Michael Böhnel

Low energy dosimetry with photon counting pixel detectors such as Medipix

The use of a semiconductor photon counting pixel detector like the Medipix detector [1] [2] in a dosimeter offers the possibility to take the photon energy dependence of the personal dose equivalents especially in the low energy range below 50 keV into account. Furthermore the measuring range can be extended down to low photon energies of about 10 keV. In this contribution we restrict our considerations to the medical diagnostic energy range from 10 to 150 keV. Due to the fact that the sensitive area of the Medipix detector is relatively large with (1.41 cm)2 while the sensitive area of one pixel is small with (55 mum)2, it is able to measure very low dose rates with high statistical precision while still processing each photon even at high dose rates. In this contribution we explain a method to determine personal dose equivalents from photon counted data, present measurement results of the air-kerma for different X-ray qualities and show simulation results of the performance of a dosimeter based on a hybrid photon counting pixel detector. We outline the advantages and perspectives of using a photon counting pixel detector in a dosimeter.

Low Energy Dosimetry With Photon Counting Pixel Detectors Such as Medipix

Hybrid semiconductor photon counting pixel detectors like the Medipix detector have several advantages for an use in X-ray dosimetry. The noiseless photon counting principle allows to monitor low photon energies down to 3.5 keV. Due to the small pixel size (55 mum in case of Medipix2) dosimetry at very high dose rates is possible still processing each photon individually. The large amount of pixels in combination with the possible thickness of the sensor layer enables dosimetry at very low dose rates. A method has been developed to determine personal dose equivalents from the number of counts in energy deposition intervals measured with a semiconductor photon counting pixel detector, despite the strong influence of charge sharing effects among pixels. We tested the method experimentally by reconstructing the air kerma free in air for different qualities of X-radiation in the energy range below 150 keV with an accuracy better than 4%. We show that the response of a dosimeter based on a hybrid photon counting pixel detector can fulfill the IEC type testing requirements. The statistical precision is high due to the thickness and the large area of the sensor layer. We estimate that a dosimeter based on the Medipix detector will be able to cope with dose rates of more than approximately 57 Sv/h for mathdot Hp (0.07) or 19 Sv/h for mathdot Hp(10) . We outline the advantages and perspectives of using this kind of detector in a dosimeter in comparison to standard active personal dosimeters.

Correlated counting and energy resolving properties of photon counting X-ray detectors like the medipix detectors

There is a strong effort in the development of semiconductor photon counting pixel detectors to determine the energy deposition of every single interacting photon. With this additional energy information, it is possible to use for example energy weighting and material reconstruction methods in medical imaging. Due to small pixel sizes needed for high spatial resolution, charge sharing has a strong impact on the counting principle and energy resolution, as it leads to multiple counts and an energy dissipation of a detected single photon. Therefore, we examined with experiments and Monte Carlo simulations the aspect of multiple counts in the average multiplicity framework in respect to photon energy and detector settings. These investigations were done with the Timepix detector and its two different counting modes. Furthermore simulations of the upcoming Medipix3 detector were performed to evaluate the energy resolving and counting properties of the new detector design.

Time resolved measurement of a pulsed X-ray source with the Timepix detector

The hybrid pixelated semiconductor Timepix detector is the successor of the Medipix2 X-ray detector. It is provided with additional functions directly integrated in the pixel electronics which allows to perform the measurement of event times of interacting X-ray photons in the silicon sensor layer in respect of the end of each acquisition. Due to the number of pixels of the device there are 65536 independently working channels available to determine the time structure of high flux X-ray sources which are operating in a pulsed mode. These X-ray sources are commonly used in medical applications and homeland security and create X-ray bursts in the range of 50 ns. We present the results of the time resolved measurement of a pulsed X-ray field and the corresponding measurement which was done in a collaboration with the Physikalisch Technische Bundesanstalt (PTB).