NEUTRON DOSE MEASUREMENT AT THE BROOKHAVEN NATIONAL LABORATORY WHOLE-BODY ACTIVATION FACILITY

The neutron dose at the Brookhaven whole-body activation facility was measured under various operational conditions. A bomab phantom was use d in the measurements. The measurements were done with a pair of a tis sue-equivalent (TE) and a graphite ionisation chambers. The TE chamber is used for neutron dose measurement and the graphite for gamma dose subtraction. The effect of a polyethylene (PE) moderator was investiga ted as well as the effect of the source-body distance. In addition to the regular phantom. the dose to an obese phantom was also measured. T he facility is made of 14 PuBe neutron sources, similar to 50 Ci each. The patient stays in the facility for 300 s, lying on a 2 cm thick po lyethylene (PE) bed, covered with a 2 cm thick PE moderator. The neutr on dose at the patient's chest under such conditions is 267 mrem. The gamma dose is 13 mrem. When the PE moderator is removed, the dose incr eases by about 20%. Thinner moderator (similar to 1 cm) also increases the dose: it also increases as the patient's skin gets closer to the sources. The increase is close to l/r(2). A 2.5 cm change in distance at 30 cm increases the dose by 22%. The dose to the patient's back was found to be 10% higher due to neutron scattering from the facility Bo or and walls. In addition to the dose, the neutron and gamma spectra w ere measured with and without the phantom and moderator. Based on the measured neutron spectrum, an effective QF=13 was found. Neutron moder ation by hydrogen was observed, the high energy neutron peaks are lowe r when more hydrogen is present in the surrounding material. In the ga mma spectra it was found, as expected, that the hydrogen in the PE mod erator and more so in the phantom, increases the intensity of the 2.2 MeV gamma rays.