In this report are discussed the characteristics of a radiation field generated by a137Cs γ-ray source and the depth-dose curve in a material placed there. A simpler formulation was achieved to describe fundamental dosimeterc quantities, the kerma and the absorbed dose. The well-known pattern of the depth-dose relation, i.e. rapid increase followed by gradual decrease due to attenuation of primary photons, could be confirmed by a numerical calculation under the condition of an exposure to parallel photon beam in vacuum. The experiments with CaSO4TL elements of 15 mg/cm2thick attached to a plastic substrate of 11 mg/cm2and aluminum foils, contrary to the theoretical prediction, resulted in a different depth-dependence of about 30% decrease by aluminum of 100 mg/cm2, which corresponds to an attenuation of 26 keV photons. Another experimental trials and a rough estimation of the absorbed dose should lead an inference that the absorbed dose near the surface may be considerably affected by secondary electrons generated in the capsule and/or the air between the source and the detector. In order to discuss more quantitatively, a Monte Carlo code EGS4 was employed to calculate the energy and the fluence of the secondary electrons with an assumption of a spherical γ-source made of glass in a stainless steel capsule. It was found out that the relative fluence of the secondary electrons was only a few thousandth of the primary photons, but that their contribution to the absorbed dose near the surface amounted to more than 70% owing to a much larger conversion coefficient. A careful consideration to the secondary electrons must be required in the case of γ-ray irradiation to materials or calibration of dosimeters with a γ-ray source.
Electromagnetic noise from radiation detectors and from an aircraft itself were examined using a field meter. It was proved that the electromagnetic noise from radiation detectors is smaller than that from the aircraft. Dose equivalent rates due to cosmic ray neutrons and ionizing radiation were measured using a moderated3He counter and a CsI (Tl) scintillation counter in a commercial aircraft at an altitude of approximately 10 km, between Narita, Japan and Vancouver, Canada. The measured data were compared with the results calculated using calculation code (CARI2), and it was found that both agree approximately.
For a next generation positron emission tomograph (PET) detector, quadrant sharing detector or detector using position sensitive photomultiplier tube (PSPMT) as well as conventional block detector will be promising candidates. Since these detectors require different position calculation circuit, it will be a considerable work to develop position calculation circuits for each type of detector. Thus, a flexible personal computer based position calculation system was developed. The system calculates the position by the personal computer event by event basis. The system made it possible to be applicable to many kinds of position calculations for different types of PET detectors.