Elsevier

Nuclear Physics A

Volume 914, 20 September 2013, Pages 99-108
Nuclear Physics A

Gamma-ray spectroscopy of hypernuclei — present and future

https://doi.org/10.1016/j.nuclphysa.2013.03.014Get rights and content

Abstract

Recent results and future directions of the γ-ray spectroscopy of Λ hypernuclei are discussed. A further analysis of the last KEK experiment (E566) has revealed a new γ transition in 12ΛC at 6048keV (preliminary), which is ascribed to the M1(1311) transition. At J-PARC, studies will be extended from p-shell to s-shell and sd-shell hypernuclei. In the first part of the J-PARC E13 experiment, we will measure the 4ΛHe(1+0+) transition to study charge-symmetry breaking in the ΛN interaction and investigate the 19ΛF hypernucleus to study ΛN spin–spin interaction further using one of the sd-shell hypernuclei. In the second part of E13, we will precisely measure the spin-flip B(M1) value for the 7ΛLi(3/2+1/2+) transition. Beyond E13, we plan to study impurity effects by observing possible changes in nuclear deformation caused by the presence of a Λ in sd-shell hypernuclei, as well as to investigate various heavier Λ hypernuclei in order to precisely determine the energies of Λ single-particle orbits.

Introduction

The spectroscopy of Λ hypernuclei has been developed using the (K,π) and (π,K+) reactions. A series of experiments with the (π+,K+) 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 A=208 [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 (e,eK+) 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 (π+,K+) and (K,π) 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]Δ=0.33MeV(A>7)or0.43MeV(A=7),SΛ=0.01MeV,SN=0.4MeV,T=0.03MeV. 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 (π+,K+) 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 gΛ

In the E13 experiment, we hope to obtain gΛ data suggesting a modification of Λ hyperon properties in the 6Li nucleus. gΛ data may be also obtained for 19ΛF. If a change in the gΛ value is detected, systematic studies of gΛ with various hypernuclei are necessary to understand the origin of the change.

Since the B(M1) 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 B(M1) 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 6048±6keV was observed and ascribed to M1(1311).

In the J-PARC E13 experiment, we will measure the 4ΛHe(1+0+) 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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