Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Proton-proton correlations in distinguishing the two-proton emission
mechanism of 23 Al and 22 Mg†
D. Q. Fang,∗1 Y. G. Ma,∗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,∗2∗5 Y. Ito,∗5N. Iwasa,∗2∗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∗2Two-proton emission is a very interesting but complicated process occurring in nuclei close to the proton
drip line. However, it is diﬃcult to distinguish between the two-body sequential and three-body simultaneous emission mechanism in two-proton emission processes.1) In these two mechanisms, the emission time of
the two protons is quite diﬀerent. For the three-bodysimultaneous emission, the two protons are emitted almost simultaneously, while the two protons are emitted
one by one in sequential emission. In heavy-ion collisions, two-particle interferometry is a well-recognizedand powerful method to extract the source size andparticle emission time and to probe and disentangle
diﬀerent reaction mechanisms.
An experiment was performed to study the protonproton momentum correlation function for the threebody decay channels 23 Al → p + p + 21 Na and 22 Mg →p + p + 20 Ne using the RIPS beamline at the RI Beam
Factory (RIBF) operated by RIKEN Nishina Centerand Center for Nuclear Study, the University of Tokyo.
The proton-proton momentum correlation function(Cpp ) was obtained by the event-mixing method withan iterative calculation. The source size and proton
emission time information was extracted by comparing the CRAB calculation with the experimental Cppdata.2) In the calculation, the space and time proﬁle of the source was assumed to be a Gaussian asS(r, t) ∼ exp(−r2 /2r02 − t/τ ), with r0 referring to thesource size and τ referring to the emission-time difference between the protons. As shown in Fig. 1, thesource sizes were approximately 2 ∼ 3 fm for both23
Al and 22 Mg based on the minimun reduced chisquare. However, the parameter τ is quite diﬀerent
for these two nuclei. For the reaction channel 23 Al →p + p + 21 Na, as shown in Fig. 1(a), τ is very large(> 600 fm/c), while for the reaction channel 22 Mg →p + p + 20 Ne, as shown in Fig. 1(b), τ is very small
†∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8Condensed from the article in Phys. Rev. C 94, 044621(2016)
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 Tokyo

Fig. 1. Contour plot of the reduced chi-square (χ2 /ν, whereν is the number of degrees of freedom) obtained fromﬁtting the experimental Cpp data by the CRAB calculation for the reaction channel of 23 Al → p + p + 21 Na(a) and 22 Mg → p + p + 20 Ne (b).

(< 50 fm/c). This implies that the emission-time difference between two protons for 23 Al and 22 Mg wasquite diﬀerent. For 23 Al, the two protons were emitted
at very diﬀerent times, i.e., the mechanism is sequential emission. For 22 Mg, the two protons were emitted
almost simultaneously, i.e., the mechanism was essentially simultaneous. Based on the previous results1)
and this work, it is possible to distinguish clearly themechanism of two-proton emission by investigating on
the proton-proton momentum correlation function, thetwo-proton relative momentum, and opening-angle distributions. The method presented in this work was applied for the ﬁrst time to two-proton emitters, and wasshown to provide new and valuable information on themechanism of two-proton emission.
References
1) Y. G. Ma et al., Phys. Lett. B 743, 306 (2015).
2) S. Pratt et al., Nucl. Phys. A 566, 103c (1994).

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