Results of a Si/CdTe Compton Telescope
Kousuke Oonukia,b , Takaaki Tanakaa,b , Shin Watanabea , Shin’ichiro Takedaa,b ,Kazuhiro Nakazawaa , Takefumi Mitania,b , Tadayuki Takahashia,b , Hiroyasu Tajimac ,Yasushi Fukazawad , Masaharu Nomachiea Institute of Space and Astronautical Science (ISAS/JAXA), Sagamihara, Kanagawa

229-8510, Japanb Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japanc Stanford Linear Accelerator Center, Stanford, CA 94309-4349
d Department of Physics, Hiroshima University, Higashi-Hiroshima, 739-8526, Japane Department of Physics, Osaka University, Toyonaka, Osaka, 560-0043, Japan

ABSTRACT
We have been developing a semiconductor Compton telescope to explore the universe in the energy band fromseveral tens of keV to a few MeV. We use a Si strip and CdTe pixel detector for the Compton telescope to cover anenergy range from 60 keV. For energies above several hundred keV, the higher eﬃciency of CdTe semiconductor incomparison with Si is expected to play an important role as an absorber and a scatterer. In order to demonstrate
the spectral and imaging capability of a CdTe-based Compton Telescope, we have developed a Compton telescopeconsisting of a stack of CdTe pixel detectors as a small scale prototype. With this prototype, we succeeded inreconstructing images and spectra by solving the Compton equation from 122 keV to 662 keV. The energy
resolution (FWHM) of reconstructed spectra is 7.3 keV at 511 keV and 3.1 keV at 122 keV, respectively. The
angular resolution obtained at 511 keV is measured to be 12.2˚(FWHM).
Keywords: gamma-ray, Compton telescope, semiconductor, CdTe, Si

1. INTRODUCTION
The hard X-ray and gamma-ray bands ranging from several tens of keV to a few MeV are important windows for
exploring the energetic universe. It is in these regions that high energy phenomena such as nucleosynthesis andparticle acceleration become dominant. However, observation sensitivity in this energy band has been still limited,
due to high background, low detection eﬃciency, and diﬃculty of imaging by means of focusing technology. A
semiconductor Compton telescope utilizing the good energy and position resolution of semiconductors is one of
the most promising ways to realize a breakthrough in this energy band.
Based on our recent achievements for high resolution CdTe and Si imaging detectors,1–5 we have proposeda Si/CdTe semiconductor Compton telescope as a next generation Compton telescope.6, 7 The basic conceptof the Si/CdTe Compton telescope is to utilize Si as a scatterer and CdTe as an absorber. Si works as a goodscatterer with energies higher than 60 keV due to large Compton scattering eﬃciency, and CdTe has high photoabsorption eﬃciency as an absorber due to the large atomic numbers of Cd (Z = 48) and Te (Z = 52). The
Si/CdTe Compton telescope surpasses scintillator-based Compton telescopes by its high energy and angularresolution. According to the measurements with our prototype which is composed of six layers of Double-sided
Si strip detector (DSSD) with a thickness of 300 µm and CdTe pixel detectors with a thickness of 500 µm,
we have successfully achieved Compton reconstructed images of gamma-rays from 81 keV to 662 keV, and anangular resolution of 3.9 ˚for 511 keV gamma-rays.8–10
In order to achieve much higher eﬃciency for gamma-rays above a few hundred keV, we need a CdTe detector
as thick as several mm. One idea to achieve such a thick device is a stacked CdTe detector.11–13 At energies
above 300 keV, the stacked CdTe detector by itself works as a Compton telescope. To understand the behavior of
Further author information: (Send correspondence to K.O.)
K.O.: E-mail: oonuki@astro.isas.jaxa.jp, Telephone: 81 42 759 8510

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Figure 1. Compton reconstructed image (left) and spectrum (right) of the 511 keV gamma-ray obtained with a prototype
Si/CdTe Compton telescope. The angular resolution is 3.9˚. The energy resolution is 14 keV (FWHM) at 511 keV.

the stacked detector, we developed a prototype, which is composed of three layers of CdTe pixel detectors alongwith one at their side. In this paper, the results obtained with the prototype are described. The comparison
with the results obtained from a Compton telescope consists of DSSD and CdTe pixel detectors are also carriedout.

2. A SI/CDTE COMPTON TELESCOPE
As described in our previous publications,8–10 we have constructed a prototype Si/CdTe Compton telescopewith diﬀerent conﬁgurations. The high energy resolution of a newly developed DSSD has proven to be essential
to obtain not only a wide energy coverage starting from 60 keV, but also a high angular resolution. With theprototype Si/CdTe Compton telescope, we obtained Compton reconstructed images of the gamma-rays from 81
keV to 662 keV. The 511 keV image is shown in the Fig. 1. The angular resolution is measured to be 3.9 ˚for
511 keV gamma-rays. Fig. 1 shows the reconstructed spectrum for 511 keV gamma-rays from a 22 Na gamma-ray
source. The energy resolution of 14 keV (FWHM) demonstrates the high potential of the Si/CdTe Compton
telescope for future missions.
Since the thickness of the CdTe pixel detector adopted as an absorber in the prototype is only 0.5 mm, thetotal eﬃciency for gamma-rays above several hundred keV is very limited. A possible improvement to this isto thicken the CdTe detector. A monolithic and thick detector is not, however, applicable if we use the current
CdTe or CdZnTe semiconductor, due to incomplete charge collection. Therefore, we are working on the conceptof a stacked CdTe detector with a capability of imaging.

3. A STACKED THIN CDTE PIXEL DETECTOR FOR COMPTON TELESCOPE
For gamma-rays above a hundred keV, adequate detection eﬃciency can be obtained by CdTe or CdZnTe with athickness of more than 5 mm. It is, however, diﬃcult to increase the thickness of the device, keeping good energy
resolution throughout the energy range from several ten keV to a few hundred keV. This is because the eﬀect ofincomplete charge collection becomes more severe due to small mobility and short lifetime of holes of CdTe or
CdZnTe. The inactive region in the detector volume degrades the energy resolution through a low energy tail inthe spectral response.14, 15
In order to achive fully active CdTe or CdZnTe detectors, the concept of using thin device is of particularimportance, since we can apply suﬃcient bias voltage to collect all charge produced in the detector. As calculated
in our paper,12 a 0.5 mm thick CdTe detector becomes fully active with a bias voltage of 800 V, and even holes
generated near the anode face can be completely collected. By using a detector with a thickness of 0.5 mm, an

Ein
Si

θ
E1

E2

CdTe

Figure 2. Conceptual drawing of our Si/CdTe
semiconductor Compton telescope. CdTe pixel
detectors are stacked into 20 layers to achieve
larger eﬀective area.

Figure 3. Calculated peak detection eﬃciency for three types ofCompton telescopes. In the simulation, the detector is deﬁned
to consist of 80 layers of thin Si or CdTe detector. It can beclearly seen that the adoption of CdTe leads to high detection
eﬃciency for a Compton telescope with stacked conﬁguration.
[Takahashi, et al, SPIE (2002)]

applied bias voltage of 1400 V, and a temperature of −20 ˚C, we have achieved an energy resolution (FWHM)
of 830 eV at 59.5 keV, and 2.1 keV at 662 keV, which is close to the Fano limit of CdTe.15
For energies up to a few MeV, stacking thin CdTe pixel detectors becomes the unique concept to realize
a detector with both high energy resolution and high eﬃciency for gamma-rays.12–14 As shown in Fig. 2, a
stacked CdTe detector is adopted in our Si/CdTe Compton telescope. In the stacked detector, several thin CdTe
pixel detectors are stacked together and operated as a single detector. By adopting fully-active thin CdTe pixel
detector in each layer, we can take advantage of the high energy resolution for gamma-rays from several tens ofkeV to a few MeV.
Another important merit of utilizing a stacked thin CdTe pixel detector is that it works as an eﬀective
Compton telescope above 300 keV, because the stacked detector gives three-dimensional information of the
gamma-ray interaction in the CdTe part. It should be noted that the cross section for Compton scattering in
CdTe becomes higher than that of photo absorption above 300 keV, and the cross section is twice larger than thatof Si. Fig. 3 clearly shows how the stacked CdTe works to improve the eﬃciency for the high energy gamma-rays.
Larger eﬃciency for the gamma-rays above a hundred keV can be achieved by optimizing the number of layersof CdTe detectors. Although the angular resolution of a stacked CdTe pixel detector is expected to be worse
than that of a Si/CdTe Compton telescope due to the larger Doppler broadening eﬀect,16 the high eﬃciency
and high energy resolution are attractive for the detection of gamma-rays up to a few MeV.

4. EXPERIMENTAL SETUP
In order to operate a stacked CdTe pixel detector as a Compton telescope, not only high resolution CdTe pixel
detector but also an individual readout system is essential. In this section, details of the CdTe pixel detector
and the detector system are described.

4.1. High Resolution 8 × 8 CdTe Pixel Detectors
The 8 × 8 CdTe pixel detector used in the prototype is based on the Schottky CdTe diode device, utilizing indium
as the anode and platinum as the cathode. The CdTe crystal is manufactured by ACRORAD in Japan with theTraveling Heater Method (THM). It features very low leakage current and high uniformity.1, 17, 18 Since position
resolution of a few mm is necessary to construct a Compton telescope, we have developed a pixel detector with
64 pixels. Fig. 4 shows the photo of the detector. The detector has dimensions of 18.55 mm × 18.55 mm and a
thickness of 500 µm. The indium side is used as a common electrode and the platinum side is divided into 8 by 8
pixels. The pixel size is 2 mm × 2 mm, and the gap between each pixel is 50 µm. A guard ring electrode with aFigure 4. A photo of a 8 × 8 CdTe pixel detector. The signal from each pixel is fed into two VA32TAs on the FEC.

width of 1 mm is attached to reduce leakage current because most of the leakage current is through the detector
perimeter. Each pixel is connected to a fanout board by bump bonding technology developed in cooperation
with Mitsubishi Heavy Industry in Japan.14 Since the leakage current of each pixel is as low as 100 pA at abias voltage of 500 V even at room temperature, the signal lines are directly connected to the input of readout
electronics, an analog LSI VA32TA.5 This LSI is developed with IDEAS in Norway, and characterized by its low
noise. The LSIs are read out via a specially designed compact readout system including an ADC and FPGA,
which is controlled with a fast serial interface ”Space Wire (IEEE 1355).19 ”
High energy resolution of each CdTe pixel detector is essential to achieve both high angular resolution andenergy resolution for Compton telescopes. The low energy threshold of the detector is also important in order
to utilize events with small-angle scattering. This is because the deposited energy of a recoiled electron becomes
lower as the scattering angle becomes smaller. Fig. 5 shows spectra of 22 Na and 57 Co with one of the CdTe
pixel detectors employed in the prototype stacked CdTe pixel detector. The spectra are drawn by summing thespectra from all channels after gain correction. The data is measured at a temperature of −20 ˚C and with abias voltage of 1200 V. The energy resolution (FWHM) is 5.0 keV and 1.6 keV for the gamma-rays of 511 keV
and 122 keV, respectively. The energy threshold is measured to be as low as 7 keV.
The homogeneity of the wafer is of particular importance for imaging detectors, because defects in the wafer
can deteriorate the energy resolution through positional variation of properties such as charge collection eﬃciency
and leakage current. As reported in our previous publications,1, 18 we have demonstrated the uniform responseof large THM-CdTe detectors with an area of 21 mm × 21 mm. Fig. 6 shows spectra of 22 Na for each channel.
The same data as in Fig. 5 (left) is used. In this detector, all 64 channels are examined to be properly connected,
and the detector shows uniform response.

4.2. Prototype Setup of CdTe Compton Telescope
Fig. 7 shows the arrangement of the detectors. Three CdTe pixel detectors are stacked with an interval of 12mm, while one detector are placed at the side of them. In order to lower the leakage current and to surpress
the so-called “polarization eﬀect”,20 the entire detector system was kept at a temperature of −20 ˚C, and theapplied bias voltage for the all detectors was set to 700 V. Under these conditions, we do not see any spectral
degradation for a long term operation of a week, as is reported in our paper.1 Gamma-ray sources are placed
370 mm above the surface of the ﬁrst layer of the detector. A trigger signal is issued from all channels unless it
is disabled due to large noise. When a trigger signal is generated from some channels, all channels of the entire
system are read out.

5. DATA ANALYSISFor Compton reconstruction, data reduction is performed as follows. First, ”two-hit events,” one hit in a CdTe
pixel detector and one hit in the other detector, were selected from the raw data. Here, one hit means that only
one channel has a pulse height above 14 keV. Fig. 8 shows an example of a scatter plot for the ”two-hit events.”
This plot is obtained when a 22 Na radiative isotope is used. There is a branch which satisﬁes E ∼ 511 keV,

Figure 5. 22 Na (left) and 57 Co spectra obtained with a 8 × 8 CdTe pixel detector. The detector is operated at atemperature of −20 ◦ C, with a bias voltage of 1200 V. Each spectrum is drawn by summing spectra of all 64 channels.
The energy resolution (FWHM) at 511 keV and 122 keV is 5.0 keV and 1.6 keV, respectively.

Figure 6. 511 keV spectra of all the 64 pixels.

370 mmradiation source

CdTe

12 mm
18 mm
Figure 7. Conﬁguration of the prototype stacked
CdTe pixel detector.

Figure 8. Scatter plot of all two-hit events. The plot is obtainedby using a 22 Na gamma-ray source.

which means incident gamma-rays of 511 keV are scattered in one detector and absorbed in the other. Other
noticeable branches are those parallel to the horizontal axis and to the vertical axis between 20 keV and 34 keV,
respectively. They are the events due to ﬂuorescence X-rays from Cd (Kα: 23.1 keV, Kβ: 26.1 keV) and Te (Kα:27.4 keV, Kβ: 31.0 keV), which escape from the CdTe pixel detectors and are absorbed in the other. Since they
are not the Compton events, we eliminate those events in the analysis.
From each two-hit event, the information of deposited energies and positions are obtained. We name thesmaller deposited energy as E1 , and the other as E2 (E1 ≤ E2 ). Using these energies, the incident energy (Ein )and the scattering angle (θcomp ) are calculated. In the calculation, we used:Eincos θcomp= E1 + E2
me c2
me c2
= 1−
+
,
E2
E1 + E2(1)(2)

assuming that incident gamma-rays are once scattered and then fully absorbed in the CdTe pixel detector. Eq.
2 assumes that a incident gamma-ray ﬁrst deposits its energy (E1 ) by a Compton scattering, and then deposits
the rest of the energy (E2 ) by photo absorption. If the energy of incident gamma-rays (Ein ) is less than me c2 /2= 255.5 keV, this assumption is valid because the energy of the recoil electron is always less than that of thescattered photon. If the energy of the incident gamma-rays exceeded 255.5 keV, we calculated two scattering
angles by swapping E1 and E2 in Eq. 2. From the calculated scattering angle and the hit positions, Compton
cones are drawn on the sky event by event. The Compton cones are weighted by using the diﬀerential crosssection of Compton scattering using the Klein-Nishina formula, and the cross section of photo absorption. We
projected the cone at the plane at the distance of 370 mm, and obtained the image of the gamma-ray sources.

5.1. Compton Reconstruction of Images
Fig. 9 shows the images obtained with four gamma-ray sources. A circle with a radius of 10 ˚is drawn together
with each image as a reference. Fig. 9 (a) is the image of 57 Co gamma-rays, by using events within 122 keV ±5 keV. The images of 133 Ba, 22 Na, and 137 Cs are also drawn in Fig. 9 (b) – (d). The energy selection is 270 keV– 390 keV for 133 Ba, 500 keV – 520 keV for 22 Na, and 650 keV – 675 keV for 137 Cs, respectively. We note thatthe images are symmetric due to the arrangement of the CdTe pixel detectors being symmetric except for the
one at a side.

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Figure 9. Compton reconstructed images. (a) 57 Co image using 122 keV ± 5 keV. (b) 133 Ba image using 270 keV – 390
keV. (c) 22 Na image using 500 keV – 520 keV. (d) 137 Cs image using 650 keV – 675 keV.

Figure 10. Compton reconstructed spectra of 22 Na (left) and 57 Co (right). Dotted lines show the simple sum of two-hit
events, while solid lines show the sum of events identiﬁed to be emitted from the source by Compton reconstruction.

5.2. Compton Reconstruction of Spectra
Background rejection using a Compton image is an important feature for a Compton telescope. Energy spectraof the incident gamma-rays are obtained by summing deposited energies. The dotted lines in Fig. 10 shows the
spectra which we obtained by simply summing the deposited energies at each detector for all ”two-hit events.” In
the spectrum obtained with a 22 Na source, there is a 511 keV gamma-ray line and a large number of scattering
components. The latter components are considered as the events scattered twice in the CdTe pixel detectors,
the events by the gamma-rays scattered in the other materials, and so forth. Then we performed an event
selection using the Compton images. The solid line spectrum in Fig. 10 is made from the events which satisfy
the condition that the drawn Compton cone corresponds to the direction of the source position. As shown inthe ﬁgure, the peak of the 511 keV gamma-rays are enhanced, and the scattering components are reduced. Theenergy resolution (FWHM) is 7.3 keV for 511 keV gamma-rays and 3.1 keV for 122 keV gamma-rays, respectively.

5.3. Angular Resolution of the CdTe compton Telescope
In order to evaluate the angular resolution of our prototype Compton telescope, we compared the calculated
scattering angles (θcomp ) with those deﬁned by the location of gamma-ray source and hit positions (θgeom ) (seeFig. 11). The diﬀerence between the two values reﬂects the angular resolution (∆θ = θcomp − θgeom ) of theCompton telescope. Fig. 12 shows an example of events within 270 keV – 390 keV for the data obtained with133
Ba. We also investigate the distribution of the angular resolution for the gamma-rays of 57 Co, 22 Na, and137
Cs. The FWHM of these distributions (∆θexp ), which is the angular resolution of the prototype, are plottedagainst the incident gamma-ray energies in Fig. 13. The angular resolution becomes better as the incident
gamma-ray energy become higher. The obtained angular resolution is 35.9 ˚at 122 keV, and 12.2 ˚at 511 keV,
respectively.
We estimated the contribution of the position and energy resolution of the CdTe pixel detectors. The
contribution of the position resolution (∆θpos ) is estimated by using the actual hit positions obtained from theexperiment, and the pixel size of the detectors. To estimate the contribution of the energy resolution (∆θene ), we
used the Compton-scattering angles of actual events and smoothed them by the energy resolution of the detector.
The estimated values are plotted in Fig. 13. The other contribution to the angular resolution is the eﬀect of the
Doppler broadening. In order to estimate the amount of them (∆θDB ), we performed Monte Carlo simulations

Position of the source(known)θ geom
CdTe pixel

CdTe pixel
Figure 11. Deﬁnition of θgeom .

Figure 12. Distribution of θcomp − θgeom for 133 Ba gammaray source.

Figure 13. Relation between incident gamma-ray energy and angular resolution (FWHM). Filled circles show the experimental data. Diamonds, open circles, and squares are estimated contributions of Doppler broadening, position resolution,and energy resolution, respectively. The total values of the three are ploted by stars.

using the Geant4 package,21 and plotted them in the same ﬁgure. The eﬀect of the Doppler broadening becomes
smaller as the incident energy becomes higher, which is consistent with the paper.16 The total value (∆θtotal )is also plotted in Fig. 13, which is calculated from:√∆θtotal = (∆θpos )2 + (∆θene )2 + (∆θDB )2 .
(3)
The experimental values can be explained with the three contributions. The angular resolution is almost limitedby the eﬀect of Doppler broadening.

6. CONCLUSIONSA Si/CdTe semiconductor Compton telescope is a promising detector for future gamma-ray missions in theenergy band from several tens of keV to a few MeV. In order to achieve higher eﬃciency, we have veriﬁed a
concept of a stacked CdTe pixel detector which acts as both a scatterer and an absorber. The prototype detector
consists of three layers of CdTe pixel detectors and one CdTe pixel detector at their side. With this prototype
detector, we succeeded in Compton reconstruction of images and spectra in the energy band from 122 keV to
662 keV. The energy resolution (FWHM) of reconstructed spectra is 7.3 keV at 511 keV and 3.1 keV at 122
keV, respectively. The high energy resolution of the reconstructed spectra is due to the high performance ofthin CdTe pixel detectors. The obtained angular resolution is 35.9 ˚at 122 keV, and 12.2 ˚at 511 keV, which ismainly dominated by the eﬀect of Doppler broadening.
The prototype stacked CdTe pixel detector shows higher eﬃciency in terms of reconstructed spectra than thatof the prototype Si/CdTe Compton telescope, while angular resolution of the stacked CdTe is worse than thatof the prototype Si/CdTe Compton telescope below 1 MeV. However, the angular resolution becomes a degree
scale, when the gamma-ray energy goes up to a few MeV. Once the stacked CdTe pixel detector is combinedwith Si layers, the detector should give us a good performance from several 10 keV up to several MeV.

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