Neutron elastic scattering from 116,118,120,122,124Sn

Accurate measurements of neutron differential elastic cross sections have been obtained from even isotopes of Sn. Data are presented for the elastic scattering of 11 MeV neutrons from ^{116, 118, 120, 122, 124}Sn, the elastic scattering for 24 MeV neutrons from ^{116, 118, 124}Sn and the neutron total cross section from ^{118, 120, 122, 124}Sn in the energy ranges 5.0–10.6 MeV and 20.0–26.0 MeV. The elastic scattering data are analyzed in terms of an empirical optical-model potential. The obtained optical-potential parameters are analyzed in terms of energy and isospin dependence and compared with those obtained from proton elastic scattering on even Sn isotopes.     

Probing of complete and incomplete fusion dynamics in heavy-ion collision

Three different types of experiments have been performed to explore the complete and incomplete fusion dynamics in heavy-ion collisions. In this respect, first experiment for the measurement of excitation functions of the evaporation residues produced in the ²⁰Ne + ¹⁶⁵Ho system at projectile energy ranges ≈2–8 MeV /nucleon has been done. Measured cumulative and direct cross-sections have been compared with the theoretical model code PACE-2, which takes into account only the complete fusion process. It has been observed that, incomplete fusion fraction is sensitively dependent on projectile energy and mass asymmetry between the projectile and the target systems. Second experiment for measuring the forward recoil range distributions of the evaporation residues produced in the ²⁰Ne + ¹⁶⁵Ho system at projectile energy ≈8 MeV /nucleon has been done. It has been observed that, some evaporation residues have shown additional peaks in the measured forwardrecoil range distributions at cumulative thicknesses relatively smaller than the expected range of the residues produced via complete fusion. The results indicate the occurrence of incomplete fusion involving the breakup of ²⁰Ne into ⁴He + ¹⁶O and /or ⁸Be + ¹²C followed by one of the fragments with target nucleus ¹⁶⁵Ho. Third experiment for the measurement of spin distribution of the evaporation residues produced in the ¹⁶O + ¹²⁴Sn system at projectile energy ≈6 MeV /nucleon, showed that the residues produced as incomplete fusion products associated with fast α and 2 α-emission channels observed in the forward cone, are found to be distinctly different from those of the residues produced as complete fusion products. The spin distribution of the evaporation residues also inferred that in incomplete fusion reaction channels input angular momentum (J ₀) increases with fusion incompleteness when compared to complete fusion reaction channels. Present observation clearly shows that the production of fast forward α-particles arises from relatively larger angular momentum in the entrance channel leading to peripheral collision.     

Absolute residue cross sections for 46–50Ti + 13C reactions at 36, 46 and 56 MeV

Absolute cross sections for nuclei produced in the reactions ^46–50Ti + ¹³C at 36, 46 and 56 MeV (lab) were measured. Complete identification in mass and atomic number for the evaporation residues was obtained by means of in-beam γ-ray spectroscopy techniques. In the entire energy range, an overall satisfactory account of the observed product nuclei is given by the predictions of the fusion-evaporation model. Direct channels like inelastic scattering and n-transfer appear to be noticeable and contribute ~ 15 % to the total cross section.     

Nuclear-level densities around Z = 50 from neutron evaporation spectra in (p, n) reactions

Neutron excitation functions, spectra, and angular distributions in the (p, n) reactions on the isotopes ¹¹⁶Sn, ¹¹⁸Sn, ¹²²Sn, and ¹²⁴Sn were measured in the proton-energy range 7–11 MeV. The measurements were performed by the time-of-flight method with the aid of a fast-neutron spectrometer at the EGP-15 pulsed tandem accelerator of the Institute of Physics and Power Engineering (Obninsk). A high resolution (about 0.6 ns/m) and a high stability of the time-of-flight spectrometer made it possible to identify reliably low-lying levels along with the continuous section of the neutron spectra. The data obtained in this way were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed with the aid of the precise Hauser-Feshbach formalism of statistical theory. The nuclear-level densities in the isotopes ¹¹⁶Sb, ¹¹⁸Sb, ¹²²Sb, and ¹²⁴Sb were determined, along with their energy dependences and model parameters. In the excitation-energy range 0–2 MeV, the energy dependence of the nuclear-level densities exhibits a structure that is associated with the shell inhomogeneities of the spectrum of single-particle states near filled shells. The isotopic dependence of the nuclear-level density is discovered and explained. It is also shown that the data obtained here for the nuclear-level density differ markedly from the predictions of model systematics of nuclear-level densities.     

化学不稳定性和同位素分布的同位旋效应

采用同位旋相关的量子分子动力学模型,研究了在轰击能量为40MeV/u和100MeV/u时,¹¹²Sn+¹¹²Sn和¹²⁴Sn+¹²⁴Sn反应产物的同位素分布.发现在40MeV/u时,同位素分布与入射体系的同位旋密切相关;而在100MeV/u时,对这两种不同同位旋体系给出几乎相同的同位素分布.这种同位素分布的能量依赖性为研究化学不稳定性及其与力学不稳定性的竞争提供了新途径 关键词： 化学不稳定性 同位旋效应 同位素分布 同位旋相关的量子分子动力学     

The barrier for compound-nucleus formation in the nearly symmetric systems86Kr+123Sb→209Fr and124Sn +94Zr→218Th

Evaporation residues from the fusion of the nearly symmetric systems⁸⁶Kr+¹²³Sb and¹²⁴Sn+⁹⁴Zr have been detected. The compound nucleus²¹⁸Th could be produced with an excitation energy as low as 21 MeV. The barriers for the formation of a compound nucleus were deduced from the evaporation residue cross sections. There is no evidence for an additional amount of energy needed to fuse these systems, if compared with the extrapolation of fusion barrier systematics or with results of fusion model calculations not including friction phenomena. One of the systems reaches a value ofZ ₁·Z ₂=2,000, about the highest value where evaporation residues have ever been observed.     

A global optical-model analysis of neutron elastic scattering data

Differential cross sections for neutron elastic scattering in the energy range 7–26 MeV from ⁴⁰Ca, ⁹⁰Zr, ⁹²Mo, ^{116, 124}Sn and ²⁰⁸Pb are analyzed in terms of a global optical-model potential. A smooth variation of the real radius parameters with mass number is investigated. Otherwise, optical-model geometrical parameters are kept fixed at values obtained by averaging individual best fits. The energy and isospin dependences of potential strengths and volume integrals per nucleon are presented.     

Inclusive and exclusive measurements of high energy γ-rays in the 101 MeV <Superscript>19</Superscript>F +<Superscript>181</Superscript>Ta fusion reaction

The high energy γ-rays from the fusion-evaporation reaction 101 MeV ¹⁹F +¹⁸¹Ta have been measured in coincidence with different γ-ray fold windows or discrete γ-rays in final residual nuclei. The line-shape analysis of the high energy γ-ray spectra confirms the large deformation of the ²⁰⁰Pb nuclei at high angular momenta. Coincidences with discrete γ-transitions evidence a strong correlation between γ-rays in the Giant Dipole Resonance region (E_γ≥ 8 MeV) and final evaporation residues. Received: 14 September 1999 / Revised version: 17 December 1999     

Linear polarization and angular correlation of continuumγ-rays

The linear polarization and angular correlation of continuumγ-rays have been measured for the reactions¹²⁴Sn(⁴⁰Ar,xn)^164?x Er and¹¹⁰Pd(⁴⁰Ar,xn)^150?x Gd. From the angular correlation experiments we find that the dominant fractions of the radiation are stretched quadrupoles (50–80%) and stretched dipoles. The linear polarization results are compatible with mainlyM1 multipolarity for the dipole part of the continuum in the energy range between 0.5 and 1.6 MeV.     

Angular momentum dependence of the giant dipole resonance width in excited nuclei of mass

Gamma ray spectra in the energy range of 4–25 MeV were measured in the reaction ²⁸Si + ¹²⁴Sn at E(²⁸Si) 150 MeV in coincidence with low energy γ-multiplicities. The spectra were analysed using a simulated Monte Carlo CASCADE code. The centroid energy and width of the giant dipole resonance were extracted for various multiplicity windows. The average angular momentum and temperature of the final states populated after the giant dipole photon emission range from 25 and 1.5 MeV to 56 and 1.3 MeV, respectively. The extracted widths are almost constant for the lower multiplicity windows and show an increase of 1.4 MeV at the highest window. The nuclei are expected to be near the liquid drop regime and the experimental results are not inconsistent with the liquid drop behaviour.     

Evaporation residues from the fusion of 32S on 27Al at E = 142–227 MeV

The evaporation residues from the reaction ³²S on ²⁷Al were measured at 142, 153, 175, 187 and 227 MeV incident sulphur energies, using a time-of-flight spectrometer. The measured fusion-evaporation residue excitation function shows a maximum around E_lab = 170 MeV, which corresponds to a limiting angular momentum of 43 ħ. Several possible mechanisms were investigated as being responsible for this limitation. It was found that both fission and entrance channel effects could be the limiting factors for the fusion-evaporation cross section at high energies.     

Charge-exchange pigmy resonances of tin isotopes

Charge-exchange states, the so-called “pigmy” resonances, which are below the giant Gamow–Teller resonance, have been studied in the self-consistent theory of finite Fermi systems. Microscopic numerical calculations and semiclassical calculations are presented for nine tin isotopes with the mass numbers A =112, 114, 116, 117, 118, 119, 120, 122, and 124, for which experimental data exist. These data have been obtained in the Sn(³He,t)Sb charge-exchange reaction at the energy E(³He) = 200 MeV. The comparison of calculations with experimental data on the energies of charge-exchange resonances gives the standard deviation δE < 0.40 MeV for microscopic numerical calculations and δE < 0.55 MeV for calculations by semiclassical formulas, which are comparable with experimental errors. The strength function for the ¹¹⁸Sn isotope has been calculated. It has been shown that the calculated resonance energies are close to the experimental values; the calculated and experimental relations between heights of pygmy resonance peaks are also close to each other.     

Break-up of 6Li and 7Li on tin and nickel nuclei

The break-up of the stable lithium nuclei on ⁵⁸Ni and ¹¹⁸Sn has been investigated in the bombarding energy range 12–24 MeV. The emerging α-particles have been analysed showing that the cross sections peak at the angle of grazing collisions. Inspection of the present data suggests the conclusion that several different reaction mechanisms contribute to the continuous spectrum of the α-particles.     

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.     

The 116Sn(d,t)115Sn reaction at 23 MeV

The levels of ¹¹⁵Sn up to 4 MeV excitation energy have been studied with 15–18 keV energy resolution, and many previously unreported levels have been observed. Angular momentum transfers and spectroscopic factors have been determined for most of the levels up to 3 MeV. The main features of the observed fragmentation of the strength are rather well reproduced by means of a weak coupling calculation.     

Target isotope effect in high energy photon production atE/A=10 MeV

A comparison has been made between the production of high energy (E _γ≧30 MeV)γ rays from the bombardment of¹²⁴Sn and¹¹²Sn targets withE/A= 10 MeV¹²C ions. The results are well explained by then-p bremsstrahlung model and do not indicate the need for any new processes.     

Evidence for large angular momentum window associated with incomplete fusion reactions

Mean values and variances of the angular momentum window of incomplete fusion reactions induced by¹⁶O on^116–124Sn targets at 90, 100 and 125 MeV have been measured. Theγ multiplicity technique has been used to extract these quantities for each reaction channel identified by discreteγ lines. Incomplete fusion seems to appear in about the same angular momentum window than complete fusion. This is confirmed by the observation of α 2p4n and 2α4n channels appearing at low angular momentum in the 125 MeV reaction.     

Direct α particle emission in incomplete fusion reaction on a spherical target124Sn

Direct α particles cross sections and angular distributions have been measured when bombarding a¹²⁴Sn target with various projectiles:¹⁶O (90, 105, 125 MeV)¹⁹F (104 MeV)²⁰Ne (112, 156 MeV). Results have been found independant of the projectile structure. In the same reactions, gamma multiplicity measurements have shown that direct α particles are emitted in collisions with intermediate impact parameters. The angular momentum is found to be about half of the valuel _cr for complete fusion.     

Complete fusion in a light heavy-ion reaction from 2.5 to 20 MeV/nucleon

Mass distributions of evaporation-residue-like fragments have been measured using a time-of-flight system for the reaction ²⁰Ne + ²⁶Mg in the energy range of 51 to 400 MeV bombarding energy for ²⁰Ne. A good mass resolution allowed for the separation of the evaporation residues and fragments from two-body reactions like e.g. damped processes. The residue distributions were compared with evaporation calculations. The analysis of velocity spectra measured at bombarding energies of 85–395 MeV showed incomplete momentum transfer for evaporation-residue-like fragments at higher energies. Statistical-model calculations and Monte Carlo methods applied to the calculation of the velocity spectra have been used to extract the complete-fusion cross section.     

Mass measurements far from stability around theN=Z line (100Sn,100In,100Cd)

Secondary ions of¹⁰⁰Ag,¹⁰⁰Cd,¹⁰⁰In and¹⁰⁰Sn were produced via the fusion-evaporation reaction⁵⁰Cr+⁵⁸Ni at an energy of 5.1 MeV/nucleon. TheA=100 secondary ions were accelerated in the second cyclotron of GANIL, which was used as a high-resolution mass spectrometer. The masses of the three latter isobars were measured with respect to the abundantly produced¹⁰⁰Ag. The known mass of¹⁰⁰Cd was very well reproduced and the masses of¹⁰⁰In and¹⁰⁰Sn were determined for the first time with precisions of 3×10^?6 and 10^?5, respectively.