II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Measuring β-decay strength distribution in the 78 Ni region using
VANDLE
M. Singh,∗1 R. Yokoyama,∗1 R. K. Grzywacz,∗1,∗2 T. King,∗2 S. Nishimura,∗3 N. T. Brewer,∗1,∗2 P. Brionnet,∗3
J. Bundgaard,∗1 I. Cox,∗1 A. Fijalkowska,∗4 L. Fraile,∗5 S. Go,∗6 A. Gottardo,∗7 M. Karny,∗4 A. Keeler,∗1A. Korgul,∗4 M. Madurga,∗1 K. Miernik,∗4 S. Neupane,∗1 M. Niikura,∗8 M. Pfutzner,∗4 M. Piersa,∗4M. Rajabali,∗9 B. C. Rasco,∗2 K. P. Rykaczewski,∗2 M. Silkowski,∗4 M. Stepaniuk,∗4 J. L. Tain,∗10 A. Tolosa,∗10M. Wolinska-Cichocka,∗4 and Z. Xu∗1 for the VANDLE Collaboration
The properties of nuclei away from the line of stability, revealed in the β-decay of the neutron-rich side,
are crucial in understanding nuclear structure evolution
and providing inputs for r-process simulations. Measurements of half-lives, one- and two-neutron emission
probabilities (Pn,2n ), and neutron energy spectra provide information on the β-decay strength distribution(Sβ ). The Sβ measurement for Ga with N > 50, 83, 84 Ga
showed that the decay properties of the r-process isotopes near 78 Ni are dominated by the Gamow-Teller
decay of the 78 Ni-core states.1) Through their work on
86, 87Ga using BRIKEN,3) R. Yokoyama et al.,2) demonstrated a need to consider the competition between oneand multi-neutron emissions to predict branching ratiosof r-process nuclei. It also strengthened the argument
for the necessity of neutron energy measurements forunderstanding the details of the neutron emission process. The Versatile Array of Neutron Detector at Low
Energy (VANDLE)4) experiment at RIBF RIKEN aims
to provide measurements of Sβ for the decay of 78 Ni
and neighboring nuclei using time-of-flight (ToF) based
neutron spectroscopy. The isotopes of interest were produced from a 345 MeV/nucleon ∼46-particle-nA 238 U
beam impinged on 4-mm-thick Be target by projectile

Fig. 2. Neutron energy spectrum of 81 Cu measured by VANDLE.

fragmentation. The nuclei identified by the BigRIPS5)
facility were supplied to the F11 focal plane, where theywere implanted in a segmented-YSO based implantation detector6) for ∼4 days. A YSO detector consistsof a segmented YSO crystal (75 × 75 × 5 mm3 ) coupled
to a position-sensitive photo-multiplier tube. YSO isused to establish ion-beta correlations and provides thestart time of the ToF. VANDLE, consisting of EJ200
scintillator bars coupled at both ends to PMTs, provides the stop time of neutron ToF. A set of 48 medium(3 × 6 × 120 cm3 ) VANDLE bars were arranged in a 100cm radius circle with YSO at the center, as shown inFig. 1. In addition, two HPGe clovers and ( ten 3” × 3”
and two 2” × 2”) LaBr3 were set up in a close geometryaround the YSO detector to record γ-transitions fromthe decays. All the signals were read using XIA Pixie16 revF digitizers at 250 MHz and 12-bit digitization.7)
Neutron spectra were measured for 78–81 Cu isotopes toestablish the role of the N = 50 shell gap on the βdecay properties. We show the first measurement of
the neutron energy spectrum of 81 Cu52 decay in Fig. 2.
The spectrum indicates that neutrons with energies of0.4–3 MeV were emitted from excited states in 81 Zn.
ReferencesFig. 1. VANDLE setup at the F11 focal plane of ZDS.
∗1Dept. of Physics and Astronomy, University of Tennessee
Physics Division, Oak Ridge National Laboratory∗3 RIKEN Nishina Center∗4 Faculty of Physics, University of Warsaw∗5 Grupo de Física Nuclear IPARCOS, Universidad Complutensede Madrid∗6 Department of Physics, Kyushu University∗7 INFN-LNL∗8 Department of Physics, University of Tokyo∗9 Department of Physics, Tennessee Tech University∗10 IFIC, CSIC-Universitat de Valencia∗21) M. Madurga et al., Phys. Rev. Lett. 117, 092502 (2016).
2) R. Yokoyama et al., Phys. Rev. C 100, 031302 (2019).
3) A. Tarifeño-Saldivia et al., J. Instrum. 12, P04006 (2017).
4) W. A. Peters et al., Nucl. Instrum. Methods Phys. Res. A
836, 122 (2016).
5) N. Fukuda et al., Nucl. Instrum. Methods Phys. Res. B
317, 323 (2003).
6) R. Yokoyama et al., Nucl. Instrum. Methods Phys. Res.
A 937, 93 (2019).
7) S. V. Paulauskas et al., Nucl. Instrum. Methods Phys.
Res. A 737, 22 (2014).

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