X-ray assisted nuclear excitation by electron capture in optical laser-generated plasmas

TL;DR

This paper explores how combining optical lasers and X-ray Free Electron Lasers can enhance nuclear excitation in plasmas, potentially enabling efficient energy release from nuclear isomers at accessible laser intensities.

Contribution

It introduces a theoretical framework for X-ray assisted NEEC in laser-generated plasmas, highlighting the efficiency of inner-shell hole recombination at low plasma temperatures.

Findings

NEEC rates are maximized at ~100 eV plasma temperature.

Inner-shell holes facilitate efficient nuclear excitation.

Potential to excite ~1 isomer per second with modest laser setups.

Abstract

X-ray assisted nuclear excitation by electron capture (NEEC) into inner-shell atomic holes in a plasma environment generated by strong optical lasers is investigated theoretically. The considered scenario involves the interaction of a strong optical laser with a solid-state nuclear target leading to the generation of a plasma. In addition, intense x-ray radiation from an X-ray Free Electron Laser (XFEL) produces inner-shell holes in the plasma ions, into which NEEC may occur. As case study we consider the $4.85$-keV transition starting from the 2.4 MeV long-lived $^{\mathrm{93m}}$Mo isomer that can be used to release the energy stored in this metastable nuclear state. We find that the recombination into $2p_{1/2}$ inner-shell holes is most efficient in driving the nuclear transition. Already at few hundred eV plasma temperature, the generation of inner-shell holes can allow optimal conditions for NEEC, otherwise reached for steady-state plasma conditions in thermodynamical equilibrium only at few keV. The combination of x-ray and optical lasers presents two advantages: first, NEEC rates can be maximized at plasma temperatures where the photoexcitation rate remains low. Second, with mJ-class optical lasers and an XFEL repetition rate of $10$ kHz, the NEEC excitation number can reach $\sim 1$ depleted isomer per second and is competitive with scenarios recently envisaged at petawatt-class lasers.