Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

Investigations of magnetic moments andspin alignment in Coulomb fission reaction

As one of the most accurate probes of the nuclear wave-functions, the magnetic moments of excitedstates in nuclei are both important and interesting.
Their measurement has several diﬃculties and particularities. One of the most crucial ones is the spin alignment of the nuclear ensemble which is produced in thereaction populating the nucleus of interest. In such an
experiment we have investigated the Coulomb ﬁssion
reaction, which was not previously studied, aiming toprobe the nuclear alignment trough the measurementof magnetic moments.
Nuclei around 132 Sn form a key region of the nuclear
chart from astrophysics and nuclear structure pointof view. In particular, below 132 Sn, several isomeric
structures emerge as for example in the 124, 125 Ag isotopes, which have interesting behaviour of the three
proton holes below Z = 50 closed-shell coupled to evenor odd amount of neutron holes from predominantly
the last three νh11/2 , d3/2 and s1/2 neutron orbitals
below N = 82.1) Measuring nuclear moments of such
excited isomeric states allows to ﬁnd what type of conﬁgurations are formed in these nuclei, and at the sametime, investigate unknown properties of the ﬁssion reaction mechanism.
The experiment was carried out in December 2019 at
the RIBF using the BigRIPS spectrometer. Primary238
U beam with an energy of 345 MeV/nucleon and an
intensity of the order of 100 pnA was used to induce
Coulomb ﬁssion onto a thin 184 W production target.
This very thin foil of only 0.1 mm was used for the ﬁrst
time in our experiment and performed well at the usedvery high ﬂux. The isotopes of interest produced in
the reaction were separated, transported and identiﬁedusing the standard methods for BigRIPS and using Al
wedges at the F1 and F5 focal planes of 7 mm and1 mm, respectively. Our detection setup was placedat the F8 focal plane where the beam was stopped ina sandwich of 2 mm thick plastic and 3 mm thick Cu
host.
The momentum distribution for ions produced in theﬁssion reaction is generally wide and it is only partiallyaccepted (e.g. ±3%) by the BigRIPS spectrometer.2)∗1∗2∗3∗4∗5∗6∗7Z

R. Lozeva,∗1 G. Häfner,∗1,∗2 M. Si,∗1 Y. Ichikawa,∗3 H. Ueno,∗3 D. S. Ahn,∗3 K. Asahi,∗3 T. Asakawa,∗3,∗4H. Baba,∗3 A. Esmaylzadeh,∗2 N. Fukuda,∗3 A. Gladkov,∗3 K. Imamura,∗3 N. Inabe,∗3 K. Kawata,∗3,∗5L. Knaﬂa,∗2 A. Kusoglu,∗6 M. Niikura,∗7 Y. Sasaki,∗3,∗4 H. Sato,∗3 Y. Shimizu,∗3 H. Suzuki,∗3 M. Tajima,∗3A. Takamine,∗3 H. Takeda,∗3 Y. Takeuchi,∗3,∗4 Y. Yanagisawa,∗3 H. Yamazaki,∗3 and K. Yoshida∗34910348
124Ag47102

125Ag

46

10

2.6

2.62

2.64

2.66

2.68

A/Q

Fig. 1. Particle-identification for the central setting.

In addition, in order to select certain parts of this momentum we have performed F1 slit cuts by accepting
only the high or only the low part of momentum distribution for the ions of interest.
Typical particle-identiﬁcation spectrum from thisexperiment for the central selection is shown in Fig. 1
for about 10% of the collected data. Similar statistics was obtained also for the wing setting. For themeasurement of the nuclear moments we have employed the well-known TDPAD method, applied successfully at RIBF.3,4) We used four HpGe detectors at90◦ with respect to each other and two LaBr3 detectorsof 1.5 × 1.5 in. They were all placed in a horizontal
plane, while the magnetic ﬁeld used to induce the precession of the nuclear spins was provided by a compact
electromagnet and applied in a vertical direction.
Data analysis for the oscillation pattern and magnetic moment for the nuclei of interest are currently inprogress.
References

1) S. Lalkovski et al., Phys. Rev. C 87, 034308 (2013).
2) T. Kubo et al., Nucl. Instrum. Methods Phys. Res. B
204, 97 (2003).
3) Y. Ichikawa et al., Nat. Phys. 15, 321 (2019).
4) F. Boulay et al., Phys. Rev. Lett. 124, 112501 (2020).

Université Paris-Saclay, CNRS/IN2P3, IJCLabIKP, University of Cologne
RIKEN Nishina Center
Department of Advanced Sciences, Hosei University
Center for Nuclear Study, University of Tokyo
Department of Physics, Istanbul University
Department of Physics, University of Tokyo- 46 -