Received: December 19, 2001; In Final Form: March 18, 2002

1. Introduction
Heavy-ion fusion reaction between massive nuclei has been extensively investigated so far. It is well known that the fusion probability between massive nuclei depends on the charge prod- uct Z p Z t of projectile and target. 1 When the charge product is larger than about 1800, the fusion probability decreases rapidly as Z ρ Z t increases. This means that even if the kinetic energy of projectile is large enough to surmount the fusion barrier, a com- pound nucleus is not always formed.  An additional kinetic en- ergy is needed to form the compound nucleus.  The compound nucleus is formed after the saddle point of the compound nu- cleus is passed through in the course of the fusion process after touching. A large kinetic energy is lost during this process. This means that the compact touching shape evolves more easily into the compound nucleus than an elongated touching shape. Thus the relative distance between the mass center of two colliding nuclei at touching with respect to the saddle point plays an es- sential role for fusion of massive reaction system.  The contact point of massive reaction system with a large charge product (>1800) usually located outside the saddle point. This situation is changed in the case of deformed nucleus and spherical projec- tile, because the distance between the mass centers at touching depends on the orientation of deformed nucleus.  This suggests that the fusion process is affected by the colliding angle θ with respect to the symmetric axis of deformed nucleus.  This was investigated by measuring evaporation residues (ERs) in the re- action 60,64 Ni + 154 Sm and 76 Ge + 150 Nd, where the nuclei 154 Sm and 150Nd are well deformed. The fusion between massive nuclei depends on not only the charge product but also the nuclear structure of projectile and target. It is reported that the number of valence nucleon outside a major shell affects the fusion probability. 1 Recently, Oganessian et al. 2 measured the evaporation residues in the fusion reactions 130Xe + 86Kr and 136Xe + 86Kr, where the nuclei 136Xe and 86Kr have the closed neutron shells N = 82 and N = 50, respectively, and the nucleus 130 Xe has no such a closed shell.  They found that the measured evaporation residue cross sections for the fu- sion reaction 136Xe + 86 Kr are almost 2 ～ 3 orders of magnitude larger than those for the fusion reaction 130 Xe + 86 Kr. This sug- gests the important role of the shell closure of the colliding nu- clei in the fusion process.  In this paper, we report the results of the ERs measurements for the two reactions 82 Se + 134 Ba and 82 Se + 138 Ba, where the nucleus 138 Ba has the neutron shell clo- sure N = 82 and the nucleus 134 Ba has the neutron number 78, four neutrons less than the closed shell. nels for the reaction 64Ni + 154 Sm as a function of the center-of-mass energy E cm and also the excitation energy E α of the compound nucleus. Each channel for the evaporation is shown as the solid circles (2n), open circles (3n) , solid triangles (4n + 5n) , and open squares (6n + 7n) . The total cross section of the measured ERs is shown as the thick solid curve with error bar. The thin dotted and the thin dashed curves are the cal- culated cross sections of the xn evaporation residues and the sum of the ERs, respectively.  The fusion cross sections are calculated by taking into account the deformation of ¹ ⁵⁴ Sm together with the 2 + and 3 − exci- tations (the solid curve), only the deformation (the long dashed curve), and without the deformation (the dotted curve). The fusion barrier V is shown as a function of the colliding angle θ in the inset.
2.  Experimental Results and Discussion
2.1. Deformation Effect.  We measured the ERs for the re- actions  60,64Ni + ¹ ⁵⁴ Sm  (Z ρ Z t = 1736)³  and  76 Ge + ¹ ⁵⁰ Nd  (Z ρ Z t = 1920) 4 by using the JAERI recoil mass separator. 5  The ex- perimental details are described elsewhere. 3.4  Figure 1 shows the xn -channel cross sections of the ERs for the fusion reaction 64 Ni + 154 Sm.  The total cross section of the measured ERs is also shown.  The nucleus 154 Sm has a large β 2 deformation of 0.32. This gives rise to a broad barrier distribution V as shown in the inset of Figure 1 and thus the fusion cross section (the solid curve) is enhanced below the Bass barrier 192.8 MeV. The standard statistical model calculation (HIVAP), 6 combined with the calculated fusion cross sections by using the code CCDEF, 7 predicts the sizable cross sections for the 2n and 3n channels (the thin dotted curves) below the Bass barrier. The reason is as follows: when the spherical 64 Ni projectile collides at the tip part (θ < 40°) of the prolate 154 Sm nucleus, where the fusion barrier is the lowest about 180 MeV as shown in the inset of Figure 1, a low excited compound nucleus of about 30 MeV is formed. This compound nucleus emits two or three neutrons in the de- excitation process. This means that the residues produced in the 2n and 3n channels are expected only at the energies below the Bass barrier and thus these cross sections are very sensitive to the fusion probability at these energies, that is, at the tip colli- sion. erably smaller than the predictions.  The total cross section of the measured ERs including the residues of Th, Ac, and Ra also deviates from the calculated result (the thin dashed curve) below 200 MeV and then increases gradually to the calculated values as the center-of-mass energy Ecm . This means that the actual fu- sion barrier at the tip collision is effectively larger than the cal- culation shown in the inset of Figure 1. In other words, an extra- push energy is needed to push a di-nuclear system formed at the tip collision into the formation of the compound nucleus.  As for the side collision (θ > 70°), the present data show that the fusion occurs without hindrance, because the predicted cross sections for the 4n + 5n and 6n + 7n channels and also the total ERs cross section at E cm > 200 MeV are consistent with the measured data. We obtained the same conclusion as mentioned here in the fu- sion reactions 60Ni + ¹³ 4 Sm (Ref. 3) and 76 Ge + ¹60 Nd (Ref. 4). The present results can be qualitatively understood by con- sidering the position of the contact point relative to the saddle point of the compound nucleus.  In the case of the reaction system 64Ni + 154Sm, the minimum distance R min /Ro = 1.48 be- tween the mass centers corresponds to the side collision and is close to the position of the saddle point R saddle / Ro ～ 1.5 of the compound nucleus 218 Th.  Here the position of the saddle point was estimated from Reference 8. R 0 is the radius of the compound nucleus.  On the other hand, the maximum distance Rmax / R 0 = 1.77 corresponding to the tip collision is well outside the saddle point. In the case of the reaction system 76 Ge + 150 Nd, the maximum distance of 1.76 corresponding to the tip collision is outside the saddle point Rsadde / R 0 ～ 1.4 of the compound nu- cleus 226 U. The minimum distance of 1.43 at the side collision is close to the saddle point. Since the compact configuration at the contact point tends to evolve into the formation of the com- pound nucleus compared with more elongated configuration, the compound nucleus may be more easily formed for the side col- lisions than the tip collision for the present reaction systems. Although the present results show that the tip collision exhibits a large fusion hindrance, some ERs corresponding to the 2 n and 3n channels still exist with small cross sections. This is because of a large survival probability against fission at low excitation energy.  On the other hand, the side collision makes the com- pound nucleus with high excitation because of a high Coulomb barrier. It turns out that the survival probability becomes small at a high excitation energy because of a large fission channel. This is a shortcoming for the side collision. 2.2. Shell Structure Effect.   We also investigated the de- pendence of the fusion probability on the nuclear structure, especially on the neutron closed shell N = 82.  The evapo- ration residues in the two fusion reactions 82 Se + 138 Ba and 82 Se + 134 Ba were measured.  Here the same compound nucleus 216Th is formed for both the fusion reactions 82 Se + 134 Ba and 130 Xe + 86 Kr (Ref. 2).  The evaporation residues were measured by the similar methods in References 3,4. The measured evapo- ration residue cross sections for the fusion reactions 82 Se + 138 Ba and 82 Se + 134 Ba are shown in Figure 2 and Figure 3, respec- tively, together with the statistical model predictions taking into account the barrier distribution due to the coupling of the first 2 + and 3 − inelastic excitations of target and projectile nuclei. In the fusion reaction 82 Se + 138 Ba, the maximum cross sections of the 2 n and 3 n channels are about 0.05 ～ 0.1 mb, while those for the fusion reaction 82 Se + 134 Ba are ～1 μb.  It should be noted that in the 2n + 3n channels for the fusion reaction 82 Se + 134 Ba, there are additional contributions from the fusion reactions due to the contamination of heavy barium isotopes A > 134 in the 134 Ba target (see the figure caption of Figure 3). The excellent agreement between the measured ERs cross sections and the calculated results in the reaction system 82 Se + 138 Ba means that there is no fusion hindrance.  On the other hand, there are large deficits in the xn channel cross sec- tions for the fusion reaction 82 Se + 134 Ba at the low energy re- gion ( E cx < 40 MeV) compared with the statistical model predic- tions. Figure 4 shows the fusion probability obtained from the sum of the ERs cross sections including xn and pxn channels. The fusion probability for the reaction system 82 Se + 134 Ba was obtained by subtracting the other contribution ascribed to the heavy barium isotopes in the target. As shown in Figure 4, the fusion probability for the reaction system 82 Se + 134 Ba is smaller than the prediction near the barrier region, which shows the sim- ilar trend with the more symmetric reaction system 124 Sn + 92 Zr (Ref. 9).  This result indicates a fusion hindrance at the barrier region. On the other hand, the fusion probability for the reaction system 82 Se + 138 Ba is close to the one for the more asymmetric reaction system 40Ar + 180Hf (Ref. 10) and clearly different from the trend for the fusion probability of the more symmetric reac- tion system 124 Sn + 96 Zr. This means no fusion hindrance for the reaction system 82 Se + 138 Ba. The present result is consistent with that of the fusion reaction 130,136 Xe + 86 Kr (Ref. 2). These results indicate the importance of the nuclear shell structure in the sub-barrier fusion. This may be related with the fission of the thorium isotopes as pointed out by Oganessian. 2  The asymmetric fission of thorium isotopes is observed for 220 Th and 222 Th but not for 216 Th (Ref. 2, 11). The both fission fragments at scission of these asymmetric fission are spherical and the atomic number of the heavy asymmetric component is concentrated around Z H = 54 with a small charge width of 4.7 (FWHM).11 The projectile and target nuclei in re- action system 82 Se + 138 Ba are close to the asymmetric fission fragments of 220 Th and also the colliding partner in the reaction system 136Xe + 86Kr is close to the asymmetric fission fragments of 222 Th. The potential energy calculated at touching also shows a minimum for these target-projectile combinations because of large negative shell energies of these projectiles and target nu- clei. 12 These target-projectile combination makes a compact touching shape and produces a cold compound nucleus when they fuse.  The present result means that the fusion of massive reaction system strongly depends on the nuclear shell structure of colliding partners and the target-projectile combination corre- sponding to the asymmetric fission product plays an important role in the fusion process.  In order to make clear the relation between fusion and fission, further experimental and theoretical investigations are needed. Acknowledgement. The authors thank the crew of the JAERI tandem-booster facility for the beam operation.