Vol. 51 (2020)

Acta Physica Polonica B

No 3

STUDYING THE EXOTIC DECAY 70 Kr → 70 Br∗
A. Vitéz-Sveiczera,b,c,† , A. Algoraa,b,‡ , A.I. Moralesa , B. Rubioa
G.G. Kissb,d,§ , G. de Angelise , F. Recchiaf , S. Nishimurad
J. Agramunta , V. Guadillaa , A. Montaner-Pizáa , S.E.A. Orrigoa
A. Horváthc , D. Napolie , S. Lenzif , A. Bosof , V.H. Phongd , J. Wud
P.-A. Söderströmd , T. Sumikamad , H. Suzukid , H. Takedad , D.S. Ahnd
H. Babad , P. Doornenbald , N. Fukudad , N. Inabed , T. Isobed , T. Kubod
S. Kubonod , H. Sakuraid , Y. Shimizud , S. Chend , B. Blankg
P. Ascherg , M. Gerbauxg , T. Goigouxg , J. Giovinazzog , S. Grévyg
T. Kurtukián Nietog , C. Magrong , W. Gelletlya,h , Zs. Dombrádib
Y. Fujitai,j , M. Tanakai , P. Aguilerak , F. Molinak , J. Eberthl , F. Diell
D. Lubosm , C. Borcean , E. Ganiogluo , D. Nishimurap , H. Oikawap
Y. Takeip , S. Yagip , W. Kortenq , G. de Francer , P. Daviess , J. Liut
J. Leet , T. Lokotkot , I. Kojouharovu , N. Kurzu , H. Shaffnerua
IFIC (CSIC-University of Valencia), Spain
Institute for Nuclear Research (Atomki), Hungaryc
ELTE Budapest, Hungaryd
RIKEN Nishina Center, Japane
INFN Legnaro, Italyf
INFN Padova, Italyg
CEN Bordeaux-Gradignan, FrancehDepartment of Physics, University of Surrey, UKi
Department of Physics, Osaka University, Japanj
Research Center for Nuclear Physics (RCNP), Osaka University, JapankComisión Chilena de Energía Nuclear, Santiago, Chilel
Institute of Nuclear Physics, Universität zu Köln, GermanymPhysik Department, Technische Universität München, GermanynIFIN-HH, Bucarest, RomaniaoDepartment of Physics, University of Istanbul, TurkeypTokyo University of Science, Japanq
CEA Saclay, Francer
GANIL, Caen, Frances
Department of Physics, York University, UKt
Department of Physics, University of Hong Kong, Chinau
GSI, Germanyb(Received January 14, 2020)∗

Presented at the XXXVI Mazurian Lakes Conference on Physics, Piaski, Poland,
September 1–7, 2019.
†
Present address: University of Debrecen, Hungary.
‡
Corresponding author: algora@ific.uv.es§
Corresponding author: ggkiss@atomki.mta.hu

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Beta-decay of the very neutron-deficient Kr isotope, 70 Kr, was studied
at RIKEN-RIBF using the EURICA cluster array. The experiment significantly increased our knowledge of the beta-decay of this isotope. Namely,
16 new γ-ray transitions were identified and the half-life was derived fromtime correlations of the beta particles (tiβ1/2 = (44.99 ± 0.16) ms) and fromthe decay curves of the observed γ-ray transitions (tiβγ1/2 = (45.16±0.71) ms),
respectively.
DOI:10.5506/APhysPolB.51.587

1. Introduction
Type I X-ray bursts, characterized by Tpeak = 1–3 GK and ρ = 106 –7
10 g cm−3 , are suggested as possible sites for the astrophysical rp-process[1–3]. In these binary systems, the hydrogen-rich material accretes from a
low-mass star, typically from a Main Sequence or a Red-Giant star, towarda neutron star. When the temperature and density in the accreted envelope
become high enough to allow breakout from the hot CNO cycle, thermonuclear ignition takes place and via a series of rapid proton capture reactions,isotopes up to 105 Te are formed [4]. As the nucleosynthesis path approaches
the drip-line, further proton capture is inhibited by strong reverse photodisintegration reactions or by direct proton emission, and the reaction flow has
to wait for the relatively slow β-decay process. According to this picture,
the modeling of the rp-process requires precise knowledge of the β-decay
half-lives and the structure of the involved isotopes.
Presently, very little is known about the β-decay of 70 Kr: its lifetime
has been determined at CERN ISOLDE by measuring the time distributionof the emitted β particles [5]. Recently the β-decay of 70,71 Kr was investigated at RIKEN Nishina Center. In the present contribution, details on thedetermination of the half-life of the 70 Kr isotope are presented.
2. Experimental setup
The 70 Kr nuclei were produced at RIKEN Nishina Center by fragmentation of a 78 Kr primary beam, with 345 MeV/nucleon energy and ∼ 40 pnA
average intensity, impinging on a 5 mm thick 9 Be target. The nuclei ofinterest were separated and identified by measuring event-by-event the energy loss (∆E), magnetic rigidity (Bρ) and time-of-flight (TOF) of the ions,in the BigRIPS spectrometer [6]. Figure 1 shows a particle identification
matrix, which was used to correlate implantation and decay events.
The fragments were transmitted to the exit of the ZeroDegree spectrometer where a setup for decay studies was installed. It consisted of the EURICA
germanium cluster detector array [7] surrounding the WAS3ABi doublesided silicon strip detector (DSSSD) system [8]. The three DSSSD detectors

Studying the Exotic Decay 70 Kr → 70 Br

589

Fig. 1. Identification plot of ions implanted in WAS3ABi, showing the atomic
number Z versus the mass-to-charge ratio A/Q of the ions.

used in the present experiment were segmented into 60 vertical (X) and 40
horizontal (Y ) strips, corresponding to 2400 pixels with an active area of1 × 1 mm2 each. The full range and the resolution of the X and Y strips
were 5 MeV/25 keV and 10 MeV/30 keV, respectively.
EURICA is an array of twelve EUROBALL cluster germanium detectors,
each containing seven crystals. The average distance between the center ofthe DSSSDs and the front face of the HPGe detectors is 22 cm, corresponding
to a total absolute detection efficiency of about 8% at the energy of 1332 keV.
3. Event selection
The event separation criteria are discussed in detail in Ref. [9], here onlya brief summary is given. Namely, we identified events as an implantationof a 70 Kr nucleus when a high-energy signal in the plastic scintillator, located after the F11 focal plane of Zero Degree spectrometer, is detected
together with an overflow energy signal in a DSSSD, but without an energy
signal from the plastic veto detector placed behind WAS3ABi. According tothese requirements, about 1.6 × 106 70 Kr nuclei were implanted into the 2nd
DSSSD. Each implantation was followed, on average, by saturated signals of
3.4 X strips and 4.3 Y strips, respectively. The strip, where the implantation took place, was identified using the time information of the strips that
fired in an implantation event. For that, the strip with the fastest signal
was used. Then the selected implantation events in WAS3ABi were corre-

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lated in time with decay events taking place in the same strips where theimplantation event was observed. The β-delayed γ-ray energy spectra were
measured by the EURICA array and the events were selected using the following conditions: the maximum time difference between an implantationand the corresponding detection of a β particle was fixed to 20 s and γ rays
were recorded up to 800 ns after the detected β events.
4. Determination of the half-life
The half-life of 70 Kr was derived first from the time distribution of
implantation-β (i-β) correlations (without any conditions on γ rays). The
i-β histogram was fitted with a function including the exponential β-decay
and a fixed background extracted from a linear fit to the backward-time
distribution of i-β correlations. Figure 2 shows the time distribution of i-β
correlations, the lines correspond to the activity of 70 Kr (dashed/green),
70 Br (dotted/blue) and to the background (long-dashed/red), respectively.
The resulting half-life from i-β correlations is


sys
stat
t1/2 = 44.99± 0.14 ± 0.06 ms = (44.99 ± 0.16) ms .

Fig. 2. (Color online) Time distribution of implantation-β correlations. The fitting
function includes the Bateman formula and a linear background, determined froma fit to the negative-time distribution.

The background-subtracted β-delayed γ-ray energy spectrum, measured
with the EURICA array, is shown in Fig. 3. The Eγ = 933 keV transition
was already assigned to the de-excitation of the 1st excited state of 70 Br [10].
Several other γ-ray transitions are attributed to the β-decay of 70 Kr. Theβ(γ–γ) coincidence spectrum gated on the Eγ = 933 keV γ-ray is shown in
Fig. 4.

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591

Fig. 3. (Color online) Compton-suppressed γ-ray spectrum. The nine strongest
γ rays used to derive the half-life are labeled, and the corresponding energies are
given in keV. They were assigned to the β-decay of 70 Kr as stated in the text. A
spectrum tagged with β particles with negative time correlations with respect toimplantation is shown in dark gray/red. These events are interpreted as a possible
contamination since they are uncorrelated to implantations.

Fig. 4. The energy spectrum of γ-ray transitions in coincidence with the Eγ =933 keV transition. The transitions in coincidence with the Compton backgroundsampled around the Eγ = 933 keV peak were subtracted from the spectrum after
normalization in order to evaluate the effect of coincidences uncorrelated with thepeak, on which the gate was taken. The labeled peaks were all verified via gatingback on them as well, and looking for significant Eγ = 933 keV coincidences in the
spectrum.

The half-life of 70 Kr was also extracted from the decay curve of the592

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strong β-delayed γ-ray transitions observed at Eγ = 933 keV, 1120 keV,
1493 keV, 1574 keV, 1630 keV, 2230 keV, 2305 keV, 2508 keV and 2563 keV.
These transitions were first used one-by-one to build decay curves, using
implantation-β–γ (i-β–γ) time correlations. Then exponential functions
with constant background were fitted using maximum likelihood method
to obtain half-lives corresponding to the different transitions. This information was used to verify that the γ rays belong to the decay of 70 Kr. Finally,
these decay curves were summed to increase statistics. Figure 5 shows thetime distribution of the i-β–γ correlations and the fitting functions. System-

Fig. 5. (Color online) The upper curve shows the β-decay time spectra of 70 Kr
based on the measurement of i-β–γ correlations. The forward and backward time
distributions are indicated with gray/blue and light gray/yellow bars, black andwhite dashed lines show the fitted exponential and the linear background, respectively. The lower figure shows the normalized residual of the fit, which should be
fluctuating randomly with a Gaussian distribution around zero.

atic uncertainties were investigated by varying the fit parameters. Figure 6
shows the resulting 70 Kr half-life as the function of the varied fitting parameters (such as bin size, fit ranges and background). The resulting half-life
from i-β–γ correlations is


sys
stat
t1/2 = 45.16± 0.68 ± 0.20 ms = (45.16 ± 0.71) ms .

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Fig. 6. Error contributions to the half-life of 70 Kr: impact of the bin size, fit andbackground ranges on the fitted half-life values.

5. Summary
Using β–γ and β–γ–γ correlations 16 new γ-ray transitions, assigned tothe β-decay of 70 Kr were identified, and the half-life was derived from thetime correlations of the β particles (tiβ1/2 = (44.99 ± 0.16) ms) and the decay
curves of the observed γ-ray transitions (tiβγ1/2 = (45.16 ± 0.71) ms).
We want to express our gratitude to the RIKEN accelerator staff forproviding a stable and high-intensity 78 Kr beam during the experiment, the
EUROBALL Owners Committee for the loan of germanium detectors, and
the PRESPEC Collaboration for the readout electronics of the cluster detectors. Part of the WAS3ABi was supported by the Rare Isotope Science
Project, which is funded by the Ministry of Science, ICT and Future Planning (MSIP) and National Research Foundation (NRF) of Korea. Support
from the Spanish Ministerio de Economía, Industria y Competitividad under
grants No. IJCI-2014-19172, No. FPA2011-24553, No. FPA2014-52823-C21-P and No. FPA2017-83946-C2-1-P, Centro de Excelencia Severo Ochoa del
IFIC SEV-2014-0398, and Junta para la Ampliación de Estudios Programme(CSIC JAE-Doc contract) co-financed by FSE is acknowledged. This work
was also financed by Programmi di Ricerca Scientifica di Rilevante Interesse Nazionale (PRIN) No. 2001024324 01302, by JSPS KAKENHI grant

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No. 25247045, 2604808, by the Chilean FONDECYT projects INICIACION
No. 11130049 and REGULAR No. 1171467, by the Istanbul University Scientific Project Unit under Project No. BYP-53195, by the Max-Planck Partnergroup, by the NSF grant No. PHY-1404442, by the UK Science and Technology Facilities Council (STFC) grant No. ST/F012012/1, by MEXT Japan
under grant No. 15K05104, and by the French–Japanese “Laboratoire international associé” FJ-NSP. This work was supported by NKFIH (NN128072),
by the New National Excellence Program of the Ministry for Innovation andTechnology (ÚNKP-19-4-DE-65). G.G. Kiss acknowledges support form the
János Bolyai research fellowship of the Hungarian Academy of Sciences.
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