Dynamical Control of Nuclear Isomer Depletion via Electron Vortex Beams

Some nuclear isomers are known to store a large amount of energy over long periods of time, with a very high energy-to-mass ratio. Here, we describe a protocol to achieve the external control of the isomeric nuclear decay by using electron vortex beams whose wave function has been especially designed and reshaped on demand. Recombination of these electrons into the isomer’s atomic shell can lead to the controlled release of the stored nuclear energy. On the example of ${}^{93m}\mathrm{Mo}$, we show theoretically that the use of tailored electron vortex beams increases the depletion by 4 orders of magnitude compared to the spontaneous nuclear decay of the isomer. Furthermore, specific orbitals can sustain an enhancement of the recombination cross section for vortex electron beams by as much as 6 orders of magnitude, providing a handle for manipulating the capture mechanism. These findings open new prospects for controlling the interplay between atomic and nuclear degrees of freedom, with potential energy-related and high-energy radiation source applications.

Figure 1

(a) A plane wave electron beam incident on a forked mask generates the electron vortex beam. Upon hitting on an ion beam with impact parameter $\mathbf{b}$, the electrons recombine into atomic vacancies. (b) At the resonant continuum electron energy, electron recombination (orange atomic shell levels on the left) will be accompanied by nuclear excitation (magenta nuclear states on the right) in the process of NEEC. (c) Partial level scheme of ${}^{93}\mathrm{Mo}$. The nuclear isomeric (IS), gateway (GW), intermediate ($F$) and ground state (GS) levels are labeled by their spin, parity, and energy in keV [3]. The transitions $\mathrm{IS}\to \mathrm{GW}$ and $\mathrm{GW}\to F$ are both of $E2$ type. Energy intervals are not to scale.

Figure 2

NEEC integrated cross section enhancement for the 4.85 keV nuclear transition depleting ${}^{93m}\mathrm{Mo}$. (a) The enhancement ratio $S_v(n{l}_j)/S_p(n{l}_j)$ comparing vortex and plane wave electron beams for recombination orbitals in the range $2p_{3/2}–4f_{7/2}$. (b) The ratio $S_v(n{l}_j)/S_p(2p_{3/2})$ of vortex beam versus maximal plane wave NEEC resonance strengths corresponding to recombination into the $2p_{3/2}$ orbital (left-hand axis, gray dashed curve with circle) and the absolute values of $S_v(n{l}_j)$ (right-hand axis, vertical colored bars). We consider (a) $m=3$, 4, 5 or (b) $m=5$, with $\zeta =p_z$ and impact parameter range $\zeta b=1$. The resonant electron energy $E_0$ is presented in color coding.