Advances in Space Research 33 (2004) 1786–1789

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Observation of solar flare hard X-ray spectra using CdTe detectors

K. Kobayashi a,*, S. Tsuneta b, T. Tamura b, K. Kumagai b, Y. Katsukawa a, M. Kubo a,Y. Sakamoto a, N. Kohara a, T. Yamagami c, Y. Saito c, K. Mori d

a Solar Physics Division, National Astronomical Observatory, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo181-8588, Japanb National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

c Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japand ClearPulse Co., 6-25-17 Chuo, Ohta-ku, Tokyo 143-0024, Japan

Received 19 October 2002; received in revised form 28 April 2003; accepted 2 May 2003

Abstract

We present the design and initial flight results of a balloon-borne hard X-ray spectrometer for observing solar flares. The in-

strument is designed for quantitative observation of nonthermal and thermal components of solar flare hard X-ray emission, and

has an energy range of 15–120 keV and an energy resolution of 3 keV. The instrument is a small (gondola weight 70 kg) system

equipped with sixteen 10� 10� 0.5 mm CdTe detectors, and designed for a 1-day flight at 41 km altitude. Detector temperature of

)15 �C was achieved through radiative cooling alone. Pre-flight tests confirmed that all detectors exceeded the target 3 keV reso-

lution. No flares were observed during the 2001 flight, but the second flight on May 24, 2002 succeeded in observing a class M1.1

flare. Preliminary analysis indicates the observed spectrum is consistent with a purely thermal plasma at an unusually high tem-

perature of 47 mK.

� 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Scientific ballooning; Solar flare hard X-ray spectra; CdTe detectors

1. Introduction

Solar flares produce nonthermal electrons, typicallyin the 10–100 keV range, but sometimes reaching 10

MeV. Thermal plasma with temperature of 20 mK or

higher are also common. The particle acceleration

mechanism remains unknown and the heating mecha-

nism is poorly understood. Hard X-ray spectra provide

important diagnostic information such as energy distri-

bution of accelerated electrons and precise temperature

measurements of thermal sources. However, there havebeen few such observations due to limitations in detector

technology. The Hinotori and Yohkoh satellites pro-

vided detailed spatial information about hard X-ray

sources, but the energy resolution is limited by the

scintillator detectors, typically 10 keV or worse. For two

decades the only high-resolution spectrum of a solar

* Correponding author. Tel.: +81-422-34-3701; fax: +81-422-34-

3700.

E-mail address: kobayashi@soalr.mtk.ac.jp (K. Kobayashi).

0273-1177/$30 � 2004 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2003.05.020

flare was the one observed by a balloon-borne germa-

nium detector with 1 keV resolution (Lin et al., 1981).

We developed a simple balloon-borne hard X-rayspectrometer using the newly developed cadmium tel-

luride (CdTe) detectors. A 10� 10 mm detector, coupled

to a low-noise preamplifier can achieve a 3 keV energy

resolution without the need for a costly and massive

refrigeration system. This is a factor of 3 better than

previous satellite observations.

The RHESSI satellite (Lin et al., 1998) was launched

in February 2002. This instrument boosts a 1 keV energyresolution and image synthesis using an array of cooled

germanium detectors. Despite this, we feel additional

independent observations are invaluable, especially for

quantitative spectral measurements which are sensitive

to calibration errors.

2. Instrument design

The main gondola measures 70� 70� 70 cm and

contains ballast, electronics and batteries. The detector

ved.

K. Kobayashi et al. / Advances in Space Research 33 (2004) 1786–1789 1787

enclosure is mounted on top of the gondola frame and

surrounded by thermal shields (Fig. 1). The detector

pointing is fixed at 45� elevation, and the gondola azi-

muth is controlled to an accuracy of �5� using a closed-

loop system. Since the sun is occulted by the balloon

during part of the flight, a magnetic sensor placed on aturntable is used for control. A separate 2-axis sun

sensor is used as backup; it uses a pinhole and analog

position-sensitive detector (PSD) and has an accuracy of

1� over a 60�� 60� field of view.

The instrument consists of sixteen 10� 10� 0.5 mm

CdTe detectors with In/Pt electrodes, fabricated by

Acrorad, Inc. of Japan. The indium electrodes act as

Schottky barriers to suppress leakage current (Takah-

Fig. 1. Left: Overview of the instrument. Righ

Fig. 2. Left: Am-241 spectrum from the 16 detectors. FWHM resolution of a

The lines at 74 and 85 keV are secondary emission lines from the lead shield w

flight calibration source.

ashi et al., 1998). Fig. 2 shows Am-241 and Co-57

spectra from all 16 detectors. The detectors were cali-

brated using calibrated Am-241, Co-57 and Cd-109

sources. At 0 �C, all detectors were shown to have 2.6

keV or better energy resolution at the 60 keV Am-241

line.The detector enclosure is pressurized. The Rohacell/

CFRP composite window was fabricated by Mitsubishi

Heavy Industries, and has an areal density of 0.1 g cm�2.

The enclosure also contains preamplifiers and high

voltage batteries for bias. The detectors are passively

shielded by 2 mm of lead. A graded-Z collimator limits

the field of view to 10�� 60�. The rear and side shields

are plain lead; secondary emissions from the shields are

t: Photograph of the entire instrument.

ll channels are below 2.6 keV. Right: Co-57 spectra from all detectors.

hich were intentionally allowed to reach the detector and act as an in-

Fig. 4. Total background spectra of all 16 detectors, from start of level

flight to just before the flare. The emission lines at 75 and 85 keV are

from the lead shield. It shows the gain and resolution were stable

1788 K. Kobayashi et al. / Advances in Space Research 33 (2004) 1786–1789

useful for validating the operation and gain of the de-

tector.

The operating temperature of the detectors is 0 �C;this is achieved by radiative cooling. The detector en-

closure surface, covered with a silver-coated Teflon tape,

acts as a radiator. The enclosure is insulated with FRPand Delrin blocks from the rest of the gondola. Shields

placed around the detector enclosure block sunlight and

infrared from the ground while maximizing the view of

the sky by the radiator surface. Thermal math models

neglecting atmospheric effects predicted a temperature

of )40 �C while calculations with atmospheric effects

predicted a temperature range of )20 to )10 �C.The events are accumulated into a separate spectrum

for each detector. This is read out every 0.54 s and

transmitted by telemetry in real time. This design insures

a constant load on the readout circuit and telemetry

during high flux solar flares.

throughout the 8-h observation.

3. Flight results

First flight of the instrument took place on August

29, 2001 from the Sanriku Balloon Center in northern

Japan. The observation was cut short by a battery

problem, but operation of the instrument was verified.

The second flight took place on May 24, 2002. Obser-

vation lasted for 9 h at an altitude of 41 km. In both

flights the thermal design worked better than expected,

with the detector enclosure temperature staying below)15 �C throughout the level flight.

The total count rate is shown in Fig. 3. Noise during

ascent is caused by acoustic input from the siren and

electromagnetic interference from the transponder, both

of which are turned off before reaching level flight. The

sharp spike at 6:41 UT (15:41 JST) is a solar flare.

Fig. 3. Total count rate of all 16 detectors throughout the flight. The

spikes during ascent are mostly electrical and acoustic noise from the

transponder and siren.

Fig. 4 shows the background spectra during levelflight. The lines at 75 and 85 keV are secondary emission

from the lead shield. This figure shows there were no

major changes in gain or resolution during the almost 8

h of observation.

A solar flare was successfully observed at 6:41 UT.

This is a class M1.1 flare which occurred in active region

9963. The flare was also observed by the Nobeyama

radio polarimeters, and the first half of the flare wasobserved by the RHESSI spacecraft. The detailed light

curves are shown in Fig. 5. A background subtracted

counts spectrum of the first minute of the flare is shown

in Fig. 6, along with a fit to thermal spectra. The spec-

trum is consistent with a purely thermal plasma of 47

mK temperature. This is an unexpectedly high temper-

Fig. 5. Observed light curves during flare.

Fig. 6. Observed flare spectrum with 1-r error bars and fits to thermal

spectra.

K. Kobayashi et al. / Advances in Space Research 33 (2004) 1786–1789 1789

ature, but our analysis of a simultaneous RHESSI

satellite data is in good agreement. Detailed analysis of

this solar flare is underway.

Acknowledgements

We thank Genzo Kato of Mitsubishi Heavy Indus-

tries, Ltd. and Koji Yamaguchi of Orbital Engineering

Inc. for help in instrument development. We also thankAki Takeda of the Solar Physics Research Corp.,

Tetsuya Watanabe of the National Astronomical Ob-

servatory of Japan, and the members of the ISAS

balloon group for their support. K. Kobayashi is

supported by the Japan Society for the Promotion of

Science.

References

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Takahashi, T., Hirose, K., Matsumoto, C., et al. Performance of a new

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