Measured Evaporation Residue Research Articles

Sub-barrier fusion induced by neutron-rich radioactive 132Sn

Evaporation residue cross-sections measured with short-lived 132Sn on 64Ni at energies near and below the Coulomb barrier were found to be enhanced as compared to those measured with stable Sn isotopes on 64Ni. Subsequent measurements of fission following fusion of 132Sn with 64Ni and extending the measurement of evaporation residues to higher energies were carried out.

Fission fragment angular distribution for theF19+Au197fusion-fission reaction at near-barrier energies

Angular distribution of fission fragments have been measured for $^{19}\mathrm{F}+^{197}\mathrm{Au}$ reaction at bombarding energies from 91 to 110 MeV. Fission fragment angular distributions have been calculated by transition state model with the transmission coefficients obtained using the coupled-channels theory. The calculated angular anisotropies are in good agreement with the experimental anisotropies. The experimental fission cross sections have also been reproduced on the basis of the coupled-channels theory. The results of angular distribution measurement do not show any significant contribution from quasifission as was reported in the literature based on the measurement of evaporation residues and mass distribution.

Effect of Closed Shell Structure on Heavy-Ion Fusion Reactions

The effect of the nuclear shell structure on the heavy-ion fusion reaction was investigated for the reaction systems 8 2 Se + 1 3 8 Ba, 8 2 Se + 1 3 4 Ba, 1 6 O + 2 0 4 Pb, 8 6 Kr + 1 3 4 Ba, 8 6 Kr + 1 3 8 Ba and 8 2 Se + n a t . Ce. Evaporation residues for these fusion reactions were measured using a recoil mass separator (JAERI-RMS) near Coulomb barrier region. The measured evaporation residue cross sections for the reactions 8 2 Se + 1 3 8 Ba and 8 6 Kr + 1 3 8 Ba were two orders of magnitude and one order of magnitude larger than those for the reactions 8 2 Se + 1 3 4 Ba and 8 6 Kr + 1 3 4 Ba, respectively, at the excitation energy region of 10 ∼ 30 MeV. The evaporation residue cross sections were compared with those of the other reaction systems that make the same or similar compound nuclei as the present reaction systems. It was found that the evaporation residue cross sections correlate strongly with the sum of the shell energies for both projectile and target nuclei, i.e. the evaporation residue cross sections increase as the sum of the shell energy decreases.

Effect of nuclear shell structure on fusion reaction

The dependence of the fusion reaction on the nuclear shell structure was investigated for the two reaction systems 82Se + 138Ba and 82Se + 134Ba, where the nucleus 138Ba has a closed neutron shell N=82, while the nucleus 134Ba has a neutron number 78. Evaporation residues for these fusion reactions were measured near the Coulomb barrier region. The measured evaporation residue cross sections for the reaction system 82Se + 138Ba were two orders of magnitude larger than those for the reaction system 82Se + 134Ba in the excitation energy region of 20–30 MeV. The evaporation residue cross sections were compared with those of the other reaction systems that produce the same compound nucleus as the present systems. It was found that the fusion reaction 82Se + 138Ba occurs without hindrance, while that of 82Se + 134Ba is considerably hindered, as commonly observed in the massive reaction system with the charge product ZpZt>1800 of projectile and target. This suggests the importance of the shell closure N=82 in the heavy-ion fusion reaction.

Effect of shell structure in the fusion reactions82Se+134Baand82Se+138Ba

The dependence of fusion on the nuclear shell structure was investigated for the two reaction systems ${}^{82}\mathrm{Se}{+}^{138}\mathrm{Ba}$ and ${}^{82}\mathrm{Se}{+}^{134}\mathrm{Ba},$ where the nucleus ${}^{138}\mathrm{Ba}$ has a closed neutron shell $N=82$ while the nucleus ${}^{134}\mathrm{Ba}$ has a neutron number $N=78,$ four neutrons less than the closed shell. Evaporation residues for these reaction systems were measured in the vicinity of the Coulomb barrier. The measured evaporation residue cross sections of $\mathrm{xn}$ and $\mathrm{pxn}$ channels for the reaction system ${}^{82}\mathrm{Se}{+}^{138}\mathrm{Ba}$ were considerably larger than those for the reaction system ${}^{82}\mathrm{Se}{+}^{134}\mathrm{Ba},$ almost 100 times larger at the excitation energy ${(E}_{\mathrm{ex}})$ region of 20--30 MeV. The fusion probabilities for these reaction systems were obtained from the evaporation residue cross sections with the aid of calculated survival probability and compared with those of the other reaction systems that make the same compound nucleus as the present systems. It was found that the fusion reaction ${}^{82}\mathrm{Se}{+}^{138}\mathrm{Ba}$ occurs without hindrance, while that of ${}^{82}\mathrm{Se}{+}^{134}\mathrm{Ba}$ is considerably hindered as commonly seen in the massive reaction system with the charge product ${Z}_{\mathrm{p}}{Z}_{\mathrm{t}}>~1800$ of projectile and target. The present result suggests that the neutron shell closure $N=82$ promotes fusion.

Charge and mass distribution in7Li induced fission of232Th

SummaryFormation cross sections of about forty fission products have been determined using recoil catcher technique followed by off line gamma-ray spectrometry in7Li induced fission of232Th atElab= 41.9, 36.6 and 31.4 MeV. The measured data have been used to deduce charge and mass distributions. Mass distribution is found to be asymmetric at all the three energies. Cross sections of evaporation residues formed in both transfer reactions (232,233,234Pa) as well as in complete fusion (234Np), have also been measured. The measured evaporation residue cross sections and the decay probabilities of target like nuclei (233,234,235Pa) formed in the various transfer reactions, as calculated by PACE2, have been used to estimate the transfer induced fission cross sections. The data indicated that the magnitude of transfer induced fission is very small.

Mass distribution in11Binduced fission of232Th

Formation cross sections of several fission products have been determined using recoil catcher technique followed by \ensuremath{\gamma}-ray spectrometry in ${}^{11}\mathrm{B}$ induced fission of ${}^{232}\mathrm{Th}$ at ${E}_{\mathrm{lab}}=72,$ 60, and 55 MeV. The data show significant admixture of fission from compound nucleus formed by complete fusion as well as targetlike nuclei formed by transfer reaction. Mass distributions for both the fissioning systems have been obtained using the systematics of charge distribution in low and medium energy fission. The mass distribution for complete fusion fission is broad, Gaussian whereas it is asymmetric for transfer induced fission. The evaporation residue cross sections of targetlike nuclei formed in ${}^{232}{\mathrm{T}\mathrm{h}(}^{11}{\mathrm{B},\mathrm{}}^{10}{\mathrm{B}\mathrm{e})}^{233}\mathrm{Pa}$ reaction were also measured. The measured evaporation residue cross sections and the decay probabilities of targetlike nucleus ${}^{233}\mathrm{Pa},$ calculated by the PACE2 code have been used to estimate the proton transfer fission cross sections which were found to be negligible compared to the total transfer cross section for all the projectile energies used. The transfer fission cross section is dominated by alpha transfer fission as inferred by measured forward to backward ratios for several fission products as well as ${Q}_{\mathrm{gg}}$ systematics for the probable transfer reactions. The proportion of transfer fission cross section to the total fission cross section was 15, 17, and 22 % at projectile energies of 72, 60, and 55 MeV, respectively.

Inverstigation of [formula omitted] and [formula omitted] reactions at low energies

The cross sections for the characteristic γ-rays of the residual nuclei following 6 Li+ 16O and 7 Li+ 16O reactions have been measured at low energies with HPGe detectors. The fusion cross sections obtained from these γ-ray cross sections for both the systems are found to be in excellent agreement with the total reaction cross sections and this indicates that there is no inhibition of fusion cross sections for these systems at low energies, which is in contradiction to the results obtained from the measurement of evaporation residues and light particles. These cross sections also show good agreement with the IWBC (Incoming Wave Boundary Condition) model and the 1-D BPM (One-Dimensional Barrier Penetration Model) calculations. The critical angular momenta ( l cr) dediced from the fusion cross sections are also found to have an energy dependence similar to other Li-induced reactions.

Fusion cross sections for 6Li+ 12C and 6Li+ 13C reactions at low energies

The partial cross sections for the 6Li+ 12C and 6Li+ 13C reactions below and above the Coulomb barrier energy have been determined from the cross sections of the characteristic γ-rays measured with HPGe detectors and using the branching factor σ γ σ ch obtained from the statistical model calculations. The fusion cross sections obtained from the sum of these partial cross sections are found to be equal to the total reaction cross sections upto an energy well above the Coulomb barrier energy and there appears to be no evidence for limitation of fusion cross sections for these systems at such energies contrary to the evaporation residue measurements. The measured cross sections are also found to agree nicely with the IWBC (Incoming Wave Boundary Condition) and one-dimensional BPM (Barrier Penetration Model) calculation.

Rapid increase in prescission GDR γ-ray emission with energy

A rapid increase in the emission of prescission giant dipole resonance (GDR) γ-rays with bombarding energy is observed in excited Th and Cf nuclei formed in the reactions 16O+ 208Pb and 32S+ natW, 208Pb. This increase begins around E exc = 40 MeV for the 16O+ 208Pb reaction and E exc = 70 MeV for the 32S-induced reactions. The excess γ-ray yield above these thresholds cannot be described within the standard statistical model. Statistical model calculations which include a temperature dependent nuclear dissipation are able to reproduce simultaneously the observed GDR γ-ray spectra and recently measured evaporation residue cross sections.

Absence of isotopic dependence in the sub-barrier fusion of 48Ti+58,60,64Ni systems.

Fusion process in the near and sub-barrier region has been investigated for the systems $^{48}\mathrm{Ti}$${+}^{58,60,64}$Ni using the heavy-ion reaction analyzer (HIRA). Fusion excitation functions and the mean angular momenta are obtained from the measured evaporation residue cross sections. Significant enhancements both in the cross section and mean angular momentum data are seen with respect to the predictions of the one-dimensional barrier penetration model. Simplified coupled channel calculations incorporating linear coupling to the inelastic channels (lowest ${2}^{+}$ and ${3}^{\mathrm{\ensuremath{-}}}$ states of both the projectile and the target) are not able to explain the observed enhancements. A systematic analysis of the data indicates lack of isotopic dependence in the sub-barrier fusion for these three systems. \textcopyright{} 1996 The American Physical Society.

A compact velocity filter for evaporation residue measurements

A velocity filter with crossed electric and magnetic fields has been built and used to measure evaporation residue angular distributions for a series of reactions using 16O, 19F and 28Si projectiles. The filter is compact, ∼ 20 cm long, and uses permanent magnets. The spatial separation between the residues and the intense elastic scattering is such that the latter can be physically blocked without affecting the observation of residues; this allows measurements to be made at angles as close as 0.5° to the beam direction. The measured efficiency of the filter is very close to 100%.

Large-scale collective motion in heavy-ion collisions

Heavy-ion fusion and deep inelastic reactions have been studied for symmetric systems in a classical dynamical model with deformation and necking as the collective shape coordinates. The calculated fusion excitation functions (for compound nucleus formation) are in good agreement with the experimental results from the evaporation residue measurements. It is observed that “nuclear molecules” are formed for not too heavy systems. The calculated reaction time for collisions of very heavy ions like 238U + 238U is found to be ~10 −21 sec only and thus the width of the positron spectra observed in these reactions can not be explained in the light of quantum electrodynamics. The extra-extra push energies for fusion of heavy nuclei have also been studied.

Dynamical hindrance to compound-nucleus formation in heavy-ion reactions

The model of a sharp-surfaced drop with one-body dissipation is used to map out the extra-extra push energy E xx( A 1, A 2) (i.e. the excess bombarding energy above the Coulomb barrier required to form a compound nucleus) in its dependence on the mass numbers A 1, A 2; of the colliding nuclei. The calculated two-dimensional function E xx( A 1, A 2) may be scaled approximately to a one-dimensional dependence on a mean fissility parameter x m = 2 3 x + 1 3 x e , where x is the total system's fissility and x e is the entrance channel “effective” fissility. Apart from indications of possible nuclear structure effects, the theoretical predictions seem consistent with experimental data on evaporation residue measurements for systems with A 1 ⋍ A 2 . For asymmetric systems the relation to experiment is unclear. A by-product is the calculation of nucleus-nucleus sticking times, which are generally found to be in the range around a few times 10 −21 s, but could be longer in special cases.

Measurements of evaporation residues to investigate lower limiting angular momenta in fusion reactions

Evaporation residues from fusion reactions have been measured in the 16O bombardment of 12C and 16O at E lab = 70–130 MeV. Atomic and mass numbers of the measured reaction products were well identified by a TOF system combined with an ionization chamber. The experimental cross sections of evaporation residues are compared with statistical model calculations with and without the low-angular momentum cutoff predicted by the time-dependent Hartree-Fock theory. No evidence for the low- l cutoff has been found in the present systems. The possible importance of incomplete fusion is discussed.

Measurements of evaporation residues and fission for 40Ar + 110Pd

Fusion cross sections for the reaction 40Ar + 110Pd have been measured in the bombarding energy range 164–262 MeV. Evaporation residues and fission fragments have been detected by ΔE- E telescopes. We compare the fusion cross sections to various theoretical models with special attention to the high-energy data. An analysis is also given for entrance-channel spin zones for evaporation residues and fission.

On the fusion dynamics of40Ar+40Ca — Fusion-fission and fusion-evaporation

Evaporation residue measurements have been made for the40Ar+40Ca system at high incident energies,E Lab=161, 190, 236 and 273 MeV. A dynamical description of the fusion process is presented, where in a trajectory model both rotational and neck degrees-of-freedom are included. Using this model maximuml-values for fusion are determined and at high incident energies are found to be significantly lower than those found when using the more usual one-dimensional effective nucleus-nucleus potential. The fusion cross sections minus the fission component yields evaporation residue cross sections that are in good agreement with the data. Also, mass distributions of the evaporation residues compare well with statistical-decay model predictions. A comparison is made with the similarly mass-symmetric40Ca+40Ca system.

Fusion and deeply inelastic collisions ofNe20withAl27

Fusion cross sections were measured for $^{20}\mathrm{Ne}$ bombardments of $^{27}\mathrm{Al}$ from 32 to 151 MeV by detecting the evaporation residues from the compound nucleus decay with gas counter telescopes. Charge, energy, and angular distributions of projectilelike fragments from the deeply inelastic collisions were also measured at 151 MeV. The fusion excitation function was analyzed through the barrier penetration model and compared to the predictions of the statistical yrast line model. The critical angular momentum and the grazing angular momentum deduced from the fusion and elastic scattering data were used in the interpretation of the kinetic energies of the deeply inelastic fragments in terms of a rotating dinuclear model.NUCLEAR REACTIONS $^{20}\mathrm{Ne}$+$^{27}\mathrm{Al}$, $E=32.5 \mathrm{to} 151$ MeV; measured evaporation residue $\ensuremath{\sigma}(E, \ensuremath{\theta})$; deduced $\ensuremath{\sigma}(E)$ total fusion excitation function; barrier and critical parameters. $E=55.7, 63, 125, 151$ MeV; measured elastic scattering $\ensuremath{\sigma}(\ensuremath{\theta})$; deduced optical potentials; reaction cross sections ${\ensuremath{\sigma}}_{R}$. $E=151$ MeV; measured charge, energy, and angular distributions for $4\ensuremath{\le}Z\ensuremath{\le}12$ fragments. Natural target. Gas ionization and Si detectors.

Fusion excitation function ofNi58+Ni62at 215—275 MeV

Experimental evaporation residue excitation functions are presented for $^{58}\mathrm{Ni}$ projectiles on $^{62}\mathrm{Ni}$ targets. These data are analyzed for fusion radii and for the nucleus-nucleus potential at the fusion barrier. The results are compared with predictions of three potential models.NUCLEAR REACTIONS Measured evaporation residue excitation function $^{58}\mathrm{Ni}$+$^{62}\mathrm{Ni}$, ${E}_{\mathrm{lab}} 215\ensuremath{-}275$ MeV. Deduced fusion barrier and radius.

Comment on fusion cross sections in the40Ar+40Ca and40Ca+40Ca systems

Cross sections for the production of evaporation residues in a fusion reaction have been extracted from measurements of cross sections for elastic, quasi-elastic and deep inelastic scattering of40Ar projectiles on40Ca target nuclei at incident energies ELAB=191, 236 and 272 MeV. The extracted data are compared with the predictions of a one-body window-friction model including a neck configuration in the interaction phase between the projectile and target nuclei. Furthermore, the results for the40Ar+40Ca system are compared to those obtained in the similarly mass-symmetric system40Ca+40Ca where directly measured evaporation residue data are available.