TOP > Report & Column > The Forefront of Space Science > 2012 > Experiment demonstrating the Viability of Easy, On-site Visualization of the Distribution of Radioactive Materials by the “Ultra-Wide-Angle Compton Camera”
The Great East Japan Earthquake of March 11, 2011 caused significant damage to Japan. The radioactive materials released from the Fukushima Number 1 nuclear power plant of the Tokyo Electric Power Company have had a huge impact on the Japanese people. The removal of these radioactive materials is of very high priority for Japan.
In early April 2011, the Institute of Space and Astronautical Science (ISAS) received an e-mail from Tokyo Electric Power Company containing the following request: "By using X-ray observation technology for astronomical observations, can you broadly and at once identify areas where strong or weak radiation exists on the ground?" In fact, our group has developed high-sensitivity gamma-ray sensors over the last 20 years with the purpose of advancing X-ray astronomy and paving the way for new astronomy in the hard X-ray or gamma-ray regime. These sensors have been tested on scientific balloons and onboard the X-ray astronomical satellite SUZAKU. Furthermore, we plan to install the sensor on the next X-ray astronomical satellite ASTRO-H. After the earthquake, we were concerned because we knew that cesium radiation affected the lives of many people. The e-mail arrived just when we were looking for ways to use our sensors to help solve the problem.
Determining the Direction of Incoming Gamma Rays with a High-Sensitivity Gamma-Ray Sensor
Because of the accident at the nuclear power plant, dust containing radioactive materials such as iodine-131, cesium-134, and cesium-137 dispersed over a broad area. The term 'Radioactive materials' refers to those that contain unstable nuclei. Gamma rays are emitted when unstable nuclei in the materials decay to become stable. Strong gamma-ray emissions have a damaging impact on the human body and the environment. Thus, quick removal of dust with radioactive materials (decontamination) is essential. To decontaminate, we need to know where the radioactive materials are present. One method is to walk around carryinga dosimeter, but this is a time consuming procedure. If we could "see" the gamma rays emitted from cesium-137, etc., we could identify the source Ealthough this is not an easy task. While it is simple in the case of visible light, determining the incoming direction of gamma rays is difficult.
The sensor under development for installation on ASTRO-H uses a technology called the Compton camera. It utilizes the Compton scattering effect discovered by Dr. A. H. Compton, who was awarded the Nobel Prize in Physics in 1937 for this work. This technology actively utilizes the nature of particles in high-energy light, i.e., gamma rays. To find the direction of incoming gamma rays using the Compton scattering effect, it measures two energies: first, the energy conveyed by gamma rays to electrons in the Compton scattering collision; and secondly, the energy left in post-collision gamma rays. The information on reaction location is combined with the energy data. This can be understood by comparison with the game of billiards. Without seeing the actual collision, if we know accurately how the balls spread when struck, we can determine the direction and speed of the ball that hit them. The crucial point in this method is that the sum of the two energies measured by the first and second reaction detectors is equal to the initial gamma-ray energy emitted from the radioactive materials (e.g., 662 keV released from cesium-137). Such agreement guarantees that the gamma rays come directly from the materials, without being scattered on the ground, buildings, etc.