An experimental approach for determination of a microdosimetric quantity in a solid material a few nanometer thick

When a material is irradiated with ionizing radiation the energy spectrum of the emitted secondary electrons has a pronounced peak well below 50 eV. Due to their short range in solids these electrons originate from a layer a few nanometer thick. The sensitive volume is thus a very thin slab and could therefore simulate for an example thin layers such as nuclear membranes in cells. The fluctuations in the emission of these low energy secondary electrons (SEE) have been proposed by Burlin (1974) to be related to microdosimetry quantities. Since there is a lack of experimental methods for determining microdosimetry quantities for objects below 0.3 μm, it is of interest to try to apply the so called variance technique (Bengtsson 1970, 1972) to the SEE-phenomena. This is done in this report, and an attempt to determine the dose mean of lineal energy in a solid material a few nm thick, irradiated with 60Co γ-rays, is described. The results indicate that useful information about the variance in energy depositions in a solid should be possible to obtain through variance measurements. The experimentally determined Y sub(D) value for a mean chord length of 12 nm in a solid is larger than for a sperical gas volume with the same chord length, which is expected due to the different track length distribution in the SEE-measurements. Further experiments are necessary with a more suitable vacuum chamber if a more accurate estimation of the actual Y sub(D) value for the solid should be made. The results indicate also preliminary that the lower limit of the mean chord length for which variance measurements in a gas with a walled ionization chamber can be made is set by the variance in the primary SEE-current from the walls. (author)