The Dosimetry Laboratory at the Norwegian Radiation and Nuclear Safety Authority (DSA) is the designated institute (DI) for ionizing radiation in Norway under the CIPM MRA1. We are a member of CCRI(I)2 on dosimetry and serve users in radiation therapy, medical diagnostics and radiation protection, including nuclear preparedness. Calibration of instruments used for measuring dose rates and doses from x-ray fields requires knowledge of x-ray spectra. These spectra depend on the generating voltage (which gives the kVp cutoff max energy of the spectrum) and inherent and added filtration (which determines how much fluence is removed from the beam at different energies). Different combinations of generating voltages and filters give different so- called x-ray qualities. Standarized qualities are specified in standards such as IEC 61267 (for diagnostics) or ISO 4037 (for radiation protection).
In the current project we want to investigate methods for establishing capabilities for x- ray spectrometry at the Dosimetry Laboratory at DSA. We want to achieve this using a commercial CdTe semiconductor detector and making measurements in the 10 kVp to 300 kVp range preferentially without the use of very small (<100 μm) pinhole collimators. We aim to investigate the use of i) larger pinhole collimators (>200 μm) in combination with filter stacks and ii) validation at long distances utilizing the inverse square law and modest air attenuation.
The project requires development of:
i) A robust and reproducible setup jig for the AmpTek CdTe spectrometer including its EXVC Collimator Kit, extra shielding and additional filter stacks
ii) A Geant4 Monte Carlo radiation transport simulations model of the setup jig
iii) A one-dimensional analytical model of electron-hole transport in the CdTe-crystal
iv) An inverse model of the system for spectral unfolding
The main aims of the project are to establish these capabilities and to characterize the performance of the system against published studies using very small (<100 μm) pinholes. Free-in-air fluence spectra will be used to calculate beam characteristics such as first and second half-value layers (HVLs) and kVp cutoff for the Dosimetry at BIPM/CCRI, IEC 61267 and ISO 4037 beam qualities.