Speaker
Description
Radiative neutron capture on rare-earth nuclei is important for applications ranging from nuclear astrophysics to reactor-related environments, yet experimental data remain limited, particularly for odd–odd systems. In our work [1], we present new results on the $^{169}$Tm$(n,\gamma)$ reaction, including an experimental determination of the capture cross section in the presence of considerable discrepancies in the unresolved-resonance region. Precise knowledge of the $^{169}$Tm$(n,\gamma)$ cross section in the keV region is essential for understanding the slow neutron-capture process in this mass region. In addition, we study the statistical $\gamma$ decay of the compound nucleus $^{170}$Tm, focusing on the scissors mode (SM) in the $M1$ photon strength function (PSF), for which data in odd–odd rare-earth nuclei are scarce.
The capture experiments were performed at the Los Alamos Neutron Science Center using the time-of-flight technique with the Detector for Advanced Neutron Capture Experiments (DANCE). DANCE provides high-efficiency detection of complete $\gamma$-ray cascades following neutron capture, enabling detailed studies of level density (LD) and PSFs. These quantities are also crucial both for efficiency determination and for Hauser–Feshbach cross-section calculations.
The capture cross section was determined from 1.8 eV to 0.97 MeV, representing the broadest neutron-energy range measured for this isotope. Eight new resonances were observed and their parameters extracted using SAMMY [2]. In the resolved-resonance region, the measured cross section agrees well with ENDF/B-VIII.0 [3], JEFF 3.3 [4], and JENDL-5 [5], while in the unresolved-resonance region it is generally lower than the evaluations.
The statistical $\gamma$ decay of $^{170}$Tm was investigated using coincident spectra constructed from individual resonances. These were compared with statistical simulations using the DICEBOX code [6] to test different LD and PSF models. Previously reported model parameters for odd–odd rare-earth nuclei [7, 8], as well as SMLO and D1M-QRPA PSF models from the Reference Database for Photon Strength Functions [9], fail to reproduce the measured spectra. The best agreement is obtained with the SM centered at 3.3 MeV, width of 1.0 MeV, and strength comparable to that observed in neighboring $^{168}$Er [10], combined with the MGLO [11] $E1$ PSF model; the Back-Shifted Fermi Gas LD model is favored.
Finally, the impact of the measured cross section on stellar nucleosynthesis was evaluated. The derived slow-process abundance of $^{169}$Tm is expected to increase by a factor of 1.26, while changes in the abundances of heavier nuclei remain at the level of approximately 0.2%.
[1] I. Knapova, K. Horcickova et al., Physical Review C 112 (2025) 014612.
[2] N. M. Larson, Updated User’s Guide for SAMMY: Multilevel R-Matrix Fits to Neutron Data Using Bayes’ Equations, Technical Report ORNL/TM-9179/R8, ENDF-364/R2, Oak Ridge National Laboratory, 2008.
[3] D. Brown et al., Nuclear Data Sheets 148 (2018) 1–142.
[4] A. J. M. Plompen et al., European Physical Journal A 56 (2020) 181.
[5] O. Iwamoto et al., Journal of Nuclear Science and Technology 60 (2023) 1–60.
[6] F. Becvar, Nuclear Instruments and Methods in Physics Research A 417 (1998) 434–449.
[7] J. Kroll et al., International Journal of Modern Physics E 20 (2011) 526–531.
[8] F. Pogliano et al., Physical Review C 107 (2023) 034605.
[9] S. Goriely et al., European Physical Journal A 55 (2019) 172.
[10] I. Knapova et al., Physical Review C 107 (2023) 044313.
[11] J. Kroll et al., Phys. Rev. C 88 (2013) 034317.