Production and measurement of fast neutrons in a compact cyclotron for use in radiotherapy

This paper relates the physical-technical fundamentals of fast neutron production for use in radiotherapy at the Deutsches Krebsforschungszentrum in Heidelberg. The available accelerator is a cyclotron of compact structure with relatively low particle energies, therefore the production of neutrons by the d → Be reaction performed usually with larger machines would not give satisfactory results with respect to dose rate and depth profile in the texture. A deuterium gas target is chosen which gives rise, besides the exothermal compound reaction, to only one competing endothermal process, when irradiated with deuterons. The gas target essentially consists of a deuterium-filled pressure vessel with a window for the ion beam which is directly cooled by circulating the target gas in a closed circuit. A computer code is described which allows the calculation of the neutron field in the beam axis on the basis of given cross-section and shopping power data, taking into account reaction kinematics and the spacial extention of the target. For different particle energies and target pressures spectral flux densities and integral quantities such as energy integrated flux densities, cerma doses, mean energies and mean cross sections are calculated. For the given deuteron energy of 11 MeV, a beam current of 100 μA and a target pressure of 13 atm (target length 30 cm) the neutron flux density amounts to 2 x 108 1/cm2s and the cerma rate is 60 rad/min in a distance of 1 m from the ion beam window. The measurement of the spectral flux density in the beam axis was performed using a recoil proton telescope with a thin radiator and semiconductor detectors.