New method to detect radiotherapy gamma field

Full text: This paper describes a novel method, which allows one to measure the intensity distribution of energetic gamma radiation field, used typically in radiation therapy. Traditionally, measurements are based on a detector system of one or more ionization chambers or an array of semiconductor diodes, which are working in the so called current mode. Therefore, in these methods the spatial resolution is limited by the size of the individual chamber- or diode unit and varies typically from one to five centimeters. The improvement in the resolution leads one to decrease the unit size, increase their amount and, consequently, the electronics (amplifiers etc.), because each detection unit needs its own channel. Better resolution is getting more important question together with the generalization of the Intensity Modulated Radiation Therapy (IMRT) but, unfortunately, the improvement in spatial resolution can only be achieved by increasing considerably the total expenses. Our detection method offers a solution to improve the spatial resolution with very low extra costs. It relies on a position sensitive avalanche counter (PSAC), which owns good position resolution and detection efficiency abilities for heavy ions. Due to the extremely high intensity and low ionization properties of gamma radiation, direct monitoring of gamma field with PSAC has not been possible so far. Therefore, a converter with an adequate cross section to photo fission reactions is used to convert the high intensity gamma field to a 'low' intensity field of heavy fission fragments, which then have the intensity distribution similar to the original gammas. The fragments have an excellent capability to ionize the chamber gas and the discrete pulses can easily be sorted out from the background. By this way the resolution of one millimeter in a typical total gamma field area and with a very simple applied electronics can be achieved. The chamber is also very robust against the radiation damages, which is not the case e.g. with the semiconductor diodes. The converter and wire thicknesses, wire voltages and gas pressure are the parameters, which allow one to optimize the operation of the detector for the certain accelerator conditions. In the final paper we will introduce our detection system and its construction in more detail. We also give the first results about the tests we have carried out in the laboratory and in a hospital accelerator and compare the resolutions to the ones existing in the recent detectors. (author)