Abstract
The Dosepix hybrid pixel detector was designed for dosimetry and radiation monitoring applications. It has three programmable modes of operation: photon counting mode, energy integration mode, and dosimetry mode. The dosimetry mode measures the energy of individual X-ray photons and automatically sorts events into pre-defined energy bins. The output is a histogram representing the measured X-ray energy spectrum, permitting a dose reconstruction that accounts for the attenuation of photons at each energy bin. This presents a potential radiation protection and dosimetry instrument in medical radiodiagnostic practices, including high flux systems such as computed tomography (CT). In this paper, we characterise the Dosepix chip by investigating the energy response and count rate capabilities when coupled to a 300μm silicon sensor under high fluxes of monochromatic synchrotron radiation. Under nominal settings, the Dosepix detector can detect photons down to 3.5keV, with an energy resolution of 16.5% FWHM for 8.5keV photons and 8% FWHM for 40keV photons. The chip can count up to 1.67Mcps/mm2 of 40keV photons whilst maintaining linear counting behaviour. This count rate range can be further increased by changing the programmable operating settings of the detector, making it suitable for a range of photon dosimetry applications.
Highlights
This paper evaluates the performance of a Dosepix detector assembly equipped with a 300 μm silicon sensor
We have characterised the energy response and high flux count rate capabilities of a Dosepix detector coupled to a 300 μm Si sensor
Because photon attenuation in semiconductor sensors as well as biological tissue depend on photon energy, the spectral information provided by Dosepix permits a means to reconstruct dose that accounts for photon attenuation in each energy bin
Summary
Voltages, kVp, of medical CT scans typically generate X-ray spectra up to 120 keV–140 keV). Each Dosepix ASIC pixel contains an analogue frontend that outputs a voltage pulse whose height and width are proportional to the energy absorbed by the detector when an X-ray photon impinges on the corresponding sensor segment. This voltage pulse is compared to an analogue threshold voltage programmed to correspond to a given energy during calibration. Since the chip contains 256 discrete channels, each with sixteen programmable thresholds, the detector can theoretically output photon spectra quantised in up to 3840 energy bins from 3.5 keV to roughly 1 MeV1 (16 thresholds gives 15 bins þ 1 overflow bin). Because up to 65k hits per bin can be stored in the memory of each pixel, the spectrum of an X-ray beam can be measured in a single shot by the Dosepix detector (i.e. without the need for time-consuming threshold scans)
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