Activation Pocesses in Medical Linear Accelerators Activation Processes in Medical Linear Accelerators and Spatial Distribution of Activation Products

Activation products have been identified by in situ gamma spectroscopy at the isocenter of a medical linear accelerators shortly after termination of a high energy photon beam irradiation with 15 x 15 cm field size. Spectra have been recorded either with open or with closed collimator. Whilst some activation products disappear from the spectrum with closed collimator or exhibit reduced count rates, others remain with identical intensity. The former isotopes are neutron-deficient and mostly decay by positron emission or electron capture, the latter have neutron excess and decay by β emission. This new finding is consistent with the assumption that photons in the primary beam produce activation products by (γ,n) reactions in the treatment head and subsequently the neutrons created in these processes undergo (n,γ) reactions creating activation products in a much larger area. PACS classification: 87.56.By; 87.52.-g