This simple method allows the rapid visualization of low-level gamma radiation sources with ambient surface and, or air dose rates over a few microsieverts per hour or less. The detector is omnidirectional, highly sensitive, inexpensive and portable, and can be used for low-level gamma radiation sources within RI facilities, and out of the ward around Fukushima. The method can serve as a next-generation technology for environmental radiation monitoring, replacing the conventional stationary dose rate monitors, currently in use in RI facilities on hospitals.
Demonstrating the procedure will be Nina Kuwata and Katsuki Kubayashi, students from my laboratory. For monitoring of a sealed radiation source within a radioisotope facility, set the Compton camera beside the wall-mounted dose rate monitor, and measure the height of the detectors from the ground. The wall-mounted dose rate monitor consists of a parallel plate ionization chamber, and can constantly monitor the air dose rate of the position at one minute intervals.
Power on the Compton camera with the online computer and initiate the simultaneous measurement with the Compton camera and the dose rate monitor. Place a Cesium-137 sealed source at the A position, 3.6 meters away from the detectors. After 30 minutes, move the sealed source to position B, 6.7 meters away from the detectors.
After another 30 minutes, move the sealed source to position C, 6.7 meters away from the detectors. After another 30 minutes, move the sealed source to position D, one meter from the detectors. After 30 more minutes, move the sealed source outside the room for 30 minutes, and stop all of the measurements.
For environmental monitoring within a PET facility, early in the morning before patients arrive, set the Compton camera in front of the reception desk in the PET facility, and measure the height of the detectors from the ground. Place the online computer in the staff room and power on the camera and computer. When all of the materials are in place, start the measurements.
Stop monitoring after all of the patients have left for the day. For radioisotope measurement outside, place the Compton camera near the building of interest, at which the existence of some radiological Cesium hotspots with service dose rates of one microsieverts per hour, or less, are suspected. Set the height of the detectors to 1.5 meters from the ground, and power on the camera and computer.
Then, acquire Compton camera measurements for 30 minutes. Here, the time variation of the trigger rate measured by the Compton camera in a radioisotope facility, after applying a time lag selection of two head counters less than one microsecond, is shown. The trigger rate changed every 30 minutes, depending on the position of the sealed source, as confirmed from the data measured by the stationary dose rate monitor.
Five periods were set to represent the five positions of the sealed source. As illustrated in these omnidirectional images, for periods one to four of monitoring, the position of the Cesium-137 sealed source can be successfully identified from the gamma ray images shown in red. The overall time variations of the trigger rates measured in a PET facility during the daytime reveals a remarkable enhancement in the trigger rate with various patterns, which could be attributed to the movement of patients injected with Fluorine-18 Fluorodeoxyglucose around the reception desk.
For example, focusing on the period from 6, 200 to 7, 000 seconds, a series of enhancements with two plateaus become apparent. In these images of the facility, the directions of the gamma ray peaks in both images correspond to the directions of the sofa and the restroom behind the wall. Considering the trigger rates of both monitoring periods, it is likely that a patient entered the restroom for two minutes, and afterwards, sat on the sofa for a few minutes before the PET scan.
In the outdoor measurement in Fukushima field, the protocol reveals that the gamma ray images concerning ground-level distribution of low-level radioactive Cesium contamination can be acquired. Our decontamination procedure can be used in areas with low-level radioactive Cesium contamination, released by the Fukushima Daiichi nuclear power plant accident, and we improved the present PET system.
