A Prototype Si/CdTe Compton Camera and thePolarization Measurement
Takefumi Mitani, Takaaki Tanaka, Kazuhiro Nakazawa, Tadayuki Takahashi, Takeshi Takashima,
Hiroyasu Tajima, Hidehito Nakamura, Masaharu Nomachi, Tatsuya Nakamoto, Yasushi Fukazawa

Abstract— A Compton camera is the most promising approachfor gamma-ray detection in the energy region from several
hundred keV to MeV, especially for application in high energy
astrophysics. In order to obtain good angular resolution, semiconductor detectors such as silicon, germanium and cadmium telluride(CdTe) have several advantages over scintillation detectors,
which have been used so far. Based on the recent advances of highresolution CdTe and silicon imaging detectors, we are working ona Si/CdTe Compton camera. We have developed 64-pixel CdTe
detectors with a pixel size of 2 mm×2 mm and double-sided
Si strip detectors(DSSDs) with a position resolution of 800µm.
As a prototype Si/CdTe Compton camera, we use a DSSD as ascatterer and two CdTe pixel detectors as an absorber. In orderto verify its performance, we irradiate the camera with 100%
linearly polarised 170 keV γ-rays and demonstrate the system
works properly as a Compton camera. The resolution of thereconstructed scattering angle is 22◦ (FWHM). Measurement ofpolarization is also reported. The polarimetric modulation factor
is obtained to be 43%, which is consistent with the prediction ofMonte Carlo simulations.
Index Terms— Compton Camera, Semiconductor Detectors,
CdTe, polarimeter.

I. I NTRODUCTION
S gamma-ray energies increase to the MeV region, thedetection of photons, the determination of the incident
direction, and the rejection of background become very difficult. The use of the technique of the Compton telescope seems
to be the only way for the detection of MeV γ-rays. The
development of the technique began in 1970s in order to explore
the γ-ray universe in this energy band [1], [2]. The COMPTEL
instrument on-board the CGRO satellite, which was launched
in 1991, achieved pioneering results with this technique [3]. In
order to go beyond the level achieved by the COMPTEL, thenext generation of Compton telescope needs to be developed.

A

Manuscript received November 15, 2003; revised April 15, 2004.
T. Mitani, T. Tanaka, and T. Takahashi are with the Institute of Space andAstronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA),
Sagamihara, Kanagawa 229-8510, Japan, and also with the Department of
Physics, the University of Tokyo, Bunkyo, Tokyo 113-0033, Japan (telephone:81-42-759-8510, e-mail:mitani@astro.isas.jaxa.jp).
K. Nakazawa and T. Takashima are with the Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA),
Sagamihara, Kanagawa 229-8510, Japan.
H. Tajima is with Stanford Linear Accelerator Center, Menlo Park, CA 94025
USA.
H. Nakamura and M. Nomachi are with the Laboratory of Nuclear Studies,
Osaka University, Toyonaka, Osaka 560-0043, Japan
T. Nakamoto, Y. Fukazawa are with Department of Physics, Hiroshima
University, Higashi Hiroshima, Hiroshima 739-8526, Japan

In a Compton telescope, an incident gamma-ray photon isscattered in the scattering part of the detector and the Comptonscattered photon is absorbed in the absorber part (see Fig. 1(a)).
The scattering angle θ of the incident gamma-ray is then written
as,
me c2
me c2
+
,
(1)
cos θ = 1 −E2
E1 + E 2
where E1 is the energy of the recoil electron in the scattering
detector and E2 is the energy deposited in the absorbing
detector. Thus, by measuring the energy and position of theindividual interactions, we can limit the direction of the incident
photon to the surface of a cone. The projection of this cone onto
the celestial sphere is the “event circle”. The γ-ray source islocated at the point of the intersection of all event circles. As
clearly seen in the equation, the angular resolution becomes
better if the energy resolution becomes higher.
Since the major goals for the next generation Compton
telescope are to increase the detection efficiency by up to
two orders of magnitude over the COMPTEL and to realize
much improved background rejection, the development of 3D position sensitive detectors with a high detection efficiency,
a high angular resolution and an excellent energy resolution
is the key. Several detector options are under consideration,
including the use of position-sensitive solid state detectors suchas germanium and silicon [4]–[12]. As to applications to nuclear
medicine, see D. Meier et al. [13] and references therein.
We have been working on a semiconductor Compton telescope based on silicon and cadmium telluride (Si/CdTe Compton camera) in view of sub-MeV to MeV detectors with highsensitivity [11], [12]. In this paper, we demonstrate the first
results from a prototype Si/CdTe Compton camera. Firstly
we describe the performance of newly-developed CdTe pixel
detectors which are used as the absorber part of the prototype.
Secondly, the reconstruction of the scattering angle and apolarization measurement are reported based on experiments
at the SPring-8 synchrotron radiation facility in Japan. Further
report on the imaging performance of the Si/CdTe Compton
camera will be described in a separate paper [14].
II. S I /C D T E C OMPTON CAMERAIn the Si/CdTe Compton camera, a stack of many thin
scatterers made of fine pitch double-sided Si strip detectors
is used to record the Compton scatterings. Additional layers
made of a heavy semiconductor such as CdTe (or CdZnTe) areused to absorb the Compton scattered photons. The order of theinteraction sequences, hence the correct energy and directionof the incident photon, can be reconstructed by examination of
the energy-momentum conservation for all possible sequences(Fig. 1(b)).
For imaging devices, their good energy resolution and theability to fabricate compact arrays are very attractive features ofsemiconductor detectors in comparison with inorganic scintillation detectors. The high density and good detection efficiency
of CdTe and CdZnTe, comparable with that of NaI(Tl) andCsI(Tl), are important additions when we design the absorberpart of the Compton telescope [11]. Based on our recent
developments, the CdTe diode now shows an energy resolution
comparable with that of a Ge detector, even at room temperature[15]–[17]. The energy resolution achieved for a planar diodedetector, which has dimensions of 2 mm × 2 mm as an active
area and a thickness of 0.5 mm, is 0.93 keV and 1.2 keV(FWHM) at 59.5 keV and 122 keV, respectively.
The merit of using Si as scatterers in the energy range from50 keV up to several hundred keV is that the cross sectionof Compton scattering is well above that of photoelectric
absorption. Therefore the efficiency of Compton scattering ishigh if the scattering part has sufficient thickness. Additionally,
Si works better than CdTe in terms of the “Doppler broadening”
effect. The standard Compton formula Eq. (1) assumes thescattering takes place with a free electron at rest. When we
take the momenta of bound electrons into account, we needto consider additional uncertainties in the energy and angleof the scattered gamma ray [18]. This phenomenon is often
referred to as Doppler broadening. The additional uncertainties
on the energy resolution and angular resolution are especiallypronounced at low gamma ray energies and become lesssignificant as the atomic number decreases. Together with thehigh absorption efficiency of CdTe, a Si/CdTe Compton camera
can be used efficiently below several hundred keV in particular.
Application of a stack of Si detectors to higher γ-ray energies,
where multiple Compton scattering has to be taken into account,is described in [4], [5], [11].
III. H IGH RESOLUTION C D T E PIXEL DETECTOR AND
D OUBLE - SIDED S I S TRIP D ETECTOR
As a key component of a Si/CdTe Compton camera, we havedeveloped a CdTe pixel detector (Fig. 2(b)). It is made of a
CdTe diode and has the dimensions of 16.5 mm × 16.5 mm as
an active area, surrounded by a 1 mm wide guard-ring, and a
thickness of 0.5 mm. On the cathode side, it is pixellated with
2 mm square electrodes. The gap between pixels is 50 µm.
A fanout board, which consists of bump pads and patterns toroute the signal from pads on the surface of the ceramic board,is shown in Fig. 2(a). Once each pixel electrode on the CdTe
diode is connected to the bump pad, we can extract signals fromthe detector. The leakage current of this device is 200 pA/pixel
at a bias voltage of 600 V and a temperature of 20 ◦ C . For thereadout of many channels from the detector, we utilize a newly
developed low noise analog ASIC, “VA32TA” [19], [20]. We

Fig. 1. (a) Concept of a Compton camera. When an incident photon undergoes
a Compton scattering and the scattered photon is absorbed completely, thescattering angle can be calculated with Eq. (1) using the energy deposited.
From the interaction position and the scattering angle, the direction the photon
comes from can be constrained. (b) Concept of a semiconductor Compton
telescope. It is a stack of position sensitive detectors made of semiconductormaterials. The upper layers work as scatterers and the lower layers work as
absorbers.

handle 64 channels as a module and read out with the Front
End Card (FEC) on which two VA32TAs are mounted.
We developed a first prototype detector and obtained spectrafrom each channel. Fig. 3 shows the sum of the energy spectra
for all channels except for seven channels at the corner inwhich no signal was obtained. The detector is operated at room
temperature (20 ◦ C ) under the bias voltage of 600 V. We obtainan energy resolution of 2.5 keV (FWHM) for the 122 keV γ-raylines. Fig. 4 shows the distribution of the energy resolution of
all channels. It ranges from 2.1 to 3.4 keV. When the operating
temperature is 0 ◦ C , the highest energy resolution of 1.7 keV
at 60 keV is obtained.
Although we have demonstrated that our bump method
provides high yield when we handle small pixel size [21],[22], no signal was obtained for seven channels out of 64
of the first detector due to a failure in the procedure ofconnecting the pixels to the fanout board. To improve theyield of the connections, we have fabricated another batchof detectors in different conditions of bump bonding. In thisversion, the yield of the connections has increased to 99.6%, but
the energy resolution deteriorates to ∼ 6 keV under the same
operating conditions at 20 ◦ C . We are working to improve thisperformance. Despite the relatively poor energy resolution, we
use detectors from the second batch in the following experimentbecause they provide larger acceptance than the case for the firstbatch.
Another key component is the low noise double-sided Si strip
detectors(DSSDs). The DSSD used in the present detector hasa thickness of 300 µm and the strips have a length of 2.56 cm
on each side. The strip pitch of DSSD is 800 µm and the strip
gap is 100µm. The signals are also processed via the same
FECs as the CdTe pixel detectors. See Tajima et al. for details

(a)Fig. 4. Distribution of the energy resolution of each pixel of the CdTe detector.

on DSSD developments [19], [20].
IV. P ROTOTYPE OF S I /C D T E C OMPTON C AMERA AND ITS
P ERFORMANCE
A. Setup
Fig. 2. (a) Layout of the fanout board for the CdTe pixel detector. It consistsof bump pads and patterns to route the signal from pads on the surface ofthe ceramic board, which has a thickness of 200 µm. (b) A CdTe 8×8 pixel
detector. The signal of each pixel is fed into VA32TAs on FEC.

Fig. 3. Summed 57 Co spectra of the CdTe pixel detector. The operating
temperature is 20 ◦ C and the bias voltage is 600 V. The energy resolution is
2.5 keV.

To demonstrate the concept of the Si/CdTe Compton camera,
two CdTe 8×8 pixel detectors are combined with one DSSD.
In Fig. 5(a), we present a schematic view of the arrangementof the detectors. In this configuration, photons scattered in the
DSSD are absorbed by the CdTe detectors if the scattering angle
is between 100◦ and 130◦ . This geometry is adopted because
the scattered photons have relatively low energy and can be
photo-absorbed efficiently in CdTe.
The CdTe detectors used in the prototype have an energy
resolution of about 7 keV on average when we operate at
a temperature of 25 ◦ C with a bias voltage of 500 V. Theenergy resolution of the DSSD used in this system is 5 keV at
a temperature of 25 ◦ C . The resolution is deteriorated dueto the rather high operation temperature. As to the DSSD,“RC chips”, which provides bias voltage via polysilicon bias
resistors and AC-coupling between each strip and the input of
VA32TA, could also be the cause of the degradation of thespectra. Another type of DSSD we have developed with a strip
pitch of 400 µm has achieved an energy resolution of 1.3 keV
at a temperature of 0 ◦ C without “RC chips” [19], [20].
Two CdTe pixel detectors and a DSSD are read out together
by the specially designed compact readout system including an
ADC, which is controlled with a fast serial interface, “Space
Wire (IEEE 1355)” [23]. During data acquisition, events aretaken when trigger signals are generated from CdTe pixel
detectors. The data from triggered CdTe detectors and the
DSSD are read out.
We conducted a photon beam experiment at the synchrotron
radiation facility, “SPring-8” [24] in Japan on 7 July, 2003. The
initial beam energy was 270 keV. To reduce the intensity of theinformation of the collimator and fix the hit channel in thefollowing analysis.
B. Reconstruction of Compton scattering angle
From the data obtained, we reconstruct the Compton scattering angle. Fig. 6 shows the distribution of θComp −θgeom for the
events under the conditions 150 keV< ESi + ECdTe <180 keV,
ESi > 30 keV and 90◦ < θComp < 180◦ . Here, ESi , ECdTe
are the energy deposited in the DSSD and the CdTe detectors,
respectively. The scattering angle θComp is calculated from
Eq. (1) using ESi , ECdTe while θgeom is that calculated fromthe hit positions under the assumption that the incident beamis orthogonal to the DSSD surface. The FWHM of the peak in
Fig. 6, i.e. the angular resolution of the Compton camera, is
22◦ . This value is consistent with the angular resolution of 20◦
obtained from an EGS4 Monte Carlo simulation [25] with a low
energy extension [26]. From the analysis of the simulation, thecontribution to this angular resolution of the detector positional
resolution is estimated to be 6.2◦ , that of the energy resolution
17◦ , and that of Doppler broadening 4.7◦ .
Although the basic performance of a Si/CdTe Compton
camera is demonstrated, the angular resolution is not so pronounced. If the energy resolution of both the DSSD and CdTe
pixel detectors reaches their present best value, 1.3 keV and2.5 keV, respectively, the contribution of the energy resolution
can be reduced to 3-4◦ . For the geometrical contribution,
improvement of positional resolution can be achieved either
by using smaller pitch devices or by increasing the distance
between the DSSD and CdTe detectors. Note that the relatively
large Doppler broadening contribution is only because of the
low incident gamma-ray energy and there is no semiconductor
currently, except for diamond, with better performance.
Fig. 5. (a) Conceptual image of the detector in top view. A DSSD is put on
the z = 0 mm plane, and two CdTe pixel detectors are put on the z = 10 mm
plane, where the beam spot on the DSSD is the origin. P-strips of the DSSD arearranged to be parallel to the x-axis. The original beam comes from y = +∞.
(b) Horizontal view of the setup for the photon beam experiment. To reducethe intensity, the original beam is scattered by an Al plate and collimated tothe prototype of a Compton camera. The photon beam enters the p-side of the
DSSD.

original beam, the beam was scattered by an Al plate. Photons
scattered at 90◦ are collimated with Pb collimators as an input
to the Compton camera (Fig. 5(b)). According to the Compton
kinematics, the estimated incident energy at the detector is
177 keV and the incident photons are nearly 100 % linearlypolarized. The electric vectors of the photons are parallel tothe p-strips of the DSSD. We irradiated the photon beam withthe geometry shown in Fig. 5(a). After a irradiation, the whole
Compton camera system is rotated by 90◦ around the z-axis. By
performing this procedure four times, we obtain 360◦ coverage.
During the experiment, the noise of n-strips of the DSSD washigh and we can not use the information from the n-strips. We
estimate the hit channel of the n-side on the basis of geometricalFig. 6. The distribution of θComp −θgeom for the events under the conditions
150 keV< ESi + ECdTe <180 keV, ESi > 30 keV and 90◦ < θComp <180◦ . The angular resolution is calculated to be 22◦ (FWHM).

Fig. 7. Experimental φ distribution corrected with a φ distribution obtained
from a simulation in which unpolarized photons are incident on the prototype.
The modulation factor is found to be 43%.

C. Performance as a polarimeter
An important feature of the Compton camera is the capabilityof measuring polarization [27]. The differential cross-section ofCompton scattering is expressed as¢dσ
r2 β 2 ¡ −1= e
β + β − 2 sin2 θ cos2 φ .
dΩ
2

(2)

Here, θ is the scattering angle, φ is the azimuthal angle ofthe scattered photon with respect to the electric vector of theincident photon, β = E2 /(E1 + E2 ) and re is the classicalelectron radius. As shown in Eq. (2), photons are likely to
be scattered in a direction orthogonal to their electric vector.
Therefore the φ distribution of scattered photons provides
information of the linear polarization of incident photons.
The azimuthal scattering angle (φ) distribution of the experiment is shown in Fig. 7 and an angular modulation isclearly observed. To correct the geometrical effect related to
the layout of the detectors and the associated quantizationof possible scattering angle, the measured φ distribution isnormalized by the model distribution (Fig. 8(a)) estimated froman EGS4 Monte Carlo simulation for an unpolarized beam.
Thus, Fig. 7 includes the statistical error of both the experimentand simulation. For comparison, a φ distribution obtained fromthe results of a simulation with a 100% linearly polarized beam
is shown in Fig. 8(b), which is corrected in the same way as theactual data. In the simulation, 108 photons are incident on thedetector and 6.3×102 photons undergo one Compton scattering
followed by photoelectric absorption. The low efficiency shown
here must be improved by stacking Si and CdTe detectors.
We fit the distribution in Fig. 7, 8(b) with a formula,N (φ) = p0 + p1 cos2 (φ + p2 ),(3)which is obtained by integrating Eq. (2) over the solid angle.

Fig. 8. (a) A φ distribution obtained from an EGS4 Monte Carlo simulationin which unpolarized photons are incident on the prototype. This shows thegeometrical effect related to the layout of the detectors and the associatedquantization of possible scattering angle. The statistical error bars arise fromthe limit of simulation time. (b) A φ distribution obtained from an EGS4
Monte Carlo simulation where a 100% linear polarized beam is incident on
the prototype. The modulation factor is found to be 41%.

The polarimetric modulation factor, Q,
Q≡

Nmax − Nmin
p1=,
Nmax + Nmin
2p0 + p1

(4)

is 43(±3)% for the experimental data, which is consistent withthat of the simulation data, 41(±2)%.
V. C ONCLUSION
The hard X-ray and γ-ray bands are important frequency
windows for exploring the energetic universe. It is in theseenergy bands that non-thermal emission becomes dominant.
In order to study the “non-thermal universe”, characterized byaccelerated high energy particles, we need an advanced mission
which has orders of magnitude better sensitivity than any
previous or current missions. To this end, we are working on a
Si/CdTe Compton Camera. With two CdTe pixel detectors anda DSSD, we have constructed a prototype Si/CdTe Compton
camera. Using the prototype, we conducted a photon beam
experiment and demonstrate the concept of Si/CdTe Compton
camera. The angular resolution is 22◦ which is limited by
the energy resolution of 7 keV for the CdTe pixel detector

and 5 keV for the Si detector employed in this camera. By
employing devices with a better energy resolution, the angularresolution is expected to be improved significantly. As the first
demonstration of the Si/CdTe Compton camera as a gammaray polarimeter, we have measured polarization by using a
synchrotron photon beam. We have succeeded to obtain a veryclear modulation pattern. The polarimetric modulation factor ismeasured to be 43%. Based on these results, a semiconductor
Compton telescope utilizing Si and CdTe is shown to posses a
potential as both a Compton camera and a polarimeter.
ACKNOWLEDGMENT
The authors would like to thank P. G. Edwards and C. Baluta
for their critical reading of the manuscript.
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