Test of a compton imaging prototype at the ELBE bremsstrahlung beam

In the context of particle therapy, particle range verification is a major challenge for the quality assurance of the treatment. One approach is the measurement of the prompt gamma rays resulting from the tissue irradiation. A Compton camera based on several position sensitive gamma ray detectors, together with an imaging algorithm, is expected to reconstruct the prompt gamma ray emission density map, which is correlated with the dose distribution. At Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and OncoRay, a Compton camera setup has been developed consisting of two scatter planes (CdZnTe cross strip detectors) and an absorber (Lu2SiO5 block detector). The data acquisition is based on VME electronics and handled by software developed on the ROOT framework. The setup was tested at the linear electron accelerator ELBE at HZDR, which was used to produce bunched bremsstrahlung photons with up to 12.5MeV. Their spectrum has similarities with the one expected from prompt gamma rays in the clinical case, and the flux is also bunched with the accelerator frequency. The spatial resolution for the CZT and LSO detector is analyzed and it showed a trend to improve for low and high energy depositions respectively. The time correlation between the pulsed prompt photons and the measured signals to be used for background discrimination exhibits a time resolution of 3 ns (2 ns) FWHM for the CZT (LSO) detector. A time walk correction and pixel calibration is applied for the LSO detector, whose resolution improved up to 630 ps. In conclusion, the detectors are suitable for time-resolved background suppression in pulsed clinical particle accelerators. Ongoing tasks are the test of the imaging algorithms and the quantitative comparison with simulations. Experiments at proton accelerators have been also performed and are now under analysis.