Speaker
Description
The structure of $fp$ shell nuclei has been the subject of intensive study for many decades. These nuclei remain of crucial importance because they provide a foundation for studies of neutron-rich nuclei currently being investigated at facilities such as FRIB (e.g. [1]). Within the $fp$ shell, large-basis shell model calculations, wherein both protons and neutrons occupy the $0f_{7/2}$, $1p_{3/2}$, $0f_{5/2}$, and $0p_{1/2}$ orbits, are now routine, and can be tested against a considerable body of data [2]. Electromagnetic transition strengths provide a primary set of observables to test shell model wavefunctions, which are usually based on effective interactions derived from fits to energy levels [2].
Through the late 1960s to the early 1980s the lifetimes of many excited states in the $fp$ shell were measured by the Doppler-shift attenuation method (DSAM) with the required stopping powers evaluated using the theory of Lindhard, Scharff and Schiott (LSS) [3]. It was recently discovered [4] that the anomalously strong literature value [5] for the experimental strength of the $4^+_1 \rightarrow 2^+_1$ transition in $^{58}$Fe could be attributed to the use of LSS electronic stopping powers in Doppler-shift measurements, which for the case of $^{58}$Fe stopping in tantalum, differ by a factor of two from more recent values given by SRIM [6]. Woodside et al. [4] re-evaluated the 1978 DSAM lifetime measurement on $^{58}$Fe [7] with the LSS stopping powers replaced by values from SRIM, which brought the transition strength of the $4^+_1 \rightarrow 2^+_1$ transition into agreement with shell model calculations [2].
This example is unlikely to be unique: we have therefore begun to re-evaluate historical DSAM measurements that used LSS stopping powers. One outcome is a re-evaluation of effective charges applicable for the $fp$ shell. For example, Honma et al. [2] adopted the standard $e_p=1.5$ and $e_n=0.5$ in their comprehensive test of the GXFP1 interaction, whereas re-evaluated lifetimes in $^{54}$Cr strongly favor the "universal" effective charges $e_p=1.33$ and $e_n=0.45$ recently proposed for the $sd$ and $fp$ shells by Ogunbeku et al. [1].
An overview of the progress and implications of this work will be given, including an evaluation of the impact of historical DSAM lifetime data on extracted electric monopole ($E0$) transition strengths such as those in the Ni isotopes [8].
References
[1] T.H. Ogunbeku et al., Phys. Rev. Lett. 135 (2025) 072501.
[2] M. Honma et al., Phys. Rev. C 69 (2003) 034335.
[3] J. Lindhard, M. Scharff and H.E. Schi{\o}tt, Mat. Fys. Medd. Dan. Vid. Selsk. 33 no.14 (1963).
[4] J.A. Woodside et al., Phys. Rev. C, in press.
[5] C.D. Nesaraja, S.D. Geraedts and B. Singh, Nucl. Data Sheets 111 (2010) 897.
[6] J.F. Ziegler, M.D. Ziegler and J. P. Biersack, Nucl. Inst. Meth. Phys. Res. B 268 (2010) 1818.
[7] H.H. Bolotin et al., Nucl. Phys. A 311 (1978) 75.
[8] L.J. Evits et al., Physics Letters B 779 (2018) 396.