Spectroscopy of the Proton Rich Nuclei by the use of Secondary Beams and Direct Observation of Giant M1 Resonance

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We have previously succeeded in commissioning the recoil separator CARP at the Research Center for Nuclear Physics, Osaka University. This device allows us to mass-separate nuclear reaction products and transport them into the detecting position in the form of a secondary beam. Considering the advantage of the use of the separator, the objective of the present investigation was set at the accurate measurement of the Gamow-Teller (GT) beta-transition rates of the proton rich T_z = -3/2 and -2 nuclei in the sd-shell. This was to promote deeper understanding of the quenching mechanism of the giant M1 resonance, by facilitating the detailed comparison of the GT-strength between the theory and experiment for a wide range of nuclei far off the beta-stability line.In the early stage of the investigation, an emphasis was placed on the achievement of a large solid angle of the recoil separator for the reaction products and on the construction of a detector system which is highly sensitive to the beta-rays and associated radiations with the decay of the products but insensitive to the external background radiations. Following these developments of the apparatus, a gain of a factor of no smaller than 20 over the previous experimental set-up was found achieved in the overall collection efficiency of the data.Taking account of the relatively large production cross sections of the T_z = -3/2 nuclei, the first experiment was determined to be carried out on ^<13>O, ^<21>Mg, ^<25>Si, ^<21>S, ^<37>Ca and ^<41>Ti. We have successfully completed the measurements for all these T_z = -3/2 nuclei by the time of preparing this report. The analysis of the data has been completed for ^<25>Si and the quenching factor of the GT-strength was determined. An unexpectedly large concentration of the GT-strength was found in the isobaric analog state of ^<25>Si in ^<25>Al. For the other nuclei, the data analysis is under way. The experiments for the T_z = -2 nuclei are about to start.