Thorsten Schneider

A method to determine the water kerma in a phantom for x-rays with energies up to 40 keV

In brachytherapy, as in any form of radiation therapy, accurate dosimetry in terms of absorbed dose to water is required. The output of brachytherapy sources has, up to now, been specified as the reference air kerma rate at 1 m distance. For this reason, the Physikalisch-Technische Bundesanstalt is developing a primary standard to determine the absorbed dose to water at the point of measurement within a water phantom. A candidate for the standard to be newly established is a water-equivalent parallel-plate ionization chamber with continuously adjustable electrode spacing, forming an extrapolation chamber in a phantom of water-equivalent material.In this work, a method is introduced to determine the absorbed dose to water with an extrapolation chamber in a phantom of water-equivalent material. Thereby, a conversion factor C(xi+1, xi) is applied to the difference of the ionization charges measured at two different plate separations xi and xi+1. In contrast to already described methods, the applied method is based on fundamental dosimetric relations without the use of a wall-less chamber approximation. It will be shown that by using the wall-less chamber approximation, a term contributing to the absorbed dose to water is overlooked. This term is given by the net energy fluence of the secondary electrons at the surface of the measuring volume.The introduced method is verified experimentally by a comparison with water kerma values obtained from air kerma measurements free in air and applying a conversion factor to water kerma obtained by Monte Carlo (MC) calculations for the conditions of the experiment. Evidence is produced showing that the new model significantly increases the consistency of experimental and MC results.

X-ray spectroscopy with photon counting imaging detectors such as Timepix

The energy response spectrum of highly pixelated photon-counting detectors with a semiconductor sensor layer (e.g. silicon) differs significantly from the impinging X-ray spectrum due to charge sharing and Compton scattering. Therefore determining the impinging X-ray spectrum from the measured energy deposition spectrum is a non-trivial task. A detailed model of all relevant physical processes in the sensor layer is necessary to simulate monoenergetic response functions, which can be used to deconvolve the measured spectrum of energy depositions to reconstruct the impinging X-ray spectrum.

Corrigendum: Experimental determination of the dose rate constant for selected 125I- and 192Ir-brachytherapy sources (2012 Metrologia 49 S219–22)

The consensus value for Λ reported in the last column of table 2 should be Λ = 1.012 × 104 not Λ = 1.021 × 104.

Towards a determination of the absorbed dose to water in water for low-energy photon-emitting brachytherapy seeds

An accurate determination of the dose produced by brachytherapy seeds emitting low-energy photons is an important component of the radiotherapeutic process. As yet, the output of these seeds has usually been specified in terms of the air kerma rate. The desired quantity in radiation therapy is, however, the absorbed dose to water inside a water phantom, for which primary standards are not available.For this reason, developments are under way in the Physikalisch- Technische Bundesanstalt to establish a primary standard to determine the absorbed dose to water within a phantom. As a fundamental step towards this aim, a method will be introduced in this publication to determine the water kerma inside a graphite phantom housing an extrapolation chamber. Experimental results will be presented and compared with water kerma values obtained from air kerma measurements in free air and applying a conversion factor to water kerma for the conditions of the experiment. First estimates indicate that the relative uncertainty is of the order of 1% (k = 1).