Variance-covariance measurements of anti y/sub D/ for 6-MeV neutrons using wall-less spherical detectors

The variance-covariance technique provides a way to measure mean microdosimetric quantities such as anti y/sub D/ for accelerator-produced radiation in simulated volumes with a wide range of sizes, including sizes an order of magnitude smaller than is possible with conventional methods of microdosimetry. The first experimental realization of the variance-covariance method was accomplished in 1984 by Lennart Lindborg of the Swedish National Institute of Radiation Protection in collaboration with members of this Laboratory. The measurements were of anti y/sub D/ for monoenergetic 6-MeV neutrons produced at the Radiological Research Accelerator Facility (RARAF). The recent measurements go beyond the earlier ones in a number of ways. It is important that the new detectors are spherical because for spherical sites there is an expression relating values of y/sub D/ in a range of site sizes to the energy transfer proximity function. In addition, good measurements with the new detectors have already been made in sites with mean chord lengths as small as 20 nm, compared with a useful minimum of 50 nm in the first experiments