Report on NRL Measurements as a Participant in the INDI.
NAVAL RESEARCH LAB WASHINGTON D C
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NRL was invited to participate in the International Neutron Dosimetry Intercomparison INDI which took place during 1973 at the Raiological Research Accelerator Facility of Brookhaven National Laboratory, under the sponsorship of the International Commission on Radiation Units and Measurements. The source was a D-T generator of 15.5-MeV neutrons. Measurements were made only along the extended axis of the deutron beam at 30 cm in free space, and at depths of 5, 10, and 20 cm in a 30 cm cubic water phantom positioned with its input face 20 cm from the neutron source. A 1-cc tissue-equivalent TE ion chamber was used with TE gas or air flowing through it to measure the total ngamma dose in tissue, with appropriate corrections. Thermoluminescent dosimeters were employed to measure the gamma-ray dose component, assuming that the ngamma sensitivity ratio measured in the NRL neutron beam approx. 15 MeV average energy was appropriate to the free-space measurement for the INDI. The gamma-ray dose component at 30 cm from the target is negligible in free space, probably not exceeding a few tenths of a percent of the total dose there. Within the phantom it is necessary to make a correction to the ngamma sensitivities of the TLDs to account for the degradation of neutron energy with depth. The neutron spectrum at 5 cm, 10 cm, and 20 cm depth has been estimated by Herling through Monte Carlo calculations.