DOI 10.1140/epja/i2007-10471-9 Special Article – HYP 2006 Eur. Phys. J. A 33, 243–246 (2007) T HE EUROPEAN P HYSICAL JOURNAL A γ -ray spectroscopy study of 11 Λ B and 12 Λ C Y. Ma 1, a , S. Ajimura 4 , K. Aoki 2 , M. Dairaku 3 , Y.Y. Fu 7 , H. Fujioka 8 , T. Fukuda 6 , K. Futatsukawa 1 , K. Hosomi 1 , W. Imoto 6 , M. Kawai 1 , Y. Kakiguchi 2 , S. Kinoshita 1 , T. Koike 1 , N. Maruyama 1 , M. Mimori 1 , S. Minami 6 , Y. Miura 1 , K. Miwa 3, b , Y. Miyagi 1 , T. Nagae 2 , D. Nakajima 8 , H. Noumi 2 , K. Shirotori 1 , T. Suzuki 1 , T. Takahashi 2 , T.N. Takahashi 8 , H. Tamura 1 , K. Tanida 5, c , N. Terada 1 , A. Toyoda 2 , K. Tsukada 1 , M. Ukai 1 , and S.H. Zhou 7 1 Department of Physics, Tohoku University, Sendai, 980-8578, Japan 2 High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, 305-0801, Japan 3 Department of Physics, Kyoto University, Kyoto, 606-8502, Japan 4 Department of Physics, Osaka University, Osaka, 560-0043, Japan 5 RIKEN, Wako 351-0198, Japan 6 Osaka Electro-Communication University, Osaka, 572-8530, Japan 7 China Institute of Atomic Energy, P.O. Box-275-30, 102413, Beijing, China 8 Department of Physics, University of Tokyo, Tokyo, 113-0033, Japan Received: 28 February 2007 Published online: 3 September 2007 – c  Societ`a Italiana di Fisica / Springer-Verlag 2007 Abstract. In this paper, a preliminary result from the latest hypernuclear γ-ray spectroscopy experiment (KEK-E566) is presented together with a short discussion. The experiment was performed at the KEK-PS K6 beam line in 2005. In this experiment, the 12 C(π + , K + ) 12 Λ C reaction was employed to populate 12 Λ C/ 11 Λ B hypernuclei. A germanium detector array, Hyperball2, was constructed to detect γ-rays emitted from the hypernuclei produced. Three hypernuclear γ-ray peaks were observed and assigned. PACS. 21.80.+a Hypernuclei – 13.75.Ev Hyperon-nucleon interactions – 07.85.Nc X-ray and γ-ray spec- trometers 1 Introduction The presence of a hyperon inside the nuclear medium could cause many interesting phenomena such as the core shrinkage in 7 Λ Li [1]. The hyperon may also serve as a tagged baryon to probe the inner structure of the nuclear core [2]. The central theme of hypernuclear physics lies in the promise of providing a unified baryon-baryon in- teraction picture. In the current work, we will focus on the spin-dependent interaction between a Λ-hyperon and a nucleon (ΛN interaction). In general, the ΛN interaction can be expressed into effective interactions as V ΛN (r)= V 0 (r)+ V σ (r)s N · s Λ + V Λ (r)l NΛ · s Λ +V N (r)l NΛ · s N + V T (r)  3(σ Λ · r)(σ N · r) r 2 - σ Λ · σ N  , where r is the relative position between a Λ and a nucleon and l NΛ stands for the relative orbital angular momentum. a e-mail: ma@lambda.phys.tohoku.ac.jp b Present address: Department of Physics, Tohoku Univer- sity, Sendai, 980-8578, Japan. c Present address: Department of Physics, Kyoto University, Kyoto, 606-8502, Japan. The s N and s Λ are the spin angular momentum operator of a nucleon and a Λ, respectively. To describe low-lying level energies of p-shell hypernuclei, the radial integrals of these five terms for p N s Λ wave functions are introduced as parameters and denoted by V (radial interaction), ∆ (spin-spin interaction), S Λ (Λ spin-dependent spin-orbit interaction), S N (N spin-dependent spin-orbit interaction) and T (tensor interaction), respectively. The V term is usually set to fit the binding energy of the Λ hyperon in- side a hypernucleus. Based on a shell model calculation, level energies of a hypernucleus can be written by a lin- ear combination of the interaction parameters [3]. Experi- mentally, these parameters can be extracted by measuring energy level spacings of hypernuclei of interest [4]. Natu- rally, γ -ray spectroscopy study provides an ideal chance to reveal the level spacing of the hypernucleus. One should also notice that the measured level spacings may contain the contribution from nuclear-medium effects and the cor- responding corrections are necessary. In the beginning of the hypernuclear γ -ray spec- troscopy experiments, NaI detectors were used. The ap- plication of the NaI was limited to the cases in which the hypernuclear levels have large spacing. For example, the γ -rays from 13 Λ C(p Λ → s Λ ) were measured in this way [5].