Abstract
This paper presents the first results of a project intended to investigate γ-radiation activity induced in dosimeters used in clinical practice during routine quality assurance of high-energy photon beams emitted by electron linear accelerators. Two aspects of the activation via photonuclear reactions (X, n) of therapeutic beam and subsequent capture of secondary neutrons (n,γ) are under considerations: the influence of activation on intrinsic background of the dosimeters and exposure of dosimetrists who operate this equipment. The activation of several types of ionization chambers as well as the silicon diodes was studied after long-time exposure (10 000 MUs) of the 15 MV photon beam (Elekta Synergy). Photon fluxes obtained from spectra of γ-rays registered by HPGe spectrometer were subsequently converted to equivalent doses using appropriate coefficients. The main contribution to the induced activity comes from the neutron capture process on Al, Mn and Cu, therefore it decays quite fast with the half-lives of the order of 15 minutes. Nevertheless, the activation of chlorine was also observed. The estimated equivalent doses to skin and eye lens were in the range 0.19 – 0.62 μSv/min. However, no influence on intrinsic background signal of all studied dosimeters was observed. The preliminary results indicate that induced radioactivity of dosimeters is strongly influenced by therapeutic beam quality and neutron source strength of particular linac. This dependence will be studied deeper in order to quantify it more precisely.
Highlights
The phenomenon of inducing radioactivity in the construction materials of medical linear accelerators working in high-energy photon mode has been widely studied over the last decades experimentally as well as using the calculations, either a Monte Carlo simulations or the equations of activation and depletion processes, e.g. [1,2,3,4]
Presented study have shown that the process of radionuclides activation takes place in materials of commonly used dosimeters, e.g. ionization chambers and silicon diodes, at a measurable level
It has been shown that this induced activity does not affect the intrinsic signal of these dosimeters
Summary
The phenomenon of inducing radioactivity in the construction materials of medical linear accelerators working in high-energy ( above ~9MV) photon mode has been widely studied over the last decades experimentally as well as using the calculations, either a Monte Carlo simulations or the equations of activation and depletion processes, e.g. [1,2,3,4]. The phenomenon of inducing radioactivity in the construction materials of medical linear accelerators working in high-energy ( above ~9MV) photon mode has been widely studied over the last decades experimentally as well as using the calculations, either a Monte Carlo simulations or the equations of activation and depletion processes, e.g. The induced radioactivity is the consequence of nuclear photo effect (X,n) as well as neutron capture reactions (n,γ), being dependent on linac beam emission time. It seems to be especially important during linac commissioning and performance of periodical quality assurance dosimetric tests.
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