An attempt has been made to calculate the response of T1 kidney cells in tissue culture to four beams of fast neutrons, based on their response to charged particles using the track segment method. The calculated RBE's of the neutron beams are lower than the observed values, and the calculated values of the oxygen enhancement ratio are higher. These differences seem too great to be explained by errors in dosimetry and in the calculated LET spectra. It is suggest that LET is not an adequate criterion of radiation quality, and that the discrepancies may be explained by more detailed consideration of the part played by delta rays and by heavy recoil tracks of short range. The notion of a biologically effective LET is shown to depend as much on the nature of the biological test used as on the physical quality of the radiation beam.

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