part number D107 A051-27) on
the other. High-voltage distribution boxes were mounted on the
side of the calorimeter stand and on the electronics rack.
5. Data Acquisition System
Since the calorimeter was intended to be used in Hall A
at JLab together with the standard Hall A detector devices,
the Data Acquisition System of the calorimeter is part of the
standard Hall A DAQ system. The latter uses CODA (CE-
BAF On-line Data Acquisition system) [17] developed by the
-1800 -1700 -1600 -1500 -1400 -1300 -1200 -1100 -1000
At the end of the experiment
High Voltage [V]
-1800 -1700 -1600 -1500 -1400 -1300 -1200 -1100 -1000
At the start of the experiment
Figure 15: The HV settings for the calorimeter PMTs.
JLab data-acquisition group. The calorimeter DAQ includes
one Fastbus crate with a single-board VME computer installed
using a VME-Fastbus interface and a trigger supervisor mod-
ule [18], which synchronizes the read-out of all the information
in a given event. The most important software components are
a Read-Out Controller (ROC), which runs on the VME com-
puter under the VxWorks OS, and an Event Builder and Event
Recorder which both run on a Linux workstation. For a detailed
description of the design and operation of the Hall A DAQ sys-
tem see [8] and references therein.
All 704 PMT signals and 56 SUM-32 signals are digitized by
LeCroy 1881M FastBus ADC modules. The 56 SUM-32 dis-
criminator pulses are also read-out by scalers and LeCroy 1877
FastBus TDCs. During the RCS experiment the calorimeter
was operating in conjunction with one of the High Resolution
Spectrometers (HRS), which belong to the standard Hall A de-
tector equipment [8]. The Hall A Data Acquisition System is
able to accumulate data involving several event types simulta-
neously. In the RCS experiment there were 8 types of trigger
signals and corresponding event types. Trigger signals from the
HRS are generated by three scintillator planes: S0, S1 and S2
(see Fig. 8 in [8]). In the standard configuration the main sin-
gle arm trigger in the spectrometer is formed by a coincidence
of signals from S1 and S2. An alternative trigger, logically de-
scribed by (S0 AND S1) OR (S0 AND S2), is used to measure
the trigger efficiency. In the RCS experiment one more proton
arm trigger was used, defined as being a single hit in the S0
plane. As this is the fastest signal produced in the proton arm,
it was better suited to form a fast coincidence trigger with the
photon calorimeter.
The logic of the Photon Arm singles trigger was described in
detail in Section 2.3. Besides this singles trigger there are two
auxiliary triggers that serve to monitor the calorimeter blocks
and electronics. The first is a photon arm cosmics trigger, which
was defined by a coincidence between signals from two plas-
tic scintillator paddles, placed on top and under the bottom of
photo tube
Laser calibration  trigger
Cosmics  trigger
top  paddle
bottom  paddle
1024 Hz  Pulser
Retiming
Retiming
ADC  gates
TDC Common Stop
Proton  Arm
ADC  gates
TDC Common Stop
Photon  Arm
Proton  Arm
Main  photon  trigger
Photon  Arm
Figure 16: Schematic diagram of the DAQ trigger logic.
the calorimeter. The other trigger is the light-calibration (laser)
trigger which was used for gain monitoring purposes.
The two-arm coincidence trigger is formed by a time overlap
of the main calorimeter trigger and the signal from the S0 scin-
tillator plane in the HRS. The width of the proton trigger pulse
is set to 100 ns, while the photon trigger pulse, which is delayed
in a programmable delay line, is set to 10 ns. As a result, the co-
incidence events are synchronized with the photon trigger, and
a correct timing relation between trigger signals from two arms
is maintained for all 25 kinematic configurations of the RCS
experiment. Finally, a 1024 Hz puls generator signal forms a
pulser trigger, which was used to measure the dead time of the
electronics.
All 8 trigger signals are sent to the Trigger Supervisor mod-
ule which starts the DAQ readout. Most inputs of the Trigger
Supervisor can be individually pre-scaled. Triggers which are
accepted by the DAQ are then re-timed with the scintillators of
a corresponding arm to make gates for ADCs and TDCs. This
re-timing removes trigger time jitter and ensures the timing is
independent of the trigger type. Table 3 includes information
on the trigger and event types used in the RCS experiment and
shows typical pre-scale factors used during the data-taking. A
schematic diagram of the overall RCS experiment DAQ trigger
logic is shown in Fig.16.
6. Calorimeter Performance
The calorimeter used in the RCS experiment had three related
purposes. The first purpose is to provide a coincidence trigger
Table 3: A list of triggers used in the RCS experiment. Typical pre-scale factors
which were set during a data-taking run (run #1819) are shown.
Trigger Trigger Description pre-scale
ID factor
T1 Photon arm singles trigger 100,000
T2 Photon arm cosmics trigger 100,000
T3 Main Proton arm trigger:
(S1 AND S2)
T4 Additional Proton arm trigger:
(S0 AND S1) OR (S0 AND S2)
T5 Coincidence trigger 1
T6 Calorimeter light-calibration
trigger
T7 Signal from the HRS S0 scintil-
lator plane
65,000
T8 1024 Hz pulser trigger 1,024
Coincidence Time (ns)
10 20 30 40
Figure 17: The time of the calorimeter trigger relative to the recoil proton trig-
ger for a production run in kinematic 3E at maximum luminosity (detected
Eγ = 1.31 GeV). The solid curve shows all events, while the dashed curve
shows events with a cut on energy in the most energetic cluster > 1.0 GeV.
signal for operation of the DAQ. Fig. 17 shows the coincidence
time distribution, where one can see a clear relation between en-
ergy threshold and time resolution. The observed resolution of
around 8 ns (FWHM) was sufficient to identify cleanly coinci-
dence events over the background, which meant that no off-line
corrections were applied for variation of the average time of in-
dividual S UM − 32 summing modules. The second purpose
is determination of the energy of the scattered photon/electron
to within an accuracy of a few percent, while the third is rea-
sonably accurate reconstruction of the photon/electron hit co-
ordinates in order that kinematic correlation cuts between the
scattered photon/electron and the recoil proton can be made.
The off-line analysis procedure and the observed position and
energy resolutions are presented and discussed in the following
two sections.
6.1. Shower Reconstruction Analysis and Position Resolution
The off-line shower reconstruction involves a search for clus-
ters and can be characterized by the following definitions:
1. a cluster is a group of adjacent blocks;
2. a cluster occupies 9 (3 × 3) blocks of the calorimeter;
3. the distribution of the shower energy deposition over the
cluster blocks (the so-called shower profile) satisfies the
following conditions:
(a) the maximum energy deposition is in the central
block;
(b) the energy deposition in the corner blocks is less than
that in each of two neighboring blocks;
(c) around 50% of the total shower energy must be de-
posited in the central row (and column) of the cluster.
For an example in which the shower center is in the middle
of the central block, around 84% of the total shower energy
is in the central block, about 14% is in the four neighboring
blocks, and the remaining 2% is in the corner blocks. Even at
the largest luminosity used in the RCS experiment the proba-
bility of observing two clusters with energies above 50% of the
elastic value was less than 10%, so for the 704 block hodoscope
a two-cluster overlap was very unlikely.
The shower energy reconstruction requires both hardware
and software calibration of the calorimeter channels. On the
hardware side, equalization of the counter gains was initially
done with cosmic muons, which produce 20 MeV energy equiv-
alent light output per 4 cm path (muon trajectories perpendic-
ular to the long axis of the lead-glass blocks). The calibration
was done by selecting cosmic events for which the signals in
both counters above and below a given counter were large. The
final adjustment of each counter’s gain was done by using cali-
bration with elastic e-p events. This calibration provided PMT
gain values which were on average different from the initial cos-
mic set by 20%
The purpose of the software calibration is to define the co-
efficients for transformation of the ADC amplitudes to energy
deposition for each calorimeter module. These calibration co-
efficients are obtained from elastic e-p data by minimizing the
function:
Ci · (A
i − Pi) − E
where:
n = 1 ÷ N — number of the selected calibration event;
i — number of the block, included in the cluster;
Mn — set of the blocks’ numbers, in the cluster;
Ani — amplitude into the i-th block;
Pi — pedestal of the i-th block;
Ene — known energy of electron;
Ci — calibration coefficients, which need to be fitted.
The scattered electron energy Ene is calculated by using the en-
ergy of the primary electron beam and the scattered electron
angle. A cut on the proton momentum-angle correlation is used
to select clean elastic events.
Following calculation of the calibration coefficients, the total
energy deposition E, as well as the X and Y coordinates of the
shower center of gravity are calculated by the formulae:
Ei , X =
Ei · Xi/E , Y =
Ei · Yi/E
where M is the set of blocks numbers which make up the clus-
ter, Ei is the energy deposition in the i-th block, and Xi and Yi
are the coordinates of the i-th block center. The coordinates cal-
culated by this simple center of gravity method are then used for
a more accurate determination of the incident hit position. This
second iteration was developed during the second test run [7],
in which a two-layer MWPC was constructed and positioned
directly in front of the calorimeter. This chamber had 128 sen-
sitive wires in both X and Y directions, with a wire spacing of
2 mm and a position resolution of 1 mm. In this more refined
procedure, the coordinate xo of the shower center of gravity in-
side the cell (relative to the cell’s low boundary) is used. An
estimate of the coordinate xe can be determined from a polyno-
mial in this coordinate (P(xo)):
xe = P(xo) = a1 · xo + a3 · x
o + a5 · x
o + a7 · x
o + a9 · x
For symmetry reasons, only odd degrees of the polynomial are
used. The coefficients an are calculated by minimizing the func-
tional:
P(an, x
o) − x
where:
i = 1 ÷ N — number of event;
xio — coordinate of the shower center of gravity inside the
cell;
xit — coordinate of the track (MWPC) on the calorimeter
plane;
an — coordinate transformation coefficients to be fitted.
The resulting resolution obtained from such a fitting proce-
dure was found to be around 5.5 mm for a scattered electron
energy of 2.3 GeV. For the case of production data, where the
MWPC was not used, Fig. 18 shows a scatter plot of events
on the front face of the calorimeter. The parameter plotted
is the differences between the observed hit coordinates in the
calorimeter and the coordinates calculated from the proton pa-
rameters and an assumed two-body kinematic correlation. The
dominant contribution to the widths of the RCS and e-p peaks
that can be seen in this figure is from the angular resolution of
the detected proton, which is itself dominated by multiple scat-
tering. As the calorimeter distance varied during the experiment
between 5.5 m and 20 m, the contribution to the combined an-
gular resolution from the calorimeter position resolution of a
few millimeters was minimal.
6.2. Trigger Rate and Energy Resolution
At high luminosity, when a reduction of the accidental co-
incidences in the raw trigger rate is very important, the trigger
threshold should be set as close to the signal amplitude for elas-
tic RCS photons as practical. However, the actual value of the
threshold for an individual event has a significant uncertainty
due to pile-up of the low-amplitude signals, fluctuations of the
signal shape (mainly due to summing of the signals from the
PMTs with different HV and transit time), and inequality of the
x (cm)δ
y (cm)
’p)γ,γp(p(e,e’p)
p)0π,γp(
Figure 18: The scatter plot of p − γ(e) events in the plane of the calorimeter
front face.
gain in the individual counters. Too high a threshold, therefore,
can lead to a loss in detection efficiency.
The counting rate of the calorimeter trigger, f , which defines
a practical level of operational luminosity has an exponential
dependence on the threshold, as can be seen in Fig. 19. It can
be described by a function of Ethr:
f = A × exp(−B × Ethr/Emax),
where Emax is the maximum energy of an elastically scat-
tered photon/electron for a given scattering angle, A an angle-
dependent constant, and B a universal constant ≈ 9±1. The an-
0.3 0.6 0.9 1.2 1.5 1.8 2.1
40200 60 80 100 120 140
elastic
Threshold  [GeV]
Threshold  [mV]
Beam energy:
Beam current:              
Target:                           
Calorimeter angle:
Calorimeter to target:
Solid angle:
10 µ A
3.3 GeV
Figure 19: Calorimeter trigger rate vs threshold level.
gular variation of the constant A, after normalization to a fixed
luminosity and the calorimeter solid angle, is less than a factor
of 2 for the RCS kinematics. The threshold for all kinemat-
ics was chosen to be around half of the elastic energy, thereby
balancing the need for a low trigger rate without affecting the
detection efficiency.
In order to ensure proper operation and to monitor the perfor-
mance of each counter the widths of the ADC pedestals were
used (see Fig. 20). One can see that these widths vary slightly
with block number, which reflects the position of the block in
the calorimeter and its angle with respect to the beam direction.
This pedestal width also allows for an estimate of the contri-
bution of the background induced base-line fluctuations to the
overall energy resolution. For the example shown in Fig. 20 the
width of 6 MeV per block leads to energy spectrum noise of
about 20 MeV because a 9-block cluster is used in the off-line
analysis.
0 100 200 300 400 500 600 700
LG block number
Figure 20: The width of the ADC pedestals for the calorimeter in a typical run.
The observed reduction of the width vs the block number reflects the lower
background at larger detector angle with respect to the beam direction.
The energy resolution of the calorimeter was measured by
using elastic e-p scattering. Such data were collected many
times during the experiment for kinematic checks and calorime-
ter gain calibration. Table 4 presents the observed resolution
and the corresponding ADC pedestal widths over the course of
the experiment. For completeness, the pedestal widths for cos-
mic and production data are also included. At high luminosity
the energy resolution degrades due to fluctuations of the base
line (pedestal width) and the inclusion of more accidental hits
during the ADC gate period. However, for the 9-block cluster
size used in the data analysis the contribution of the base line
fluctuations to the energy resolution is just 1-2%. The measured
widths of ADC pedestals confirmed the results of Monte Carlo
simulations and test runs that the radiation background is three
times higher with the 6% Cu radiator upstream of the target than
without it.
The resolution obtained from e-p calibration runs was cor-
rected for the drift of the gains so it could be attributed directly
to the effect of lead glass radiation damage. It degraded over
the course of the experiment from 5.5% (for a 1 GeV photon
energy) at the start to larger than 10% by the end. It was es-
timated that this corresponds to a final accumulated radiation
dose of about 3-10 kRad, which is in agreement with the known
level of radiation hardness of the TF-1 lead glass [19]. This
observed radiation dose corresponds to a 500 hour experiment
with a 15 cm LH2 target and 50 µA beam.
6.3. Annealing of the radiation damage
The front face of the calorimeter during the experiment was
protected by plastic material with an effective thickness of
10 g/cm2. For the majority of the time the calorimeter was lo-
cated at a distance of 5-8 m and an angle of 40-50◦ with respect
to the electron beam direction. The transparency of 20 lead-
glass blocks was measured after the experiment, the results of
which are shown in Fig. 21. This plot shows the relative trans-
mission through 4 cm of glass in the direction transverse to
the block length at different locations. The values were nor-
malized to the transmission through similar lead-glass blocks
which were not used in the experiment. The transmission mea-
surement was done with a blue LED (λmax of 430 nm) and a
Hamamatsu photo-diode (1226-44).
0 10 20 30 40
Distance from the calorimeter face [cm]
Figure 21: The blue light attenuation in 4 cm of lead-glass vs distance from
the front face of calorimeter measured before (solid) and after (dashed) UV
irradiation.
A UV technique was developed and used in order to cure ra-
diation damage. The UV light was produced by a 10 kW total
power 55-inch long lamp9, which was installed vertically at a
distance of 45 inches from the calorimeter face and a quartz
plate (C55QUARTZ) was used as an infrared filter. The inten-
sity of the UV light at the face of the lead-glass blocks was
found to be 75 mW/cm2 by using a UVX digital radiometer10.
In situ UV irradiation without disassembly of the lead-glass
stack was performed over an 18 hour period. All PMTs were
removed before irradiation to ensure the safety of the photo-
cathode. The resultant improvement in transparency can be
seen in Fig. 21. An alternative but equally effective method
to restore the lead-glass transparency, which involved heating
of the lead-glass blocks to 250◦C for several hours, was also
9Type A94551FCB manufactured by American Ultraviolet, Lebanon, IN
46052, USA
10 Manufactured by UVP, Inc., Upland, CA 91786, USA
Table 4: Pedestal widths and calorimeter energy resolution at different stages of the RCS experiment for cosmic (c), electron (e) and production (γ) runs in order of increasing effective luminosity.
Runs L e f f Beam Current Accumulated Detected Ee/γ σE /E σE /E at Eγ=1 GeV Θcal σped
(1038 cm−2/s) (µA) Beam Charge (C) (GeV) (%) (%) (degrees) (MeV)
1517 (c) - - - - - - - 1.5
1811 (e) 0.1 2.5 2.4 2.78 4.2 7.0 30 1.7
1488 (e) 0.2 5 0.5 1.32 4.9 5.5 46 1.75
2125 (e) 1.0 25 6.6 2.83 4.9 8.2 34 2.6
2593 (e) 1.5 38 14.9 1.32 9.9 11.3 57 2.0
1930 (e) 1.6 40 4.4 3.39 4.2 7.7 22 3.7
1938 (γ) 1.8 15 4.5 3.23 - - 22 4.1
2170 (γ) 2.4 20 6.8 2.72 - - 34 4.0
1852 (γ) 4.2 35 3.0 1.63 - - 50 5.0
tested. The net effect of heating on the transparency of the lead-
glass was similar to the UV curing results.
In summary, operation of the calorimeter at high luminos-
ity, particularly when the electron beam was incident on the
bremsstrahlung radiator, led to a degradation in energy resolu-
tion due to fluctuations in the base-line and a higher accidental
rate within the ADC gate period. For typical clusters this effect
was found to be around a percent or two. By far the largest con-
tributor to the observed degradation in resolution was radiation
damage sustained by the lead-glass blocks, which led to the res-
olution being a factor of two larger at the end of the experiment.
The resulting estimates of the total accumulated dose were con-
sistent with expectations for this type of lead-glass. Finally, it
was found that both UV curing and heating of the lead-glass
were successful in annealing this damage.
7. Summary
The design of a segmented electromagnetic calorimeter
which was used in the JLab RCS experiment has been de-
scribed. The performance of the calorimeter in an unprece-
dented high luminosity, high background environment has been
discussed. Good energy and position resolution enabled a suc-
cessful measurement of the RCS process over a wide range of
kinematics.
8. Acknowledgments
We acknowledge the RCS collaborators who helped to oper-
ate the detector and the JLab technical staff for providing out-
standing support, and specially D. Hayes, T. Hartlove, T. Hun-
yady, and S. Mayilyan for help in the construction of the lead-
glass modules. We appreciate S. Corneliussen’s careful reading
of the manuscript and his valuable suggestions. This work was
supported in part by the National Science Foundation in grants
for the University of Illinois University and by DOE contract
DE-AC05-84ER40150 under which the Southeastern Universi-
ties Research Association (SURA) operates the Thomas Jeffer-
son National Accelerator Facility for the United States Depart-
ment of Energy.
References
[1] C. Hyde-Wright, A. Nathan, and B. Wojtsekhowski, spokespersons, JLab
experiment E99-114.
[2] Charles Hyde-Wright and Kees de Jager Ann.Rev.Nucl.Part.Sci. 54, 217
(2004).
[3] A.V. Radyushkin, Phys. Rev. D 58, 114008 (1998).
[4] H.W. Huang, P. Kroll, T. Morii, Eur. Phys. J. C 23, 301 (2002); erratum
ibid., C 31, 279 (2003).
[5] R. Thompson, A. Pang, Ch.-R. Ji, Phys. Rev. D 73, 054023 (2006).
[6] M.A. Shupe et al., Phys. Rev. D 19, 1929 (1979).
[7] E. Chudakov et al., Study of Hall A Photon Spectrometer. Hall A internal
report, 1998.
[8] J. Alcorn et al., Nucl. Instr. Meth. A 522, (2004) 294.
[9] D.J. Hamilton et al., Phys. Rev. Lett. 94, 242001 (2005).
[10] A. Danagoulian et al., Phys. Rev. Lett. 98, 152001 (2007).
[11] Yu.D. Prokoshkin et al., Nucl. Instr. Meth. A 248, 86102 (1986).
[12] M.Y. Balatz et al., Nucl. Instr. Meth. A 545, 114 (2005).
[13] R.G. Astvatsaturov et al., Nucl. Instr. Meth. 107, 105 (1973).
[14] R.R. Crittenden et al., Nucl. Instr. Meth. A 387, 377 (1997).
[15] V. Popov et al., Proceedings of IEEE 2001 Nuclear Science Symposium
(NSS) And Medical Imaging Conference (MIC). Ed. J.D. Valentine IEEE
(2001) p. 634-637.
[16] V. Popov, Nucl. Instr. Meth. A 505, 316 (2003).
[17] W.A. Watson et al., CODA: a scalable, distributed data acquisition sys-
tem, in: Proceedings of the Real Time 1993 Conference, p. 296;
[18] E. Jastrzembski et al., The Jefferson Lab trigger supervisor system, 11th
IEEE NPSS Real Time 1999 Conference, JLab-TN-99-13, 1999.
[19] A.V. Inyakin et al., Nucl. Instr. Meth. 215, 103 (1983).
	1 Introduction
	2 Calorimeter
	2.1 Calorimeter Design
	2.1.1 Air Cooling
	2.1.2 Cabling System
	2.2 Lead-Glass Counter
	2.2.1 Design of the Counter
	2.2.2 HV Divider
	2.3 Electronics
	2.3.1 Trigger Scheme
	2.4 Gain Monitoring System
	3 Veto Hodoscopes
	4 High Voltage System
	5 Data Acquisition System
	6 Calorimeter Performance
	6.1 Shower Reconstruction Analysis and Position Resolution
	6.2 Trigger Rate and Energy Resolution
	6.3 Annealing of the radiation damage
	7 Summary
	8 Acknowledgments

  A lead-glass hodoscope calorimeter that was constructed for use in the
Jefferson Lab Real Compton Scattering experiment is described. The detector
provides a measurement of the coordinates and the energy of scattered photons
in the GeV energy range with resolutions of 5 mm and 6%/\sqrt(E{\gamma} [GeV]).
Features of both the detector design and its performance in the high luminosity
environment during the experiment are presented.

Introduction
A calorimeter was constructed as part of the instrumentation
of the Jefferson Lab (JLab) Hall A experiment E99-114, “Ex-
clusive Compton Scattering on the Proton” [1], the schematic
layout for which is shown in Fig. 1. The study of elastic pho-
ton scattering provides important information about nucleon
structure, which is complementary to that obtained from elastic
electron scattering [2]. Experimental data on the Real Comp-
ton Scattering (RCS) process at large photon energies and large
scattering angles are rather scarce, due mainly to the absence of
high luminosity facilities with suitable high-resolution photon
detectors. Such data are however crucial, as the basic mecha-
nism of the RCS reaction is the subject of active debate [3, 4, 5].
The only data available before the JLab E99-114 experiment
were obtained at Cornell about 30 years ago [6].
The construction of the CEBAF (Continuous Electron Beam
Accelerator Facility) accelerator has led to an extension of
many experiments with electron and photon beams in the GeV
energy range and much improved precision. This is the result of
a number of fundamental improvements to the electron beam,
∗Tel.: +44-141-330-5898; Fax: +44-141-330-5889
Email address: d.hamilton@physics.gla.ac.uk (D. J. Hamilton)
1deceased
2present address: Applied Biosystems/MDS, USA
3present address: University of Virginia, Charlottesville, VA 22901, USA
4present address: NIKHEF, 1009 DB Amsterdam, The Netherlands
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Beam dump
Cable lines
Calorimeter
Front−end electronics
Beam line
Magnet
Radiator and target
Figure 1: Layout of the RCS experiment in Hall A. An electron beam incident
on a radiator produces an intense flux of high energy photons.
including a 100% duty cycle, low emittance and high polariza-
tion, in addition to new dedicated target and detector systems.
The CEBAF duty factor provides an improvement of a factor of
15 compared to the best duty factor of a beam extracted from a
synchrotron, at a similar instantaneous rate in the detectors.
In 1994 work began on the development of a technique for an
RCS experiment at JLab, leading in 1997 to the instigation of
a large-scale prototyping effort. The results of the subsequent
test runs in 1998 and 1999 [7] provided sufficient information
Preprint submitted to NIM A August 16, 2018
http://arxiv.org/abs/0704.1830v3
Figure 2: A photograph of the experimental set-up for E99-114, showing the calorimeter (center) and part of the proton spectrometer (rear).
for the final design of the apparatus presented in the present
article. The fully realized physics experiment took place in
2002 (see Fig. 1) at a photon-nucleon luminosity which was
a factor of 1300 higher than in the previous Cornell experi-
ment. The experimental technique involves utilizing a mixed
electron-photon beam which is incident on a liquid hydrogen
target and passes to a beam dump. The scattered photons are
detected in the calorimeter, while the recoiling protons are de-
tected in a high resolution magnetic spectrometer (HRS-L). A
magnet between the hydrogen target and the calorimeter de-
flects the scattered electrons, which then allows for clean sep-
aration between Compton scattering and elastic e-p scattering
events. The Data Acquisition Electronics (DAQ) is shielded by
a 4 inch thick concrete wall from the beam dump and the target.
Figure 2 shows a photograph of the experimental set-up with
the calorimeter in the center.
The experiment relied on a proton-photon time coincidence
and an accurate measurement of the proton-photon kinematic
correlation for event selection. The improvement in the event
rate over the previous measurement was achieved through the
use of a mixed electron-photon beam, which in turn required
a veto detector in front of the calorimeter or the magnetic de-
flection of the scattered electron [1]. In order to ensure redun-
dancy and cross-checking, both a veto and deflection magnet
were designed and built. The fact that a clean photon beam was
not required meant that the photon radiator could be situated
very close to the hydrogen target, leading to a much reduced
background near the beam line and a dramatic reduction of the
photon beam size. This small beam size in combination with
the large dispersion in the HRS-L proton detector system [8]
resulted in very good momentum and angle resolution for the
recoiling proton without the need for a tracking detector near
the target, where the background rate is high.
Good energy and coordinate resolutions were key features
of the photon detector design goals, both of which were sig-
nificantly improved in the JLab experiment as compared to the
Cornell one. An energy resolution of at least 10% is required to
separate cleanly RCS events from electron bremsstrahlung and
neutral pion events. In order to separate further the background
from neural pion photo-production, which is the dominant com-
ponent of the high-energy background in this measurement, a
high angular resolution between proton and photon detectors is
crucial. This was achieved on the photon side by constructing a
highly segmented calorimeter of 704 channels. The RCS exper-
iment was the first instance of a calorimeter being operated at
an effective electron-nucleon luminosity of 1039 cm2/s [9, 10]
(a 40 µA electron beam on a 6% Cu radiator upstream of a
15 cm long liquid hydrogen target). It was observed in the test
runs that the counting rate in the calorimeter fell rapidly as the
threshold level was increased, which presented an opportunity
to maintain a relatively low trigger rate even at high luminosity.
However, on-line use of the calorimeter signal required a set of
summing electronics and careful equalizing and monitoring of
the individual channel outputs during the experiment.
As the RCS experiment represented the first use of such a
calorimeter at very high luminosity, a detailed study of the
calorimeter performance throughout the course of the experi-
ment has been conducted. This includes a study of the rela-
tionship between luminosity, trigger rate, energy resolution and
ADC pedestal widths. An observed fall-off in energy resolu-
tion as the experiment progressed allowed for characterization
of radiation damage sustained by the lead-glass blocks. It was
possible to mitigate this radiation damage after the experiment
by annealing, with both UV curing and heating proving effec-
tive.
We begin by discussing the various components which make
up the calorimeter and the methods used in their construction.
This is followed by a description of veto hodoscopes which
were used for particle identification purposes. An overview
of the high-voltage and data acquisition systems is then pre-
sented, followed, finally, by a discussion on the performance
of the calorimeter in the unique high-luminosity environment
during the RCS experiment.
2. Calorimeter
The concepts and technology associated with a fine-
granularity lead-glass Čerenkov electromagnetic calorimeter
(GAMS) were developed by Yu. Prokoshkin and collaborators
at the Institute of High Energy Physics (IHEP) in Serpukhov,
Russia [11]. The GAMS type concept has since been employed
for detection of high-energy electrons and photons in several
experiments at JLab, IHEP, CERN, FNAL and DESY (see for
example [12]). Many of the design features of the calorimeter
presented in this article are similar to those of Serpukhov. A
schematic showing the overall design of the RCS calorimeter
can be seen in Fig. 3. The main components are:
• the lead-glass blocks;
• a light-tight box containing the PhotoMultiplier Tubes
(PMTs);
• a gain-monitoring system;
• a doubly-segmented veto hodoscopes;
• the front-end electronics;
• an elevated platform;
• a lifting frame.
The calorimeter frame hosts a matrix of 22×32 lead-glass
blocks together with their associated PMTs and High Voltage
(HV) dividers. Immediately in front of the lead-glass blocks
is a sheet of UltraViolet-Transmitting (UVT) Lucite, which is
used to distribute calibration light pulses for gain-monitoring
purposes uniformly among all 704 blocks. The light-tight box
provides protection of the PMTs from ambient light and con-
tains an air-cooling system as well as the HV and signal cable
systems. Two veto hodoscopes, operating as Čerenkov coun-
ters with UVT Lucite as a radiator, are located in front of the
calorimeter. The front-end electronics located a few feet be-
hind the detector were assembled in three relay racks. They are
comprised of 38 analog summers, trigger logic and patch pan-
els. The elevated platform was needed to bring the calorimeter
to the level of the beam line, while the lifting frame was used
to re-position the calorimeter in the experimental hall by means
of an overhead crane. This procedure, which took on average
around two hours, was performed more than 25 times during
the course of the experiment.
platform
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system
monitoring
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Veto counters
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Figure 3: Schematic side view of the RCS calorimeter detector system.
2.1. Calorimeter Design
The main frame of the calorimeter is made of 10 inch wide
steel C-channels. A thick flat aluminum plate was bolted to the
bottom of the frame, with a second plate installed vertically and
aligned to 90◦ with respect to the first one by means of align-
ment screws (see Fig. 4). Another set of screws, mounted inside
and at the top of the main frame on the opposite side of the ver-
tical alignment plate, was used to compress all gaps between the
lead-glass modules and to fix their positions. The load was ap-
plied to the lead-glass blocks through 1 inch× 1 inch × 0.5 inch
plastic plates and a 0.125 inch rubber pad. In order to further as-
sist block alignment, 1 inch wide stainless steel strips of 0.004
inch thickness running from top to bottom of the frame were
inserted between every two columns of the lead-glass modules.
2.1.1. Air Cooling
All PMTs and HV dividers are located inside a light-tight
box, as shown in Fig. 5. As the current on each HV divider
is 1 mA, simultaneous operation of all PMTs would, without
cooling, lead to a temperature rise inside the box of around
50-70◦C. An air-cooling system was developed to prevent the
PMTs from overheating, and to aid the stable operation of the
calorimeter. The air supply was provided by two parallel oil-
less regenerative blowers of R4110-2 type5, which are capable
5Manufactured by S&F Supplies, Brooklyn, NY 11205, USA.
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Figure 4: Front cross-section of the calorimeter, showing the mechanical com-
ponents.
of supplying air at a maximum pressure of 52 inches water and
a maximum flow of 92 CFM. The air is directed toward the HV
divider via vertical collector tubes and numerous outlets. When
the value on any one of the temperature sensors installed in sev-
eral positions inside the box exceeds a preset limit, the HV on
the PMTs is turned off by an interlock system. The air line is
equipped with a flow switch of type FST-321-SPDT which was
included in the interlock system. The average temperature in-
side the box during the entire experimental run did not exceed
the preset limit of 55◦C.
2.1.2. Cabling System
A simple and reliable cabling system is one of the key fea-
tures of multichannel detectors, with easy access to the PMTs
and HV dividers for installation and repair being one of the key
features. The cabling system includes:
• 1 foot long HV and signal pig-tails soldered to the HV
divider;
• patch panels for Lemo and HV connectors;
• 10 feet long cables from those patch panels to the front-
end electronics and the HV distribution boxes;
• the HV distribution boxes themselves;
• BNC-BNC patch panels for the outputs of the front-end
modules;
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Figure 5: A schematic showing the calorimeter air cooling and cabling systems.
• BNC-BNC patch panels on the DAQ side for the analog
signals;
• BNC-Lemo patch panels on the DAQ side for the veto-
counter lines.
Figure 6 shows the cabling arrangement inside the PMT box.
The patch panels, which are custom-built and mounted on the
air supply tubes, have the ability to swing to the side in order to
allow access to the PMTs and the HV dividers. The box has two
moving doors, the opening of which leads to activation of an in-
terlock system connected to the HV supply. In order to reduce
the diameter of the cable bundle from the PMT box, RG-174
cable (diameter 0.1 inch) was used for the PMT signals, and a
twisted pair for the HV connection (two individually insulated
inner 26 AWG conductors with an overall flame-retardant PVC
jacket, part number 001-21803 from the General Wire Product
company). The box patch panels used for the HV lines each
convert 24 of the above twisted pairs (single HV line) to the
multi-wire HV cables (the part 001-21798 made by General
Wire Product), which run to the HV power supply units located
in the shielded area near DAQ racks.
2.2. Lead-Glass Counter
The basic components of the segmented calorimeter are the
TF-1 lead-glass blocks and the FEU 84-3 PMTs. In the 1980s
the Yerevan Physics Institute (YerPhI) purchased a consignment
of TF-1 lead-glass blocks of 4 cm × 4 cm ×40 cm and FEU 84-3
PMTs of 34 mm diameter (with an active photo-cathode diam-
eter of 25 mm) for the construction of a calorimeter to be used
Lemo connector
from HV divider
HV connector
Patch panel
Al T−channel
HV cables
Signal cables
HV&Sig. cables
Figure 6: A photograph of the cabling inside the PMT box.
in several experiments at the YerPhI synchrotron. In January
of 1998 the RCS experiment at JLab was approved and soon
after these calorimeter components were shipped from Yerevan
to JLab. This represented the YerPhi contribution to the exper-
iment, as the properties of the TF-1 lead-glass met the require-
ments of the experiment in terms of photon/electron detection
with reasonable energy and position resolution and radiation
hardness. The properties of TF-1 lead-glass [12, 13] are given
in Table 1.
Table 1: Important properties of TF-1 lead-glass.
Density 3.86 gcm−3
Refractive Index 1.65
Radiation Length 2.5 cm
Moliére Radius 3.50 cm
Critical Energy 15 MeV
All PMTs had to pass a performance test with the follow-
ing selection criteria: a dark current less than 30 nA, a gain of
106 with stable operation over the course of the experiment (2
months), a linear dependence of the PMT response (within 2 %)
on an incident optical pulse of 300 to 30000 photons. 704 PMTs
out of the 900 available were selected as a result of these per-
formance tests. Furthermore, the dimensional tolerances were
checked for all lead-glass blocks, with strict requirements de-
manded on the length (400±2 mm) and transverse dimensions
(40±0.2 mm).
2.2.1. Design of the Counter
In designing the individual counters for the RCS calorime-
ter, much attention was paid to reliability, simplicity and the
possibility to quickly replace a PMT and/or HV divider. The
individual counter design is shown in Fig. 7. A titanium flange
is glued to one end of the lead-glass block by means of EPOXY-
190. Titanium was selected because its thermal expansion coef-
ficient is very close to that of the lead glass. The PMT housing,
which is bolted to the Ti flange, is made of an anodized Al
flange and an Al tube. The housing contains the PMT and a µ-
metal shield, the HV divider, a spring, a smaller Al tube which
transfers a force from the spring to the PMT, and a ring-shaped
spring holder. The optical contact between the PMT and the
lead-glass block is achieved by use of optical grease, type BC-
630 (Bicron), which was found to increase the amount of light
detected by the PMT by 30-40% compared to the case without
grease. The PMT is pressed to the lead-glass block by means
of a spring, which pushes the HV base with a force of 0.5-1 lbs.
Such a large force is essential for the stability of the optical
contact over time at the elevated temperature of the PMTs. The
glue-joint between the lead glass and the Ti flange, which holds
that force, failed after several months in a significant fraction
(up to 5%) of the counters. An alternative scheme of force
compensation was realized in which the force was applied to
the PMT housing from the external bars placed horizontally be-
tween the PMT housing and the patch-panel assembly. Each
individual lead-glass block was wrapped in aluminized Mylar
film and black Tedlar (a polyvinyl fluoride film from DuPont)
for optimal light collection and inter-block isolation. Single-
side aluminized Mylar film was used with the Al layer on the
opposite side of the glass. Such an orientation of the film limits
the diffusion of Al atoms into the glass and the non-oxidized
surface of aluminum, which is protected by Mylar, provides a
better reflectivity. The wrapping covers the side surface of the
lead-glass block, leaving the front face open for the gain mon-
itoring. The signal and the HV cables are each one foot long.
They are soldered to the HV divider on one end and terminated
with Lemo c©00 and circular plastic connectors (cable mount re-
ceptacle from Hypertronics) on the other end. The cables leave
the PMT housing through the open center of the spring holder.
2.2.2. HV Divider
At the full luminosity of the RCS experiment (0.5 ×
1039 cm2/s) and at a distance of 6 m from the target the back-
ground energy load per lead-glass block reaches a level of
108 MeVee (electron equivalent) per second, which was found
from the average value of anode current in the PMTs and the
shift of the ADC pedestals for a 150 ns gate width. At least
30% of this energy flux is due to high energy particles which
define the counting rate. The average energy of the signals for
that component, according to the observed rate distribution, is
in the range of 100-300 MeVee, depending on the beam en-
ergy and the detector angle. The corresponding charge in the
PMT pulse is around 5-15 pC collected in 10-20 ns. The elec-
tronic scheme and the selected scale of 1 MeV per ADC chan-
nel (50 fC) resulted in an average anode current of 5 µA due to
background load. A high-current HV base (1 mA) was there-
fore chosen to reduce the effect of the beam intensity variation
on the PMT amplitude and the corresponding energy resolu-
tion to the level of 1%. The scheme of the HV base is shown
in Fig. 8. According to the specification data for the FEU 84-
3 PMTs the maximum operation voltage is 1900 V. Therefore
a nominal voltage value of 1800 V and a current value in the
voltage divider of 1 mA were chosen.
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grease metal
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Figure 7: Schematic of the lead-glass module structure.
7.5ns
VME  Scalers
Fastbus  TDC
Fastbus  ADC
Fastbus  ADC
Fastbus  ADC
DAQ  Platform100m cable lines
Calorimeter Platform
LG  PMT  signals
600  central  channels
LG  PMT  signals
104  rim  channels
SUM−32  analog  signals
56  channels
SUM−32  logic signals
56  channels
LRS428F PS706D
LRS3412
PHOTON  TRIGGER
To Trigger Superviser2ns6ns
Sum−8
PS755
PS757
Figure 9: A block diagram of the calorimeter electronics.
R1 R2
R9 R10
D9 D10
R11 R12
Figure 8: Schematic of the high-voltage divider for the FEU 84-3 PMT. The
values of the resistors are R(1 − 10) = 100 kΩ, R11 = 130 kΩ,R12 =
150 kΩ, R13 = 200 kΩ, R14 = 150 kΩ, R15 = 10 kΩ,R16 = 10 MΩ, R17 =
4 kΩ. The capacitance C is 10 nF.
2.3. Electronics
The calorimeter electronics were distributed over two loca-
tions; see the block diagram in Fig. 9. The first group of
modules (front-end) is located in three racks mounted on the
calorimeter platform in close vicinity to the lead-glass blocks.
These are the trigger electronics modules which included a mix
of custom-built and commercially available NIM units:
• 38 custom-built analog summing modules used for level-
one signal summing 6;
• 14 linear fan-in/fan-out modules (LeCroy model 428F) for
a second-level signal summation;
• 4 discriminator units (Phillips Scientific model 706);
• a master OR circuit, realized with Phillips Scientific logic
units (four model 755 and one model 757 modules);
• several additional NIM modules used to provide auxiliary
trigger signals for the calorimeter calibration with cosmics
and for the PMT gain-monitoring system.
The second group of electronic modules, which include
charge and time digitizers as well as equipment for the Data
Acquisition, High Voltage supply and slow-control systems, is
6This module was designed by S. Sherman, Rutgers University.
placed behind a radiation-protecting concrete wall. All 704
lead-glass PMT signals and 56 SUM-32 signals are digitized by
LeCroy 1881M FastBus ADC modules. In addition, 56 SUM-
32 discriminator pulses are directed to scalers and to LeCroy
1877 FastBus TDCs. Further detailed information about the
electronics is presented in Section 5.
The signals between these locations are transmitted via
patch-panels and coaxial cables, consisting of a total number
of 1040 signal and 920 HV lines. The length of the signal ca-
bles is about 100 m, which serve as delay lines allowing the
timing of the signals at the ADC inputs to be properly set with
respect to the ADC gate, formed by the experiment trigger. The
width of the ADC gate (150 ns) was made much wider than
the duration of PMT pulse in order to accommodate the wider
pulses caused by propagation in the 500 ns delay RG-58 signal
cables. The cables are placed on a chain of bogies, which per-
mits the calorimeter platform to be moved in the experimental
hall without disconnecting the cables. This helped allow for a
quick change of kinematics.
2.3.1. Trigger Scheme
The fast on-line photon trigger is based on PMT signals from
the calorimeter counters. The principle of its operation is a sim-
ple constant-threshold method, in which a logic pulse is pro-
duced if the energy deposition in the calorimeter is above a
given magnitude. Since the Molière radius of the calorimeter
material is RM ≈ 3.5 cm, the transverse size of the electro-
magnetic shower in the calorimeter exceeds the size of a single
lead-glass block. This enables a good position sensitivity of
the device, while at the same time making it mandatory for the
trigger scheme to sum up signals from several adjacent coun-
ters to get a signal proportional to the energy deposited in the
calorimeter.
From an electronics point of view, the simplest realization of
such a trigger would be a summation of all blocks followed by
a single discriminator. However, such a design is inappropri-
ate for a high-luminosity experiment due to the very high back-
ground level. The opposing extreme approach would be to form
a summing signal for a small group including a single counter
hit and its 8 adjacent counters, thus forming a 3 × 3 block struc-
ture. This would have to be done for every lead-glass block, ex-
cept for those at the calorimeter’s edges, leading to an optimal
signal-to-background ratio, but an impractical 600 channels of
analog splitter→analog summer→discriminator circuitry fol-
lowed by a 600-input fan-in module. The trigger scheme that
was adopted and is shown in Fig. 10 is a trade-off between the
above extreme cases. This scheme contains two levels of ana-
log summation followed by appropriate discriminators and an
OR-circuit. It involved the following functions:
• the signals from each PMT in the 75 2×4 sub-arrays of ad-
jacent lead-glass blocks, excluding the outer-most blocks,
are summed in a custom-made analog summing module
to give a SUM-8 signal (this module duplicates the signals
from the PMTs with less then 1% integral nonlinearity);
• these signals, in turn, are further summed in overlapping
groups of four in LeCroy LRS428F NIM modules to pro-
duce 56 SUM-32 signals. Thus, each SUM-32 signal is
proportional to the energy deposition in a subsection of the
calorimeter of 4 blocks high and 8 blocks wide, i.e. 16×32
cm2. Although this amounts to only 5% of the calorime-
ter acceptance, for any photon hit (except for those at the
edges) there will be at least one segment which contains
the whole electromagnetic shower.
• the SUM-32 signals are sent to constant-threshold discrim-
inators, from which the logical pulses are OR-ed to form
the photon singles trigger T1 (see Section 5). The discrim-
inator threshold is remotely adjustable, and was typically
set to around half of the RCS photon energy for a given
kinematic setting.
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141312
56 57 58 59 60
73 75747271
66 67
07 08 09
01 0403
S06 S07 S08
S50 S51
S11S09 S10
S03S01
Figure 10: The principle of two-level summation of signals for the hardware
trigger: 75 eight-block sub-arrays and 56 overlapping groups of four sub-arrays
forming SUM-32 signals labeled as S01-S56. In the highlighted example the
sums 02,03,07, and 08 form a S02 signal.
2.4. Gain Monitoring System
The detector is equipped with a system that distributes light
pulses to each calorimeter module. The main purpose of this
system is to provide a quick way to check the detector opera-
tion and to calibrate the dependence of the signal amplitudes on
the applied HV. The detector response to photons of a given en-
ergy may drift with time, due to drifts in the PMT gains and to
changes in the glass transparency caused by radiation damage.
For this reason, the gain monitoring system also allowed mea-
surements of the relative gains of all detector channels during
the experiment. In designing the gain-monitoring system ideas
developed for a large lead-glass calorimeter at BNL[14] were
used.
The system includes two components: a stable light source
and a system to distribute the light to all calorimeter modules.
The light source consists of an LN300 nitrogen laser7, which
provides 5 ns long, 300 µJ ultraviolet light pulses of 337 nm
7 Manufactured by Laser Photonics, Inc, FL 32826, USA.
wavelength. The light pulse coming out of the laser is atten-
uated, typically by two orders of magnitude, and monitored
using a silicon photo-diode S1226-18BQ8 mounted at 150◦ to
the laser beam. The light passes through an optical filter, sev-
eral of which of varying densities are mounted on a remotely
controlled wheel with lenses, before arriving at a wavelength
shifter. The wavelength shifter used is a 1 inch diameter semi-
spherical piece of plastic scintillator, in which the ultraviolet
light is fully absorbed and converted to a blue (∼ 425 nm) light
pulse, radiated isotropically. Surrounding the scintillator about
40 plastic fibers (2 mm thick and 4 m long) are arranged, in or-
der to transport the light to the sides of a Lucite plate. This plate
is mounted adjacent to the front face of the lead-glass calorime-
ter and covers its full aperture (see Fig.11). The light passes
through the length of the plate, causing it to glow due to light
scattering in the Lucite. Finally, in order to eliminate the cross-
talk between adjacent counters a mask is inserted between the
Lucite plate and the detector face. This mask, which reduces
the cross-talk by at least a factor of 100, is built of 12.7 mm
thick black plastic and contains a 2 cm × 2 cm hole in front of
each module.
Light  distributor
Optic fibers
Lead−glass modules
Light  source
Figure 11: Schematic of the Gain-monitoring system.
Such a system was found to provide a rather uniform light
collection for all modules, and proved useful for detector test-
ing and tuning, as well as for troubleshooting during the exper-
iment. However, it was found that monitoring over extended
periods of time proved to be less informative than first thought.
The reason for this is due to the fact that the main radiation dam-
age to the lead-glass blocks occurred at a depth of about 2-4 cm
8Manufactured by Hamamatsu Photonics, Hamamatsu, Japan.
from the front face. The monitoring light passes through the
damaged area, while an electromagnetic shower has its maxi-
mum at a depth of about 10 cm. Therefore, as a result of this
radiation damage the magnitude of the monitoring signals drops
relatively quicker than the real signals. Consequently, the re-
sulting change in light-output during the experiment was char-
acterized primarily through online analysis of dedicated elastic
e-p scattering runs. This data was then used for periodic re-
calibration of the individual calorimeter gains.
3. Veto Hodoscopes
In order to ensure clean identification of the scattered pho-
tons through rejection of high-energy electrons in the compli-
cated environment created by the mixed electron-photon beam,
a veto detector which utilizes UVT Lucite as a Čerenkov radi-
ator was developed. This veto detector proved particularly use-
ful for low luminosity runs, where its use made it possible to
take data without relying on the deflection magnet (see Fig. 1).
The veto detector consists of two separate hodoscopes located
in front of the calorimeter’s gain monitoring system. The first
hodoscope has 80 counters oriented vertically, while the second
has 110 counters oriented horizontally as shown in Fig. 12. The
segmentation scheme for the veto detector was chosen so that
it was consistent with the position resolution of the lead-glass
calorimeter. An effective dead time of an individual counter is
about 100 ns due to combined double-pulse resolution of the
PMT, the front-end electronics, the TDC, and the ADC gate-
width.
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PMT XP2971
Honeycomb plate
Lucite radiator
Figure 12: Cut-off view of the “horizontal” veto hodoscope.
Each counter is made of a UVT Lucite bar with a PMT glued
directly to one of its end, which can be seen in Fig. 13. The Lu-
cite bar of 2×2 cm2 cross section was glued to a XP2971 PMT
and wrapped in aluminized Mylar and black Tedlar. Counters
are mounted on a light honeycomb plate via an alignment
groove and fixed by tape. The counters are staggered in such
a way so as to allow for the PMTs and the counters to overlap.
The average PMT pulse generated by a high-energy electron
corresponds to 20 photo-electrons. An amplifier, powered by
the HV line current, was added to the standard HV divider, in
order that the PMT gain could be reduced by a factor of 10
XP2971
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Figure 13: Schematic of the veto counter.
[15, 16]. After gain-matching by using cosmic ray data a good
rate uniformity was achieved, as can be seen in the experimental
rate distribution of the counters shown in Fig. 14. The regular
variation in this distribution reflects the shielding effect result-
ing from the staggered arrangement of the counters. A signif-
0 25 50 75 100 125
0 20 40 60 80
Horizontal Veto Detector
Vertical Veto Detector
Counter  number
Figure 14: The counting rate in the veto counters observed at luminosity of
1.5 · 1038 cm−2/s.
icant reduction of the rate (by a factor of 5) was achieved by
adding a 2 inch polyethylene plate in front of the hodoscopes.
Such a reduction as a result of this additional shielding is con-
sistent with the observed variation of the rate (see Fig. 14) and
indicates that the typical energy of the dominant background
is around a few MeV. The veto plane efficiency measured for
different beam intensities is shown in Table 2. It drops signif-
icantly at high rate due to electronic dead-time, which limited
the beam intensity to 3-5 µA in data-taking runs with the veto.
An analysis of the experimental data with and without veto de-
tectors showed that the deflection of the electrons by the magnet
Table 2: The efficiency of the veto hodoscopes and the rate of a single counter
at different beam currents. The detector was installed at 30◦ with respect to the
beam at a distance 13 m from the target. The radiator had been removed from
the beam path, the deflection magnet was off and the 2 inch thick polyethylene
protection plate was installed.
Run Beam Rate of the Efficiency Efficiency
current counter V12 horizontal vertical
[µA] [MHz] hodoscope hodoscope
1811 2.5 0.5 96.5% 96.8%
1813 5.0 1.0 95.9% 95.0%
1814 7.5 1.5 95.0% 94.0%
1815 10. 1.9 94.4% 93.0%
1816 14. 2.5 93.4% 91.0%
1817 19 3.2 92.2% 89.3%
provided a sufficiently clean photon event sample. As a result
the veto hodoscopes were switched off during most high lu-
minosity data-taking runs, although they proved important in
analysis of low luminosity runs and in understanding various
aspects of the experiment.
4. High Voltage System
Each PMT high-voltage supply was individually monitored
and controlled by the High Voltage System (HVS). The HVS
consists of six power supply crates of LeCroy type 1458 with
high-voltage modules of type 1461N, a cable system, and a set
of software programs. The latter allows to control, monitor,
download and save the high-voltage settings and is described
below in more detail. Automatic HV monitoring provides an
alarm feature with a verbal announcement and a flashing signal
on the terminal. The controls are implemented over an Ethernet
network using TCP/IP protocol. A Graphical User Interface
(GUI) running on a Linux PC provides access to all features
of the LeCroy system, loading the settings and saving them in
a file. A sample distribution of the HV settings is shown in
Fig. 15.
The connections between the outputs of the high-voltage
modules and the PMT dividers were arranged using 100 m long
multi-wire cables. The transition from the individual HV sup-
ply outputs to a multi-wire cable and back to the individual
PMT was arranged via high-voltage distribution boxes that are
located inside the DAQ area and front-end patch panels outside
the PMT box. These boxes have input connectors for individual
channels on one side and two high-voltage multi-pin connec-
tors (27 pins from FISCHER part number D107 A051-27) on
the other. High-voltage distribution boxes were mounted on the
side of the calorimeter stand and on the electronics rack.
5. Data Acquisition System
Since the calorimeter was intended to be used in Hall A
at JLab together with the standard Hall A detector devices,
the Data Acquisition System of the calorimeter is part of the
standard Hall A DAQ system. The latter uses CODA (CE-
BAF On-line Data Acquisition system) [17] developed by the
-1800 -1700 -1600 -1500 -1400 -1300 -1200 -1100 -1000
At the end of the experiment
High Voltage [V]
-1800 -1700 -1600 -1500 -1400 -1300 -1200 -1100 -1000
At the start of the experiment
Figure 15: The HV settings for the calorimeter PMTs.
JLab data-acquisition group. The calorimeter DAQ includes
one Fastbus crate with a single-board VME computer installed
using a VME-Fastbus interface and a trigger supervisor mod-
ule [18], which synchronizes the read-out of all the information
in a given event. The most important software components are
a Read-Out Controller (ROC), which runs on the VME com-
puter under the VxWorks OS, and an Event Builder and Event
Recorder which both run on a Linux workstation. For a detailed
description of the design and operation of the Hall A DAQ sys-
tem see [8] and references therein.
All 704 PMT signals and 56 SUM-32 signals are digitized by
LeCroy 1881M FastBus ADC modules. The 56 SUM-32 dis-
criminator pulses are also read-out by scalers and LeCroy 1877
FastBus TDCs. During the RCS experiment the calorimeter
was operating in conjunction with one of the High Resolution
Spectrometers (HRS), which belong to the standard Hall A de-
tector equipment [8]. The Hall A Data Acquisition System is
able to accumulate data involving several event types simulta-
neously. In the RCS experiment there were 8 types of trigger
signals and corresponding event types. Trigger signals from the
HRS are generated by three scintillator planes: S0, S1 and S2
(see Fig. 8 in [8]). In the standard configuration the main sin-
gle arm trigger in the spectrometer is formed by a coincidence
of signals from S1 and S2. An alternative trigger, logically de-
scribed by (S0 AND S1) OR (S0 AND S2), is used to measure
the trigger efficiency. In the RCS experiment one more proton
arm trigger was used, defined as being a single hit in the S0
plane. As this is the fastest signal produced in the proton arm,
it was better suited to form a fast coincidence trigger with the
photon calorimeter.
The logic of the Photon Arm singles trigger was described in
detail in Section 2.3. Besides this singles trigger there are two
auxiliary triggers that serve to monitor the calorimeter blocks
and electronics. The first is a photon arm cosmics trigger, which
was defined by a coincidence between signals from two plas-
tic scintillator paddles, placed on top and under the bottom of
photo tube
Laser calibration  trigger
Cosmics  trigger
top  paddle
bottom  paddle
1024 Hz  Pulser
Retiming
Retiming
ADC  gates
TDC Common Stop
Proton  Arm
ADC  gates
TDC Common Stop
Photon  Arm
Proton  Arm
Main  photon  trigger
Photon  Arm
Figure 16: Schematic diagram of the DAQ trigger logic.
the calorimeter. The other trigger is the light-calibration (laser)
trigger which was used for gain monitoring purposes.
The two-arm coincidence trigger is formed by a time overlap
of the main calorimeter trigger and the signal from the S0 scin-
tillator plane in the HRS. The width of the proton trigger pulse
is set to 100 ns, while the photon trigger pulse, which is delayed
in a programmable delay line, is set to 10 ns. As a result, the co-
incidence events are synchronized with the photon trigger, and
a correct timing relation between trigger signals from two arms
is maintained for all 25 kinematic configurations of the RCS
experiment. Finally, a 1024 Hz puls generator signal forms a
pulser trigger, which was used to measure the dead time of the
electronics.
All 8 trigger signals are sent to the Trigger Supervisor mod-
ule which starts the DAQ readout. Most inputs of the Trigger
Supervisor can be individually pre-scaled. Triggers which are
accepted by the DAQ are then re-timed with the scintillators of
a corresponding arm to make gates for ADCs and TDCs. This
re-timing removes trigger time jitter and ensures the timing is
independent of the trigger type. Table 3 includes information
on the trigger and event types used in the RCS experiment and
shows typical pre-scale factors used during the data-taking. A
schematic diagram of the overall RCS experiment DAQ trigger
logic is shown in Fig.16.
6. Calorimeter Performance
The calorimeter used in the RCS experiment had three related
purposes. The first purpose is to provide a coincidence trigger
Table 3: A list of triggers used in the RCS experiment. Typical pre-scale factors
which were set during a data-taking run (run #1819) are shown.
Trigger Trigger Description pre-scale
ID factor
T1 Photon arm singles trigger 100,000
T2 Photon arm cosmics trigger 100,000
T3 Main Proton arm trigger:
(S1 AND S2)
T4 Additional Proton arm trigger:
(S0 AND S1) OR (S0 AND S2)
T5 Coincidence trigger 1
T6 Calorimeter light-calibration
trigger
T7 Signal from the HRS S0 scintil-
lator plane
65,000
T8 1024 Hz pulser trigger 1,024
Coincidence Time (ns)
10 20 30 40
Figure 17: The time of the calorimeter trigger relative to the recoil proton trig-
ger for a production run in kinematic 3E at maximum luminosity (detected
Eγ = 1.31 GeV). The solid curve shows all events, while the dashed curve
shows events with a cut on energy in the most energetic cluster > 1.0 GeV.
signal for operation of the DAQ. Fig. 17 shows the coincidence
time distribution, where one can see a clear relation between en-
ergy threshold and time resolution. The observed resolution of
around 8 ns (FWHM) was sufficient to identify cleanly coinci-
dence events over the background, which meant that no off-line
corrections were applied for variation of the average time of in-
dividual S UM − 32 summing modules. The second purpose
is determination of the energy of the scattered photon/electron
to within an accuracy of a few percent, while the third is rea-
sonably accurate reconstruction of the photon/electron hit co-
ordinates in order that kinematic correlation cuts between the
scattered photon/electron and the recoil proton can be made.
The off-line analysis procedure and the observed position and
energy resolutions are presented and discussed in the following
two sections.
6.1. Shower Reconstruction Analysis and Position Resolution
The off-line shower reconstruction involves a search for clus-
ters and can be characterized by the following definitions:
1. a cluster is a group of adjacent blocks;
2. a cluster occupies 9 (3 × 3) blocks of the calorimeter;
3. the distribution of the shower energy deposition over the
cluster blocks (the so-called shower profile) satisfies the
following conditions:
(a) the maximum energy deposition is in the central
block;
(b) the energy deposition in the corner blocks is less than
that in each of two neighboring blocks;
(c) around 50% of the total shower energy must be de-
posited in the central row (and column) of the cluster.
For an example in which the shower center is in the middle
of the central block, around 84% of the total shower energy
is in the central block, about 14% is in the four neighboring
blocks, and the remaining 2% is in the corner blocks. Even at
the largest luminosity used in the RCS experiment the proba-
bility of observing two clusters with energies above 50% of the
elastic value was less than 10%, so for the 704 block hodoscope
a two-cluster overlap was very unlikely.
The shower energy reconstruction requires both hardware
and software calibration of the calorimeter channels. On the
hardware side, equalization of the counter gains was initially
done with cosmic muons, which produce 20 MeV energy equiv-
alent light output per 4 cm path (muon trajectories perpendic-
ular to the long axis of the lead-glass blocks). The calibration
was done by selecting cosmic events for which the signals in
both counters above and below a given counter were large. The
final adjustment of each counter’s gain was done by using cali-
bration with elastic e-p events. This calibration provided PMT
gain values which were on average different from the initial cos-
mic set by 20%
The purpose of the software calibration is to define the co-
efficients for transformation of the ADC amplitudes to energy
deposition for each calorimeter module. These calibration co-
efficients are obtained from elastic e-p data by minimizing the
function:
Ci · (A
i − Pi) − E
where:
n = 1 ÷ N — number of the selected calibration event;
i — number of the block, included in the cluster;
Mn — set of the blocks’ numbers, in the cluster;
Ani — amplitude into the i-th block;
Pi — pedestal of the i-th block;
Ene — known energy of electron;
Ci — calibration coefficients, which need to be fitted.
The scattered electron energy Ene is calculated by using the en-
ergy of the primary electron beam and the scattered electron
angle. A cut on the proton momentum-angle correlation is used
to select clean elastic events.
Following calculation of the calibration coefficients, the total
energy deposition E, as well as the X and Y coordinates of the
shower center of gravity are calculated by the formulae:
Ei , X =
Ei · Xi/E , Y =
Ei · Yi/E
where M is the set of blocks numbers which make up the clus-
ter, Ei is the energy deposition in the i-th block, and Xi and Yi
are the coordinates of the i-th block center. The coordinates cal-
culated by this simple center of gravity method are then used for
a more accurate determination of the incident hit position. This
second iteration was developed during the second test run [7],
in which a two-layer MWPC was constructed and positioned
directly in front of the calorimeter. This chamber had 128 sen-
sitive wires in both X and Y directions, with a wire spacing of
2 mm and a position resolution of 1 mm. In this more refined
procedure, the coordinate xo of the shower center of gravity in-
side the cell (relative to the cell’s low boundary) is used. An
estimate of the coordinate xe can be determined from a polyno-
mial in this coordinate (P(xo)):
xe = P(xo) = a1 · xo + a3 · x
o + a5 · x
o + a7 · x
o + a9 · x
For symmetry reasons, only odd degrees of the polynomial are
used. The coefficients an are calculated by minimizing the func-
tional:
P(an, x
o) − x
where:
i = 1 ÷ N — number of event;
xio — coordinate of the shower center of gravity inside the
cell;
xit — coordinate of the track (MWPC) on the calorimeter
plane;
an — coordinate transformation coefficients to be fitted.
The resulting resolution obtained from such a fitting proce-
dure was found to be around 5.5 mm for a scattered electron
energy of 2.3 GeV. For the case of production data, where the
MWPC was not used, Fig. 18 shows a scatter plot of events
on the front face of the calorimeter. The parameter plotted
is the differences between the observed hit coordinates in the
calorimeter and the coordinates calculated from the proton pa-
rameters and an assumed two-body kinematic correlation. The
dominant contribution to the widths of the RCS and e-p peaks
that can be seen in this figure is from the angular resolution of
the detected proton, which is itself dominated by multiple scat-
tering. As the calorimeter distance varied during the experiment
between 5.5 m and 20 m, the contribution to the combined an-
gular resolution from the calorimeter position resolution of a
few millimeters was minimal.
6.2. Trigger Rate and Energy Resolution
At high luminosity, when a reduction of the accidental co-
incidences in the raw trigger rate is very important, the trigger
threshold should be set as close to the signal amplitude for elas-
tic RCS photons as practical. However, the actual value of the
threshold for an individual event has a significant uncertainty
due to pile-up of the low-amplitude signals, fluctuations of the
signal shape (mainly due to summing of the signals from the
PMTs with different HV and transit time), and inequality of the
x (cm)δ
y (cm)
’p)γ,γp(p(e,e’p)
p)0π,γp(
Figure 18: The scatter plot of p − γ(e) events in the plane of the calorimeter
front face.
gain in the individual counters. Too high a threshold, therefore,
can lead to a loss in detection efficiency.
The counting rate of the calorimeter trigger, f , which defines
a practical level of operational luminosity has an exponential
dependence on the threshold, as can be seen in Fig. 19. It can
be described by a function of Ethr:
f = A × exp(−B × Ethr/Emax),
where Emax is the maximum energy of an elastically scat-
tered photon/electron for a given scattering angle, A an angle-
dependent constant, and B a universal constant ≈ 9±1. The an-
0.3 0.6 0.9 1.2 1.5 1.8 2.1
40200 60 80 100 120 140
elastic
Threshold  [GeV]
Threshold  [mV]
Beam energy:
Beam current:              
Target:                           
Calorimeter angle:
Calorimeter to target:
Solid angle:
10 µ A
3.3 GeV
Figure 19: Calorimeter trigger rate vs threshold level.
gular variation of the constant A, after normalization to a fixed
luminosity and the calorimeter solid angle, is less than a factor
of 2 for the RCS kinematics. The threshold for all kinemat-
ics was chosen to be around half of the elastic energy, thereby
balancing the need for a low trigger rate without affecting the
detection efficiency.
In order to ensure proper operation and to monitor the perfor-
mance of each counter the widths of the ADC pedestals were
used (see Fig. 20). One can see that these widths vary slightly
with block number, which reflects the position of the block in
the calorimeter and its angle with respect to the beam direction.
This pedestal width also allows for an estimate of the contri-
bution of the background induced base-line fluctuations to the
overall energy resolution. For the example shown in Fig. 20 the
width of 6 MeV per block leads to energy spectrum noise of
about 20 MeV because a 9-block cluster is used in the off-line
analysis.
0 100 200 300 400 500 600 700
LG block number
Figure 20: The width of the ADC pedestals for the calorimeter in a typical run.
The observed reduction of the width vs the block number reflects the lower
background at larger detector angle with respect to the beam direction.
The energy resolution of the calorimeter was measured by
using elastic e-p scattering. Such data were collected many
times during the experiment for kinematic checks and calorime-
ter gain calibration. Table 4 presents the observed resolution
and the corresponding ADC pedestal widths over the course of
the experiment. For completeness, the pedestal widths for cos-
mic and production data are also included. At high luminosity
the energy resolution degrades due to fluctuations of the base
line (pedestal width) and the inclusion of more accidental hits
during the ADC gate period. However, for the 9-block cluster
size used in the data analysis the contribution of the base line
fluctuations to the energy resolution is just 1-2%. The measured
widths of ADC pedestals confirmed the results of Monte Carlo
simulations and test runs that the radiation background is three
times higher with the 6% Cu radiator upstream of the target than
without it.
The resolution obtained from e-p calibration runs was cor-
rected for the drift of the gains so it could be attributed directly
to the effect of lead glass radiation damage. It degraded over
the course of the experiment from 5.5% (for a 1 GeV photon
energy) at the start to larger than 10% by the end. It was es-
timated that this corresponds to a final accumulated radiation
dose of about 3-10 kRad, which is in agreement with the known
level of radiation hardness of the TF-1 lead glass [19]. This
observed radiation dose corresponds to a 500 hour experiment
with a 15 cm LH2 target and 50 µA beam.
6.3. Annealing of the radiation damage
The front face of the calorimeter during the experiment was
protected by plastic material with an effective thickness of
10 g/cm2. For the majority of the time the calorimeter was lo-
cated at a distance of 5-8 m and an angle of 40-50◦ with respect
to the electron beam direction. The transparency of 20 lead-
glass blocks was measured after the experiment, the results of
which are shown in Fig. 21. This plot shows the relative trans-
mission through 4 cm of glass in the direction transverse to
the block length at different locations. The values were nor-
malized to the transmission through similar lead-glass blocks
which were not used in the experiment. The transmission mea-
surement was done with a blue LED (λmax of 430 nm) and a
Hamamatsu photo-diode (1226-44).
0 10 20 30 40
Distance from the calorimeter face [cm]
Figure 21: The blue light attenuation in 4 cm of lead-glass vs distance from
the front face of calorimeter measured before (solid) and after (dashed) UV
irradiation.
A UV technique was developed and used in order to cure ra-
diation damage. The UV light was produced by a 10 kW total
power 55-inch long lamp9, which was installed vertically at a
distance of 45 inches from the calorimeter face and a quartz
plate (C55QUARTZ) was used as an infrared filter. The inten-
sity of the UV light at the face of the lead-glass blocks was
found to be 75 mW/cm2 by using a UVX digital radiometer10.
In situ UV irradiation without disassembly of the lead-glass
stack was performed over an 18 hour period. All PMTs were
removed before irradiation to ensure the safety of the photo-
cathode. The resultant improvement in transparency can be
seen in Fig. 21. An alternative but equally effective method
to restore the lead-glass transparency, which involved heating
of the lead-glass blocks to 250◦C for several hours, was also
9Type A94551FCB manufactured by American Ultraviolet, Lebanon, IN
46052, USA
10 Manufactured by UVP, Inc., Upland, CA 91786, USA
Table 4: Pedestal widths and calorimeter energy resolution at different stages of the RCS experiment for cosmic (c), electron (e) and production (γ) runs in order of increasing effective luminosity.
Runs L e f f Beam Current Accumulated Detected Ee/γ σE /E σE /E at Eγ=1 GeV Θcal σped
(1038 cm−2/s) (µA) Beam Charge (C) (GeV) (%) (%) (degrees) (MeV)
1517 (c) - - - - - - - 1.5
1811 (e) 0.1 2.5 2.4 2.78 4.2 7.0 30 1.7
1488 (e) 0.2 5 0.5 1.32 4.9 5.5 46 1.75
2125 (e) 1.0 25 6.6 2.83 4.9 8.2 34 2.6
2593 (e) 1.5 38 14.9 1.32 9.9 11.3 57 2.0
1930 (e) 1.6 40 4.4 3.39 4.2 7.7 22 3.7
1938 (γ) 1.8 15 4.5 3.23 - - 22 4.1
2170 (γ) 2.4 20 6.8 2.72 - - 34 4.0
1852 (γ) 4.2 35 3.0 1.63 - - 50 5.0
tested. The net effect of heating on the transparency of the lead-
glass was similar to the UV curing results.
In summary, operation of the calorimeter at high luminos-
ity, particularly when the electron beam was incident on the
bremsstrahlung radiator, led to a degradation in energy resolu-
tion due to fluctuations in the base-line and a higher accidental
rate within the ADC gate period. For typical clusters this effect
was found to be around a percent or two. By far the largest con-
tributor to the observed degradation in resolution was radiation
damage sustained by the lead-glass blocks, which led to the res-
olution being a factor of two larger at the end of the experiment.
The resulting estimates of the total accumulated dose were con-
sistent with expectations for this type of lead-glass. Finally, it
was found that both UV curing and heating of the lead-glass
were successful in annealing this damage.
7. Summary
The design of a segmented electromagnetic calorimeter
which was used in the JLab RCS experiment has been de-
scribed. The performance of the calorimeter in an unprece-
dented high luminosity, high background environment has been
discussed. Good energy and position resolution enabled a suc-
cessful measurement of the RCS process over a wide range of
kinematics.
8. Acknowledgments
We acknowledge the RCS collaborators who helped to oper-
ate the detector and the JLab technical staff for providing out-
standing support, and specially D. Hayes, T. Hartlove, T. Hun-
yady, and S. Mayilyan for help in the construction of the lead-
glass modules. We appreciate S. Corneliussen’s careful reading
of the manuscript and his valuable suggestions. This work was
supported in part by the National Science Foundation in grants
for the University of Illinois University and by DOE contract
DE-AC05-84ER40150 under which the Southeastern Universi-
ties Research Association (SURA) operates the Thomas Jeffer-
son National Accelerator Facility for the United States Depart-
ment of Energy.
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	1 Introduction
	2 Calorimeter
	2.1 Calorimeter Design
	2.1.1 Air Cooling
	2.1.2 Cabling System
	2.2 Lead-Glass Counter
	2.2.1 Design of the Counter
	2.2.2 HV Divider
	2.3 Electronics
	2.3.1 Trigger Scheme
	2.4 Gain Monitoring System
	3 Veto Hodoscopes
	4 High Voltage System
	5 Data Acquisition System
	6 Calorimeter Performance
	6.1 Shower Reconstruction Analysis and Position Resolution
	6.2 Trigger Rate and Energy Resolution
	6.3 Annealing of the radiation damage
	7 Summary
	8 Acknowledgments
