What is 2-protons radioactivity ?
When the number of protons and neutrons of an atomic nucleus is too largely unbalanced, the strong nuclear interaction, which is the main component of nuclear binding, cannot keep the exceeding nucleons (protons or neutrons). For nuclei with a large proton excess, the last proton(s) may nevertheless be kept inside the nucleus for a short time, due to the Coulomb barrier (potential barrier created by the charges of all the protons of the nucleus). That’s why theoretical descriptions of the atomic nucleus predicted, in the 1960’s, that 1-proton (for odd proton number isotopes) and 2-proton (for even proton number isotopes) radioactivities should exist. The last proton(s) are kept inside the nucleus by this Coulomb barrier, before being ejected by the tunnel effet throught it.
In the case of 2-proton radioactivity, the pairing effect plays an additional role. The pairing forces inside the nucleus give an extra stability for isotopes with even numbers of protons and/or neutrons, compared to neighbour isotopes. Thus, for 2-proton radioactivity, one proton emission is not possible, and the nucleus has to emit both exceeding protons simultaneously. The best candidates for this new decay mode were predicted in the Titanium (Z=20) to Zinc (Z=30) region, at the proton drip-line, and those predictions have been confirmed by more recent theoretical models.
On the experimental side, 1-proton radioactivity has been evidenced in the early 1980’s at GSI (Darmstadt), while 2-proton radioactivity has been observed only in the early 2000’s at GANIL (Caen) and GSI.
Discovery of 2-proton radioactivity
The group performed several experiments, mainly at GANIL with the LISE spectrometer, in order to produce and to study the nuclei in the mass region where good candidates were expected by predictions.
Those experiments first resulted in the discovery of doubly-magic 48Ni nucleus. In addition to be a candidate for the 2-proton radioactivity, this nucleus is the most neutron deficient isotope of the nuclear chart, and it is of special interest for nuclear shell structure due to its proton and neutron closed shells (see experiment E312a).
We could observe for the first time the 2-proton radioactivity in the decay of 45Fe (see experiment E312b), from an indirect analysis. The nuclei are implanted in a silicon detector, that is also used to measure the total energy of charged particles emitted during the decay process (positrons for beta decay and protons for 2-proton decay). The conclusion that the decay could only proceed via 2-proton radioactivity is based on global observables from the decay : half-life and transition energy (from the emitted protons), absence of coincident positrons in the decay, half-life of the daughter nucleus that only matched the half-life of 43Cr (which results from the removal of 2 protons from 45Fe),...
In the following experiments (E312c, E312d, E312e) we could confirm this observation for 45Fe, observe the 2-proton radioactivity of 54Zn with the same type of indirect analysis, and to get a first possible 2-proton event in the decay of 48Ni.
First direct observation of 2-proton radioactivity
In order to go further in the understanding of this new decay mode (comparing experimental results with theoretical descriptions), it is necessary to observe individually both emitted protons and to measure the energy sharing between the two particles and their angular correlation.
Indeed, since the sub-system made of the 2 protons is not bound outside of the nucleus (the 2He nucleus does not exist), the 2 protons escape the nucleus as "separated" particles. The correlations observed outside the nucleus may lead to information about the correlations inside, and therefore about the proton pairing and the decay process.
That was the purpose when the group started the construction of a new type of detector: a time projection chamber (TPC) to measure the tracks of individual protons. With this detector, we could make the first direct observation of the new decay mode, in the 2-proton radioactivity of 45Fe (exp. E457a, 2005) and of 54Zn (exp. E457b, 2008).
This experimental program now follows 2 main directions:
the search for new candidates like59Ge, 63Se or 67Kr, most likely with experiments at RIKEN (Japan) ;
the development of a new generation TPC.