Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 52 (2019)

Inelastic scattering of neutron-rich Ni and Zn
isotopes oﬀ a proton target†
M. L. Cortés,∗1,∗2 P. Doornenbal,∗3 M. Dupuis,∗4 S. M. Lenzi,∗5 F. Nowacki,∗6 A. Obertelli,∗7,∗3 S. Péru,∗4
N. Pietralla,∗1 V. Werner,∗1 K. Wimmer,∗8 and the NP1312-RIBF118 collaboration.
One of the most important insight on nuclear structure obtained from studies performed at radioactive ion
beam facilities is the modiﬁcation of the shell closureswith varying number of protons and neutrons.1) Thisshell-evolution has been widely studied, in particularon the neutron-rich side of the nuclear chart. Experimental evidence suggests, for example, the appearanceof new magic numbers at N = 32, 34 in Ca isotopes,2,3)
as well as the disappearance of the shell closures at
N = 8, 20 and 281) in various neutron-rich isotopes.
For the Ni isotopic chain, the reduced transition probability, B(E2)↑, measured between N = 28 and N = 40,
shows a parabolic trend which indicates a subshell closure at N = 40. A measurement of the B(E2)↑ value of70
Ni indicated an enhanced collectivity for N = 42,4)
thus a possible weakening of the Z = 28 gap towards
78Ni. In contrast, measurements of 72, 74 Ni5,6) show a
reduced B(E2)↑ value for these isotopes, corroborating
the magic character of the N = 50 and Z = 28 shellgaps.
In the experiment, performed at the RIBF as partof the ﬁrst SEASTAR campaign, a 238 U primary beamwith an energy of 345 MeV/nucleon and an averageintensity of 12 pnA impinged on a 3 mm thick 9 Be target at the entrance of BigRIPS.7) After selection andidentiﬁcation, 72, 74 Ni and 76, 80 Zn ions were focused on
the MINOS device,8) composed of a 102(1) mm longliquid hydrogen target surrounded by a Time Projection Chamber (TPC). Due to the low eﬃciency of MINOS for the (p, p′ ) reaction, the information of the TPC
was not used, leading to a decrease of the resolution of1 MeV γ-rays from a typical 9% using MINOS, to 14%.
Reaction products were identiﬁed using the ZeroDegree spectrometer,7) and γ rays were detected with theDALI2 array,9) composed of 186 NaI(Tl) detectors. The
full-energy-peak eﬃciency of the array was determinedusing a detailed Geant4 simulation and was found to
be 14% at 1.33 MeV with an energy resolution of 6.2%(FWHM) for a stationary source.
Direct proton inelastic scattering cross sections to the+2+
1 and 41 states were inferred from the γ-ray spectrum of each isotope. Such cross sections were analyzed
considering two reaction models. First, a microscopic†

∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8

Condensed from the article in Phys. Rev. C 97, 044315 (2018)
Institut für Kernphysik, Technische Universität Darmstadt
GSI Helmoltzzentrum für Schwerionenforschung GmbHRIKEN Nishina Center
CEA, DAM, DIF
Dipartimento di Fisica e Astronomia, Universitá di Padova
IPHC, CNRS/IN2P3, Université de Strasbourg
IRFU, CEA, Université Paris-Saclay
Department of Physics, University of Tokyo- 45 -approach, based on transition densities obtained from
Quasiparticle Random Phase Approximation (QRPA)
and the Jeukenne-Lejeune-Mahaux (JLM) potential,10)
was used to calculate theoretical inelastic scatteringcross sections. To deal with the high energies of thebeam in front of the target (≈ 270 MeV), an extensionof the JLM folding model above 200 MeV/nucleon wasdeveloped. This approach provided the theoreticalcross sections for inelastic scattering to the 2+
1 stateof 72, 74 Ni and 80 Zn. By comparing the theoretical andexperimental cross sections, neutron-to-proton matrix
element ratios, Mn /Mp , were obtained. The results
suggest that for the Ni isotopes (Mn /Mp ) > (N/Z),
which implies that the contribution of the neutrons tothe collectivity is enhanced. For 80 Zn, the calculation
yields (Mn /Mp ) < (N/Z), which is in agreement withan increased role of the protons to the collectivity. Second, deformation lengths for the ﬁrst quadrupole excitation of each isotope were obtained using the ECIS9711) code. The calculations included a collective vibrational model and used the KD02 global optical potential.12) From the deformation lengths a matter deformation parameter β2 (p, p′ ) was obtained and compared with previously measured charge deformations,
β2 (EM). For the Ni isotopes β2 (p, p′ ) is slightly higher
than β2 (EM), consistent with a previous measurementon 74 Ni13) and with the conservation of the Z = 28 gap
for neutron-rich Ni isotopes. The opposite eﬀect is observed for 80 Zn, thus suggesting that the shell closure
for N = 50 is conserved when approaching Z = 28.
This work represents a step towards a consistent interpretation of the (p, p′ ) data and will be beneﬁcial forthe extraction of useful physics parameters linked tonuclear structure calculations.
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