Dosimetry of the boron neutron capture reaction for BNCT and BNCEFNT

The use of paired proportional counters, constructed from A-150 tissue equi valent plastic (TEP) and A-150 TEP loaded with an appropriate amount of B-1 0 (50 to 200 ppm), for the dosimetry of the boron neutron capture reaction has been investigated for several years at the Gershenson Radiation Oncolog y Center. This method has been used for determining the dose components (fa st neutron, gamma ray and boron capture product dose) in both Boron Neutron Capture Therapy (BNCT) beams and in beams proposed for boron neutron captu re enhancement of fast neutron therapy (BNCEFNT). A disadvantage of this me thod, when standard 1/2 " diameter Rossi type proportional counters are use d, is that the beam intensity must be relatively low in order to avoid satu ration effects (pulse pile-up) in the counter. This is a major problem if m easurements are to be made in a reactor beam, since reducing the beam inten sity generally results in a change in the neutron spectrum. In order to ove rcome this problem, miniature cylindrical proportional counters have been d eveloped which may be used in high intensity beams. The operational charact eristics of these counters are compared with the standard 1/2 " spherical c ounters. A further disadvantage of proportional counters is the relatively long time it takes to collect data, particularly if detailed information (d epth-dose curves and beam profiles) is required. This problem could be over come by using a set of ionization chambers (an A-150 TEP chamber, a Mg cham bar and a Mg chamber with a 25 mu m boron loaded inner wall) which can be scanned in a water phantom. After calibration against the paired proportion al counters it should be possible to extract the fast neutron, gamma ray an d boron neutron capture product doses from measurements made with these thr ee ionization chambers. A set of such chambers has been used to make prelim inary measurements in a fast neutron beam and the results of these measurem ents are presented. (This work has been supported in a part by a grant from the US Department of Energy, Grant # DE-FG02-96ER62217.).