Models for Accelerated Nuclear De-excitation: Dicke-enhanced excitation transfer on the 14.4 keV transition in Fe-57

Numerous anomalies have been reported over the decades in experiments with metal deuterides and metal hydrides, including excess heat production and low-level energetic nuclear particle emission. Corresponding experimental reports suggest a change of reaction rates and a change of reaction products compared to conventional expectations. Nonradiative excitation transfer among coupled quantum systems is a known mechanism that can account for the majority of anomalies. The basic principle is that the emission of energy from a reaction — such as a fusion reaction — is accelerated through the resonant absorption of released energy by a nearby acceptor system. Secondary reactions can then lead to a range of different reaction products. Such energy transfer requires the enhancement of ordinarily weak couplings between nuclei, as enabled by collective effects related to superradiance.

We seek quantitative agreement between this theoretical picture and experiments that exhibit anomalies. However, samples used in such experiments are complex and often incompletely characterized, posing challenges for modelling efforts. Our approach in recent years has been to work with conceptually simpler experiments focused on the above-described mechanism, ideally under conditions where all relevant physical parameters can be controlled. In these experiments, the transfer of excitation from one Fe-57 nucleus to another can be much faster than the decay rate, resulting in an externally induced modification of the natural decay scheme.

In Fe-57, the lowest excited state at 14.4 keV is coupled to the ground state via the magnetic dipole interaction, which allows for quantitative models for excitation transfer in the presence of an externally driven oscillating magnetic field. If the magnetic field is uniform over many nuclei, then excitation transfer is enhanced by cooperative Dicke factors, which greatly increase the transfer rate. Destructive interference is reduced due to shifts of the nuclear energy levels off of resonance.

The fastest excitation transfer rates are predicted for Dicke state to Dicke state transfer, originating from an initial delocalized state of collective nuclear excitation. This motivates an experiment where largescale delocalization of gamma emission is produced by excitation transfer across a single-crystal sample. An initial Dicke state can be produced by synchrotron excitation. For experiments in which initially localized excitation is produced following the electron capture decay from Co-57, stronger couplings are need to yield comparable effects. In such experiments, strong internal oscillating magnetic fields capable of mediating excitation transfer can be obtained through spin wave excitation.