Extrapolation Chamber for the Measurement of Beta Sources

Abstract

Construction and operation of a variable‐spacing, parallel‐plate ionization chamber is described. It has been designed for the measurement of beta‐particle sources of medical and biological interest. It has concentric collecting electrodes of 1, 3, 10, and 30 mm diameter. The air gap in which ionization is measured can be adjusted from 0.04 to 10 mm. Ionization current is measured with a commercial vacuum tube voltmeter which has a full scale sensitivity of 2×10⁻¹³ amp. The instrument sensitivity is about 0.4 esu/cc‐sec using a one mm diameter collecting electrode with a collecting air gap of 0.2 mm, and about 4×10⁻⁵ esu/cc‐sec using a 30 mm diameter collecting electrode with an air gap of 2 mm. Conditions are described in detail for the determination of ionization per cc in vanishingly small air gaps between polystyrene electrodes, with an accuracy of a few percent.