Entrance Channel Dependence Research Articles

Entrance channel dependence of quasi fission in reactions leading to 206Po compound nucleus

The evaporation residue (ER) cross sections for the reaction F19+187Re→206Po⁎ are measured, in the excitation energy range of 86.1 to 118.4 MeV. The measured cross sections are compared with that of 30Si + 176Yb reaction populating the same compound nucleus. Theoretical calculations are performed using the coupled - channels calculations for the capture cross sections and statistical model calculations for the ER cross sections. The dependence of quasi fission on entrance channel parameters such as charge product (Z1Z2), mass asymmetry (α), target deformation (β2) and effective fissility (χeff) are studied.

Disentangling complete and incomplete fusion events in 12C + 169Tm reaction by spin-distribution measurements

The spin-distributions of different reaction products populated via x n and α/2αx n channels in + system have been measured at E lab ≈ 6, 7 and 7.5 MeV A−1 to disentangle complete and incomplete fusion events. Particle (p, α)-γ-coincidences were recorded to identify evaporation residues channel-by-channel. The spin-distributions of fusion evaporation and direct-α-emitting channels are found to be distinct corroborating the involvement of entirely different reaction dynamics in their production, respectively termed as complete (CF) and incomplete fusion (ICF). The values of mean input angular momenta, obtained from the analysis of the spin-distributions, involved in ICF-α/2α xn channels are estimated to be larger than that observed in CF-xn/α xn channels. For ICF-α/2αxn channels, the multiplicity of fast-α particle emission increases with the magnitude of input angular momentum imparted into the system. The CF residues display a population of broad spin range while the ICF residues are confined to a narrow spin range generally localized in the higher spin states. Findings of the present work conclusively demonstrate the possibility to populate high spin states in final reaction products using ICF, which are otherwise not accessible. Further, an attempt has been made to study the variation of input angular momentum with the choice of different entrance-channel parameters. It has been observed that the value of input angular momentum involved in ICF-α/2α xn channels increases with the deformation and entrance-channel mass-asymmetry parameter, indicating strong entrance-channel dependence of ICF dynamics.

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.

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 160Er*(54 MeV) is investigated in a cross-bombardment employing the reactions 160 + 144Nd and 64Ni + 96Zr. Evaporation residue cross sections and entry state 7-ray fold distributions of the dominant exit channels were obtained for each reaction, using a 4π 7-ray detection system. An entrance-channel dependence of the 7-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 164Yb*, 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.

Dependence of the prompt fission γ-ray spectrum on the entrance channel of compound nucleus: Spontaneous vs. neutron-induced fission

Prompt γ-ray spectra were measured for the spontaneous fission of 240,242Pu and the neutron-induced fission of 239,241Pu with incident neutron energies ranging from thermal to about 100 keV. Measurements were made using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments using a parallel-plate avalanche counter. The unfolded prompt fission γ-ray energy spectra can be reproduced reasonably well by Monte Carlo Hauser–Feshbach statistical model for the neutron-induced fission channel but not for the spontaneous fission channel. However, this entrance-channel dependence of the prompt fission γ-ray emission can be described qualitatively by the model due to the very different fission-fragment mass distributions and a lower average fragment spin for spontaneous fission. The description of measurements and the discussion of results under the framework of a Monte Carlo Hauser–Feshbach statistical approach are presented.

Entrance channel systematics of pre-scission neutron multiplicities

Statistical model analysis has been performed for the available neutron multiplicity ( \(\nu_{pre}\) data in the literature. Larger \(\nu_{pre}\) values for more symmetric reactions have been observed in comparison with asymmetric reactions forming the same compound nucleus, in most cases. A reverse trend has also been noticed in a few cases. A systematic entrance channel dependence of fission timescale is brought out in this work. Fission timescales calculated using the experimental \(\nu_{pre}\) values fall into two distinct groups according to the entrance channel mass asymmetry of the reaction with respect to the Businaro-Gallone critical mass asymmetry. The difference in the delay between these two groups ranges between 20 and 100zs, which is larger than that reported in some cases.

Fission fragment mass distribution studies in [formula omitted] reaction

Fission fragment mass–angle and mass ratio distributions have been measured for the 30Si+180Hf reaction in the beam energy range 128–148 MeV. Quasifission signature is observed in this reaction, forming the compound system 210Rn. The results are compared with a very asymmetric reaction 16O+194Pt, forming the same compound nucleus. Calculations assuming saddle point, scission point and DNS models have been performed to interpret the experimental results. The results strongly suggest the entrance channel dependence of quasifission in heavy ion collisions.

Entrance channel effect with stable and radioactive beams using dynamical cluster decay model

Abstract The decay of hot and rotating 172Yb*, formed in two entrance channels 124Sn + 48Ca and 132Sn + 40Ca, is studied using the dynamical cluster-decay model. The effect of entrance channel, deformations (up to β 2 ), barrier modification and fusion enhancement are addressed. The decay pattern of compound system, formed in different channels at comparable energy around the barrier, shows change in magnitude with structure remains almost same. There is an increase in the fusion probability with decrease in barrier modification, which leads to fusion enhancement at low energies. The higher l values are contributing for 132Sn + 40Ca channel at lower energies as compare to 124Sn + 48Ca. It is inferred that with the use of stable and radioactive beam, forming same compound nucleus, the entrance channel dependence changes with the excitation energy.

Nuclear temperatures from the evaporation fragment spectra and possible lifetime effect

The temperature of a compound nucleus should decrease for the higher values of its spin angular momentum and so the spectra of the heavier evaporation fragments emitted from the compound nucleus should have progressively steeper slopes. To test this idea, we formed the same compound nucleus 105Ag at the same excitation energy (EX=76 MeV) and with very similar spin distribution by 16O+89Y and 12C+93Nb reactions and studied back-angle alpha to carbon particle emissions. The compound nucleus character of the reaction was established from the angular distribution of the emitted fragments and the lack of any entrance channel dependence of the angle-integrated yields of the fragments. It was found that the temperatures of the ensembles of the residual nuclei as obtained from the slopes of alpha, lithium, boron, carbon spectra remain about 3 MeV in most cases and higher (about 4.2 MeV) for lithium emission, whereas the statistical model codes predict decrease of the corresponding temperatures from 3 MeV (for alpha) to 1.65 MeV (for carbon). The result could not be understood by adjusting the parameters of the statistical models and might imply the effect of the lifetime of the exit channel fragment plus residual dinuclear system.

Protactinium neutron-induced fission up to 200 MeV

The theoretical evaluation of 230-233Pa(n,F) cross sections is based on direct data, 230-234Pa fission probabilities and ratios of fission probabilities in first-chance and emissive fission domains, surrogate for neutroninduced fission. First chance fission cross sections trends of Pa are based on consistent description of 232Th(n,F), 232Th(n,2n) and 238U(n,F), 238U(n,xn) data, supported by the ratio surrogate data by Burke et al., 2006, for the 237U(n,F) reaction. Ratio surrogate data on fission probabilities of 232Th(6 Li,4 He)234Pa and 232 Th(6 Li,d)236U by Nayak et al., 2008, support the predicted 233Pa(n, F) cross section at En=11.5-16.5 MeV. The predicted trends of 230-232Pa(n, F) cross section up to En=20 MeV, are consistent with fissilities of Pa nuclides, extracted by 232Th(p,F) (Isaev et al., 2008) and 232Th(p,3n) (Morgenstern et al., 2008) data analysis. The excitation energy and nucleon composition dependence of the transition from asymmetric to symmetric scission for fission observables of Pa nuclei is defined by analysis of p-induced fission of 232Th at Ep=1-200 MeV. Predominantly symmetric fission in 232Th(p,F) at En( p)=200 MeV as revealed by experimental branching ratios (Dujvestijn et al., 1999) is reproduced. Steep transition from asymmetric to symmetric fission with increase of nucleon incident energy is due to fission of neutron-deficient Pa (A≤229) nuclei. A structure of the potential energy surface (a drop of f f symmetric and asymmetric fission barriers difierence (EfSYM - EfASYM) from ~3.5 MeV to ~1 MeV) of N-deficient Pa nuclides (A≤226) and available phase space at outer fission saddles, are shown to be responsible for the sharp increase with En( p) of the symmetric fission component contribution for 232Th(p,F) and 230-233 Pa(n, F) reactions. That is a strong evidence of emissive fission nature of moderately excited Pa nuclides, reliably quantified only up to En( p)~20(30) MeV. Predicted fission cross section of 232Pa(n,F) coincides with that of 232Th(p,F) at En(p)≥80 MeV, that means that entrance channel dependence of fission cross section with increase of nucleon incident energy diminishes.

Influence of Ground State Spin of Projectile-Target on Fission Anisotropies

Fission fragment anisotropies have been investigated for various systems produced in heavy-ion reactions at near and sub-barrier energies. In particular, special attention has been paid to the entrance channel dependence of fragment angular anisotropies. The results of our analysis of the fragment angular anisotropies induced by boron, carbon, and oxygen ions on Thorium and Neptunium targets as well as Fluorine ions on Neptunium target indicate strong dependence of fragment anisotropies on the channel spin, in consistence with the predication of the pre-equilibrium model.

Fission fragment anisotropies for theC13+U235system at near-Coulomb barrier energies

The fission fragment angular distribution measurements for the {sup 13}C+{sup 235}U system have been carried out to study the entrance channel dependence and spin dependent effects in the fission fragment anisotropies. This particular combination of target and projectile leads to {sup 248}Cf compound nucleus that has been studied earlier using three other entrance channels viz. {sup 11}B+{sup 237}Np, {sup 12}C+{sup 236}U, and {sup 16}O+{sup 232}Th. A comparison of the measured anisotropy data for all the four systems, with the statistical saddle-point model and pre-equilibrium fission (PEF) model, brings out a clear signature of entrance channel dependence in fission anisotropy values, characteristic of the PEF model.

Nuclear Orbiting At Low Energy Nuclear Collisions

The emissions of projectile-like fragments from light and medium heavy ion reactions have been discussed. In the case of certain nuclear reactions, such as, 24Mg+16O, 28Si+12C etc. among light heavy ion systems and 16O+89Y, 16O+93Nb etc. among medium heavy ion systems, both a 1 / sin θ c . m . angular distribution for projectile-like fragments and strong entrance channel dependence favoring breakup into the entrance channel with large excitation energy have been observed. We think these observations imply the formation of a long-lived orbiting dinuclear complex where the nuclei maintain their identities and nucleon exchange between them is highly suppressed. The analysis of fragment spectra show that the projectile-like fragments coming from such an orbiting dinuclear complex give significantly lower temperature compared to that of the respective compound nucleus.

Entrance channel dependence in compound nuclear reactions with loosely bound nuclei⋆

The measurement of light charged particles evaporated from the reaction 6,7Li + 6Li has been carried out at extreme backward angle in the energy range 14–20 MeV. Calculations from the code ALICE91 show that the symmetry of the target-projectile combination and the choice of level density parameter play important roles in explaining the evaporation spectra for these light particle systems. In the above barrier energy region the fusion cross-section is not suppressed for these loosely bound nuclei.

Entrance Channel Dependence of Production Cross Sections ofSuperheavy Nuclei in Cold Fusion Reactions

Production cross sections of superheavy nuclei Rf and Hs forasymmetric and nearly symmetric projectile-target combinations aresystematically studied within the framework of the dinuclear systemmodel. The calculated results show that the production crosssections are strongly dependent on the symmetry of reactionsystems. The obtained results are in good agreement with theavailable experimental data for asymmetric reaction systems. Fornearly symmetric systems, the model gives opposite results withcoupled channel model in which surface vibration andnucleon transfer are included.

Entrance Channel Dependence of the Isospin Effects of theNuclear Stopping in Intermediate Energy Heavy-Ion Collisions

The entrance channel dependence of the isospin effects of nuclear stoppingin intermediate energy heavy-ion collisions has been studied by using anisospin-dependent quantum molecular dynamics with three different kindsof symmetry potentials. It is shown that nuclear stopping is sensitiveto the beam energy, the impact parameter and the mass of the collidingsystem, especially very sensitive to the isospin dependence of thein-medium nucleon-nucleon cross section, but insensitive to the symmetrypotential and the neutron-to-proton ratio of the colliding system.From this investigation, it is proposed that nuclear stopping can be used asa new probe to extract the information on the isospin dependence of the in-mediumnucleon-nucleon cross section in intermediate energy heavy-ion collisions.

Entrance channel dependence and isospin dependence of preequilibrium nucleon emission in intermediate energy heavy ion collisions

Using isospin-dependent quantum molecular dynamical model, the studies of the isospin effect of preequilibrium nucleon emission in heavy-ion collisions under different entrance channel conditions show that the ratio of preequilibrium neutron number to proton number depends strongly on symmetry potential, beam energy, and the ratio of neutron to proton of the colliding system, but weakly on isospin-dependent in-medium nucleon–nucleon cross sections, impact parameter, Pauli potential, and momentum-dependent interaction in the energy region from 45 MeV/u up to 150 MeV/u where the dynamics is dominated by nucleon–nucleon collisions. In addition, the ratio of preequilibrium neutron number to proton number for a neutron-rich colliding system is larger than the initial value of the ratio of the colliding system, but the ratio for a neutron-deficient system is less than the initial value.

Search for entrance channel effects in the decay of compound nucleus

The α-particle energy spectra have been measured at different laboratory angles from the fusion reaction 12 C + 45 Sc at 83 and 97 MeV incident energies. These spectra are consistent with the predictions of the statistical model calculations using the Rotating Liquid Drop Model (RLDM) values of the moment of inertia and the optical model transmission coefficients for the respective inverse absorption channels. The results of 12 C + 45 Sc asymmetric system were compared with the 28 Si + 27 Al symmetric system at 140 MeV studied earlier as both the systems lead to the formation of nearly same compound nucleus ( Z=27) with the same angular momentum and the excitation energy. The comparison was made in order to understand the symmetric and the asymmetric entrance channel effects in the formation and the decay of the compound system. In the case of 28 Si + 27 Al symmetric system, the experimental spectra deviate at higher as well as at lower energies from the statistical model calculations. Alterations were required in the moment of inertia as well as in the calculation of the transmission coefficients to explain the experimental data. However, in the case of 12 C + 45 Sc asymmetric system there was no need for such changes and the experimental spectra can be well explained by the statistical model calculation using RLDM values of moment of inertia and the optical model transmission coefficients. The dynamical trajectory model calculations are done to explain the experimental data. The comparison of the decay times with the formation times implies that the light charged particles are preferentially emitted prior to the full relaxation of the compound nucleus in the case of 28 Si + 27 Al symmetric system. The results of the present study suggest an entrance channel dependence in the formation and the decay of the compound nucleus.

Nuclear orbiting and anomalies in nuclear reactions

In this paper, we report our measurements of back-angle oxygen and carbon particle yields from 16O+89Y, 12C+93Nb reactions forming the same compound nucleus 105Ag at the same excitation energy and spin distribution. We find anomalously large oxygen yield and entrance channel dependence at high excitation energies from 16O+89Y reaction implying formation of a dinuclear orbiting complex. Possible connection between nuclear orbiting and fast fission is also discussed.

Entrance channel dependence of fragmentation dynamics in heavy-ion collisions

We study the mechanism of intermediate energy heavy-ion collisions in a picture of tripartition of the total system, using the framework of statistical simultaneous multifragmentation model taking into account the entrance channel characteristics, with no other parameter except density. The relative excitations of the three parts and their contributions to the physical observables such as mean multiplicity for different fragments and total fragment multiplicity are shown as functions of entrance channel properties; namely, the impact parameter, the bombarding energy, and the mass of projectile and target nuclei. As an application we apply the model to describe the data on {sup 40}Ar+{sup 40}Ca and {sup 40}Ar+{sup 197}Au reaction with incident energy varying from 42 to 137 MeV/nucleon, and the intermediate mass fragment production in the {sup 197}Au+{sup 197}Au reaction with incident energies 100 and 400 MeV/nucleon. The importance of the contributions from the spectator parts is clearly demonstrated. It is also shown that the role of the interfragment interaction is important up to bombarding energy 100 MeV/nucleon and a temperature of about 10 MeV. {copyright} {ital 1997} {ital The American Physical Society}