Be BeTe BeO Gamma-ray spectroscopy of cluster hypernuclei : 9  Be K. Shirotori for the Hyperball collaboration, Tohoku Univ. 8 Be is known as the  -  cluster nucleus and it has an importance of having the simplest  -cluster nuclei. The ab initio calculation predicts a well developed  -  cluster structure, and the recent experiment[2] shows that the B(E2) value of 4 + →2 + transition agrees with this calculation. 8 Be is an unbound nucleus and the direct B(E2) measurement is experimentally very difficult. The  -ray spectroscopy experiment of 9  Be, in which a  was added to 8 Be, was performed in BNL-E930(‘98). We obtained the upper limit for the lifetime less than 0.1 ps[3] from the measurement via DSAM of the corresponding 2 + →0 + transition in 8 Be. However, the measured lifetime is much shorter than that predicted by the cluster model[4] since the calculation resulted in a sizable reduction in B(E2) value reflecting a shrinkage in the inter  cluster distance with a presence of . This inconsistency between the data and the theory shows that the theoretical prediction based on the cluster picture of 9  Be fails to apply to 9  Be, while a good agreement is seen for 7  Li. The experimental indicates a small change in the core 8 Be structure and suggests that 8 Be has rather shell-like structure than the 2  cluster. Thus measurement of B(E2) of 9  Be is important not only to understand the 8 Be structure but also to test the cluster model in light nuclei. Nuclear shrinkage effect in 7  Li 19% shrink Tanida et al., Phys. Rev. Lett. 86 (2001) 1982  -ray energy spectrum of E2(5/2 + →1/2 + ) transition  (5/2 + )=5.8± ±0.7 ps 7  Li level scheme 0 Study of 8 Be structure from the  -ray spectroscopy of 9  Be E [MeV]  -  [fm] B(E2) [e 2 fm 4 ]  [ps] 8 Be 理  Be 理  Be 実 3.05?22><0.1 Red : T. Motoba et al., Prog. Theor. Phys. 70 (1983) 189 Blue : R.B. Wiringa et al., Phys. Rev. C62 (00) How many hyperisotope can we produce ? : 5 Be hyperisotope 9 Be(K -,  - ) 9  Be : Aiming for B(E2) measurement 10 B(K -,  0 ) 10  Be : Need to construct  0 spectrometer 11 B(K -,  0 ) 11  Be : Need to construct  0 spectrometer 12 C(K -,  + ) 12  Be : DCX (double charge exchange reaction), small cross section (~0.1  b/sr) 13 C(K -,  + ) 13  Be : DCX, small cross section, optimum enrich carbon target  Level energy : change in excitation energy  B(E2) : 9  Be, 11  Be, 13  Be Experimentally an impurity effect of  was observed for the first time from the  -ray spectroscopy of 7  Li. Lifetime measurement via the Doppler shift attenuation method (DSAM) showed a much smaller B(E2) value of E2(5/2 + →1/2 + ) transition than the E2(3 + →1 + ) transition in the 6 Li core. The theoretical calculation[1] attributes 19% shrinkage of the  -n-p cluster distance to reproduce the measured B(E2) value. B(E2: 3 + →1 + : 6 Li) = 9.3 ±0.5 e 2 fm 4 ⇒ B(E2: 5/2 + →1/2 + : 7  Li) = 3.6 ±2.1 e 2 fm 4 ＊ B(E2) ∝ R 4, 1/  ∝ B(E2)E  5 The study of hypernuclei is one of the ways to understand the baryon-baryon (BB) interactions, through 1) the investigation of hyperon-nucleon interactions, 2) the properties of baryons in the nuclear matter, and 3) impurity effects of  on the core nucleus. The  N interaction is studied through the  hypernuclear level structure and its precise structure can only be observed from the  -ray spectroscopy by using germanium (Ge) detectors. The method of  -ray spectroscopy with Ge detectors has been successfully used to study structure of light p-shell  hypernuclei. We discuss the third subject, the effects of  as an impurity. Even a single  added to a nucleus may drastically change the properties of the nucleus, such as size, deformation, collective motions, shell /cluster structure, and etc. These effects had not been observed until the shrinkage of 7  Li was experimentally confirmed. Thus the hypernuclear  ray spectroscopy offers a new area in nuclear physic, that is, “impurity nuclear physics”. The change in the core nucleus or the core polarization induced by a  arises from its “glue-like role” in a nucleus. In particular, this glue-like aspect of  can be used to probe light nuclei as to whether they posses cluster-like or shell-like structure. If the core structure is forming clusters, inter cluster distance can be shrunk leading to a dramatic structural change. On the other hand, the core nucleus is close to the shell structure, the inclusion of  is expected to have a small effect. By looking at the changes of the core of a hypernucleus, it will be possible to study the structure of a nucleus. The  two-body potential is attractive and provides extra binding to a hypernucleus that brings an unbound normal nucleus to a bound hypernucleous. Therefore, we gain access to energy levels and the B(E2) measurement of the otherwise unbound core nucleus via  -ray spectroscopy experiment. Introduction B(E2) measurement of 9  Be by Doppler shift attenuation method (DSAM) 9 Be (K -,  -  ) 9  Be 43±5 keV E  (keV) Akikawa et al., Phys. Rev. Lett. 88 (2002)  Be level scheme 9  Be : experimental result     Cluster-like picture Shrink Shell-like picture 9  Be 8 Be Which ? Charge distribution Shrink 9  Be 8 Be Which is the true picture of core 8 Be ? Comparison between theory and experiment [1] E. Hiyama et al., Phys. Rev. C 59 (1999) 2351 [2] V. M. Datar et al., Phys. Rev. Lett. 94 (2005) [3] H. Tamura et al., NPA 754 (2005) 58c [4] T. Motoba et al., Prog. Theor. Phys. 70 (1983) 189 unbound +92 keV,  ~7 eV Nucleon density distribution (g.s 0 + ) 8 Be Level scheme Intrinsic frame Lab. frame 4 fm 8 Be : level and structure R.B. Wiringa et al., Phys. Rev. C62 (00) B(E2) values are extracted from a lifetime of excited states. We use DSAM to measure the lifetime. The hypernuclei produced recoils and are slowed down and stopped by loosing the kinematic energy in a target. When the stopping time is comparable to lifetime of the excited state, the energy peak shape has two comportments, namely the shifted and stopped component from  -ray emitted event during the slow down and after the stopping, respectively. The lifetime can be extracted from a fitting of the peak shape using a lifetime as a parameter. Summary Future possibilities Neutron → Evolution of cluster structure  -orbital  -orbital 9 Be (2  +1n)  2 config.  2 config.  config.  2  2 config. M. Seya, et al., PTP65, 204 (1981). Y. Kanada-En’yo, et al., PRC60, (1999). N. Itagaki, et al., PRC , (2000). … 10 Be (2  +2n) 12 Be (2  +4n) 8 Be (2  ) Systematic measurements of Be hyper isotopes A systematic measurement of nuclear isotopes is one of interesting studies of the nuclear clustering using  as a probe. The Be isotopic chain is the most suitable so that evolution of clustering nuclei as a function of neutron numbers can be studied systematically through five Be hypernuclei. In addition, by adding neutron to hypernuclei, the isospin dependent  N-  N coupling effect can be stronger. We may study a unique structure change of cluster hypernuclei due to the  N-  N coupling effect. For a production of Be isotopes for the systematic studies, three kinds of  hypernuclear productions must be used. These are challenging experiment pursued at J-PARC. Simulated response of  -ray spectrum GeV/c (0°) 1.1 GeV/c (0°) 1.1 GeV/c (5°) 1.5 GeV/c (5°) 1.1 GeV/c (10°) BeO Be BeTe BeO (Natural product) [5] M. Ukai et al., Phys. Rev. Lett. 93 (2004) Recoil velocity v.s. stopping time Stopping time vs initial beta For an ideal DSAM measurement, a stopping time should be the same or at most 4 times longer than a lifetime. Following factors are important to select a target. 1. Density : determine the stopping time 2. Crystal or uniform material : decrease the ambiguity of stopping time 3. Background of a compound : S/N ratio and the same energy  ray In the previous BNL-E930(‘98) experiment, a pure Beryllium (1.85 g/cm 3 ) was used, and the stopping time was too short for DSAM using the K - beam of 0.93 GeV/c. At J-PARC, we can use beams of the same momentum, but a much higher density target has to be used. Beryllium oxide (BeO, 3.01 g/cm 3 ) is one of the suitable target. It is of uniform crystal and has higher density than the pure Beryllium. Oxygen in the compound target should not be a contaminant as the 16  O  -ray background because the 16  O experiment was performed in BNL- E930(‘00) experiment[5]. The oxide is one of the safe to use target for the DSAM measurement. For the same reason, Li 2 O will be used as a target for the B(M1) measurement of 7  Li in the J-PARC E13 experiment. The stopping time of 9  Be in various targets is estimated from the SRIM code and shows that the stopping time is too short. To measure the B(E2) of 9  Be, we have to use lower beam momentum (0.8 GeV/c) as well as an optimal configuration of the Ge array with high statistics. (BeTe (5.1 g/cm 3 ) is a reference of heavier target)  Nuclear structure studies with an extra  binding as a probe Study of the cluster-like or shell-like structure Study of an unbound nucleus from a bound hypernucleous via a precise  -ray spectroscopy measurement   -ray spectroscopy of 9  Be Study of two  cluster nucleus, 8 Be B(E2) : inconsistency between theory and experiment Possibility to measure B(E2) by using the Beryllium oxide (BeO) target Need low beam momentum, an optimal configuration of Ge detector array, and high statistics  Future possibilities Systematic measurement of Be hyper-isotopes Study of 2  + neutron cluster nuclei  N-  N coupling effect via  -ray spectroscopy