Dosimetry of High Energy Neutrons and Protons by 209Bi Fission

Fission of Bi by neutrons and protons of energies above 50 MeV provides a detector for the high energy part of cosmic ray neutron and proton doses received by personnel in commercial aircraft. A stack of damage track detectors and Bi radiators, of large total area, has been tested and has adequate sensitivity. Knowledge of the fission cross sections for neutrons and protons is critical for such measurements. Contrary to a common assumption that these are nearly equal, both old and recent measurements and recent calculations show that σ Bi (p,f) is more than twice σ Bi (n,f). Calculations indicate that Bi fission can probably determine dose equivalents from high energy cosmic ray neutrons to within ±30%, and from cosmic ray protons to within ± 25%. The response is nearly the same for equal dose equivalents of cosmic ray neutrons and protons.The accuracy for neutrons could be improved if cosmic ray neutron spectra were better determined.