AbstractID: 6986 Title: Monte Carlo characterization of a highly efficient photon detector for
megavoltage imaging
The imaging characteristics of an arc-shaped xenon gas ionization chamber for the purpose of
megavoltage tomography were investigated. The detector consists of several hundred 0.3 mm thick
gas cavities separated by thin tungsten plates of the same thickness. Dose response, efficiency and
resolution parameters were characterized using Monte Carlo simulations and measurements in a
4 MV and 6 MV photon beam.
The measured response profiles for narrow and broad incident photon beams could be well
reproduced with Monte Carlo calculations. For the 4 MV beam, the quantum efficiency is 29.2%
and the detective quantum efficiency at zero frequency DQE(0) 15.7% for a detector placed in
focus with the photon source. For a detector placed out of focus as well as for the 6 MV beam,
these numbers even increase. Hence, the efficiencies for megavoltage radiation surpasse the
reported efficiencies of existing detector technologies by a considerable margin.
The resolution was quantified by calculated and measured line spread functions. The results show
that the resolution is only slightly dependent on the plate thickness but is predominantly determined
by the channel size of the detector.
In conclusion, a gas ionization chamber of the described type is a highly efficient megavoltage
radiation detector, allowing CT images to be obtained with very little dose for a sufficient image
quality. Furthermore, the combination of a dense, high-atomic material with a low-density signal
generating medium might serve as a model for a future generation of highly efficient photon
radiation detectors.