Sensitivity enhancement for low level activities by complete syntheses of liquid scintillation solvents

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Abstract
In order to take full advantage of liquid scintillation measurements for low level radioactivity, a large part of the solvent should be synthesized from the material of the sample to be counted. This procedure has been applied to three natural radioactivity dating a large part of the solvent should be synthesized from the material of the sample to be counted. This procedure has been applied to three natural radioactivity dating methods. For carbon-14, the most sensitive and practical of the various liquids investigated is benzene. The compound, in addition to being a good liquid scintillation solvent, contains 92% carbon. The work on the synthesis of benzene for carbon-14 dating is now complete with yields approaching 100% and less than eight hours total chemical preparation time. Furthermore, the isotope effect for carbon has been proven to be small and reproducible to better than 1/2%. A synthesis of benzene for low level tritium measurements has also been developed. Benzene contains 8 % hydrogen, but the chemical technique is considerably simpler than that used for radiocarbon dating. Due to the faster preparation, large quantities of benzene for tritium dating can be produced and the sensitivity increasedin this way. An isotope effect of −34% has been observed, but it seems to be sufficiently reproducible not to pose a problem to the use of the liquid scintillation counter for tritium dating. The possibility of age determinations with naturally occurring chlorine −36 is attractiye due to the long half-life of the isotope. Dating of samples with ages of more than a million years is possible. The procedure now being developed consists of the synthesis of an inorganic liquid, silicon tetrachloride, from the chlorine of the sample to be dated. The chemical preparation is rapid and has an average yield of 75%, but the compound is not suitable for use without the presence of an organic solvent in which the scintillator must be dissolved. Despite this, with silicon tetrachloride much larger quantities than were previously possible can be counted and the method offers yroniise of eventually making chlorine -36 dating practical.