Dose calculation and measurement for radiation protection at the Finnish BNCT facility

Abstract

The dose absorbed by tissues irradiated with epithermal neutrons of a boron neutron capture therapy (BNCT) beam is due not only to epithermal, thermal and fast neutrons but also to gamma-rays. The gamma dose, which originates mainly from the capture reaction 1 H(n,g) 2 H but also from a gammaray component present in the beam, exposes both cancer and healthy tissue to the same irradiation. The other signi®cant dose component arises from the capture reaction 14 N(n,P) 14 C. The main dosemeters for characterizing and controlling BNCT beams are activation foils and paired ionization chambers. Thermoluminescent (TL) dosemeters are also of interest because of their small size and suitability for large-scale measurements. With TL dosemeters it is possible to measure depth±dose curves and pro®les at the same time, with one irradiation.In this study, gamma dose and neutron¯uence distributions were measured in cylindrical PMMA and water phantoms (diameter 20 cm, length 24 cm) at the BNCT beam using TL detectors. Measurements for the determination of the depth±dose curve were made 1, 2, 2.5, 3, 6, 10 and 15 cm from the beam aperture and phantom surface. At depths of 2.5 and 6 cm in the water phantom the beam pro®les in both the x-and y-direction were also measured. In these measurements, a beam delimiter aperture of 14 cm was used.A TL dosemeter [MCP-7s ( 7 LiF:Mg,Cu,P) from TLD Niewiadomski, Krakow, Poland] was used for gamma detection because of its high sensitivity to gamma radiation compared with LET radiation [1]. However, thermal neutron sensitivity and thus an appropriate correction factor for 7 LiF:Mg,Cu,P TL material still remains a problem. A MTS-Ns TL dosemeter from the same manufacturer with an ultrathin active LiF:Mg,Ti layer for small self-shielding of thermal neutrons was selected for use as a neutron sensitive dosemeter. According to the preliminary results, the measured gamma doses and neutron uences correlate with those calculated using the DORT code and measured with ionization chambers [2].