Tailoring Laser-Generated Plasmas for Efficient Nuclear Excitation by Electron Capture

Abstract

The optimal parameters for nuclear excitation by electron capture in plasma environments generated by the interaction of ultra-strong optical lasers with solid matter are investigated theoretically. As a case study we consider a 4.85 keV nuclear transition starting from the long-lived 93m Mo isomer that can lead to the release of the stored 2.4 MeV excitation energy. We find that due to the complex plasma dynamics, the nuclear excitation rate and the actual number of excited nuclei do not reach their maximum at the same laser parameters. The nuclear excitation achievable with a high-power optical laser is up to twelve and up to six orders of magnitude larger than the values predicted for direct resonant and secondary plasma-mediated excitation at the x-ray free electron laser, respectively. Our results show that the experimental observation of the nuclear excitation of 93m Mo and the subsequent release of stored energy should be possible at laser facilities available today.Novel coherent light sources open unprecedented possibilities for the field of laser-matter interactions [1]. The X-ray Free Electron Laser (XFEL) [2,3] for instance can drive low-energy electromagnetic transitions in nuclei. Ultra-strong optical laser systems with up to few petawatt power [4][5][6][7][8] are very efficient in generating plasma environments [9], which host complex interactions between photons, electrons, ions and the atomic nucleus. Nuclear excitation in laser-generated hot plasmas involving optical lasers [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26], or cold high-density plasmas [27] at the XFEL [28,29] have been under investigation. Special attention has been attracted by nuclear transitions starting from long-lived excited states. Such states are also known as nuclear isomers and are particularly interesting due to their potential to store large amounts of energy over long periods of time [30][31][32][33][34][35][36][37]. A typical example is 93m Mo at 2.4 MeV, for which an additional excitation of only 4.85 keV could lead to the depletion of the isomer and release on demand of the stored energy.For both optical-and x-ray laser-generated plasmas, the process of nuclear excitation by electron capture (NEEC) [38,39] into the atomic shell has proven to have a significant contribution. As secondary process in the cold plasma environment generated by the interaction of the XFEL with solid-state targets, NEEC can exceed the direct nuclear photoexcitation by six orders of magnitude [28,29] for the 4.85 keV excitation starting from the 93m Mo isomeric state. In this Letter, we show that by tailoring optical-laser-generated plasmas to harness maximum nuclear excitation via NEEC, a further six orders of magnitude increase in the nuclear excitation and subsequent isomer depletion compared to the case of cold XFEL-generated plasmas can be reached. As an interesting point, we find that due to the complexity of the processes involved, the plasma and correspondingly laser parameters for reaching the maximal NEEC ra...

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

“…We calculate NEEC rates as function of plasma temperature for selected electron densities, for both TE conditions as previously discussed in Refs. [36,37] and for capture into vacant inner-shell holes in the x-ray assisted process. For the latter we consider in first approximation the TE free electron flux and charge distribution with an additional inner-shell hole as the capture state.…”

“…The resonance energy for L-shell capture requires free electron energies between 52 eV and 597 eV [37], which should be well available at plasma temperatures of a few hundred eV. However, with such low temperatures, under the TE steady state L-shell vacancies are inexistent, thus allowing only for the capture into M shell or further outer shells [36,37]. The inner-shell hole created by the XFEL could optimise the NEEC by decoupling the free electron energy condition from the capture state generation condition.…”

“…As a general feature of NEEC, capture is most efficient and the excitation cross sections are highest for electron recombination into inner-shell vacancies. This holds true also for the case of 93m Mo, for which the largest NEEC cross section occurs for the capture into the L-shell vacancies, while capture into the inner-most K-shell is energetically forbidden [24,25,36,37]. However, in a plasma scenario it is difficult to simultaneously facilitate both the existence of inner-shell vacancies and the corresponding free electron energies that are allowed by NEEC.…”

“…In Refs. [36,37], parameters of petawatt-class optical lasers were adopted for the calculations. However, the two types of facilities do not match well [45,46]: (i) Petawatt-class optical lasers are much stronger than XFELs (i.e., contain a larger number of photons); (ii) XFELs have higher repetition rates than petawatt-class lasers.…”

“…where N iso is the number of isomers in the plasma, and λ TE neec is the total NEEC rate assuming the plasma is in TE steady state [36,37]. Here, N h iso is the number of isomers with an x-ray generated inner-shell h hole in the atomic shell (h: 2s, 2p 1/2 , or 2p 3/2 ) in the plasma, λ h neec is the NEEC rate for the capture into the inner-shell h hole, and τ h is the lifetime of the h hole.…”