Abstract In space-based experiments, detector mass is a critical concern. To allow an acceptable geometric acceptance, calorimeters used in such situations must be thin, and will thus not fully contain hadron showers. We discuss the benefits of using a low-Z target in front of thin calorimeters to force nuclear interactions and improve resolution. We report simulation results comparing several target options and a &apos;monolithic&apos; configuration. The results demonstrate how calorimeter configuration, available mass, required energy resolution and &apos;good event&apos; definitions can affect the optimal division of mass between the target and the calorimeter. Finally we suggest a method to optimize design choices based on the parameters of the experiment

E S Seo

J Z Wang

O Ganel

CiteSeerX

OG.4.1.09 1
On the Use of Low-Z Targets in Space-Based Hadron
Calorimetry
O. Ganel, E.S. Seo, and J.Z. Wang
IPST, University of Maryland, College Park, MD 20742, USA
Abstract
In space-based experiments, detector mass is a critical concern. To allow an acceptable geometric acceptance,
calorimeters used in such situations must be thin, and will thus not fully contain hadron showers. We discuss
the benefits of using a low-Z target in front of thin calorimeters to force nuclear interactions and improve
resolution. We report simulation results comparing several target options and a ’monolithic’ configuration.
The results demonstrate how calorimeter configuration, available mass, required energy resolution and ’good
event’ definitions can affect the optimal division of mass between the target and the calorimeter. Finally we
suggest a method to optimize design choices based on the parameters of the experiment.
1 Introduction:
The driving concern in space-based high energy cosmic-ray experiments is collection power. The flux of
particles requires large-area detectors with high acceptance to collect a sufficiently large data sample within
an acceptable length of time. With a power-law spectrum falling roughly by a factor of 50 for every decade
in particle energy above 1 GeV, experiments with a geometric factor (GF) on the order of 1 m2 sr require
three years to collect 10 protons above 1015 eV. The only practical way to measure a proton’s energy above
about 1 TeV is through ionization calorimetry, but calorimeters deep enough for nearly full containment of
hadron showers are too massive to be incorporated into space-based experiments. A workable compromise
is provided by thin calorimeters, but several issues must be addressed, as detailed below. A new generation
of experiments, e.g., the balloon-borne Advanced Thin Ionization Calorimeter (ATIC) (Guzik et al., 1999),
the Advanced Cosmic-ray Composition Experiment for the Space Station (ACCESS) (Israel et al., 1999) and
the Cosmic Ray Energetics And Mass (CREAM) experiment (Seo et al., 1999), employing calorimeters with
depths less than 2.5 nuclear interaction lengths (int) are planned to fly during the coming decade. In this
paper we discuss the process of optimizing the configuration of such experiments, including the incorporation
of a target, choosing the target material, and dividing the stack thickness between target and calorimeter.
2 Thin Calorimetry:
A proton incident on a block of matter will interact inelastically after 1int on average, generating many
secondary particles, mostly+,   and0 in roughly equal proportions. The neutral pions decay almost
immediately into pairs of photons, initiating electromagnetic (EM) cascades. A calorimeter must force at least
one such interaction to provide an energy measurement. The energy of the resulting shower must then be
measured with sufficient accuracy. TheeffectiveGF is thus the GF for particles in the detector’s geometric
acceptance that also interact sufficiently early to allow their energy to be measured reliably. The energy
resolution of thin calorimeters, driven by leakage, is crude by the standards of ground-based devices, but
sufficient for cosmic ray spectral measurements. For the purpose of reconstructing the rapidly falling spectra of
various particles, it is more important to have energy independent resolution and response to hadron showers.
This is achievable with thin calorimeters (Ganel et al., 1999a).
3 Space-Based Hadronic Calorimetry:
A space-based hadronic calorimeter should be deep enough in bothint and radiation lengths (X0) to
generate interactions and contain EM showers produced by secondary neutral pions, be light enough to fly, and
yet retain an acceptable GF. At first glance, with limited mass one is tempted to use a monolithic device with
every gram exploited for the calorimeter. However, low-Z materials such as carbon generate more interactions
for the same mass (in g/cm2) compared to higher-Z materials, thereby providing improved resolution.
The mass limitation of space-based experiments implies that a configuration with a deep calorimeter and
a thick target would be extremely limited in lateral extent and thus in collection power. The ATIC, ACCESS
and CREAM baseline designs have proposed the use of 0.5 – 1.0int-thick low-Z (carbon) targets preceding
a 20 – 27X0 deep calorimeter as optimal.
4 Low-Z Targets and the Effective Geometry Factor:
Low-Z targets present a least-mass solution to the problem of forcing nuclear interactions. A 1int thick-
ness of either a low-Z material (e.g. carbon) or high-Z material typically used in calorimeters (e.g. Fe, Pb, W,
U, BGO, etc.) will provide similar interaction probabilities. However, the low-Z target will have a smallerint
value in g/cm2 and will thus require less mass. This leaves a greater mass allowance for the calorimeter itself,
allowing greater lateral size and increasing the effective GF. The lower target density increases the overall
stack thickness, but the impact on the effective GF is much smaller. Another advantage of low-Z materials is
the low ratio ofint toX0, which minimizes the dependence of calorimeter response on first-interaction depth.
4.1 Comparison of Low-Z Targets and a Monolithic Configuration: Balloon launch limits con-
strain payload masses to 1 – 2 tons, with various support systems taking up about a third of the mass. A custom-
written code was used to compare different target materials and a monolithic calorimeter option (calorimeter
only, with no target) for balloon payloads. The combined calorimeter and target mass (calorimeter only, for
the monolithic case) was set at 668 kg as the likely allowance to fit within the current 1000 kg mass limit of
NASA’s Ultra-Long Duration Balloon (ULDB) program. The calorimeter was assumed to be comprised of
alternating layers of 3.5 mm tungsten and 0.5 mm scintillators, but the results of this study should apply to any
high-Z calorimeter.
The target thickness was set at 0.5int, corresponding to 19.1 cm, 14.4 cm, and 20.4 cm for carbon,
alumina and beryllium, respectively. In the monolithic case one would be tempted to position the charge
detector immediately atop the calorimeter, thereby maximizing
the angular acceptance. Unfortunately, such a design would de-
grade data quality. Particles scattered back from the calorimeter
would overlay the signal from the primary in the charge detec-
tor, making charge identification uncertain for protons at high
energies (Ganel et al., 1999b). To avoid this, one must separate
the charge detector vertically from the tungsten. The charge de-
tector’s lateral extent will then define the experiment’s field of
view. It is possible to make the charge detector very large, or to
use a ’hat-like’ design with active panels extending downwards
to cover high-angle trajectories. This, however, would increase
the mean incident angle, thereby reducing data quality by in-
creasing atmospheric overburden (for balloon experiments), ac-
Figure 1: Schematic representing a space-
based calorimeter preceded by an inverted
trapezoidal target.
cepting more background events and degrading the tracking resolution. Such a design would also increase the
size, cost and complexity of the charge detector, complicate the calorimeter readout, and make data interpre-
tation much more difficult. To allow a fair comparison one must thus require comparable fields of view for the
monolithic case and for the low-Z target cases.
The opening angle (see Fig. 1) of all targets was optimized for highest effective GF (35). The same
angle was assumed for the monolithic case to allow similar data quality. The calorimeter depth was similarly
optimized. The calorimeter’s lateral dimensions were set to fit the overall mass limit based on the opening
angle and the target and calorimeter densities and thicknesses. Events were counted as good if the proton
entered through the top of the target, interacted in the target or in the top of the calorimeter at least 20X0
Table 1: Effective GF comparison for target options with target opening angles and calorimeter depth opti-
mized, and calorimeter width set to fit in mass allowance.
Configuration Calorimeter depth [cm] Calorimeter width [cm] Effective GF [m2sr]
Carbon target 8.0 57.4 0.32
Alumina target 8.0 55.5 0.31
Beryllium target 8.0 58.5 0.33
Monolithic calorimeter 14.4 52.2 0.26
above its bottom and exited through the bottom or side of the calorimeter. The trajectory and first-interaction
position were required to allow the shower development to have a vertical extent of at least 20X0 from the
first interaction to its point of exit from the calorimeter. This allows both sufficient containment of energy and
track reconstruction. In the monolithic case the top-of-target requirement was substituted with a requirement
that the proton pass through a charge detector positioned above a 19.1 cm air-gap, and defining the same 35
field of view as for the low-Z target cases.
From Table 1 we see that the three low-Z materials provide similar effective GF values. Of the three
however, carbon is the most common, easiest to machine, and least toxic. Comparing the case of a carbon
target to the monolithic case shows that the effective GF in the former case is about 25% higher. To achieve a
comparable effective GF with the monolithic design requires the charge detector to be positioned with no air-
gap, degrading the data quality and increasing cost and complexity, as described above. The conclusion from
this study for a 1000 kg balloon payload is that a carbon target provides an optimal solution for cosmic-ray
hadron calorimetry in terms of data quantity and quality, detector cost and complexity, and simplicity of data
analysis. This conclusion would similarly apply to space-based calorimeters with mass limits of several tons.
4.2 Optimizing Calorimeter Depth vs. Target Thickness: Once a target material is selected, the
target thickness can be optimized for greatest effective GF. Two trends compete in this process. On one hand,
with thicker targets the interaction fraction for particles in the geometrical acceptance increases. On the other
hand, the lateral extent possible with thicker targets is reduced, thereby limiting the geometrical acceptance.
The result of the optimization depends on the available mass, the definition of good events (determined by
the required energy resolution) and on the target opening angle.
To illustrate this dependence, the same custom-written code was
used to optimize the calorimeter depth vs. carbon target thickness
for a space-based BGO calorimeter. The total mass of calorime-
ter and target was set equal to the expected mass for the baseline
ACCESS calorimeter (2730 kg). The total stack thickness was
set to 2.4int. Simulations using GEANT/FLUKA 3.21 (Brun et
al., 1984; Arino et al., 1987) show that keeping this overall depth
keeps the energy resolution, a measure of data quality, between
32%1% and 34%1%. These overall (mass and thickness) pa-
rameters were kept constant to allow a fair comparison between
all configurations. Thus, for each opening angle, as the BGO
depth was varied from 30 cm to 50 cm, the carbon target thick-
ness was varied from 39.4 cm to 4.7 cm, respectively, to complete
the required 2.4int stack depth. The calorimeter lateral dimen-
sions were determined by the materials and depths to meet the
overall mass. Isotropically incident protons were simulated with
random positions of incidence, interacting in the target or the top
30 35
40
45 50
0
10
20
30
40
50
60
70
0.2
0.3
0.4
0.5
0.6
0.7
Figure 2: Effective GF as a function of
targ t opening angle () and BGO depth for
simulated protons interacting at least 30 cm
above the calorimeter bottom.
of the calorimeter (at least 30 cm above its bottom), and exiting through its bottom (see Fig. 1 for a schematic
of such a configuration). Figure 2 shows the effective GF for ranges of opening angles and BGO depths. As
can be seen, for the above parameters, the optimal values are a 30 opening angle and 30 cm BGO depth, or
a 35 opening angle and 35 cm BGO depth, giving an effective GF of 0.74 m2sr. This is almost 28% higher
than the best achievable with a 0 opening angle (0.58 m2sr). From Fig. 2 we see, however, that many com-
binations of BGO depth and target opening angle provide an effective GF close to the maximal value. For a
BGO depth of 30 cm, for example, the target opening angle can be set between 15 a d 45 while keeping the
effective GF above 0.67 m2sr. Thus, if other considerations (e.g., charge detector power or cost per channel)
constrain the design to targets with smaller opening angles, one could opt, for example, for a configuration
with a 30 cm deep BGO calorimeter preceded by a 39.4 cm thick carbon target with a 15 opening angle,
providing an effective GF of 0.69 m2sr. Configurations combining a thicker target (thinner calorimeter) and
large opening angles (far left corner in Fig. 2) suffer greatly reduced effective GF as a result of too much mass
being dedicated to the target. In such cases, the calorimeter width is so reduced that the effective GF shrinks.
For thinner targets this effect is more limited, and the optimal opening angle slowly increases as BGO depth
increases. If the required energy resolution allows, events can be accepted that interact deeper in the calorime-
ter. Simulations have shown that such a change increases the effective GF, shifting the optimal configuration
to slightly smaller opening angle and calorimeter depth, and somewhat thicker target. A significant increase
in payload mass will increase the effective GF and slightly increase the optimal target opening angle.
5 Discussion and Conclusions:
The optimal solution for a space-based hadron calorimeter is to employ a high-density calorimeter preceded
by a carbon target. The target increases the effective GF by 25% compared to a monolithic configuration, for
a comparable data quality. Shaping the target as an inverted truncated pyramid (Fig. 1) increases the effective
GF by 28% or more, compared to a box-like target with lateral dimensions matching those of the calorimeter.
The optimal configuration depends in a complicated way on the available mass and definition of acceptable
event quality and energy resolution. To correctly optimize the design one must first determine the allowed
mass and the data quality, i.e., energy resolution, required for the scientific goals of the experiment. Once
these are known, one can determine the total stack depth, maximal depth of first interaction, minimal shower
path-length, and number of tracking points required. Given these, the optimal design parameters and expected
collection power can be determined with a relatively simple Monte Carlo program. Other constraints (e.g.,
charge detector size) also play a role when choosing among configurations with similar collection power.
For the 2730 kg ACCESS baseline BGO calorimeter and carbon target combination with a required energy
resolution of 30%–35%, the optimal configuration is a 30 cm deep calorimeter preceded by a 39.4 cm thick
target with a 30 opening angle.
6 Acknowledgment:
This work was supported by NASA grant NAG5-5155.
References
Arino, P.A. et al. 1987, FLUKA User’s Guide, CERN, TIS-RP-190
Brun, R. et al. 1984, GEANT User’s Guide, CERN DD/EE/84-1
Ganel, O. et al. 1999a,26th ICRC , OG.4.6.01
Ganel, O. et al. 1999b, STAIF-99 Space Tech. & Appl. Int. Forum AIP 1-56396-846-0/99, 272
Guzik, T.G., et al. 1999,26th ICRC , OG.4.1.03
Israel, M.H., Streitmatter, R.E. and Swordy, S.P., STAIF-99 Space Tech. & Appl. Int. Forum AIP 1-56396-
846-0/99, 114
Seo, E.S. et al. 1999,26th ICRC , OG.1.2.30


On the Use of Low-Z Targets in Space-Based Hadron Calorimetry

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On the Use of Low-Z Targets in Space-Based Hadron Calorimetry

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