Plaque type60Co γ-ray sources having wide radiation field with high absorbed dose rates ranging of about 100-10000 Gy/h is widely used in radiation processing e.g. radiation sterilization of health care products. Calibration of such radiation processing doses or dose rates is not easy to be performed at the present national standard dosimetry laboratories, since characteristics of most of their radiation fields can provide collimated gamma-rays from a point source and/or lower dose rates, which are quite different from those of panoramic (uncollimated) radiation fields used in radiation processing. The high-dose calibration technique using a parallel plate ionization chamber system has been developed in the JAERI dose calibration facility. The facility equips two60Co γ-ray plaque sources covering wide dose-rate range of 5-20 000 Gy/h which have overlapping dose-rate region with that at the National Institute of Advanced Industrial Science and Technology (AIST), the national standard dosimetry laboratory in Japan. Consistency of dose calibration results at JAERI and those at AIST was verified without additional correction regarding to difference in characteristics of radiation fields involving current/exposure-rate conversion coefficients given in the dose rate level of 10-100 Gy/h at the AIST and high-dose rates of 5-20 000 Gy/h at JAERI. This consistency demonstrates our dose traceability to the AIST from the technical point of view. The dose calibration at JAERI covers the high-dose range of 0.5 to 1.60 × 105 Gy with uncertainty of 2.2% at 95% confidence level.
Peak efficiency of a Ge detector for any volume sources with different matrices can be determined by integration of disk source peak efficiency as a function of distance from the detector end-cap. In comparison with the conventional method using many standard volume sources, advantages of the disk to volume integration method are higher precision of peak efficiency because of exact geometry and good homogeneity in radioactivity, available for any kind of source matrix using one standard disk source, facile storage of disk sources and low cost. An easy method to prepare the disk source is described, and a problem was found in correction of summing coincidence effect for volume source.
Some toy badges that we obtained emitted fluorescence strongly, when there were irradiated by an ultra violet lamp in a darkroom. They were left on X ray negative films for several hours and one of them blackened films. Owing to these facts, we doubted if it is a radioactive material. In order to search if it is a radioactive material or not, a portion of the badge paint was examined by a liquid scintillation analyzer and an ammeter which is connected to a photo diode. Consequently, the badge paint was not identified a radioactive material. Then, to find the reason why some badges emitted under an ultra violet lamp and one badge paint blackened X ray negative films, neutron activation analysis was applied to the badge paint sample. As a result, we finally found that the badge paint was made of phosphorescent phosphor because it contains aluminum.
The effect of the weak leakage magnetic field on the response of a scintillation survey meter for125I is studied. The counting rate and the magnetic flux density are measured on the planes above the surface of an analog meter. The meter produces the weak leakage magnetic field. With the presence of the magnetic field, the counting rate for background radiations increases, whereas that for radiation from125I decreases considerably. The spectra of these radiations are observed to be shifted to the lower energy side. An amplification factor of the photomultiplier tube connected with the scintillator is reduced by the magnetic field. The variation of the counting rate is therefore ascribed to the shift of the spectra. When the main part of the spectrum enters the window set by the single channel pulse height analyzer of the survey meter, the counting rate increases. On the other hand, when the part leaves the window, the counting rate decreases. These results show that when radioactive contamination is checked with the scintillation survey meter, it is necessary to be more careful about the effect of the leakage magnetic field.