Thursday, 8 September 2005 - 10:50 AM

This presentation is part of: New Innovations and Revolutionary Concepts

Improving the detection limit for ¹⁸²Hf

The long-lived radionuclide ¹⁸²Hf (t_1/2=9.0Ma [1]) is one of a number of candidates for the detection of a supernova isotope signature [2]. The most suitable site for detecting such a signature would be a deep-sea sediment of slow deposition rate. Both the expected amount of ¹⁸²Hf and the ¹⁸²Hf/Hf isotope ratio are very low, and hence the highest possible detection efficiency and effective separation from the stable isobar ¹⁸²W are crucial requirements.

A suppression factor of ~6000 for ¹⁸²W relative to ¹⁸²Hf is achieved by injecting the HfF₅^- ion [3]. Even with optimum chemistry, however, the ¹⁸²W counting rate limits the sensitivity to ¹⁸²Hf/Hf ratios of ≥10^-11 [3]. This detection limit is substantially above the value expected for the supernova signal. Hence, an isobar separation in the detector is required.

We have investigated techniques of isobar separation that can be used on the ANU 14UD tandem. The energies available do not allow for effective separation using either multiple energy-loss measurements in an ionization chamber or a gas-filled magnet. A number of other methods have been explored. Amongst these were projectile x-ray AMS (PXAMS) and the use of the differences in energy loss of ¹⁸²Hf and ¹⁸²W ions in a degrading foil measured via the residual energy in a subsequent ionization chamber. Data will be presented for both methods, and their potential and limitations for detecting a supernova signature in a typical deep sea sediment core will be discussed.

[1] C. Vockenhuber, F. Oberli, M. Bichler, I. Ahmad, G. Quitté, M. Meier, A. N. Halliday, D.-C. Lee, W. Kutschera, P. Steier, R. J. Gehrke, and R. G. Helmer. Phys. Rev. Lett. 93 (2004), 2501.

[2] J. Ellis, B.D. Fields, D.N. Schramm. ApJ 470 (1996), 1227.

[3] C. Vockenhuber, M. Bichler, R. Golser, W. Kutschera, V. Liechtenstein, A. Priller, P. Steier, S. Winkler. Nucl. Inst. And Meth. in Phys. Res. B 223-224 (2004), 823.

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