Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 50 (2017)

Two-Proton Radioactivity of 67 Kr†β decay is the predominant decay mode in protonrich nuclei close to stability, but further away from stability valley the binding energy of excess protons decreases and β-delayed proton emission becomes more
likely. When the one or two-proton separation energies
Sp and S2p become negative, the dripline is reached andone- or two-proton emission from the ground state forodd- and even-Z elements, respectively, competes withβ decay.
Two-proton (2p) radioactivity is a unique tool tostudy nuclear structure beyond the proton dripline.
Predicted in 1960,1) this direct emission of two protons
was discovered in 2002 in the decay of 45 Fe.2,3) The
other known medium-mass cases 48 Ni 4) and 54 Zn 5)
were discovered in the same decade.
According to mass predictions, the heavier nuclei
59
Ge, 63 Se and 67 Kr are candidates for 2p emission.
They were successfully produced and identiﬁed during
the 78 Kr beam campaign in 2015 6) at RIBF. 63 Se and67
Kr were observed for the ﬁrst time and 59 Ge for thesecond.
The nuclei of interest 7) were implanted in WAS3ABi,a set of three DSSSDs to measure the energy of β particles and protons. The vertical and horizontal strips
allowed ion-decay position correlations, greatly reducing the background in the energy spectra. WAS3ABi
was surrounded by the EURICA γ-ray array.8)
No 2p evidence was found for 59 Ge and 63 Se. Fig-(a)59Ge

10Counts / ms

Counts / 40 keV

T. Goigoux,∗1 P. Ascher,∗1 B. Blank,∗1 M. Gerbaux,∗1 J. Giovinazzo,∗1 S. Grévy,∗1 T. Kurtukian Nieto,∗1C. Magron,∗1 J. Agramunt,∗2 A. Algora,∗2,∗3 V. Guadilla,∗2 A. Montaner-Piza,∗2 A. I. Morales,∗2 S. E. A. Orrigo,∗2B. Rubio,∗2 D. S. Ahn,∗4 P. Doornenbal,∗4 N. Fukuda,∗4 N. Inabe,∗4 G. G. Kiss,∗4 T. Kubo,∗4 S. Kubono,∗4S. Nishimura,∗4 H. Sakurai,∗4,∗5 Y. Shimizu,∗4 C. Sidong,∗4 P.-A. Söderström,∗4 T. Sumikama,∗4 H. Suzuki,∗4H. Takeda,∗4 P. Vi,∗4 J. Wu,∗4 Y. Fujita,∗6,∗7 M. Tanaka,∗6 W. Gelletly,∗2,∗8 P. Aguilera,∗9 F. Molina,∗9 F. Diel,∗10D. Lubos,∗11 G. de Angelis,∗12 D. Napoli,∗12 C. Borcea,∗13 A. Boso,∗14 R. B. Cakirli,∗15 E. Ganioglu,∗15J. Chiba,∗16 D. Nishimura,∗16 H. Oikawa,∗16 Y. Takei,∗16 S. Yagi,∗16 K. Wimmer,∗3 G. de France,∗17 S. Go,∗18 and
B. A. Brown∗19520(b)T1/2 = 13.3(17) ms

15
10502000

4000

6000

8000

020406080(c)63Se

6428(d)T1/2 = 13.2(39) ms

64202000

4000

6000

8000

020406080(e)67Kr

643(f)

T1/2 = 7.4(30) msCounts / ms

823T1/2 = 6.5(33) ms

2
105

100

2000

4000

6000

8000

Energy (keV)015202530Time (ms)12100Time (ms)Counts / ms

Energy (keV)Counts / 40 keV

100Time (ms)Counts / ms

Counts / 40 keVEnergy (keV)

2040

60

80

100

Time (ms)Fig. 1. Charged-particle energy spectra (left) and decaytime distributions (right) for events above 1 MeV to reject β decays without protons, for 59 Ge, 63 Se and 67 Kr.
For the charged-particle spectra: in blue all decay events
correlated within a 100 ms time window, and in red incoincidence with β particles detected in neighbouring
detectors. The inset in (f) is obtained from the 1690keV peak events of spectrum (e).7)

ure 1(a) and (c) do not show any peak without coincident β detection. However, the 67 Kr spectrum (e)shows a clear peak at 1690(17) keV originating from
2p radioactivity without any coincident β particle or511-keV γ ray. A 2p branching ratio of 37(14)% and ahalf-life of 7.4(30) ms were found, leading to a 2p partial
half-life of 20(11)ms, in strong disagreement with thethree-body half-lives 9) for diﬀerent 2 conﬁgurations.

†Condensed from the article in Phys. Rev. Lett. 117, 162501(2016)∗1 CEN Bordeaux Gradignan∗2 IFIC, CSIC-Universidad de Valencia∗3 Atomki, Debrecen∗4 RIKEN Nishina Center∗5 Department of Physics, University of Tokyo∗6 Department of Physics, Osaka University∗7 Research Center for Nuclear Physics, Osaka University∗8 Department of Physics, University of Surrey∗9 Comisı́on Chilena de Energia Nuclear∗10 Institute of Nuclear Physics, University of Cologne∗11 Physik Department E12, Technische Universität München∗12 Laboratori Nazionali di Legnaro dell’ INFN
∗13 IFIN-HH∗14 INFN Sezione di Padova and Dipartimento di Fisica∗15 Department of Physics, Istanbul University∗16 Department of Physics, Tokyo University of Science∗17 Grand Accélérateur National d’Ions Lourds∗18 Dept. of Physics and Astronomy, University of Tennessee∗19 Department of Physics and Astronomy, and NSCL, MSU

References
1) V.I. Goldansky, Nucl. Phys. 19, 482 (1960).
2) J. Giovinazzo et al., Phys.Rev. Lett. 89, 102501 (2002).
(2002).
3) M. Pfützner et al., Eur. Phys. J. A 14, 279 (2002)
4) C. Dossat et al., Phys. Rev. C 72, 054315 (2005).
5) B. Blank et al., Phys. Rev. Lett. 94, 232501 (2005).
6) B. Blank et al., Phys. Rev. C 93, 061301(R) (2016).
7) T. Goigoux et al., Phys. Rev. Lett. 117, 162501 (2016).
8) P.-A. Söderström et al., Nucl. Instrum. Methods Phys.
Res. Sect. B 317, 649 (2013).
9) L. V. Grigorenko et al., Phys. Rev. C 68, 054005 (2003).

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