Search for X-Ray Induced Decay of the 31-yr Isomer of 178-Hf at Low X-Ray Energies

The 31-yr isomer of {sup 178}Hf at excitation energy 2.446 MeV, and with J{sup {pi}}, K= 16+, 16, has been the object of several studies for possible mechanisms that might trigger isomer decay to the ground state. Potential for the control of nuclear energies (MeV) with atomic energies (keV) is the driving interest. The 178Hf isomer is a favorite nucleus in the search for triggered decay because this isomer is long-lived, available in microgram quantities, the decay scheme is well known, the excitation energy is high, and a sample enriched in the isomer can be fabricated for irradiation. In fact, accelerated decay of the {sup 178}Hf isomer when irradiated with photons from a dental x-ray machine has been reported by Collins et al. The triggering x-ray energies were reported to be in the 20-60 keV range. Using the synchrotron radiation at the Advanced Photon Source (APS) we published limits on such accelerated emission approximately 5 orders of magnitude lower than those in ref. 1. Very recently, a new measurement of triggering has been reported by Collins et al., using monochromatic x rays from the SPring-8 synchrotron but this time for (several) much lower incident x-ray energies in the 9-13 keV region. In order to verify this observation, we undertook a new measurement at the APS, with significantly increased focus on incident x-rays below 20 keV, as described below.