Speaker
Description
In atomic nuclei, the term pygmy dipole resonance (PDR) has been commonly used for the electric dipole (E1) strength around and below the neutron-separation energy. It has been shown that the PDR strength strongly impacts neutron-capture rates in the s- and r-process, which synthesize the majority of heavy elements in our universe. A precise understanding of the PDR’s microscopic structure is essential to pin down how it contributes to the gamma-ray strength function (γSF) often used to calculate the neutron-capture rates. In fact, the different responses to isovector and isoscalar probes highlighted the complex structure of the PDR and emphasized that different underlying structures would indeed need to be disentangled experimentally if stringent comparisons to microscopic models wanted to be made.
Featuring our recent study of $^{208}\mathrm{Pb}$ [1] and $^{62}\mathrm{Ni}$ [2], I will present how the neutron one-particle- one-hole structure of the PDR can be studied with high-resolution magnetic spectrographs. The data on $^{208}\mathrm{Pb}$ were obtained from $(d,p)$ one-neutron transfer and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany, while the data on $^{62}\mathrm{Ni}$ were measured at Florida State University with the Super-Enge Split-Pole Spectrograph. In this contribution, the new data will be compared to the large suite of complementary, experimental data available for 208Pb highlighting how we established $(d,p)$ as an additional, valuable, experimental probe to study the PDR and its collectivity. Besides the single-particle character of the states, different features of the $(d,p)$ strength distributions will be discussed.
To highlight future possibilities, I will also briefly present first results from a new experimental setup recently commissioned at the Super-Enge Split-Pole Spectrograph at Florida State University for particle-γ coincidence experiments [3].
References
[1] M. Spieker, A. Heusler, B. A. Brown, T. Faestermann, R. Hertenberger, G. Potel, M. Scheck, N. Tsoneva, M. Weinert, H.-F. Wirth, and A. Zilges, Accessing the Single-Particle Structure of the Pygmy Dipole Resonance in $^{208}\mathrm{Pb}$, Phys. Rev. Lett. 125, 102503 (2020). https://link.aps.org/doi/10.1103/PhysRevLett.125.102503.
[2] M. Spieker, L. T. Baby, A. L. Conley, B. Kelly, M. Mu ̈scher, R. Renom, T. Schüttler, and A. Zilges, Experimental study of excited states of $^{62}\mathrm{Ni}$ via one-neutron $(d,p)$ transfer up to the neutron-separation threshold and characteristics of the pygmy dipole resonance states, Phys. Rev. C 108, 014311 (2023) . https://link.aps.org/doi/10.1103/PhysRevC.108.014311.
[3] A. Conley, B. Kelly, M. Spieker, R. Aggarwal, S. Ajayi, L. Baby, S. Baker, C. Benetti, I. Conroy, P. Cottle, I. D’Amato, P. DeRosa, J. Esparza, S. Genty, K. Hanselman, I. Hay, M. Heinze,
D. Houlihan, M. Khawaja, P. Kielb, A. Kuchera, G. McCann, A. Morelock, E. Lopez-Saavedra, R. Renom, L. Riley, G. Ryan, A. Sandrik, V. Sitaraman, E. Temanson, M. Wheeler, C. Wibisono, and I. Wiedenhöver, The CeBrA demonstrator for particle-γ coincidence experiments at the FSU Super-Enge Split-Pole Spectrograph, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 1058,
168827 (2024) . https://www.sciencedirect.com/science/article/pii/S0168900223008185.
