Identification of Neutron-Induced Isotopes in Varian LINAC System.

To investigate the neutron-induced radioactivity, we have measured and analyzed the energy spectrum of decay gammas in external radiotherapy system. The measurements has performed with two of Clinac 21EX and Clinac 21EX platinum (Varian Medical System, USA) using a NaI crystal detector placed inside LINAC heads. After the spectrum had taken for 1 hour of data acquisition (DAQ) time, sequentially we took another spectrum under the same condition for 58 ~ 59 hours of the DAQ time. The ratios of full peak areas in the two gamma spectrums was compared with the ratio calculated with the lifetimes of isotopes corresponding to the decay gamma energy. The comparison method provides the identification of the induced isotopes using not only the decay energy of secondary gammas but also the lifetime of isotopes. The ratios of full peak areas for candidates was calculated as 40.6 (²⁰³Pb), 27.7 (¹⁸⁷W), 34.6 (⁸²Br), 58.8 (⁵⁴Mn) and 34.7 (⁵⁷Ni). The area ratios of full peaks tagged by gamma energy spectrum were measured to be 39.5, 25.3 28.0, 29.3, 25.2 and 35.9 38.8 respectively, which are consistent within errors. [ABSTRACT FROM AUTHOR]