High-resolution Compton cameras based on Si/CdTe double-sided strip detectors
Hirokazu Odakaa , Yuto Ichinohea,b , Shin’ichiro Takedaa , Taro Fukuyamaa,b , Koichi Haginoa,b , Shinya Saitoa,b , Tamotsu Satoa,b ,Goro Satoa , Shin Watanabea , Motohide Kokubuna , Tadayuki Takahashia,b , Mitsutaka Yamaguchic , Takaaki Tanakad ,Hiroyasu Tajimae , Kazuhiro Nakazawab , Yasushi Fukazawafa Department of High Energy Astrophysics, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai,

Chuo, Sagamihara, Kanagawa, 252-5210, Japanb Department of Physics, Graduate School of Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo, 113-0033, Japanc Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency (JAEA), 1233 Watanuki-machi, Takasaki, Gunma, 370-1292, Japand Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA, USA.

e Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8601, Japanf Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan

Abstract
We have developed a new Compton camera based on silicon (Si) and cadmium telluride (CdTe) semiconductor double-sided strip
detectors (DSDs). The camera consists of a 500-µm-thick Si-DSD and four layers of 750-µm-thick CdTe-DSDs all of which havecommon electrode configuration segmented into 128 strips on each side with pitches of 250 µm. In order to realize high angularresolution and to reduce size of the detector system, a stack of DSDs with short stack pitches of 4 mm is utilized to make the camera.
Taking advantage of the excellent energy and position resolutions of the semiconductor devices, the camera achieves high angularresolutions of 4.5 degrees at 356 keV and 3.5 degrees at 662 keV. To obtain such high resolutions together with an acceptabledetection eﬃciency, we demonstrate data reduction methods including energy calibration using Compton scattering continuum anddepth sensing in the CdTe-DSD. We also discuss imaging capability of the camera and show simultaneous multi-energy imaging.
Key words: gamma-ray imaging, Compton camera, CdTe double-sided strip detector
1. Introduction
A Compton camera using high-resolution semiconductor detectors is a promising gamma-ray imaging spectrometer in theenergy band from 100 keV up to 10 MeV, in which Compton
scattering is dominant over other interactions of a photon withmatter. In astrophysics, gamma rays in this energy band are important probes of energetic phenomena in the universe such ascosmic-ray acceleration and nucleosynthesis. In this field, thefirst Compton telescope in orbit, COMPTEL [1], provided pioneering results of the gamma-ray sky. The gamma-ray imaging
technique is also useful for medical imaging, nondestructive inspection, and search for radioactive isotopes.
The authors have been developing Compton cameras basedon advanced technologies of silicon (Si) and cadmium telluride(CdTe) semiconductor detectors with high energy and position
resolutions [2, 3, 4]. Adoption of such devices are great advantage to the angular (spatial) resolution since it depends mainly
on precision of the energy and position measurements. Si isgood scatterer since it yields a large Compton-scattering probability and has a small Doppler broadening eﬀect; this eﬀect degrades the angular resolution due to nonzero momentum of thetarget electron [5]. On the other hand, CdTe is used as absorberowing to its large photoelectric-absorption cross section. Using
our camera prototypes [6], we have achieved an excellent angular resolution of 2.5 degrees at 511 keV [7], and have demonstrated high-resolution Compton imaging for diﬀuse emission
as well as point sources, resolving a-few-millimeter scale strucPreprint submitted to Nuclear Instruments and Methods in Physics Research A

ture at a distance of several centimeters from the detector head[8]. In addition, high-precision polarimetry was conducted using a synchrotron photon beam [9].
In this work, we have developed a new Compton camera thatsimultaneously realizes high angular resolution and compactness of the entire detector size. The high-density integrationof the camera is essential to obtain acceptable detection eﬃciency for all applications and to achieve fine spatial resolution
for near-source imaging such as medical applications. The camera consists of a compact multiple-layer stack of Si and CdTe
double-sided strip detectors (DSDs) which have common geometrical configurations. This modular design allows us to optimize the scale (e.g. the number of the stacks) of the camera for
future practical applications. In the present paper, we describe
the detector design and analysis methods to obtain the high angular resolution with the ultra-compact instruments, and thenshow achieved performances. The detector concept is adopted
as the key technology for the Hard X-ray Imager (HXI) and theSoft Gamma-ray Detector on board the ASTRO-H X-ray observatory, which is scheduled for launch in 2014 [10].
2. Detector design
The new Compton camera consists of a 500-µm-thick SiDSD (collaboration with Hamamatsu Photonics) and four layers of 750-µm-thick CdTe-DSDs (collaboration with ACRORAD), as shown in Fig. 1. The CdTe detector holds aluminum
October 8, 2013

3. Data reconstruction(Al) electrodes on the anode (high-voltage side) and platinum(Pt) electrodes on the cathode. As we adopt a modular design
to assemble the detector system, the camera is designed as a
stack of “standardized” detector trays with detector-to-detector
pitches of 4 mm. Figure 2 shows the tray on which each detector is integrated with front-end readout electronics including eight signal processing ASICs (VATA 460). Each detector
has the same active area of 32×32 mm2 and the same electrodeconfiguration that is segmented into 128 strips on each sidewith strip pitches of 250 µm. The readout ASIC simultaneously processes signals from 32 strips and digitizes the heightsof the shaped pulses. The HXI onboard ASTRO-H adopts similar module, but it has diﬀerent detector numbers (four Si-DSDs
and one CdTe-DSD) and diﬀerent readout ASICs (VATA 461).
In the experimental evaluation of the camera, these detectors were operated about −15◦ C. A bias of 250 V was applied
to the Si-DSD for full depletion; a bias of 250 V was applied
to the CdTe-DSDs, balancing between increasing charge collection eﬃciency and suppressing leakage currents. The typical energy resolution of the Si-DSD was 2.3 keV (full widthat half-maximum or FWHM) at 59.5 keV. The CdTe-DSD
achieved energy resolutions of 3.8 keV (FWHM) at 81.0 keVand ∆E/E = 3% (FWHM) at 511 keV. Since VATA 460 is lowgain or high-dynamic-range type, its spectral performance ismodest comparing to an energy resolution of 1.7 keV at 59.5
keV obtained by an equaivalent CdTe-DSD with high-gain type
VATA 461. Detailed description of CdTe-DSDs are given by
Watanabe et al. (2009) and Ishikawa et al. (2010) [11, 12].

As shown in Fig. 1, a typical Compton event consists of ascattering in Si and an absorption in CdTe. By using the energies of the recoil electron (E1 ) and of the scattered photon (E2 ),the scattering angle θK and the energy E of the incident photon
are given by the Compton kinematics,cos θK = 1 −

me c2
me c2
+
,
E2
E1 + E2

E = E1 + E2 ,(1)

where me is the electron mass and c is the speed of light. Whenthe scattering angle θG is geometrically calculated by the positions of the two interactions and the given incident direction,
the diﬀerence between the scattering angles calculated in twoways, ∆θ = θK − θG , provides the angular resolution measure(ARM) of the Compton camera. Obviously, uncertainties ofthe measured energies and positions aﬀects the performance ofCompton imaging. In this section, we describe analysis methods to fulfill simultaneously high angular resolution and suﬃcient eﬃciency.
3.1. Energy calibration
Accurate gain calibration are essential to make the best use of
the high-resolution semiconductor detectors. Though we usually apply gamma-ray radioactive sources to energy calibration,
it is not realistic to use photoelectric peaks at high energies forthis purpose. Above 150 keV, photoelectric absorption hardly
occurs in Si; furthermore, absorption events would be infrequent even in CdTe when the trigger logic requires event coincidence in both the Si and CdTe detectors.
We propose a calibration method using Si-CdTe two-hit
events, which are major events of the Compton camera. The
following is brief description of the calibration procedure. First,
a temporary gain correction function from the pulse heightvalue to the energy is determined by using photoelectric peaks
below Esup = 136 keV ( 57 Co) for every readout channel. Above
Esup , the functions are linearly extrapolated. At this step, we
have obtained gain functions which are valid below Esup . Second, by using the two-hit events that satisfy E1 + E2 = Eline
within a certain error and E1 < Esup , we fit gain functions of
the CdTe detectors represented as third-order polynomials, regarding a true value of E2 as Eline − E1 . Here, we have applied
gamma-ray lines of 356 keV (133 Ba), 511 keV (22 Na), and 662
keV (137 Cs). Final gain correction functions of the CdTe detectors are obtained in the second step. Third, we fit gain functionsof the Si detector above Esup , regarding a true value of E1 as
Eline − E2 , where the calibrated values of E2 at the second step
are used. Consequently, we have obtained well-calibrated gain
functions of all the detectors in the entire energy range.
Figure 3 shows correlations between E1 (Si) and E2 (CdTe)
using the temporary gain functions at the first step and thoseat the final step for the 137 Cs data. After the calibration, thetwo-hit events are distributed along the line E1 + E2 = Eline(662 keV). This calibration method can be cross-checked bycalibration test pulse input to the readout ASIC.

Figure 1: Schematic drawing of the Si/CdTe Compton camera.

Figure 2: Photograph of the CdTe-DSD tray.

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gies of Kα or Kβ lines of Cd or Te are merged. Otherwise,
there are two sequence possibilities: Si-CdTe1-CdTe2 or SiCdTe2-CdTe1. Since the second interaction obeys the Compton
kinematics and the positions of the first and second interactions
give the incident direction, we can calculate ∆θ of the second
scattering for the two possibilities. We select the sequence thatyields the smaller value of ∆θ, which is physically more consistent with the true sequence.
To check the correctness of the three-hit event analysis,Monte Carlo simulations were performed using the same detector setup for 662 keV photons. We selected three-hit events
showing no escape from the camera, but eliminated events including a hit at the top CdTe detector because such events arecontaminated by hits at the Si detector of photoelectron or fluorescence from CdTe. The simulation results show that the 95%
of Si hits is generated via Compton scattering. In such events(including a Compton at Si), 61% is correctly sequenced, 23%
contains a Si hit as the first Compton but wrong CdTe-hit order,and 16% contains a Si hit as the second Compton. The reconstruction accuracy of the three-hit events is 0.5-0.6, and this is
acceptable for imaging applications because wrong events only
contribute a pedestal level of the back-projection images.

Figure 3: Correlations between E1 (Si) and E2 (CdTe) of 137 Cs data. The left
panel uses the temporal gain function at the first calibration step; the right panel
uses the final calibration functions. The lines E1 + E2 = Eline (662 keV) aresuperposed.

3.2. Event reconstruction and depth sensing
Before reconstruction of the scattered events, multiple signals in a DSD are reconstructed into a single hit. In the simplest
case, a photon interaction generates two signals on one side andthe other side. The hit position is determined as the intersectionof the two strips, and the energy is taken from the P-side signal
for the Si-DSD and from the Al-side signal for the CdTe-DSDs
because these sides yield better spectral performance than the
opposite sides. Since a charge-shared event, in which adjacentstrips generate signals from a single photon hit, is not negligible
for CdTe detectors[13], it is necessary to cluster such adjacentstrip signals. By clustering, we improve the detection eﬃciencyof the 662 keV Compton events by a factor of 3.7 and that of
the 356 keV Compton events by a factor of 1.7.
Depth sensing in the CdTe-DSD is an optional but interesting data reconstruction to improve the angular resolution. Dueto the ineﬀective charge transport properties of holes in CdTe
semiconductor and distorted weighting potential by small strip
electrodes [14], the charge collection eﬃciency on the cathode(Pt-side) is significantly lower than that of anode (Al-side) (SeeFig. 8 of Odaka et al. (2010) [15]). By utilizing this eﬀect, weare able to estimate the interaction depth in the detector to improve the position resolution. In this work, we equally divided
the 750-µm-thick CdTe detector into three volumes with thicknesses of 250 µm, which is equal to the strip pitch, and assigned
the hit positions according to the ratio of the cathode-measuring
energy to the anode-measuring energy.
After the hit reconstruction, we perform event reconstructionof a set of hits in multiple detectors. The object of the analysis
is events that hit in Si once and CdTe once (two-hit events) andevents that hit Si once and CdTe twice (three-hit events). An
event including four or more hits is ineﬀective and negligible.
For the two-hit events, we can safely assume that the Si hit isCompton scattering and the CdTe hit is absorption when thesum of the energies agrees with the line energy.
For the three-hit events, we determine the hit sequence asfollows. The Si hit is regarded as the first Compton scattering,and the unsettled two CdTe hits should be ordered. Since fluorescence of CdTe (fluorescence yields ∼0.5) can constitute thethree-hit events, two hits in CdTe-DSDs that agree with ener-

4. Imaging performance
We evaluated the angular resolution of the Compton camera
by measuring gamma-ray point sources located at a distanceof 315 mm from the detector head. Figure 4 shows the ARM
distribution at 662 keV (137 Cs). Event reconstruction was performed as described in §3, and the depth sensing analysis was
also applied to assign the hit position in the CdTe-DSDs. To
quantify the resolution, we fit the ARM distribution to a Voigt
function (a convolution of a Lorentzian with a Gaussian function), and obtained its FWHM of 3.5 ± 0.1 degrees. Due tothe well-calibrated energy scale, the ARM distribution does not
display apparent systematic biases. In Figure 5, we extract aback-projection image of the cesium source from the same dataof Fig. 4. The image shows the spatial resolution of 20 mm(FWHM) at a distance of 315 mm, being consistent with theangular resolution of 3.5◦ . At lower energy, the camera still has
a high angular resolution of 4.5◦ at 356 keV (133 Ba).
Figure 6 shows the angular resolution as a function of the distance between the detectors of the first (Si) and second (CdTe)interactions for the 662 keV events, 356 keV events, and 662
keV events without the depth sensing analysis . We also show
the contributions by the position uncertainty, which can be estimated by the detector position resolution [16], for two diﬀerent
depth resolutions in the CdTe-DSDs. The angular resolution
improves with the distance between the two detectors because
the angular uncertainty of the scattered photon direction getssmaller. Thus, the depth sensing analysis of the CdTe-DSD isparticularly important for the top layer and promising approach
to higher imaging performance. However, the experimental results show slight improvement probably because the depth accuracy is not suﬃcient and other contributions such as energy
uncertainty and the Doppler broadening eﬀects hide them.
3

250
200
150
100
50
0
-10

-5

0

5
10
ARM [degree]

Figure 4: ARM distribution of 137 Cs 662 keV events. FWHM of the fitted Voigt
function (solid line) is 3.5 ± 0.1 degrees.

Figure 7: Energy-selected gamma-ray images extracted from the simultaneous
measurements of three radioactive isotopes labeled in this figure. The distanceto the sources from the camera is 1−1.3 m. The image is reconstructed by the
LM-EM-ML method and translated to a combined fisheye picture with 180◦
field of view.

y [mm]

1.4

100

Finally, we demonstrate simultaneous multi-energy-source
imaging using the Compton camera. We measured three different point sources, 133 Ba, 22 Na, and 137 Cs at the same time,and separately reconstructed Compton images by selecting theline energies. Figure 7 shows the gamma-ray images superposed on a 180◦ field-of-view fisheye picture of the laboratory. To make the image, a list-mode expectation-maximizing
maximum-likelihood method [17, 8] is applied, and only a
few hundred events are suﬃcient to reconstruct the point-likesources. This is advantage of the high resolution and recovers relatively low detection eﬃciency of 1.7 × 10−4 at 662 keV.
Basic performance tests of this camera aimed at practical applications have been conducted by Takeda et al. [18].

1.2
1

50

0.8
0
0.6
-50
0.4
-100

0.2
-100

-50

0

50

100
x [mm]

Figure 5: Back projection image of a point source of 137 Cs (662 keV) located at
a distance of 315 mm. The image is presented in ten levels of grey linear scalewith contours, showing a spatial resolution of 20 mm (FWHM).

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Angular resolution [degree]

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Distance between Si and CdTe [mm]16

Figure 6: FWHMs of the ARM distributions for diﬀerent event groups as functions of the distance between the Si detector of the first interaction and the
CdTe detector of the second interaction. The top CdTe layer and the bottom
CdTe layer correspond to values of 4 mm and 16 mm, respectively. The filled
circles and filled squares shows the events of 662 keV and of 356 keV, respectively. The open circles are extracted from the events of 662 keV without thedepth sensing analysis. The thick solid line represents estimated contributionsof position uncertainties of detectors without depth resolution. The dotted line
is similar estimation if the detector has depth resolution of 250 µm.

4