Improving the detection limit for ¹⁸²Hf

A nearby supernova would deposit radionuclides on earth. The long-lived radionuclide ¹⁸²Hf (t_1/2 = 8.9 Ma) is one of a number of candidates for an isotopic signature of such an event. Together with ⁶⁰Fe, observation of ¹⁸²Hf would be direct evidence for a supernova site of the r-process. The most suitable site for searching for such a signature would be a deep-sea sediment of slow deposition rate. Measurement of ¹⁸²Hf at the anticipated level requires very effective suppression of the interfering stable isobar ¹⁸²W. Chemical separation and the injection of HfF₅^- allow for suppression by several orders of magnitude, but more is needed for detection of ¹⁸²Hf as a supernova isotope signature. We are currently developing AMS methods for measuring ¹⁸²Hf/¹⁸⁰Hf isotope ratios at the required level using a 15 MV tandem accelerator . Both projectile X-ray emission and the use of a solid passive absorber with a subsequent measurement of the residual energy are being explored. The former suffers from low efficiency, but the latter looks promising.