Experiments to investigate phonon-nuclear interactions

Abstract

This thesis presents a series of experiments conducted by the author between 2016 and 2018 that were designed to test for and investigate a proposed phonon-nuclear coupling interaction and an associated nuclear excitation transfer mechanism. Electric and magnetic interactions of phonons with atomic nuclei have been studied for several decades. However, such second-order interactions are too weak to induce nuclear state changes. Hagelstein and Chaudhary recently identified the possibility of a stronger, first-order phonon-nuclear interaction, based on the boost correction associated with the nucleon-nucleon potential for nuclei embedded in a condensed matter environment. Because the newly proposed interaction follows from the relativistic (Dirac) treatment of nucleons, Hagelstein and Chaudhary refer to this interaction as relativistic phonon-nuclear coupling.

Relativistic phonon-nuclear coupling implies the possibility of phonon-mediated nuclear excitation transfer where in the process of absorbing and emitting phonons, energy can transfer from excited state nuclei to nearby ground state nuclei, analogous to widely studied excitation transfer at the atomic and molecular level. To test for and investigate these theoretical conjectures, we prepared samples with a combination of ground state and excited state Fe-57 nuclei (from beta-decaying Co-57) attached to a steel substrate. Samples then underwent treatment by inducing vibrations via ultrasound or mechanical stress. Simultaneously, time histories of radioactive emission were recorded at different locations. Early experiments with vibrations induced at the MHz level via ultrasound transducers yielded negative results and no variations in radioactive emission were observed. However, in conjunction with mechanical stress, deviations from expected emission were observed.

After applying mechanical stress to a sample, we observed a 19% enhancement above expected levels of 14.4 keV gamma emission from Fe-57 and a 17% enhancement above expected levels of Fe K-alpha emission (which to a large extent is driven by internal conversion from the 14.4 keV nuclear transition). The enhancements decayed away with a time constant of about 2.5 days. At the same time, emission on the Sn K-alpha line (driven by fluorescence of Sn in the steel) was consistent with the expected exponential decay of Co-57 at the 1% level, suggesting detector integrity. Similar deviations from expected emission were observed by two additional detectors in different locations. Further experimentation exhibited a high level of reproducibility of the observed effects. By now, evidence for the effects have been seen in seven different detectors and in six different experimental configurations. In some experiments, reductions instead of enhancements can be observed.

Moreover, we observe differences in the ratio of 14.4 keV gamma and Fe K-alpha emissions across experiments. To explain reported observations, we propose that the temporary enhancements and reductions of emission originate from phonon-mediated nuclear excitation transfer and are caused by resulting delocalization and angular anisotropy effects. Delocalization can result from excitation transferring into the steel substrate and across the Co-57/Fe-57 residue. Angular anisotropy can follow from phase coherence at neighboring sites as a result of resonant excitation transfer. Furthermore, observed differences in the incremental emission of 14.4 keV gamma and Fe K-alpha emission suggests that a new channel for internal conversion is opened in off-resonant states present in excitation transfer. We motivate and discuss the conjectured mechanisms as well as alternative candidate explanations and conclude that the latter do not suffice to account for the reported observations.

Finally, we present limitations of this work to date and point at avenues for further research and clarification. Relativistic phonon-nuclear coupling and nuclear excitation transfer have the potential to form new tools in the toolbox of nuclear engineers. The further pursuit of research in this area could lead to the use of phonons in a wide range of applications: for mixing nuclear states; for generating angular anisotropy or inducing beam formation; and potentially for exciting or de-exciting atomic nuclei in applications otherwise reliant on photons. This, in turn, could lead to many nuclear engineering applications becoming more economical as well as less hazardous.