Abstract
The principles of In Vivo Neutron Activation Analysis, IVNAA, and the present status of IVNAA methods and other in vivo elemental composition techniques have been outlined. Description has been given of the modifications made to the IVNAA facility used in the present study and the subsequent effects on the performance of the system have been discussed. Detection limits, using the ’prompt’ neutron activation technique for the major body elements, sodium, chlorine and nitrogen were found to be 220 ppm, 140 ppm and 1.55% by weight, respectively. It was found that neither calcium nor phosphorus could be measured using the ’prompt’ technique at acceptable dose levels to the subject. It has been shown that measurement of body chlorine concentrations through the 37Cl (n,γ) 38mCl reaction is not feasible using ’cyclic’ activation analysis at acceptable dose levels delivered to the subject. Determination of cadmium and selenium concentrations in a liver phantom was carried out, during the same experiment, using the technique of alternate ’prompt’ and ’cyclic’ activation analysis. This allows for the collection of the ’prompt’ gamma-ray data with no further dose delivered to the subject. Detection limits of 13 ppm and 5.8 ppm for Cd and Se were obtained, respectively. The origin of the interfering photopeak in 77mSe measurements was not conclusively identified but some of the possible sources have been outlined and suggestions have been made for further investigations. Neutron inflicted damage of germanium based semiconductor detectors has been discussed and a method of Ge(Li) crystal repair has been described that is expected to lead to full fast neutron damaged detector regeneration, provided that the necessary active outgassing of the detector vacuum enclosure is incorporated. A Monte Carlo aided Fortran-77 computer programme for the calculation of the average solid angle subtended by a collimated detector at the photon emitting source which addresses the collimator edge penetration was developed and tested. The strength of the simulation technique in providing pre-experimental information in a variety of investigations where collimated detectors are used for gamma-ray measurements has been demonstrated. The goal of performing IVNAA as an ’absolute’ method has been partially addressed; it has been shown that the programme can be used to: (i) determine the volume of the activated target viewed by the collimated detector and (ii) to estimate the effect of the neutron flux non-uniformity within the activated volume of interest. Finally, the photoneutron field around a medical electron accelerator has been determined using ’bare’ activation detectors. A review has been given of the past photoneutron measurements and the results of the present study have been discussed. Evidence of photoneutron production within the patient body has been presented. The ’in beam’ photoneutron dose equivalent contribution on the patient plane was found to be 8.7 +- 30% mSv per Gy of photons.