Abstract:The production of fermium isotopes was attempted by complete fusion of different targets and projectiles spanning a wide range of effective entrance channel fissilities below and above the predicted threshold value Xef fthr~(l'/-v... For the most asymmetric systems where fusion is expected to occur without dynamical hindrance we investigate to what extent the expected amount of sub-barrier fusion contributes to the production of fermium evaporation residues. For increasingly symmetric systems the experimental … Show more
“…For the intermediate system 4~176 this new systematics [15] predicts an extra-push energy for formation of a compound nucleus inside the true fission saddle point of the order of 10 MeV. This is at variance with work by G/iggeler et al [20], who concluded on the basis of cross sections for fusion-evaporation residues [20] and for fission [22] that the 4~ Pb reactions did not show any evidence for an extra push.…”
Section: Introductionmentioning
confidence: 67%
“…From these observations and from the Z-dependent asymmetries we conclude that a significant fraction of the cross section for fission in the 4~176 reaction is due to quasi-fission reactions which do not involve compound-nucleus formation inside the fission barrier. As is evident from the observation of fusion-evaporation residues [20] true fusion followed by fission must be present as well. Thirdly, again in agreement with our previous findings for the reactions of S~ and S6Fe with 2~ [18], there are products from deep-inelastic scattering of 4~ from 2~ with atomic numbers close to Z=82 and sideward peaked angular distributions centered close to the grazing angle (0 = 55 ~ for targetlike products at Ecru = 172 MeV), see Fig.…”
Section: Z-dependent Asymmetries and Ko-valuesmentioning
confidence: 92%
“…Previous analyses of cross sections for fusion-evaporation residues in the reactions of 4~ with 2o4, 2o6,207, 2Oapb revealed [20,28] no evidence for a shift of the barrier relative to Collisions near the Coulomb-Barrier 321…”
Section: Scaling Of the Extra-extra Push Energiesmentioning
confidence: 95%
“…This indicates the presence of non-equilibrium fission (quasi-fission) which seems to be characterized by the lack of some kind of a steady state in the accumulation of tilting angular momentum and/or the absence of a well-defined effective decision point at which the K-distribution is frozen in. From the observation of fusion-evaporation residues [20] it is evident that, at the same time, there exists a true fusion-fission component in the 4~ + 2~…”
Section: Z-dependent Asymmetries and Ko-valuesmentioning
Fission-fragment angular distributions were measured in the reaction of 4~ with 2~ near the fusion barrier. For nearly symmetric mass-/charge splits we find angular distributions symmetric around 0 = 90 degrees, however, with unusually large anisotropies. These develop gradually into forward-backward asymmetric distributions as one moves away from mass-/charge symmetry. This indicates that non-compound fission ('quasi-fission') competes with true fusion-fission. The relative contribution of quasi-fission to the total fission cross section is somewhere between 51 and 85%. In the framework of the extrapush model this is equivalent to an extra-extra push energy for compound-nucleus formation inside the true fission saddle point of 4 < Exx < 9 MeV in agreement with a recent empirical parametrization of fusion-barrier shifts based on fusion-fission cross sections. On the basis of cross sections for fusion-evaporation residues it had previously been concluded that fusion of 4~ with Pb isotopes occurs unhindered. Possible reasons for this apparent discrepancy are discussed. PACS: 25.70.Jj
“…For the intermediate system 4~176 this new systematics [15] predicts an extra-push energy for formation of a compound nucleus inside the true fission saddle point of the order of 10 MeV. This is at variance with work by G/iggeler et al [20], who concluded on the basis of cross sections for fusion-evaporation residues [20] and for fission [22] that the 4~ Pb reactions did not show any evidence for an extra push.…”
Section: Introductionmentioning
confidence: 67%
“…From these observations and from the Z-dependent asymmetries we conclude that a significant fraction of the cross section for fission in the 4~176 reaction is due to quasi-fission reactions which do not involve compound-nucleus formation inside the fission barrier. As is evident from the observation of fusion-evaporation residues [20] true fusion followed by fission must be present as well. Thirdly, again in agreement with our previous findings for the reactions of S~ and S6Fe with 2~ [18], there are products from deep-inelastic scattering of 4~ from 2~ with atomic numbers close to Z=82 and sideward peaked angular distributions centered close to the grazing angle (0 = 55 ~ for targetlike products at Ecru = 172 MeV), see Fig.…”
Section: Z-dependent Asymmetries and Ko-valuesmentioning
confidence: 92%
“…Previous analyses of cross sections for fusion-evaporation residues in the reactions of 4~ with 2o4, 2o6,207, 2Oapb revealed [20,28] no evidence for a shift of the barrier relative to Collisions near the Coulomb-Barrier 321…”
Section: Scaling Of the Extra-extra Push Energiesmentioning
confidence: 95%
“…This indicates the presence of non-equilibrium fission (quasi-fission) which seems to be characterized by the lack of some kind of a steady state in the accumulation of tilting angular momentum and/or the absence of a well-defined effective decision point at which the K-distribution is frozen in. From the observation of fusion-evaporation residues [20] it is evident that, at the same time, there exists a true fusion-fission component in the 4~ + 2~…”
Section: Z-dependent Asymmetries and Ko-valuesmentioning
Fission-fragment angular distributions were measured in the reaction of 4~ with 2~ near the fusion barrier. For nearly symmetric mass-/charge splits we find angular distributions symmetric around 0 = 90 degrees, however, with unusually large anisotropies. These develop gradually into forward-backward asymmetric distributions as one moves away from mass-/charge symmetry. This indicates that non-compound fission ('quasi-fission') competes with true fusion-fission. The relative contribution of quasi-fission to the total fission cross section is somewhere between 51 and 85%. In the framework of the extrapush model this is equivalent to an extra-extra push energy for compound-nucleus formation inside the true fission saddle point of 4 < Exx < 9 MeV in agreement with a recent empirical parametrization of fusion-barrier shifts based on fusion-fission cross sections. On the basis of cross sections for fusion-evaporation residues it had previously been concluded that fusion of 4~ with Pb isotopes occurs unhindered. Possible reasons for this apparent discrepancy are discussed. PACS: 25.70.Jj
“…As we have chosen identical or very similar compound nuclei in our comparative study between hot and cold fusion reactions, we should be able to deduce information on the possible influence of the fusion dynamics by comparison with experimental cross sections. One of the most spectacular experimental results of the lead based reactions, besides the discovery of the heaviest elements known today rl, 2], was certainly the fact that the 2n cross section, which was several nanobarn in the fusion of 4OAr+ z~ [43], was several microbarn following the fusion of ~SCa with 2~ [44] leading to an element (102) that has an atomic number 53 higher by two charge units. Could this be understood quantitatively?…”
Section: Results For Fusion Reactions Using Targets Around Pbmentioning
Experimental cross sections to synthesize heavy ion induced fusion reactions are confronted with calculations using the conventional picture of complete fusion after passing the barrier plus statistical deexcitation after complete equilibration. The sensitivity of the calculations to some of the parameters is demonstrated. Despite this sensitivity, a single parameter set is found that reproduces well the large body of data from experiments using actinide targets. By comparison, for the more symmetric entrance channels using targets around 2~we find a fusion hindrance that increases steeply with increasing compound nuclear charge. Predictions for a few reactions of future interest for the synthesis of heavy elements are also presented.
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