Neutron scattering by hydrogen in cold neutron prompt gamma-activation analysis

The effects of neutron scattering by hydrogen within targets for cold neutron prompt -ray activation analysis (CNPGAA) have been characterized. For most targets studied, the probability for neutron absorption, and hence CNPGAA sensitivities (counts·s^–1·mg^–1), decrease with increasing H content and with target thickness. Comparisons with results from thermal neutron PGAA indicate that the effects of cold neutron scattering differ from those of thermal neutron scattering. CNPGAA sensitivities for l/v nuclides show similar sensitivity decreases, while Sm sensitivities show smaller decreases.     

Determination of the chain conformation of polypropylene in the crystalline state by neutron scattering

Summary Wide and small angle neutron scattering from melt-crystallised isotactic polypropylene mixtures of deuterated and protonated molecules were measured in aQ range of 0.003 Å^–1 to 0,8 Å^–1. The scattering curves between 0.1 Å^–1 and 0.8 Å^–1 are very sensitive to the amount of folding of the chains within a lamella. Different annealing conditions which influence the degree of crystallinit between 0.39 and 0.67 and the long spacing between 125 Å and 245 Å had an insignificant influence on the scattering in this range. For various arrangements of crystalline sequences scattering functions were calculated. Best agreement with the experimental scattering curves was obtained for a mixture of single and double crystalline sequences of equal proportion. An adjacent reentry folding model for the molecular conformation is not consistent with the experimental data.The neutron scattering experiments have been performed at the Institute Laue-Langevin in Grenoble/France and at the Kernforschungsanlage Jülich/Germany.     

Structure factors of polyelectrolyte solutions revealed by neutron scattering

The solute structure in concentrated polyelectrolyte solutions is discussed in terms of the following observations: 1) The broad peak in the global structure function measured by radiation scattering (neutron, X rays or light), in the interval q < 2 nm⁻¹, 2) The separate intra and intermolecular structure functions obtained by neutron scattering, 3) The separate polyion-counterion partial structure factors derived from neutron scattering, 4) The variation with wave vector of neutron and light quasi-elastic broadening.     

Micro- and mesoscopic structure of poly(vinyl alcohol) gels determined by neutron and light scattering

We studied the structure of poly(vinyl alcohol) (PVA) gels formed in mixtures of dimethyl sulfoxide (DMSO) and water using several scattering techniques such as wide-angle neutron scattering (WANS), small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (U-SANS) and light scattering (LS) to cover a very wide Q range from 10^-4 to 10 Å^-1. The WANS measurements have revealed that the cross-linking points of the gels are crystallites, and the size and its distribution have been evaluated by the SANS measurements. The SANS results have also shown that the structure observed in the low Q range below 10^-2 Å^-1 is dominated by a liquid–liquid-phase separation. The early stage of the phase separation has been studied in detail using the time-resolved LS technique, while the late stage has been investigated by the U-SANS technique because the LS measurements cannot access the opaque samples. On the basis of the results, we present a quantitative sketch of the structure of the PVA gel.     

Measurements of the neutron spectra from the 7Li(p,n) accelerator based neutron source: Position and angular dependences

Thick target ⁷Li(p,n) neutron spectra were measured using a ³He ion chamber in the proton energy range of 1.95 to 2.30 MeV. The fast neutron spectra were collected for various distances from the lithium target as well as for various neutron emission angles. By unfolding the ³He raw data with the iterative van Cittert algorithm, the neutron fluence spectra were obtained. The ³He measured neutron spectra were compared with both analytically computed and Monte Carlo simulated spectra to account for neutron scatterings in the lithium target assembly and in the experimental area. To verify the accuracy of the neutron dose computation, the fast neutron kerma was obtained for each neutron spectrum using the fluence to kerma conversion coefficients and was compared with the measured neutron dose using tissue-equivalent proportional counters. In the position dependence investigation at the 0° emission angle, the analytically computed neutron kerma overestimates the experimental kerma by a factor of two mainly due to neutron moderation. The corresponding neutron kerma from the ³He measured spectra were in agreement with the neutron doses measured using tissue-equivalent proportional counters within 20% for lower proton energies, but the discrepancy increased to ～50% for higher proton energies. In the angular distribution investigation, a notable discrepancy between measured and computed neutron spectra were observed due to the neutron scattering effects in the target assembly and experimental room.     

Wide-angle neutron scattering studies of mixed crystals of polyethylene and deuteropolyethylene

Wide-angle neutron scattering studies have been made of a series of mixed crystal blends of polyethylene and deuterated polyethylene in the range 0.68 ≤ K = 4πλ^?1 sinθ ≤ 2.02 Å^?1. Recent calculations by Stamm have shown that regularly folded subunits of small numbers (two to ten) of stems should produce an observable modulation of the diffuse scattering pattern of such mixtures in this K range. For both melt- and solution-crystallized blends, no extra signal was observed above the background scattering level. The data are used to place an upper limit (of about four) on the number of stems that could be regularly folded in the (100) and (010) planes for these materials.     

Systematic errors in 14-MeV neutron activation analysis for oxygen: Part I. Neutron and γ-ray attenuation effects

A detailed account is given of neutron and γ-ray attenuation effects in 14-MeV neutron activation analysis of oxygen. Appropriate neutron cross-section values have been determined in two different ways and compared with literature values. It appears that the attenuation process is best described in terms of nonelastic scattering cross-sections. It is also shown that the narrow beam total γ-ray attenuation coefficients at 6 MeV, given in the literature are suitable for correction purposes if ¹⁶N γ-rays are counted with a window of 4.5–6.5 MeV. Attention was paid to the contribution of β-rays when the ¹⁶N activity is counted in this energy interval with a NaI(Tl) detector.     

Inelastic neutron scattering spectra of the hydrogen tungsten bronze H0.4WO3

Inelastic neutron scattering spectra of metallic H_0.4WO₃ show only the vibrations that would be expected of a metal hydroxide. The diffusion coefficient of the proton could not be detected from quasi-elastic scattering with the best available neutron spectrometer setting an upper limit to its value of 10^?7 cm²/sec. Both these results confirm that this material is correctly described as a hydroxide.     

Comparison between neutron and quasielastic light scattering by polyacrylamide gels

The authors describe small-angle neutron scattering measurements of the screening length ζ in polyacrylamide-water gels. Although these are inhomogeneous systems, the screening length is clearly observable and is in good numerical agreement with the relation E = 3kT/4πζ³, where E is the longitudinal elastic modulus of the gel obtained from measurements of the intensity of qu-asielastically scattered light. Static light scattering observations reveal a larger-scale (ca. 30 nm) superstructure in the gel.     

On the use of bismuth as a neutron filter

A formula is given which, for neutron energies in the range 10⁻⁴<E<10 eV, permits calculation of the nuclear capture, thermal diffuse and Bragg scattering cross-sections as a function of bismuth temperature and crystalline form. Computer programs have been developed which allow calculations for the Bi rhombohedral structure in its poly-crystalline form and its equivalent hexagonal close-packed structure. The calculated total neutron cross-sections for poly-crystalline Bi at different temperatures were compared with the measured values. An overall agreement is indicated between the formula fits and experimental data. Agreement was also obtained for values of Bi-single crystals, at room and liquid nitrogen temperatures. A feasibility study for use of Bi in powdered form, as a cold neutron filter, is detailed in terms of the optimum Bi-single crystal thickness, mosaic spread, temperature and cutting plane for efficient transmission of thermal-reactor neutrons, and also for rejection of the accompanying fast neutrons and gamma rays.     

Incoherent inelastic neutron scattering studies of the torsional modes of hexachloroethane

From incoherent inelastic neutron scattering studies of solid C₂Cl₆ the in- and out-of-phase torsions about the C-C axis are assigned at 56 and 95 cm^?1, respectively. Using a model for the potential barriers in the solid the torsional frequency in the gas has been calculated to be 76.7 cm^?1 and the internal barrier to be 67.8 kJ mol^?1.     

A study on neutron energy spectrum estimation by LaBr3:Ce detector

A novel approach to estimation of neutron energy spectrum by LaBr₃:Ce detector which was usually used for gamma rays measurement was presented. In this approach, energy distribution of the neutron flux was devided into several bins, and simultaneous equations of these bins were setup based on the net counts of gamma peaks induced by neutron inelastic scattering with LaBr₃:Ce detector. With these equations, the neutron energy spectrum was derived by a deconvolution algorithm. This approach was tested using a 3 in.?×?3 in. LaBr₃:Ce detector exposured to an ²⁴¹Am-Be neutron source and proved practicable.

     

Incoherent inelastic neutron scattering studies of potassium bicarbonate and caesium hydrogen dinitrate

Inelastic neutron scattering spectra of KHCO₃ and CsH(NO₃)₂ have been obtained in the region 400 → 2400 and 400 → 2800 cm^?1 respectively. The in- and out-of-phase bending vibrations of the hydrogen bonds have been observed and assigned. For CsH(NO₃)₂ the two bending modes are closer in frequency than in KHCO₃ and they are not resolved from the antisymmetric stretch.     

Low frequency vibrations in crystalline biphenyl: Model calculations and raman and neutron spectra

Vibrational frequencies of crystalline biphenyl have been calculated in the rigid phenyl approximation using the crystal structure at 110 K. The calculated results explain successfully the experimental infrared and Raman frequencies as well as the polarization data of the Raman bands at 80 K that have been observed in the present study for the first time. The frequency dispersion curves calculated in the b^* direction show that the coupling between the intramolecular CC torsion and the translational lattice vibrations changes drastically as the wave vector changes. The existence of a minimum frequency point located away from the Brillouin zone boundary indicates that the pulse transition near 40 K is a commensurate—incommensurate one in accordance with the results of neutron diffraction that have recently been reported. Incoherent inelastic neutron scattering spectra have also been investigated above and below the phase transition temperature.     

The ferroelectric phase of CdTiO3: A powder neutron diffraction study

The synthesis of bulk samples of polycrystalline CdTiO₃ in both the rhombohedral ilmenite and orthorhombic perovskite forms is described and the structures of these have been refined using powder neutron diffraction data. This involved the preparation of samples enriched in ¹¹⁴Cd. Cooling perovskite-type CdTiO₃ to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna2₁. Mode analysis shows the polar mode to be dominant at low temperatures. The ilmenite-structure of CdTiO₃ is compared with that of ZnTiO₃. The refined scattering length of the ¹¹⁴Cd is estimated to be 5.56 fm. Attempts to dope CdTiO₃ with Ca and Sr are described.     

The diminution of the neutron odd-even effect due to prompt neutron emission in low energy nuclear fission

Two fissioning systems [²³⁵U(n_th, f) and²⁵²Cf(sf)] have been studied to see the consequence of prompt neutron emission on odd-even effects. Monte Carlo calculations have been carried out for prompt neutron emission. Fragment and product yields have been analyzed by the WahlZ _p Model, the Method of Differences and Model Independent Calculations. The result of the wash-out effect of prompt neutron emission on the neutron odd-even effect has been shown; however, this was not seen with the protons.     

Advances in 14 MeV neutron activation analysis by means of a new intense neutron source

A new intense 14 MeV neutron generator with cylindrical acceleration structure has been put in operation at the GKSS Research Center Geesthacht. The sealed neutron tube is combined with a fast pneumatic rabbit system with particular capabilities for neutron activation analysis involving shortlived reaction products. The sample transfer time is less than 140 ms. The maximum neutron flux available for activation is 5.2·10¹⁰ n/cm²s. Theoretical sensitivity predictions made in a previous study have been verified for some important trace elements. As a first application, samples of freeze-dried suspended matter and fishes of the Elbe river were analyzed.     

A small-angle neutron scattering study of the ethyl (n-octyl) phosphate micelles in water

Summary Mixed-double chain anionic surfactants, barium- and lithium-salts of ethyl(n-octyl) phosphate (EOP), which are asymmetric in the molecular shape, and a series of identical chain di-n-alkyl phosphate lithium salts have been synthezized. The limiting partial molar volume of a PO ₄ ^– group (23.43±0.41 cm³ mol^–1) for use in small-angle neutron scattering analysis was determined by density measurements of a series of identical chain di-n-alkyl phosphate lithium salts. For lithium EOP-D₂O system, a critical micellar concentration (2.3 wt%) was determined by³¹P NMR spectra. The micellar shape and size in the EOP-water binary system has been investigated by using small-angle neutron scattering (SANS) spectra. It has been found that the micelles of barium EOP in water have the shape of a prolate spheroid and aggregation numbers (n) equal to 48 at 23°C and 52 at 50°C. For the lithium EOP-micellar system, it has been found that the minimum micelle with an aggregation numbern=21 is spherical and micellar growth and variation from the spherical to the prolate shape might occur with an increase in concen tration above the CMC.     

Small-angle neutron scattering study of homopolymer blends and blockcopolymers of polystyrene/polyparamethylstyrene

Small angle neutron scattering (SANS) experiments were carried out at one mixture and two block copolymers of polystrene (PS) and poly(p-methylstyrene)(PpMS) at different temperatures ranging from 107 to 295°C. Both block copolymers show a maximum in scattering intensity, which increases with decreasing temperature approaching the spinodal point. Theoretical curves from Leibler's mean field theory agree very well with the experimental points with (XN) as the only fitting parameter, where χ is the Flory-Huggins interaction parameter and N is the degree of polymerization. The reciprocal value of I(q_m)⁻¹ of the maximum intensity for the block copolymers as well as the reciprocal intensity at zero scattering vector (I(q=0)⁻¹) for the mixture obey well the ansatz I = A + B/T within the experimental temperature range. The spinodal values of (X^N)_S are in good agreement with the theoretical values from Leibler.