Mirror β-transitions should have similar strengths as long as binding energy, Coulomb energy and other explicitly breaking effects are small. However, large β-decay asymmetries have been observed in the decay of 9 C [1] and the decay of its mirror nucleus 9 Li [2]. These asymmetries could be the indication of structural differences between mirror states. We have focussed on the beta-decays of 9 C and 9 Li to the high lying region (between 11-12 MeV) in the daughter nuclei 9 B and 9 Be. The difficulty in the study of these decays is that they are accompanied by the emission of three particles and one needs to determine the breakup mechanism to extract information about the position, width and feeding of the states. The β-decays of 9 C [3] and 9 Li [4] to the high lying region of the daughter nuclei have been remeasured with setups that allow the study of the complete kinematics of the processes and, hence, the breakup mechanism of the states in the daughter nuclei can be determined. The most recent experiments on 9 C [3,5] gave B GT values of 1.58( 16) and 1.20 (15) for the transition 9 C → 9 B (12.2 MeV) whereas the only early determination for 9 Li→ 9 Be (11.81 MeV) [2] gave 5.6 (1.2). The experiments on 9 C employed coincidence detection of charged particles thereby allowing a detailed analysis, whereas the older 9 Li experiment only included singles spectra. The only previous 9 Li experiment where αparticle coincidences were measured [6] did not have sufficient angular resolution and no detailed analysis of the breakup mechanism was performed.
We have remeasured the β-decay of 9 Li with a setup similar to the one used for 9 C [3] in two different occasions. The experiments were carried out at the ISOLDE facility at CERN. The 9 Li beam has been stopped in a C-foil placed in the middle point of the geometry. The activity was viewed by two Double Sided Silicon Strip Detectors (DSSSD) in a back-to-back geometry. In the first experiment the energy of the beam was 60 keV. The beam was completely stopped in a C-foil of 100 µg/cm 2 thickness. Two standard DSSSDs of thickness 300 µm and 64 µm were used. The deadlayer of these detectors was equivalent to 630 nm of Si which constitutes a serious problem to detect the low energy α-particles. The combination of the trigger thresholds and the energy losses in the Nuclear Physics A 746 (2004) 518c-521c