Two new methods of determining radon diffusion in fish otoliths

Otoliths are bony structures found in the ears of fish and used in the 210Pb/226Ra dating method for age determination. This paper checks the assumption that 222Rn is not lost from or added to orange roughy fish otoliths by diffusion, which would invalidate the technique. The first method of monitoring diffusion relies on measuring the gamma activity of daughter radionuclides. Otoliths were exposed to an atmosphere enriched in 222Rn for 10 days, and the supported gamma activity inside them measured allowing for various decay corrections. The calculated radon addition was (0.5 ±0.5)% of the activity of the 226Ra present. The second method used an alpha spectrometer and attempted to detect 222Rn directly outguessed from otoliths in the detector vacuum chamber. The results were consistent within errors with those of the first method and showed no loss or gain of 222Rn, supporting previous estimates of a long life-span for the orange rough y. In contrast it was found that approximately 10% of 222Rn formed in orange roughy fish scales was lost to an evacuated environment, (hence perhaps to an aqueous environment) and that for this species it could be difficult to base a dating method on analysis of scales. Nevertheless a preliminary minimum age of 57 years was obtained. The methods could be used with non-biological samples to determine 222Rn diffusion rates. (author). 17 refs., 5 figs