Nuclear excitation by electron capture in optical-laser-generated plasmas

Abstract

The process of nuclear excitation by electron capture in plasma environments generated by the interaction of ultrastrong optical lasers with solid-state samples is investigated theoretically. With the help of a plasma model, we perform a comprehensive study of the optimal parameters for the most efficient nuclear excitation and determine the corresponding laser setup requirements. We discern between the low-density plasma regime, modeled by scaling laws, and the high-density regime, for which we perform particle-in-cell calculations. As a nuclear transition case study we consider the 4.85-keV nuclear excitation starting from the long-lived ^{93m}Mo isomer. Our results show that the optimal plasma and laser parameters are sensitive to the chosen observable and that measurable rates of nuclear excitation and isomer depletion of ^{93m}Mo should be already achievable at laser facilities existing today.

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

“…However, the comparison with the more detailed hydrodynamic expansion model in Ref. [37] has shown that this lifetime approximation provides reasonable NEEC results, with a ∼ 50% of deviation for the case of low temperatures and ∼ 5% for high temperatures ( 6 keV). In addition, our calculations here predict that both the NEEC rate and the resulting number of excited nuclei have a smooth dependence on density and temperature.…”

“…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.…”

“…Following the studies in Refs. [24,25,36,37], we consider the 4.85-keV nuclear transition starting from a longlived excited state of 93m Mo at 2.4 MeV, which has a half-life of 6.85 h. The 93m Mo isomer can be depleted by driving the 4.85 keV electric quadrupole (E2) transition from the isomer to a triggering level (T), which subsequently decays via a cascade to the ground state. The partial level scheme of 93m Mo is shown in Fig.…”

“…The free-electron flux φ e (E, T e , n e ) is obtained by modelling the plasma by a relativistic Fermi-Dirac distribution. This approximation based on TE is appropriate starting shortly after the end of the optical laser pulse, since thermalization occurs on a much shorter timescale much than the plasma lifetime over which NEEC takes place [37]. As already mentioned, we neglect here the shift in electron temperature induced by the XFEL which is expected to be few tens of eV for the case under consideration.…”