Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 53

Strong one-neutron emission from two-neutron unbound states inβ decays of r -process nuclei 86, 87 Ga†

Delayed neutron emission after β decays is found inneutron-rich nuclei in which the energy window of β −decay (Qβ ) is high enough to populate excited statesof the daughter nucleus above the neutron separationenergy (Sn ). All of the neutron-rich nuclei on the rprocess path are either one or multi-n β-delayed precursors. Delayed neutron emission shapes the ﬁnalabundance pattern owing to the modiﬁcation of theisotopic population through the modiﬁcation of the decay path back to stability and by contributing significantly to the neutron ﬂux after freeze-out. However,experimental data that enable the evaluation of therole of multi-n emission for the r-process nuclei arealmost non-existent. Often, predictions for neutron
emission probabilities are based on a simpliﬁed cut-oﬀ
model that neglects γ-decay competition and assumes
that only the higher-multiplicity neutron emission prevails in the energy regions open to multiple neutronemission channels.
We studied neutron-rich Ga isotopes by means ofβ-neutron-γ spectroscopy at RIBF using the in-ﬂight
ﬁssion of a 345 MeV/nucleon 238 U86+ beam on a 4mm-thick 9 Be production target. For decay measurement, 7 × 104 and 6 × 103 ions of 86 Ga and87
Ga, respectively, were transported to the ﬁnal focal
plane. Double-sided silicon-strip detectors (DSSSDs),
AIDA,1) WAS3ABi,2) and a YSO scintillator3) wereemployed for ion and β correlation measurements. The3He neutron counter array, BRIKEN,4) was used forneutron measurement.
The half-lives and Pxn values were obtained throughthe analysis described in Ref. 6). The P1n and P2nvalues obtained in this work for 86 Ga are consistent
with the data reported by Miernik et al.7) We discovered new β-delayed two-neutron emitters, 84, 85, 87 Ga,
and measured their two-neutron branching ratio forthe ﬁrst time. Figure 1 shows a comparison of the experimental neutron branching ratio with shell-model
calculations based on the cut-oﬀ model reported byMadurga et al.5) and shell-model calculations based onthe Hauser-Feshbach statistical model.8) When comparing the new experimental results with the predictions from the shell-model calculations, we notice adiscrepancy between the cut-oﬀ model and experimental data for all investigated β2n gallium precursors,
†∗1
∗2
∗3
∗4
∗5Neutron branching ratio (%)

R. Yokoyama,∗1 R. Grzywacz,∗1,∗2 B. C. Rasco,∗1,∗2 N. T. Brewer,∗1,∗2 K. P. Rykaczewski,∗2 I. Dillmann,∗3J. L. Tain,∗4 and S. Nishimura∗5 for the BRIKEN collaboration
100P1n GT (cutoff)
P1n GT+FF (cutoff)
P1n GT+FF (stat.)
P1n EXP50

P2n GT (cutoff)
P2n GT+FF (cutoff)
P2n GT+FF (stat.)
P2n EXP

05152

53

54

55
56
Neutron number NFig. 1. Comparison of the experimental data and the predictions from the shell-model calculations by Madurga
et al.5) “GT” and “GT+FF” in the legend denote shellmodel calculations with pure Gamow-Teller and GT +ﬁrst forbidden transitions, respectively. The GT+FF
predictions, when coupled with the statistical model(stat.), provide a much better agreement with the data
than the cutoﬀ model.

which is most dramatically manifested in 87 Ga.
In contrast to the cut-oﬀ model, the inclusion of
the statistical model correctly reproduces the dominating role of one-neutron emission from two-neutron
unbound states. This result is the ﬁrst clear case inmedium/heavy nuclei where the eﬀects of statisticalemission must be considered to model β-delayed multineutron emission, which is of particular importance for
r-process modeling. The strong 1n emission from 2n
unbound states will require more detailed studies ofneutron and γ-ray spectra to establish the details of
the emission process. The detection of neutron energies and γ-ray energies is required to better constrain
both the strength-distribution models and statisticalmodel parameters.
References

1) C. J. Griﬃn et al., PoS NIC XIII, 097 (2014).
2) S. Nishimura et al., RIKEN Accel. Prog. Rep. 46, 182(2013).
3) R. Yokoyama et al., Nucl. Instrum. Methods Phys. Res.
A 937, 93–97 (2019).
4) A. Tarifeño-Saldivia et al., J. Instrum. 12, P04006(2017).
5) M. Madurga et al., Phys. Rev. Lett. 117, 092502 (2016).
6) B. C. Rasco et al., Nucl. Instrum. Methods Phys. Res.
A 911, 79 (2018).
7) K. Miernik et al., Phys. Rev. Lett. 111, 132502 (2013).
8) T. Kawano et al., Nucl. Phys. A 913, 51 (2013).

Condensed from article Phys. Rev. C 100, 031302(R)University of Tennessee, Knoxville
Oak Ridge National Laboratory
TRIUMF
IFIC (CSIC, Universitat de Valencia)
RIKEN Nishina Center-1-