A Portable Liquid Scintillation Counter for General LSC and High Sensitivity Alpha-Counting Applications

Abstract

Applicability of a new portable, single tube liquid scintillation (LS) spectrometer was demonstrated for general LS applications, such as wipe tests and low level alpha counting, especially radon (222Rn) measurements in water. Wipe tests were performed with wad sticks. They were counted conventionally in a small volume in Eppendorf tubes with less than 200µl of LS cocktail, thus minimizing costs and waste. Small volume is specially recommended because low background can be achieved without heavy lead shielding, thus maintaining portability. Typical NRC recommended LLD's were reached for 3H and other typical LS isotopes. For 222Rn in water a biphasic extraction system was adopted where radon is extracted from water into a water-immiscible LS cocktail. The method is sensitive because radon can be extracted from a large water volume. It was observed that common non-evaporating "safe" cocktails with di-isopropyl naphtalene solvent are convenient to use and quite suitable for extraction. Also the isotope 226Ra can be reliably measured via production of its daughter 222Rn. The instrument includes pulse shape electronics to perform alpha/beta separation. This is based on the fact that in LS cocktails alphas generate pulses with longer duration than betas. The alpha/beta separation can be visualized with a two dimensional graph where the x-axis represents pulse amplitude (MCA channels) and the y-axis its length. The graphical operations are all done in standard Excel/Windows environment. Due to their longer pulses, alphas have greater y-coordinates than betas with the same x-coordinate (amplitude). With this graph, one can select a region occupied only by alphas and exclude betas. The above mentioned "safe" cocktails posses good alpha/beta separation properties. Because natural background (cosmic-rays and environmental gammas) produces beta-like pulses, they can be stripped away giving low background for alpha counting, typically a few counts per hour for the extraction samples. The LLD for 222Rn was 0.1 Bq/I.