Speaker
Description
Random Matrix Theory provides a comprehensive framework for the description of complex, chaotic quantum systems [1,2]. It is exploited across various domains of physics as for instance in the statistical treatment of nuclear reactions within the Hauser-Feshbach formalism [3]. One important aspect in the practical application of Hauser-Feshbach codes is the fluctuation property of partial transition widths. Experimentally, the applicability of so-called Porter-Thomas (PT) fluctuations [4] has been extensively studied in thermal neutron capture experiments [5]. Former analyses of the nuclear data ensemble (NDE) of neutron resonances [5] validate the PT distribution. Recent studies and thorough reanalyses of the NDE revealed significant deviations from PT predictions [6] partially explained by non-statistical $\gamma$ decays. Neutron resonances provided a vast amount of precision data on fluctuation properties of nuclear resonances. However, no experimental data exist, to date, for width fluctuations \textit{below} neutron separation thresholds. This region is particularly interesting because it contains the onset of the quasicontinuum region. There, the nuclear spectra transition from a few individual states at low excitation energies to an ensemble of states at high excitation energies. The latter are assumed to be well described by the ansatz of the Hauser-Feshbach statistical model.
In this contribution, we introduce a new method for the study of fluctuations of partial transition widths based on nuclear resonance fluorescence experiments with quasimonochromatic linearly-polarized photon beams below particle separation thresholds [7]. Assuming (\chi^2)-distributed partial transition widths, average branching ratios of internal $\gamma$ decay transitions are related to the degree of freedom $\nu$ of the (\chi^2) distribution. Recent studies with $^{150}$Nd result in a degree of freedom of (\nu = \num{1.93(12)}) in clear disagreement with the PT distribution [8,9]. The recent findings will be discussed in the context of non-statistical effects in the (\gamma)-decay behavior that are potentially connected to the survival of good $K$ quantum numbers in the covered excitation-energy region.
This work is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project-ID 279384907 - SFB~1245, Project-ID 460150577 - ZI 510/10-1, and Project-ID 499256822 - GRK~2891 “Nuclear Photonics”, by the State of Hesse under the grant “Nuclear Photonics” within the LOEWE program (LOEWE/2/11/519/03/04.001(0008)/62), by the BMBF under grant number 05P21RDEN9. Furthermore, this work is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under grants DE-SC0023010, DE-FG02-97ER41041 (UNC), and DE-FG02-97ER41033 (Duke, TUNL), and by the UK-STFC (Grant No. ST/P005101/1).
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[6] P. E. Koehler, Phys. Rev. C 84, 034312 (2011).
[7] A. Zilges, D. Balabanski, J. Isaak, and N. Pietralla, Prog. Part. Nucl. Phys. 122, 103903 (2022).
[8] O. Papst, J. Isaak, V. Werner, D. Savran, N. Pietralla, G. Battaglia, T. Beck, M. Beuschlein, S. W. Finch, U. Friman-Gayer, K. E. Ide, R. V. F. Janssens, M. D. Jones, J. Kleemann, B. Löher, M. Scheck, M. Spieker, W. Tornow, R. Zidarova, and A. Zilges, accepted for publication in Phys. Rev. Lett. (2025).
[9] O. Papst, J. Isaak, V. Werner, D. Savran, N. Pietralla, G. Battaglia, T. Beck, M. Beuschlein, S. W. Finch, U. Friman-Gayer, K. E. Ide, R. V. F. Janssens, M. D. Jones, J. Kleemann, B. Löher, M. Scheck, M. Spieker, W. Tornow, R. Zidarova, and A. Zilges, (2025), arXiv:2501.19185 [nucl-ex].
