High resolution γ spectroscopy of hypernuclei with large Ge detector system

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The purpose of this research project was to construct a Ge-ball detector and to measure g-transitions of hypernuclei with keV resolution and to open the new field of hypernuclear physics, namely γ-spectroscopy of hypernuclei. After the R&D of the Ge detector and its fast electronics by using π-beams, we have constructed the Ge-ball detector called "Hyperball" with collaboration of Chinese physicists. It consists of 14 pure 60% Ge crystals with BGO Compton suppressors. We have carried out the first experiment at KEK with π-beam and observed γ transitions of hypernuclei with keV resolution for the first time. The "Hyperball" is now wellknown by its great success.The M1 transition between the ground spin-doublet states (3/2 -> 1/2) of ィイD17ィエD1LiィイD2ΛィエD2 was measured to be 690 keV. This splitting is mainly due to the ΛN spin-spin interaction. The experiment, therefore, provided a key data to determine the spin-spin interaction. By using Doppler attenuation method, we measured B(E2) of the transition (5/2 -> l/2) of ィイD17ィエD1LiィイD2ΛィエD2 for the first time. The shrinkage in the nuclear size by Λ particle was first confirmed by this data. This success claims the great ability of γ-spectroscopy of hypernuclei. The γ-transitions of ィイD19ィエD1BeィイD2ΛィエD2 were also observed in the second experiment done at BNL. They are they from the doublet states (5/2, 3/2) to the ground (1/2) which are almost purely due to the LS interaction between Λ and nucleon. The preliminary data shows that the splitting is as small as 30 keV. It indicates that the LS interaction of ΛN is about 100 times smaller than that of the NN interaction.This project was quite successful in its achievement and first results. The γ spectroscopy of hypernuclei is now established as a usable and great tool for the study of hypernuclei.