An experimental study of the relative biological effectiveness of neutrons of various energies for the inactivation of hamster cells has been reported previously; the present article contains a detailed statistical analysis of the data. Repeated determination of X-ray survival curves made it possible to assess the statistical fluctuations of survival rates. Within individual experiments the statistical variations are close to those expected on theoretical grounds. However, between experiments substantial variations in the reaction to X rays occur; these fluctuations can be approximately described by dose-modifying factors. An idealized survival curve for X-irradiation has been derived. The survival curves for neutrons have been compared to the linear-quadratic dose-effect relation and the two coefficients in this equation have been derived together with their joined confidence regions. The linear coefficient for neutrons reaches a maximum for neutron energies of several hundred thousand electron volts; the coefficient of the quadratic component is increased at neutron energies below 1 MeV. The dependence of RBE on neutron energy and on neutron dose is derived from the estimated dose-effect relations. In addition, the more rigorous nonparametric method is used to derive confidence intervals for the RBE of neutrons.

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