CYRIC Annual Report 2005
IV. 3. Measurement and Monte Carlo Calculation of the Response
Function of a Schottky CdTe Detector with a Guard Ring Electrode
for Medical X-ray Field
Mohammadi A.1, BabaA M.1, Ohuchi H.2, and Yamaguchi Y.1
1
2
Cyclotron and Radioisotope Center, Tohoku University
Graduate School of Pharmaceutical sciences, Tohoku University
The CdTe detectors have a great advantage for use at room temperature because
they are compact and have large photon absorption cross section in comparison with Si and
Ge.
Therefore, they are convenient for x-ray spectroscopic systems. However, the CdTe
detectors have problems of poor charge transport properties because of the charge carrier
trapping1).
This problem has been improved significantly in a Schottky CdTe diode that is
made of a high quality CdTe crystal, where a high Schottky barrier is formed by using
indium2), 3) for the holes on a CdTe surface.
The Schottky CdTe diode with guard ring can
be operated with higher bias voltage, which leads to high resolution γ-ray spectra free from
tail structure, even at room temperature4).
In this study, we use a Schottky CdTe detector with a guard ring electrode for
analysis of medical X-ray field.
thick.
The effective area of the detector is 4x4 mm2 and 0.3 mm
The response function of the detector has been investigated at various operating
condition for an
241
Am source.
The spectra of mentioned source is shown in Fig. 2 at
various bias voltage and shaping time.
From this figure, the optimum applied bias voltage
and shaping time are obtained to be 200 V and 2 µsec respectively.
The response function of detectors can be calculated using the MCNP4C code5).
Our detector response was calculated by the code for
241
Am,
133
Ba and
152
Eu sources and
then the results were compared with the measured responses at optimum operating
condition. Figure 2 shows the experimental results in comparison with calculation for
three sources.
From these results, the energy resolution (FWHM) is estimated to be 1.53
keV, 1.6 keV and 2.07 keV for 59.54 keV, 81keV and 121.78 KeV, respectively.
The full-energy-peak efficiency of the detector has also been calculated and
47
241
compared with the measurement for 60 keV and 81 keV lines of
Am and
133
Ba sources,
respectively. Figure 3 shows the calculated and measured full-energy-peak efficiency of
the detector.
The general agreement between the calculation and the measurement confirms the
validity of the calculation and that the carrier trapping effect is not serious for the present
CdTe detector.
The detector will be applied to the characterization of medical X-ray fields
with the calculated response and peak efficiencies.
References
1) Richter M., Siffert P., Nucl. Instr. Meth. A322 (1992) 529.
2) Matsumoto C., Takahashi T., Takizawa K., Ohno R., Ozaki T., Mori K., IEEE Trans. Nucl.
Sci. 45 (1998) 428.
3) Takahashi T., Watanabe Sh., IEEE Trans. Nucl. Sci. 48 (2001) 950.
4) Nakazawa K., Oonuki K., Tanaka T., et al., IEEE Trans. Nucl. Sci. in press (2004).
5) Gallardo S., Rodenas J., Verdu G., Med. Phys. 31 (2004) 2028.
2500
2500
Shaping time=2µSec
Bias voltage 200V
Bias voltage 100V
Bias voltage 50V
2000
1500
Counts
counts
2000
Bias voltage 200V
Shaping time 0.5µSec
Shaping time 2.0µSec
Shaping time 5.0µSec
Shaping time 10µSec
(a)
1000
1500
(b)
1000
500
500
0
0
0
10
20
30
40
50
0
60
10
20
30
40
50
60
Energy (kev)
Energy (kev)
Figure 1. Spectra of 241Am gamma-rays obtained with the Schottky CdTe detector with guard ring (a) at
fixed shaping time of 2 µs and various bias voltage and (b) at fixed bias voltage of 200 V and various
shaping time.
6000
241Am
Bias voltage 200V & Shaping time 2µsec
Measured spectra
Simulated spectra
5000
C o u n ts
4000
3000
FWHM 1.53KeV
2000
1000
0
23
28
33
38
43
Energy (kev)
48
53
58
Figure 2. Comparison of the measured gamma-ray spectra for 241Am,
with the calculated spectra.
48
63
152
Eu and
133
Ba
1600
1400
152Eu
1200
Bias voltage 200V & Shaping time 2μsec
Measure spectra
Simulated spectra
C o u n ts
1000
800
600
400
FWHM =2.07keV
200
0
C o u n ts
0
20
40
60
Energy (kev)
80
100
120
3000
133Ba
2500
Bias voltage 200V & Shaping time 2μsec
Measure spectra
Simulated spectra
2000
1500
1000
500
FWHM=1.6keV
0
0
10
20
30
40
Energy (kev)
50
60
70
80
1
Calculated
Efficiency
0.8
Measured
0.6
0.4
0.2
0
0
50
100
Energy (kev)
Figure 3. Comparison of calculated and measured full-energy peak efficiency of the
Schottky CdTe detector with guard ring.
49