Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

15C/2003G095

Long-term observations of x-ray energy responses of semiconductor detectors
after neutron irradiation
Junko KOHAGURA*1, Teruji CHO1, Mafumi HIRATA1, Tomoharu NUMAKURA1,Noboru YOKOYAMA1, Yamato TOMII1, Serina KIMINAMI1, Suguru TOKIOKA1,1
1
2
2
Yasuhiro MIYAKE , Kiyoaki SHIMIZU , Makoto NAKAO , Takashi KONDOH ,Takeo NISHITANI2, Keiichi HIRANO3, Hideki MAEZAWA3, Syoichi MIYOSHI1
1Plasma Research Center, University of Tsukuba, Ibaraki 305-8577, Japan
2
Japan Atomic Energy Research Institute, Ibaraki 319-1195, Japan
3KEK-PF, Tsukuba, Ibaraki 305-0801, Japan

Experimental Apparatus
X rays monochromatized with a double-crystal[Si(111)] monochromator (BL-15C) are incident on
semiconductor detectors irradiated with the neutron
13
15
2
fluence of 10 -10 n/cm by the use of the fusion
neutronics source (FNS) of Japan Atomic Energy
Research Institute. Semiconductor detectors investigatedin the present work are similarly designed pin photodiodearrays (n-type wafer) for position-sensitive X-ray
tomography diagnostics in the Joint European Torus(JET) tokamak. Each detector array consists of 35
elements with an individual anode of 4.5 mm by 0.96 mm.
Experimental Results
It is well known that detectors show radiation-induced
degraded performance after irradiation such as anincrease of leakage current, and changes in effective
doping concentration. It is also known that the observed
degraded characteristics of the detectors change graduallywith time even after the end of the irradiation process.
The effects are known as “annealing effect” and “reverseannealing effect”.
Figure 1 shows the long-term observations of X-ray
energy responses of the JET detector arrays as a functionof time kept at room temperature after the neutron
irradiation experiments. X-ray energy responses η/E of122
the detectors with neutron fluences of 8.9×10 n/cm and132
4.5×10 n/cm are plotted by the circles and the squares
in Fig. 1, respectively. Here, open symbols and closed

1

η/E

Introduction
X-ray tomographically reconstructed data analyses by
the use of silicon semiconductor detectors play animportant role in controlled thermonuclear fusionresearch in order to investigate plasma electron behavior[1]. However, recent harsh radiation environments infusion experiments by using deuterium-tritium and/ordeuterium-deuterium reactions pose the serious problemof radiation-induced degradation in X-ray detection
characteristics of semiconductor detectors. In this article,
the effects of fusion produced neutron irradiation on Xray energy responses of silicon semiconductor detectors
are investigated in terms of dependence on time after theend of neutron irradiation process [2].

0.150100Time (days)

1000

Fig.1 Long term changes in X-ray energy responses
η/E as a function of time after irradiation
12213with fluences of 8.9×10 n/cm and 4.5×102n/cm .
symbols represent the data with a detector bias Vb of 1.5
V and 6.0 V, respectively. The incident X-ray energy of
8.0 keV is applied by the use of the synchrotron radiation
at the Photon Factory.
The result that the differences between the data in eachdata set are only less than 4% is consistent with theresults obtained for other characteristic properties [3]; thatis, the annealing behavior of effective doping andvolume-generated current drastically changes within aperiod of roughly one week after irradiation, and afterthat changes with time become moderate. This isinterpreted as follows: Various kinds of defects anddefect complexes produced by irradiation affect thedetector performance and have different annealing timeconstants. After some period of time the annealing effectof detects having longer annealing time constant remains
dominantly, which makes the annealing changes withtime moderate.
References[1] J. Kohagura et al., Phys. Rev. E 56, 5884 (1997).
[2] J. Kohagura et al., Nucl. Instr. Meth. A513, 300(2003).
[3] W. R. Wunstorf et al., Nucl. Instr. Meth. A315, 149(1992).
*kohagura@prc.tsukuba.ac.jp

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