The 10,11B(n, p)10,11Be reactions at En=96 MeV

Double-differential cross sections of the ^10,11B(n, p)^10,11Be reactions have been measured at 96 MeV in the angular range 0°–30° for excitation energies up to 35 MeV. The spectra have been decomposed into different multipolarities using sample angular distributions calculated within the distorted-wave Born approximation. From the identified Gamow–Teller strength, S_β⁺ values were obtained for ¹⁰B and ¹¹B. At higher excitation energies, the spectra are dominated by L=1 strength in broad distributions with maxima around 22 and 12 MeV in ¹⁰B and ¹¹B, respectively.     

Investigation of the fusion-fission reaction 12C+208Pb

For the first time the cross section and angular distributions of fission fragments have been measured for the compound nucleus 220 Ra produced in the reaction 12 C+ 208 Pb at three energy values in the range from 0.5 to 8.0 r MeV below the fusion barrier calculated in terms of the Bass model. In the investigated energy range there is a plateau characterized by the relation W (180°)/ W (90°)=1.2 in the angular anisotropy of fission fragments.     

High spin selectivity in α-particle induced reactions

Distinct maxima have been observed in the spectra of protons emitted from the ²⁴Mg(α, p)²⁷Al reaction in the region of the excitation energies of ²⁷Al between 5 and 15 MeV, where continuous spectra are expected according to the statistical model. To investigate the reaction mechanism responsible for the selective excitation of several states of ²⁷Al, angular distributions of protons in the angular range from 20° to 170° in the lab system were measured at the α-particles energies E_lab = 25.4 25.8 and 26.2 MeV. Thirteen out of thirty levels observed in proton spectra, exhibit symmetry of their angular distributions around 90° the c.m. system, when averaged over energy. The contribution of preequilibrium emission was calculated and found to be negligible. A Hauser-Feshbach analysis was carried out to determine spins of states of ²⁷Al whose angular distributions were symmetric around 90° in the c.m. system. Results of this analysis indicated high spin selectivity in the ²⁴Mg(α, p)²⁷Al reaction.     

Direct and compound nuclear contributions to the 13C(6Li,t) reaction at Elab = 25 MeV

The reaction ¹³C(⁶Li, t)¹⁶O has been studied in the incident energy range 24–26 MeV. Complete angular distributions have been measured at E_⁶Li, = 25 MeV in the angular range θ_lab = 8°–172°, with the reaction ⁶Li(¹³C, t)⁶O being used for the backward angle measurements. Cross sections for evaporation residues from the fusion of the ⁶Li + ¹³C system have been measured in the incident ⁶Li energy range 9.2–35.1 MeV. Compound nuclear contributions to the transfer cross sections have been calculated using the Hauser-Feshbach statistical theory with the assumption that the compound-nucleus formation cross section is equal to the measured fusion cross section. By comparison of the compound nuclear calculations with backward angle data it is found that the sharp cutoff approximation commonly used to represent the initial angular momentum distribution of the compound nucleus is not adequate for the ¹³C(⁶Li, t)¹⁶O reaction. Good fits to the backward angle data can be obtained by using a smooth cutoff approximation. The forward angle cross sections have been compared with exact finite-range distorted-wave Born approximation calculations to extract transferred angular momenta and spectroscopic strengths. The present results differ from those of an earlier study. These differences are due to the inclusion of forward angle data in the present study.     

Elastic and inelastic scattering of 185 MeV protons from 12C

The polarization of 185 MeV protons in elastic scattering and in the excitation of the 2⁺ state at 4.44 MeV and the 0⁺ state at 7.65 MeV in ¹²C has been measured in the angular region 2°–60°. Optical model calculations are performed for the elastic scattering. Angular distributions for the inelastic scattering from the 2⁺ state at 4.44 MeV and the 3^? state at 9.64 MeV are calculated in the distorted-wave impulse approximation (DWIA) as well as in the distorted-wave Born approximation (DWBA).     

Angular distributions of photoprotons from 208Pb

Angular distributions of photoprotons following the (e, e′p) reaction in ²⁰⁸Pb have been measured at incident energies of 25.0 MeV and 40.0 MeV. The proton energy distributions were measured at several angles and the angular distributions were obtained for several proton energy ranges. Proton groups at around 11.0 MeV and 15.3 MeV are separated for the angular distributions and the results show almost isotropic distributions, which indicate that these proton groups are emitted through IAR. The angular distributions for other slowly varying parts of the proton spectra proved strongly asymmetrical, indicative of interference between E1 and E2 transition.     

Microscopic interpretation of the (<Emphasis Type="Italic">t</Emphasis>, <Superscript>3</Superscript>He) reaction at 130 MeV on <Superscript>90</Superscript>Zr and isovector monopole strength

The 130-MeV primary tritium beam of the AGOR facility with an intensity of up to 10⁸ pps and the Big Bite Spectrometer experimental setup have been used to study the (t, ³He) reaction between 0° and 5° lab angles on ¹²C and ⁹⁰Zr targets. The standard ray-tracing procedure has allowed us to obtain excitation-energy spectra up to 30 MeV in six angular bins for each residual nucleus, with an average energy resolution of 350 keV. We have used the DWBA reaction mechanism model to reproduce those spectra and their angular distributions. In this approximation, the form factor was treated as a folding of an effective projectile-nucleon interaction with a transition density. The effective projectile-nucleon interaction has been adjusted to reproduce the 0° cross section of the 1⁺ ground state of ¹²B populated in the ¹²C(t, ³He) reaction. We have employed RPA wave functions of excited states to construct the form factors. This DWBA+RPA analysis is used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in the ⁹⁰Y nucleus. In this talk, we give some details on this analysis. We show that there are important contributions of L = 2 transitions in the observed cross sections for the 1⁺ final states that explain the previous difficulties in clearly identifying the monopole strength distributions. We then have a better indication of where the L = 0 part is located with this reaction and its microscopic analysis. The text was submitted by the author in English.     

7Li(α,n)10B differential cross-section measurements from threshold to Eα = 5.1 MeV

Differential cross sections for the ⁷Li(α, n)¹⁰B reaction have been measured at lab angles of 0°, 20°, 31°, 50°, 60°, 70°, 80°, 90°, 100° and 114° for α-particle energies between 4.385 and 5.1 MeV. A thick natural lithium target was bombarded with a 5.2 MeV, nanosecond-pulsed ⁴He⁺ beam and neutron velocity spectra at each angle were measured by time-of-flight techniques. These data have been converted to cross sections at 10 keV intervals in α-particle energy. Angular distributions have been fitted with a series of Legendre polynomials. Angle-integrated cross sections, the 0° excitation function, and angular distributions are compared to past measurements and R-matrix calculations.     

Measurement of double-differential (n,xp) cross sections of natural nickel in 14.6 MeV neutron energy

The energy spectra and angular distributions of proton emission in the reaction of ^natNi(n,xp) at neutron energy 14.6 MeV have been measured by the USTC multitelescope system. The double-differential cross sections of 16 reaction angles from 25° to 164.5° have been obtained in this measurement. The statistical error can be reduced because of the thicktarget used. The angular distributions show a slightly energy-dependent forward-backward asymmetry. The angle-integrated proton spectrum is compared with ENDF/B-VI evaluation and Grimes' result. The total p-emission cross section is in fair agreement with prediction and evaluation.     

High resolution gamma-ray spectroscopy of the27Al(p,γ)28Si resonance reaction

High resolution gamma-ray spectra have been measured from the²⁷Al (p,γ)²⁸Si reaction for the resonances atE _p=2·482, 2·511 and 2·735 MeV at ϑ _pγ=0°, 30°, 55° and 90° using a Ge (Li) gamma spectrometer. From the spectra and the angular distributions the properties of the resonance states have been obtained. These states are the isobaric analogues of the levels at 4·69, 4·75 and 4·93MeV levels respectively in the parent nucleus A²⁸l.     

From quasi-elastic to quasi-compound nucleus reactions induced by a 140 MeV 19F beam bombarding 89Y target nuclei

Various exit channels ranging from ⁶Li to ²⁷Al for the ⁸⁹Y(¹⁹F, x)y reaction have been studied at 140 MeV incident energy. Energy spectra and angular distributions were measured between 10° and 160° laboratory angles. Three different reaction mechanisms were observed: quasi-elastic collisions, deep-inelastic collisions and, with weaker cross sections and longer lifetimes, compound nucleus-like processes.     

Differential cross sections of proton compton scattering at photon laboratory energies between 700 and 1000 MeV

Differential cross sections of proton Compton scattering have been measured at the Bonn 2.5 GeV synchrotron. 78 data points are presented as angular distributions at photon lab energies of 700, 750, 800, 850, 900, and 950MeV. The c.m. scattering angle ranges from 40°–130°, corresponding to a variation of the four momentum transfer squared betweent=?0.10 tot=?0.96 GeV² at 700 and 950 MeV, respectively. Two additional differential cross sections have been measured at 1000MeV, 35.6° and 47.4°. The angular distributions show forward peaks whose extrapolations to 0° are consistent with calculated forward cross sections taken from literature. The small angle data (|t| ?0.2 GeV²) together with the calculated cross sections at 0° are also consistent with the assumption of a slope parameterB of 5 GeV^?2. For the first time a rerise of the angular distributions towards backward angles has been observed. It becomes less steep with increasing energy. The most interesting feature of the angular distributions is a sharp structure which appears betweent=?0.55 GeV² at 700MeV andt=?0.72 GeV² at 950 MeV. Such a rapid varation of the differential cross section witht has never been ovserved in elastic hadron-hadron scattering or photoproduction processes. It indicates the existence of a dynamical mechanism which could be a peculiarity of Compton scattering.     

Interaction of loosely bound radioactive 7Be and stable 7Li with 9Be

Quasielastic scattering angular distributions have been measured for the ⁷Be + ⁹Be system at E _lab = 17 , 19 and 21MeV in the angular range $ \theta_{{cm}}^{}$ = 24^° - 57^° . An optical model (OM) analysis of these data has been carried out in order to extract optical potential parameters and reaction cross-sections. One-proton stripping cross-sections were also measured for this system at E _lab = 19 and 21MeV. These transfer angular-distribution data were compared with the finite-range distorted-wave Born approximation (FRDWBA) calculations. For the ⁷Li + ⁹Be system quasielastic scattering angular distributions were measured and emitted light charged particles were detected at E _lab = 15.75 , 24.00 and 30.00MeV in the angular range $ \theta_{{cm}}^{}$ = 7^° - 70^° . Fusion cross-sections were obtained by reproducing the measured $ \alpha$ -evaporation spectra from the compound nucleus at backward angles with the statistical model calculations. The ratios of the experimental fusion cross-sections to the total reaction cross-sections (obtained from OM analysis) were found to be small. This result suggests that the break-up process has a strong influence on the fusion process leading to a reduction in the fusion cross-section.     

Positive pion production at threshold on A 10B target

The angular distribution of low-energy positive pions (E_π ≈ 11 MeV) produced by 154 MeV protons on a ¹⁰B target has been measured between 16.2° and 54.8° c.m. angles. Only pions corresponding to the ground state of ¹¹B were observed. The data have been compared with theoretical angular distributions calculated in the framework of the one-nucleon mechanism using the DWBA formalism.     

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The present work represents the results of angular distribution measurements performed for promt gamma-rays in the range of 1.1 … 1.2. MeV due to the ²³³U(n, f) and ²³⁹Pu(n,f) reactions. The measurements yielded anisotropy values found to be consistent with values previously measured and using the same technique at different gamma energy bands. Such consistency, also found to exist between anisotropy values previously measured for the ²³⁵U(n,f) reaction, is a strong evidence about the energy independence of the anisotropy and indicates that the gamma spectra (at 180° and 90° to the fission direction) are essentially the same. The average values of the fragment angular momentum were calculated according to Strutinsky and Nix-Swiatecki theories. It was found that the values of the average angular momentum calculated for ²³⁴U, ²³⁶U and ²⁴⁰Pu according to Strutinsky,s formula (at different gamma energy bands) are consistent and yield average values which are in good agreement with those obtained from direct measurements.     

Excitation of the analogue of the T> giant dipole resonance of90Zr via the (n,p) reaction

Proton spectra have been measured at lab angles ofθ=0° to 100° for the reaction Zr(n, p)Y at 21.8 MeV incident neutron energy. Spectra over an angular spread of 80° were accumulated simultaneously using a detection system consisting of proportional counterΔE detectors and a curved plastic scintillator as energy detector. Resonant behaviour is observed in the proton spectra for the Zr(n, p)Y reaction but not for the Fe(n, p)Mn reaction. The resonant structure is attributed to the excitation of the analogues ofT _> giant dipole states of Zr. The resonant structure is shown to correspond in position, gross and fine structure, and, in strength and angular dependence of the cross sections, with expectations for the⁹⁰Zr(n, p)⁹⁰Y reaction. DWBA cross sections are derived and fitted to the experimental data with a Lane potential of 150 MeV.     

Proton polarization in the14N(3He,p)16O reaction near 10 MeV and reaction mechanisms

The angular distribution of the proton polarization has been measured between 30° and 110° in the CM system near 10 MeV laboratory energy. It provides a sensitive test of theoretical fits to the cross section angular distribution. The reaction mechanism is largely direct below 70° and evidence has been found for a deuteron-cluster transfer. Nuclear Reactions¹⁴N³(He,p),E=9.8 MeV; measuredP(E,θ), σ(E,θ); DWBA and deuteron transfer analysis; deduced dominant mechanism of forward angle amplitudes.     

On the mechanism of the (n, 2n) reaction with 14MeV projectiles on127I and209Bi

The angular and spectral distributions of coincident neutrons from the reactions¹²⁷I(n, 2n)¹²⁶I and²⁰⁹Bi(n, 2n)²⁰⁸Bi have been measured with two time-of-flight detectors at the incident energy 14.1 MeV. Neutron emission has been studied for reaction anglesθ _lab between 10° and 150° (relative angles from 60° to 270°). Energy spectra, angular distributions and coincidence yields are compared with statistical model calculations including preequilibrium decay modes, and nuclear level density and spin cut off parameters are derived.     

The total reaction cross section for α-induced deuteron break-up in the energy range 20–28 MeV

Alpha-particle spectra resulting from the bombardment of douterons with α-particles have been measured at energies of 20.4, 24.2 and 28.1 MeV for all kinematically possible scattering angles θ_1ab > 5°. Angular distributions for the break-up cross section have been obtained by integration over the α-particle energy. The total reaction cross section is computed by extrapolating the angular distributions to 0°. The results are compared with predictions calculated from α-d phase shifts and Faddeev theory.     

A study of the (α, t) reaction on 19F, 27Al, 51V and 59Co

The angular distributions of tritons from the (α, t) reaction on ¹⁹F, ²⁷Al, ⁵¹V and ⁵⁹Co nuclei corresponding to the (0⁺) ground states and (2⁺) excited states in the final nuclei have been measured in the angular range between 15° and 170° at α-particle energies of 25 MeV. For reactions on ²⁷Al and ⁵¹V nuclei, the differential excitation functions have also been obtained at different angles of outgoing tritons at E_α from 20 to 25 MeV. The experimental angular distributions are analysed by the DWBA approximation on the assumption of a nucleon stripping mechanism. The analyses of the present results and the data obtained earlier for the (α, t) reaction on the 1 p shell nuclei, A ? 30, reveal that the distinguishing feature of the reaction under study is the presence of backward angle peaks in the reaction cross section, which appear to be associated with exchange processes. For the (α, t) reaction on the heavier nuclei (A > 30), the dominant mechanism is nucleon stripping.