Experimental limitations in microdosimetry measurements using the variance technique.

Microdosimetry measurements give information about the energy imparted to a volume of given size. With the most frequent method only data for object sizes above 0.2 micrograms are obtainable. Although restricted to the dose mean values, the variance technique can, however, be applied to objects sizes an order of magnitude less. In this report some limiting parameters for this technique are discussed and results down to about 20 nm in a 60Co gamma-ray beam are reported. the resulting data exceed earlier reported values at objects size below 0.1 microgram, but are in good agreement for larger object sizes.