Fusion-fission Cross Sections Research Articles

Theoretical study of evaporation residue cross sections in the decay of Cn*286 formed via the U238+Ca48 reaction using Skyrme forces

The dynamical cluster-decay model (DCM) is employed to study the excitation functions and fusion evaporation residue (ER) cross sections for the production of $^{283}\mathrm{Cn}$ and $^{282}\mathrm{Cn}$ isotopes via $3n$ and $4n$ decay channels from the $^{286}\mathrm{Cn}^{*}$ compound nucleus. The study includes quadrupole deformation ${\ensuremath{\beta}}_{2i}$ and hot-optimum orientations ${\ensuremath{\theta}}_{i}$ at various $^{48}\mathrm{Ca}$-beam energies ${E}_{\mathrm{lab}}=233.3\text{--}240\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ (equivalently, excitation energies ${E}^{*}=34.4\text{--}39.8\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$), supporting symmetric fission, which agrees well with the experimental data. The reaction was investigated by using a hot compact configuration. The Skyrme forces used are new forces GSkI and KDE0(v1), and the conventional force SIII. Apparently, the DCM with the pocket formula for nuclear proximity potential reproduces the measured data on fusion ER nicely within single-parameter fitting of $\mathrm{\ensuremath{\Delta}}R$, independently of the nuclear interaction potential and Skyrme force used. We have also calculated the fusion-fission cross section (${\ensuremath{\sigma}}_{ff}^{\mathrm{predicted}}$) at ${E}^{*}=35\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ and ER cross sections for the experimentally unobserved neutron emission channels $1n$, $2n$, and $4n$ at different excitation energies ${E}^{*}$. Further, we have proposed new target-projectile (t-p) combinations for the synthesis of $^{286}\mathrm{Cn}$ for future experiments.

Sub-barrier fusion of 34,36S+204,206,208Pb: Signature of isotopic dependence of repulsive core potential in heavy-ion fusion reactions

The measurements of the excitation function for the heavy-ion fusion-fission cross sections in negative Q-value systems 34S+204,206,208Pb and 36S+204,206,208Pb have been analyzed by coupled-channels (CC) calculations based on the M3Y double-folding (DF) potential that is supplemented with a short-ranged repulsive potential. The standard Aage Winther potential is also employed to explore the occurrence of the unexpected steep fall-off phenomenon of the measured cross sections in fusion between sulfur and lead nuclei. Our analysis based on the deep phenomenological potentials clearly demonstrates that the fusion data are strongly hindered at incident energies well below the Coulomb barrier for the four heavy-ion systems 34S+206Pb, 34S+208Pb, 36S+206Pb and 36S+208Pb. In addition, a first indication of a maximum in the trend of the experimental data of the S-factor representation has been observed for each of the fusions of 34,36S with 206,208Pb targets. It is shown that the M3Y+Repulsion potential accompanied by the CC calculations provides a satisfactory description of the experimental fusion cross sections and S factors in the whole energy range under study for all the six cases of interest. A discussion is also presented concerning the isotopic dependence of the parameters existing in simulation of the repulsive core potential. Our detailed study reveals that the strength of the repulsive interaction and incompressibility of the nuclear matter (NM) increases and decrease linearly with addition of neutrons in the isotopic systems, respectively. Further, we show that the repulsive part of the density distribution becomes more diffuse with increasing neutrons from 34S+204Pb to 36S+208Pb.

Ca48 -induced reaction on the lanthanide target Gd154 and its decay to ground and metastable states within the dynamical cluster-decay model

The decay mechanism of compound nucleus (CN) $^{202}\mathrm{Po}^{*}$, formed in $^{48}\mathrm{Ca}+^{154}\mathrm{Gd}$ reaction, is studied within the dynamical cluster-decay model (DCM) at various excitation energies ${E}_{\mathrm{CN}}^{*}$, where neutron emission $xn$, $x=3\text{--}5$, are the predominant decay modes. The study is of interest since $^{202}\mathrm{Po}^{*}$ decays to the ground state (g.s.) of $^{198}\mathrm{Po}$ by emission of $4n$ and to metastable states $^{199m}\mathrm{Po}$ and $^{197m}\mathrm{Po}$ via ($3n,5n$) emission, respectively. The DCM is applied here for the first time to the decays of metastable states. Both types of decays are analyzed separately, using neck-length $\mathrm{\ensuremath{\Delta}}R$ (equivalently, barrier-lowering) parameter, the only parameter in the DCM, to best fit the evaporation residue or channel cross section (${\ensuremath{\sigma}}_{xn}$) data and predict the quasifissionlike (qf-like) noncompound (${\ensuremath{\sigma}}_{\mathrm{qf}}$) and fusion-fission (${\ensuremath{\sigma}}_{\mathrm{ff}}$) cross sections. For g.s. to g.s. decay of $^{202}\mathrm{Po}^{*}$, possibly due to involving the deformed rare-earth lanthanide target $^{154}\mathrm{Gd}$, the observed $4n$ decay channel requires the noncompound nucleus (nCN) contribution, treated as the qf-like process. On the other hand, the decay mechanism of $^{202}\mathrm{Po}^{*}$ to metastable states (m.s.) $^{199m}\mathrm{Po}$ and $^{197m}\mathrm{Po}$, is a pure CN decay, i.e., the ${\ensuremath{\sigma}}_{\mathrm{qf}}$ is zero. In this study, we have included the deformation effects up to quadrupole deformations ${\ensuremath{\beta}}_{2i}$ and optimum orientations ${\ensuremath{\theta}}_{i}^{\mathrm{opt}.}$ for coplanar ($\mathrm{\ensuremath{\Phi}}={0}^{0}$) nuclei, using hot-compact configurations, supporting asymmetric fission of CN $^{202}\mathrm{Po}^{*}$. The variation of CN formation probability ${P}_{\mathrm{CN}}$ and survival probability ${P}_{\mathrm{surv}}$ with excitation energy ${E}_{\mathrm{CN}}^{*}$ is in complete agreement with the known systematic of other radioactive CN studied so far, thereby giving credence to the predicted ${\ensuremath{\sigma}}_{\mathrm{qf}}$ in g.s. to g.s. decay and fusion-fission cross section ${\ensuremath{\sigma}}_{\mathrm{ff}}$ of $^{202}\mathrm{Po}^{*}$. Interestingly, both the observed g.s. to g.s. and g.s. to m.s. processes occur at a fixed $\mathrm{\ensuremath{\Delta}}R=2.45\ifmmode\pm\else\textpm\fi{}0.20\phantom{\rule{0.28em}{0ex}}\mathrm{fm}$, which is within the nuclear proximity limit of $\ensuremath{\sim}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{fm}$, and hence useful for making predictions.

Distinguishing Features of Radioactive Compound Nucleus Decays within the Dynamical Cluster Decay Model

In this paper, we are interested to study the distinguishing features of the decaying radioactive compound nuclei 246Bk* and 220Th*, using the Dynamical Cluster-decay Model (DCM) with deformation β and non-coplanar degree-of-freedom Φ. 246Bk* and 220Th* have so-far been studied within the DCM, using quadrupole deformations (β2i), “optimum” orientations (θopt) of the two nuclei lying in the same plane (Φ=0o), which shows that there is a non-compound nucleus (nCN) content in the observed data. The first turning point Ra (equivalently, the neck-length ∆R in Ra=R1+R2+∆R), which fixes both the preformation and penetration paths, is used to best fit the measured evaporation residue (ER) and fusion-fission (ff) cross sections, σER, σff, respectively, in 220Th* and 246Bk*, formed via different entrance channels. In this work, we subsequently add higher multipole deformations, the octupole and hexadecupole (β3i, β4i), `compact’ orientations θci and Φ≠00, and look for their effects on the nCN contribution predicted by the DCM calculations referenced above.

Synthesis of the Z=122 superheavy nucleus via Fe58 - and Ni64 -induced reactions using the dynamical cluster-decay model

Within the framework of the dynamical cluster-decay model (DCM), we have studied the nuclear system with $Z=122$ and mass number $A$ = 306 formed via two ``hot'' fusion reactions $^{58}\mathrm{Fe}+\phantom{\rule{0.16em}{0ex}}^{248}\mathrm{Cm}$ and $^{64}\mathrm{Ni}+\phantom{\rule{0.16em}{0ex}}^{242}\mathrm{Pu}$. The up-to-date measured data are available only for the first reaction, and for fusion-fission cross section ${\ensuremath{\sigma}}_{\mathrm{ff}}$ and quasifission cross section ${\ensuremath{\sigma}}_{\mathrm{qf}}$, only at one compound nucleus (CN) excitation energy ${E}^{*}=33\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. In this study, we have included the deformation effects up to quadrupole deformations ${\ensuremath{\beta}}_{2i}$ and with ``optimum'' orientations ${\ensuremath{\theta}}_{i}^{\mathrm{opt}.}$ for coplanar ($\mathrm{\ensuremath{\Phi}}={0}^{0}$) configurations. The only parameter of the model is the neck-length parameter $\mathrm{\ensuremath{\Delta}}R$ whose value, for the nuclear proximity potential used here, remains within its range of validity ($\ensuremath{\sim}2\phantom{\rule{0.28em}{0ex}}\mathrm{fm}$). Using the best fitted $\mathrm{\ensuremath{\Delta}}R$'s to the observed data for ${\ensuremath{\sigma}}_{\mathrm{ff}}$, calculated for mass region $A/2\ifmmode\pm\else\textpm\fi{}20$, and ${\ensuremath{\sigma}}_{\mathrm{qf}}$ for the incoming channel of Fe-induced reaction at ${E}^{*}=33\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$, we have extended the DCM calculations to the other Ni-induced reaction, and to ${E}^{*}$'s in the energy range 25--68 MeV. The interesting result is that the predicted evaporation residue cross section ${\ensuremath{\sigma}}_{\mathrm{ER}}$ for 1--4 neutrons is largest for 4n decay at ${E}^{*}=45\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$, having the value ${\ensuremath{\sigma}}_{\mathrm{ER}}\ensuremath{\equiv}{\ensuremath{\sigma}}_{4n}\ensuremath{\sim}{10}^{\ensuremath{-}5}$ pb for both reactions, and that the $\mathrm{\ensuremath{\Delta}}R$'s for the three processes (ER, ff, and qf) are different, i.e., they belong to different time scales where ff occurs first, then qf and the ER at the end. Other results of interest are the predictions of the magic $N=82\phantom{\rule{4pt}{0ex}}^{136}\mathrm{Xe}$ fragment in the ff region of mass $A/2\ifmmode\pm\else\textpm\fi{}20$, and the doubly magic $^{208}\mathrm{Pb}$ in the qf region, in near close agreement with observed data (the observed fission fragment is of mass 132, instead of the predicted mass 136). The role of the weakly bound neutron-rich intermediate mass fragments and of the nucleus in the neighborhood of deformed magic $Z$ = 108 are also indicated in the DCM calculations, which need experimental verification. For the predicted ${\ensuremath{\sigma}}_{\mathrm{ER}}$, the largest value of CN fusion probability ${P}_{\mathrm{CN}}=0.2$, and its survival probability against fission ${P}_{\mathrm{surv}}\ensuremath{\rightarrow}0$. Further experiments are called for.

Role of rotational energy component in the dynamics of 16O+198Pt reaction

The role of rotational energy is investigated in reference to the dynamics of 16 O+198 Pt →214 Rn∗ reaction using the sticking (I S ) and the non-sticking (I NS ) limits of moment of inertia within the framework of dynamical cluster decay model. The decay barrier height and barrier position get significantly modified for the use of sticking or non-sticking choice, which in turn reproduce the evaporation residue and the fusion-fission cross-sections nicely by the I S approach, while the I NS approach provides feasible addressal of data only for evaporation residue channel. Moreover, the fragmentation path of decaying fragments of 214 Rn∗ compound nucleus gets influenced for different choices of moment of inertia. Beside this, the role of nuclear deformations i.e. static, dynamic quadurpole (β2 ) and higher order static deformation up to β4 are duly investigated for both choices of the moment of inertia.

Charged particle decay of hot and rotatingMo88nuclei in fusion-evaporation reactions

A study of fusion-evaporation and (partly) fusion-fission channels for the $^{88}$Mo compound nucleus, produced at different excitation energies in the reaction $^{48}$Ti + $^{40}$Ca at 300, 450 and 600 MeV beam energies, is presented. Fusion-evaporation and fusion-fission cross sections have been extracted and compared with the existing systematics. Experimental data concerning light charged particles have been compared with the prediction of the statistical model in its implementation in the Gemini++ code, well suited even for high spin systems, in order to tune the main model parameters in a mass region not abundantly covered by exclusive experimental data. Multiplicities for light charged particles emitted in fusion evaporation events are also presented. Some discrepancies with respect to the prediction of the statistical model have been found for forward emitted $\alpha$-particles; they may be due both to pre-equilibrium emission and to reaction channels (such as Deep Inelastic Collisions, QuasiFission/QuasiFusion) different from the compound nucleus formation.

Theoretical study of odd-mass Fr isotopes using the collective clusterization approach of the dynamical cluster-decay model

The reaction dynamics of various odd-mass Fr isotopes is studied over a wide range of incident energies, spread across the Coulomb barrier. The specific reactions analyzed are ${}^{18}$O+${}^{197}$Au and ${}^{19}$F+${}^{192,194,196,198,200}$Pt, forming odd-mass ${}^{211\ensuremath{-}219}$Fr${}^{*}$ compound systems where some data are available for three of these isotopes: ${}^{213,215,217}$Fr${}^{*}$. Based on the dynamical cluster-decay model (DCM), we have extended our calculations of the evaporation residue (ER) cross sections to the mainly fissioning ${}^{215}$Fr${}^{*}$, using the systematics of ${}^{213,217}$Fr${}^{*}$ isotopes where the available ER cross sections (as well as fusion-fission cross sections) were studied earlier within the DCM. In order to obtain a clear picture of the dynamics involved, including entrance channel effects, the variations of fragmentation potential, preformation factor, and decay barrier height are analyzed. The relevance of barrier modification effects is also explored in the decay of ${}^{213,215,217}$Fr${}^{*}$ nuclei. In addition, fusion-fission (ff) cross sections are extended to ${}^{213,217}$Fr${}^{*}$ systems where some more data has recently become available. Also, the fission fragment anisotropies (so far measured and studied for ${}^{215}$Fr${}^{*}$ alone) are estimated for ${}^{213,217}$Fr${}^{*}$ using DCM for the use of nonsticking moment of inertia, and relevant comparison with the sticking moment-of-inertia approach is analyzed. Furthermore, the shell closure effects of the decay fragments are investigated for odd-mass ${}^{211\ensuremath{-}219}$Fr${}^{*}$ isotopes.

One-neutron and noncompound-nucleus decay contributions in the12C+93Nb reaction at energies near and below the fusion barrier

The dynamical cluster-decay model (DCM), with effects of deformations and orientations of nuclei included in it, is used to study the decay of the hot and rotating compound nucleus ${}^{105}$Ag${}^{*}$ formed in a ${}^{12}\mathrm{C}+{}^{93}\mathrm{Nb}$ reaction at near and below barrier energies. The only parameter of the model is the neck-length parameter, which varies smoothly with the temperature of the compound nucleus, and its value remains within the range of validity ($\ensuremath{\sim}$2 fm) of the proximity potential. The emissions of both the observed light particles (${A}_{2}=1--4$) representing the evaporation residue (ER) and the (energetically favored) intermediate mass fragments (IMFs; $5\ensuremath{\le}{A}_{2}\ensuremath{\le}13$), together with the so far unobserved fission channel, are considered as the dynamical collective mass motions of preformed fragments or clusters through the barrier. A best fit to data is shown to be obtained only if a large noncompound-nucleus (nCN) contribution, calculated as a quasifission or capture process, is allowed in the DCM. Another interesting result is that the one-neutron contribution is large, rather the largest, when best fits are attempted only for the ER cross sections, but, in turn, it reduces to zero or becomes relatively small if the data on the total fusion cross section ${\ensuremath{\sigma}}_{\mathrm{fus}}$, i.e., ${\ensuremath{\sigma}}_{\mathrm{ER}}+{\ensuremath{\sigma}}_{\mathrm{IMFs}}$, are considered. Furthermore, the fusion-fission (ff) cross section ${\ensuremath{\sigma}}_{\mathrm{ff}}$, consisting of symmetric and near-symmetric fragments, at the considered three energies of the experiment, is predicted to be of the order of 10${}^{1}$ to 10${}^{3}$ mb, depending on the choice of neck-length parameter for fission region. Further experimental studies are called for both the nCN and ff channels in this reaction.

Fusion excitation function revisited

We report on a comprehensive systematics of fusion-evaporation and/or fusion-fission cross sections for a very large variety of systems over an energy range 4A-155A MeV. Scaled by the reaction cross sections, fusion cross sections do not show a universal behavior valid for all systems although a high degree of correlation is present when data are ordered by the system mass asymmetry. For the rather light and close to mass-symmetric systems the main characteristics of the complete and incomplete fusion excitation functions can be precisely determined. Despite an evident lack of data above 15A MeV for all heavy systems the available data suggests that geometrical effects could explain the persistence of incomplete fusion at incident energies as high as 155A MeV.

Radiochemical study of sub-barrier fusion hindrance in the 19F+209Bi reaction

Abstract Production cross sections of fission product 99Mo produced in the reaction of 19F + 209Bi have been measured at the incident energies 135–83 MeV near and below the fusion barrier by radiochemical methods. The excitation function for the fusion-fission cross sections in this reaction were deduced down to 80 μb from the measured production cross sections. A systematic analysis of the fusion-fission excitation function shows that sub-barrier fusion hindrance in the heavy mass system 19F + 209Bi is different from that in the medium-heavy mass systems but similar to that in the medium-light mass system and in the heavy mass system. This work is the first radiochemical study of sub-barrier fusion hindrance in heavy mass systems.

Evaporation and fission decay of 132Ce compound nuclei at Ex=122 MeV: some limitations of the statistical model

Light charged particle (LCP) emission in the evaporation residue (ER) and fusion fission (FF) channels have been studied for the 200 MeV 32S + 100Mo reaction, leading to 132Ce composite nuclei at E x =122 MeV. The main goal was to study the decay of 132 Ce on the basis of an extended set of observables, to get insights on the fission dynamics. The proton and alpha particle energy spectra, their multiplicities, ER-LCP angular correlations, ER and FF angular distributions, and ER and FF cross-sections were measured. The measured observables were compared with the Statistical Model (SM). Using standard parameters, the model was able to reproduce only the pre-scission multiplicities and the FF and ER cross-sections. The calculation was observed to strongly overestimate the proton and alpha particle multiplicities in the ER channel. Disagreements were also observed for the ER-LCP correlations, the LCP energy spectra and the ER angular distribution. By varying the SM input parameters over a wide range of values, it is shown that it is not possible to reproduce all the observables simultaneously with a unique set of parameters. The inadequacy of the model in reproducing the ER particle multiplicities is also observed analysing data from the literature for other systems in the A ≈ 150 and E x ≈ 100−200 MeV region. These results indicate serious limitations about the use of the SM in extracting information on fission dynamics.

Fusion-fission of superheavy compound nuclei produced in reactions with heavy ions beyond Ca

Total Kinetic Energy - Mass distributions of fission-like fragments for the reactions of 22 Ne, 26 Mg, 36 S, 48 Ca, 58 Fe and 64 Ni ions with actinides leading to the formation of superheavy compound systems with Z=108-120 at energies near the Coulomb barrier have been measured. Fusion-fission cross sections were estimated from the analysis of mass and total kinetic energy distributions. It was found that the fusion probability drops by three orders of magnitude for the formation of the compound nucleus with Z=120 obtained in the reaction 64 Ni+ 238 U compared to the formation of the compound nucleus with Z=112 obtained in the reaction 48 Ca+ 238 U at the excitation energy of the compound nucleus of about 45 MeV. From our analysis it turns out that the reaction 64 Ni+ 238 U is not suitable for the synthesis of element Z=120.

FUSION-FISSION DYNAMICS IN SUPERHEAVY MASS REGION

In heavy nucleus collision experiments, the fusion-fission cross section is derived from counting mass-symmetric fission events. However, a discrepancy exists between the experimental and theoretical estimations of the fusion cross section. We attempt to clarify the origin of the discrepancy and remove it by performing a dynamical calculation. The trajectory calculation has been performed in three-dimensional coordinate space with the Langevin equation.

DECAY OF 202Pb* FORMED IN 48Ca+154Sm REACTION USING THE DYNAMICAL CLUSTER-DECAY MODEL

The decay of compound nucleus 202 Pb *, formed in entrance channel reaction 48 Ca +154 Sm at different incident energies, is studied by using the dynamical cluster-decay model (DCM) where all decay products are calculated as emissions of preformed clusters through the interaction barriers. The calculated results show an excellent agreement with experimental data for the fusion-evaporation residue cross-section σ ER together with the fusion-fission cross-section σ FF (taken as a sum of the energetically favored symmetric [Formula: see text] and near symmetric A=65–75 plus complementary fragments), and the competing, non-compound-nucleus quasi-fission cross-section σ QF where the entrance channel is considered not to loose its identity (and hence with preformation factor P0=1). The interesting feature of this study is that the three decay processes (ER, FF and QF) are quite comparable at low energies, ER being the most dominant, whereas at higher energies FF becomes most probable followed by ER and QF. The prediction of two fission windows, the symmetric fission (SF) and the near symmetric fission (nSF) whose contribution is more at lower incident energies, suggests the presence of a fine structure effect in the fusion-fission of 202 Pb *. This result is attributed to the shell effects (magic shells) playing effective role in the fragment preformation yields for 48 Ca +154 Sm reaction at lower excitation energies, giving rise to "shoulders", to an otherwise Gaussian FF mass distribution, responsible for the QF process. As a further verification of this result, absence of "shoulders" (hence, the QF component) in the decay of 192 Pb * due to 48 Ca +144 Sm reaction is also shown to be given by the calculations, in agreement with experiments. The only parameter of the model is the neck-length ΔR which shows that the ER occurs first, having the largest values of ΔR, and the FF and QF processes occur almost simultaneously at lower incident energies but the FF takes over QF at higher incident energies. In other words, the three processes occur in different time scales, QF competing with FF at lower incident energies.

Decay ofBa118,122*compound nuclei formed inKr78,82+Ca40reactions using the dynamical cluster-decay model of preformed clusters

Application of the preformed clusters based dynamical cluster-decay model (DCM) is made to the recent data on decay of the compound systems $^{118,122}\mathrm{Ba}{}^{*}$ at a relatively low bombarding energy of 5.5 MeV/$A$. The same model has been applied earlier to the intermediate mass fragment (IMF) data of $^{116}\mathrm{Ba}{}^{*}$, observed at medium and higher incident energies. For the heavier $^{118,122}\mathrm{Ba}{}^{*}$ systems, however, a complete mass fragmentation spectrum is observed experimentally. Except for a small narrow region of heavier mass fragments ($8\ensuremath{\leqslant}{Z}_{L}\ensuremath{\leqslant}15$), the DCM gives an overall reasonable description of the observed data on both the intermediate mass fragments and the fusion-fission cross-sections, whereas the statistical model calculations based on BUSCO and GEMINI codes describe the intermediate mass fragment data and the heavier mass fragment and fusion-fission data, respectively. Within the DCM (with preformation factor ${P}_{0}=1$), the possibility of non-compound-nucleus decay contributing to the region $8\ensuremath{\leqslant}{Z}_{L}\ensuremath{\leqslant}15$ of heavier mass fragments is also explored. All three models use the maximum angular momentum ${\ensuremath{\ell}}_{\mathrm{max}}$ as a fitting parameter, which in the DCM is fixed via a neck-length parameter for the penetrability $P\ensuremath{\rightarrow}1$.

Fusion ofCa+Sm near the Coulomb barrier: enhancement vs. suppression

Fusion-evaporation and fusion-fission cross sections have been measured in the reaction 48 Ca+ 154 Sm from well below to well above the Coulomb barrier. The comparison of fusion cross sections for this reaction with previous data corresponding to 16 O+ 186 W, leading to the same compound nucleus 202 Pb * , puts in evidence an interesting competition between sub-barrier enhancement, due to the strong couplings related to the 154 Sm deformation, and above barrier suppression for the 48 Ca induced reaction. The fusion hindrance mechanism has been attributed to the strong quasi-fission component observed in the mass-energy distributions of fission fragments.

Fusion ofCa+Sm near the Coulomb barrier: enhancement vs. suppression

Fusion-evaporation and fusion-fission cross sections have been measured in the reaction 48 Ca+ 154 Sm from well below to well above the Coulomb barrier. The comparison of fusion cross sections for this reaction with previous data corresponding to 16 O+ 186 W, leading to the same compound nucleus 202 Pb * , puts in evidence an interesting competition between sub-barrier enhancement, due to the strong couplings related to the 154 Sm deformation, and above barrier suppression for the 48 Ca induced reaction. The fusion hindrance mechanism has been attributed to the strong quasi-fission component observed in the mass-energy distributions of fission fragments.

Fusion of 48Ca+ 154Sm near the Coulomb barrier: enhancement vs. suppression

Fusion-evaporation and fusion-fission cross sections have been measured in the reaction 48Ca+ 154Sm from well below to well above the Coulomb barrier. The comparison of fusion cross sections for this reaction with previous data corresponding to 16O+ 186W, leading to the same compound nucleus 202Pb *, puts in evidence an interesting competition between sub-barrier enhancement, due to the strong couplings related to the 154Sm deformation, and above barrier suppression for the 48Ca induced reaction. The fusion hindrance mechanism has been attributed to the strong quasi-fission component observed in the mass-energy distributions of fission fragments.

Fusion Hindrance and Quasi-Fission in48Ca Induced Reactions

Fusion-fission and fusion-evaporation cross sections have been measured in a large energy range for the 4 8 Ca+ 1 6 8 Er, 1 5 4 Sm reactions. The comparison of the reduced evaporation data for such reactions and for collisions induced by light projectiles leading to the same compound nuclei 2 1 6 Ra* and 2 0 2 Pb* puts in evidence a fusion hindrance, which is particularly large for the first composite system. This fusion suppression is consistent with a noticeable contribution coming from quasi-fission events observed in the mass-energy distribution of fission fragments. The asymmetric shape of the quasi-fission component has been explained in terms of shell effects in the exit channel, favouring the population of closed shell fission fragments. The comparison of 4 8 Ca+ 1 5 4 Sm with preliminary data on 4 8 Ca+ 1 4 4 Sm seems to suggest that the target deformation plays a role in the onset of the quasi-fission mechanism.