Comparison of the microdosimetric event-size method and the twin-chamber method of separating dose into neutron and gamma components.

Microdosimetric measurements of event-size spectra, made with a proportional counter, are being used increasingly for separation of dose components in mixed n-gamma fields. Measurements in fields produced by 8.3 MeV deuteron bombardment of thick beryllium and deuterium targets were made in air and at 6 and 12 cm depth in water with a spherical tissue-equivalent (TE) proportional counter and with a pair of calibrated ion chambers (TE-TE and Mg-Ar). The dose results obtained with the two methods agree well for the neutron components, but the gamma components do not demonstrate consistent agreement. An important source of error in the microdosimetric method is the matching of the spectra measured at different gain settings to cover the large range of event sizes. The effect of this and other sources of error is analysed.