Visualization of Radioactive Substances witha Si/CdTe Compton Camera
Tadayuki Takahashi, Shin’ichiro Takeda Shin Watanabe, and Hiroyasu Tajima

Abstract—Dust containing radioactive materials dispersed following the Fukushima nuclear power plant accident in March
2011. Gamma-rays are emitted in the process when unstable
nuclei in the materials decay. Based on the technology of Si/CdTe
Compton Camera, we have manufactured a quick prototype
model for the use in the field. The camera, now called a“Ultra-Wide-Angle Compton Camera” was successfully applied
to visualize the distribution of radio-active substances in the
Fukushima area.
Index Terms—Gamma-ray imaging, Semiconductor Compton
camera, Silicon double-sided strip detector, CdTe double-sided
strip detector , Hotspot monitoring

T

I. I NTRODUCTION

he Great East Japan Earthquake of March 11, 2011
caused significant damage to Japan. The radioactive
materials released from the Fukushima nuclear power plant
have had a huge impact on the Japanese people. The removalof these radioactive materials is of very high priority for Japan.
In early April 2011, we received an e-mail from Tokyo
Electric Power Company containing the request of possible
contribution to provide a detector which can broadly and atonce identify areas where strong or weak radiation exists on
the ground. Because of the accident at the nuclear power plant,
dust containing radioactive materials such as 134 Cs and 137 Cs
dispersed over a broad area [1]. Gamma rays are emitted
in the process when unstable nuclei in the materials decay
to become stable. Quick removal of dust with radioactive
materials (decontamination) is essential. To decontaminate, weneed to know where the radioactive materials are. A gammaray imaging system which is capable of directly locatingradiation sources, which enables monitoring of hotspots inotherwise inaccessible radiation sites and with considerable
radiation exposure, and is namely more effective than manual
surveys, if such a detector exists.
Over the past 15 years, we have been working on thenext generation gamma-ray detector, “Si/CdTe semiconductor
Compton camera,” with the aim of high-sensitivity observations for gamma-ray astronomy. The Soft Gammar-ray Detector (SGD) onboard the 6th X-ray Astronomy satellite, ASTROH, is now being built to cover the soft gamma-ray region up to
600 keV [2], [3]. In order to lower the background dramatically
T. Takahashi, S. Takeda and S. Watanabe are with the Department ofSpace Astronomy and Astrophysics, Institute of Space Astronautical Science,
JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan(http://www.astro.isas.jaxa.jp/ takahasi).
H. Tajima is with Cosmic-ray Research Facility, Solar-Terrestrial Environment Laboratory, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464-8601,
Japan
Manuscript received Nov. 16, 2012.

and thus to improve the sensitivity as compared to the HXDof Suzaku, the SGD design combines a stack of Si and CdTe
pixel detectors to form a Compton camera (Si/CdTe Compton
Camera) [4], [5]. SGD measures soft gamma-rays via reconstruction of the Compton scattering in the Compton camera
and covers an energy range of 40 600 keV. By combining
another new concept, “narrow FOV Compton telescope”, SGD
will have sensitivity at 300 keV of more than 10 times better
than that of the Suzaku Hard X-ray Detector [6], [7], [8].
Upon receiving a consultation from Tokyo Electric Power
Company, we swiftly constructed a prototype of the ultrawide-angle Compton camera for imaging tests in Fukushima
prefecture based on the SGD concept combined with elemental
technology for another device onboard ASTRO-H, the Hard
X-ray Imager (HXI) [9].
II. R EQUIREMENTS OF G AMMA - RAY C AMERA
Prior to the measurement by our Compton camera, wevisited the site carrying dosimeters and portable gamma-ray
detectors. Then we found that there were places with a strong
ambient dose as publicly reported and, at 5 cm above thoseareas, there were hot spots with a dose approximately 10
times the ambient dose. Our on-site observations also revealed
that, in addition to the constant-energy gamma-rays directly
emitted from 134 Cs and 137 Cs, the sites were dominated by
low-energy gamma rays (gamma rays that had been scatteredby the ground, buildings, etc., and changed to lower-energyones).
According to the on-site experiences, we realized that thefollowings are requirements for gamma-cameras to be used forvisualization of gamma-ray emitting radioactive substances;
1) Spectral Resolution:
In order to take a gamma-ray image by using gammarays directly emitted from radioactive material suchas 134 Cs and 137 Cs, it is important to avoid possible
contamination from scattered components, the camera
should have a spectrum resolution capable of distinguishing individual lines. Thus energy resolution better
than 3 % ( E/E) is necessary.
2) Field of View (FOV):
Urgent tasks at the region close to or inside the evacuation zone is to decontaminate the radioactive substances.
In order for this, efficient survey of hot spots that could
contribute the increase of the environmental radiationlevel, is useful for the decontamination operation. In
order to make a map of hot spots in short time, wider
field of view is better.

3) Angular Resolution:
In order to identify radioactive substances that emit
gamma-ray lines, it would be useful to have an angularresolution of <5 degrees that corresponds to about 1 m’s
size at 10 m away from the camera.
4) Efficiency:
The sensitivity should be sufficient to pick up hot spots
in the area where the radiation level is 0.5–a few µSv/h.
5) Portability: The detector should be easily handled andcould be carried by hand, because people would like to
use detectors in the various environment, in forest or in
mountain areas.
Gamma-rays emitted directly from 137 Ce and 134 Ce haveenergies ranging from 600 to 800 keV. A conventional gamma
camera uses a simple pinhole for imaging observations inthis energy range1 . By limiting the direction of incident
gamma-rays beforehand, the camera captures images of theradioisotopes. The maximal field of view is limited to 40
to 60 degrees due to the aperture angle of the pinhole. Thisimaging method is effective when the surrounding backgroundradiation is low, and when the pinhole mask is thick enough toblock gamma-rays that fall outside the pinhole. However, when
gamma-ray energies exceed several hundred keV, the shielding
turns transparent, and, even if one can visualize radioisotopes,
it could be difficult to obtain a clear contrast. In order toimprove imaging performance, the camera has to be madeheavier for sufficient shielding.
In order to meet the requirements listed above, we decided
to construct a new detector which utilizes the Compton scattering effect.
III. S I /C D T E C OMPTON C AMERA
Compton scattering typically plays a dominant role in theenergy band from a few keV s to 10 MeV. It entails elastic
collision between an incident photon and an electron in the
scattering medium. In conventional Compton cameras, theincident gamma-ray is identified by successive interactions in
two detector layers. The ideal case would be that a gammaray photon emitted from the source is Compton-scattered in
the first layer and then photo-absorbed in the second layer.
Once the locations and energy deposites of both interactions
are measured, Compton kinematics allows us to calculate theenergy and direction (as a cone in the sky) of the incident
gamma-ray by using the following Compton equations:cos ✓ = 1

Ein = E1 + E2 ,(1)

1
me c2 (
E2

(2)

1
),
E1 + E2

where E1 denotes the energy of the recoil electron, E2 theenergy of the scattered gamma-ray photon, and ✓ the scattering
angle. For every event, a cone can be reconstructed as an
opening angle of 2 ✓. The source is somewhere on the conesurface. Compton cameras offer the advantage of needing only
small number of photons to recover the position of sources
without mechanical collimators in front of the camera. If the1 Coded masks that extend the idea of a pin hole camera are often used forhard X-ray observations in space. But the situation listed here is almost same

direction of a recoil electron can be measured, the Compton
cone is reduced to a segment of the cone, the length of which
depends on recoil electron measurement accuracy.

Fig. 1. Principle of a Si/CdTe Compton Camera Gamma rays are scatteredthrough the Si semiconductor detector and then absorbed by the CdTe
semiconductor detector. Based on the values measured by semiconductor
detectors, we can retrace the initial direction of incoming gamma rays. .

Semiconductor imaging detectors are desired for Compton
imaging. As expressed in Equations (1) and (2) above, theenergy and position resolution provided with semiconductors
should improve the angular resolution and hence the sensitivity of Compton cameras. From this perspective, several
semiconductor-based Compton cameras have been proposed.
However, most of these cameras have been developed based on
combining a semiconductor such as silicon with scintillators.
Taking advantage of significant progress, we have performed
in CdTe technology [10], [11], [14], we have been developing
a new generation of Compton telescope [4], [15], [16], thesemiconductor Compton telescope by combining Si and CdTe,
as the Si/CdTe semiconductor Compton camera. Since CdTe
has large atomic numbers (48, 52) and high density (5.8g/cm3 ), it offers the potential to replace scintillators and toform a full-semiconductor Compton camera.
Figure 1 shows the conceptual design of the Si/CdTe semiconductor Compton camera. In the Si/CdTe Compton camera,
events involving the incident gamma-ray being scattered in the
Si detector and fully absorbed in the CdTe detectors are used
for Compton imaging. The direction of the gamma-ray is calculated by solving the Compton kinematics with information
concerning deposit energies and interaction positions recordedin the detectors. In principle, each layer could act not only asa scattering part but also as an absorber part.
A very compact, high-angular resolution (fineness of image)camera could be realized if we were able to fabricate successfully semiconductor imaging elements made of Si and CdTe,
which have excellent performance in position resolution, highenergy resolution, and high-temporal resolution. The field ofview of the camera is determined from the gap between eachlayers, because events coming from large angle with resect tothe detector plane, if we can cover the large angle scattering
The effect of the doppler broadening, which degrades theangular resolution of the imaging system, is smaller in the Si
devices than other semiconductor devices [23], allowing thedifference between the measured and actual scattering angles
to be constrained. The effect of the Doppler broadening becomes smaller as the incident energy becomes higher, because
the binding energy and momentum of electrons in materials
become relatively small for gamma-ray photons with higher
energies.
In our previous work, we developed a prototype Si/CdTe
Compton camera, and verified its performance. The camera
consists of a single layer of double-sided silicon strip detector(DSSD) and four layers of CdTe pice detectors. The stacked
CdTe module is placed 14.5 mm under the DSSD module.
Each CdTe layer contains four CdTe detectors in a 2⇥2
arrangement. The Compton reconstruction was successfully
performed and the gamma-ray images of point sources wereobtained from 662 keV down to 59.5 keV. The resolution ofcatering angle (Angular Resolution Measure or ARM) was 3.5
degree (FWHM) and 2.5 degree (FWHM) at 356 keV and 511
keV, respectively. The performance and the responses of thecamera has been studied quite intensively and reported in otherliteratures [16], [17], [18], [19].

we employed the one developed for SGD with large dynamic
range. We prepared two cameras and installed them into a
portable refrigerator.

IV. T HE PROTOTYPE CAMERA FOR HOT SPOT MONITORING
Although the principle of a Compton camera is well established for some time, no working model has been manufactured that is both relatively easy to use and also able toshoot images on site with sufficient efficiency and resolution.
The ASTRO-H program was in the detailed design phaseat that time and the design of its Compton camera wascomplete. Furthermore, we had begun partial fabrication of
the prototype model. It was technically difficult to use themodel in its existing condition in Fukushima, however, so we
had to build a new model dedicated to observations there. We
decided to secure the research budget and manufacture thededicated model. This was considered to be a project that
could drive our future astronomy research and, eventually,
also drive the development of space technology that would
contribute to the national disaster. We promptly established ateam for prototype-model manufacture and on-site imaging.
The proto-type model assembly started in October 2011 andwas completed in January 2012.
According to the measurements we performed with adosimeter, we decided to fabricate the prototype which issuitable for gamma-ray monitoring in the area where the environment background level is 2 3 µSv/h. The ultra-wide-angle
Compton camera, we brought to Fukushima, is structured ina way that gamma-rays scattered by Compton scattering are
unlikely to escape from the system; therefore, it can achieve
an ultra wide angle view. We fabricated a hybrid camera by
utilizing the ASTRO-H HXI structure (i.e., a 5-layer sensor),
but a change was made to the mix of layers: from 4 layers of
Si and 1 of CdTe used in HXI to 2 and 3 layers, respectively.
For the analog read-out LSI (Large Scale Integrated circuit),

Fig. 2. Photo of the prototype Compton camera. (top) five-layers of Si and
CdTe imagers (bottom) two camera modules and readout electronics with aportable refrigerator.

Figure IV shows a photograph of the gamma-ray camera,
which was brought to Fukuhshima. The top detector module
consists of a 0.5 mm thick Si-Double Sided Detector (DSD).
The Si-DSD used here has strip electrodes with pitch of
250 µm on the P- and N-sides, respectively. The 0.75 mm
thick CdTe-DSDs are stacked under the Si-DSD module. The
CdTe-DSD has been developed based on the technology ofhigh resolution CdTe diode detector [10], [12], [13]. On the
CdTe-DSD, electrodes with a strip pitch of 250 µm are placedorthogonally on both sides. Details of the CdTe-DSD arereported in Watanabe et al. [14] and Ishikawa et al. [20].
The direction of the incident gamma-ray is back-projected
as a cone by using the measured scattering angle ✓ andscattering direction ~s. The coordinate for the back-projection is
selected by taking account of the distance between the camera
and targets. Cases for the near-field, such as medical imaging, are reported in some papers [21], [22]. In astrophysicalobservations, where the source is located in extremely farfield, a 2-dimensional celestial sphere is generally used as
the back-projection coordinate. If the angle of the Compton
camera as viewed from the targets is much smaller than
the angular resolution of the imaging system, the celestialFig. 3.

Concept of image reconstruction by the back projection method

Fig. 4. Gamma-ray spectrum taken together with an image during the imaging
test in Iidate Village, Fukushima.

sphere is regarded as an appropriate coordinate, even for otherapplications. For hotspot monitoring, the sources typicallyexist within the range of a few meters to several tens ofmeters. When the source is located at 5 m, the camera angle asviewed from the source becomes 0.37 degrees in our system,
which is small enough when compared with the typical angularresolution of a few degrees, hence the celestial sphere is used
for the back-projection [18]. The probability distribution of theestimated position by a single event becomes a circle, as in Fig.
3. The location of the sources is determined by accumulating
many cones.
The camera has a wide field of view, about 180 deg., andwas thus named the Ultra-Wide-Angle Compton Camera. The
camera has an angular resolution of several degrees in theenergy range of 500 to 800 keV. This means that we canlocalize hot spots to several tens cm size at a distance of 10m.
V. M EASUREMENT
After we carried out initial performance tests in our laboratory, we performed an imaging experiment at Iidate Village

Fig. 5. Imaging test in Iidate Village, Fukushima. (top) Optical image takenwith a digital camera with a fish-eye lens. (bottom) Contour plot generated
from the back projection image by Compton reconstruction from the dataobtained at the site.

in Fukushima in cooperation with the Japan Atomic Energy
Agency (JAEA) and Tokyo Electric Power Company in Feb.
2012 [24].
Figure 4 shows the spectrum obtained with the Ultra-wideangle Compton Camera at the site. Four lines from 134 Cs
and 137 Cs are seen in the spectrum. One crucial point of the
Si/CdTe Compton camera is that the sum of the two energies
measured by the first and second reaction in the detector
is equal to the gamma-ray energy directly emitted from theradioactive materials (e.g., 662 keV released from 137 Cs). Suchagreement guarantees that the gamma rays come directly fromthe materials, without being scattered on the ground, buildings,
etc.
Since the Ultra-wide angle Comptin Camera has a FOV of
180 degrees, it is convenient to superimpose the image onto the optical image taken with a camera with a fisheye lens(Figure 5 (top)). Figure 5 (bottom) shows the simple imageof back projected cones calculated from events taken by theCompton camera. In order to obtain the image, we extracted
events obtained from energy regions that contain these lines,sensitivity should be improved by about 30 times. We expectthat the 20 to 30 minutes required for “gamma-ray imaging”
with the current prototype could be significantly shortened.
To introduce a new model to visualize the distribution ofradioactive materials in Fukushima, a plan to create a newcamera based on the design of ASTRO-H SGD is underwaywith the support of the Japan Science and Technology Agency(JST), jointly with Mitsubishi Heavy Industries.
Space science observations require the most advanced technology. By realizing our own cutting-edge space observation
technology, we were able to contribute to current and futureradioactive decontamination, and widely expand the role of
ASTRO-H from space to the earth.
ACKNOWLEDGMENT
Fig. 6. The intensity (flux) distribution of gamma rays with energies of 605,
662, 796, and 802 keV directly emitted from cesium-134 and 137. The color
image is extracted by requiring above three sigma excess from our statisticalanalysis. Red is high intensity while blue is low, superimposed on an image
taken with a digital camera with a fish-eye lens. The exposure time is 60 min.

with energies of 605, 662, 796, and 802 keV. Smoothingalgorithm is applied to draw contours.
We can clearly see the high intensity region in the imageof the back projection. In order to extract peaks from theimage, we have developed a peak-finding algorithm basedon our experiences of source finding in the astronomical
application. We have also studied statistical significances ofthe excess by studying the offset components appeared in
the sky region of the back projection image. The resultant
image that corresponds to the concentration of radio active
substances is shown in Fig. 6. The independent measurementsof the radiation level at 5cm cm away from the surface of theground was carried out by using the detector developed by
JAEA. Both results are consistent with each other [24].
VI. C ONCLUSION
Based on the technology of Si/CdTe Compton Camera, wehave manufactures a prototype model for the use in the site, inorder to verify whether this type of camera is useful to decontaminate the radio active material. With this prototype, named
Ultra-Wide-Angle Compton Camera, we have succeeded tovisualize the distribution of radio-active substances in areas
where the background radiation level ranges from 1 µSv/h to⇠30 µSv/h. The camera has an angular resolution of a coupleof degrees in the energy range of 500 to 800 keV. This meansthat we can localize hot spots to several tens cm size at a
distance of 10 m.
The challenge with future decontamination work is to improve the sensitivity of the camera several times over, up to 10fold, in order to enable fast imaging in a few minutes, and sofacilitate the operatorfs work at on-site decontamination. The
SGD for ASTRO-H has a multiple-layer structure (32-layer
Si detector and 8-layer CdTe detector) to detect faint gammaray signals coming from celestial bodies. If we can apply thatstructure to the gamma-ray camera used in Fukushima, theThe keys to realization of the prototype Ultra-Wide Angle
Compton Camera were detector technology employing CdTe
semiconductors with excellent energy resolution and extremely
high-density packaging technology. These were jointly developed with Acrorad Co. Ltd. and Nagoya Guidance Systems
Works of Mitsubishi Heavy Industries, Ltd. over 15 years. The
HXI and SGD for ASTRO-H are under incessant development
toward launch in 2014 by the same team that developed hard
X-ray/gamma-ray observation instruments for Suzaku. The
team consists of JAXAfs ISAS, University of Tokyo, Nagoya
University, Hiroshima University, Waseda University, et al.
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