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Cluster structure of 16 O, 18 O and 20 O is investigated by the antisymmettrized molecular dynamics (AMD) plus generator coordinate method (GCM). We have found the K π =0 + 2 and 0 − 1 rotational bands of 18 O that have the prominent 14 C+α cluster structure. Clustering systematics becomes richer in 20 O. We suggest the K π =0 + 2 band that is the mixture of the 12 C+α+4n and 14 C+ 6 He cluster structures, and the K π =0 − 1 band that has the 14 C+ 6 He cluster structure. The K π =0 + 3 and 0 − 2 bands that have the prominent 16 C+α cluster structure are also found.

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It was recently suggested that the odd-even staggering of reaction cross sections is an evidence of the pairing anti-halo effect on projectile radii. We define the dimensionless staggering parameters, Γ rds and ΓR, for projectile radii and reaction cross sections, respectively, and analyze the relation between Γ rds and ΓR for the scattering of 14,15,16 C from a 12 C target at 83 MeV/A by taking account of projectile-breakup and nuclearmedium effects newly with the microscopic version of the continuum discretized coupled-channels method. The value of ΓR is deviated from that of Γ rds by the projectile-breakup effect, the nuclear-medium effect and an effect due to the fact that the scattering are not the black-sphere scattering (BSS) exactly. The projectile-breakup and nuclear medium effects are nearly canceled for ΓR. The remaining non-BSS effect becomes small as an incident energy decreases, indicating that nucleus-nucleus scattering at lower incident energies are a good probe of evaluating Γ rds from measured reaction cross sections.PACS numbers: 24.10. Eq, 25.60.Gc, 25.60.Bx Introduction. Interaction cross section σ I and reaction cross section σ R are an important tool of determining radii of unstable nuclei. Actually, the halo structure as an exotic property was reported for unstable nuclei like 11 Li through analyses of measured σ I [1, 2]. Very recently, σ I was measured for Ne isotopes [3] and it is suggested by the analyses [4,5] that 31 Ne is a halo nucleus with large deformation.The difference between σ I and σ R is considered to be small for scattering of unstable nuclei at intermediate energies [6]. The reaction cross section is nearly proportional to a raidus of projectile; for example, see Ref.[6] for detailed analyses. Meanwhile, it is well known that pairing correlations are important particularly in even-N nuclei. The correlations become essential in weakly bound nuclei, since they are not bound without the correlations. Effects of the correlations on nuclear radii of unstable nuclei were investigated by the Hartree-Fock Bogoliubov (HFB) method [7]. In the meanfield picture, the correlations make the quasi-particle energy larger and hence reduce the root-mean-square radius of the HFB density. Obviously, this effect is conspicuous for unstable nuclei with the separation energy smaller than the gap energy. Thus, the pairing correlation suppresses the growth of halo structure for even-even unstable nuclei. This is now called the pairing anti-halo effect.The pairing anti-halo effect is an interesting phenomenon, but any clear evidence is not shown for the effect yet. Very recently, however, Hagino and Sagawa suggested that observed odd-even staggerings of σ R are possible evidence of the effect [8][9][10]. They introduced the staggering parameter [10]

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