Entrance Channel Dependence Research Articles

Charged-particle evaporation from hot164Yb compound nuclei and the role of5He emission

A systematic study of the decay properties of ${}^{164}$Yb compound nuclei with excitation energies of 100--300 MeV was performed. The emission patterns of light charged particles detected in coincidence with evaporation residues produced in ${}^{64}$Ni+${}^{100}$Mo and ${}^{16}$O+${}^{148}$Sm reactions have been measured and compared to statistical model predictions. There is found to be significant disagreement between the experiment and the model calculations in the magnitude of the multiplicities and the peak position in the energy spectra. For deuterons and tritons, significant nonstatistical components were present at all excitation energies. The proton energy spectrum did not exhibit any entrance-channel dependence at an excitation energy of $\ensuremath{\sim}170$ MeV. However for $\ensuremath{\alpha}$ particles, there is an enhancement in the spectrum at the lowest kinetic energies for the more symmetric reaction. It is suggested that the emission and subsequent decay of ${}^{5}$He fragments, can account for a large fraction of the apparent, and often reported, decrease in the Coulomb barrier for $\ensuremath{\alpha}$ particle emission.

Disappearance of entrance channel dependence of fission fragment anisotropies at well-above-barrier energies.

Fission fragment anisotropies have been measured to energies extending well above the barrier for three entrance channels leading to the same compound nucleus $^{248}\mathrm{Cf}$. For the $^{12}\mathrm{C}$ + $^{236}\mathrm{U}$ and $^{16}\mathrm{O}$ + $^{232}\mathrm{Th}$ systems, lying on either side of the Businaro-Gallone critical asymmetry, the mean square angular momentum is matched at an excitation energy of 62 MeV. The anisotropies, although different at lower energies, are in agreement within experimental errors at this excitation energy and thus show no entrance channel dependence.

Entrance-channel effects in quasifission reactions.

The entrance-channel dependence of the distribution of reaction strength has been studied for three systems, namely $^{32}\mathrm{S}$${+}^{182}$W, $^{48}\mathrm{Ti}$${+}^{166}$Er, and $^{60}\mathrm{Ni}$${+}^{154}$Sm, which all lead to the compound system $^{214}\mathrm{Th}$ in complete fusion reactions. The cross sections for elastic/quasielastic scattering, deeply inelastic, and fissionlike processes were measured at beam energies of ${\mathit{E}}_{\mathrm{lab}}$ =166, 177, 222, 260 MeV for $^{32}\mathrm{S}$${+}^{182}$W, ${\mathit{E}}_{\mathrm{lab}}$=220, 240, 270, 298 MeV for $^{48}\mathrm{Ti}$${+}^{166}$Er, and ${\mathit{E}}_{\mathrm{lab}}$=339, 390, 421 MeV for $^{60}\mathrm{Ni}$${+}^{154}$Sm, respectively. The maximum contribution of complete-fusion fission processes to the fissionlike cross section is estimated on the basis of expected angle-mass correlations for such reactions. The results show a strong entrance-channel dependence as predicted by the extra-push model. \textcopyright{} 1996 The American Physical Society.

Preequilibrium fission for low angular momentum

We have compared the fragment anisotropies of 11B + 237Np and 16O + 232Th which both populate the same fissioning system 248Cf. The data clearly show the evidence of the entrance-channel dependence of fission-fragment anisotropies. A new version of the preequilibrium fission model is put forward. In the framework of this model, the discrepancy between the experimental mean-square angular momentum 〈 J 2〉 exp. and the prediction of the coupled-channels model, 〈 J 2〉 theory is reasonably removed.

Search for entrance channel dependence in the population of superdeformed bands in 191Hg

Measurements have been made of the population intensities of superdeformed (SD) bands and of the average entry points and spin distributions of normal and SD states in 191Hg from the ( 36S,5n) and ( 64Ni,3n) reactions. The entry points and spin distributions for 191Hg are the same and so are the SD intensities in the two reactions. Hence, no entrance channel effect is observed in the population of the SD bands in 191Hg, in contrast with data for SD bands in the mass 150 region.

Decay of 160Er* in 16O+144Nd and 64Ni+96Zr fusion reactions.

The population of evaporation residue entry states in the decay of the compound nucleus $^{160}\mathrm{Er}^{\mathrm{*}}$(54 MeV) is investigated in a cross-bombardment employing the reactions $^{16}\mathrm{O}$${+}^{144}$Nd and $^{64}\mathrm{Ni}$${+}^{96}$Zr. Evaporation residue cross sections and entry state \ensuremath{\gamma}-ray fold distributions of the dominant exit channels were obtained for each reaction, using a 4\ensuremath{\pi} \ensuremath{\gamma}-ray detection system. An entrance-channel dependence of the \ensuremath{\gamma}-ray fold distributions of the xn products is observed. This effect is described successfully by the statistical model making use of compound nucleus angular momentum distributions obtained with a fusion model that provides a good description of the bombarding energy dependence of fusion data for both reactions. In accordance with recent findings on the decay of $^{164}\mathrm{Yb}^{\mathrm{*}}$, it is suggested that the observed differences in the population of the dominant exit channels originate from the primary spin distributions rather than a possible dependence of the compound nucleus decay on the formation mode.

Search for entrance channel dependence in the population of superdeformed bands in 191Hg

Search for entrance channel dependence in the population of superdeformed bands in 191Hg

The role of nuclear dynamics in the population of highly- and super-deformed bands

The population of the highly-deformed band in 133Nd has been studied using the reactions 32S + 105Pd and 60Ni + 77Se to form the compound nucleus 137Sm at comparable excitation energy. Experimental results do not show any dependence on the entrance-channel mass asymmetry, in agreement with recent results for 135Nd and at variance with those for the 147,148Gd and 152Dy nuclei. The experimental data on the entrance-channel dependence of the population of highly- and super-deformed bands in the mass regions A ˜130,150and190 are discussed in terms of dynamical effects which occur during the formation process of the compound nucleus as well as in the fission channel.

Effects of prefission neutron emission on the fission fragment angular distributions in heavy-ion-induced fission.

Earlier calculations of fission fragment anisotropies made with the use of the standard saddle point statistical model have been corrected to include the effects of prefission neutron emission. These calculations are compared with the data reported earlier for the fissioning systems formed by projectile-target combinations of $^{10}\mathrm{B}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, and $^{19}\mathrm{F}$ on $^{232}\mathrm{Th}$ and $^{237}\mathrm{Np}$ and $^{9}\mathrm{Be}$ on $^{232}\mathrm{Th}$. It is seen that for these systems the conclusions reached earlier regarding the entrance channel dependence of the fission fragment anisotropy as a function of mass (charge) asymmetry are not affected by the inclusion of corrections due to prefission neutron emission.

Evolution towards equilibration in orbiting interactions.

We have studied the evolution towards mass equilibration in $^{24}\mathrm{Mg}$${+}^{16}$O orbiting interactions. $^{24}\mathrm{Mg}$ beams of energies between 75 and 115 MeV were used to bombard targets of $^{16}\mathrm{O}$. Targetlike particles were detected at forward angles to determine the yields from orbiting interactions. The ratio of the orbiting yields of $^{12}\mathrm{C}$ and $^{16}\mathrm{O}$ exit channels rises continuously with energy from values below unity to above. The data are interpreted as demonstrating a strong entrance channel dependence at low energies, and an evolution towards mass equilibration with increasing energy.

Compound nucleus origin of back-angle yields in the 31P+16O and 35Cl+12C reactions.

The back-angle yields of the oxygen and carbon particles from the {sup 31}P+{sup 16}O reaction have been measured at {ital E}{sub lab}({sup 31}P)=135.6 MeV by using reverse kinematics. Comparison with similar data for the {sup 35}Cl+{sup 12}C reaction forming the same compound nucleus at the same excitation energy and with a very similar spin distribution shows very small entrance-channel dependence of back-angle yields. These results are in contrast to a similar study of the reactions {sup 28}Si+{sup 12}C and {sup 24}Mg+{sup 16}O. It is evident that the oxygen and carbon yields from the {sup 31}P+{sup 16}O and {sup 35}Cl+{sup 12}C reactions have a predominantly compound nucleus origin in contrast to those from the {sup 28}Si+{sup 12}C and {sup 24}Mg+{sup 16}O reactions. Possible connection of this nonequilibrium entrance channel dependence with the presence of nuclear molecular resonances is discussed.

Dissipative reaction mechanisms in medium-mass nuclear collisions

Mass, energy, charge and angular distributions from fully damped reactions of the 40Ca + 40Ca system have been measured at 197 and 231 MeV, in both singles and coincidence. The data display features of compound nucleus fission. Comparison with similar, but asymmetric systems, however, shows an entrance-channel dependence which invalidates a fusion-fission description. A discussion in terms of fission barriers calculated using the finite-range liquid drop model suggests a fast-fission mechanism. Re-analyses of systems studied in the literature are presented in terms of the finite-range liquid drop model, indicating fast fission — a process previously assumed only to be relevant for heavier systems — to be significant in most cases. Furthermore, using the finite-range liquid drop model, we find that the systematic behaviour of the mass distributions from systems where complete fusion-fission is expected to be dominant is well described in terms of the revised locus of the Businaro-Gallone transition from symmetric to asymmetric fission.

Fission fragment angular distributions for the system 19F+232Th.

The fission fragment angular distributions for the system $^{19}\mathrm{F}$${+}^{232}$Th have been measured at several bombarding energies between 94 and 108 MeV. Even though the anisotropy values measured in the present work are considerably smaller than the ones reported by Zhang et al. for the same system at similar energies, they are still anomalous when compared with the predictions of the standard saddle-point statistical model and fit into the systematics of entrance-channel dependence of fission anisotropies reported by us earlier.

Damped reactions in medium mass nuclei

The distributions in mass, energy and angle of fully damped fragments from the A≈80 system produced in the symmetric 40Ca+ 40Ca reaction at 197 and 231 MeV and in the asymmetric 28Si+ 50Cr reaction at 150 MeV have been studied. The mass distributions show a strong entrance channel dependence. A discussion in terms of barriers calculated using the finite-range liquid drop model suggests a fast-fission mechanism previously only ascribed to much heavier systems.

Fluctuation and resonance analysis for 11B+12C, 7Li+16O, and 10B+13C induced reactions.

Some heavy-ion resonance studies have suggested that entrance channel binding energies and, to a lesser extent, moments of inertia are important factors in determining whether or not a given entrance channel will exhibit resonances. To further investigate this question, two channels which lead to the $^{23}\mathrm{Na}$ compound nucleus, $^{10}\mathrm{C}$ and $^{7}\mathrm{O}$, have been studied. When combined with the results from an earlier $^{11}\mathrm{C}$ investigation, an extensive set of data is now available to study the entrance channel dependence of heavy-ion resonance formation in $^{23}\mathrm{Na}$.

Entrance-channel effects on the average angular momentum of compound nuclei produced in fusion reactions.

Haas et al. recently observed a strong entrance-channel dependence in the angular momentum content of compound nuclei produced in fusion reactions. We explain the origin of these effects and analyze the cases of $^{16}$O${+\mathrm{}}^{144}$Nd, $^{37}$Cl${+\mathrm{}}^{123}$Sb, $^{64}$Ni${+\mathrm{}}^{96}$Zr, and $^{80}$Se${+\mathrm{}}^{80}$Se presented by Haas et al.

Entrance and exit channel phenomena ind- andHe3-induced preequilibrium decay

Activation techniques were used to measure more than 30 excitation functions for single and multiple nucleon and/or $\ensuremath{\alpha}$ particle emission for $d$+$^{64,66}\mathrm{Zn}$, $^{89}\mathrm{Y}$ with ${E}_{d}=9\ensuremath{-}26$ MeV and $^{3}\mathrm{He}$+$^{63,65}\mathrm{Cu}$, $^{93}\mathrm{Nb}$ with $E(^{3}\mathrm{He})=10\ensuremath{-}44$ MeV. The excitation functions are generally in agreement with the results of a combined equilibrium and preequilibrium hybrid model calculation applying initial exciton numbers ${n}_{0}=3$ for $d$ and ${n}_{0}=4$ for $^{3}\mathrm{He}$ reactions. The composite system $^{66}\mathrm{Ga}$ has been produced via $d$+$^{64}\mathrm{Zn}$ and $^{3}\mathrm{He}$+$^{63}\mathrm{Cu}$ at excitation energies between 22 and 36 MeV. An entrance channel dependence shows up in the yields for single $p$- and $n$-emission when compared in the double ratio $R=\frac{[\frac{\ensuremath{\sigma}(^{3}\mathrm{He}, p)}{\ensuremath{\sigma}(^{3}\mathrm{He}, n)}]}{[\frac{\ensuremath{\sigma}(d, p)}{\ensuremath{\sigma}(d, n)}]}$. It approaches a value of about 2, indicating enhanced $p$ emission for the $^{3}\mathrm{He}$-induced reaction. This value disagrees with the equilibrium isospin formalism and is best reproduced by initial particle exciton numbers ${n}_{0p}={n}_{0n}=1.5$ for $d$ and ${n}_{0p}=2.5$, ${n}_{0n}=1.5$ for $^{3}\mathrm{He}$ projectiles, indicating conservation of charge asymmetry in the entrance channel. Isomeric ratios have been measured for $^{89}\mathrm{Y}(d, 2n)^{89}\mathrm{Zr}$ and $^{93}\mathrm{Nb}(^{3}\mathrm{He}, xn)^{96\ensuremath{-}x}\mathrm{Tc}$ ($x=1, 2, 3$). Calculations with a full statistical model fail to reproduce $\frac{{\ensuremath{\sigma}}_{g}}{{\ensuremath{\sigma}}_{m}}$ as well as ${\ensuremath{\sigma}}_{g}$ and ${\ensuremath{\sigma}}_{m}$ for reasonable values of the spin cutoff parameter. Inclusion of a preequilibrium decay mode improves the fit, in particular if the angular momentum depletion of the composite system due to preequilibrium decay is increased over that of the equilibrium decay at the same channel energy.NUCLEAR REACTIONS $^{64,66}\mathrm{Zn}$, $^{89}\mathrm{Y}(d, \mathrm{xnypz}\ensuremath{\alpha})$, ${E}_{d}=9\ensuremath{-}26$ MeV, $^{63,65}\mathrm{Cu}$, $^{93}\mathrm{Nb}(^{3}\mathrm{He}, \mathrm{xnypz}\ensuremath{\alpha})$, ${E}_{^{3}\mathrm{He}}=10\ensuremath{-}44$ MeV, $x<~4$, $y<~1$, $z<~2$; measured $\ensuremath{\sigma}(E)$ by activation, enriched targets. Statistical model analysis including preequilibrium decay, deduced reaction mechanism, charge asymmetry conservation, spin depletion.