Neutron Dose Measurement at the Brookhaven National Laboratory Whole-Body Activation Facility

Abstract

The neutron dose at the Brookhaven whole-body activation facility was measured under various operational conditions. A bomab phantom was used in the measurements. The measurements were done with a pair of a tissue-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 investigated 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. The facility is made of 14 PuBe neutron sources, ∼50 Ci each. The patient stays in the facility for 300 s, lying on a 2 cm thick polyethylene (PE) bed, covered with a 2 cm thick PE moderator. The neutron dose at the patient's chest under such conditions is 267 mrem. The gamma dose is 12 mrem. When the PE moderator is removed, the dose increases by about 20%. Thinner moderator (∼1 cm) also increases the dose: it also increases as the patient's skin gets closer to the sources. The increase is close to 1/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 floor and walls. In addition to the dose, the neutron and gamma spectra were measured with and without the phantom and moderator. Based on the measured neutron spectrum, an effective QF=13 was found. Neutron moderation by hydrogen was observed, the high energy neutron peaks are lower when more hydrogen is present in the surrounding material. In the gamma spectra it was found, as expected, that the hydrogen in the PE moderator and more so in the phantom, increases the intensity of the 2.2 MeV gamma rays.