A comparison of the gamma component of two high energy neutron therapy beams.

Reciprocal neutron dosimetry intercomparisons have been undertaken between the National Accelerator Centre (NAC), South Africa, and the Université Catholique de Louvain (UCL), Belgium. A comparison of the gamma components measured in the respective therapy beams using the classic twin-detector method formed part of the study. This is important in clinical applications since the biological effects of neutrons and gamma rays are different. The gamma component under various irradiation conditions was derived from pairs of measurements with tissue equivalent ionization chambers and Geiger-Müller counters. The gamma component expressed as a fraction of the gamma dose relative to the total dose agreed within less than 0.01, with the measurements with the NAC Geiger-Müller counters being consistently higher than those with the UCL counter. A re-evaluation of the kU value of the UCL Geiger-Müller counter reduces the differences to less than 0.005 at NAC and less than 0.002 for the measurements at UCL. The results obtained are highly satisfactory in view of the complexity and difficulty of the measurements.