Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)
68“ASTROCAM 7000HS”
Radioactive Substance Visualization Camera
DAISUKE MATSUURA*1

KEI GENBA*2

YOSHIKATSU KURODA*2

HIROSHI IKEBUCHI*2

TETSUYA TOMONAKA*3

Mitsubishi Heavy Industries, Ltd. (MHI) released the ASTROCAM 7000HS, a radioactive
substance visualization camera, in March 2013, following the accident at the Fukushima Daiichi
Nuclear Power Plant caused by the Great East Japan Earthquake. The ASTROCAM 7000HS
incorporates the technologies for the gamma-ray detector used for the ASTRO-H satellite, which
we have been developing under entrustment from and together with scientists at the Institute ofSpace and Astronautical Science (ISAS) at the Japan Aerospace Exploration Agency (JAXA), andthe design was modified for use on land to commercialize the product. We performed on-site tests
in June 2013 in the area where living is restricted in Fukushima Prefecture and succeeded in thevisualization of hot spots. The outline of the ASTROCAM 7000HS, the measurement principle andthe test results are reported below.

|1. Introduction
The contamination of a wide range of areas with radioactive substances, which is attributable
to the accident at the Nuclear Power Plant caused by the Great East Japan Earthquake, is a serious
problem today. The removal of radioactive substances is essential, but the substances that need tobe removed are not visible to the naked eye, which is one of the issues preventing the easy removalof such substances. Cesium 134 and cesium 137 (134Cs, 137Cs), which are particularly problematic
at present, release gamma rays featuring a high amount of energy ranging from approximately 600
kilo electron volt (keV) to 800 keV. Conventionally, the air dose rate was normally measured bysurvey meters to detect the existence of hot spots where radioactive substances are concentrated.
However, there are some issues in using survey meters to measure every corner of the wide rangeof areas that may be contaminated. For example, it is difficult to measure high areas that cannot bereached, the individuals that perform measurement could be exposed to radiation for a long timeand some of the hot spots could be overlooked.
MHI has been manufacturing X-ray and gamma-ray detectors onboard ASTRO-H satellite1,2,
which is the next X-ray astronomical satellite by Institute of Space (ISAS) and Astronautical
Science in JAXA. The detector, called “Si/CdTe Compton Camera”, is based on silicon (Si)semiconductors and cadmium telluride (CdTe) semiconductors and has functions to image gamma
rays. The technology of the camera has been developed in ISAS/JAXA for more than 10 years
aiming at future astronomical satellites, including ASTRO-H3. By incorporating the technologies
for space development in the device for use on land, we commercialized the ASTROCAM
7000HS, the first camera that can visualize radioactive substances including 137Cs, in March20134,5, 6. Issues regarding gamma ray measurement and the key technologies to solve such issues,
as well as the results of on-site tests performed in Fukushima Prefecture, are introduced in thisreport.

*1 Space & Integrated Defense Systems Engineering Department, Guidance & Propulsion Division, Integrated Defense &Space Systems*2 Chief Staff Manager, Space & Integrated Defense Systems Engineering Department, Guidance & Propulsion Division,
Integrated Defense & Space Systems*3 Chief Staff Manager, Takasago Research & Development Center, Technology & Innovation Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)69|2. Interaction between gamma rays and substances
Rays are classified into infrared rays, visible rays, ultraviolet rays, X-rays, etc., depending on
the wavelength and the formation principle (Figure 1). Gamma rays are one type of rays
(electromagnetic waves). The visualization of gamma rays released from radioactive substances,which are the targets of removal, was difficult for the following two reasons. (1) It is difficult toconcentrate gamma rays using a lens, etc., and (2) Gamma rays are characterized by a high amountenergy ranging from several hundred keV to several thousand keV, which is approximately
100,000 times the energy of visible rays, and penetrate CCD elements used in digital cameras.

Figure 1

Classification of rays

The following three types of physical phenomena are observed as interactions between
gamma rays and substances. The gamma ray visualization camera is works using these principles.
Examples of the application of such principles to the camera are introduced in section 3.
(1) Photoelectric absorption
Photoelectric absorption is the reaction where gamma rays are absorbed completely in thedetecting portion and disappear in a single cycle of interaction. Gamma rays have a highamount of energy and easily penetrate the detector, and the rate of reaction to the interaction islow.
(2) Compton scattering
Incident gamma rays scatter, without being absorbed completely in the detecting portion,and create a scattering gamma ray and a recoil electron, like billiard balls, as shown in
Figure 2. The reaction is called Compton scattering. The interaction is the highest in terms of
the rate of reaction against gamma rays.
(3) Pair production
The pair production is a reaction where gamma rays generate pairs of electrons andpositrons. For pair production, the energy needs to exceed two times 511 keV, which is the rest
mass of electrons, and the reaction is caused by gamma rays featuring energy higher than 600
keV to 800 keV, which are released from 134Cs and 137Cs, the targets of removal.

Figure 2Compton scattering|3. Imaging principle of gamma ray camera
Usually pinhole system is used to image gamma rays. The comparison with the Compton
Camera method is summarized below. The methods are compared in Table 1, and the outline of theimaging principle is shown in Figure 3.

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Table 1 Comparison of pinhole camera with Compton camera
Imaging
methodMajorinteraction

Disadvantages

Advantages

Pinhole camera

Compton camera

Photoelectric effectCompton scattering･ Detected portion needs to be enclosed
by a heavy shield, and the equipment
is large and heavy.
･ View angle is limited to 40 to 60
degrees.
･ Increase in noise due to gamma rays
that pass through the shield.
The noise is especially remarkable
near pinhole collimator where theshield is thin.
･ The structure at the detecting portionis comparatively simple.
･ Gamma image can be createdcomparatively easily from theobtained data.

･ Production of an gamma-ray image requires
calculation based on Compton reconstruction
kinematics. Dedicated analysis code should
be prepared.

･ No need to use a shield because the directionof incoming gamma rays can be identified,making it possible to downsize and reduce theweight of the equipment.
･ View angle is extremely large at 180 degrees(half sphere).
･ By requiring the condition of Compton
kinematics, signal to noise ration becomeshigh.
･ High background rejection capability.

Figure 3 Imaging principles of pinhole camera and Compton camera(1)

Pinhole camera method
In the pinhole camera method, the photoelectric effect is utilized to image gamma rays.
The detector of the camera is covered by a pinhole collimator and a shield, as shown in Figure 3(a). Gamma rays that pass through the pinhole collimator and enter the camera are detected by
the pixel detector. The direction of incoming gamma rays is identified from the geometrical
relationship between the measured detector position and the pinhole.
(2) Compton camera method 3
In the Compton camera method, Compton scattering is utilized to image gamma rays.
The ASTROCAM 7000HS released by MHI incorporates this method. The camera has a pixel
detector consisting of the scattering layer and the absorbing layer, as shown in Figure 3(b).
Gamma rays that enter the camera go through Compton scattering at the scattering layer, andthe scattered gamma rays undergo photoelectric absorption at the absorption layer. The energy
measured at the scattering layer is defined as (E1), the 3D coordinate positionon the detector
surface where Compton scattering occurred is defined as position vector ( X 1 ), the energy
measured at the absorbing layer as (E2), and the position where photoelectric absorption
occurred as ( X 2 ). The energy of gamma rays that enter is obtained by E1+E2. As for the
direction of incoming gamma rays, it is estimated that they pass through one of the points alongMitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)71

the ring of the cone shown in Figure. The ring is obtained from the scattering angle (θ)
calculated from the energy information and the central axis of the cone calculated from thepositional information.
If multiple gamma rays are detected from the same gamma ray source, the direction ofincoming gamma rays can be obtained from the intersection point where the rings overlap. Theintersection point of the rings is emphasized in red as the events of detected gamma rays
increase from three to 16 and more and the images of gamma rays become clearer, as Figure 4
shows.
The Compton camera can narrow down the direction of incoming gamma rays by themethod explained above, and there is no need to limit the direction of incoming gamma rays
using a pinhole collimator, etc., realizing a super wide-angle measurement field covering all
directions in a hemisphere.

Figure 4

Number of detected gamma ray events and change in gamma image|4. Outline of ASTROCAM 7000HS
4.1

Configuration of the productFigure 5 shows the overall structure of the ASTROCAM 7000HS and the basic
specifications are shown in Table 2. The components are explained below.

Figure 5

System configuration of ASTROCAM 7000HS

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Table 2 Basic specifications of ASTROCAM 7000HS
Model No.
Outer dimensions
View angle
NuclideEnergy resolution
Angle resolution
WeightPower supply
Temperature
Operating temperature
Accessories(1)

ASTROCAM 7000HS
L445×W340×H235 (mm)(Camera unit, excluding projections)
180 degrees (super wide angle) Detecting efficiency varies
depending on the angle
12 types, can be selected by user
2.2% (FWHM) at 662keV
5.4 degrees (FWHM) at 662keV
ARM (Angular Resolution Measure) evaluation value
Approx. 8-13kg (Camera unit)
Depends on the number of sensor layers that constitute the camera
AC100V-240V/battery
Operating temperature: 0 to +40°C, storage temperature: 0 to +50°C
35-80%, to be free from condensation
Camera control box, notebook PC, measurement software

Camera unit
The camera unit consists of Si/CdTe Compton camera, a high voltage power supply tothe camera, an interface circuit, an electronic cooler, visible light fisheye camera, etc. The
camera unit is 445 mm in depth, 340 mm in width and 235 mm in height, and the weight is 8 to
13 kg. The unit was downsized and the weight was reduced so that the product can be carriedby one individual, assuming its use for the removal of contamination.
The Compton camera structurally takes over the camera mounted on the ASTRO-H
satellite. The number of sensor layers in the camera, however, is less than that of the camera
intended for the satellite, and the camera has 12 layers in total, 8 layers of Si semiconductor forthe scattering layer and 4 layers of CdTe semiconductor for the absorbing layer. The
configuration was decided by performing a simulation and analysis to determine the layer
configuration appropriate for contamination removal. The sensitivity of the camera can be
adjusted according to the application and the sensitivity can be increased to about 16 times thecurrent specification if the number of sensor layers is increased to the maximum.
A fisheye camera is installed in front of the camera, and the direction of incoming
radioactive rays is identified by superimposing the optical image of the fisheye camera over the
gamma ray image.
(2) Control unit
The control unit houses a camera control circuit, a temperature control circuit, a highvoltage control circuit, a power supply circuit, etc. The control unit receives commands fromthe operation/image processing unit and transmits voltage supply control commands andmeasurement control signals to the camera unit. The control unit features a safety design andconstantly monitors temperature/humidity abnormalities and communication between theoperation unit and the image processing unit, as well as a function to prevent failures byshifting the camera to safe mode upon error detection.
(3) Operation/image processing unit
All measurements can be controlled on the user interface screen of a PC. Therefore,
remote operation is possible if the remote operation function of a PC is used, reducing theexposure of measurers to radiation.
The user interface screen is shown in Figure 6. Operations on the screen are very simple,
and the measurer simply needs to turn on the voltage supply to the camera and startmeasurement using the “operation buttons” on the screen. When the measurement is started, thering of gamma rays detected by the Compton camera is overwritten on the optical image at thecenter of the screen imaged by the fisheye camera in nearly real time, and the hot spot isemphasized gradually. The camera status, energy spectrum and change in the number ofdetections over time can be checked on the screen.
The nuclide selection button is on the top right of the screen and the nuclide for image
display can be switched in real time during measurement. The user can register up to 12 typesof arbitrary nuclides in the database. The selection function is one of the special features that

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)73

characterize image processing by the ASTROCAM 7000HS. Gamma rays generated fromradioactive substances, such as 137Cs featuring 662keV, have energies specific to each nuclide.
The image processing unit processes the data for visualization only when the measured energy
matches the energy information specific for each nuclide in the database. Secondary gamma
rays scattered by or absorbed in the ground, buildings, etc., are excluded by this processing
method, and only the gamma rays coming directly from radioactive substances are processed,
realizing accurate analysis of radioactive substance locations.

Figure 6 User interface screen

4.2

Figure 7 Energy spectrum of 137Cs

Basic performance

The energy resolution and angle resolution of the camera are essential for the generation ofclear gamma images. The energy resolution of the ASTROCAM 7000HS is 2.2% in respect to 662
keV gamma rays at FWHM (Full Width at Half Maximum) (Figure 7). The angle resolution is 5.4
degrees (FWHM) in ARM (Angular Resolution Measure) evaluation value, and a 10 m distancefrom the camera corresponds to 1 m.

|5. Results of validation test performed on ASTROCAM 7000HS
5.1

Simultaneous measurement of multiple radiation source

Three types of radioactive substances were used for measurement, aiming to prove thatdifferent types of nuclides can be distinguished when multiple radioactive substances are measured
simultaneously. The radioactive substances that we used were barium 133 (133Ba), natrium 22(22Na) and 137Cs. The image obtained in a 10-minute measurement is shown in Figure 8. The
locations of the three types of radioactive substances were clearly distinguished on one screen. We
also confirmed visualization by a test using europium 154 and cobalt 60, which is a target nuclide
for the decommissioning of nuclear reactors at nuclear power plants.

Figure 8

Results of simultaneous measurement of three types of radioactive substancesMitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)74

5.2

Evaluation of sensitivity

To evaluate the sensitivity on the side with lower radiation source strength, we performed
measurement by setting radioactive substances at a position where the air dose rate at the camera
position was equivalent to the air dose. For the measurement conditions, 137Cs with an air dose of
0.09 microsieverts/hour (µSv/h) and a strength of 1MBq was set at a distance of 3.2 m from thecamera. We confirmed in an eight-hour measurement that the locations of radioactive sources canbe identified under the same conditions as for air dose, which means that visualization is possible
even if the difference in dose rates is so small that the existence of radioactive substances cannot be
detected by a survey meter.

5.3

Influence of obstacles

To prove that the locations of radioactive substances can be identified even if there is an
obstacle such as a pipe or a metallic tank between the radioactive substances and the camera, we
performed a measurement using a 24-mm stainless steel plate as an obstacle. The measurement
result indicated that the existence of an obstacle does not largely affect the size of gamma images,
which means the influence of gamma rays scattered by the obstacle is minimized and the locationsof radioactive substances hidden by the obstacle can be identified clearly. The ratio of detectedgamma ray events between a measurement with an obstacle and a measurement without an obstacle
was 100:15, which is nearly the same as 100:12, which was obtained by a calculation based on
damping due to the simplified obstacle.

5.4

3D gamma image

For the creation of 3D gamma images, we performed measurements from two directions toprove that information on distance between the camera and radioactive substances can be measured
when the same radioactive substances are measured from multiple locations. It was confirmed thatthe distance was measured accurately, as Figure 9 shows, because of the disparity (difference inviews between cameras A and B), which was realized by the superior angle resolution of the
ASTROCAM 7000HS3, 7.

Figure 9

5.5

Measurement results of distance from radioactive substances based on parallax

On-site test

To prove that actual hot spots can be visualized, on-site tests were performed in the area
where living is restricted in Fukushima Prefecture in June 2013 (Figure 10 (a)). The 137Cs gamma
image obtained by a 30-minutre measurement under a 1.5 µSv/h air dose is shown in Figure 10 (b).
The red areas in the image are the areas where gamma rays were strong according to themeasurement. The dose rate of the measured area was checked by a survey meter, and it was
confirmed that the red areas in the gamma image coincide with a 20 µSv/h hot spot located 1 mfrom the camera and a 30 µSv/h hot spot located 10 m from the camera. At another test site,
locations before and after contamination removal were distinguished by a 30-minute measurement
under the same air dose rate of 1.5 µSv/h (Figure 10 (c)).

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Figure 10 Results of measurement in the area where living is restricted in Fukushima
Prefecture|6. Conclusion
As a result of the accident at the Fukushima Daiichi Nuclear Power Plant, gamma ray
visualization cameras have been released by a number of companies in recent years. MHI released
the “ASTROCAM 7000HS,” a radioactive substance visualization camera, and has been
performing verification and validation of the system. Through the on-site test performed in the area
where living is restricted in Fukushima Prefecture, we succeeded in visualizing the hot spot anddistinguishing between places before and after contamination removal, and confirmed that the
product is moving toward commercialization. We hope that the product will support operations forcontamination removal and contribute to the reconstruction of Fukushima Prefecture.
The future development will be as follows:・ Technological development: Application to reactor decommissioning operations and control
at nuclear power plants through the creation of 3D gamma images and a network ofmultiple cameras, as well as an increase in the upper limit energy value of gamma rays thatcan be measured
・ Business development: Measurement service business using the ASTROCAM 7000HS
Acknowledgment: MHI appreciates the support of the Japan Science and Technology
Agency (JST) in developing the product.
JST Development of Systems and Technology for Advanced Measurement and Analysis“Development of Next-generation Ultra-wide Angle Compton Camera”
Team Leader: Tadayuki Takahashi (Professor, Institute of Space and Astronautical Science(ISAS)/JAXA, Department of Space Astronomy and Astrophysics)
Sub-Team Leader: Yoshikatsu Kuroda (Chief Staff Manager, Mitsubishi Heavy Industries,
Ltd.)

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Tadayuki Takahashi et al., “The ASTRO-H Mission”, SPIE, 7732, pp. 77320Z-77320Z-18, (2010)
Hiroyasu Tajima et al., “Soft Gamma-ray Detector for the ASTRO-H Mission”, SPIE, 7732, (2010)
Takahashi Group Homepage; see also references therein
http://www.astro.isas.jaxa.jp/~takahasi/English-Version/Detectors/Compton/Compton_E.html
Tadayuki Takahashi, Shin'ichiro Takeda, and Shin Watanabe, “Visualization of Radioactive Substanceswith an Ultra-wide Angle Compton Camera”, BUTSURI, The Physical Society of Japan (JPS), vol.68,
No.6 pp.382-386, (2013) (Japanese)
Tadayuki Takahashi, Shin'ichiro Takeda, Shin Watanabe, and Hiroyasu Tajima, “Visualization ofRadioactive Substances with a Si/CdTe Compton Camera”, Nuclear Science Symposium and Medical
Imaging Conference (NSS/MIC), 2012 IEEE, PP. 4199-4204(DOI 10.1109/NSSMIC.2012.6551958)
Mitsubishi Heavy Industries, LTD. News Release，(2012)
http://www.mhi.co.jp/news/story/121115-1.html
Shin’ichiro Takeda, Yuto Ichinohe, Tadayuki Takahashi, Shin Watanabe,
Hiroyasu Tajima, Kei Genba, Daisuke Matsuura, Hiroshi Ikebuchi, Yoshikatsu Kuroda, “A portable
Si/CdTe Compton camera for visualizing radioactive substances”, Nuclear Science Symposium andMedical Imaging Conference (NSS/MIC), Seoul, Korea, 2013