Gamma-ray spectroscopy of hypernuclei — present and future
Introduction
The spectroscopy of Λ hypernuclei has been developed using the () and () reactions. A series of experiments with the () reaction spectroscopy carried out first at BNL-AGS and then at KEK-PS successfully revealed the Λ single-particle orbits for wide range mass numbers up to [1], [2]. In the KEK experiments, the Superconducting Kaon Spectrometer (SKS) with a large acceptance and good momentum resolution, constructed by O. Hashimoto and his collaborators, played a particularly important role. However, the mass resolution for those experiments was limited to 1.5 MeV (FWHM) at best, while much better resolution is necessary to resolve fine structure with a splitting less than 1 MeV due to spin–spin coupling and spin–orbit coupling between the Λ and a core nucleus. In order to improve the resolution, the () reaction spectroscopy at JLab and the γ-ray spectroscopy with germanium (Ge) detectors have been developed at the end of the last century.
In 1998, we developed a dedicated Ge detector array called Hyperball and started a project of hypernuclear γ-ray spectroscopy [3]. Since then, we have accumulated γ-ray data for almost all the p-shell hypernuclei that can be produced by the non-charge-exchange () and () reactions at KEK-PS and BNL-AGS. Fig. 1 shows level schemes and γ transitions of Λ hypernuclei as of 2012. By comparing some of these level energies measured by γ-ray spectroscopy with shell-model calculations, the spin-dependent ΛN interaction strengths have been determined to be [4], [5], [6] It is found that this parameter set reproduces all of the level energies in Fig. 1 quite well except for a few spacings between levels based on different core states in 10ΛB, 11ΛB, 12ΛC, and 13ΛC.
Section snippets
New data on 12ΛC (KEK E566)
The γ-ray spectroscopy experiment, KEK E566, was performed in 2005 at KEK-PS using the () reaction with the SKS spectrometer and an upgraded Ge detector array, Hyperball2. This experiment observed several γ transitions in 12ΛC and 11ΛB, as already reported [7].
Recently, we have improved data analysis particularly for the energy-loss correction in a thick polyethylene target and for a γ-ray energy calibration curve for each Ge detector. Details are described in Ref. [8]. As a result, the
J-PARC E13
At J-PARC, we will extend our studies on p-shell hypernuclei to s-shell and sd-shell hypernuclei. The first γ-ray spectroscopy experiment, E13, has been approved as one of the day-one experiments.
Status of Hyperball-J
We have recently constructed a new Ge detector array, Hyperball-J, for hypernuclear γ-ray spectroscopy experiments at J-PARC. Our previous Ge detector arrays, Hyperball and Hyperball2, were developed to overcome problems caused by a high counting rate and a high energy-deposit rate from high-energy secondary meson beams and the thick material of the target. The Hyperball-J array is characterized by tolerance to higher counting and energy-deposit rates caused by a higher beam intensity at
Systematic study of
In the E13 experiment, we hope to obtain data suggesting a modification of Λ hyperon properties in the 6Li nucleus. data may be also obtained for 19ΛF. If a change in the value is detected, systematic studies of with various hypernuclei are necessary to understand the origin of the change.
Since the value is expected to be changed by Σ mixing in Λ hypernucleus, dependence on nuclear isospin will reveal the effect of Σ mixing. It is desirable that measurements for
Summary
A series experiments for the hypernuclear γ-ray spectroscopy have accumulated p-shell hypernuclear data. Recently, a further analysis of the KEK experiments on 12C target (E566) has been carried out and a new γ transition in 12ΛC at was observed and ascribed to .
In the J-PARC E13 experiment, we will measure the 4ΛHe() transition to study charge-symmetry breaking in the ΛN interaction and investigate the 19ΛF hypernucleus to study ΛN spin–spin interaction in sd-shell
Acknowledgements
The authors thank to all the members of the KEK E566 and J-PARC E13 experiments. This work is supported by Grants-in-Aid Nos. 17070001, 23244043, and 24105003 for Scientific Research from the Ministry of Education of Japan.
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