VARIANCE COVARIANCE MEASUREMENTS IN PHOTON BEAMS FOR SIMULATED NANOMETER OBJECTS

The dose mean lineal energy, <(gamma)over bar>(D), has been measured w ith the variance and the variance-covariance method for simulated obje ct diameters down to 6 nm in photon beams. The measurements were made both in a conventional X ray beam (100 kV and a HVL of 0.14 mm Cu) and in a Co-60 gamma ray beam. The measurements were made with electromet ers designed to have low electronic noise. For small simulated object diameters the electronic noise and the variance from the secondary emi tted electrons (SEE variance) increase in importance. In the present i nvestigation those variances start to dominate over the variance from the ionisation of the gas (microdosimetric variance) at about 20 nm.