E U R I C A!

~ Secrets Revealed via Decay Spectroscopy of Exotic Nuclei ~EURICA: A state-of-the-art decay spectrometer~ Exploring exotic nuclei with high-efficiency andhigh-resolution equipment ~The RIBF in Japan, one of the world’ s most powerfulradioactive beam facilities, has been providing intense
beams of exotic nuclei since 2007. Following the successof the first decay spectroscopy experiments in 2009, which
focused on the study of neutron-rich nuclei with masses ofaround A = 100-117, a world-class decay project called
EURICA (Euroball-RIKEN Cluster Array)† was launchedowing to the efforts of 190 international collaborators. This

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state-of-the-art spectrometer, which consists of twelve
Euroball HPGe cluster detectors (formerly GSI RISING) andthe highly-segmented RIKEN β-ray detector WAS3ABi,
markedly improves the performance of high-resolution
decay spectroscopy of exotic nuclei with fast radioactive
beams. EURICA has been in operation since 2012, and has
harvested the decay properties of several hundred exotic
nuclei. The lifetimes and level schemes of isotopes
produced in BigRIPS can be deduced by measuring thetimes of flight, trajectories, and energies of the isotopes,
and their subsequent β and γ rays emitted in the β-γ decay.

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β-decay half-lives and nuclear astrophysics~ How and where were the heavy elements produced in the universe? ~The β-decay half-lives of isotopes withproduction rates as low as several events per
day were measured using the EURICA
spectrometer. In the fall of 2012, thehalf-lives of neutron-rich nuclei in thevicinity of 78Ni (Z=28, N=50) wereinvestigated using WAS3ABi, a device thatconsists of 8 layers of double-sided
silicon-strip detectors (shown in the image
on the right). The results are summarized inFigure 1. The systematics of the half-lives
indicate a sudden decrease beyond N=50
(79,80Ni) and below Z=28 (77Co), thus
81
providingevidence
for a doubly
ves of 76,77Co, 79,80
Ni androbustCu:experimental
magic 78Ni. 78 †ndication of doubly magic

Ni

G. Lorusso,∗2 F. Browne,∗3,∗2 P. Doornenbal,∗2 G. Gey,∗4,∗5,∗2 H.-S. Jung,∗6.-A. Söderström,∗2 T. Sumikama,∗8 J. Taprogge,∗9,∗10,∗2 Zs. Vajta,∗11,∗2Yagi,∗13 K. Yoshinaga,∗14 H. Baba,∗2 S. Franchoo,∗15 T. Isobe,∗2 P. R. John,∗16o,∗2 N. Kurz,∗17 I. Matea,∗15 K. Matsui,∗1 D. Mengoni,∗16 P. Morfouace,∗15
H. Nishibata,∗13 A. Odahara,∗13 E. Şahin,∗20 H. Sakurai,∗1,∗2 H. Schaffner,∗17. G. Stefan,∗15 D. Suzuki,∗15 R. Taniuchi,∗1 and V. Werner∗19. Rev. Lett. 113, 032505
of Tokyo

CNRS/IN2P3

of Notre Dameniversity
niversityeria, CSIC

103

Half-life (ms)shell evolution aroundutron-rich nuclei, i.e.,measured for the first
med as part of an EUility, RIKEN in 2012.
accelerated up to an
KEN cyclotron accel3-mm-thick beryllium
ms via in-flight fission.
at an average current
t position. During thet 1.2 × 104 78 Ni nuclei
he experimental decay
gree spectrometer.
tal results (solid symature (open symbols)mber. Due to the fifth
life and its Qβ value,g10 T1/2 and the neuleus is expected phees smoothly in an isoarity is clearly visible
a sudden reduction isin due to the shorterce to the systematicscesses in 79,80 Ni couldside the N = 50 shell,and β-decay rates of.

Figure 1: Experimental half-lives as afunction of neutron number for the
Z=27–31 isotopic chains. The filled
and open symbols represent half-lives
measured by EURICA and otherexperimental setups, respectively.

10231Ga30Zn

29Cu

1028Ni

27Co

444648
Neutron number5052

The decay properties of exotic nuclei are also key to uncovering some of the long-standing mysteries ofFig. 1. Experimental half-lives as a function of neutron
the astrophysical rapid-neutron capture process (r-process), namely, the main astronomical site(s) and its
number for isotopes with Z = 27 − 31. All the solidmechanism.
The r-process
is a determinedsequence inofthis
neutron-captureand β-decay processes that is known to besymbols represent
the half-lives
work
responsible
for thesynthesis
approximately
while the open
symbols
are theof
half-lives
taken fromhalf
the of the elements heavier than iron. Nuclear physics
1–4)
neutron-capture
rates, β-decay half-lives, and β-delayed neutron
inputs,for example,
masses(Qinliterature. Thenuclear
systematictrendsdifferentβ ),theisotopicprobabilities
chains are highlighted
lines connecting
theemissionof verybyneutron-richnuclei,are expected to play a significant role in thedata points witha smaller
uncertainty.
understandingof themechanismand site(s) of the r-process. Figure 2 summarizes the nuclei surveyed
with EURICA at the RIBF. A first attempt to study the universality of the astrophysical r-process wasperformed by measuring the β-decay half-lives of 110 neutron-rich nuclei around mass A=100-140,In addition, a large gap can be noticed in Fig. 1
where
the first
between40thehalf-lives
half-lives wereof the measured
Co and Ni forisotopes
from time. As shown in Figure 3, the reaction-network
calculations
ofsupernova
nucleosynthesis
withthe new half-lives indicate that the (n,γ)↔(γ,n) equilibrium
N = 44 to N = 50. According to shell model calcuislations,still valid,
andbe
reproduces
universalityof thethis canexplained theby thefilled proton
f7/2elements with Z > 56.
single particle orbit (SPO) in Ni isotopes. In this case,the proton produced in the β decay of Ni isotopes fills
the πf5/2 SPO above πf7/2 , leading to a reduction of
the Qβ value and longer half-lives of Ni isotopes thanthose of Co isotopes. The newly measured half-lives of76,77
Co follow the decreasing trend with considerable
gaps relative to those of the corresponding Ni isotones,indicating an almost constant Z = 28 shell gap without significant quenching up to N = 50.

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Figure 2: Summary of the nuclei surveyed with EURICA, where known half-lives are presented incolor. The open circles indicate the data collected prior to the Kr beam campaign in 2015. The results
are presented in S. Nishimura et al. PRL 106, 052502 (2011), Z.Y. Xu et al. PRL 113, 032505 (2014),and G. Lorusso et al. PRL 114, 192501 (2015).

st1 Peak

Theoretical Prediction
with new half-lives (RIBF)

nd2 Peak130Xe

rd3 Peak195

r-process element

Pt

Rare earth
232

Th
238

80

100

120

140

160

180

Mass A

200

220

U240

Figure 3: Abundances of r-process elements compared with estimations of reaction-networkcalculations of supernova nucleosynthesis.

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Magicity and deformation~ The magic numbers 28, 50, and 82, and regions far from the valley of stability ~Understanding nuclear shell structure and its evolution toward the drip-lines is one of the major
topics in nuclear structure research as well as nuclear astrophysics. One of the main goals ofEURICA is to study the shell gaps around the doubly magic nuclei 78Ni (Z=28, N=50), 132Sn (Z=50,N=82), and 100Sn (Z=50, N=50), where a possible weakening of the magicity and shell quenchingeffects have been discussed. Figure 4 provides a compilation of the energies of the first excited statesin even-even nuclei obtained via γ rays from isomers and β-delayed γ rays from daughter nuclei. Inaddition, detailed level schemes were deduced for 116,118Ru, 129In, and 131In via the β-delayed γ raysof 116,118Tc, 129Cd, and 131,132Cd, respectively, and long-lived isomers in 126Pd and 131Cd wereidentified via internal conversion. Some of the EURICA results from 2012 are related to low-lying
states in neutron-rich nuclei that contribute to the formation of the r-process peak around mass
A~130. Further systematic studies on shell evolution of both neutron- and proton-rich nuclei havebeen conducted using β-γ spectroscopy with EURICA at the RIBF and will be reported in the future.

P.-A. Söderström et al.
PRC C 88,, 024301 (2013)

136

Sn

H. Watanabe et al.
PLB 704, 270 (2011)

G. Simpson et al.
PRL 113, 132502 (2014)

T. Sumikama et al.
PRL106, 202501 (2011)
H. Watanabe et al.
PRL111, 152501 (2013)
PRL 113, 042502 (2014)

126

Pd128

Pd

136

Sn138

SnFigure 4: Overview of first excited state energies in even-even nuclei obtained via γ rays from isomers
and β-delayed γ rays from daughter nuclei.

†EURICA was supported by the EUROBALL Owners Committee for the loan of the germanium
detectors, and the PreSpec collaboration for the use of the readout electronics. WAS3ABi was partly
funded by the Priority Centers Research Program in Korea (2009-0093817). Ten international Ph.D.
students participating in the EURICA collaboration were supported by RIKEN IPA (InternationalProgram Associates).

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