Measurement of H³ and C¹⁴ with a Liquid Scintillation Counter

Abstract

A liquid scintillation counter for measuring low level C¹⁴ and H³ beta activities has been constructed and its performance characteristics studied.
The instrument is of the coincidence type. Scintillation induced in the sample, usually 140 cc in volume, contained in Terex glass ampoules, are observed with two photomultipliers (EMI 6262). The output pulses are fed to two respective linear amplifiers (gain 10, 000) and the coincidence count is taken. For reducing background count, the output of the coincidence circuit is anticoincidenced with the pulses that passed through the upper discriminator. The resolving time of the coincidence was 0.2μpsec, and as for the stability of the circuit in measuring C¹⁴, the variation of the counting efficiency was 1% in 24 hours.
The counting efficiency of the instrument was of the order of 10% for H³ and about 40% for C¹⁴.
For measuring the distribution of C¹⁴ in nature, quenching of various organic materials is studied. Ethanol is one of the best samples, for the quenching is not large and its preparation is easy.
Various side-effects are also studied, such as 1) the temperature dependency of the thermal noise of photomultiplier, 2) the increase of the dark noise of photomultiplier after being exposed to light of the room, 3) the phosphorescence of the glass ampoule and that of the reflector, 4) the effect of the ampoule material on the background counting rate.