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**Clustering in 12Be: Determination of the Enhanced monopole strength**

Yanlin Ye School of Physics and State Key Lab. of Nucl. Phys.&Tech. Peking University ARIS, , Tokyo

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Collaborators

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Outline I. Some background II. Observation Discussion IV. Summary

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alternating independent-particle and clustering structure — a unique feature of nuclear many-body system.

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**Idea of the relative tightness of a cluster inside a nucleus**

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for unstable nuclei Unstable nuclei: much more cluster configurations (and thresholds) within a small Ex interval. — richness of nuclear clustering

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**Recent progress Theory: AMD, GCM(RGM), GTCM, FMD, TCSM, TCHO(DHO), …**

Experiment: 0+2 (7.65 MeV) state in 12C, 0+2 (6.05 MeV) and 0+3 (12.05 MeV) in 16O; 0+4 (8.03 MeV) in 20Ne …...

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AMD 6He + 6He resonance

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**Monopole excitation method**

T. Yamada et al., PRC85,034315(2012); PTP120,1139(2008) Isoscaler monopole excitation, a jump of about 35 MeV in a simple single-particle picture.

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GTCM Explicit cluster-degree of freedom Good up to high excited levels

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Outline I. Some background II. Observation Discussion IV. Summary

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**Main experimental observables for determining the cluster formation**

Ex - spin systematics: high moment of inertia ii) Large cluster decay width: large ΓCluster/Γ ; γ2Cluster ; θ2Cluster Characteristic transition strength large M(IS) !! M(IS) ΓCluster 15

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**no small angle (low Erel) detection**

Freer et al., PRL82(1999) 1383; PRC63 (2001)034301 Charity et al., PRC76(2007)064313 no small angle (low Erel) detection 6He+6He,CH2 4He+8He,with P 4He+8He, with C Freer Charity

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**A new exp. at RIBLL1@HIRFL, Lanzhou**

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Beam： 12Be，29.0MeV/u，~3000pps Target：Carbon, 100 mg/cm2 DSSD：32 2mm-stip, 300μm , covering Lab. CsI(Tl)： 4 x 4， 2.5cm*2.5cm*3cm, Detection focused on the most forward angles

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**PID for 2-xHe fragments 4He 6He 8He**

Uniform calibration of the Si strips and treatment of PID under intense direct beam: [IEEE-NS 61(2014)596, NIMA728(2013)52]

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**Reconstruction (insensitive to Ebeam)**

Ex = Erel + Ethr 2-xHe events for 12Be 4He+4He 4He+6He 6He+6He 4He+8He

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**verification for 4He + 4He channel**

107.6KeV 8Be(g.s) 504.1KeV: 9Be->8Be(2+) Ref1 Excitation Energy---4He+4He Ref1: PLB580(04)129-14Be breakup 21

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**Resolution and efficiency**

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**Our exp: 6He+6He 11.7 13.3 MeV 4He+8He 10.3 12.1 13.6 MeV**

Large and unusual 10.3 MeV state; 不能三峰拟合，一定有连续态成分 Moment of inertia Our exp: 6He+6He MeV 4He+8He MeV 23

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Outline I. Some background II. Observation Discussion IV. Summary

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**Determine the spin and MR band**

Large and unsual 10.3 MeV state; pure 0+ !! 不能三峰拟合，一定有连续态成分 Moment of inertia Our exp: 6He+6He MeV 4He+8He MeV 25

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**Spin: Angular correlation analysis for the **

10.3 MeV state in 4He + 8He channel Ex: MeV For small angle inelastic scattering leading to a resonant state with an angular momentum J, which subsequently breaks up into spin-0 fragments, the projected angular correlation spectrum is proportional to | PJ(cos(Ψ) |2, with Ψ being the fragment c.m. angle relative to the beam direction. 26

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10.3 MeV state 13.6 MeV state

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10.3 MeV state |cosΨ| resolution

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**Confirming the MR band with large moment of inertia**

10.3 MeV state

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**Determining the monopole strength**

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**DWBA calculation Fraction: 0.034(10) x 2.2. EWSR：6727.9 fm4 MeV,**

M(IS)： /- 1.0 fm2, 31

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**Determining the cluster decay width**

10.3 MeV (0+) state: 32

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**All possible decay channels**

10.3 MeV(0+) state: Γ = 1.5(2) MeV; Γ = ΓHe+ΓBe 33

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**Overall dominance of the 0+ state**

Kosheninnilov ours DWBA calculations for the excitation of 12Be from its ground state to the 10.3 MeV excited state, when interacting with a C target. 34

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**10.3 MeV(0+) state: Γ = 1.5(2) MeV; Γ = ΓHe+Γbe **

Kosheninnilov[12]: ΓBe/Γ= 0.28±0.12 ΓHe/Γ = 1−ΓBe/Γ = 0.72(12); ΓHe = 1.1(2) MeV γ2He= 0.50(9); θ2He= 0.53(10) （comparable to 8Be) 35

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Outline I. Some background II. Observation Discussion IV. Summary

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Summary For the first time a strong monopole strength has been determined in unstable nuclei, providing a clear evidence for cluster formation in 12Be. Angular-correlation method is very sensitive to the spin of the resonance. MR band in 12Be has been justified based on the Ex-spin systematics . A large cluster-decay width, and consequently a large cluster SF, are obtained for the MeV state in 12Be. All these findings demonstrate a dominating cluster structure in 12Be just above the 4He+8He threshold. iv) detection around 0-degrees is essential in in order to measure near-threshold resonances in breakup exp.

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**Thank you for your attention**

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comparison ours Freer Charity

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Korsheninnikov et al., 1995, 50 AMeV 12Be + p; inelastic 10 MeV state, L >= 4 Saito et al., 2004, 60 AMeV 12Be + α;

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**Recent comprehensive review**

238 pages

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