Abstract
The aim of this research was to investigate the relationship between the collimator aperture and fast-neutron flux, neutron-energy spectrum and absorbed dose rate. For remote therapy, rather large fluxes of fast neutrons are needed which can create dose levels in the tissues of at least 0.1 Gy/min with a source-patient distance of 1 m. Advantageously for these purposes, the 9Be(d, n) reaction was investigated with deuteron energy of 13.6 MeV. The mean energy of the outgoing neutrons was obtained using the code PACE 4 (LISE++) which gave the value of about 5.2 MeV. The maximum neutron flux was at an energy of about 2.5 MeV. Samples activation analysis was deployed to measure the neutron flux in the energy-region [0–14 MeV]. The experimental works were carried out using Al, Fe, Cu and Cd foils which installed on the collimator apertures. To investigate the neutron spectrum, fluxes, and dose rates absorbed at the position of patients, experiments were conducted for four different neutron irradiation-field sizes, which can be modified by the removable-polyethylene parts. Simulation results obtained by the code MCNP-4C and PACE4 (LISE++) were comparable with the experimental data to some extent with consideration of some uncertainties of PACE4 results. It can be concluded that the neutron flux is depended on the irradiation-field size where the neutron flux output for bigger aperture size was about +25% comparing with the smaller ones. These results could play a significant role in improving the neutron flux and optimizing the collimation system utilized in fast neutron therapy. In addition, this can lead to optimization of irradiation canals installed in the nuclear reactors which employed for production of medical isotopes, material testing and many other applications.
Highlights
The reason of supporting the utilization of neutrons for treatment is their radiobiological effectiveness (RBE)
The extracted results from experiments and MCNP-4C simulations for neutron fluxes in different energy ranges of neutrons and the neutron doses absorbed are presented in Tables 1 and 2
The average energy of neutrons calculated using the code PACE4 is about 5.2 MeV, and the value resulting from some previous experiments is about 5 MeV, as in reference [3]. Both experimental and simulation works deploying MCNP4C and PACE4 codes were carried out to investigate the dependency of neutronenergy spectrum, neutron flux and the absorbed dose rate on the collimation apertures of collimators constructed for fast neutron therapy
Summary
The reason of supporting the utilization of neutrons for treatment is their radiobiological effectiveness (RBE). For the neutron energies provided by the cyclotron beam, 1/3 fewer doses are required to accomplish an indistinguishable clinical impact with neutrons compared with customary photons. They react ineffectively to ordinary photon treatment. In these cases, neutrons are more effective by the factor of three in RBE [1]. If the neutron energy is too low, the dose of radiation on healthy tissue is so high that healthy tissue may not recover. Radiation therapists, who have extensive experience using beams from 60Co sources and good knowledge about their tissue attenuation, may wish to have a neutron source that gives equivalent or better tissue penetration
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