Thermal and epi-thermal neutron fluence rates around the isocenter of the electron linear accelerator at Hiroshima Prefectural University were measured using indium foils when the 10 MV X-rays were generated at the copper target. Gold foils are frequently used for measurements of thermal and epi-thermal neutron fluence rates. The indium foils are more sensitive than gold ones, and the half-life (54 min) of 116mIn is convenient for measuring the gamma rays. The thermal and epi-thermal neutron fluence rates in a tissue-equivalent block phantom were measured not only by indium foils but also by gold ones. The both results were in approximately agreement with each other. Thermal neutron fluence in phantom human was, for example, 2.6×1010 m−2 per 1 Gy of treatment dose at 5 cm depth from the surface along the base line. The epi-thermal neutron fluence rate was less than 1/10 of the thermal one. The results suggest that the neutrons induce few activities in patient body. The results also suggest that the 28Al radioactivities induced in the couch may cause about 0.4 mGy of annual radiation doses to the medical staffs.
The numerical tables of dose rate constants and penetration factors of radioisotopes, published in “Manual for shield calculation of radiation facilities, Nuclear Safety Technology Center (2007)”, are widely employed in estimating the effective dose of radiations from radioisotopes. In order to extend the expedient shield calculation method with the tables in the “Manual”, we have prepared new tables of dose rate constants and penetration factors. Main improvements in the present tables are as follows: 1) Number of radioisotopes available for the calculation is increased from 66 to 429 and that of decay chains are increased from 8 to 48. 2) Some incorrectness in the “Manual”, which is related to the estimation for β-ray bremsstrahlungs, are pointed out and discussed: β-ray spectrum, β-ray bremsstrahlung spectrum and bremsstrahlung emission efficiency ratio. 3) Dose rate constants and penetration factors of bremsstrahlungs caused by mono-energetic electrons (internal conversion and Auger electrons) are newly estimated. The incorrectness in the “Manual” is explained in this paper. Notable differences between the present tables and the “Manual” are also discussed.
For a direct measurement method using a GM survey-meter, the detection efficiency and detection limit against surface contamination with nuclides emitting β-rays on a polyethylene-coated paper were examined. Three nuclides, 14C, 35S and 32P were used as a source. The detection efficiency against 14C, 35S and 32P at the place of 5.0 mm distance from the detection face of the survey-meter was approximately 1.4, 1.9 and 34.5%, respectively. Although the detection efficiency obtained for 32P was the same as the value showed in the instruction manual, that of 14C was smaller than that of the instruction manual. It is considered that such a difference in detection efficiency between 32P and 14C is ascribed to difference in source efficiency. Furthermore the detection limit against 14C, 35S and 32P at the distance of 5.0 mm was about 3.6, 2.6 and 0.2 Bq/cm2, respectively. Hence it appears that the direct measurement method using the GM survey-meter is useful to check the surface contamination with 14C or 35S in a controlled area since the surface contamination density limit in a controlled area is 40 Bq/cm2.
Radioisotope concentrations in air in radioisotope working area were surveyed by questionnaire. 108 concentration data for 10 radionuclides were obtained from 13 radioisotope facilities. The aggregate analysis shows that all the data were less than the concentration limits regulated for radioisotope workplaces. Radioisotope concentrations in air exceeding the detection limits were observed in 10 of 51 data obtained by sampling during handling radioisotopes. For measurements before and after handling, the detection rates were 0 and 3%, respectively. For limited 3H, 35S, and 125I, the detection rates were 57, 40, and 19%, respectively, during handling each radioisotope. The concentrations exceeded the detection limits were observed, but almost all data were apparently less than 1/100 of the concentration limits regulated for the respective nuclides in radioisotope workplaces.