We converted the measured values to the measurement value of 1 m height obtained by the calibrated survey meter by using the regression of the car-borne survey measurements on existing survey meter measurements of the absorbed dose rate in air. Detailed investigation was conducted for all factors considered to potentially influence the measured value. The equation made by these factors was nearly accordance with this regression. The result of converting the measurement results into the survey meter measurement value was 22-111 nGy h-1. Comparing the distribution map of environmental gamma-ray dose rate by car-borne survey with a geological map shows that the area with high gamma-ray dose rate coincides well with areas of Granitoid, Paleogene system and Tertiary Volcanic Rocks. The area with low gamma-ray dose rate coincides with areas of Quaternary Volcanic Rocks and their surroundings. The source of these rocks is considered to be volcanic ejecta. The annual effective dose from the geometric mean value in Aomori prefecture was estimated to be about 0.21 mSv y-1. This value is lower than the annual external dose that would be obtained by exposure to the average terrestrial gamma-ray dose rate in Japan.
A high environmental dose rate observed at Rokkasho-mura on December 17, 2015, which was reported to be caused by the wet deposition of radon decay products, was analyzed to depict its spatial distribution and to understand mechanisms of the unprecedentedly high dose rate increase of 0.151 µGy/h. It was pointed out that the high dose rate was observed widely on the Sea of Japan side of the main island of Japan, with areas of slight increases where the precipitation was light. It was also pointed out that the large increases corresponded with the precipitation intensity exceeding about 3 mm/h. However, precipitation events exceeding the value did not necessarily correspond to large increases of the dose rate. According to the analysis using a long-range atmospheric transport model and weather charts, the causal 222Rn was transported from the northeastern part of China and the southeastern part of Siberia, where existed a cold low pressure system transporting 222Rn vertically to form a 1 to 2 km-deep high concentration air layer. This air layer was transported by the winter monsoon to Japan without significant decay of 222Rn. It was found in this analysis that the large dose rate increase was observed at the locations where the significant level of precipitation occurred when the air layer existed.
The characteristic of radiation exposure in medicine is to irradiate a human body directly. Therefore, the benefits of radiological practice in medicine have to largely exceed its risks. The risks need to be considered not only for the use of radiation but also the case of not having an examination. Moreover, those risks have to be reviewed in terms of both medical workers and patients. Also, radiation exposure in medicine is generally not uniform but ununiform for both sides. Although it is difficult to measure doses, in recent years, simulation software using computers has begun to be widely used. There are also unique radiation units in medicine based on quality control, such as CTDIvol, Ka,e, PKA and others. On the other hand, exposure in medicine can be also characterized by no upper limit having been specified, such as dose limits or dose constraints. Instead, some efforts have been made in the establishment of diagnostic reference levels to narrow a gap between facilities. We should apply and take “justification” and “optimization” for radiological protection in medicine, “dose limits” for medical workers and “diagnostic reference levels” for patients into account.
Evaluation of radiation dose from medical exposure is important because the use of ionizing radiation in medical field contributes significantly to the exposure of the population. In plain radiography, the entrance skin dose, which is absorbed by the skin as it reaches the patient, is generally estimated. It is calculated from the air kerma at the same focus skin distance on the beam central axis measured with a dosimeter. In fluoroscopy, the indirect monitoring using dose-area product meter is generally performed for estimating the entrance skin dose in real-time to avoid skin injuries. In mammography, the average glandular dose is estimated because mammary glands have more sensitive to radiation than skin. The European Organization for Quality-Assured Breast Screening and Diagnostic Services protocol has been used to estimate average glandular dose from full-field digital mammography in Japan. Although volume CT dose index or dose-length product, as seen on CT consoles, do not represent the actual dose for the patient, they are measured to assist in quality control and optimization as well as the air kerma rate at the patient entrance reference point in fluoroscopy and the average glandular dose in mammography. For the purpose of patient dose evaluation, physical dose measurements using an anthropomorphic phantom and Monte Carlo simulations can estimate patient organ doses from medical exposure.
In medical field, radiation is used for various examinations and treatments. The number of radiological examinations has also increased internationally. Radiation is regulated by laws in each country in order to secure the safety of workers and the public in effective use. As a recent trend in occupational exposure, the International Commission on Radiological Protection (ICRP) has issued a statement to lower the equivalent dose limit of the crystalline lens of the eye to 20 mSv per year on average for five years. They released their scientific basis at ICRP Publication 118. From this statement, there is an increasing interest in the lens exposure in the medical field, its evaluation, and protection methods. In recent years surgical imaging machines used in surgery have become widespread. The use of radiation outside the imaging diagnosis department has increased, and radiation management of patients and radiation workers has been taken up in ICRP Publication 117 as a problem. This paper reports on the current situation of exposure of radiation workers at medical fields and how to manage them.
Radiation Protection is reviewed in the context of Ethics. The discussions of importance of ethics in radiation protection induced by long lived waste disposal and ignited by three major NPP accident, TMI, Chernobyl, and Fukushima. In this report, the ethical philosophy transformation through those accidents is featured, extending its application to radioactive waste disposal.
On June 6, 2017, in the hood of the analyzing room at Plutonium Fuel Facility of Oarai Research and Development Center in Japan Atomic Energy Agency, five workers were checking the storage container of fast reactor nuclear fuel material. Around 11:15 a.m., vinyl bags in the container of the fuel material including plutonium and enriched uranium burst during the inspection work. All the workers heard the bang; which caused misty dust leakage from the container. This event caused significant skin α-contamination for four workers and nasal cavity α-contamination for three workers. Decontamination was conducted for workers in the shower room. Then, the five workers were transferred to the Nuclear Fuel Cycle Engineering Laboratory to evaluate inhalation intake of plutonium etc. in lungs. The maximum values of 2.2 × 104 Bq for 239Pu and 2.2 × 102 Bq for 241Am were estimated by the lung monitor. From these results, injection of chelate agent was conducted for prompt excretion of plutonium etc. Next morning, the five workers were transferred to the National Institute of Radiological Sciences for treatments including decontamination of their skin and measurement by lung monitor. Then no obvious energy peak was confirmed for plutonium. The Japan Health Physics Society launched the ad-hoc working group for plutonium intake accident around the middle of June to survey issues and to extract lessons on radiological protection. We will report the activity of the working group.
New dose conversion factors (DCF) for radon progeny inhalation have been presented in the latest ICRP publication 137. There used to be a large difference in the DCF between those derived from epidemiological (ICRP 65) and from dosimetric approaches (ICRP 66). In parallel, UNSCEAR has presented their own DCF. The UNSCEAR DCF fell in the two results given by ICRP. This revision results in a higher DCF than before. This is based on the recently new scientific findings obtained by pooled analyses of epidemiological data from European studies on residential radon and lung cancer. Although the publication 137 is used only for occupational exposures, it will be able to be applied because of the consistency. For occupational exposures the new DCF is two times higher than the previous value and is estimated to be 17 nSv per Bq h m-3. It may be different from the previous one by a factor of more than 3 for public exposures (approximately 21 nSv per Bq h m-3). Using the mean indoor radon concentration in Japan, an annual effective dose due to radon progeny inhalation indoors is estimated to be 0.9 mSv a-1. As the DCF is calculated according to aerosol characteristics, site-specific DCFs can be provided.
Total beta measurements at Fukushima Daiichi Nuclear Power Station of Tokyo Electric Power Company (TEPCO) were studied using the egs5 Monte Carlo code. Number of β-rays from various calibration sources emitted to 2π direction, β-ray counts rates by the low background gas-flow counter (LBC) were calculated for calibration sources and imitation sources. Detector efficiencies obtained using these calculated values were compared with measured ones by TEPCO. Total beta values for imitation sources were compared between measurement and calculation to study dependence for radionuclide to get detector response, for imitation source radionuclides and the thickness of LBC wall. Self absorption effects inside source were also studied using measured count rates from KCl.