G. Shani, NEUTRON DOSE MEASUREMENT AT THE BROOKHAVEN NATIONAL LABORATORY WHOLE-BODY ACTIVATION FACILITY, Radiation protection dosimetry, 67(4), 1996, pp. 263-270
Citations number
7
Categorie Soggetti
Radiology,Nuclear Medicine & Medical Imaging","Nuclear Sciences & Tecnology
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.