Radon escape from New Zealand speleothems

Abstract

When calculating the environment radiation absorbed dose rate in ESR or TL dating, it is usual to allow for the possibility of dissequilibrium in the uranium-series decay chain, due to the escape of the radioactive gas, radon, from the sample before its decay to non-gaseous daughter products. Partial or complete radon escape may reduce the internal radiation dose to less than half that which would be received if no radon escaped. In this study a number of speleothems from a temperate climate, varying in age from 6 ka to more than 730 ka (ages determined by 234U/ 230Th and 235U/ 231Pa dating and by paleomagnetic analysis) have been analysed for thorium and polonium isotopes. As radon lies between thorium and polonium in the uranium decay chain, a comparison of thorium and polonium activities with the expected values, derived from the age of the sample and the initial concentrations, gives an estimate of the radon escape factor and its impact on environmental dose rates. The activity rates of thorium, uranium and polonium were measured separately by counting their α particle emissions after electroplanting the elements onto rotating stainless steel discs. Polonium was plated directly after dissolution of the sample and before loading the solution onto a heated cation column to separate the uranium and thorium fractions. The uranium fraction was further purified on an anion before plating. The recovery rates of uranium and thorium were monitored by spiking but the recovery rate of polonium must be regarded as a minimum; estimates of radon escape are therefore likely to be overestimates rather than underestimates. Of 19 speleothems studied, only two gave results inconsistent with values predicted on the basis of no radon loss (Chauvenet's test): one, a relatively young sample of 6 ka, showed evidence of radium coprecipitation with uranium, and one, a dog-tooth spar with an open crystal structure and a high porosity showed evidence for radon escape. A third sample, also a flowstone/spar with a higher porosity than normal, was marginally significant. As the degree of radon escape will vary from sample to sample, the environment dose rate must be regarded as having a high level of uncertainty for samples of this type, unless radon escape is determined experimentally. The remaining samples (12 stalagmites, 3 stalactites and 1 flowstone), of a more massive structure, showed no significant radon escape (Student's t-test), and therefore the environmental dose rate for such samples should be calculated on the basis of radon retention.