$\beta$ - decay asymmetry in mirror nuclei: A = 9

Investigations of light nuclei close to the drip lines have revealed new and intriguing features of the nuclear structure. The occurrence of halo structures in loosely bound systems has had a great impact on the nuclear physics research in the last years. As intriguing but not yet solved is the nature of transitions with very large $\beta$ - strength. \\ \\We report here on the investigation of this latter feature by an accurate measurement of the $\beta$ - decay asymmetry between the mirror nuclei in the A=9 mass chain.\\ \\The possible asymmetry for the decay to the states around 12 MeV is interesting not only due to the fact that the individual B$_{GT}$ values are large (with large overlap in wave-functions, an unambiguous interpretation is much easier made), but also due to the special role played by this transition for the $^{9}$Li decay. It seems to belong to a class of high-B$_{GT}$ transitions observed at the neutron drip line and has been suggested to be due either to a lowering of the giant Gamow-Teller resonance or to the occurrence of "two-neutron $\rightarrow$ deuteron" transitions. Knowing whether the mirror transition on the proton rich side has a similar strength would help greatly in identifying what causes the large transition strengths. This type of "superallowed" transition has been observed in other light nuclei as $^{6}$He, $^{8}$He and $^{11}$Li but their mirror partners $^{6}$Be, $^{8}$C and $^{11}$O respectively are particle unbound. This makes the decay $^{9}$C $\stackrel{\beta}{\to}$ $^{9}$B* the unique case to study the preservation of superallowed GT-transition in mirror nuclei.\\ \\The $^{9}$C ions, produced by proton bombardment of a MgO-target at the ISOLDE facility at CERN, were stopped in a thin C-foil and the $^{9}$B* decay products were registered by two Double Sided Si Strip Detectors (DSSSD) (5 x 5 cm$^{2}$, 16 + 16 strips), triggered by a annular Si-detector measuring $\beta$-particles from the decay of $^{9}$C. To detect the high energy protons the two DSSSD (500 and 300 $\mu$m thick) detectors were complemented by a 700 $\mu$m thick 2000 mm$^{2}$ Si-detector and a 1000 $\mu$m thick 5 x 5 cm$^{2}$ Si-PAD detector respectively (see figure). The energies and angles of all fragments are thus measured in order to obtain the proton + $\alpha$ + $\alpha$ - correlations. The sum of the three measured energies will directly give us the excitation energy in $^{9}$B.