Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Measurement of the neutron-decay lifetime of the 26 O ground state at
the SAMURAI setup at RIBF
C. Caesar,∗1,∗2 J. Kahlbow,∗3,∗2 V. Panin,∗2 D. S. Ahn,∗2 L. Atar,∗3,∗2 T. Aumann,∗3 H. Baba,∗2K. Boretzky,∗1,∗2 H. Chae,∗4 N. Chiga,∗2 S. Choi,∗4 M. L. Cortes Sua,∗2 D. Cortina-Gil,∗5 Q. Deshayes,∗6P. Doornenbal,∗2 Z. Elekes,∗2,∗7 N. Fukuda,∗2 I. Gašparić,∗8,∗2 K. I. Hahn,∗9 Z. Halász,∗7 A. Hirayama,∗11J. Hwang,∗4 N. Inabe,∗2 T. Isobe,∗2 S. Kim,∗4 T. Kobayashi,∗10,∗2 D. Körper,∗1 Y. Kondo,∗11,∗2 Y. Kubota,∗2I. Kuti,∗7 C. Lehr,∗3,∗2 S. Lindberg,∗12,∗2 M. Marques,∗6 M. Matsumoto,∗11 T. Murakami,∗13,∗2 I. Murray,∗2T. Nakamura,∗11,∗2 T. Nilsson,∗12 H. Otsu,∗2 S. Paschalis,∗14 M. Parlog,∗6 M. Petri,∗14 D. Rossi,∗3 A. Saito,∗11M. Sasano,∗2 H. Scheit,∗3 P. Schrock,∗15 Y. Shimizu,∗2 H. Simon,∗1 D. Sohler,∗7 S. Storck,∗3,∗2 L. Stuhl,∗15H. Suzuki,∗2 I. Syndikus,∗3 H. Takeda,∗2 H. Törnqvist,∗3,∗2 T. Togano,∗11,∗2 T. Tomai,∗11 T. Uesaka,∗2H. Yamada,∗11 Z. Yang,∗2 M. Yasuda,∗11 K. I. Yoneda,∗2 for the SAMURAI20 Collaboration
In December 2016, the NP1306-SAMURAI20 experiment was conducted. A new technique to measure
lifetimes of possible neutron-radioactive nuclei1) wasapplied for the ﬁrst time to study 26 O. The technique
is based on the production of the neutron-unboundnucleus of interest in a target with large Z and highdensity that slows down the produced nucleus and theresidual nucleus after (multi-) neutron emission. The
spectrum of the velocity diﬀerence between neutron(s)and the residual nucleus has a characteristic shape thatallows to extract the lifetime.
2)
TheThe experimentexperimentwas
wascarried
carriedout
outatatthe
theSAMURAISAMURAI2)
setup
withananinvariant-massinvariant-mass configuration
conﬁguration together
setup
withwith an experiment speciﬁc target region. Secondary
beams of 27 F and 26 F were produced in BigRIPS by
projectile fragmentation of a 48 Ca primary beam at 345
MeV/nucleon on a 20 mm thick beryllium production
target. The combination of the incoming energy of thesecondary beam and the reaction-target thickness is
crucial for the sensitivity to a speciﬁc lifetime region.
For this reason, the beam energy was reduced withdegraders. At the ﬁrst (second) momentum dispersive
focal plane F1 (F5) of BigRIPS, a 15 mm (10 mm)
thick aluminum wedge was installed. In addition, a24O beam with a very narrow energy spread has beenused for calibration purposes to emulate the fragmentsof interest in the analysis.
At the SAMURAI setup, incoming beam particles
were identiﬁed by two 1 mm thick plastic scintillators∗1GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt∗2 RIKEN Nishina Center∗3 Institut für Kernphysik, TU Darmstadt∗4 Department of Physics, Seoul National University∗5 Departamento de Fìsica de Partìculas, Universidade de Santiago de Compostela∗6 LPC, Caen
∗7 ATOMKI, Debrecen∗8 Ruđer Bošković Institute, Zagreb∗9 Department of Physics, Ewha Womens University∗10 Department of Physics, Tohoku University∗11 Department of Physics, Tokyo Institute of Technology∗12 Department of Physics, Chalmers University of Technology∗13 Department of Physics, Kyoto University∗14 Department of Physics, University of York
∗15 Center for Nuclear Studies (CNS), University of Tokyo(SBTs), two drift chambers (BDC1 and BDC2), andone ionization chamber (ICB). In addition, one siliconpin diode was mounted in front of the target stack andtwo behind it for energy loss measurements.
Unbound 26 O was produced by proton removal fromthe 27 F beam in a target stack consisting of 6 foils separated by 0.8 mm, with decreasing thickness in the beam
direction. The foils of thickness 2.04 mm, 1.59 mm,1.35 mm, and 1.06 mm were made of tungsten. Thetwo foils of lower thickness were platinum foils of thickness 0.77 mm and 0.61 mm. The total area density ofthe target amounted to 14.6 g/cm2 . The heavy target material causes a broad neutron-fragment velocitydiﬀerence distribution for prompt decays, due to thelarge energy loss of the fragments. In contrast, a sharp
peak will be observed for out-of target decays, whichallows to distinguish between these two types of events.
The decay products 24 O and the two neutrons wereseparated by the SAMURAI dipole magnet. Multiwire drift chambers (FDC1 and FDC2) in front of themagnet and behind it and the plastic scintillator hodoscope (HODF24) were used for the detection andtracking of charged fragments. For neutron detection,
the NeuLAND demonstrator in conjunction with NEBULA was used. The energy loss measurements in thesilicon pin diodes can be used to select events with
Z=9 (8) directly in front (behind) the target to exclude possible background contributions. The remaining ‘background’ was estimated in a ’null’ measurement. For this purpose, the one-neutron decay of 25 O
produced by proton removal from 26 F as a reference
was measured. The ground state resonance of 25 O
has a width of around 88 keV3) corresponding to an
extremely short lifetime. The analysis of the SAMURAI20 data is currently in progress.
References
1) J. Kahlbow et al., Nucl. Instrum. Methods,(DOI:10.1016/j.nima.2017.06.002).
2) T. Kobayashi et al., Nucl. Instrum. Methods Phys. Res.,
Sect. B 317, 294 (2013).
3) Y. Kondo et al., Phys. Rev. Lett. 116, 102503 (2016).

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