208Pb Reaction Research Articles

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.

Study of Decay Properties of 260Sg* Nucleus Formed in 52Cr + 208Pb Reaction by Using GSkI Skyrme Force

Study of Decay Properties of 260Sg* Nucleus Formed in 52Cr + 208Pb Reaction by Using GSkI Skyrme Force

Searching for treasures at sub-barrier energies: the case of 8B and 7Be

Unexpected and challenging experimental results, at below barrier energies for weakly bound nuclei, are briefly reviewed in this article. The emphasis will be on our recent breakup results for 8B+208Pb at deep sub-barrier energies, indicating a dominance of direct mechanisms at this low energy regime. We will also present, a preliminary analysis of the 4He and 3He - particle production events for the 7Be + 208Pb reaction. These data were collected at the same experiment and at similar deep sub-barrier energies, exhibiting large yields compatible with cluster transfer processes. To confirm these results a new experiment to be performed at TriSol is planned, as soon as the upgrading of this facility will be completed.

Fission of Relativistic Nuclei with Fragment Excitation and Reorientation.

Experimental studies of fission induced in relativistic nuclear collisions show a systematic enhancement of the excitation energy of the primary fragments by a factor of ∼2, before their decay by fission and other secondary fragments. Although it is widely accepted that by doubling the energies of the single-particle states may yield a better agreement with fission data, it does not prove fully successful, since it is not able to explain yields for light and intermediate mass fragments. State-of-the-art calculations are successful to describe the overall shape of the mass distribution of fragments, but fail within a factor of 2-10 for a large number of individual yields. Here, we present a novel approach that provides an account of the additional excitation of primary fragments due to final state interaction with the target. Our method is applied to the ^{238}U+^{208}Pb reaction at 1 GeV/nucleon (and is applicable to other energies), an archetype case of fission studies with relativistic heavy ions, where we find that the large probability of energy absorption through final state excitation of giant resonances in the fragments can substantially modify the isotopic distribution of final fragments in a better agreement with data. Finally, we demonstrate that large angular momentum transfers to the projectile and to the primary fragments via the same mechanism imply the need of more elaborate theoretical methods than the presently existing ones.

Role of Charge Equilibration in Multinucleon Transfer Reactions

Charge equilibration is analyzed in the context of the dynamic approach based on the Langevin equations. The time and energy dependences of this process are discussed. Analysis of the isotopic distributions of the final products obtained in the 40Ca, 58Ni and 64Ni + 208Pb shows that charge equilibration process strongly affects the multinucleon transfer in them. Yields of the most neutron-rich heavy nuclides obtained in the 58Ni and 64Ni + 208Pb reactions are discussed.

Neutron transfer and nuclear breakup in 208Pb(11Li, 9Li) reaction

Neutron transfer and nuclear breakup in 208Pb(11Li, 9Li) reaction

Role of charge equilibration in multinucleon transfer in damped collisions of heavy ions

In this work the charge equilibration process has been analyzed within the Langevin-type dynamical approach. Its duration and energy dependence are discussed. We have analyzed the isotopic distributions of final products obtained in the isospin-asymmetric 58Ni,40Ca + 208Pb reactions. Comparison of 58Ni,64Ni + 208Pb systems have been done in order to analyze the final yields of neutron-rich heavy nuclides.

Millisecond 23/2+ isomers in the N=79 isotones Xe133 and Ba135

Detailed information on isomeric states in A ≈ 135 nuclei is exploited to benchmark shell-model calculations in the region northwest of doubly-magic nucleus 132Sn. The N = 79 isotones 133Xe and 135Ba are studied after multinucleon transfer (MNT) in the 136Xe + 208Pb reaction employing the high-resolution Advanced GAmma Tracking Array (AGATA) coupled to the magnetic spectrometer PRISMA at the Laboratori Nazionali di Legnaro, Italy and in a pulsed-beam experiment at the FN tandem accelerator of the University of Cologne, Germany utilizing a 9Be+130Te fusion-evaporation reaction at a beam energy of 40 MeV. Isomeric states are identified via delayed γ-ray spectroscopy. Hitherto tentative excitation energy, spin, and parity assignments of the 2107-keV Jπ = 23/2+ isomer in 133Xe are confirmed and a half-life of T1/2 = 8.64(13) ms is measured. The 2388-keV state in 135Ba is identified as a Jπ = 23/2+ isomer with a half-life of 1.06(4) ms. The new results show a smooth onset of isomeric Jπ = 23/2+ states along the N = 79 isotones and close a gap in the high-spin systematics towards the recently investigated Jπ = 23/2+ isomer in 139Nd. The resulting systematics of M2 reduced transition probabilities is discussed within the framework of the nuclear shell model. Latest large-scale shell-model calculations employing the SN100PN, GCN50:82, SN100-KTH, and a realistic effective interaction reproduce the experimental findings generally well and give insight into the structure of the isomers.

Coupled-channels analyses for 9,11Li + 208Pb fusion reactions with multi-neutron transfer couplings

We discuss the role of two-neutron transfer processes in the fusion reaction of the 9,11Li + 208Pb systems. We first analyze the 9Li + 208Pb reaction by taking into account the coupling to the 7Li + 210Pb channel. To this end, we assume that two neutrons are directly transferred to a single effective channel in 210Pb and solve the coupled-channels equations with the two channels. By adjusting the coupling strength and the effective Q-value, we successfully reproduce the experimental fusion cross sections for this system. We then analyze the 11Li + 208Pb reaction in a similar manner, that is, by taking into account three effective channels with 11Li + 208Pb, 9Li + 210Pb, and 7Li + 212Pb partitions. In order to take into account the halo structure of the 11Li nucleus, we construct the potential between 11Li and 208Pb with a double folding procedure, while we employ a Woods–Saxon type potential with the global Akyüz–Winther parameters for the other channels. Our calculation indicates that the multiple two-neutron transfer process plays a crucial role in the 11Li + 208Pb fusion reaction at energies around the Coulomb barrier.

Decay analysis of compound nuclei formed in reactions with exotic neutron-rich 9Li projectile and the synthesis of 217At⁎ within the dynamical cluster-decay model

The decay of various compound nuclei formed via exotic neutron-rich 9Li projectile is studied within the dynamical cluster-decay model (DCM). Following the earlier work of one of us (RKG) and collaborators (M. Kaur et al. (2015) [1]), for an empirically fixed neck-length parameter ΔRemp, the only parameter in the DCM, at a given incident laboratory energy ELab, we are able to fit almost exactly the (total) fusion cross section σfus=∑x=16σxn for 9Li projectile on 208Pb and other targets, with σfus depending strongly on the target mass of the most abundant isotope and its (magic) shell structure. This result shows the predictable nature of the DCM. The neck-length parameter ΔRemp is fixed empirically for the decay of 217At⁎ formed in 9Li + 208Pb reaction at a fixed laboratory energy ELab, and then the total fusion cross section σfus calculated for all other reactions using 9Li as a projectile on different targets. Apparently, this procedure could be used to predict σfus for 9Li-induced reactions where experimental data are not available. Furthermore, optimum choice of “cold” target-projectile combinations, forming “hot” compact configurations, are predicted for the synthesis of compound nucleus 217At⁎ with 8Li + 209Pb as one of the target-projectile combination, or another (t,p) combination 48Ca + 169Tb, with a doubly magic 48Ca, as the best possibility.

Goodness of isospin in neutron rich systems from the fission fragment distribution

We present the results of our calculations for the relative yields of neutron-rich fission fragments emitted in 208Pb (18O, fission) reaction by using the concept of the conservation of isospin and compare with the experimental data. We take into account a range of isospin values allowed by the isospin algebra and assume that the fission fragments are formed in isobaric analog states. We also take into account the neutron multiplicity data for various neutron-emission channels in each partition, and use them to obtain the weight factors in calculating the yields. We then calculate the relative yields of the fission fragments. Our calculated results are able to reproduce the experimental trends reasonably well. This is the first direct evidence of the isospin conservation in neutron-rich systems and may prove a very useful tool in their studies.

Stretched configuration of states as inferred from $ \gamma$ γ -ray angular distributions in 40Ar + 208Pb neutron transfer reactions

Angular distributions of $ \gamma$ -rays for selected transitions in 40, 41, 42Ar isotopes have been studied with the PRISMA magnetic spectrometer coupled to the CLARA $ \gamma$ array. These transitions were populated in Ar isotopes reached via neutron transfer in the 40Ar + 208Pb reaction. By comparison with the shape of the experimental angular distribution of the known E2 transitions we established more firmly the spin and parity of excited states. In particular, in 41Ar for the $ (11/2^-)$ state through the $ (11/2^-) \rightarrow 7/2^-$ transition whose structure was discussed in terms of a phonon-fermion coupled state. The comparison with the expected fully aligned spin indicated that a high level of spin alignment has been reached.

Analysis of fission-fragment mass distribution within the quantum-mechanical fragmentation theory

The fission-fragment mass distribution is analysed for the 208Pb(18O, f) reaction within the quantum-mechanical fragmentation theory (QMFT). The reaction potential has been calculated by taking the binding energies, Coulomb potential and proximity potential of all possible decay channels and a stationary Schrodinger equation has been solved numerically to calculate the fission-fragment yield. The overall results for mass distribution are compared with those obtained in experiment. Fine structure dips in yield, corresponding to fragment shell closures at Z = 50 and N=82, which are observed by Bogachev et al., are reproduced successfully in the present calculations. These calculations will help to estimate the formation probabilities of fission fragments and to understand many related phenomena occurring in the fission process.

The effects of density-dependent form factors for (e, e’p) reaction in quasi-elastic region

Within the framework of a relativistic single particle model, the effects of density-dependent electromagnetic form factors on the exclusive $ (e,e'p)$ reaction are investigated in the quasi-elastic region. The density-dependent electromagnetic form factors are generated from a quark-meson coupling model and used to calculate the cross sections in two different densities, either at the normal density of $ \rho_0 \sim 0.15$ fm^-3 or at the lower density, $ 0.5\rho_0$ . Then these cross sections are analyzed in the two different kinematics: One is that the momentum of the outgoing nucleon is along the momentum transfer. The other is that the angle between the momentum of the outgoing nucleon and the momentum transfer is varied at fixed magnitude of the momentum of the outgoing nucleon. Our theoretical differential reduced cross sections are compared with the NIKHEF data for the 208 Pb(e, e’p) reaction, which is related to the probability that a bound nucleon from a given orbit can be knocked-out of the nucleus. The effects of the density-dependent form factors increase the differential cross sections for both knocked-out proton and neutron by an amount of a few percent. Moreover they are shown to be almost the same within only a few percent, i.e., nearly independent of the shell location of knockout nucleons. These results are quite consistent with the characteristics of double magic nuclei which have relatively sharp smearing in the density distribution.

Nuclear matter incompressibility effect on the cross-section of fusion reactions with a weakly bound projectile

Fusion reactions with a weakly bound projectile are studied using the double-folding model along with a repulsive interaction modifying term. Using this modified potential, including nuclear matter incompressibility effects, the fusion reaction cross sections and suppression parameters are calculated for 9Be+ 209Bi, 208Pb, 29Si and 27Al reactions. The results show that applying these effects at energies near the Coulomb barrier improves the agreement between the calculated and experimental cross sections, and modifies the mean values of the suppression parameter.

Observation of bound and unbound 1− states in 208Pb by particle spectroscopy

Using the Q3D magnetic spectrograph of the Maier-Leibnitz-Labaratorium at Garching (Germany), experiments on the 208Pb(d,d'), the 207Pb(d,p), the resonant and the non-resonant 208Pb(p,p') reactions were performed. The 208Pb(p,p') reaction was investigated near all seven known isobaric analog resonances in 209Bi. The excitation energies of about 300 states in 208Pb at Ex < 8MeV were determined with an accuracy of about 100 eV. The mean distance between states in 208Pb is about 10keV up to the neutron threshold (S(n) = 7368 keV). The number of 1− states observed by particle spectroscopy up to the proton threshold (S(p) = 8.00 MeV) is close to the number predicted by the schematic shell model without residual interaction. The structure of several 1− states is deduced from particle spectroscopy. Four out of the seven lowest 1− states contain about 80% strength of a single configuration.

Role of the diagonal and off-diagonal continuum–continuum couplings in the breakup of 8B and 19C on 58Ni and 208Pb targets

Abstract The role of the diagonal and off-diagonal continuum–continuum couplings on Coulomb + nuclear, Coulomb and nuclear breakup cross sections is investigated for the 8B + 58Ni, 8B + 208Pb and 19C + 208Pb at 29.3, 170.3 and 1273 MeV incident energies respectively. Qualitatively, we found that, the diagonal continuum–continuum couplings are responsible for the large reduction of the differential Coulomb + nuclear and nuclear differential breakup cross sections at backward angles. At forward angles, this reduction is due to the off-diagonal continuum–continuum couplings. In the absence of these couplings, the nuclear breakup is the more dominant process, while when they are included, the Coulomb breakup becomes dominant. This shows that, the nuclear breakup is more affected by the continuum–continuum couplings than its Coulomb counterpart. Quantitatively, we found that, the off-diagonal continuum–continuum couplings reduce by 13.39%, 12.71% and 11.11% the Coulomb + nuclear breakup cross sections for the 8B + 58Ni, 8B + 208Pb and 19C + 208Pb reactions, respectively.

Role of neutron transfer in asymmetric fusion reactions at sub-barrier energies

The measured complete fusion (capture) excitation function is presented for the 28Si + 208Pb reaction at deep sub-barrier energies. This excitation function is compared with the one predicted with the quantum diffusion approach.

Isospin character of low-lying pygmy dipole states in 208Pb via inelastic scattering of 17O ions.

The properties of pygmy dipole states in 208Pb were investigated using the 208Pb(17O, 17O'γ) reaction at 340 MeV and measuring the γ decay with high resolution with the AGATA demonstrator array. Cross sections and angular distributions of the emitted γ rays and of the scattered particles were measured. The results are compared with (γ, γ') and (p, p') data. The data analysis with the distorted wave Born approximation approach gives a good description of the elastic scattering and of the inelastic excitation of the 2+ and 3- states. For the dipole transitions a form factor obtained by folding a microscopically calculated transition density was used for the first time. This has allowed us to extract the isoscalar component of the 1- excited states from 4 to 8 MeV.

Single-particle widths in 208Bi and 209Bi determined from the inelastic proton scattering by 207Pb and 208Pb

Single-particle widths in 209Bi for all particle orbits s1/2, d3/2, d5/2, g7/2, g9/2, i11/2, j15/2 and for the hole orbits p1/2, p3/2, f5/2, f7/2 are deduced from the study of about fifty states in 208Pb with rather pure particle-hole configurations. Relative single-particle widths of the 0+ isobaric analog resonance (IAR) in 208Bi for the orbits p3/2, f5/2, f7/2 are deduced from the 207Pb(p, p′) reaction. The 207Pb(p, p′) and 208Pb(p, p′) reactions via IAR were studied in a scattering chamber experiment of the Max-Planck institute for nuclear physics (Heidelberg, Germany) at scattering angles 40° ≤ Θ ≤ 170°. The semiconductor detectors yielded a resolution of 11–15 keV. Proton energies E p = 14.99, 16.30, 16.45, 16.60, 17.40, 17.47, 17.75 MeV covered the g9/2, j15/2, d5/2, g7/2, and d3/2 IARs in 209Bi, and 11.0 < E p < 11.8 MeV the 0 + IAR in 208Bi. The differential cross sections for nearly 70 levels in 208Pb were determined with an uncertainty of the solid angle of about 2%. Similar measurements of 208Pb(p, p′) with the Q3D magnetic spectrograph of the Maier-Leibnitz-Laboratorium (Garching, Germany) 35–45 years later yielded a resolution of about 3 keV. The uncertainty of the differential cross section was 10–30%. Scattering angles covered 20° ≤ Θ ≤ 115° and Θ = 139°. The nonresonant (p, p′) reaction often dominates at scattering angles Θ ≲ 100°. Therefore, only the combination of the two data sets, i) backward angles, precise solid angles but modest resolution, ii) medium scattering angles and high resolution, allows to determine in an iterative manner both the structure of particle-hole states in 208Pb and the relevant single-particle widths in 209Bi .