Primary Barrier Calculations with an Effective Build-up Factor

Radiotherapy X-ray facilities require structural barriers to reduce radiation. To calculate the thickness of these barriers, rather conservative limits are normally set to ensure that the radiation limits are not exceeded. Recently, members of our department published a more exact method for calculating the thickness of primary barriers, which took into account the scatter radiation generated in a particular barrier, by using an effective build-up factor. We have employed this method to calculate the thickness of a primary barrier for a bunker for our Varian-600c accelerator, with a maximum nominal energy value of 6 MV. The calculation was carried out for concrete and iron materials. We repeated the calculations based on the DIN 6847 standard from the tenth-value layer of concrete and from the corresponding attenuation of iron provided in ICRP 33. The results obtained by both methods were very similar. However, by adapting our method to the particular conditions of our accelerator we find it more useful for those cases in which, for reasons of space or economy, a calculation more in keeping with actual conditions needs to be made. INTRODUCTION Therapeutic X-ray facilities require structural barriers to reduce the levels of radiation to the recommended limits [1]. The method for estimating the thickness of a barrier consists of calculating the attenuation that must be provided by that barrier, according to the material, the structure of the installation and the use of the equipment. Subsequently, the reading of the attenuation curve of the barrier thickness has to be referred to, or either the tenth-value layer of the material used in the construction of the barrier. We can find this data in national and international guidelines [2, 3, 4, 5] or by more recent estimations, such as those based on the Monte Carlo method. However, these values are given for nominal maximum energies of general spectra of X-rays and are not specific to each accelerator. In those cases where, for reasons of space or economy, a more exact calculation of barrier thickness is wanted, members of our department have published a method for calculating the thickness of primary barriers, specific for the spectrum of the accelerator and taking into account the dispersion generated in the barrier by using an effective build-up factor [6]. Recently we improved this method by introducing a coefficient of attenuation of the material, also specific to the spectrum of the accelerator. We have carried out calculations with this new method for two materials normally used in the construction of barriers, such as concrete and iron, for a primary barrier of the bunker for our 6 MeV nominal maximum energy Varian Clinac 600c accelerator. MATERIALS AND METHODS 1. Method based on the DIN 6847 and ICRP 33 recommendations Using the method based on the DIN 6847 standard [5] the thickness of the barrier required is given by       ⋅ = W i i i i H q WUTK z s log i i W q WUTK H gives us the maximum transmission allowable for the barrier.