Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 52 (2019)

Magnetic-moment measurement of the isomeric state of 130 Sn
in the vicinity of doubly-magic nucleus 132 Sn
G. Georgiev,∗1,∗2,∗3 Y. Ichikawa,∗1 D. S. Ahn,∗1 T. Asakawa,∗4 K. Asahi,∗1 H. Baba,∗1 K. Doi,∗4 N. Fukuda,∗1A. Gladkov,∗1,∗5 T. Gray,∗3 G. Häfner,∗2,∗6 K. Kawata,∗1,∗7 A. Kusoglu,∗8 W. Kobayashi,∗4 G. J. Lane,∗1,∗3
R. L. Lozeva,∗2 C. Mihai,∗9 R. Mihai,∗9 Y. Nakamura,∗10 M. Niikura,∗11 H. Nishibata,∗1 A. Odahara,∗12S. Pascu,∗9 M. Sanjo,∗4 Y. Sasaki,∗4 T. Sato,∗1 Y. Shimizu,∗1 C. Sotty,∗9 A. Stuchbery,∗3 T. Sumikama,∗1H. Suzuki,∗1 M. Tajima,∗1 A. Takamine,∗1 Y. Takeuchi,∗4 H. Takeda,∗1 A. Toda,∗5 A. Turturica,∗9 H. Ueno,∗1and H. Yamazaki∗1in thickness and 30 × 30 mm2 in area. The dipole magnet provided a static magnetic ﬁeld of B0 = 0.150 T.
130m
Sn was implanted into the Cu stopper and γ rays
were detected with four Ge detectors located in a plane
perpendicular to B0 at a distance of 7.0 cm from thestopper and at every 90 degrees. Two LaBr3 detectors were also placed at 90 degrees with respect toeach other so as not to interfere with the Ge detectors.
The plastic scintillator with a thickness of 0.1 mm wasplaced upstream of the stopper. Its signal provided thetime-zero trigger. The TDPAD apparatus enabled us
to determine the g-factor of 130m Sn by observing thechanges in anisotropy of the de-excitation γ rays emitted from spin-aligned 130m Sn in synchronization withthe spin precession in the presence of an external ﬁeld.
In this experiment, we observed two cascade γ raysfrom 130 Sn with energies of 97 keV and 391 keV. TheR(t) ratio representing the change in the anisotropy ofγ rays was obtained according toThe 130 Sn nucleus is located in the vicinity of thedoubly-magic nucleus 132 Sn and known to have an isomeric state at an energy level of 2435 keV with a halflife of 1.61 µs.1–3) Its spin parity has been tentatively
assigned to be 10+ where a conﬁguration of two neutron holes coupled in parallel in the h11/2 orbital isexpected to be predominated. The purity of the conﬁguration can be a good indicator of the magicity near
132Sn. Therefore, we measured the magnetic momentof the isomeric state of 130 Sn.
The experiment was conducted at the BigRIPS at
the RIBF. The two-step fragmentation scheme4) was
employed to produce spin-aligned 130 Sn beam. In thereaction at F0, 132 Sn was produced by a ﬁssion reaction of a 345-MeV/nucleon 238 U beam on a 9 Be target
with a thickness of 6 mm. A wedge-shaped aluminium
degrader with a mean thickness of 6 mm was placed atthe ﬁrst momentum-dispersive focal plane F1, where
the momentum acceptance at F1 was ±1.4%. The secondary 132 Sn beam was introduced to a second targetof wedge-shaped aluminum with a mean thickness of2 mm, placed at the momentum-dispersive focal plane
F5. The 130 Sn nuclei including those in the isomericstate 130m Sn, were produced by removing two neutrons
from 132 Sn. The 130 Sn beam was subsequently transported to F7 under the condition that the momentum
dispersion between F5 and F7 was matched to thatbetween F3 and F5. The slit width at F7 was set to±9 mm. The intensity and purity of 130 Sn in the tertiary beam were 30 cps and 30%, respectively.
The 130 Sn beam was introduced to the experimental
apparatus for time-diﬀerential perturbed angular distribution (TDPAD) measurement, which was placed at
the focal plane F12. The TDPAD apparatus consistedof a dipole magnet, a Cu crystal stopper, Ge detectors,
and a plastic scintillator. The Cu stopper was 3.0 mm

R(t) =

N13 (t) − ϵN24 (t),N13 (t) + ϵN24 (t)(1)

where N13 (t) and N24 (t) are the aggregates of thephoto-peak count rates in the two pairs of Ge detectors
placed diagonally to each other, and ϵ denotes a correction factor for the diﬀerence in detection eﬃciency.
The same applies to the LaBr3 detectors. Theoretically, R(t) is expressed as a function of t as,
R(t) =

3A22cos 2(ωL t + α),
4 + A22(2)in terms of the rank-two anisotropy parameter A22 =AB2 F2 , where A denotes the degree of spin alignment,
B2 is the statistical tensor for complete alignment, andF2 is the radiation parameter. The Larmor frequency
ωL is given by ωL = gµN B0 /h̄, where g is the g-factorof 130m Sn in the unit of nuclear magneton µN . α is theinitial phase of the precession depending on the detector arrangement. The detailed analysis and deductionof the g-factor are in progress.

∗1

RIKEN Nishina Center
CSNSM, CNRS/IN2P3, Université Paris-Saclay∗3 Department of Physics, Australian National University∗4 Department of Advanced Sciences, Hosei University∗5 Department of Physics, Kyungpook National University∗6 Department of Physics, University of Cologne∗7 Center for Nuclear Study, University of Tokyo∗8 Department of Physics, Istanbul University∗9 NIPNE, IFIN-HH∗10 Department of Physics, Meiji University∗11 Department of Physics, University of Tokyo∗12 Department of Physics, Osaka University∗2References
1) R. L. Lozeva et al., Phys. Rev. C 77, 064313 (2008).
2) D. Kameda et al., Phys. Rev. C 86, 054319 (2012).
3) S. Pietri et al., Phys. Rev. C 83, 044328 (2011).
4) Y. Ichikawa et al., Nat. Phys. 8, 918 (2012).

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