Experimental determination of thermal neutron fluence around Elekta Versa HD linear accelerator for various photon energies

A complex neutron spectrum generated along with a useful photon beam imposes an additional radiation protection risk around medical linear accelerators (linac). The thermal neutron component of this complex neutron spectrum formed during different photon modes of operation of Elekta Versa HD linac has been quantified using Indium foil activation technique. The thermal neutron fluence (Phi th) at isocenter for 15 MV, 10 MV and 10 MV FFF beams was found to be 2.45 x 10(5), 4.35 x 10(4)and 3.2 x 10(4)neutrons cm(-2)Gy(-1), respectively. The analysis shows a reduction in the Phi th as the flattening filter is being taken out from the beam path. A negative correlation in Phi th with respect to field size has been observed with an average 18% reduction in Phi th per monitor units as field size changes from 10 cm x 10 cm to 40 cm x 40 cm. For particular field size and photon energy, Phi th was found to be uniform across the patient plane. From the measured gamma ray spectrum inside the treatment room six major isotopes have been identified which were Sb-122,W-187,Br-82,Mn-56,Na-24 and Al-28.