Investigation of the <Emphasis Type="Italic">n-γ</Emphasis> angular correlation in the <Superscript>9</Superscript>Be(α,nγ)<Superscript>12</Superscript>C reaction by analysis of the 4.44-MeV <Emphasis Type="Italic">γ</Emphasis>-line shape

A method for evaluating the angular correlation function (ACF) of neutrons and γ quanta produced in nuclear reactions has been developed based on the analysis of Doppler-broadened γ-line shapes. The lineshape analysis of the 4.44-MeV_γ line from the ⁹Be(α, nγ)¹²C reaction at E _α = 4.03 MeV and θ_γ = 0°, 15°, 30°, ..., 150°, taking into account the angular n and γ angular distributions, made it possible to unambiguously fit the ACF parameters. In view of the importance of this reaction for γ diagnostics of plasma, the γ-line shape has been calculated for the monoenergetic component of isotropic α-particle distribution, characteristic of d-t experiments.     

锗、硅、锑化铟和砷化镓的热膨涨——用X射线衍射法测量

本工作是用X射线衍射法测量锗、硅和合金InSb及GaAs在不同温度的点阵常数,观察它们的热膨涨,并求得它们的膨涨系数。     

Elastic Scattering of α-Particles on Be9

The differential cross-section for the elastic scattering of He⁴ particles on Be⁹ has been studied in the bombarding energy range 1.4-2.5 MeV. The excitation functions for the elastically scattered α-particles from Be⁹ were measured at six angles 90°, 125°, 132°, 140°, 150° and 160° in the centre of mass system. A clear and pronounced resonance is observed in the measured cross-section at incident α-particle energy 1.93 MeV. Analysis of the data in terms of the single level approximation theory leads to the assignment Jπ = 5+/2 for the observed resonance. Reduced and partial widths for elastic scattering were determined.     

Natural Abundance 17O NMR Study of Substituted α,α,α-Trifluoromethoxybenzenes

Natural abundance ¹⁷O NMR chemical shift data for meta- and para-substitued α,α,α-trifluoromethoxybenzenes recorded in acetonitrile at 75° C are reported. The ¹⁷O NMR signals for the trifluoromethoxy compounds are deshielded by greater than 65 ppm compared to analogous methoxy compounds. A quantitative relationship between ¹⁷O NMR chemical shifts for the trifluoromethoxy and methoxy benzenes is reported.     

Study of the fission decay of the isoscalar monopole resonance via the 238U(α, α′) reaction at 0°

The fission decay of ²³⁸U has been investigated using inelastic scattering of 120 MeV ga-particles to excite the ²³⁸U nucleus. Angular correlations of the fission fragments have been measured for excitation energies between 5.7 and 15.7 MeV in coincidence with inelastically scattered α-particles between 0 and 3°. The difference in yield for fission in coincidence with inelastically scattered α-particles between 0–1.35° and 1.35–3° was used to deduce the fission decay of the giant monopole resonance. It was found that in the fission decay channel (22 ± 5)% of the E0 EWSR strength is located between 8 and 15 MeV excitation energy. The distribution of the deduced monopole strength is in agreement with recent theoretical calculations predicting splitting of the giant monopole resonance in deformed nuclei.     

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.     

Reinvestigation of proton threshold states in26Al

The²⁷Al(³He,α)²⁶Al reaction has been used to determine 1-transfers for states in²⁶Al between 6.3- and 6.5- MeV excitation. Angular distributions of α-particles between 5° and 15°, when added to our previously published results confirm a strong 1=0 component for a state at 6363 keV but indicate pure 1=2 for states at 6410 and 6498 keV. We discuss implications for nucleosynthesis.     

Radiation enhanced diffusion in UO2 and (U,Pu)O2

Abstract

The radiation enhanced diffusion (coefficient D*) of U-233 and Pu-238 in UO₂ and (U, Pu)O₂ with 2.5 and 15% Pu was measured during fission in a nuclear reactor. Normal diffusion sandwiches with a thin tracer layer were used. A radio-frequency furnace allowed the temperatures to be varied between 130 and 1400°. Neutron fluxes (7 × 10¹² to 1.2 × 10¹⁴ n cm^?2 s^?1) and irradiation times (56 to 334 h) were also varied to cover ranges of fission rates [Fdot] between 7× 10¹¹ and 6.4 × 10¹³ f cm^?3 s^?1 and of doses F between 4.2 × 10¹⁷ and 3.1 × 10¹⁹ f cm³. Below ～1000°, D* was completely athermal and increased linearly with [Fdot]. It was described by D* = A[Fdot] with A = 1.2× 10^?29cm⁵. A possible temperature dependence was indicated between ～1000and 1200°. The results are explained in terms of thermal and pressure effects of fission spikes and are related with other studies of radiation damage as well as with technologically interesting processes occurring in UO₂ during irradiation.     

Decay of the isoscalar giant monopole resonance in 208Pb

The decay of the isoscalar giant monopole resonance (GMR) in ²⁰⁸Pb has been studied with the ²⁰⁸Pb(α, α'n)²⁰⁷Pb reaction at θ_α′ = 0°. Comparison of the experimental data with detailed statistical model calculations shows a good overall agreement for the decay of both the GMR and the underlying continuum. Interpreting small discrepancies between the calculations and data for the GMR as indications for the presence of pre-equilibrium decay we find a direct decay branch of the GMR ⩽ 10% and a pre-equilibrium decay branch ⩽ 30%.     

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.     

The study of the (α, α'f) reaction at 120 MeV on 232Th and 238U: (I). Fission probabilities and angular distributions in the region of the giant quadrupole resonances

The fission decay channel of ²³²Th and ²³⁸U has been investigated, using the (α, α'f) reaction at 120 MeV bombarding energy. The angular distributions of the fission fragments and the fission probabilities up to around 15 MeV excitation have been measured. No evidence for the fission decay of the giant quadrupole resonance has been found, although for ²³⁸U, a weakly excited structure is seen in the (α, α'f) spectrum at about 9.5 MeV excitation at backward angles with respect to the recoil axis. This effect is similar to what has been found in a (⁶Li, ⁶Li'f) experiment reported recently. The over-all feature of the fission probability for excitation energies above the fission barrier are well reproduced by statistical calculations.     

Isobaric Analogue State of the 0.211 MeV Level of Mn56 in Fe56

Gamma-ray spectrum from the reaction Mn⁵⁵(p, γ) Fe⁵⁶ at the isobaric analogue resonance at E_p = 1537 ± 4keV has been measured with a 15 cm³ Ge(Li) detector. Double and triple angular correlations at this resonance state have been measured. Proton elastic scattering was studied also at this resonance at the angles 135° and 150°. Analysis of these data yields a spin and parity assignment 2+, proton radiative width 2 ± 0.2 keV and total width 9 keV to this resonance state which is the isobaric analogue of the third excited state of Mn⁵⁶ at excitation energy 0.211 MeV.     

<Emphasis Type="Italic">T</Emphasis>-odd angular correlations in the emission of prompt gamma rays and neutrons in nuclear fission induced by polarized neutrons

Study of the T-odd three-vector correlation in the emission of prompt neutrons from ²³⁵U fission by polarized cold neutrons has been continued at the facility MEPHISTO of the FRM II reactor (Technical University of Munich). The sought correlation was not found within experimental error of 2.3 × 10⁻⁵. The upper limit for the asymmetry coefficient has been set to |D _n | < 6 × 10⁻⁵ at 99% confidence level, whereas for ternary fission correlation coefficient D _α = (170±20) × 10⁻⁵. This limit casts doubt on a model that explains the three-vector correlation in ternary fission by the Coriolis mechanism. At the same time, five-vector correlation in the emission of prompt fission neutrons has been measured, which describes the rotation of the fissioning nucleus at the moment it breaks (ROT effect). At the angle 22.5° to the fission axis, the correlation coefficient was found to be (1.57 ± 0.20) × 10⁻⁴, while at the angle of 67.5° it is zero within the experimental uncertainty. The existence of ROT effect in the emission of prompt fission neutrons can be explained by the anisotropy of neutron emission in the rest frame of the fragment (fission fragments are aligned with respect to the axis of deformation of the fissioning nucleus), similar to the mechanism of ROT effect in the emission of prompt γ-rays.     

Investigation of backward α-particle scattering in 24Mg and 28Si through α-γ angular correlations

Alpha-gamma angular correlations have been measured for the reaction ²⁴Mg(α, α'γ) in the energy range 15.50 to 17.00 MeV in steps of 300 keV and for ²⁸Si(α, α'γ) from 15.00 to 16.20 MeV in steps of 200 keV to provide energy averaged results. The scattered α-particles were detected for lab angles θ_α = 115°–178° in a multidetector arrangement. The coincident γ-rays were observed with a Ge(Li) detector placed at 90° in the reaction plane. Data were obtained for the 1st, 2nd and 3rd excited states of ²⁴Mg and for the 1st, 2nd, 3rd and 4th excited states of ²⁸Si. The absolute doubledifferential cross sections were analysed with Hauser-Feshbach calculations. The analyses show that compound nuclear processes are responsible for the rise of backward α-scattering in ²⁴Mg(α, α'γ)²⁴Mg and ²⁸Si(α, α'γ)²⁸Si at the energy studied.     

Pressure-dependent phase transition in the ordered BaBi0.7Nb0.3O3 perovskite

BaBi_0.7Nb_0.3O₃, an ordered perovskite, crystallizes in a centrosymmetric rhombohedral structure with the space group R3¯. The refined cell parameters obtained from synchrotron powder X-ray diffraction data for the rhombohedral phase at ambient pressure are a=6.109 (2) Å and α=60.3 (1)°. The pressure-dependent synchrotron powder X-ray diffraction studies show a phase transition around 8.44±1 GPa, where it transforms from rhombohedral structure to a monoclinic structure. The lattice parameters obtained for the monoclinic phase at a pressure of 15±1 GPa are a=5.91 (2) Å, b=6.25 (3) Å and c=8.22 (1) Å with monoclinic angle, β=88 (1)°.     

The emission of Long-Range α-particles in the thermal neurtron induced fission of233U,235U,and239Pu

The yields of the long-range α-particles (LRA) in the thermal neutron induced fission 0f²³³U,²³⁵U, and²³⁹Pu have been determined relative to the corresponding binary fiassion yield. The measurements were performed with surface harrier detectors on a thermalized reactor beam. We obtained for²³³U,²³⁵U and²³⁹Pu binary-to-LRA ratios the values 485±20 respectively 615±20, 475±20. Some of the discrepancies between the published data were explained and experimental indications for²³⁵U(n, α)- and²³⁹Pu(n, α)-reactions are present. Finally the LRA-yield was correlated with some other fission characteristics.     

Study of the (p,pn) reaction in light nuclei

The (p, pn) reaction on ²H, ⁶Li, ⁷Li, ⁹Be has been studied at 47 MeV bombarding energy. Excitation energy spectra and energy sharing spectra are presented. Fragmentary information on ¹⁰B, ¹¹B and ¹²C was also obtained. Sequential decay contributions to the ⁶Li(p, pn)⁵Li¹ reaction suggest an admixture of parentage $\text{(α)π(}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)π(}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ for the (3^?), 23 MeV excited state of ⁶Be. A possible 15.5 MeV excited state in ⁶Be is reported. An extensive study of the ⁹Be (p, pn)⁸Be reaction for 45 MeV protons was made. Results for θ_p = θ_n = 25°, 30°, 35°, 40°, 45°, 55°, 65°; θ_n = 40°, θ_p = 25°, 30°, 35°, 40°, 45°, 55°, 65°; θ_p/θ_n = 40°/45°, 38.5° are reported and compared to DWIA calculations. Agreement is good when the lower momentum components of the wave function are sampled, and deteriorates as higher momenta are required. Results are in agreement with the Cohen and Kurath spectroscopic factors for ⁹Be.     

Correlation between energies and angles of α-particles emitted in thermal-neutron fission of U

The energy distribution and yields of the α-particles emitted in the thermal-neutron fission of ²³⁵U were measured with the same detector system for the cases when the average angles between fragments and α-particles were 90°, 46°, 27° and 11°. The data were analysed by the Monte Carlo method to take into account the effects of the finite size of the source and the various detectors, and the following results were obtained: (i) Even at small angles with respect to the fission axis the yield of the α-particles is about 2–3 % of the yield at 90°, and does not go to zero as would be expected for a true Gaussian angular distribution of a variance which fits the data around 90°. (ii) At these angles where most of the α-particle yield belongs to the non-Gaussian component, the α-particle energy distribution has a significantly higher most probable energy but the FWHM of the energy distribution is not significantly different, (iii) The rms width σ_θ of the Gaussian angular distribution is found to increase for very high (E_α > 20 MeV) α-particle energies and also to a lesser extent for very low (E_α < 15 MeV) energies. The origin of the yield of the α-particles at small angles, and the dependence of the rms width σ_θ on the energy are discussed.     

Alpha-heavy-ion angular correlations from 28Si + 12C

Alpha particles have been measured in coincidence with heavy recoil nuclei from the ²⁸Si + ¹²C reaction. At E_lab = 87 MeV angular correlations for alphas between 15° and 55° and heavy ions at angles ?9°, ?12° and ?15° have been taken. An excitation function of coincidence events with θ_α = 30° and θ_HI = ?12° has been measured for 84 MeV < E_lab < 91.5 MeV. The results are well described by a statistical-model calculation for compound nucleus decay. No evidence is found for additional processes.