Measurements of neutron doses produced in therapeutical media

This paper presents the measurements of the neutron doses generated by the 40 MeV Medical Betatron. Measurements were performed by means of the thermoluminescent (TL) detectors i.e. the TL detector, for photon (γ) radiation by the DTL-y type and the TL detector for neutron (n) and photon radiation by DTL-γ + n type, both made in the Institute of Atomic Physics (IAP). The Sγ=1.028 x 105 pulse/Gy photon sensitivity of the two detectors was determined by a 60 Co reference calibration source and Sn=17.223 Gy, γ/Gy neutron sensitivity of the DTL-γ+n detector was determined by the method of induced activation in gold foils at the 2 MW Nuclear Reactor of IAP. The thermal neutron fluency conversion was made with a factor of 1 Gy=3 x 1011 n/cm2. As therapeutical medium, a water phantom (SSD=1085 mm) irradiated with high energy photons on 10 x 10 cm2 standard field was used. The determination method of neutron absorbed dose consisted in the simultaneous irradiation of the two detectors, located inside and near the phantom, and the calculation of the difference between the two detector responses. The thermal neutron detection in DTL-γ+n detector was based on recording of the charged particles (alpha and triton) from the nuclear reaction 6 Li(n,α)3 H. The measurements showed that the ratio between neutron dose and photon dose is less than 0,1277% at the depth of the maximum absorbed photo dose. (authors)