Abstract

We describe here a simple method of determining Th-230 and Pa-231 contents of ocean sediments using alpha scintillation counting. The Pa-231 content is determined by counting pairs of alpha particles emitted within 4 ms of each other. The Th-232 content is determined by counting pairs of alpha particles emitted within 0.4 s of each other. The Th-230 content is found from the total alpha count rate after contributions from the Pa-231, Th-232, U-238, and U-235 decay chains are subtracted. The uranium content is obtained from independent measurements or from alpha counting measurements on samples deep enough that the Th-230 content is negligible.The method is checked by comparing measured and calculated count rates on six standards and then by comparing our results on a set of core samples with those obtained by alpha spectrometry. The agreement is generally satisfactory except for a possible discrepancy of ~20% in the pair count rates. Results are presented for two other cores; these did not show a simple decrease of Th-230 content with depth. One showed three oscillations superimposed on an overall decrease; the other showed fluctuations of at least 20%/cm along with an overall decrease. It is suggested that these fluctuations may be associated with variations in the manganese oxide content.

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