Accelerator Based Radiation Measurements of In Vivo Total Body Nitrogen

Abstract

Accurate measurement of protein in the human body is essential to the management and maintenance of health in individuals when correct balance cannot be achieved without intervention. It can be measured in a number of ways but the least invasive method is by neutron activation analysis, in this case using the 14N(n,γ)15N reaction and measuring the prompt γ-rays in the region of 9 – 11 MeV, the single most prominent γ-ray being at 10.83 MeV. During the course of investigations, using phantoms containing different ratios of nitrogen to water, flux suppression was observed as the nitrogen content increased. This was attributed primarily to the 14N(n,p)14C reaction, which cannot be measured directly by the methods used in these investigations. The suppression of flux at 2.6% nitrogen content, typical of human body composition content, was found to be 7.0+/-1.0%, which should be taken into account when using hydrogen as an internal standard to quantify nitrogen. The optimum proton beam energy and current from the KN accelerator used to liberate neutrons from a thick lithium target was determined to be 2.5 MeV and 0.3 μA respectively. Over the course of the experiments, an overall improvement in precision was achieved by the addition of a fast pile-up rejector and pulsing of the proton beam. High energy γ-rays were observed, which can interfere with the counts in the nitrogen region via Compton scattering. The origin of these γ-rays was identified and then reduced by the effective use of beam pulsing. Investigations to achieve this reduction included shielding, filtering, and optimization of the pulsing parameters using a purpose-built multi-scalar unit, specifically designed to identify the timing of the arrival of the thermal neutrons.