Fast neutrons from a 25-MeV betatron.

Since published results for the fast-neutron dose per x-ray rad from high-energy therapy installations have differed by as much as a factor of 300, we have measured the neutron production from our 25-MeV betatron. Fast-neutron activation of aluminum foils was the method used. The effect of photoneutron production in the detectors, which has affected some past work, has been eliminated. A major source of neutrons in the treatment field was the platinum target. The neutron spectra used in the calculations of fluence were obtained by interpolation between published energies and between neighboring elements. Fluences per rad of x rays without a phantom were largely independent of field size and energy in the range 18-23 MeV. At 100 cm SSD and 23 MeV a large beam flattener contributed 15% of the neutrons, the remainder coming equally from the target and background. A phantom increased the neutron fluence/rad of x rays by 0%-10% depending on the field size. At 23 MeV we estimated the neutron dose to a patient to be 2.2 X 10(-4) rad per rad of x rays inside the treatment field and 3 X 10(-5) at 20 cm outside the field. The uncertainty in these figures is believed to be +/- 50%. In the electron beam the neutron dose per rad was about 50 times smaller than in the x-ray beam. Estimates were made of neutron fluences at other energies and target thicknesses. We discuss our results in comparison with those of others.