Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 50 (2017)

First Spectroscopy of 110 Zr∗1,∗2∗1 ∗3
∗1,∗2
∗1,∗2
N.N.
Paul,for forthethe
SEASTAR2015
Collaboration(spokespersons:P. Doornenbal,and andAlexandre
Obertelli,)Paul,
SEASTAR2015
Collaboration(spokespersons:P. Doornenbal
Alexandre
Obertelli
∗3∗4
∗4 ∗5
∗5 ∗6
∗5 ∗6
∗6∗5
∗7
∗1,∗2
∗7∗6
Delaroche,
Girod,Heenen,T. Otsuka,)andandB.
B.Bally,Bally, M.M.Bender,
Bender, J.-P.
J.-P.
Delaroche, M. M.
Girod,P.-H.
P.-H.
Heenen,J. Libert,
J. Libert,
T. Otsuka ∗8∗9∗9∗8∗10T. T.
R. R.
Rodrı́guez,
N.N.Shimizu,andY.Y.Tsunoda,Tsunoda∗9∗10Shimizu,∗10T. Togashi,∗9andRodrı́guez,

The ﬁrst spectroscopy measurement of 110 Zr wasperformed as part of the SEASTAR campaign in 20151). This nucleus has been much debated in the literature as its 40 protons and 70 neutrons correspondwith magic numbers of both the harmonic oscillator
and tetrahedral symmetry. Whether or not signaturesof these symmetries emerge in the structure of 110 Zr is
key information to help constrain our understanding ofshell evolution far from stability. Furthermore, a stabilization eﬀect associated with these magic numbers
at 110 Zr has been proposed in the literature as a potential explanation of deﬁciencies in r-process simulations
near mass 1102) .
The experiment was performed at the RIBF using
a 30 pnA 238 U primary beam at 345 MeV/nucleon.
111
Nb was produced via in-ﬂight ﬁssion on a thin Be
production target at the F0 focal plane of the BigRIPS
spectrometer, and focused onto a 10 cm thick liquidhydrogen target at F8 with an intensity of 20 particles
per second. 110 Zr was produced via proton knockoutin the MINOS3) hydrogen target, emitted gamma rays
were detected with the DALI2 array, and the MINOS
TPC3) tracked the outgoing protons to provide a precise doppler correction of the gamma rays. The reaction residues were identiﬁed in the ZeroDegree spectrometer.
The observed transitions in 110 Zr lie close in energy
to the Bremsstrahlung background generated fromhigh velocity ions colliding with electrons in the hydrogen target. This Bremsstrahlung component wasmeasured, normalized according to the number of nuclei incident on the target, and subtracted from thegamma ray spectrum. The subtracted spectrum wasﬁt with GEANT4 simulated DALI2 response functions,
including lifetime and feeding eﬀects. Gamma-gamma
coincidences and systematics in the region were used+
to identify the three visible transitions as the 2+
1 → 01 ,++++
41 → 21 , and 22 → 01 transitions, and construct a++level scheme consisting of a 2+
1 , 41 , and 22 at 185(11),565(21), and 485(11) keV, respectively .

400

Data, literature
Data, this work
Gogny D1S + 5DCH
Gogny D1S + PCM
SLyMR0 + PCM
MCSM

1000
800

1Energy 2+ (keV)

1200

Zoom

300
200
38 40 42 44600
40020030 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64

ZFig. 1.: E2+ systematics for the N=70 isotones comparedwith theory: 5DCH4) and PCM5) with the Gogny D1S interaction, PCM with Skyrme SLyMR06) , and MCSM calculations7) . Experimental data are taken from Ref8) andthis work.

These results were compared with state-of-the-art
beyond mean ﬁeld calculations, as well as the mostadvanced Monte Carlo shell model (MCSM) calculations7) . E2+ systematics are shown in Figure 1. We
ﬁnd that 110 Zr shows no increased E2+ energy thatwould indicate a stabilization associated with a spherical or tetrahedral symmetry, but rather it continues thesharp downward trend of E2+ energies going towards
midshell. The R42 = E4+ /E2+ ratio for this nucleus
is 3.1(2), approaching the rigid rotor value of 3.33.
Both these trends are even more pronounced than predicted by beyond-mean-ﬁeld calculations. MCSM calculations provide the best agreement with our data.
The constructed level scheme is not consistent with atetrahedral symmetry in the ground state. Thus ourdata shows that 110 Zr is a well deformed nucleus withno indications of special stability either isotonically or
isotopically associated with its 40 protons and 70 neutrons. This result also discredits a stabilized 110 Zr as
an explanation for the r-process model deﬁciencies nearmass 110.
References∗1∗2∗3∗4∗5∗6∗7∗8∗91) condensed from N. Paul et al., Phys. Rev. Lett, 118,032501 (2017).
2) K. L. Kratz et al., Eur. Phys. J. A 25, 633 (2005).
3) A. Obertelli et al., Eur. Phys. J. A 50, 8 (2014).
4) J. P. Delaroche et al,al.,Phys.
Phys.Rev.
Rev.CC81,81,014303014303(2010).
(2010).
5) M. Borrajo et al., Phys. Lett. B 746, 341 (2015).
6) B. Bally et al., Phys. Rev. Lett. 113, 162501 (2014).
7) T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016).
8) NNDC database, Nucl. Instrum. Methods Phys. Res.,Sect. A 213 (2011).

RIKEN Nishina Center
SPhN, CEA Saclay
ESNT, CEA Saclay
IPNL Lyon
Nuclear Theory Group, CEA DAM/DIF
Department of Physics, University Libre of Bruxelles
Department of Physics, University of Tokyo
Department of Theoretical Physics, University Autonomaof MadridCenter for Nuclear Study, University of Tokyo-3-