Design and construction of equipment for point-source, low energy electron microdosimetry measurements

Actual measurements of dose as a function of distance for point-source, discrete energy spectra are essential to the selection of radionuclides for radioimmunotherapy, as the present theoretical basis provides extrapolations into the low energy region where assumptions about the energy-loss mechanisms are believed to break down. The authors present a discussion of design criteria for an apparatus aimed at measuring dose in gases for low energy (10 to 100 keV) electron sources, as well as description of the equipment that emerged from this analysis. Given the short ranges involved, an almost-wall-less ionization chamber with cylindrical symmetry was selected. The chamber is fixed in position and a radioactive point-source is moved along its axis by remote control, preventing air disturbances in the chamber while obtaining ionization current measurements. A laser-aided positioning mechanism maintains alignment of the axis within 0.5 mm, and chamber-to-source distance is known within 0.3 mm. Special attention was given to the open-air ionization chamber design, and two different models were built. Guarding, grounding, and insulating schemes are proposed. The apparatus can be easily modified for use with gases other than air. This equipment is presently being used to provide data for validation of, or corrections to, available theoretical calculations