Evaporation Residue Production Research Articles

Reaction dynamics at the barrier for heavy compound systems

To investigate basic properties of the fusion reaction dynamics for heavy compound systems, the partial wave distribution σl extracted from measured γ multiplicities can be employed as an alternative to the classically used fusion/fission excitation functions. A variety of reactions leading to compound nuclei in the Pb region can be used to investigate features like the fusion-fission competition, the role of deformation in the fusion of heavy systems, and a possible effect of the Z=82 shell on the enhancement of evaporation residue production. The measured spin distribution can provide information on the single partial wave cross sections, which is hidden in the integral fusion cross section. Moreover, it can reveal signatures in the high-spin region, which could be an indication of a stabilization due to an increase in the potential hole by shell correction energies Eshell in the vicinity of a closed shell. The systematic investigation and understanding of the fusion-fission reaction dynamics, together with the understanding of the structure of transfermium nuclei, which are stabilized only via shell effects, are essential for a successful program aiming at the synthesis of new elements at GSI, Dubna, or elsewhere. We started a series of experiments to measure those properties for the reactions at the Laboratori Nazionali di Legnaro, Italy. In order to extract the compound nucleus spin distribution, γ multiplicities are measured using the γ-detector array GASP and its inner ball in the multiplicity filter mode.

Reaction mechanism studies using the CN/ER spin distribution

To study basic properties of the fusion reaction dynamics for heavy compound systems the partial-wave distribution \(\sigma_\ell \) can be employed as an alternative to the classically used fusion/fission excitation functions. A variety of reactions leading to compound nuclei (CN) in the Pb region can be used to investigate features like the fusion-fission competition, the role of deformation in the fusion of heavy systems and a possible effect of the Z = 82 shell on the enhancement of evaporation residue (ER) production.

Heavy residues and intermediate-mass fragment production in dissipative197Au+86Krcollisions atE/A=35 MeV

Neutrons and charged light and intermediate-mass products from the ${}^{197}\mathrm{Au}{+}^{86}\mathrm{Kr}$ reaction at ${E}_{\mathrm{lab}}/A=35 \mathrm{MeV}$ were measured with $4\ensuremath{\pi}$ angular coverage, in coincidence with projectilelike (PLFs) and targetlike fragments (TLFs). The characteristics of PLF and TLF yields and their correlation with neutron, light-charged particles, and intermediate-mass fragments (IMFs) are consistent with a scenario where essentially all collisions proceed through a dissipative stage. It is found that the damping of the available kinetic energy into thermal excitation is essentially incomplete, with no positive evidence for complete damping. Slow, massive reaction products are observed with significant yields. These products, termed also heavy residues, are identified as TLF evaporation residues and as TLF fission fragments. The TLF fission mode is seen to fade away in favor of TLF evaporation residue production for excitation energies above 3--4 MeV/nucleon. The suppression of the TLF fission can be traced to the dynamical IMF production process which reduces considerably the sizes of the primary reaction fragments and, hence, their fissility.

Capture and fusion dynamics in heavy-ion collisions

The dependence of the difference between the processes of capture and complete fusion is studied as a function of the entrance channel in the framework of the combined dynamical and statistical model. The excitation functions for the 126 Ba compound nucleus formed in the 19 F + 107 Ag and 28 Si + 98 Mo reactions were calculated to compare the E min and E max values of the beam energy window for the fusion cross section. The importance of the competition between quasi-fission and complete fusion in interpreting the experimental data obtained for the 40 Ar + nat Ag reaction is discussed. The increase in the difference between the experimental evaporation residue production and the extracted fusion cross section at increasing 40 Ar beam energy is explained as an increasing effect of the quasi-fission process.

Fusion-fission dynamics in the superheavy nucleus production

The fusion-fission reaction mechanism leading to the massive nucleus formation is studied. We investigate the superheavy nucleus formation in heavy-ion induced reactions by analysing the evaporation residue (ER) production in order to study the fusion dynamics and the decay properties of nuclei close to the stability island at Z=114. We consider the 61Ni+208Pb, 48Ca+238U and 48Ca+244Pu reactions that lead to the Z=110, 112 and 114 superheavy elements respectively. By using the dinuclear system (DNS) concept of the two interacting nuclei we calculate the quasifission-fusion competition in the entrance channel and the fission-evaporation competition along the de-excitation cascade of the compound nucleus. The dynamics of the entrance channel allows us to determine the beam energy window which is favorable to the fusion, while the dynamic evolution of the compound nucleus on the shell correction to the fission barrier and the dissipative effects influence the fission-evaporation competition in order to obtain the residue nuclei from the superheavy nucleus formation. We also calculate the τ n /τtot ratio at each step of the de-excitation cascade of the compound nucleus and we present a systematics of τ n /τtot (at first step of the cascade) for many reactions that lead to nuclei with Z=102–114.

The GDR width in hot Sn nuclei

Gamma-rays, charged particles and evaporation residue products from 18O+ 100Mo collisions at E( 18O)=125 to 217 MeV have been measured in order to understand better the width evolution of the GDR in hot nuclei. Detailed angular distributions of all reaction products are used to determine important preequilibrium and bremsstrahlung effects and the giant-dipole parameters as a function of temperature. Results show the GDR width increasing with final-state temperature up to T=2.5 MeV, in contrast with previous works which argue for a saturation of the width in this energy range.

Limiting excitation energy in fusion-evaporation reactions?

The systematics of fusion-evaporation reactions in the mass-symmetric system ${}^{40}\mathrm{Ar}{+}^{40}$Ca are studied for incident energies of 5--30 MeV/nucleon. The measurement of the resulting evaporation residues in coincidence with emitted neutrons permits a clear identification of central collisions and a separation of equilibrium and preequilibrium contributions. The total neutron multiplicities and the total average mass loss of the system both show a tendency of soft saturation. Above about 20 MeV/nucleon the increase of the energy removal is entirely due to preequilibrium emission of light particles. The total excitation energy of the evaporation residues approaches a maximum value, but the excitation energy per nucleon rises linearly without any sign of saturation. At even higher projectile energies the production of evaporation residues is dramatically reduced and they become indistinguishable from spectatorlike fragments resulting from less central collisions. Thus, the long-standing question of a limiting excitation energy in hot nuclei can probably not be solved by experimental studies of fusion-evaporation reactions.

Production of fast evaporation residues by the reaction $^{20}$ Ne + $^{208}$ Pb at projectile energies of 8.6, 11.4 and 14.9 A.MeV

Production of fast evaporation residues by the reaction $^{20}$ Ne + $^{208}$ Pb at projectile energies of 8.6, 11.4 and 14.9 A.MeV

Production of fast evaporation residues by the reaction20Ne+208Pb at projectile energies of 8.6, 11.4 and 14.9 A.MeV

Velocity distributions and production cross sections of evaporation residues have been measured in the reaction20Ne+208Pb at projectile energies of 8.6, 11.4, 14.9 A.MeV. Essential deviations from statistical model of deexcitation have been observed. Monte Carlo simulations involving emission of non-equilibrium particles have been used in order to reproduce experimental velocity, charge and mass distributions of evaporation residues and to estimate indirectly multiplicities of pre-equilibrium particles.

Coincidence correlations between light charged particles in the matched reactions 905 and 1030 MeV and 550 and 750 MeV

Abstract Coincidence measurements have been made between 4 He 1 H and a second particle ( 4 He 1 H ) produced in the reactions 905 and 1030 MeV 121 Sb + 27 Al and 550 and 750 MeV 86 Kr + 63 Cu . All four of the reactions lead to composite systems of 149Tb∗ or 148Gd∗ which facilitates comparisons of matched reactions. In the two 121 Sb + 27 Al reactions and the 550 MeV 86 Kr + 63 Cu reaction, the particle-particle coincidences are dominated by processes that lead to evaporation residues. In the 750 MeV 86 Kr + 63 Cu reaction, about half of the 1H and 4He are associated with evaporation residue production. These particle-particle coincidences arise from cross sections that are subsets of the total evaporation residue cross sections. The derived individual particle chain lengths are ∼ 2 for both protons and alphas in each of the four reactions. The particle multiplicities and subset cross sections associated with the evaporation residues are quite similar for the matched reactions, in contrast to fission-like processes where spin-dependent entrance channel effects were observed.

Sensitivity of the evaporation residue production to dynamical effects in highly fissile compound nuclei

Abstract We investigate the sensitivity of statistical model calculations of individual evaporation residue (ER) production on the nuclear shape evolution and viscosity. To this aim we analyze the de-excitation of the 217 Ac compound nucleus formed at the excitation energies between 36 and 90 MeV. We find a strong dependence of the calculated results on the nuclear dissipation and shape evolution, which make evaporation residues a useful tool for investigating dynamical effects. We indicate a new method of estimating the nuclear viscosity and claim that the statistical model interpretation of the ER's does not exclude high values for the viscosity parameter.

Yields of evaporation residues and average angular momentum in heavy ion induced fusion reactions leading to compound nucleus96Ru

Cross-sections for production of evaporation residues from the compound nucleus96Ru* formed by fusion reactions28Si+68Zn,32S+64Ni,37Cl+59Co and45Sc+51V have been obtained from the yields of their characteristicγ-rays. The measurements span an excitation energy range of 55 MeV to 70 MeV of the compound nucleus. The evaporation residue (ER) cross-sections have been analysed in terms of statistical model for the decay of the compound nucleus. A good agreement is found between statistical model calculation and the experimental evaporation residue cross-sections in all the four cases. It is shown that the average angular momentum\(\bar \ell \) of the compound nucleus can be deduced from a comparison of the experimentally measured and the statistical model predictions for the ER cross-sections. The validity of this method of deriving\(\bar \ell \) has been discussed for the case of16O+154Sm system.

Investigation of the velocity distribution of evaporation residues produced in20Ne+197Au reactions atE/A=5.7?11.4 MeV/u

The velocity distributions of evaporation — residues produced in complete fusion reactions of20Ne+197Au at bombarding energies ofE/A=5.7, 8.6 and 11.4MeV/u were measured with use of the velocity filter SHIP. It could be shown, that the production cross section for evaporation residues is influenced by emission of fast, light particles having an angular distribution, that is peaked in forward direction (preequilibrium particles). This contribution is, however too small to explain the enhancement of the evaporation residue production cross section by about two orders of magnitude compared to statistical model calculations. This latter effect is interpreted as mainly due to a delayed onset of compound fission.

Fusion of 16O + 40Ca at Elab(16O)=13.4 MeV/nucleon.

Mass and velocity distributions have been measured for the evaporation residue and fusion-fission products from the $^{16}\mathrm{O}$${+}^{40}$Ca reaction at 214 MeV. Comparisons of Monte Carlo statistical evaporation simulations to the observed angle and mass dependences of the evaporation-residue velocity distributions were used to set limits on the maximum complete-fusion cross section and to extract information about the magnitude and character of incomplete-fusion processes. The extracted value of the complete fusion evaporation-residue cross section is discussed in the framework of previous results and existing models.

Probing sub-barrier fusion and extra-push by measuring fermium evaporation residues in different heavy ion reactions

The production of fermium isotopes was attempted by complete fusion of different targets and projectiles spanning a wide range of effective entrance channel fissilities below and above the predicted threshold valuex eff thr ≃0.7. For the most asymmetric systems where fusion is expected to occur without dynamical hindrance we investigate to what extent the expected amount of sub-barrier fusion contributes to the production of fermium evaporation residues. For increasingly symmetric systems the experimental fusion barriers are found to exceed the fusion barriers predicted by the proximity formalism. The barrier heights are discussed in the framework of both the extra-push model and the surface friction model.

Cross sections for evaporation residue production near theN=126 shell closure

In fusion reactions of40Ar with isotopes of Ho, Tm, Yb, Lu, Hf and Ta, cross sections for the production of proton-rich evaporation-residues near the 126 neutron shell were measured. This first comprehensive study of very fissile spherical residues reveals a surprisingly low stabilizing influence of the sphericalN=126 shell on the survival probability. The experimental results are compared with evaporation calculations. Conclusions for the production of superheavy nuclei are drawn.

Formation and decay of the compound nucleus studied in the reaction20Ne+27Al

Energy and velocity spectra, angular and mass distributions have been measured for evaporation residue products of the20, 22Ne+27Al system in the energy range ofEL (20Ne)=51 to 395 MeV and in the angular rangeθ=2° and 30°. Calculations with simple assumptions of the velocity and angular distributions of the evaporation residues are presented and compared to the data. The structure seen in the mass distributions, a competition between α-particle and nucleon evaporation in the deexcitation of the compound nucleus, is described well by calculations with the computer code CASCADE. The evaporation residues exhibit mass distributions varying systematically as a function of the excitation energy. The excitation function of the evaporation residue cross section is compared with theoretical models. At higher incident energies contributions of incomplete momentum transfer (incomplete fusion) are observed. A limitation for complete compound nucleus formation with following light particle evaporation is found.

Energy dependence of fusion cross section forSi28+C12by evaporation residue measurements

The excitation function for production of evaporation residues from the fusion of /sup 28/Si+/sup 12/C has been studied in two experiments. In the first experiment the ..gamma..-ray yields of transitions populating final state evaporation residues were measured, thereby allowing a detailed study of the bombarding energy dependence of the production of specific isotopes. More accurate absolute cross sections for evaporation residue production were measured at several energies with a gas proportional ..delta..E solid state E telescope. In this manner absolute cross sections were determined against which the ..gamma..-ray data could be normalized. The fusion cross section is found to vary smoothly with bombarding energy. Further, absolute cross sections agree well with model predictions currently in vogue.

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.

The sub-barrier fusion of40Ar with144,148,154Sm

The cross sections for production of evaporation residues (σ er) and for fusion-fission (σ ff) have been measured for40Ar+144, 148, 154Sm at sub-barrier energies by observation of x-ray emission from radioactive products and by direct,ΔE−E identification of fission fragments, respectively. These isotopes span the transition region from spherical (144Sm) to strongly deformed (154Sm) equilibrium shapes. The cross section for fusion,σ fus=σ er+σ ff, is found to vary markedly at low energies with the isotope number and, hence, with the quadrupole collectivity of the target. The thresholds for fusion of148Sm and144Sm are, respectively, ∼3.5 MeV and ∼7 MeV (c.m.) higher than for fusion with154Sm. These differences and the energy dependence of the fusion cross sections are discussed in terms of the effect of nuclear deformation on heavy-ion fusion. A comparative analysis of results for16O+Sm and40Ar+Sm in terms of static deformation indicates thatσ fus for the Ar+Sm system at very low energies is enhanced relative to the prediction for a one-dimensional barrier based on a fit toσ fus for16O+Sm. This may be an indication that additional degrees of freedom (such as formation of a neck or fragment elongation) may be important for fusion with the larger projectile. At energies above the fusion barrier, values ofσ fus for144, 148Sm are nearly equal, but are significantly smaller than for154Sm. This is in contrast to the results of previous experiments with16O projectiles in whichσ fus (16O+148Sm) andσ fus (16O+154Sm) were nearly equal above the barrier. These differences, observed for144, 148Sm and154Sm at energies above the barrier may reflect a new mechanism which is not encompassed by a static theory.