Analysis of body size effects on the WBC

In an accidental intake of radionuclides, identification of the nuclides and determination of the exact amount in the body are important. The identification can be done by measurements using high resolution Ge semiconductor detectors in a whole body counter (WBC). Body size affects the determination of amount. Various BOMAB phantoms were measured to confirm the effects. The measurements were carried with the WBC owned by the National Institute of Radiological Sciences (NIRS). This WBC is composed of six Ge semiconductor detectors. First, adult male BOMAB phantoms which uniformly enclosed Ba-133, Cs-137, Co-60 and K-40 sources were measured. Then the detection efficiencies of various energies were gotten and size influence was calculated by extrapolation from the results of similarity measurements of five BOMAB phantoms which uniformly enclosed a K-40 source; the five phantoms were of a 4 year-old, a 10 year-old, an adult female, an adult male and a 95th percentile male. Moreover, it is necessary to clarify the dependence of the efficiency on the distribution of the nuclides just after they are taken into the body. Point sources were affixed between the chest and the waist as a local distribution. Although the ratio was generally 1 near 80 keV, it was 1.3 to 1.6 times that from 350 keV to 1333 keV. Furthermore, the point sources were affixed on the surface of the BOMAB phantoms. This situation was assumed as the case where surface contamination cannot be removed such as when there is a wound. For the same radioactivity, it turned out that the internal contamination had rather higher detection efficiency than surface contamination on the adult BOMAB. On the contrary, the detection efficiency of surface contamination became higher than that for internal contamination for smaller body size