Towards a Scintillator Based Digital Hadron
Calorimeter for the Linear Collider Detector
A. Dyshkant, Member, IEEE, D. Beznosko, G. Blazey, D. Chakraborty, K. Francis, D. Kubik,
J. G. Lima, M. I. Martin, J. McCormick, V. Rykalin, V. Zutshi

Abstract-- We report on the feasibility of a scintillator-based
(semi) digital hadron calorimeter for the proposed Linear Collider
Detector (LCD). A finely segmented, (semi) digital hadron
calorimeter, combined with energy flow algorithms, represents
one of the most promising approaches to attaining the
unprecedented jet energy resolutions required to fully exploit the
physics potential of a future Linear Collider. At the Northern
Illinois Center of Accelerator and Detector Development at
Northern Illinois University we have made a number of initial
studies of a scintillator-based (semi) digital tile calorimeter. These
studies include determination of optimum cell size, comparison of
light yield of cast and extruded scintillator cells, optimization of
cell processing and performance of candidate photo-detectors.
Initial results are encouraging and indicate that a scintillatorbased (semi) digital hadron calorimeter for the Linear Collider
Detector merits continued study.
I.
INTRODUCTION
ability to distinguish between physics signals that
THE
involve multi-jet final states will be crucial to the
success of a future Linear Collider Detector (LCD). This
translates into a requirement for unprecedented precision in jet
energy measurements. For instance, to effectively separate W
and Z bosons in their hadronic final states, based on a
reconstruction of their invariant masses, fractional jet energy
resolution  30% / E will be needed [1]. The socalled Energy-Flow Algorithms (EFA) have the best potential
of achieving such an ambitious goal and are now an integral
part of the general approach toward LCD design.
The basic premise of EFAs (also called Particle Flow
Algorithms) [2] is based on separating in a jet, energy
deposited by charged particles, from that left by neutrals. The
former is then substituted by the more precisely determined
momentum from the magnetized central tracker. A calorimeter,
optimized for EFAs, must therefore have fine lateral and
longitudinal segmentation to track individual charged particles
Manuscript received November 15, 2003. This work was supported in part
by the U.S. Department of Education under Grant No. P116Z010035, the
Department of Energy, and the State of Illinois Higher Education
Cooperation Act.
A. Dyshkant, D. Beznosko, G. Blazey, D. Chakraborty, K. Francis, D.
Kubik, G. Lima, M. I. Martin, J. McCormick, V. Rykalin, V. Zutshi are with
Northern Illinois University, DeKalb, IL 60115 USA (telephone: 815-7533504, email: dyshkant@fnal.gov).
in the calorimeter and separate the energy deposited by them
from the hits that belong to nearby photons and neutral
hadrons. Once this separation, or assignment, is achieved, the
tracker momentum is substituted for the calorimeter energy that
is associated with the charged tracks, and the electromagnetic
calorimeter is used to measure the photons. Each system
provides high precision energy measurements for the
respective components. The neutral hadrons that constitute on
average only 11% of a jet’s energy are then mainly measured
in the hadron calorimeter with the traditional 50% / E
resolution. Thus, in principle, a net jet energy resolution of
~ 30% / E could be achieved with a calorimeter that is
optimized for EFAs.
The large number of channels, required for fine lateral and
longitudinal segmentation, poses a significant challenge in the
form of complexity and cost of signal processing and data
acquisition. Reducing the dynamic range of the readout is a
potential solution. A (semi) digital (i.e. one or two-bit readout)
approach trades dynamic range to achieve finer spatial
resolution at an affordable price. Thus in the (semi) digital
paradigm one records whether a cell is or is not above a certain
threshold while in traditional analog calorimetry the total
energy deposited in the cells (usually 12-15 bit digitization) is
read out. In the following sections of this paper, we examine
the feasibility of a finely segmented scintillator-based hadron
calorimeter with one or two-bit readout.
II. HADRONIC ENERGY RESOLUTION
We have used a GEANT4 [3] simulation of the nominal
LCD configuration [4] to understand the behavior of a digital
hadron calorimeter and establish the feasibility of the digital
approach. The main detector elements, going out in radius
from the interaction point are:
a) Silicon tracker
b) 22.5 radiation lengths Si-W electromagnetic calorimeter
(15 cm deep, with 1 cm x 1 cm cells)
c) Steel-Scintillator sandwich hadron calorimeter (40 layers
deep with 2 cm steel and 0.5 cm scintillator in each layer)
d) 5T solenoid coil (tracker and calorimeter are immersed in
its field)
e) Iron-scintillator muon/tail-catcher system.
0
20
40
60
Number of Cells Above Threshold
800
800
4cm^2
6cm^2
600
600
9cm^2
12cm^2
16cm^2
2cm^2
400
400
200
200
0
0
0
20
40
Incident Energy (GeV)
60
Fig. 1. Mean number of cells above threshold (0.25 MIP) vs. incident energy
of single charged pions.
The feasibility of hit counting (see Fig. 3) as a measure of
hadron energy can be confirmed with a simple algorithm based
on assigning sampling weights to the calorimeter layers. The
sampling weights can be calculated by minimizing the
expression:
( E0   ai Li ) 2
where E0 is the incident energy
ai is the sampling weight for layer i
(1)
Li is the energy, sampled in ith layer, or the number of
cells above threshold for the analog and digital approaches
respectively.
Number of entries per 1000 events
0
50
100
150
1600
1600
1200
1200
800
800
400
400
0
0
0
50
100
150
Momentum (GeV/c)
Fig. 2. Momentum spectrum of hadrons inside jets produced in
e+e- collisions at s  500 GeV. The events were generated
using PYTHIA [5].
5
Number of Entries per Thousand Events
To study the single particle hadron resolution, we generated
charged pions of different energies at 90 to the beam axis and
random azimuthal angle. The steel-scintillator sandwich
geometry of the hadron calorimeter described previously was
run successively with different transverse segmentations for
charged pions in the 2-50 GeV range. The transverse
segmentation was achieved by dividing the scintillator into
square cells of a user specified size. Once specified all cells in
all layers of the hadron calorimeter had the same size (i.e. the
calorimeter had non-projective geometry). Thus simulations
were first run with all hadron calorimeter cells having the
dimensions 1.414 cm x 1.414 cm (2 cm2) then jobs were run
with cell dimensions of 2.45 cm x 2.45 cm (6 cm2) and so on.
An indication that the digital approach can work for hadrons
is evident in Fig. 1. Shown there is the mean number of cells
above threshold, which was chosen to be 0.25 times the energy
deposited by a minimum ionizing particle (MIP), as a function
of the incident energy. The different symbols correspond to
different cell sizes (in cm). The proportionality between the
number of cells and particle energy clearly indicates that
simply counting cells above threshold will give a reasonable
estimate of the incident energy. At higher energies, with
increasing cell size the relationship becomes non-linear. This is
not of fundamental concern because the spectrum of hadrons
inside a jet is peaked towards low momenta with a mean
around 9-10 GeV (Fig. 2).
35
65
95
125
155
185
250
250
200
200
150
150
100
100
50
50
0
0
5
35
65
95
125
155
185
Number of Cells
Fig 3. Total number of cells above threshold for a 1000-event sample of 5
GeV charged pions.
Fig. 4 shows the fractional energy resolution for single
charged pions as a function of their energy, using the analog
(only one set of points is shown as the answer is largely
independent of cell size) or digital approach for different cell
sizes. It can be seen that the ‘digital’ single particle resolution
compares favorably with the analog resolution, especially
below 25 GeV. For higher energies, the resolution in the cell
counting approach begins to degrade. Simulations indicate that
this degradation can be controlled by using multiple thresholds
(three thresholds, or two-bit readout for instance). For initial
prototyping studies we picked a cell size in the mid-range (~ 9
cm2), for which the resolution is reasonably well behaved, the
total number of channels is manageable, the dimension of the
cells is close to the effective Moliere radius of a steelscintillator sandwich and the 9 cm2 area does not pose
problems in bending fibers for embedding inside the
scintillator.
20
40
60
0.5
0.5
2cm^2 Digital
4cm^2 Digital
0.4
Counts
0
0
600
900
25
20
20
15
15
10
10
5
5
0.4
6cm^2 Digital
9cm^2 Digital
12cm^2 Digital
0.3
300
25
0.3
/E
16cm^2 Digital
0
Analog
0.2
0.2
0
0
300
600
900
ADC Channels
0.1
0.1
0
0
20
40
Incident Energy (GeV)
60
Fig. 4. Energy resolution as a function of the incident energy for single
charged pions. The different symbols represent different cell sizes.
III.
LIGHT YIELD
We tested the light response for our nominal 9 cm2 cells
(actual size of the fabricated cells was 9.4 cm2) by studying
their response to cosmic ray muons. The cells were made with
5mm thick Bicron [6] BC-408 scintillator. Kuraray [7] Y11,
0.94mm diameter wavelength shifting (WLS) fiber was
embedded and glued into the scintillator in a sigma-shaped
groove (see Fig. 5). The groove radius was 12mm, with a width
of 1mm and a depth of 4.5mm, angled to the exit. Both ends of
the fiber were polished using the fly-diamond cutting technique
[8]. The end of the fiber inside the scintillator was aluminum
mirrored [9], while the other end was spliced to a clear fiber. A
Hamamatsu [10] 16-channel multi-anode photo multiplier tube
(MAPMT) H6568 was used as the photo-detector, and its
output was digitized by a 32 channel ADC (CAEN V792),
sitting in a VME crate. A clear MIP signal, cleanly separated
from the pedestal, can be seen in Fig. 6 for a typical channel.
The light yield for the 14 cells studied was found to be between
10-12 photoelectrons.
Fig. 5. Schematics of square cells with straight (left) and sigma-shaped (right)
grooves.
A high-resolution detector requires consistent light yield for
a large number of cells. While we have not studied light yield
for a very large number of cells, we have made a systematic
effort to track the dispersion within a particular sample over
the hundreds of cells we have fabricated.
0.9
Number of Fibers
0
Fig. 6. ADC count distribution for a 9.4 cm 2 cell showing a clear MIP signal
separated from the pedestal.
1
1.1
1.2
1.3
40
40
30
30
20
20
10
10
0
0
0.9
1
1.1
1.2
1.3
Anode Current (microAmps)
Fig. 7. Response distribution for about 100 fibers.
For example, the dispersion obtained for a sample of about
100 fibers, is shown in Fig. 7. The fibers were cut, polished
and mirrored using the same procedure. One by one, they were
inserted in the same cell and their response to 90Sr source
measured with a picoammeter. Over the hundred fibers we see
dispersion of about 5%. This sets the lower limit in the
uniformity of response that can be expected. Our
measurements with the fully processed cells indicate that the
net dispersion amongst cells, taking into account all the
intermediate processing steps (fiber insertion, gluing,
wrapping, painting etc.), is less than 10% and, therefore, well
under control.
We have also studied the response of the 9.4 cm2 tiles for
different thicknesses. This will give us a good idea of the
allowed range of thickness of the scintillator within the final
calorimeter design. Fig. 8 shows the response of cells, relative
to the 3mm thick cell, with a 137Cs source. The thickness of the
scintillator can be reduced by (0.5-1.0) mm, if needed, and we
can still stay within 10-20% of the mentioned above light yield.
Responce Normalized To 3mm Thick Cell
2.5
3.5
4.5
5.5
1.75
1.75
1.5
1.5
1.25
1.25
1
1
0.75
0.75
2.5
3.5
4.5
5.5
TABLE II
NORMALIZED CELL RESPONSES FOR DIFFERENT COATINGS OR WRAPPINGS
Cell Thickness (mm)
Fig. 8. Response of cells with thicknesses of 3, 4, and 5 mm.
radioactive source was used.
137Cs
IV. CELL PROCESSING
2
Even with ~9 cm cells, a few million scintillator tiles will
be required. Processing such a large number represents a
formidable challenge. In the following, we present some results
from studies that allow the cell preparation to be simplified to a
large extent, thereby opening up the possibility of semiautomated tile treatment. A minimum of 8-10 cells was used
for each set of measurements.
TABLE I
NORMALIZED CELLS RESPONSES FOR DIFFERENT SURFACE TREATMENTS
Surface
treatment
Unpolished top Polished top
and polished
and polished
bottom
bottom
Unpolished
top and
unpolished
bottom
Response
0.98
1.02
1.00
A similar study was done on the effectiveness of various
reflective treatments available for tiles. Table II shows the
response of the cells with various treatments relative to Tyvek
wrapping. Each cell was first wrapped in Tyvek [12] and its
response measured. The wrapping was then removed and the
cells were subsequently wrapped in the other reflective
materials. Paint was applied last, using an airbrush to spray a
specific number of coats on the tile. After comparative studies
of paint available in the market (acrylic, enamel and vinyl
paints were tried out), a white acrylic paint with titanium
dioxide pigmentation from Liquitex, England, was chosen.
While Tyvek and VM2002 (3M [13] super-radiant mirror film)
give the best response, wrapping can be a daunting operation
for the number of tiles under consideration. Spray-painting,
however, is a convenient way to apply a reflective coating to
these cells, and it has the potential to be easily automated.
Furthermore, the response for painted cells is within 10% of
that obtained for cells wrapped in Tyvek. Continuous
observation of the painted cells over a six-month period has
shown no chipping or aging effects. We also intend to do
radiation damage testing in the near future.
Unless otherwise specified, these and the following
measurements were made with a 74 MBq (2mCi) 90Sr source.
A Keithley [11] picoammeter, connected to a Hamamatsu
R580 PMT, was used to make absolute current measurements.
Table I shows the response for 9.4 cm2 cells with polished and
unpolished top and bottom surfaces. Given that the accuracy
for these measurements was 2%, the light response for the
cells is independent of the surface treatment, thereby
eliminating costly and manpower intensive surface polishing.
Coating
Response
Tyvek
1.00
Paint
0.89
VM2002
1.08
Coating
Response
CM590a
0.28
CM500
0.44
Alum. Foil
0.63
Mylar
0.83
V. PHOTO-DETECTORS
Due to the presence of a 4-5T magnetic field in the proposed
LCD, conventional PMTs cannot be used unless the light is
transported far from the detector (~10m), which can degrade
the signal substantially. Consequently, we have been exploring
the use of solid-state photo-detectors like Avalanche
Photodiodes (APD) [14] and Silicon Photo Multipliers (SiPM) [15]. These devices operate at room temperature and have
complementary strengths. The APDs have high quantum
efficiency (70-80%) but low to moderate gains (see Fig.9),
while the Si-PMs have low quantum efficiencies (12-15%) but
high gains (~106). Fig. 10 shows a cosmic ray event, as
captured on a scope, using a 32-channel Hamamatsu APD
matrix (S8550). Thus the APDs have a reasonable signal-tonoise performance and, in addition, exhibit low intrinsic noise
and cross-talk. However, other factors, such as cost, high
sensitivity of the gain to temperature and voltage variations
make APDs less attractive at the moment, when compared to
Geiger-mode detectors like the Si-PMs.
a
CM590 and CM500 are color films from 3M.
100
1000.0
200
300
400
1
1000.0
for 587nm, yellow
151
301
451
601
751
901
1000
1000
750
750
500
500
250
250
565nm, green
100.0
486nm, blue
100.0
Gain
Counts
660nm red light
10.0
10.0
0
1.0
100
1.0
200
300
0
1
151
301
451
601
751
901
ADC Channels
400
Bias Voltage (V)
Fig. 11. Pulse height spectrum, obtained with a Si-PM (bias voltage = 51 V)
using Ru-106 source.
Fig. 9. Hamamatsu Si APD S8550 gain vs. bias voltage for different
wavelength of incident light.
Number of PEs
51
Fig. 11 shows the photoelectron spectrum, obtained with a
Si-PM, mounted on a scintillator exposed to a 106Ru source.
Since the Si-PM operates in Geiger-mode, an optimal working
voltage needs to be set for them.
52
53
54
16
16
12
12
8
8
4
4
0
0
51
52
53
54
Bias (V)
Fig. 12. Number of photoelectrons as a function of operating voltage for SiPM, from cosmic ray muons.
VI. COST OPTIMIZATION CONSIDERATIONS
Fig. 10. MIP signal from a cosmic ray event, obtained from a 5 mm thick
scintillating cell with APD S8550, bias voltage of 393 V. Signal amplitude
here is ~8 mV with noise of ~2 mV; signal width is ~100 nsec.
Fig. 12 shows the mean response for cosmic ray muons, in
number of photoelectrons, as a function of the operating
voltage. We obtain ten or more photoelectrons for cosmic ray
muons using Si-PMs. The high photoelectron yield indicates
that Si-PMs are very promising devices for use in a (semi)
digital hadron calorimeter.
Extruded scintillator is five to ten times cheaper than
commercially available cast scintillator. To date, extruded
scintillator has mostly been used in the form of long strips. We
are, however, investigating its potential for small tiles. An
extruder line run jointly by Fermi National Accelerator
Laboratory and Northern Illinois Center for Accelerator and
Detector Development (NICADD) [16] produced the extruded
scintillator used for our studies. Our studies have shown that
the tiles made from extruded material have ~70% of the light
yield obtained with cast BC-408. Given that we are
consistently getting more than 10 photoelectrons, extruded
scintillator, cut into tiles (see Fig. 13), can already be used to
instrument a (semi) digital hadron calorimeter. Since the
extruded scintillator light yield is dependent on dopant
concentration in the extruded material, we expect to improve
the light yield. The mixture we are currently using was
optimized for long strips, where the smaller attenuation length
of the extruded scintillator is the primary concern. The small
size of our cells implies that the attenuation length is not an
overriding issue for us, and studies are underway to optimize
the light yield.
solution to both of these problems. Other groups [18] have
suggested mounting the Si-PM (since it is only 1 mm x 1 mm)
directly onto the scintillator itself. Since the photo-detection
occurs at the tile itself, the signal can be brought out by
electrical cables instead of the much more fragile fibers. This
will significantly reduce the amount of fiber required and
eliminate the need for fiber routing. Additionally, the issues of
attenuation length associated with the wavelength shifting
fibers and the cumbersome process of splicing them to clear
fibers, is eliminated.
VII. CONCLUSIONS AND PLANS
Fig. 13. Different shapes and sizes of cells fabricated from extruded and cast
scintillator for studies.
While we have used a sigma groove for much of our studies,
we have also studied straight grooves as they can be easily
machined or extruded. We have found that a straight groove
has slightly higher non-uniformity across the tile [17] (this is
not a big issue for digital or semi-digital approach since all you
are looking for is whether a cell is above or below a specified
threshold). However, a straight groove offers obvious
construction advantages as it can be easily extruded. For
example, with the extruded strip, shown in Fig. 14, cells can be
automatically machined.
Initial studies indicate that a scintillator based (semi) digital
calorimeter represents an attractive alternative for the LCD.
Simulations indicate that simply counting cells above MIP
threshold will be sufficient for single particle resolution. This
is a prerequisite for EFAs needed to achieve jet resolutions
required for the LCD. Other studies, not discussed in this
paper, are now underway to determine the sensitivity of
complete EFAs. Nonetheless, the simulations have suggested
initial cell sizes for prototype studies.
These prototype studies show that both cast and extruded 3
cm x 3 cm scintillator cells have sufficient light yield and
uniformity for operation in a digital hadron calorimeter. Our
work also indicates that the cells can be manufactured with a
number of labor saving features conducive to automated
processing. These features include minimal surface
preparation, painted surfaces, straight fiber grooves and,
perhaps, surface mounted photo-detectors.
Our simulation and cell studies have led us to assemble a
hadron calorimeter stack for study with cosmic ray muons. The
stack has twelve layers, each instrumented with seven cells.
The cells are initially being readout with PMTs, but gradually
solid-state photo-detectors will be phased in. We will
characterize the cell and tower response over the next few
months in preparation for the construction of a prototype for
beam tests.
VIII. ACKNOWLEDGMENTS
Fig. 14. An extruded strip with 10 holes, the holes run along the length of the
strip.
For both cast and extruded scintillator, a straight groove also
allows one to choose smaller cells, if desired, since the
minimum bending radius for fiber is no longer an issue. The
light yield is not an issue given that the light yield for a 6 cm2
cell is 93% of a 9 cm2 cell.
Fibers present problems in terms of the cost and routing out
of the calorimeter. Interestingly, the Si-PMs offer a potential
The authors are thankful to Peter Torres and Daniel
Ruggiero for their help during the cell tests.
We would like to express our appreciation to B. Dolgoshein
and E. Popova, from Moscow Engineering and Physics
Institute, Moscow, Russia, for samples of Si-PMs.
Also, we would like to thank Phil Stone for providing
excellent mechanical shop support.
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