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Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 41 (2008)(LNS Experiment : #2593)

Response of Electron Calorimeters for SCRIT Experiments
A. Kuwajima1 , T. Tamae1 , T. Suda2 , K. Ishii3 , K. Kurita3 , T. Shirai4 ,M. Wakasugi 2 , and S. Wang2
1

Laboratory of Nuclear Science, Tohoku University, Sendai, 982-0826
2

3
4

RIKEN Nishina Center, Wako, Saitama 351-0198

Department of Physics, Rikkyo University, Toshima, Tokyo 171-8501

Institute of Chemical Research, Kyoto University, Uji, Kyoto 611-0011

We investigated the response of pure CsI and BaF2 electron calorimeters for SCRIT experiments
using electron beams and cosmic rays. We compared the measured results with calculations by GEANT3,
and found that the inclusion of housings for crystals into GEANT3 calculations is essential in order toobtain the correct response of the detectors. Energy distribution of the cosmic rays also affects theresults of the calibration very much, and the use of the accurate energy spectrum is important when thecalorimeters are calibrated using cosmic rays.

§ 1. Introduction
The feasibility study of SCRIT (Self-Conﬁning Radioactive Isotope ion Target) [1, 2] has been doneusing 121 MeV electron beams at KSR (Kaken Storage Ring) in Kyoto University. In the experiment, theenergy of electrons elastically scattered from trapped Cs ions was measured using electron calorimetersof pure CsI and BaF2 . Those calorimeters were calibrated using cosmic rays and scattered electrons froma tungsten wire by comparing the measured spectra with results of GEANT3 simulations. Experimentswith electron beams were performed at KSR in Kyoto University and LNS in Tohoku University. As
data obtained at LNS have problems due to light leak for the CsI detectors, the results obtained fromthe measurements at KSR and from the cosmic rays are included in this report.

§ 2. Experimental Setup
Figure 1 shows a schematic view of the SCRIT experiment, which has been set up at KSR in Kyoto
University. Cesium (Cs) ions injected from an ion source are trapped in the potential produced by the
SCRIT electrodes and an electron beam. A tungsten wire with 50 μm diameter can be inserted onto
the electron beam at 6 cm upstream from the center of SCRIT. The energy of scattered electrons ismeasured using CsI at 30◦ and BaF2 at 40◦ and 60◦ . The distance of the front of the calorimeters fromthe center of SCRIT is 1750, 1180 and 790 mm for 30◦ , 40◦ and 60◦ detectors, respectively. Two plastic
scintillation counters have been installed between the drift chamber and each calorimeter. Anotherplastic scintillators have been distributed around the calorimeters for measurement of the cosmic rays,
which are used as veto counters during the measurements with the electron beam. The drift chamber

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determines the vertex point and the angle of the scattered electrons. Each calorimeter consists of seven
optically isolated crystals; each crystal has a hexagon cross section and its length is 20 cm. The crosssection of the CsI and BaF2 crystal is 41.6 cm2 and 39.9 cm2 , respectively. Each crystal is contained in a
1.5 mm thick aluminum housing, and seven detectors are set in a cylindrical case of stainless steel. The
light signal generated in the crystals is converted to the current signal by photomultipliers.

Fig.1. Prototype SCRIT and experimental setup.

§ 3. Calibration of Calorimeters using Cosmic Rays and a Wire Target
The calorimeters were calibrated using the cosmic rays and scattered electrons off a tungsten wire
target at KSR. The calibration using cosmic rays alone was applied before the wire target was installed.
The calorimeter at 60◦ was calibrated using only cosmic rays because the number of the scattered electrons was not enough for the calibration. Aluminum housings with a thickness of 1.5 mm for the crystals
were included in the calculation; they inﬂuence the simulated spectrum very much, as shown in the following section.
The propriety of the calorimeters was evaluated using spectra of electrons scattered off the tungsten
wire target. The ADC spectrum of each crystal was ﬁtted with that calculated using GEANT3 program
to determine the calibration parameter for each crystal. The simulation includes the radiative correction
at the scattering point, the energy loss in materials between the target and the calorimeters, and theformation of the shower and the energy dissipation in the calorimeters. The energy spectra measured
by the CsI calorimeter at 30◦ and BaF2 calorimeter at 40◦ are compared with that calculated using
GEANT3 simulation in Fig. 2; the calibration parameter for each crystal was determined by ﬁtting

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the measured spectrum to the calculation. The ﬁgure indicates that the shapes of the elastic peak ofmeasured and calculated spectra are almost identical. A considerable background exists below 50 MeV.

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Fig.2. Total energy loss distribution in 7 crystals of the calorimeters for electrons scattered by atungsten wire. (left: CsI calorimeter at 30◦ , right: BaF2 calorimeter at 40◦ ). (dashed line:
spectrum calculated with GEANT3, solid line: measured spectrum where the calibration
parameter for each crystal was obtained from ﬁtting the data to the calculation.
The calibration of the calorimeters using cosmic rays is also useful. The high-energy cosmic rays
penetrate the crystals; the main part of the cosmic rays is thought to be the muon. At the ﬁrst stage,
the energy distribution of muons was assumed to be constant from 0.5 to 10 GeV. The intensity of thecosmic rays was supposed to have the zenith angle dependence of cos2 θ. The ﬁtting for each crystal andthat for addition of the signals were fairly well (Fig. 3). However, the spectrum of electrons elasticallyscattered off the tungsten wire, which were measured by the CsI calorimeter with a 121 MeV electron
beam at KSR, is 20 MeV higher than the calculation using GEANT3, as shown in Fig. 4, when we use
the calibration parameter determined using cosmic rays for each crystal. The result indicates that thecalibration using the parameters obtained using cosmic rays cannot reproduce the absolute value of theenergies of electrons scattered off the tungsten wire.
One of the reasons is thought to be unrealistic assumption for the energy distribution of the cosmic
rays. Figure 5 shows a dE/dx curve for the muon; it has a minimum around 2 GeV and increases towards lower energy. It means that the low energy component of the muons affects the energy dissipationof the cosmic rays in the calorimeters very much. Parameters for calibration were deduced by comparing
the measured spectrum and calculated ones by GEANT3, where the ﬂat spectrum of muons from 0.5
to 10 GeV (Fig.6) and a more realistic spectrum were used. The realistic muon spectrum was obtained
from the BESS data [3] including the energy loss in the concrete ceiling of the experimental hall. In Fig.
7 spectra for the CsI calorimeter deduced using calibration parameters using cosmic rays are comparedwith the spectrum shown in Fig. 2; the solid lines in Figs. 2 and 7 are same. The energy spectrum using

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Fig.4. Total energy loss distribution in 7 crystals of the CsI calorimeter for electrons
scattered by a tungsten wire. (solidline: measured spectrum, dashed line:
GEANT3 calculation) Parameters foreach crystal were obtained by ﬁtting themeasured spectrum of cosmic rays with
GEANT3 calculation.

Fig.3. Total energy loss distribution of cosmic
rays in 7 crystals of the CsI calorimeter.
(solid line: measured spectrum, dashedline: GEANT3 calculation) Parameters
for each crystal were obtained by ﬁtting
the measured spectrum with GEANT3
calculation.

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Fig.5. dE/dx curve for the muon.

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Fig.6. Muon energy spectra used for simulation. A ﬂat distribution from 0.5 to 10
GeV and a realistic distribution wereused in the calculation.

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Fig.7. Total energy loss distribution in 7 crystals of the CsI calorimeter for electrons scatteredby a tungsten wire. (solid line: spectrum using calibration parameters obtained fromﬁtting with the data themselves, dashed line: spectrum using calibration parameters
obtained from cosmic rays.) (left: a ﬂat spectrum of cosmic rays is used, right: a realistic
spectrum of cosmic rays is used.
parameters obtained by assuming the ﬂat muon spectrum shifts to higher energies than the spectrum
using parameters obtained from data of scattered electrons off the tungsten wire (left panel in Fig. 7).
The spectrum using parameters assuming a realistic muon spectrum largely improves the reproduction
although the difference of 3 MeV still remains (right panel in Fig. 7). It means that the use of theaccurate spectrum is important when the calorimeters are calibrated using cosmic rays.

§ 4. Inﬂuence of Housing for Crystal
Each crystal of the calorimeters was contained in aluminum housings. Some parts of particles
dissociate their energies in the housings, and their energies are removed from the signal. It worsens
the energy resolution of the detectors. As effective thickness of the housings for the particles that movenearly parallel to the axis is much larger than the actual thickness, inﬂuence of the housings is larger
for scattered electrons than for cosmic rays. The energy dissipation in the CsI crystals calculated by
GEANT3 is shown for no housings (solid line) and for 1.5 mm thick aluminum housings in Fig. 8. In
the calculation, electrons of 121 MeV are distributed in the area of 13 cm × 13 cm. It indicates that the
thickness of the housings should be as thinner as possible in order to improve the resolution.

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Fig.8. Inﬂuence of housings for crystals. Total energy loss distribution in 7 crystals of the CsI
calorimeter was calculated for 121 MeV electrons by GEANT3. (solid line: spectrum withno housings, dashed line: spectrum with 1.5 mm thick aluminum housings.)

References[1]

M. Wakasugi, T. Suda, and Y. Yano: Nucl. Instrum. and Meth. A532 (2004) 216.

[2]

M. Wakasugi et al.: Phys. Rev. Lett. 100 (2008) 164801.

[3]

http://bess.kek.jp.