Measurement of neutron polarization from (d,n) reactions on Mg,Si and Ca and comparison to DWBA calculations

Angular distributions of the neutron polarization produced in (d, n) reactions on ²⁴Mg and ²⁸Si were obtained in about 300 keV steps from 3.9 MeV down to 2.2 and 2.9 MeV, respectively. Excitation functions of the polarization were measured at 20° and 40° (lab) over these energy ranges. Polarization angular distributions were also measured for (d, n) reactions on ²⁸Si at 8.1 MeV and ⁴⁰Ca at 3.8 MeV. DWBA calculations are compared to the latter distributions as well as to the (d, n) cross-section data. Fluctuations in the low-energy polarization d ata from the ²⁴Mg and ²⁸Si targets made DWBA comparison of questionable value.     

Evidence for nonequilibrium particle emission before symmetric disintegration of a composite system formed in the 16O+40Ca reaction at 230 MeV

Measurement of fragment-fragment correlations in the reactions of 230 MeV ¹⁶O with ⁴⁰Ca and 280 MeV³²S with ²⁴Mg have been used to isolate processes in which symmetric decay follows nonequilibrium emission of one or two alpha particles. At the higher energy per nucleon. in contrast to previous observations for lower velocity projectiles, nonequilibrium emission followed by symmetric decay has approximately the same probability as the symmetric fission following complete fusion.     

Interfering proton and neutron transfer in the reaction C(N, N)C

Elastic scattering and one-nucleon transfer reactions induced by ¹⁶O have been investigated in the energy region 45–60 MeV on targets of ²⁶Mg, ²⁷Al, ³⁰Si and ⁴⁸Ca. Angular distributions were measured in the angular range 4°–40°. Optical model parameters were derived from the elastic scattering data and the transfer reaction data were analysed using both no-recoil and full-recoil DWBA codes. In the case of proton transfer reactions on ⁴⁸Ca, good agreement was obtained between the data and the DWBA calculations while the data for the lighter targets could not be satisfactorily reproduced. The oscillatory pattern of the angular distributions is discussed in terms of the three-parameter model of Kahana, and it is found that the model qualitatively explains the observed transition from smooth to oscillatory angular distributions.     

The (12C, 8Be) reaction on 12C, 16O, 24,26Mg, 40,48Ca, 54Fe AND 58Ni between 50 and 65 MeV bombarding energy

Absolute cross sections have been measured for the (¹²C, ⁸Be_g.s.) reaction from the target nuclei ¹²C, ¹⁶O, ²⁴Mg, ²⁶Mg, ⁴⁰Ca, ⁴⁸Ca, ⁵⁴Fe and ⁵⁸Ni at various energies between 50 and 65 MeV bombarding energy (lab) using a highly efficient detection system for ⁸Be. The results are presented in form of particle spectra and angular distributions. Except for the lightest target nuclei ¹²C and ¹⁶O, the cross sections decrease rapidly with angle and a one-step direct reaction mechanism is indicated. Satisfactory agreement of the data is obtained with DWBA calculations, using the finite range computer code LOLA of DeVries which treats recoil effects exactly. The spectroscopic factors extracted for the (¹²C, ⁸Be) reaction are close to those obtained from (⁶Li, d), (⁷Li, t) and (¹⁶O, ¹²C) reactions. The selective excitation of the same final states in all of these reactions, as far as data are available, and the close agreement of the spectroscopic factors are interpreted as evidence for a rather simple α-transfer in these reactions in contrast to a more complicated transfer of four nucleons.     

Measurement and interpretation of heavy ion fusion excitation functions

Fusion excitation functions for ³²S induced reactions on ²⁴Mg, ²⁷Al, ⁴⁰Ca and ⁵⁸Ni are reported at incident ³²S ion energies of 65 to 132.5 MeV. Measurements were made using counter-telescopes with beams from Van de Graaff accelerators. From these data barrier heights and radii for fusion are extracted. These results are interpreted in terms of the nuclear diftuseness, and the nuclear attractive potentials at the fusion radii are deduced. Relative density overlaps at the fusion radius are estimated from electron scattering density distributions. Several parameterizations for the fusion radii and barrier heights are presented. Fusion cross sections are compared with reaction cross sections based on elastic scattering measurements coupled with optical model analysis. It is found that for the systems investigated, 90±10 % of the reaction cross section results in fusion.     

Unelastische Elektronenstreuung im Bereich der Riesenresonanz von24Mg,26Mg und40Ca mit Primärenergien kleiner als 60 MeV

Inelastic electron scattering at low primary energies and scattering angles smaller than 180° represents a successful method for the investigation of giant resonances, even though the elastic radiation tail is not known exactly. Energy resolution and statistical accuracy are superior to other methods. It is shown, that the giant resonances in²⁴Mg,²⁶Mg and⁴⁰Ca are excited mainly via E1-absorption. Integrated cross-sections agree with results of photon-experiments. A splitting of the²⁶Mg-giant resonance into two broad bumps is interpreted as isospin-splitting.     

Simultaneousg 9/2 proton and neutron alignments in76Kr

Yrast states with spins as high as 22 ? have been identified in⁷⁶Kr with the reactions⁵⁸Ni(²⁴Mg,α2p) at 110 MeV and⁴⁰Ca (⁴⁰Ca, 4p) at 155 MeV. Simultaneous band crossings due to g_9/2 proton and neutron quasiparticles have been observed in the frequency range 0.60≦?ω≦0.75 MeV. The moment of inertia increases rapidly at the crossing frequency. The systematics of proton and neutron alignments in the Kr isotopes are discussed.     

Transfer and fusion reactions of unstable nuclei

The transfer and fusion reactions are studied for reactions between a stable and an unstable nucleus with neutron skin, taking as examples the reactions ^16,28O+⁴⁰Ca. The two-dimensional time-dependent Hartree-Fock method is used. It is shown that, in such reactions, the nucleon transfer is enhanced enormously for both protons and neutrons. The neutron skin does not enhance the fusion cross section contrary to the usual expectation.     

Lifetime measurement in 74Kr and 76Kr

Lifetimes of excited states in the ground-state bands of ⁷⁴Kr and ⁷⁶Kr were measured using the recoil-distance Doppler-shift and the differential decay curve methods. The states were populated in the ⁴⁰Ca(⁴⁰Ca, α2p) and ⁴⁰Ca(⁴⁰Ca, 4p) reactions. Gamma rays were detected with the GASP array which was coupled to the Cologne Plunger device. The results resolve discrepancies between earlier lifetime measurements and a recent Coulomb excitation experiment. Experimental transition rates are compared to theoretical calculations. The results support a strong mixing between prolate and oblate configurations for the low-spin states, and represent an important basis for the interpretation and understanding of the shape coexistence phenomenon in this mass region.     

(d, 6Li) reactions on intermediate mass nuclei

(d, ⁶Li) reactions have been studied at 28 and 36 MeV on ^{40, 42}Ca, ⁵⁶Fe, ⁵⁸Ni and ¹¹⁴Sn. In most reactions levels were identified up to 3–4 MeV. A DWBA analysis has been carried out in the zero-range approximation, assuming simple configurations for the final state. The spectroscopic factors for the ground-state transitions remain fairly constant from ⁴⁰Ca to ¹¹⁴Sn. Transitions leading to excited states are discussed.     

Statistical analysis of the (d,p) and (d,d) reactions on 40Ca

Excitation functions of the ⁴⁰Ca(d, p)⁴¹Ca and ⁴⁰Ca(d, d)⁴⁰Ca reactions have been measured at 45°, 90°, 135° and 170° from E_d = 4.50 to 5.43 MeV in 10 keV steps. Angular distributions of these reactions have been taken at E_d = 4.70, 5.00 and 5.30 MeV from 25° to 170° in 5° steps. Transitions were observed to the excited states for the range 0.0 ≦ E_x ≦ 3.74 MeV in ⁴¹Ca. Rapid fluctuations in the excitation functions and strong variations of the angular distributions with the incident energy were observed, suggesting that the contribution from compound nucleus processes is very large. Various quantities extracted from the experimental data were compared to the predictions of the statistical theories combined with the DWBA theory for the calculation of the direct reaction amplitudes. The results of the present analysis are consistent with the predictions of the standard statistical theories based on the neglect of the channel-channel correlation.     

Formation of 24Na in “fission-like” reactions

Formation of ²⁴Na in reactions induced by protons of energy 40–85 MeV on ⁴⁰Ca, ⁴⁵Sc, ⁴⁸Ti, and naturally occurring V was measured. Evidence is presented that ²⁴Na is formed in a binary break-up reaction. The results are compared with transition state (fission) calculations.     

Neutron density distributions of neutron-rich nuclei studied with the isobaric yield ratio difference

The isobaric yield ratio difference (IBD) between two reactions of similar experimental setups is found to be sensitive to nuclear density differences between projectiles. In this article, the IBD probe is used to study the density variation in neutron-rich ⁴⁸Ca . By adjusting diffuseness in the neutron density distribution, three different neutron density distributions of ⁴⁸Ca are obtained. The yields of fragments in the 80A MeV ^{40, 48}Ca + ¹²C reactions are calculated by using a modified statistical abrasion-ablation model. It is found that the IBD results obtained from the prefragments are sensitive to the density distribution of the projectile, while the IBD results from the final fragments are less sensitive to the density distribution of the projectile.     

The Two-Nucleon Stripping Process with Skyrme Type Potential

Theory of two-nucleon stripping reactions is reconsidered. The nucleon-nucleon interactions are taken as Skyrme type potentials. Differential cross-sections for two-nucleon stripping reactions are calculated using DWBA approximation with different Skyrme type potentials of different parameters. The angular distributions for different two-nucleon stripping reactions (t, p) with incident triton on the different targets of ³¹P, ⁴⁰Ca, ⁹⁶Zr, ¹¹⁸Sn and ²⁰⁶Pb are calculated giving good agreement with the experimental data. Better values are extracted for the spectroscopic factors.     

Three-body model for stripping nuclear reactions

A three-body model for the deuteron stripping nuclear reactions is presented. A set of three integral equations is obtained for the wave functions of the three-body problem by introducing a decomposition into angular momentum states into the Lippmann-Schwinger equation. Simple two-particle interactions with separable potentials are used. These separable potentials reduce the three-body problem to the solution of coupled sets of one-dimensional Fredholm integral equations. The angular distributions for²⁸Si(d,p)²⁹Si and⁴⁰Ca(d, p)⁴¹Ca stripping reactions are calculated. From the extracted spectroscopic factors, good agreement with the experimental measurements is obtained.     

Theoretical study of fusion reactions 32S + 94,96Zr and 40Ca + 94,96Zr and quadrupole deformation of 94Zr

The dynamic coupling effects on fusion cross sections for reactions~(32)S + ~(94,96)Zr and ~(40)Ca + ~(94,96)Zr are studied with the universal fusion function formalism and an empirical coupled channel(ECC) model. An examination of the reduced fusion functions shows that the total effect of couplings to inelastic excitations and neutron transfer channels on fusion in ~(32)S +~(94)Zr(~(40)Ca +~(94)Zr) is almost the same as that in ~(32)S +~(96)Zr(~(40)Ca +~(96)Zr). The enhancements of the fusion cross section at sub-barrier energies due to inelastic channel coupling and neutron transfer channel coupling are evaluated separately by using the ECC model. The results show that effect of couplings to inelastic excitations channels in the reactions with94 Zr as target should be similar as that in the reactions with ~(96) Zr as target. This implies that the quadrupole deformation parameters β_2of ~(94)Zr and~(96) Zr should be similar to each other.However, β_2 's predicted from the finite-range droplet model, which are used in the ECC model, are quite different. Experiments on~(48) Ca +~(94)Zr or~(36) S +~(94)Zr are suggested to solve the puzzling issue concerning β_2for~(94)Zr.     

On the excitation of isovector dipole strength by inelastic proton scattering in the giant resonance region in light nuclei

A detailed comparison between inelastic α and p scattering in the giant resonance region of ^{24, 26}Mg, ²⁸Si and ⁴⁰Ca shows that there is no evidence for ΔT = 1, E1 excitation in the (p, p′) spectra. This is consistent with DWBA calculations using a recently obtained isovector interaction potential.     

Correlation phenomena in the fragmentation of 32s-induced reactions

Fragmentation data from the reactions ³²S + ²⁸Si, ⁴⁰Ca, ⁵⁸Ni at 10 MeV/u are used to deduce the scission configurations. Assuming three fragments the possible scission configurations are expanded into two basic configurations (closed and stretched configurations) which can be distinguished by means of the relative fragment momenta. The majority of the analyzed reactions is in agreement with the assumption of random fragmentation. However, in a few reactions consisting of one heavy and two light fragments the fragmentation appears to pass through the closed configuration, whereas the stretched configuration is formed in reactions containing one light and two heavy fragments.