1

Experimental results of the gamma-ray imaging
capability with a Si/CdTe semiconductor
Compton camera
Shin’ichiro Takeda, Hiroyuki Aono, Sho Okuyama, Shin-nosuke Ishikawa, Hirokazu Odaka, Shin Watanabe,
Motohide Kokubun, Tadayuki Takahashi, Kazuhiro Nakazawa, Hiro Tajima, Naoki Kawachi

Abstract—A semiconductor Compton camera that combines
silicon (Si) and Cadmium Telluride (CdTe) detectors was developed, and its imaging capability was examined with various
kinds of gamma-ray targets such as a point source, arranged
point sources and an extended source. The camera consists of
one double-sided Si strip detector and four layers of CdTe pad
detectors, and was designed to minimize the distance between ascatterer and the target. This is because the spatial resolution
with Compton imaging improves as the target approaches thescatterer. This new camera realizes a minimum distance of
25 mm. By placing the target at a distance of 30 mm from thedetector, resolving power better than 3 mm was demonstrated
experimentally for a 364 keV (131 I) gamma-ray. Positional determination with accuracy of 1 mm was also demonstrated. As adeconvolution method, we selected the iteration algorithm (called
List-Mode Expectation-Maximizing Maximum Likelihood), andapplied it to several kinds of experimental data. The Compton
back projection images of the arranged point sources and an
extended object were successfully deconvolved.
Index Terms—Gamma-ray imaging, Silicon strip detector,
CdTe detector, Compton Camera

I. I NTRODUCTION
OMPTON imaging is an attractive technology used in
gamma-ray detection for various applications. In highenergy astrophysics, high-sensitivity gamma-ray observation isrequired for studying nucleosynthesis and particle acceleration,
the phenomena of which are widely observed throughout thevast universe. The technology is also applicable to nuclear
medical imaging and/or non-destructive inspection [1] [2],since it can directly locate the position of gamma-ray signals
with a wide field of view, as well as a wide energy band.
We have developed a Compton camera – the Si/CdTe
Compton camera [3]–[6] – based on the combination of
Silicon (Si) and Cadmium Telluride (CdTe) semiconductor
detectors. It consists of many layers of thin Si and CdTeCS. Takeda, H. Aono, S. Ishikawa, H. Odaka, S. Watanabe, M. Kokubun,
and T. Takahashi are members of the Institute of Space and Astronautical
Science, Japan Aerospace Exploration Agency, Kanagawa 229-8510, Japan,
and also the department of physics, University of Tokyo, Tokyo 113-0033,
Japan (e-mail: takeda@astro.isas.jaxa.jp)
S. Okuyama and K. Nakazawa are members of department of physics,
University of Tokyo.
H. Tajima is with the Stanford Linear Accelerator Center in Menlo Park,CA 94025 USA.
N. Kawachi is from the Quantum Beam Science Directorate, Japan Atomic
Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan.
This work was supported in part by Gumma University’s Faculty of
Medicine and the Japan Atomic Energy Agency under grant JSPS COE(biomedical research using accelerator technology).

Fig. 1.

Photograph of the Compton Camera

position-sensitive detectors. The design leads to a high-angularresolution thanks to the high-energy and position resolutionof these semiconductor detectors, including dedicated lownoise analog ASICs. Si is a suitable scatterer material, given
its smaller Doppler broadening effect than that of any othersemiconductor devices [7]. This effect is caused by the nonzero momenta of electrons, and imposes a theoretical limit
on angular resolution. CdTe works very nicely as an absorber
thanks to its large atomic numbers (48, 52) and high density(5.8 g/cm3 ).
In our previous work, we developed a prototype Compton
camera that was primarily designed for future balloon-borne
astrophysics experiments [8], and verified its performance.
The Compton reconstruction was successfully performed andthe gamma-ray images of point sources were obtained from662 keV down to 59.5 keV. The typical resolution of scattering
angle (the Angular Resolution Measure or ARM) was 3.5 ◦Full-Width-at-Half-Maximum (FWHM) and 2.5 ◦ (FWHM) at
356 keV and 511 keV, respectively.
A Compton camera with an angular resolution of a few
degrees is an attractive detector applicable not only to astrophysics, but also to nuclear medical imaging. Such applications require spatial resolution at the mm level. In this paper,
we report the experimental results of Compton imaging using
a new detector designed for high spatial resolution in shortdistance imaging. Fig.1 shows a photo of the detector system.
Section II describes our approach to obtaining high spatial
resolution. Section III describes the key detectors mounted
on the Compton camera and the camera’s design. Section IV

2

VA64TA
ΔX1 = (d1/d2)(ΔX2)

ΔX2

d1

A
VA64Tverse)(the red2Δθ

1st scatterer

2nd scatterer

1st scattererΔθ

2nd scatterer

absorber(a)absorber(b)Fig. 2. Schematic of a Compton camera consisting of two scatterers and one
absorber. Case (a) has a factor of (d1/d2) better spatial resolution, as derived
from the angular resolution multiplied by the distance.

Fig. 3.

Photograph of a DSSD board

Fig. 4.

Photograph of a 4-layer stacked CdTe module

describes the procedure of Compton imaging, and Section V
summarizes the imaging results.
II. D ESIGN APPROACH TO HIGH SPATIAL RESOLUTION
In order to achieve the high spatial resolution of a shortdistance target by using Compton imaging, designing the arrangement of detectors requires special care. Fig. 2 shows theschematic of a Compton camera consisting of two scatterers
and one absorber. A gamma-ray photon emitted from a target
is detected through two major scenarios: (a) scattered in the 1st
scatterer or (b) in the 2nd scatterer, and then absorbed in theabsorber detector. The direction of incident photons is obtained
as a cone in the sky (Compton cone) based on information
about the interaction positions and related energy deposits inboth the scatterer and absorber. By simply assuming that cases(a) and (b) have the same angular resolution, case (a) has
a factor of (d1/d2) better spatial resolution, as derived fromthe angular resolution multiplied by the distance. Generally
speaking, the spatial resolution in Compton imaging gradually
deteriorates as the distance increases between the target andscatterer.
In our previous Compton camera [8], we used a Si scatterer
consisting of four layers of a double-sided silicon strip detector (DSSD described in Section III-A) in order to improvedetection efficiency [8] [9]. For astrophysical imaging, only
the angular resolution and detection efficiency are essential
since the targets are located virtually indefinitely in the far
distance. Conversely, in a short-distance application, the 2nd to
4th layer of the scatterer degrades the spatial resolution, given
the increasing distance to the target. Therefore, we employed
one DSSD layer for the scatterer in the Compton camera
described in this paper. It is also important to minimize thedistance between 1st scatterer and a target. When considering
the few degrees of angular resolution, the interval must be
limited within a few tens of mm to obtain mm-scale spatial
resolution. In our case, a minimum distance of 25 mm to atarget was achieved.
III. C OMPTON CAMERA SETUP
A. Double-sided silicon strip detector (DSSD)
As a scattering part of the Compton camera, we developed alow-noise double-sided silicon strip detector (DSSD) [9]–[12].

Fig. 3 shows a photograph of the DSSD board. It consistsof one DSSD and two low-noise, low-power consumption
analog VLSIs (VA64TA1s [13] developed in collaboration
with Gamma-Medica Ideas) and mounted on a small circuitboard. The DSSD used here is a 2.56-cm wide and 500µm thick device developed in conjunction with Hamamatsu
Photonics, Japan. The strip pitch is 400 µm with a strip
gap of 100 µm on each side. The N-side has a floating pimplantation between the strips to isolate the adjacent strips.
The Al electrodes are directly coupled on each strip withohmic contact.
In order to extend the imaging band down to low-energy
gamma-rays, low-noise read-out to achieve a lower detection
threshold is needed on both the P-side and N-side strips of
the DSSD. This is because we need both the P-side and Nside signals to obtain two-dimensional positional information.
Thus, a DC-coupled readout is employed for both sides. To
apply a reverse bias, the N-side circuit board is biased as awhole. The applied bias voltage is 250 V. The decouplingis held on the digital interface after the analog signal isdigitalized by the ADC. The energy resolution at 59.5 keV isfound to be 1.6 keV (FWHM) for the P-side and 2.8 keV for
the N-side. From the noise level measured on the N-side, we
calculated the possibility of incorrectly identifying the position
due to random noise, and found that it is less than 10% for a
5 keV energy deposit for this device.

3
Z

C. Setup of the Compton camera
We constructed the new Compton camera (shown in Fig.1)
by combining the DSSD and 4-layer stacked CdTe module.
Fig. 5 shows a cross-sectional view of the camera. The 4-layer
stacked CdTe module is arranged 14.7 mm underneath the
DSSD. The minimum distance to the target (dmin ) is 25 mm
due to the entrance window provided for thermal decoupling.
Fig. 6 shows a schematic diagram of the readout system.
The readout VA64TA1s are divided into four groups: DSSDPside, DSSD Nside, CdTe-A, and CdTe-B. In the CdTe-A and
CdTe-B groups, the eight VA64TA1s are arranged in a daisy
chain configuration. The readout sequence of each group iscontrolled by a dedicated Read Out CTL Unit (ROU). The
ROU starts the readout sequence by using an external readstart trigger. The event trigger from the Compton camera
is fed to the TRIG CTL Unit (TCU) that consists of NIM
modules in this version. In the TCU, a coincidence judgment
is made between the trigger of DSSD and that of CdTe forthe effective acquisition of data on Compton events. The
coincidence judgment output is then returned to the ROU asa read start trigger. Since this trigger is sent to all ROUs, thepulse heights of all channels are acquired for a coincidence
event. The pulse heights converted into digital data in the ROU
are then sent to DATA&Command Unit controlled by a PC via
the VME bus.
IV. P ROCEDURE FOR C OMPTON IMAGING
Data reduction and image reconstruction are performed forCompton imaging. First we select ”two hit events”, meaning
one hit in a DSSD and one in a CdTe detector. Here, the energy
threshold to determine a hit is set to 10 keV for both the
DSSD and CdTe detectors. Obtaining the Compton events of

d

X
DSSD
14.7

The absorber consists of a total 16 CdTe pad detectors
based on the technology of a high-resolution CdTe Schottky
diode [14]–[16]. The production and pixelation of CdTe were
performed by ACRORAD, Japan. Each detector is 13.2-mm
wide and 500-µm thick. In the CdTe detector, a common
electrode made of Indium (In) is used as the anode, andpixelated electrodes divided into 64 pads made of Platinum(Pt) are used as the cathode. The size of each pad of thecathode electrode is 1.35 mm2 , with a gap of 50 µm between
the electrodes. A guard-ring electrode surrounds the pads. Dueto a high Schottky barrier and the guard-ring electrode, leakage
current as low as 10 pA per pad is obtained under bias voltageof 700 V. Good energy resolution is realized thanks to thehigher efficiency of charge collection.
Fig. 4 shows the 4-layer stacked CdTe module with a stack
pitch of 2 mm [16]. One layer consists of four CdTe pad
detectors configured in 2×2 geometry and two FECs (Front
End Cards) on which four VA64TA1s are mounted. Energy
resolution of 2.0 keV (FWHM) is achieved at 81 keV and thatof ∆E/E ∼1% (FWHM) is at 511 keV under temperature of–20◦ C, and bias voltage of 600 V. Under these conditions, nosignificant degradation in spectrum performance was observed
during continuous operation of up to two weeks.

(d m in = 25 m m )B. 4-layer stacked CdTe module

CdTe

Fig. 5.

Cross-sectional view of the Compton camera

incident gamma-rays entails the selection of ”two hit events”
as follows:• The sum of energy deposits in Si (Edssd ) and CdTe (Ecdte )is equal to the incident gamma-ray energy (Ein ), that
is, Edssd + Ecdte = Ein . By considering the energy
resolution of the detectors, the events where Edssd +Ecdte
equal Ein ± 5 keV are selected.
• The events where Edssd is from 20 keV to 35 keV are
excluded. This region is contaminated by fluorescence Xray events, which are X-rays from Cd (Kα : 23.1 keV,Kβ : 26.1 keV) and Te (Kα : 27.4 keV, Kβ : 31.0 keV)
escaping from the CdTe detectors and then absorbed in
the DSSD.
With these selected events, the Simple Back Projection(SBP) image was obtained. We assumed that incident gammarays are scattered in the DSSD and absorbed in the CdTe
detector. Based on this assumption, the direction of incident
photons is calculated as;
11
cos θ = 1 − me c2 (
−
).
(1)
Ecdte
Edssd + Ecdte
The back projection of the Compton cone into the RI source
plane is performed event by event. The SBP image is thesimple accumulation of Compton cones over all events. The
overlapping position of the Compton cones is the origin of the
gamma-rays.
Conversely, in the SBP image, most parts of a cone notoverlapping with a source become the background. Imaging
capability should be improved using a deconvolution methodthat properly accounts for the response of the Compton
camera. We selected a deconvolution algorithm called ListMode Expectation-Maximizing Maximum Likelihood (LMEM-ML). Briefly, this is an iteration algorithm between image
space and data space.
λlj ∑t∑ ij l
(2)
λl+1
=
j
sj i
k tik λkwhere, λlj denotes the image-bin content at iteration level l, sj
the probability that an event emitted from j is measured, and4

TRIG CTL.Unit

Pside group
DSSD Pside signal

VA64TA

Dead Tim e
Pside Trigger

Nside group
DSSD Nside signal

VA64TADSSD ( 64ch * 2 )Coincidence
G ATE

CdTe_A Trigger
CdTe_B Trigger

Read O utCTL.Unit* 4

CdTe-A group

Pside,Nside,CdTe-A,CdTe-B
Coincidence

CdTe signal

VA64TA * 8
Read O utCTL.

CdTe ( 64ch * 16 )CdTe-B group
Trigger

Com pton Cam era

Read StartTrigger

Read
CdTe signal

VA64TA * 8

Start

Busy

DATA & Com m and Unit

ADC

VM E BusDATA
Trigger
Analog Data

DATA

DATA
BufferPC

Read
Com m andFig. 6.

Com m and

Schematic diagram of the readout system

tij the probability that a photon emitted from j is measured
with the parameters of event i (response). From image space,estimated data space is calculated with a response matrix
derived from list-mode data. By comparing the expected data
and measured data, corrections are made to image space. A
detailed description of the LM-EM-ML method is given in[17], [18].
V. R ESULTS
A. Distance dependence
As mentioned in Section II, the spatial resolution is afunction of the distance between the scatterer and target. To
demonstrate this concept, we obtained point source data inthree cases of d = 30 mm, 60 mm and 150 mm. Here, ddenotes the distance between the DSSD and the point source,
as shown in Fig. 5. The 356 keV gamma-ray source is placedon the Z-axis penetrating the center of the DSSD.
The left panel in Fig. 7 shows the distance dependence of
spatial resolution of the SBP image; the right panel showsthat of the angular resolution defined by FWHM of the ARM
distribution. We evaluated the spatial resolution as FWHM of
the SBP image by fitting using a two-dimensional Voigt function. The function is a convolution of Gaussian and Lorentzian
distribution, and offers good approximation of a point spreadfunction. We also determined the angular resolution as theARM distribution in FWHM, by using a one-dimensional
Voigt function. The obtained angular resolution was 3.6 ◦ andindependent of the distance. The spatial resolution (proportional to distance) was about 6 mm and 18 mm at d = 30 mm
and d = 150 mm, respectively. This relation is consistent withthe description given in Section II.
B. Field of view
One feature of Compton imaging is a wide field of view.
By using 356 keV and 511 keV gamma-ray point sources,
we investigated the position dependence relative to detection
efficiency. The point source was placed on the plane of d = 60
mm (see Fig. 5), and measurements were performed for several

Fig. 7. Left panel showing the distance dependence of spatial resolutionof the SBP image; right panel showing that of angular resolution defined by
FWHM of the ARM distribution for a 356 keV gamma-ray.

Fig. 8. The position dependence of detection efficiency for 356 keV and
511 keV at d = 60 mm plane. The efficiency at the center position (x,y,z) =(0,0,60) in Fig. 5 is set to 1. The radius denotes the distance from (x,y) =(0,0). The radius 100 mm corresponds to a 120◦ field of view.

positions on the plane. The detection efficiency for eachposition is obtained from the counts of events where the totalenergy deposit (EDSSD + ECdTe ) is within 356 keV±5 keV or
511 keV±5 keV.
Fig. 8 shows the position dependence of detection efficiency
normalized at the center position of (x,y,z) = (0,0,60) for
the 356 keV (left) and the 511 keV (right) gamma-rays. The
radius denotes the distance from (x,y) = (0,0). The normalized
efficiency at a radius of 100 mm (corresponding to a 120◦ fieldof view) is 40 % for 356 keV and 30 % for 511 keV.

5

C. Imaging of grid sources
To investigate the resolving power and accuracy of positional determination, an imaging test was conducted using anacrylic plate with 2 mm diameter holes arranged regularly(Fig. 9). The liquid 131 I (364 keV, 85 kBq/hole) sources were
mounted at 20-mm pitch. The variability of strength of eachsource is less than 10 %. Imaging is performed in three cases:d = 30, 45 and 60 mm.

Simple Back Projection (z=30)
9

100

8
7

50
[mm]

6
5

0

4
-50

3
2

-100

1
-100

-50

0
[mm]

50

100

Simple Back Projection (z=45)
6
100
5
50
4
[mm]

For a detailed evaluation of detection efficiency, we developed a simulator [19] based on the Geant4 toolkit. In thesimulation, we included the G4LECS [20] extension in order toestimate the effect of Doppler broadening. The mass model isconstructed not only with the detector material, but also withvarious passive support structures. The energy and position
resolutions of the detectors are parameterized based on theexperimental results. Fig. 8 shows the normalized efficiency
calculated by the simulator, together with the experimental
data. The simulation reproduced the experimental results very
well. The absolute efficiency derived from the simulator ison the order of 10−2 % at the center position for 356 keVgamma-rays.

0

3
2

-50

1

-100
-100

-50

0
[mm]

50

100

Simple Back Projection (z=60)
4100

Fig. 10 summarizes the SBP images. The point sourceswere clearly resolved. The outside sources become visible as dincreases, since the sources enter the field of view. Resolving
power better than 20 mm was demonstrated up to a distanceof d = 60 mm, and larger than an 80◦ field of view as well.
We performed image deconvolution by using the LM-EMML method. A total of 1×105 events are utilized in thisprocess. A correction factor (sj ) in Equation 2 is derived fromthe position dependence of detection efficiency calculated by
the simulator mentioned in Section V-B. Fig. 11 shows thedeconvolution results for the experimental data obtained at d
= 60 mm. The left panel shows the SBP image; the rightpanel shows the deconvolved image after the 100th iteration.
In the deconvolved image, the point sources placed at theedge become apparent, while only sources located around thecentral region are resolvable in the SBP image. The bottom
panel is the projection of the rectangular region shown ineach image to the Y-axis in Fig. 11. Following deconvolution,
the peak area of each source becomes stable, thanks to
the deconvolution technique that included the correction ofefficiency.

3

50

2.5
[mm]

Fig. 9. Acrylic plate with 2 mm diameter holes arranged regularly. The
liquid 131 I (364 keV) sources were mounted at 20-mm pitch.

3.5

0

2
1.5

-50

1
-100

0.5
-100

-50

0
[mm]

50

100

Fig. 10. SBP image of the grid sources in case of d = 30 (top), 45 (center)and 60 (bottom) mm. The liquid 131 I (364 keV) sources were mounted witha pitch of 20 mm.

It is also important to study the accuracy of positional
determination. Therefore, we first derived the center positionof the sources from the deconvolved image shown in the rightpanel in Fig. 11. For the 25 sources arranged in 5×5, the center
position of each source was determined by calculating theweighted center around the largest pixel. The center positions
are plotted as shown Fig. 12 with open squares. Then basedon the assumption that the sources are at the intersection of

6

Simple Back Projection

iteration 100
14

60

60
300

40
20

250
20

10

0

8

-20

[mm]

[mm]

40

12

200

0

150

-20

100

6
-40
-60

-40
4
-60

-40

-20

0
[mm]

20

40

60

[mm]

-60

50

-60

-40

-20

0
[mm]

20

40

60

[mm]

Fig. 11. Deconvolution results for experimental data (d = 60 mm in Fig. 10). The left panel shows the SBP image; the right panel shows the deconvolved
image after the 100th iteration. The bottom panel is the projection of the rectangular region of each image to the Y-axis.

cross lines at a right angle, we fit the center positions. The red
grid in Fig. 12 shows the fitting results. The bottom panel ofFig.12 summarizes the differences of these positions of all 25
sources as distributed within ∼1 mm. Thus, this experimentdemonstrates positional determination accuracy of 1 mm forthe point source.

Fig. 12. Top: measurement values of source positions (open square) derived
from the deconvolved image shown in the right panel of Fig. 11 in the grid(cross lines). Bottom: distribution of the difference of all 25 sources.

7

Simple Back Projection

Since the ability to image extended object is a key requirement for both astrophysical and medical applications, weprepared an extended target as shown in Fig.13. The liquid 131 I
source (364 keV, 1.6 MBq) is soaked in paper and shaped likean inverted ”C” with a gap of 3 mm. The target was placedon the d = 30 mm plane.

30
12

20

10

10
[mm]

D. Extended object

8

0

6

-10

4

-20
-30

3 mm

-30

-20

-10

0
[mm]

10

20

30

2

iteration 10
30
2500
20

Fig. 14 shows the images obtained. The top panel shows
the SBP image; the bottom panel shows the 10th iteration
using the LM-EM-ML method. The shape with a gap of 3 mm
is recognized even in the SBP image. With the LM-EM-ML
method, the shape is successfully deconvolved. Thus, resolving
power better than 3 mm is demonstrated experimentally.

VI. C ONCLUSION
By combining the DSSD and a 4-layer stacked CdTe
module, we developed a new Compton camera designed toachieve high spatial resolution. As explained in Section II,
the spatial resolution gradually deteriorates as the distanceincreases between the target and scatterer. Therefore, thecamera is designed to minimize this distance. A minimum
distance of 25 mm was realized. Studies were conducted on
the field of view, spatial resolution, the accuracy of positional
determination, and resolving power. Using the experimental
data, resolving power better than 3 mm was demonstrated fora 364 keV (131 I) gamma-ray placed 30 mm away from thesensor. Positional determination accuracy of 1 mm was also
demonstrated experimentally. Furthermore, the imaging of an
extended object was also successfully performed.
As the deconvolution method, we selected an iteration algorithm called List-Mode Expectation-Maximizing Maximum
Likelihood (LM-EM-ML). It was applied to several kindsof experimental data. The Compton back projection images
derived from the arranged point sources and the extended
object were successfully deconvolved.
The results of this imaging test suggest that the Si/CdTe
Compton camera is an attractive detector not only for astrophysics application, but also for medical imaging. The
data obtained by the experiments also proved effective ininvestigating the detailed response of a Compton camera usedin astrophysics.

2000

10
[mm]

Fig. 13. Photograph of the extended target. The liquid 131 I source in soakedin paper, and shaped like an inverted ”C” with a gap of 3 mm.

1500

0

-10

1000

-20

500

-30

-30

-20

-10

0
[mm]

10

20

30

Fig. 14. Results of extended target imaging. The distance to the target is d
= 30 mm. The top panel shows the SBP image; the bottom panel shows thedeconvolved image after the 10th iteration.

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