Dosimetric Aspects of Fast Neutron Irradiations of Cells Cultured in Monolayer

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Abstract
Perturbations of charged particle equilibrium at interfaces of materials of different atomic compositions can lead to considerable differences in the energy deposition by photons and neutrons. Cell-survival characteristics were determined for cultured cells of human kidney origin. The survival curves obtained for the 14.5 MeV neutron irradiation in 2 geometries are presented. For a given tissue kerma, a higher level of survival is observed for the irradiations through the polystyrene bottom of the flasks. This indicates that a smaller amount of energy is absorbed in the cells in this situation. The relative absorbed doses in tissue layers irradiated through water or through polystyrene differ by a factor of 1.16. This ratio is somewhat lower than the factor expected on the basis of the kerma ratios. The discrepance has to be attributed to small contributions from energy deposition processes inside the cell and from back-scattered secondary charged particles. Cell irradiations with 15 MeV neutrons are generally performed with the neutron beam reaching the cells through the medium. Under these conditions, the slight increase in neutron dose is accounted for by applying a correction of 5%. Larger correction factors will be required for neutron irradiations through the bottom of the flasks. The situation of incomplete charged particle equilibrium at an interface can have considerable consequences for practical neutron dosimetry in radiobiology.

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