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

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

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

“…The excitation N exc per laser pulse is up to six orders of magnitude larger than the one [∼ 10 −6 , recalculated for the parameters considered here] in the XFEL-generated cold (T =350 eV) plasma [28,29]. The largest value of 1.9 excitations per pulse should be reached with the PETAL laser which provides both high laser power and long pulse duration.…”

“…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.…”

“…Then the total number of excited nuclei is N exc = N iso λ neec (T e , n e )τ p , with N iso the number of isomers in the plasma. Assuming a spherical plasma the plasma lifetime is approximatively given by τ p = R p m i /(T eZ ) [29,45] with the ion mass m i , the average charge stateZ and the plasma radius R p . N iso can be estimated introducing the isomer fraction embedded in the original solid-state target f iso , [46].…”

“…The dependence φ e (T e ) determines the quantitative contribution of the NEEC resonances. The theoretical formalism for the calculation of the NEEC cross section σ neec has been presented elsewhere [28,29,43,44]. The total NEEC excitation number N exc is connected to the rate λ neec via…”

“…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.…”