Fine Structure of Isobaric Analog State Studied by (p, np) Reaction

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Studies of the decay property for giant resonances following charge-exchange reactions limited to a few cases, especially for the (p, n) reaction, no reliable data has been reported due to experimental difficulties encountered with measuring neutrons and decay protons with a sufficient efficiency, and with reasonable resolution. In this report, we describe the newly equipped coincidence measurement system with the (p, np) reaction for further investigation of microscopic decay properties of highly excited states such as isobaric analog state.The experiment was performed using a 35 MeV momentum-analyzed proton beams obtained from the AVF cyclotron at CYRIC,Tohoku University. Decay protons were detected in coincidence with neutrons using a SSD-telescope of Si surface-barrier type mounted at the length of 8 cm from the target, and at rhetap=135 degree relative to the beam direction. The SSD-telescope consisted of DELTAE (250mum thick), E (1000mum thick) and veto counters with a 100 mm_2 active surface area. The veto detector was used to reject energetic protons scattered at backward angles. The solid angle for the proton detection was 15.6 msr.Direct measurement of the proton decay from the IAS in <@D1208@>D1Bi was performed by means of the neutron-proton coincidence experiment through the (p, n) reaction at 35 MeV.The branching ratio for the proton decay of the IAS has been determined to be (57(]SY.+-。[)21)%, which is in consistent with the previous results within the statistical error. Further measurements have been carried out for the (p, np) reactions on <@D1140@>D1Ce and <@D1174@>D1Yb. The branching ratio for the proton decay of the IAS has been determined to be (23(]SY.+-。[)3)% for the former case, while it is (35(]SY.+-。[)8)% for the latter. These data are expected to provide significant information for the isospin mixing in nuclei.