II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Proton removal and lifetimes in the Ca isotopes: Spectroscopy andreaction studies
H. L. Crawford,∗1 S. Paschalis,∗2 and M. Petri∗2 for the RIBF170R1 Collaboration
Nucleon knockout reactions are an essential andpowerful tool to study the single-particle structure ofnuclei far from stability. Through comparison of measured (σexp ) and theoretical (σth ) inclusive and exclusive cross-sections they provide a method to assign speciﬁc single-particle conﬁgurations and occupancies ofstates in nuclei. However, the theoretical descriptionof knockout reactions is challenging, requiring both atreatment of the reaction dynamics and the nuclear
structure of initial and ﬁnal states.
The ratio R = σexp /σth has been found, in stable
nuclei and speciﬁcally for (e, e′ p) and single-nucleon
transfer reactions1,2) to be consistently less than 1(R ∼ 0.6), a reduction attributed to short and longrange correlations not captured in the shell-model.
However, moving away from β-stability, a study by
Tostevin and Gade3) using available one-nucleon removal data on unstable isotopes performed at intermediate energies (∼100 MeV/nucleon) shows a dependence of R on the diﬀerence in the proton and neutron separation energies of the initial system. Thisdependence is in contrast to both transfer reaction
data taken at lower energies and to new data reported
recently by the R3 B collaboration4) and RIBF5) for
quasi-free one-proton knockout at higher energies.
Given the important implications of disentangling
the reaction and structure eﬀects related to the originof the ∆S dependence of R observed in knockout reactions, we have performed an experiment to directly
explore the isospin dependence of proton knockout on
both C and H targets at RIBF energies. In addition tothe reaction cross-sections, we were also able to takeadvantage of the opportunity presented by the HiCARI
high-resolution γ-ray array6) at RIBF to simultaneously explore lifetimes in the neutron-rich Ca isotopes,
which provide an important testing ground for interactions derived from chiral Eﬀective Field Theory andmany-body methods.
The experiment was performed in December 2020 atthe RIBF as part of the HiCARI campaign. A 70 Zn
primary beam was accelerated to 345 MeV/nucleon
and fragmented on a 9 Be primary target to produce
secondary beams centered on 38, 48, 54 Ca. Fragments
were separated and identiﬁed within BigRIPS using
the standard ∆E-TOF-∆E method. The secondary
beam impinged on 1 g/cm2 thick C and 1.2 g/cm2
thick CH2 targets at the F8 focal plane, which induced
knockout reactions. The reaction residues were identiﬁed in the ZeroDegree spectrometer. The preliminary∗1∗2Fig. 1. (Top) Particle identification plot for incoming beam
particles in BigRIPS for the setting of 54 Ca. (Bottom)
Reaction residue particle identification in the ZeroDegree spectrometer, gated on incoming 54 Ca particles.

particle identiﬁcation plots for the setting on 54 Ca areshown in Fig. 1. De-excitation γ-rays were detected
within the HiCARI array.
Currently, the proton removal inclusive and exclusive cross-sections from 38, 48, 54 Ca to ﬁnal states in37, 47, 53
K are under analysis. In addition, γ-ray spectrain the most neutron-rich Ca isotopes, namely 53, 54 Caare being carefully analyzed. Preliminary energy spectra show evidence for lifetimes within the sensitivity
range for line-shape analysis, which will be pursued inthe next stages of analysis.
References

1) B. P. Kay et al., Phys. Rev. Lett. 111, 042502 (2013).
2) G. Kramer et al., Nucl. Phys. A 679, 267 (2001).
3) J. A. Tostevin, A. Gade, Phys. Rev. C 90, 057602(2014).
4) L. Atar et al., Phys. Rev. Lett. 120, 052501 (2018).
5) S. Kawase et al., Prog. Theor. Exp. Phys. 2018, 021
D01 (2018).
6) K. Wimmer et al., in this report.

Nuclear Science Division, Lawrence Berkeley National Lab
Department of Physics, University of York-4-1