Measured Evaporation Residue Cross Sections Research Articles

Evaporation residue cross-section measurements for 30Si+142Ce system

Evaporation residue (ER) cross-sections were measured for 30Si+142Ce reaction at laboratory energies in the range of 105 - 132 MeV using the HYbrid Recoil mass Analyzer (HYRA) at IUAC, New Delhi. The transmission efficiency of the HYRA was estimated using the calibration reaction 28Si+142Ce. Sub-barrier fusion enhancement was observed when compared with the one-dimensional barrier penetration model predictions for the present system. To explore the phenomena responsible for sub-barrier fusion enhancement, coupled-channels calculations were performed using the CCFULL code. The effect of couplings and the role of positive Q-value neutron transfer (PQNT) channels on sub-barrier fusion cross-sections were investigated. Coupled-channels calculations were able to reproduce the data by including the coupling in both projectile and target nucleus along with two neutron-transfer channels having positive Q-value. The measured ER cross-sections were compared with the nearby systems to explore the role of deformation of the colliding nuclei.

Effects of isospin and shell on probing nuclear dissipation with survival probabilities of heavy systems

Using the Langevin model, we calculate the survival probability (Peva) of 228U, 234U and 240U as a function of the presaddle dissipation strength (β). It is shown that Peva displays a greater sensitivity to β with increasing the isospin of the U system and with decreasing excitation energy and angular momentum, and that shell effects can significantly enhance the sensitivity. Furthermore, we compare calculated and measured evaporation residue cross sections of heavy compound nuclei 224Th produced in the 16O + 208Pb reaction and find that the best-fit value of β required to satisfactorily describe the measured data has an apparent drop as shell effects are taken into account in the calculation compared to that neglecting the effects, clearly indicating a strong influence of shell effects on the precise determination of β. These results demonstrate that to more stringently constrain the strength of presaddle dissipation with survival probabilities of heavy nuclei, on the experimental side, it is best to produce high-isospin compound systems with low energy and low spin; theoretically, it is important to account for the shell effects in the calculations.

Evaporation residue cross section measurements for the Si30+Yb176 reaction

Evaporation residue (ER) cross sections are measured for the reaction $^{30}\mathrm{Si}+^{176}\mathrm{Yb}$, which forms the compound nucleus $^{206}\mathrm{Po}^{*}$, over the excitation energy range from 47.68 to 113.73 MeV. Dependence of noncompound nuclear reaction on entrance channel parameters such as charge product (${Z}_{1}{Z}_{2}$), mass asymmetry ($\ensuremath{\alpha}$), deformation (${\ensuremath{\beta}}_{2}$) of the target, and isospin asymmetry ($\mathrm{\ensuremath{\Delta}}\frac{N}{Z}$) is explored. To analyze the experimental data the coupled-channels and the statistical model calculations are used. The measured ER cross sections are compared with the system,$^{12}\mathrm{C}+^{194}\mathrm{Pt}$ forming the same compound nucleus and also with $^{28}\mathrm{Si}+^{176}\mathrm{Yb}$, forming $^{204}\mathrm{Po}^{*}$ in the neighborhood. Observed suppression in the evaporation residue cross sections at higher energies may be attributed to the presence of the noncompound nuclear process.

Entrance channel dependent hot fusion reactions for superheavy element synthesis

We have studied the compound nucleus probability, the survival probability, and the evaporation residue cross sections for 6645 projectile-target combinations to synthesize the superheavy nuclei with atomic numbers $Z=110$--126 in the light of the entrance channel effects such as mass asymmetry, charge asymmetry, isospin asymmetry, non-compound nuclear fission probability, and Businaro-Gallone mass asymmetry. The role of quasifission in every reaction has also been considered. Decay chains of various superheavy nuclei produced from the reactions are portrayed as well. The most striking results are that the measured evaporation residue cross sections are of the order of picobarns, whereas the proposed reactions can yield up to microbarn cross sections for the superheavy nuclei with $110\ensuremath{\le}Z\ensuremath{\le}126$. We suggest future experiments may utilize these hot fusion reactions to synthesize new elements ($Zg118$) or to study the properties of the known superheavy nuclei in greater detail. In particular, a proposed reaction containing both projectiles and targets that are naturally abundant, for example, $_{36}^{82}\mathrm{Kr}+_{90}^{232}\mathrm{Th}\ensuremath{\rightarrow}^{314}126+0n$, yields evaporation residue cross sections as high as 31 nb. The presently available experimental setup can test the prediction with ease to make a dream come true by putting a footstep into the eighth row of the periodic table.

Fusion studies in Cl35,37+Ta181 reactions via evaporation residue cross section measurements

The fusion evaporation residue (ER) excitation function has been measured for $^{35,37}\mathrm{Cl}+^{181}\mathrm{Ta}$ reactions at energies above the Coulomb barrier. The measurements were performed using the HYbrid Recoil mass Analyzer at IUAC, New Delhi. Comparable ER cross sections have been observed in both reactions and there is no isotopic dependence. Measured ER cross sections were compared with theoretical calculations employing the dinuclear system model at projectile and target nuclei interaction and statistical model for the deexcitation of the formed compound nucleus. Larger ER cross sections at the complete deexcitation cascade of the formed compound nucleus are noticed in both reactions at higher excitation energies $({E}^{*}g80 \mathrm{MeV})$ over the calculated results. Fusion probability varies from $95%$ to $40%$ in the excitation energy range of the study. No appreciable difference in the fusion probability is noticed in the two reactions. Comparison of our results with other reactions populating $^{216}\mathrm{Th}$ shows a very strong entrance channel dependence.

Search for stabilizing effects of the Z=82 shell closure against fission

Presence of closed proton and/or neutron shells causes deviation from macroscopic properties of nuclei which are understood in terms of the liquid drop model. It is important to investigate experimentally the stabilizing effects of shell closure, if any, against fission. This work aims to investigate probable effects of proton shell ($Z = 82$) closure in the compound nucleus, in enhancing survival probability of the evaporation residues formed in heavy ion-induced fusion-fission reactions. Evaporation residue cross sections have been measured for the reactions $^{19}$F+$^{180}$Hf, $^{19}$F+$^{181}$Ta and $^{19}$F+$^{182}$W from $\simeq9\%$ below to $\simeq42\%$ above the Coulomb barrier, leading to formation of compound nuclei with same number of neutrons ($N = 118$) but different number of protons across $Z = 82$. Measured excitation functions have been compared with statistical model calculation, in which reduced dissipation coefficient is the only adjustable parameter. Evaporation residue cross section, normalized by capture cross section, is found to decrease gradually with increasing fissility of the compound nucleus. Measured evaporation residue cross sections require inclusion of nuclear viscosity in the model calculations. Reduced dissipation coefficient in the range of 1\textendash3 $\times$ $10^{21}$ s$^{-1}$ reproduces the data quite well. No abrupt enhancement of evaporation residue cross sections has been observed in the reaction forming compound nucleus with $Z = 82$. Thus, this work does not find enhanced stabilizing effects of $Z = 82$ shell closure against fission in the compound nucleus. One may attempt to measure cross sections of individual exit channels for further confirmation of our observation.

Survival-mediated capture and fusion cross sections for heavy-element synthesis

The cross section for producing a heavy evaporation residue ${\ensuremath{\sigma}}_{\mathrm{EVR}}$ in a fusion reaction can be written as a product of three nonseparable factors, i.e., the capture cross section, the fusion probability ${P}_{\mathrm{CN}}$, and the survival probability ${W}_{\text{sur}}$. Each of these factors is dependent on the spin. However, one must remember that the ${W}_{\text{sur}}$ term is zero or very small for higher spin values, thus effectively limiting the capture and fusion terms. For a series of $\ensuremath{\sim}287$ reactions leading to heavy evaporation residues with ${Z}_{\text{CN}}\ensuremath{\le}110$, we point out the implications of this fact for capture cross sections for heavy element formation reactions. From a comparison of calculated and measured evaporation residue cross sections we deduce values of the fusion probability ${P}_{\text{CN}}$ for some of these reactions.

Fusion measurements for the O+ reaction and search for neutron shell closure effects

Evaporation residue (ER) and fission cross sections were measured for the reaction O+ forming the compound nucleus 212Rn. Fission fragment angular distributions and anisotropies are consistent with the saddle point model predictions. Measured ER cross sections were fitted with statistical model calculations assuming shell-corrected free energy fission barrier height. The non-zero dissipation strength required to fit the ER cross section supports the dissipative nature of fission dynamics in heavy ion fusion. In the present study shell closure effects are not observed in the dissipation strength.

Evaporation residue excitation function measurement for the16O+194Ptreaction

Evaporation residue cross sections for the reaction ${}^{16}\text{O}+{}^{194}\text{Pt}$ were measured at beam energies in the range 75.4--103.1 MeV using the gas-filled separator Hybrid Recoil Mass Analyzer at the Inter University Accelerator Centre, New Delhi. The transmission efficiency of the separator was obtained using the calibration system ${}^{16}\text{O}+{}^{184}\text{W}$ and a Monte Carlo simulation code. Measured evaporation residue cross sections were fitted with statistical model calculations using Kramers' formula. The present measurement provides further evidence for the onset of dissipative forces in fission in the mass $\ensuremath{\sim}$200 region.

Extraction of probability of compound-nucleus formation

We propose an empirical barrier distribution for a unified description of heavy-ion fusion and quasi-elastic scattering based on the Skyrme energy-density functional approach. The capture (fusion) cross sections and the quasi-elastic scattering cross sections of a large number of reactions can be well reproduced. Based on these systematic studies and the HIVAP calculations for the survival probability, the probability P CN of the compound-nucleus formation is extracted from the measured evaporation residue cross sections for “cold” and “hot” fusion reactions.

Heavy-ion fusion and scattering with Skyrme energy density functional

In our recent studies, an empirical barrier distribution was proposed for a unified description of the fusion cross sections of light and medium-heavy fusion systems, the capture cross sections of the reactions leading to superheavy nuclei, and the large-angle quasi-elastic scattering cross sections based on the Skyrme energy-density functional approach. In this paper, we first give a brief review of these results. Then, by examining the barrier distributions in detail, we find that the fusion cross sections depend more strongly on the shape of the left side of the barrier distribution while the quasi-elastic scattering cross sections depend more strongly on the right side. Furthermore, by combining these studies and the HIVAP calculations for the survival probability, the formation probability of the compound nucleus is deduced from the measured evaporation residue cross sections for “cold” and “hot” fusion reactions.

Fission hindrance studies inPb200: Evaporation residue cross section and spin distribution measurements

Evaporation residue cross sections and spin distributions have been measured for $^{200}\mathrm{Pb}$ compound nucleus formed in $^{16}\mathrm{O}+^{184}\mathrm{W}$ reaction at the laboratory beam energies of 84, 92, 100, 108, 116, and 120 MeV. The evaporation residues have been selected using the recoil mass spectrometer, HIRA and detected using a 2D position sensitive silicon detector. The evaporation residue spin distributions have been measured by detecting gamma rays with 14 element BGO multiplicity filter. Measured evaporation residue cross sections and spin distributions are compared with the values predicted by a standard statistical model code. Comparison shows that, in the energy region studied, the nuclear viscosity parameter $\ensuremath{\gamma}=3$ is required to explain total evaporation residue cross sections and evaporation residue spin distributions.

Mass distribution studies in the 19F + 197Au reaction

Mass distribution and evaporation residue measurements have been carried out in the reaction 19F + 197Au using the recoil catcher technique followed by off-line γ-ray spectrometry. The random neck rupture model (RNRM) has been used to compute the variance of the mass distribution ( σ2A) and the average kinetic energy ( ¯) of the fission fragments for the present system. The results of model calculations have been found to be in good agreement with the experimental observations. Measured evaporation residue cross-sections have been compared with the statistical model calculations.

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.

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.