Experimental studies of the pygmy dipole resonance

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-captures 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 often used to calculate neutron-capture rates with statistical approaches. In fact, the different responses to isovector and isoscalar probes highlighted earlier 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.

In this invited contribution, I will give a broad overview on recent experimental investigations of the PDR and how associated results help to better understand its structure. I will specifically feature recent one-neutron (d,p) transfer studies of the PDR in ²⁰⁸Pb, ¹²⁰Sn, and ⁶²Ni, which eluminate its neutron single-particle character. For ²⁰⁸Pb, the new data will be briefly compared to the large suite of complementary, experimental data available highlighting how our collaboration established (d,p) as an additional, valuable, experimental probe to study the PDR and its collectivity.

To highlight future possibilities, I will also present first results from a new experimental setup recently commissioned at the Super-Enge Split-Pole Spectrograph at Florida State University for particle-gamma coincidence experiments, called CeBrA, and highlight the value other particle transfer reactions could add to studying the microscopic structure of the PDR. Ideas for performing similar experiments at Argonne National Laboratory and the Facility for Rare Isotope Beams (FRIB) at Michigan State University will be discussed.