Title: Title
Author: Philippe CHOMAZ
Last saved by: blank
Company: GANIL

Ground-state two-proton radioactivity

Abstract  

Ground-state two-proton radioactivity allows for the study of nuclear structure beyond the proton drip line. In particular, the pairing force can be investigated and the sequence of single-particle levels as well as the j content of the nuclear wave function can be studied. In addition, the tunneling process with a changing deformation can be subject of investigations.

Keywords 

Two-proton radioactivity, exotic decay modes, nuclear pairing, single-particle levels

Physics case

Nuclear structure studies far from the valley of stability revealed a lot of new phenomena, which are not observed closer to the stability line. These studies allow for a deeper understanding of nuclear structure, as the concepts of its description are pushed to their limits. 
	One of the newly observed phenomena is ground-state two-proton radioactivity. This new nuclear decay mode was first predicted in 1960 to occur in very proton-rich even-Z isotopes, where the nuclear forces can no longer bind all protons. However, due to the Coulomb barrier, the protons are kept for a certain time in the nucleus to yield measurable half-lives. An additional requirement is that one-proton emission is energetically forbidden.
	The first case of two-proton radioactivity fulfilling the above mentioned requirements was ^45Fe in 2002. Meanwhile ^54Zn and most likely also ^48Ni were observed as other two-proton emitters. Up-to-now, only the decay energies and the half-lives for two-proton decay, their branching ratios as well as the absence of beta particles in the decay could be established. Additionally, the decay of the two-proton daughters has been evidenced serving as an additional proof of two-proton radioactivity. However, no direct observation of the two protons was attempted. These studies should be performed soon.
	In order to study details of the two-proton decay process to extract information about pairing, the single-particle structure in the region of the two-proton emitters and other information, higher statistics is needed and other two-proton emitters have to be studied. In addition, details about the decay process have to be investigated by measuring the individual proton energies and their relative emission angle.

Observables

	To identify new two-proton emitters, the decay energy, the half-life and the 2p emission branching ratio has to be measured. Additionally, the absence of beta radiation needs to be evidenced.
	To study the details of two-proton radioactivity, the individual proton energies and the proton-proton angle in the center of mass need to be detected.

Proposed experiment

	The known 2p emitters ^45Fe, ^54Zn, and ^48Ni need to be studied with increased statistical precision. Therefore, high-intensity primary ^58Ni beams at about 100-150 MeV/nucleon or proton-rich radioactive beams (e.g. ^54Ni and ^62Zn) are needed. Production rates of the order of a few per minute should be enough to significantly improve our knowledge about two-proton radioactivity.
	New 2p emitter candidates are ^59Ge, ^63Se, ^67Kr, ^71Sr etc. To produce them, projectile fragmentation is the best tool. Therefore, high-intensity stable beams like ^78Kr or ^92Mo should be developed. Reaccelerated proton-rich radioactive species in the vicinity of the 2p candidates can also be used.

Requirements

Beam properties

High-intensity proton-rich stable or radioactive beams at about 100-150 MeV/nucleon

Detection

	To produce the above mentioned isotopes by projectile fragmentation, a high-resolution, high-acceptance fragment separator is needed to separate the exotic species from the bulk part of less exotic nuclei. At the end of this separator, a silicon detector telescope with a highly pixilated silicon-strip detector will be used to search for new two-proton emitters. To study the details of this decay mode, a high-resolution time-projection chamber can visualize the tracks of the individual protons and therefore give access to their individual energies and their emission angle.

Theoretical support

The interpretation of the experimental data requires sophisticated theoretical models which describe coherently the nuclear structure part and the nuclear dynamics of the emission process. Most of the models available today treat only one of the two parts reasonably well. These models have to be refined and new concepts like time-dependent approaches have to be implemented. In particular, the experimental observables to study e.g. the pairing force are not very well established.