Direct and secondary nuclear excitation with x-ray free-electron lasers

TL;DR

This paper models how x-ray free-electron lasers can induce nuclear excitation in solid targets, highlighting the dominance of secondary processes like electron capture over direct photoexcitation depending on specific nuclear transitions and plasma conditions.

Contribution

It introduces a realistic theoretical model to quantify plasma dynamics and secondary nuclear excitation, revealing conditions where secondary processes dominate.

Findings

Secondary excitation can surpass direct photoexcitation by several orders of magnitude.

The dominance of secondary processes depends on nuclear transition energy and plasma conditions.

Results are relevant for future nuclear quantum optics experiments at x-ray FELs.

Abstract

The direct and secondary nuclear excitation produced by an x-ray free electron laser when interacting with a solid-state nuclear target is investigated theoretically. When driven at the resonance energy, the x-ray free electron laser can produce direct photoexcitation. However, the dominant process in that interaction is the photoelectric effect producing a cold and very dense plasma in which also secondary processes such as nuclear excitation by electron capture may occur. We develop a realistic theoretical model to quantify the temporal dynamics of the plasma and the magnitude of the secondary excitation therein. Numerical results show that depending on the nuclear transition energy and the temperature and charge states reached in the plasma, secondary nuclear excitation by electron capture may dominate the direct photoexcitation by several orders of magnitude, as it is the case for the 4.8 keV transition from the isomeric state of $^{93}$Mo, or it can be negligible, as it is the case for the 14.4 keV M\"ossbauer transition in $^{57}\mathrm{Fe}$. These findings are most relevant for future nuclear quantum optics experiments at x-ray free electron laser facilities.