A monitoring post is continuously working for the environmental radiation measurement around a nuclear facility.A measured dose rate is sometimes higher than usual values.In this case, it is necessary to judge whether the radiation leakage has occurred in the nuclear facility. An effective method is to know the direction of incident radiation.Two type of direction-finding detectors have been developed to obtain incident direction.One is the detector with three kinds of scintillators, NaI, CsI and BGO, and the other is the detector with three divided NaI scintillators. The principle and performance have been examined by both computer simulations and laboratory experiments. The results show that both types can measure incident direction and that errors of direction measurement are within three degrees.

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Radioactive nuclides included in eight kinds of car goods which were declared radioactive consumer products (RCP) were analyzed qualitatively and quantitatively. All samples were Naturally Occurring Radioactive Materials which included daughter radionuclides of thorium series and uranium series. The maximum specific activity was 19 Bq·Eg^-1 of ²²⁶ Ra included in smashed stones which declared a fuel efficiency improvement. The maximum radiation exposure dose per one hour from granular sample of 500g at 1m was 2.3×10^-3μ Sv. The radiation exposure dose by working of 2, 000 h per year did not exceed ten μ Sv which has been proposed by the National Radiological Protection Board for one RCP per year.Because one sample has emitted alpha rays about 900 cpm per 14g, the misuse of the sample must be avoided. There were no samples breaking the nuclear reactor and fuel regulation law. By comparison with the Basic Safety Standards, the specific activities of three kinds of samples exceeded a regulation exemption level.

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The results of regular working environment measurements in our radioisotope facilities were about background levels. However, most of these are the data measured when unsealed radioisotopes were not used. High concentrations of radioisotopes were detected when measured during the time when unsealed ³⁵S or ¹²⁵I was used. These results were estimated far under the regulation levels. Thus, safety working environments are sufficiently guaranteed by suitable estimation and suitable handling under the strict regulation. Therefore, regulation would be relaxed in the field of education and research in such ways as estimation by calculation in place of the actual measurements, decrease of the number of monthly measurements, and measurements exemption for nonvolatile or low levels of isotopes. Working environment measurements would be still effective in cases where volatile radioisotopes are used in largequantities. They may provide a useful index if improvement of safe handling technique or equipments is required.

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A GM survey-meter probe is usually covered with a polyethylene or poly (vinylidene chloride) sheet to prevent radioactive contamination. We investigated the effects of the thickness of these protective covers on the counting values of the survey-meter. A source was put on a detection face of the probe wrapped with the protective cover, and the counting values of the radiation from the source (¹⁴C, ³⁵S, ⁴⁵Ca, ⁵¹Cr and ³²P) were measured. As the result, the decay rate of the counting values of ¹⁴C and ³⁵S were high, that of ³²P was low, and that of ⁴⁵Ca was between these nuclides, in any thickness of these protective covers. On the other hand, in the case of ⁵¹Cr, the decay rate of the counting values with the poly (vinylidene chloride) cover was far higher than that with the polyethylene in any thickness. From calculation of transmittance of radiation from ⁵¹Cr, such difference in the decay rate between these covers was ascribed to the difference in constituent elements of these covers. Further, it was confirmed that the differences in the decay rates of the counting values by using the polyethylene sheets were available to identify certain of nuclides.

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OSL personal dosimetry has been widely used in our country.Stimulation of luminescence with a green laser or green light emitting diodes (LED) is realized in the design of OSL readers.New high power LED is the most attractive light source because of their simplicity and reliability.In this paper we will report the performance of the OSL reader equipped with“amber”(590nm) LED.The OSL reader with amber LED was used without additional glass filter and had twice the noise level in comparison with the reader equipped with green LED.The minimum detectable dose of the OSL reader using the bare amber LED was 0.02m Sv.It was found that this reader can be used not only for personal dosimetry but also for environmental monitoring.The OSL reader equipped with amber LED does not need an additional optical filtration of LED light and therefore has a more compact design.

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