Improved ionisation chamber system for indoor exposure measurement

The development and testing of a large ionisation chamber for indoor exposure measurements is described. It has a 120 litre sensitive volume operated at normal atmospheric pressure and made from nearly air equivalent plastics material. The wall thickness of 5mm (0.45 g.cm-2) is sufficient for secondary electron equilibrium up to several MeV. Due to a thin internal layer of higher atomic number, the wall absorption for low energy photons is compensated. In this way an energy response of +- 2% in the range of 30 keV to 1.25 MeV is obtained, much lower than present high pressure ionisation chambers. With a digital current integrator, environmental dose rates in the order of 0.1 μSv.h-1 (10 μR.h-1) can be measured with a standard deviation of 1 nSv.h-1 (0.1 μR.h-1) within a 1000 s integration time. The instrument is particularly useful for measurement of spatial indoor exposure variations. Measurements in three different test buildings of well defined construction material (wood, concrete and bricks) and a comparison with the results of TLD exposure and track etching foil radon measurements are reported. (author)