Development of α/β detector

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.

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