2D dosimetry in a proton beam with a scintillating GEM detector

A two-dimensional position-sensitive dosimetry system based on a scintillating gas detector is being developed for pre-treatment verification of dose distributions in particle therapy. The dosimetry system consists of a chamber filled with an Ar/CF4 scintillating gas mixture, inside which two gas electron multiplier (GEM) structures are mounted (Seravalli et al 2008b Med. Phys. Biol. 53 4651–65). Photons emitted by the excited Ar/CF4 gas molecules during the gas multiplication in the GEM holes are detected by a mirror–lens–CCD camera system. The intensity distribution of the measured light spot is proportional to the 2D dose distribution. In this work, we report on the characterization of the scintillating GEM detector in terms of those properties that are of particular importance in relative dose measurements, e.g. response reproducibility, dose dependence, dose rate dependence, spatial and time response, field size dependence, response uniformity. The experiments were performed in a 150 MeV proton beam. We found that the detector response is very stable for measurements performed in succession (? = 0.6%) and its response reproducibility over 2 days is about 5%. The detector response was found to be linear with the dose in the range 0.05–19 Gy. No dose rate effects were observed between 1 and 16 Gy min?1 at the shallow depth of a water phantom and 2 and 38 Gy min?1 at the Bragg peak depth. No field size effects were observed in the range 120–3850 mm2. A signal rise and fall time of 2 ?s was recorded and a spatial response of ?1 mm was measured.Radiation, Radionuclides and ReactorsApplied Science

Construction, test and operation in a high intensity beam of a small system of microstrip gas chambers

We describe the construction, test and installation procedures, and the experience gained with the operation of a small but complete system of high rate Micro-Strip Gas Chambers, made on thin boro-silicate glass with a diamond-like coating with chromium or gold strips. A set of detectors, fully equipped with readout electronics and each with an active area of 100x100 mm2, was exposed during six months to a high intensity muon beam at CERN with a peak intensity of ~104 mm-2s-1. Continuous monitoring of the performance of the chambers during the beam runs allowed the evaluation of detection efficiency and the monitoring of accidental rates, as well as the study of ambient induced variations and aging in realistic beam conditions. No significant difference has been found in the operation of under- and over-coated plates. Efficiencies could reach ~98% in best operating conditions, although local lower values were often observed due to missing channels (open strips, broken bonds and dead electronic channels). The long term operation of the chambers has been more difficult than expected, with the appearance of break-downs and loss of efficiency in some detectors, possibly induced by the presence of small gas leaks, to water permeation or to residual reactivity of the quencher gas (dimethylether)