Radiation Dosage Due to Neutrons Produced in a Vehicle by Protons in the Trapped Radiation Belt

The rate of production of neutrons by geomagnetically trapped protons incident on a vehicle was measured by a neutron counting system carried into the trapped radiation belt by a pod flow piggy back on an Atlas rocket on December 19, 1961. The flux of neutrons produced by radiation belt protons incident on the pod was determined to be at least 700 neutrons/(cm2 sec); the actual value depends somewhat on the energy spectrum of the neutrons. This flux was estimated to be equivalent to a dose rate in tissue of 0.10 rems/hr. On the basis of proton flux measurements made in the radiation belt by Freden and White, a calculation was made of the tissue dosage which would have been received in the same environment directly from protons. These calculations were made by obtaining a numerical integration of the dE/dx times RBE times flux product over the entire energy spectrum. The total dose calculated amounted to 2.78 rems/ hr. Further calculations were made to estimate the dose rates which would have been received by tissue in the same environment but with varying amounts of shielding around the vehicle. The proton dose is, of course, reduced by the shield but the neutron dose actually increases as the shielding thickness is increased. It is seen that the neutron dose equals the proton dose at .3 rems/hr. when aluminum shielding of 2.6" surrounds the vehicle and it exceeds the proton dose with thicker shielding.