Development of α/β detector
Dr Yuki Morishita
Research purpose and contents
In the decommissioning site of the Fukushima Daiichi Nuclear Power Station (hereafter referred to as FDNPS), α particle-emitting nuclides with severe internal radiation exposure effects, such as plutonium (239Pu) and strontium (90Sr), are present. Therefore, to proceed with the decommissioning works, it is important to specify the position and distribution of α particle-emitting particles. The conventional methods take time and effort to perform the measurements because each sample is measured separately using a portable-type α particle or β particle detector. In addition, these methods only obtain counting rate (intensity of radiation); thus, the energy for radioactive materials had not been measured. To solve the above problem, we have developed a “α/β detector” that can detect α- and β particle-emitting nuclides simultaneously. Using the developed detector has enabled recording measurements smoothly and effectively compared with conventional methods.
The developed α/β detector (left) and the principle of nuclide-visualization detector (right).
Improvements by the pulse shape discrimination (PSD) method
In the α/β detector, the measurements are conducted using a scintillator, in which the light emission property of an α particle varies from a β particle. A variation appears when converting scintillation light to electric signals using a photomultiplier, particularly in the rising/falling parts between a α particle and β particle. It has become possible to measure the α- and β particles simultaneously while identifying each radiation using the pulse shape discrimination (PSD) method when analyzing the pulse shapes. If the intensity of β particles is too high, discriminating α particles becomes difficult for the developed detector. To avoid such a problem, the measurements were conducted for samples (smear samples) that were collected from the floor in the FDNPS reactor building. From the results of the smear samples, we worked to improve the accuracy of measurements while verifying whether α particle emitting nuclides can be discriminated from β particle emitting nuclides.
Output voltage waveform for α- and β particles. α particles can be discriminated from β particles through the pulse shape discrimination (PSD) method because there is a variation in the rising/falling parts between the α- and β particles.
Utilization as a surface contamination detector and dust monitor.
A α/β detector can be used for two kinds of applications, viz. surface contamination detection and dust monitoring. For the detection of surface contamination, a α particle has a particularly short flight distance of about 4 cm in air. Therefore, a measurement must be conducted as close to the contaminant as possible. For a dust monitor, the measurements are conducted while radioactive materials in air are continuously collected on the filter paper where the detector and the filter paper are facing each other. Therefore, it becomes possible to continuously monitor the concentration of radioactive materials in air.
Contributions to various fields such as environmental and medical field.
The evaluation of particle size of α particle and β particle emitting nuclides at the FDNPS working site for nuclear fuel debris retrieval is applied to improve the accuracy in evaluating the internal radiation exposure. In addition, it can be applied to the radiation management for the establishing working environment and protection of workers from radiation exposure.
In the future, the developed α/β detector can be used as an environmental monitoring instrument to measure radon (222Rn), and its daughter nuclides that emit α- and β particles. Further, the detector can be applied to the measurement instruments used in the cancer treatments in nuclear medicine (targeted radionuclide therapy).
Detection of alpha particle emitters originating from nuclear fuel inside reactor building of Fukushima Daiichi Nuclear Power Plant
Yuki Morishita*1, Tatsuo Torii*1, Hiroshi Usami*1, Hiroyuki Kikuchi*2, Wataru Utsugi*2 and, Shiro Takahira*2
Belong：*1 Japan Atomic Energy Agency, *2 Tokyo Electric Power Company
Morishita, Y., Di Fulvio, A., Clarke, S. D., Kearfott, K. J., & Pozzi, S. A. (2019). Organic scintillator-based alpha/beta detector for radiological decontamination. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 935, 207-213.