Speaker
Description
Level density (LD) and photon strength functions (PSFs) are fundamental quantities describing $\gamma$ decay in the statistical regime, where individual nuclear levels cannot be experimentally resolved. When neutron-capture cross sections -- crucial inputs for heavy element nucleosynthesis calculations -- are unavailable, LD and PSFs provide essential inputs for Hauser-Feshbach calculations. An open question in our understanding of the PSFs is the pygmy dipole resonance (PDR), a low-energy oscillation manifesting in neutron-rich nuclei. One leading theoretical interpretation describes the PDR as an oscillation of the neutron skin against the nuclear core; consequently, the PDR is expected to be more prominent in neutron-rich nuclei far away from stability, which are important for $r$-process calculations. Currently, no reliable systematics of PDR properties is available across nuclei.
This work presents a study of the PSFs and PDR from $^{203}$Tl(n,$\gamma$)$^{204}$Tl reaction measured with the DANCE detector. DANCE -- a highly segmented 4$\pi$ BaF$_2$ scintillator array -- provides high-efficiency coincidence $\gamma$-ray spectra validated against simulations of $\gamma$ cascades using DICEBOX. This analysis benchmarks models of LD and PSFs in $^{204}$Tl and characterizes properties of the PDR. No experimental PSF data currently exist in the PDR region for thallium isotopes in the Photon Strength Function database [1].
A strong PDR centered at 5.4 MeV was identified in the dataset, with a PSF shape similar to previous measurements in $^{206}$Pb and $^{196}$Pt. Notably, none of the recommended PSF models in the database reproduce the experimental spectra. Due to strong parity asymmetry in low-lying levels of $^{204}$Tl, $s$-wave and $p$-wave $\gamma$-ray spectra exhibit different sensitivities to E1 and M1 components of the PSF, providing clear distinctions between two groups. In addition to the measured $\gamma$-ray spectra, the implications of the observed PDR on neutron-capture cross section calculations will also be presented. Together with previous measurements in the region of nuclei near doubly magic $^{208}$Pb, these results will help to benchmark the PSF models and improve predictive power for neutron-rich nuclei away from stability.
[1] IAEA Photon Strength Function Database, version v2024.1, available from: https://nds.iaea.org/PSFdatabase
| Contribution category | Experiment |
|---|---|
| Presenter status | Postdoc |