II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

High-resolution spectroscopy and lifetime measurements inneutron-rich Zr and Mo isotopes
W. Korten,∗1 B. Moon,∗2,∗3 and K. Wimmer∗4,∗3 for the RIBF187 Collaboration
One of the most interesting cases of shape evolution
in nuclei is encountered along the semi-magic (Z = 40)
Zr isotopes. While 90 Zr, at neutron number N = 50,
shows properties of a doubly-magic nucleus, neutrondeﬁcient Zr isotopes become well-deformed towards
80Zr. On the neutron-rich side, a sudden onset of deformation is also indicated by the dramatic lowering of
the ﬁrst excited 2+ state from 98 Zr to 100 Zr.
When going towards even more neutron-rich isotopes the question about the further shape evolution
arises since the theoretical predictions diverge. Many
theoretical calculations have been performed for 110 Zr
since it combines the magic numbers Z = 40 and
N = 70 of the harmonic oscillator potential and could
be another quasi doubly-magic nucleus. This question
was answered in a previous SEASTAR experiment atthe RIBF,1) which measured the ﬁrst excited states of110
Zr and showed that this isotope is rather well deformed. However, several questions remain open, suchas the possibility of shape coexistence or triaxial deformation at Z = 40, N = 70 as predicted by diﬀerent
theoretical models.3–5)
We performed high-resolution spectroscopy of nuclei around 110 Zr in an experiment with the HiCARI
array2) in order to measure lifetimes of their (ﬁrst) excited states . The high-resolution γ-ray detectors fromHiCARI will allow to resolve level schemes by measuring γ rays from ∼100 keV, and to measure lifetimes ofexcited states between ∼20 ps and 1 ns. The results
will also allow to conﬁrm the level scheme of 110 Zr andto measure decay branching ratios of states beyond the2+1 for the ﬁrst time.
The nuclei of interest were populated by protonremoval reactions from projectiles around 112 Mo, produced from a primary 238 U beam, impinging on a secondary Be target. Particle identiﬁcation of the radioactive beams for selecting the proper reaction channel was performed with the BigRIPS and the ZeroDegree spectrometers. Examples of the particle identiﬁcation plots are shown in Fig. 1. Average intensities
and purities of the projectiles of interest amounted to81 and 3000 pps and 0.8% and 34% for 111 Nb and113
Tc, respectively. Additionally, 112 Mo was transmitted at a high rate as well. 110 Zr was populated throughproton removal reactions from 111 Nb and 112 Mo. The
high intensity of 113 Tc allowed to obtain signiﬁcantly
more statistics for 112 Mo than in the previous exper∗1∗2∗3∗4Fig. 1. Preliminary particle Identification (PID) for BigRIPS (top) and ZeroDegree (ZD) spectrometer (bottom).

iment.1) Additionally, the neighbouring even-even nuclei, 108 Zr and 110 Mo, have been populated strongly toallow for a detailed lifetime analysis.
In order to optimize the experiment for the long lifetimes of the 2+ states, expected to be several hundred
ps, the Be target was positioned 10 cm upstream of
the center of HiCARI. Lifetimes will be extracted using the line-shape method6) and will give access to thecollective properties. The experiment will allow to distinguish between predictions of diﬀerent nuclear models concerning the shape of 110 Zr, the key isotope forthe evolution of collective properties along the N = 70
isotones.
References
1) N. Paul et al., Phys. Rev. Lett. 118, 032501 (2017).
2) K. Wimmer et al., in this report.
3) M. Borrajo et al., Phys. Lett. B 746, 341 (2015).
4) J. Libert, Phys. Rev. C 60, 1 (1999).
5) T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016).
6) P. Doornenbal et al., Nucl. Instrum. Methods Phys. Res.
A 613, 218 (2010).

IRFU, CEA Paris-Saclay
CENS, IBSRIKEN Nishina Center
IEM-CSIC-8-1