Linac W target close-in geometry photoneutron study by nuclear track distribution function.

During electron beam stopping on W target in a linear accelerator, photonuclear (γ, xn) reactions occur; the tungsten converter provides a non-negligible neutron yield with an energy spectrum that significantly depends on surrounding mass nuclei. Reduction of the neutron radiation field is convenient to limit the side-effects that accompany the tumor or cancer radiotherapy. A close-in irradiation geometry is proposed to improve therapy effectiveness. The convenience of the proposed experimental arrangement is assessed using Monte Carlo simulation and experimental results based on nuclear track-etch methodology. Photoneutron yield for two energy groups (thermal and epithermal) are determined experimentally via boron (98%) converter and cadmium-filter employing a passive detector (poly allyl di-glicol carbonate polimer). Etched track diameter histograms are described by distribution functions to determine the ratio between thermal and higher energy neutrons. New insights are given into therapy beam quality and radiotherapy dose delivery based on bar histograms unfolding.