Progress of experimental studies on two-proton emission

Abstract

<p indent=0mm>Radioactivity is one of the most significant characteristics of unstable nuclei, the familiar modes including α, β, γ decay and fission. Two-proton (2p) emission is an exotic radioactivity mode of nuclei in the proton drip-line area, which is very significant for studying the structure, mass and correlation between proton pair of extremely proton-rich nuclei and one of the frontiers in radioactive ion beam physics. In this article, progress of experimental studies on two-proton emission was briefly reviewed. Several experimental measurement methods were introduced. The identification method for the mechanism of two-proton emission was discussed. Two-proton emission was predicted to be existed in extremely proton-rich nuclei in 1960. For 2p emission from the ground state of a nucleus with long half-life, it was observed for the first time in 2002 due to the difficulty in the production of a candidate. Only few decay events were observed for the four proton drip-line nuclei with a long half-life ground state. For the detection of 2p emission process, the in-flight decay, implantation decay and optical time projection chamber (OTPC) plus CCD method are the three main experimental methods. The in-flight decay method is suitable for the candidate with very short half-life and almost inapplicable for the candidate with a half-life longer than 1 μs. The implantation decay method is suitable for the candidate with long half-life, especially for β-delayed decay nuclei. In this case, the measured decay signal will be very clear with low background due to the separation recording of the implantation and decay events. The OTPC plus CCD method could record a picture of 2p emission process directly for the candidate. Generally, there are three mechanisms for proton-rich nuclei to emit two protons: (1) ²He cluster emission; (2) three-body emission; (3) sequential emission. The ²He cluster emission is basically two protons constrained by the pair correlation into a quasi-bound singlet, i.e., ¹S₀ configuration. Such a quasi-bound state can only exist for very short time and then becomes separated after penetrating through the Coulomb barrier. Usually, the two-proton relative momentum and the opening angle distributions are used to identify the mechanism of 2p emission in experimental investigations. However, this method can only distinguish it being ²He cluster emission or not. For the three-body and sequential emissions, the key difference is the emission time between the two protons. The proton-proton momentum correlation function (also called HBT) method is a widely used analysis technique in nuclear physics, which can extract the emission time of a particle. It has been demonstrated that the three-body and sequential emissions could be distinguished by using the HBT method. The difficulty in the study of exotic radioactivity is the low production rate of the unstable nuclei in the drip-line area. The high-intensity heavy-ion accelerator facility (HIAF) under construction in China has an advantage of high beam intensity, the most frontier work in the field of exotic radioactivity for nuclei around the drip-line could be performed on it by adopting the superior detection technique and investigation method.