II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Constraining multi-neutron emission models with spectroscopy ofneutron-rich Ga isotopes using BRIKEN array
R. Yokoyama,∗1 R. Grzywacz,∗1,∗2 B. C. Rasco,∗1,∗2 N. T. Brewer,∗1,∗2 K. P. Rykaczewski,∗2 I. Dillmann,∗3
J. L. Tain,∗4 S. Nishimura,∗5 and the BRIKEN collaboration

∗1
∗2
∗3
∗4
∗5

daughters calculated by the shell model were replacedwith experimental data. Compared with the feedingof the experimental 624-keV state in 84 Ge, the statistical model predicted the γ intensity as 33%, whichis in good agreement with the experimental value wemeasured. The statistical model also reproduced the γbranching ratio of other γ rays in the decay. The detailed β-n-γ analysis of other neutron-rich Ga isotopes
is in progress.
(a) 100

400n505107

365

151n1n 0 - 80 ms

473

101n 200 - 280 ms

598

250

51032 1063

773

2n 0 - 80 ms

624 (84Ge 2+→0+)
Iγ_stat. = 33%

20086Ga

Excitation energy (keV)

2n 200 - 280 msIγ_exp = 37(8)%2n00(b)0n 200 - 280 ms

1024

20015
100n 0 - 80 ms

400

600

800

1000

1200

1400

Gamma-ray energy (keV)

0β-β-nβ-2nP0n=24(3)%P1n=61(3)%P2n=14.7(14)%

1000
500
0(2+)
0+1024.3
527.3
086Ge

85Ge

881.4
472.9
249.9
107.9
02+0+623.9 [5.5(12)]50Ga decay

773.5 [2.8(8)]472.9 [3.2(7)]365.0 [3.9(7)]249.9 [7.6(9)]107.9 [13.3(12)]0100

865271024.3 [2.8(9)]527.3 [7.3(9)]Counts per keV

As we move further from the line of β-stability towards the neutron drip line, Qβ values become larger
than the two-neutron separation energy (S2n ), whichallows multi-neutron emissions after β-decay. In theneutron-rich nuclei along the astrophysical r-process
path, multi-neutron emissions are considered to be
dominant decay modes.1) The numbers of neutrons
emitted in the β-decays of exotic nuclei modify thedecay path back to stability and aﬀect the ﬁnal abundance calculation. However, experimental data thatenable the evaluation of multi-neutron emissions forthe r-process nuclei are almost non-existent. Until
quite recently, theoretical predictions of the neutronemission branching ratio (Pn ) were based on a cut-oﬀ
model that assumes only the higher-multiplicity neutron emission prevails in the energy regions open to
multiple neutron-emission channels. In order to include the competition of one-neutron emission from thestates above S2n , Kawano et al. developed a HauserFeshbach statistical model calculation.2)
We studied neutron-rich Ga isotopes by means of
β-n-γ spectroscopy at RIBF using the high-eﬃciency
neutron counter array BRIKEN,3–5) with two Ge
clover detectors. The result that one-neutron emission
is dominant for all the four Ga isotopes was interpreted
as competition in one-neutron emission among twoneutron unbound states, and we demonstrated thatthe inclusion of the statistical model reproduces thebranching ratio better than the cut-oﬀ model.6)
Recently, we analyzed γ-ray spectra of the decay of
84–87
Ga. Figure 1 shows γ spectra gated by the neutron multiplicity of the BRIKEN array for the decay of86Ga. γ rays from 84, 85, 86 Ge are clearly identiﬁed in
the 2n, 1n, and 0n gated spectra. The 2+ → 0+ γ rayfrom 84 Ge in the two-neutron branches was observed
at 624 keV. The intensity of the 624-keV γ ray per 100
two-neutron decays was 37(8)%. We performed ourHauser-Feshbach statistical model calculation by using
the strength distribution and level densities from shellmodel calculations. This model was used to validate
the measurement of the neutron spectra in the decay of83, 84Ga.7) The shell model produced less states below
S2n than the number of the default levels generated by
the statistical model using the Gilbert-Cameron formula. When known, the low-lying states in the decay

623.9084GeFig. 1. (a) γ-ray spectra of the decay of 86 Ga gated byneutron multiplicities in the BRIKEN array. (b) Decayscheme. γ branching per 100 decays is shown in thesquare brackets.

References

1) P. Möller et al., At. Data Nucl. Data Tables 125, 1 (2019).
2) T. Kawano et al., Nucl. Phys. A 913, 51 (2013).
3) A. Tarifeño-Saldivia et al., J. Instrum. 12, P04006(2017).
4) A. Tolosa-Delgado et al., Nucl. Instrum. Methods
Phys. Res. A 925, 133 (2019).
5) B. C. Rasco et al., Nucl. Instrum. Methods Phys. Res.
A 911, 79 (2018).
6) R. Yokoyama et al., Phys. Rev. C 100, 031302(R) (2019).
7) M. Madurga et al., Phys. Rev. Lett. 117, 092502 (2016).

Department of Physics and Astronomy, the University ofTennessee, Knoxville
Physics Division, Oak Ridge National Laboratory
TRIUMF
IFIC, CSIC-Universitat de Valencia
RIKEN Nishina Center- 22 -1