Search for low-energy induced depletion of Hf at the SPring-8 synchrotron

Electromagnetic transitions within nuclei reflect specific aspects of nuclear structure. This is particularly true for metastable excited states, or isomers, like ¹⁷⁸Hf^m2 (T_1/2=31${\mathrm{T}}_{1/2}=31$ years, excitation energy 2446 keV). The interaction of external radiation with isomers can be used to study atomic and nuclear properties and, perhaps, to induce a release of the stored energy. Some experiments indicated that low-energy photons near the L₃ edge (9.561 keV) of hafnium could cause this to occur for ¹⁷⁸Hf^m2, but the lack of a viable physical model and null experiments by other groups have left these claims in doubt. The present work describes a new experiment to examine this process by closely duplicating the irradiation conditions in positive studies, but using a more advanced multi-detector γ array. No support for an induced depletion of ¹⁷⁸Hf^m2 by low-energy photons was obtained, with an upper limit for the integral cross section that is eight orders-of-magnitude below the reported value.