In Vivo Neutron Activation Analysis and Associated Dosimetry

Abstract

The use of neutron activation analysis in-vivo for the measurement of calcium in the human body has proved to be a more accurate and less hazardous technique than many other methods currently used. The health hazard associated with this technique is determined in this study by a method of neutron dosimetry based on the radiation damage in plastics caused by neutron induced fission fragments. The lower legs of a phantom were irradiated for 10 minutes with four 23Q 49 Pu-Be neutron sources immersed in water. The 3.1 MeV Ca gamma rays from the legs were counted by two 8" dia. by 4" Nal(Tl) scintillation detectors in the Whole Body Counter at the Toronto General Hospital. The activity was such that there was a statistical error of 2%. The reproducibility of the results was determined from a series of 10 irradiations and the standard deviation was found to be +/- 2.3%. The expected interference from the 3.1 MeV peak in 37S produced by the 37Cl(n,p)37S reaction was estimated to be about 2% and an experimental assessment of this interference showed that the contribution made by 37S to the 49Ca peak was about 4% . For the neutron dosimetry, the plastic damaged by the fission fragments was etched in a 6N solution of NaOH, and the resulting holes counted by an electrical sparking method. Experiments were carried out to find optimum processing procedures and to check the range of linearity of the system. It was found that 232th foils were adequate for determining the significant neutron dose, which was about 0.8 rem during the irradiation of the legs. If required, 235U foils may also be used to monitor thermal fluences. A comparison of fast neutron doses measured by this technique with a completely independent method at Brookhaven National Laboratories showed very good agreement.