Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Lifetime measurements of excited states in 102, 104 Zr with a
LaBr3 (Ce) array
F. Browne,∗1,∗3 A. M. Bruce,∗1 T. Sumikama,∗2 S. Nishimura,∗3 P. Doornenbal,∗3 G. Lorusso,∗3 Z. Patel,∗3,∗4S. Rice,∗3,∗4 L. Sinclair,∗3,∗5 P.-A. Söderström,∗3 H. Watanabe,∗3,∗6 J. Wu,∗3,∗7 Z. Y. Xu,∗8 H. Baba,∗3N. Chiga,∗2 R. Carroll,∗4 R. Daido,∗10 F. Didierjean,∗11 Y. Fang,∗10 N. Fukuda,∗3 G. Gey,∗12,∗19,∗3
E. Ideguchi,∗10 N. Inabe,∗3 T. Isobe,∗3 D. Kameda,∗3 I. Kojouharov,∗13 N. Kurz,∗13 T. Kubo,∗3 S. Lalkovski,∗14Z. Li,∗7 R. Lozeva,∗11 I. Nishizuka,∗2 H. Nishibata,∗10 A. Odahara,∗10 Zs. Podolyák,∗4 P. H. Regan,∗4
O. J. Roberts,∗1 H. Sakurai,∗3 H. Schaffner,∗13 G. S. Simpson,∗12 H. Suzuki,∗3 H. Takeda,∗3 M. Tanaka∗10J. Taprogge,∗15,∗16,∗3 V. Werner,∗17 O. Wieland,∗18 and A. Yagi∗10Developments of novel scintillator materials haveoffered a step-change in performance characteristics
of scintillation detectors for γ ray measurements.
In particular, cerium-doped lanthanum tri-bromide(LaBr3 (Ce)) has proven to be a promising candidate
for measuring lifetimes of low-lying excited nuclear
states in the ps-to-ns range. Such information is a
powerful tool in extracting, for example, nuclear deformations.
An array of 18 LaBr3 (Ce) detectors was installed at
the F11 focal plane of the BigRIPS spectrometer, augmenting the existing EURICA array1)2) . In order toexamine the performance of the LaBr3 (Ce) array, the102, 104
Zr were meaknown lifetimes of the 2+1 states insured by means of β-γ spectroscopy. The parent nuclei
were produced by the in-flight fission of a 345 MeV/A238
U beam on a 555 mg/cm3 thick 9 Be target. The fission fragments were transported through BigRIPS andthe ZeroDegree spectrometer before being implanted
into the WAS3ABi active stopper (5 highly segmentedDSSSDs), which lies between two plastic scintillators(β-plastics). To correlate a β-decay event with an implanted ion, a signal in the same DSSSD pixel to theimplant was required. A time condition was placed onthe ion implantation to β-decay time to reduce contamination from granddaughter decays.
The level lifetime was obtained by measuring thetime difference between the β-plastic, and a signal inCounts per 400 ps

Counts per 400 ps

the LaBr3 (Ce) array. A systematic uncertainty of 10%
was added to the measured 2+
1 lifetimes to account for
the lifetimes of higher-lying levels. This was estimated+from the time difference spectra for the 4+
1 → 21 transitions. Figure 1 shows preliminary results of the background subtracted time difference spectra gated on the+2+
1 → 0g.s. transitions, the energies of which are givenin Tab. 1 along with the mean lifetime of the levels,
which are in good agreement with literature values3) .
The energy systematics indicate increased collectivity as N increases, however, the dependence of thetransition probability on Eγ results in a longer lifetime104
Zr than for 102 Zr. Future workfor the 2+
1 state inwill concentrate on a more complete characterisation of
the low-energy background, the prompt-response function and the contribution of systematic uncertainties.
The lifetimes of the 2+
1 states of more exotic Zr isotopes
will also be measured.
a)102Zr10b)104Zr

101011110-110-2

10-1-2 -50
0∗1CEM, University of Brighton
Department of Physics, Tohuku University∗3 RIKEN Nishina Center∗4 Department of Physics, University of Surrey∗5 Department of Physics, University of York
∗6 Beihang University∗7 Department of Physics, Peking University∗8 Department of Physics, University of Tokyo∗10 RCNP, Osaka University∗11 IPHC/CNRS and University of Strasbourg∗12 LPSC, UJF-INPG-IN2P3, Grenoble∗13 GSI Helmholtzzentrum für Schwerionenforschung GmbH
∗14 Department of Physics, Sofia University∗15 Departamento de Fı́sica Teórica, Universidad Autónoma deMadrid∗16 Instituto de Estructura de la Materia∗17 Institut fuer Kernphysik, Technische Universitaet Darmstadt∗18 INFN Sezione di Milano∗19 NPP, ILL, Grenoble

2468-45

10 12-4014
Time (ns)-2-35 024 -306

810 -25
12 14Time (ns)Fig. 1.: Preliminary, background subtracted time difference spectra for, a) 102 Zr and b) 104 Zr. The extracted mean lifetimes of the 2+1 states are listed below.

∗2Table 1.: Comparison between τ values derived in thiswork and adopted values3) .
Nuclide
102
Zr
104
Zr

E(2+1 ) [keV]
151.8(1)139.3(3)τ [ns]
2.7(3)
3.2(3)

ENSDF τ [ns]2.6(6)
2.9(4)

References
1) P.-A. Söderström et al., Nucl. Instr. and Meth. B317,
649 (2013).
2) Z. Patel et al.: In this report.
3) Evaluated Nuclear Structure Data File, http://www.nndc.
bnl.gov/ensdf-5-