Angular distributions of photoneutrons from the 207, 208Pb(γ, n0) reactions

Angular distributions of photoneutrons from the ^{207, 208}Pb(γ, n₀) reactions were measured at 11 angles around θ = 90°. The γ-source, E_γ = 7–11.4 MeV, contained discrete lines (ΔE ? 30 eV) obtained from n-capture and was used in conjunction with a high-resolution ³He spectrometer. Strong evidence for an E2 contribution and for E2-E1 and possibly E1-M1 interference was obtained in both ²⁰⁷Pb and ²⁰⁸Pb. The results are compared with calculations using a direct-semidirect model which involved an E1 and isoscalar E2 giant resonances. The results indicate that this model could explain only certain features of the data while most of the other features remain unexplained.     

Branching ratios for stopped pions in deuterium

The ratios $\text{S =}\text{ω(π}{}^{\mathrm{?}}\text{d}\text{→}\text{nn}\text{)}\text{ω(π}{}^{\mathrm{?}}\text{d}\text{→}\text{nn}\text{γ)}$ has been measured for negative pions stopping in deuterium gas at pressures up to 100 atm. The result is S = 2.83 ± 0.04 which is a factor of two more accurate than previous measurements.     

The polarization of MeV neutrons elastically scattered from 4He

The analyzing power of ⁴He for neutron elastic scattering has been measured at four angles between 20° and 80° (lab) throughout the energy range 1.5–6.0 MeV using a doublescattering method. The intense flux of polarized neutrons was generated via the reactions Pb(γ, n) → ¹²C(n, $\text{n}$)¹²C, and the magnitude of the polarization of the neutron beam measured absolutely in a separate double-scattering experiment. Neutron energies were determined with a nanosecond time-of-flight spectrometer, and the generalized neutron spin-precession method was used to minimize systematic uncertainties.     

Hydrogen and deuterium total neutron cross sections in the MeV region

Total neutron cross sections, using a continuous-energy source of neutrons, were measured for hydrogen and deuterium. The neutron energy range covered was from less than 1 MeV, to about 30 MeV. No structure is observed for either isotope. The precision of the cross-section magnitudes appears to be within 1 % over much of the measured energy interval.     

Proton polarization observables for the elastic and inelastic scattering of 500 MeV protons from Ca and Pb

Measurements for the elastic and inelastic scattering of 500 MeV protons from ⁴⁰Ca and ²⁰⁸Pb at small momentum transfers are reported. The induced proton polarization P and the spin rotation parameter Q were measured for the elastic scattering. The spin rotation parameters D_SS′, D_SL′, D_LL′, d_LS′ and the induced polarization P were measured for transitions to the 3⁻(3.37 MeV) and 5⁻(4.48 MeV) states in ⁴⁰Ca and the 3⁻(2.61 MeV) state in ²⁰⁸Pb. Comparisons of the data with the theoretical calculations were carried out in the framework of nonrelativistic and relativistic approaches. We extracted the spin-up and spin-down channels for the elastic scattering cross sections and found that predictions of the relativistic and nonrelativistic approaches agree well with the spin-up channel data and differ considerably in the spin-down channel.     

Core polarization effects on the Coulomb form factor

Coulomb form factors for the transitions between single-particle (or -hole) states with LS closed shells are investigated. Core polarization effects are calculated by using microscopic models. Information on the effective interaction which cannot be provided by γ-transition data alone is obtained from the momentum transfer dependence of the form factor. An effective interaction is required not to be repulsive in triplet-odd two-particle states in order to explain the experimental data on ¹⁵N. The bare G-matrix gives a reasonable fit to the data while the phenomenological interaction with Rosenfeld mixture does not. It is also shown that the core polarization effect for the Coulomb form factors with high multipolarity is sensitive to the mixing character of the effective interaction.     

The elastic and inelastic scattering of intermediate energy protons on deuterium at small momentum transfer

The differential cross sections for the elastic and inelastic scattering of protons on deuterium have been measured for scattering angles less than 14° at 198.5, 297.6 and 456.6 MeV. These quantities were determined relative to dσ / dΩ for pp elastic scattering with a precision of typically 2%. The range of excitation energies for the (p, p') reaction was chosen to emphasize the region near the np threshold dominated by the final-state interaction in the ¹S₀ channel. Particular attention was given to the dependence on excitation energy of the spectra at 198.5 MeV to examine the sensitivity to the ¹S₀ scattering length, a_np In this paper all data are compared with a new, detailed formulation of a simple model of the reaction mechanism based on the impulse approximation. The experimental results differ from the predictions by typically 10% and the differential cross sections exhibit a sensitivity to the intermediate-energy nucleon-nucleon amplitudes. If the impulse approximation is used to estimate a_np from the data at 198.5 MeV a value of − 24.7 ± 0.4 fm is obtained.     

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.     

Nuclear polarization and signs of quadrupole moments of high-spin Gd isomers

Time-differential perturbed-angular-distribution (TDPAD) experiments have been performed on polarized intermediate- and high-spin isomers ${}^{144}\text{Gd}\text{(10}{}^{\mathrm{+}}\text{)}\text{and}{}^{147}\text{Gd}\text{(}\text{13}\text{2}{}^{\mathrm{+}}\text{,}\text{27}\text{2}{}^{\mathrm{?}}\text{,}\text{49}\text{2}{}^{\mathrm{+}}\text{)}$. Excited Gd nuclei populated by $\text{(}{}^{28}\text{Si}\text{, x}\text{n}\text{)}$ reactions were polarized by the tilted-multifoil technique and implanted in a Gd single-crystal host. Observation of the subsequent electric quadrupole interaction with the known electric field gradient yielded negative values of the sign of the deformation for all isomers studied. The magnitude of the induced nuclear polarization P_I was also determined from the TDPAD data and compared to model calculations in order to deduce the average atomic angular momentum and polarization of Gd ions at v/c ～ 0.018.     

Spin polarization in 16O-induced heavy-ion reactions

The circular polarization P_γ of γ-rays from ¹⁶O-induced reactions on ²⁷Al, ⁴⁸Ti, ⁵⁸Ni and ⁶²Ni at 100 MeV incident energy has been measured in coincidence with projectile-like fragments detected at angles behind the grazing angle. The dependence of P_γ on the reaction Q-value, on the fragment atomic number and, for ¹⁶O+⁴⁸Ti, on the scattering angle was investigated.The results show that deep-inelastic (DI) collisions in these fairly light heavy-ion systems are associated with predominantly negative scattering angles. The average value of P_γ(DI) = 0.6 obtained for ¹⁶O + Ni at θ_lab= 35° is consistent with pure negative-angle scattering. The magnitude of P_γ(DI) is smaller for the two lighter systems which is interpreted as mainly arising from the increasing probability for orbiting trajectories leading to scattering angles beyond 180°Trajectory calculations within a classical friction model were performed, and the parameter dependence of the results was studied. Strong radial friction forces were found necessary to simulate the observed negative-angle cross sections.     

Self-screening of the particle-hole interaction and core polarization of the effective two-particle interaction

The diagrams of the Tamm-Dancoff and random-phase approximations (TDA and RPA) have been summed to calculate the core polarization of the effective interaction between two nucleons in the 1s0d and Ip0f shells. The large core-polarization corrections obtained in the TDA and particularly the RPA are strongly reduced when the self-screening corrections to the particle-hole and ground-state correlation vertices are included. The screening corrections serve to suppress the collectivity of the core phonons and in particular stabilize the isoscalar monopole phonon in ⁴⁰Ca against collapse. When screening is included the TDA and RPA give core-polarization corrections which are similar in magnitude and in many cases lead to good agreement with experiment. Because of neglected effects and other uncertainties the qualitative rather than the quantitative features of the results are stressed.     

A new experimental study of the circular polarization of np capture γ-rays

An installation using a light-water neutron trap in the reactor core as a proton target is described. Results of the main and control measurements are presented which permit one to conclude that the parity-violating circular polarization of the γ-rays from the np → dγ reaction is P_γ = (1.8 ± 1.8) × 10^?7.     

The polarization of 3He elastically scattered from 27Al and 28Si at 21 MeV

The polarization of 21 MeV ³He elastically scattered from ²⁷Al and ²⁸Si has been measured in an angular range of θ_lab = 25°–55°. Differential cross-section data have been obtained out to angles of approximately 100°. The small values observed in the polarization distributions differ somewhat from optical-model predictions based on fits to the differential cross-section data and cannot be used to obtain significant information about the optical-model spin-orbit potential. It is concluded that statistically significant, non-zero ³He polarization measurements will not be possible using traditional double scattering techniques for ³He energies ≦ 27 MeV and targets of $\text{Z ≧ 13}$.     

Circular polarization of gamma quanta in the radiative capture of polarized thermal neutrons by protons

An experiment to measure the circular polarization of γ-quanta, P_γ, in the reaction of radiative capture of polarized thermal neutrons by protons is described. Solid parahydrogen was employed as a proton target.The circular polarization was found to be P_γ = ?(2.90 ± 0.87) × 10^?3 which is in agreement with the theoretical estimates of P_γ arising from the admixture of the triplet capturing state to the ¹S₀ principal capturing state.     

An analysis of polarization and cross-section data for the p-He system at 30 and 50 MeV

Differential cross-section and polarization measurements on the p-³He system at 30 and 50 MeV have been analysed in terms of real phase shifts. It is shown that there is not sufficient data at either energy to define a unique solution.     

Differential cross section for elastic π± helium isotopes scattering from 25 to 65 MeV

Measurements of absolute elastic-scattering differential cross sections for π^±He at T^lab_π = 30.2 MeV, 45.1 MeV and 65.0 MeV and for π^±-⁴?⁴He at T^lab_π = 25.1 MeV and 51.1 MeV have been made in the angular range 40° to 145°. An overall accuracy of about ±5% was achieved. The results are compared with available theoretical calculations.     

Differential cross section and polarization for 2.63 MeV neutrons scattered from 12C

The differential cross section and polarization of 2.63 MeV neutrons scattered from ¹²C have been measured at eight angles between 17° and 118° in the laboratory system. By simultaneously fitting the cross section and polarization data, a set of scattering phase shifts was obtained. The values of the resulting d-wave phase shifts were larger than those of other existing sets of phase shifts in the energy region. A subsequent R-function analysis, reflecting these larger d-wave phase shifts, gave excellent fits to other experimental data below 3 MeV neutron energy region. The influence of narrow states at 7.50 and 7.55 MeV excitation energy in ¹³C is discussed.     

Polarization and polarization transfer in the 2H(d,n)3He reaction at 10 MeV

Angular distributions of six polarization transfer coefficients $\text{K}{}_{\mathrm{x}}{}^{\mathrm{x\prime }}\text{(θ), k}{}_{\mathrm{x}}{}^{\mathrm{z\prime }}\text{(θ), K}{}_{\mathrm{z}}{}^{\mathrm{<mtext>x</mtext><mtext>?</mtext>}}\text{(θ), K}{}_{\mathrm{z}}{}^{\mathrm{<mtext>z</mtext><mtext>?</mtext>}}\text{(θ),}\text{and}\text{K}{}_{\mathrm{yy}}{}^{\mathrm{<mtext>y</mtext><mtext>?</mtext>}}\text{(θ)}$; of the four analyzing powers A_y(θ), A_xx(θ), A_yy(θ), and A_zz(θ); and of the polarization function P^ý(θ), have been measured atE_d = 10.00 MeV for the reaction ²H(d, n)³He. Measurements were made for neutron lab angles between 0° and 80° in 10° steps. Additionally the y-axis associated quantities were measured at θ_1ab = 99°. Most of the measured coefficients are large at some angles and all show considerable variation with angle.