Coherent Smith-Purcell $\gamma$-Ray Emission

TL;DR

This paper explores how coherent excitation of nuclei in crystal lattices can produce narrow-band, directional gamma-ray emission via Smith-Purcell radiation, with potential applications in photon sources and energy storage.

Contribution

It introduces a novel mechanism for gamma-ray Smith-Purcell emission using nuclear coherence in crystals, highlighting its spectral and directional properties.

Findings

Coherent nuclear excitation leads to sharp angular gamma-ray patterns.

Emission at 14.4 keV from Fe-57 nuclei is spectrally narrow and delayed.

The mechanism enables separation from broader emission processes.

Abstract

We investigate the Smith-Purcell emission produced by electron- or ion-beam-driven coherent excitation of nuclei arranged in periodic crystal lattices. The excitation and subsequent radiative decay of the nuclei can leave the target in the initial ground state after $\gamma$-ray emission, thus generating a coherent superposition of the far-field photon amplitude emanating from different nuclei that results in sharp angular patterns at spectrally narrow nuclear transition energies. We focus on Fe-57 as an example of two-level nuclear lossy system giving rise to Smith-Purcell emission at 14.4\,keV with a characteristic delay of 1.2\,ns relative to the excitation time. These properties enable a clean separation from faster and spectrally broader emission mechanisms, such as bremsstrahlung. Besides its fundamental interest, our study holds potential for the design of high-energy, narrow-band, highly-directive photon sources, as well as a means to store energy in the form of nuclear excitations.