Abstract
Airborne radioactivity must be monitored at nuclear facilities. The workers for nuclear power plants are at risk of internal exposure by airborne radioactive materials. In particular, artificial radioactive materials (uranium, plutonium, etc.) have severer damage on human bodies. Therefore, it is necessary to measure airborne radioactivity concentration of these materials. Radionuclides that emit alpha-rays exist in natural, 212Po, 214Po, and 212Bi. Airborne particulate radioactivity monitor ideally should detect only the concentration of the artificial materials. The energy of the alpha rays is important to discriminate the artificial (4.0~5.8 MeV) and natural (over 6.0 MeV) radioactive materials. Therefore, it is possible to identify the two materials from the alpha-ray energy information and detect only the artificial materials.
A method for on-site radionuclide analysis is utilizing the energy spectra. Currently, ZnS scintillator or Si semiconductor detector are widely adopted to the detector of the airborne particulate monitor. The ZnS scintillator does not have enough good energy resolution to perform the analysis. Although the silicon semiconductor detector itself has sufficient energy resolution, alpha-ray energy spectra become distorted due to the energy loss of alpha-particle in the air and dust accumulated on filter paper. Therefore, it is difficult to analyze the original alpha-ray energy by either kind of detector in current methods.
This method is to estimate the source information from the detector response and obtained energy spectra. In this study, the detector response was created by radiation simulation, and the successive approximation method was adopted as an algorithm for estimating the source information.
We have fabricated an airborne particulate monitor equipped with an energy spectrum spectroscopy that applies the unfolding method. In order to evaluate the performance of this monitor, we performed a spectrum measurement using the radioactive source. From the measurement results, it was confirmed that the unfolding operation can obtain the radioactivity of the source within an uncertainty range. For further evaluation, the monitor collected dust and measured radioactivity continuously for a month. As a result of dust collection, it was confirmed that the decision threshold of artificial radionuclides had achieved less than 10-7 Bq/cm3 under the natural radioactivity 10-5 Bq/cm3. Furthermore, we confirmed that natural radionuclides 212Bi, 212Po and 214Po (6.0, 7.7 and 8.8 MeV) can be measured separately.
