Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Different mechanism of two-proton emission from excited states ofproton-rich nuclei 23Al and 22Mg†
Y. G. Ma,∗1 D. Q. Fang,∗1 X. Sun,∗1 P. Zhou,∗1 Y. Togano,∗2 N. Aoi,∗2 H. Baba,∗2 X. Z. Cai,∗1 X. G. Cao,∗1J. G. Chen,∗1 Y. Fu,∗1 W. Guo,∗1 Y. Hara,∗3 T. Honda,∗3 Z. Hu,∗4 K. Ieki,∗3 Y. Ishibashi,∗5 Y. Ito,∗5N. Iwasa,∗6 S. Kanno,∗2 T. Kawabata,∗7 H. Kimura,∗8 Y. Kondo,∗2 K. Kurita,∗3 M. Kurokawa,∗2T. Moriguchi,∗5 H. Murakami,∗2 H. Ooishi,∗5 K. Okada,∗3 S. Ota,∗7 A. Ozawa,∗5 H. Sakurai,∗2 S. Shimoura,∗7R. Shioda,∗3 E. Takeshita,∗2 S. Takeuchi,∗2 W. Tian,∗1 H. Wang,∗1 J. Wang,∗4 M. Wang,∗4 K. Yamada,∗2Y. Yamada,∗3 Y. Yasuda,∗5 K. Yoneda,∗2 G. Q. Zhang,∗1 and T. Motobayashi∗2For proton-rich nuclei, the proton decay mechanism
is complicated, especially for two-proton (2p) radioactivity1) . The proton-rich nucleus 23 Al has attracted
a lot of attention in recent years since it may play a
crucial role in understanding the depletion of the NeNa
cycle in ONe novae2) . The measurement of its reaction
cross section and fragment momentum distribution has
shown that the valence proton in 23 Al is dominated by
the d wave but with an enlarged core3) . The spin andparity of the 23 Al ground state was found to be J π =5/2+4) . Also of great interest is 22 Mg because of its
importance in determining the astrophysical reaction
rates for 21 Na(p,γ)22 Mg and 18 Ne(α,p)21 Na reactionsin the explosive stellar scenarios.
An experiment was performed to study the twoproton decay channels of 23 Al and 22 Mg using theRIPS beamline at the RI Beam Factory (RIBF) operated by RIKEN Nishina Center and Center for Nuclear
Study, University of Tokyo.
In this study, we examined the relative momentum
spectrum (qpp ) and opening angle (θpp ) of the two protons in the three-body decay system for 23 Al and 22 Mg.
Without any cut in the excitation energy, a broad qpp
spectrum and structure-less θpp distribution are observed. These results indicate that the main mechanism of two proton emission from 23 Al and 22 Mg aresequential or three-body emission with very weak correlation between the two protons. However, since thedecay mode for different excited states or excitationenergies is different, it is interesting to check qpp andθpp spectrum at different excitation energies. For thediproton or 2 He emission, a clear peak should appear
at small relative momentum as well as opening angle. Fig. 1 show the experimental results of the above
two distributions for 23 Al in the excitation energy window 10.5 < E ∗ < 15 MeV. Evident peaks at qpp ∼ 20
MeV/c (a) and small opening angle (b) are absent. Instead, the qpp spectrum is broad and the θpp distribu†
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Condensed from the article in Phys. Lett. B 743, 306 (2015)
Shanghai Institute of Applied Physics, CAS
RIKEN Nishina Center
Department of Physics, Rikkyo University
Institute of Modern Physics, CAS
Institute of Physics, University of Tsukuba
Department of Physics, Tohoku University
Center for Nuclear Study (CNS), University of Tokyo
Department of Physics, University of TokyoFig. 1. Relative momentum (a) and opening angle (b) distributions between two protons by the decay of 23 Al
into two protons plus 21 Na for 10.5 < E ∗ < 15 MeV;(c) and (d) are the results by the decay of 22 Mg into
two protons plus 20 Ne for 12.5 < E ∗ < 18 MeV.

tion is structure-less. Similar analysis has been done indifferent E∗ windows other than 10.5 < E ∗ < 15 MeVand similar behaviors for qpp and θpp are observed.
For 22 Mg, Fig. 1 shows the qpp (c) and θpp (d) datain the excitation energy window 12.5 < E ∗ < 18 MeV.
The peaks of the qpp distribution at 20 MeV/c andof the corresponding small θpp are clearly observed.
These features are consistent with the diproton emission mechanism. However, no significant enhancements for qpp ∼ 20 MeV/c and small θpp are observedfor other E ∗ windows.
In order to quantitatively understand the qpp and θpp
spectra, a Monte Carlo simulation has been performed.
A mixture of diproton and simultaneous three-bodydecay or sequential decay was used to fit the qpp andθpp data. As shown in the figure, the sequential decay
is overwhelmingly dominant for 23 Al. For 22 Mg, on
the other hand, the diproton or 2 He emission peaks
are well reproduced by the simulation. The fraction ofthe diproton emission is determined to be around 30%.
References
1) V. I. Goldansky, Nucl. Phys. 19 (1960) 482.
2) V. E. Iacob et al., Phys. Rev. C 74 (2006) 045810.
3) D. Q. Fang et al., Phys. Rev. C 76 (2007) 031601(R).
4) A. Ozawa et al., Phys. Rev. C 74 (2006) 021301(R).

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