Measurement and MIA analysis of the neutron differential cross section and polarization in the α-D break-up at 33 MeV c.m.

Differential cross sections at 0° and 20° and polarizations at 20° have been measured for the ²H(α, n) αp reaction at E_α = 100 MeV and the ⁴He(d, n) αp reaction at E_d = 50 MeV. Data have been analyzed by the Modified Impulse Approximation (MIA). Satisfactory agreement was obtained but some discrepancy remained in the analysis of polarization data.     

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.     

Light charged particle emission induced by neutrons with energies between 25 and 65 MeV on oxygen. I. Protons and deuterons

Double-differential cross sections (energy spectra) for proton and deuteron emission in fast neutron induced reactions on oxygen are reported for nine incident neutron energies between 25 and 65 MeV. Angular distributions were measured at 15 laboratory angles between 20° and 160°. Procedures for data taking and data reduction are presented. Deduced energy-differential, angle-differential and total cross sections are also reported. Experimental cross sections are compared with existing data and with theoretical model calculations. Received: 24 April 1998 / Revised version: 1 July 1998     

Neutron induced light-ion production from iron and bismuth at 175 MeV

We have measured light-ion (p, d, t, ³He and α) production in the interaction of 175 MeV neutrons with iron and bismuth with low-energy thresholds and for a wide angular range (from 20° to 160°, in steps of 20°). Measurements have been performed with the Medley setup, semi-permanently installed at the The Svedberg Laboratory, Uppsala (Sweden), where a quasi-monoenergetic neutron beam is available and well characterized. Medley is a conventional spectrometer system and consists of eight telescopes, each of them composed of two silicon surface barrier detectors, to perform particle identification, and a CsI(Tl) scintillator to fully measure the kinetic energy of the produced light-ions. We report preliminary double-differential cross sections for production of protons, deuterons and tritons in comparison with model calculations using TALYS-1.0 code. These show better agreement for the production of protons, while the theoretical calculations seem to overestimate the experimental production of deuterons and tritons.     

Inclusive π4He scattering and the πN interaction in the nuclear medium

Double differential cross sections for the inclusive reaction ⁴He(π, π′)X have been measured for six pion energies between 90 and 320 MeV and an angular range from 30° to 135°. The data are interpreted in terms of the Δ-hole formalism. Medium corrections to the free π-nucleon scattering operator are important for a quantitative understanding of our results. Total inelastic cross sections are obtained and estimates for the absorption cross section are given.     

Two-body photodisintegration of 3He and 4He in the Δ(1236) region

Differential cross sections for the reactions ³He(γ, p)d and ⁴He(γ, p)t were measured at proton c.m. angles of 60° and 90° for photon energies ranging from 150–450 MeV with an average resolution of 8 MeV. Bremsstrahlung was used as the photon source; deuterons/tritons were analyzed in a magnetic spectrometer whereas coincident protons were detected in a plastic scintillator telescope. The experimental method includes a calibration by means of ¹H(γ, π⁰)p differential cross section measurement at 90° c.m. in the same photon energy range. The ³He and ⁴He two-body photodisintegration differential cross sections show a monotonically decreasing variation with photon energy. In addition, partial data on the differential cross section of the reaction ⁴He(γ, n)τ at 90° and 120° neutron c.m. angle are given.     

Quasi-free scattering of pions on 3, 4He

Differential cross sections for ^{3, 4}He(π, π') are presented for angles of 60° and 120° at 200 MeV and 120° at 295 MeV. The momentum spectra for the scattered pions are dominated by a peak attributed to quasi-free scattering from individual nucleons. There are significant differences between the scattering on ³He and ⁴He as well as between the scattering of π⁺ and π^? on ³He. The data are compared to a simple model incorporating the free π+N scattering amplitudes. Modifications for the structure and dynamics of the target nucleus are discussed.     

The C(α, X)11C cross sections at 1.59 and 4.19 GeV

The absolute cross sections for the production of ¹¹C by 1.59 GeV and 4.19 GeV α-particles incident on natural carbon have been measured to be 46.4 ± 1.3 mb and 42.5 ± 1.1 mb respectively. These results, together with data reported at other energies, indicate that the C(α, X)¹¹C cross section becomes approximately constant at a value of about 43 mb for energies above 3 GeV (750 MeV/n). A similar energy dependence is exhibited by the C(p, X)¹¹C reaction whose cross section has been measured previously over an extensive energy range. The C(α, X)¹¹C cross sections are found to be in good agreement with predictions of a semi-empirical model developed to describe nuclear fragmentation.     

A study of the 2H(d,p)3H AND 2H(d,n)3He reactions and the excited state of 4He at 23.9 MeV

The relative differential cross sections have been measured for the ²H(d, p)³H and the ²H(d, n)³He reactions from 300 keV to 700 keV in 50 keV steps. Angular distributions of all charged particles from the reactions were taken from 20° to 160° in the laboratory system. Energy-dependent asymmetry coefficients from the expansion of the centre-of-mass angular distributions in terms of even powers of cos θ were obtained as were the branching ratios between the two reaction modes. The different energy dependences of the moments of the two cross sections were used to test the need for the existence of a recently reported T = 0 state in ⁴He.     

Two-spectator quasifree processes

Two-spectator quasifree processes (graphs with two spectator particles) for the ³He(³He, dd)pp reaction at the symmetric angle pairs θ = 30.1° at E_³He = 50 MeV and θ = 37° at E_³He = 78 MeV are investigated. The theoretical cross sections are calculated with the PWIA model. The measured cross sections are smaller but have shapes which are in good agreement with the calculation. The ratio N = experiment/theory is approximately 0.05 at 50 MeV and 0.1 at 78 MeV. The reaction ²H(d, pp)nn also was studied at the symmetric angle pair θ = 34.8° and E_²H = 34.7 MeV and the ratio is about 0.14.     

Resonance spectroscopy of44Ti in the excitation energy range 9.05 to 10.28 MeV

The absolute differential cross sections for the⁴⁰Ca(α,α)⁴⁰Ca reaction have been measured in the bombarding energy range from 4.33 to 5.68 MeV at the four laboratory angles 85°, 125°, 141° and 165°. An analysis of the data using multilevelR matrix theory has provided the spin-parities and widths of 29 levels in the compound nucleus⁴⁴Ti. A comparison has been made with the levels deduced from the (α,γ) reaction and existing theoretical calculations of⁴⁴Ti levels.     

The reaction 14C(3He,n)16O and the coexistence model of 16O

The reaction ¹⁴C(³He, n)¹⁶O has been measured at a ³He bombarding energy of 25.4 MeV. The zero-degree differential cross section for the excitation of the three low lying 0⁺T = 0 states, at energies 0.0, 6.05 and 12.05 MeV are, respectively, 1.33 ± 0.10, 0.49 ± 0.10, and 0.50 ± 0.10 mb/sr These measured cross sections are in rough agreement with single-step zero-range DWBA calculations using an empirically determined ¹⁴C ground state wave function and in which the Brown and Green coexistence-model wave functions are used to describe the ¹⁶O 0⁺ states. The angular distribution of the transition to the ground state is measured between 0° and 32°.     

Comparison of the noncoplanar 6Li(p,pd)4He reactions at 120 and 200 MeV

The ⁶Li(p, pd)⁴He reaction was studied at 200.2 MeV, at the quasi-free angle pair (θ_p, θ_d) = (54°, ?48.9°), for noncoplanarity angles φ from 0° to 28°. ⁶Li αd spectroscopic factors of 0.84 and 0.76 are deduced from our coplanar data at this energy and 120 MeV, respectively, for ground-state 2S Woods-Saxon wave functions. A recent microscopic three-body calculation predicts spectroscopic factors from 0.70 to 0.75; using the ground-state wave functions from this study, we deduce a factor of 0.76 from the 200 MeV data. DWIA calculations fit the measured integrated cross sections versus φ for spectator momenta P_α ? 100 MeV/c at both bombarding energies, but underpredict them for larger P_α. Momentum form factors were better reproduced with 1S αd cluster wave functions for a soft-core bound-state potential than with the 2S Woods-Saxon wave functions, but the former wave functions generate unphysically large (～1.25) spectroscopic factors.     

Investigation of the mechanism of inelastic alpha-particle scattering on 28Si nuclei by the method of angular αγ correlations at E α = 30.3 MeV

The angular dependence of the differential cross sections for alpha-particle scattering on ²⁸Si nuclei and double-differential cross sections for the reaction ²⁸Si(α, αγ)²⁸Si at E _α = 30.3 MeV is measured for the case of alpha-particle emission angle between 20° and 160° and the excitation of low-lying states of the ²⁸Si nucleus (0⁺, ground state; 2⁺ state at 1.78 MeV; 4⁺ state at 4.62 MeV; 0⁺ state at 4.96 MeV; and 3^? + 4⁺, 6.88 MeV + 6.89 MeV). The spin-tensor components of the density matrix for the 2⁺ state at 1.78 MeV and the 4⁺ state at 4.62 MeV in the ²⁸Si* nucleus are reconstructed in a modelindependent way. Seven rank-6 components are reconstructed for the 3^? state at 6.88 MeV. Orientation features of ²⁸Si* are determined. The experimental data in question are compared with the results of the calculations performed under the assumption of the collective-excitation mechanism and by the coupled-channel method.     

Measurement of the two-body photodisintegration of3He in theΔ(1236) resonance region

The³He(γ, p)d reaction has been measured in the photon energy region between 200 MeV and 450 MeV at proton c.m. angles between 20° and 150°. Protons and deuterons were detected in coincidence with two time-of-flight spectrometers consisting of scintillation counters; both particles were identified and their energies and angles were measured. The angular distributions show a strong forward peak. The differential cross sections fall off with increasing photon energy without showing a significant influence of theΔ resonance.     

Elastic scattering of 3He nuclei on 13C nuclei at 50 and 60 MeV and V-W ambiguity in choosing optical potentials

At energies of 50 and 60 MeV, the elastic scattering of ³He nuclei on ¹³C nuclei is investigated at laboratory angles in the range 10°–170°. The measured differential cross sections are analyzed on the basis of the optical model of the nucleus by using Woods-Saxon potentials, including both volume and surface absorption. The potential parameters are determined by fitting the computed cross sections to experimental data. It is found that, even in the region of sensitivity, the values of the real and imaginary parts of the potentials (V and W, respectively) show considerable scatter, with extreme values differing by a factor greater than two. This scatter is explained by the existence of a V-W ambiguity in choosing optical potentials.     

Differential cross section,analyzing power and phase shifts for p-3He elastic scattering below 1.0 MeV

Differential cross sections and analyzing powers for the elastic scattering of polarized protons by unpolarized ³He nuclei have been measured at eight energies between 0.3 MeV and 1.0 MeV for scattering angles θ_c.m. = 52.4°–173.3°. The cross-section values were normalized to the Rutherford cross section for proton-krypton scattering. The analyzing powers have been measured with a statistical accuracy of about 0.001. The phase-shift analysis based on these data included all phases for orbital angular momenta l ≦ 1 and the channel-spin mixing parameter for the P waves. An energy parametrization of the phase shifts by an effective-range approximation allowed a simultaneous utilization of all data.     

A measurement of neutron-proton bremsstrahlung near 130 MeV

We have measured the double differential cross section for bremsstrahlung production in neutron-proton collisions near 130 MeV. Outgoing nucleons were detected at eight pairs of angles simultaneously. The cross sections agree with several theoretical predictions for those four angle pairs in which the proton emerged at 20° (θ = 20°), but are larger than expected in the four cases that θ_p = 32°. The method of data reduction, and the sources of uncertainty, are discussed in detail.     

The elastic scattering of 3He on 4He at medium energies

The differential cross section of the elastic scattering ³He(⁴He, ⁴He)³He has been measured at center of mass energies E_c.m. between 28 and 44 MeV and in the c.m. angular range of 20° to 160°. The ³He polarization P at E_c.m. = 42 MeV and θ_c.m. = 132° was determined in a double scattering experiment. The analysis of the cross section data with the optical and the cluster model has been described previously. The phase shifts obtained in the cluster model calculations were used as starting values in a phase shift analysis. The resulting final real phase shifts and elasticity parameters give good fits to the cross section data. At 44 MeV the elasticity parameters show a pronounced odd-even dependence on angular momentum which had been found already in the case of the real phase shifts. The result of the polarization experiment |P| < 0.22 is consistent with cluster model predictions.     

Photoemission of protons from4He in theΔ-resonance region

High resolution proton energy spectra from the⁴He(γ,p) reaction have been measured with tagged photons in the range E_γ = 130?525 MeV using the large Mainz NaI(T1) spectrometer atΘ _p ^lab =37.1°. Three separate reaction channels were identified, viz.⁴He(γ,p)t two-body breakup,⁴He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi motion is taken into account.