Speaker
Description
The γ-ray strength function ($\gamma$SF) is a statistical nuclear property that describes the likelihood of $\gamma$-ray emission as a function of $\gamma$-ray energy. Investigations into the $\gamma$SF have identified prominent features in its shape, such as the giant dipole resonance, pygmy dipole resonance, scissors mode, and low energy enhancement (LEE). The LEE is a fundamental property of atomic nuclei, and it has been shown to have significant impact on astrophysical reaction rates [1]. The electromagnetic nature of the LEE has puzzled the nuclear physics community since it was first discovered in $^{56,57}$Fe [2], and despite two decades of theoretical and experimental efforts, it remains unclear if the LEE is due to electric dipole or magnetic dipole transitions [3]. Here, we present the results from an experiment conducted at the Facility for Rare Isotope Beams (FRIB) at Michigan State University, where we use a novel combination of experimental and analytical techniques to probe the electromagnetic nature of the LEE in the nucleus $^{70}$Zn. At FRIB, beams of the ground and second isomeric states of 70Cu (J$^π$=6$^-$ and J$^π$=1$^+$, respectively) were isolated with the Low Energy Beam and Ion Trap (LEBIT) Penning trap mass spectrometer [4] and delivered to the upgraded Summing NaI(Tl) Total Absorption Spectrometer [5]. These two $\beta$-decaying states populate different levels in $^{70}$Zn, with the 6$^-$ ground state favoring E1+M1 transitions and the 1$^+$ isomeric state favoring M1 transitions. In this contribution, we present the comparison of the $^{70}$Zn γSF extracted from both $\beta$-decaying states of $^{70}$Cu with the $\beta$-Oslo [6] and Shape [7] methods. From these results we are able to make a conclusive determination about the electromagnetic nature of the LEE in $^{70}$Zn.
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