Dear colleagues and friends of the HI-Jena,

we are glad to provide you with the March 2023 issue of the newsletter of the Helmholtz Institute Jena.
Below you find informations and news about recent activities of our institute.

Kind Regards,
Helmholtz Institute Jena

New Regimes of Nuclear Resonance Excitation: First Experiments at the European XFEL

Ralf Röhlsberger for the MCLS* collaboration and for the ⁴⁵Sc collaboration

The nuclear resonances of Mössbauer isotopes provide extremely narrow energy references for high-resolution spectroscopy. This qualifies them for applications in condensed matter physics as well as for studies in fundamental physics and extreme metrology. Since the pioneering experiment by E. Gerdau et al. [1] at the storage ring DORIS (DESY, Hamburg), the technique of nuclear resonance scattering has found widespread applications at hard X-ray synchrotron radiation sources worldwide. Its implementation and use at hard X-ray laser sources started with one experiment performed at SACLA in Japan reported so far [2].

In 2022 we could perform the first experiments on nuclear resonance scattering at the European XFEL. One study was conducted at the 14.4 keV resonance of ⁵⁷Fe aiming at applications in x-ray quantum optics in a collaboration with researchers from Friedrich-Schiller-Universität Jena, the Max Planck Institute of Nuclear Physics in Heidelberg, the University of Freiburg and DESY. The other experiment, initiated by researchers from Argonne National Laboratory and Texas A&M University in College Station, USA, succeeded to achieve the first photonic excitation of the ultranarrow 12.4 keV resonance of ⁴⁵Sc for future applications in extreme metrology.

Employing the 14.4 keV nuclear resonance of ⁵⁷Fe we aimed at the realization of multiphoton excitation in forward scattering from an ensemble of Mössbauer nuclei. Due to the radiative coupling between the nuclei in the ensemble, an enhanced decay rate (superradiance, SR) and collective energy shifts (collective Lamb shift, CLS) appear as characteristic signatures in the scattered light. So far, SR and CLS have been extensively studied in the single-photon excitation regime at synchrotrons. Here we were aiming to reveal the spectral properties of the nuclear exciton under multiphoton excitation conditions, especially with the goal to determine the CLS as function of the number of photons that resonantly excite the nuclear ensemble. So far, the spectral properties under multiphoton excitation conditions have neither been approached theoretically nor experimentally. Our experiment in 2022 was mainly devoted to reliably detect the nuclear forward scattering signal in the presence of the extremely intense radiation pulses of the European XFEL. To our great surprise, under these conditions a 8 mm thick ⁵⁷Fe foil appeared to be only 1 mm thick, as illustrated in Fig.1. The microscopic mechanism for this unique behavior is still under investigation.

The 12.4 keV resonance of ⁴⁵Sc is one of the narrowest nuclear resonances known with a half width of 1.4 femto-eV. In several aspects this isotope is superior to the 8.3 eV ^229mTh isomer for extreme metrology applications. The scientific potential of the ⁴⁵Sc resonance together with the possibility to resonantly excite it by photons from modern accelerator-based hard x-ray sources was identified more than 30 years ago [3]. However, it escaped detection until now, mostly due to the lack of hard x-ray sources with sufficient spectral flux. In this experiment the nuclear resonance of ⁴⁵Sc was successfully observed by irradiation of a Sc metal foil with 12.4-keV x-ray pulses of sub-millisecond duration and detection of the 4 keV delayed K-fluorescence that followed internal nuclear conversion. The resonance energy, which was known before the experiment to an uncertainty of +/- 50 eV only was determined with sub-eV accuracy. Our results set the stage for further studies of this isotope with promising applications in fundamental physics and extreme metrology.

Both experiments greatly benefitted from the high pulse repetition rate in conjunction with hard x-ray self-seeding (HXRSS) at the respective nuclear resonance energies, providing the spectral flux for efficient nuclear resonance excitation. A unique pulse pattern will be extremely beneficial for future studies at the ⁴⁵Sc resonance, in particular for the analysis of hyperfine interactions via nuclear forward scattering.

References

* Multiphoton Collective Lamb Shift
[1] E. Gerdau et al., Nuclear Bragg diffraction of synchrotron radiation in yttrium iron garnet, Phys. Rev. Lett. 54, 835 (1985).
[2] A. I. Chumakov et al., Superradiance of an ensemble of nuclei excited by a free electron laser, Nature Physics 14, 261 (2018).
[3] Yu. V. Shvyd’ko and G. V. Smirnov, On the direct measurement of nuclear resonance parameters of long-lived (>=1s) isomers. Nucl. Instrum. Methods Phys. Res. B 51, 452 (1990).

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