Position-independent ionization chember for large-size planar beta sources

An ionization chamber with a thin end-window was designed in order to make possible the comparison of large-size planar beta sources. The chamber has a rotational symmetry with a total diameter of 40.5 cm. The collecting electrode in the form of a spherical cap decreased the position dependence to less than 1% in the central region of the diameter of 25 cm. Using this chamber it was possible to compare beta sources with emission rates ranging from 103 to 108 s-1. The chamber sensitivity depends on the source construction and on the particular radionuclide, approximately being 1.9x10-16 C/particle for 90Sr. The response reproducibility at the lower limit of the measurement range 104 s-1, equals 0.3% at 99% confidence level, with a background level of 9x10-14 A. (orig.)