A Geant4 study on the comparison of the absorption of low energy X-rays in water and PMMA phantoms

The aim of this study was to validate a Geant4 Monte Carlo application for dosimetry measurements at low energies when using water and PMMA phantoms. To measure the water-equivalence in plastic materials, relative dosimetry can be performed by deploying the calculation of depth dose profiles and output factors. We simulated a quasi-uniform monochromatic X-ray beam, with dimension of 2×2 cm², perpendicularly impinging on a phantom, with energy ranging from 30 to 140 keV. A comparison of the attenuation of the beam after passing through slabs of PMMA and/or water is presented and validated against experimental measurement performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The use of PMMA as a substitute of water at low energies is not recommended for dosimetric studies as the depth dose profiles show discrepancies of at least 3%.