In this work we present the results of the first part of a research project aimed at offering a complete response to dosimeter manufacturers and users of the nuclear industry demand for high energy (6 MeV–9MeV) photon radiation beams for radiation protection purposes. Classical facilities allowing for the production of high energy photonic radiation (proton accelerators, nuclear reactors) are very rare and need large investment for development and use. We thus propose a novel solution, consisting in the use of a medical linear accelerator, allowing for a significant decrease of all costs. Using Monte-Carlo simulations (MCNP5 and PENELOPE codes), we have built a specifically designed electron-photon conversion target allowing for obtaining a high energy photon beam (with an average energy weighted by fluence of 6.17 MeV) for radiation protection purposes. Due to the specific design of the target, this “realistic” radiation protection high energy photon beam presents a uniform distribution of air kerma at a distance of 1 m, over a (30 × 30) cm2 area. Two graphite cavity ionization chambers for ionometric measurements have been built. For one of these chambers we have measured the charge collection volume allowing for its use as a primary standard. The second ionization chamber is a transfer standard, as such it has been calibrated in a 60Co source, and in the high energy photon beam for radiation protection. The measurements with these ionization chambers allowed for an evaluation of the air kerma rate in the high energy photon beam for radiation protection: the values cover a range between 80 mGy·h-1 and 210 mGy·h-1, compatible with radiation protection purposes. Finally, we have calculated using Monte-Carlo simulations conversion coefficients from air kerma to dose equivalents in the range between 10 keV and 22.4 MeV, in specific geometrical set-ups, and for the spectral distribution of the fluence in the beam produced by the linear accelerator of LNE-LNHB.

Key words

conversion coefficients
high energy photon beam
radiation protection
electron-photon conversion target
primary standard
transfer standard
ionization chamber