The macromolecular neutron diffractometer MaNDi is currently under construction at the first target station of the Spallation Neutron Source at Oak Ridge National Laboratory. This instrument will collect neutron diffraction data from small single crystals (0.1–1 mm3) with lattice constants between 100 and 300 Å, as well as data from less well ordered systems such as fibers. A focusing neutron guide has been designed to filter the high‐energy neutron component of the spectrum and to provide a narrow beam with a wide spectral window and angular divergence almost insensitive to neutron wavelength. The system includes a final interchangeable section of neutron guide and two slits, which enable tuning of the horizontal and vertical beam divergence between 0.12 and 0.80° (full width at half‐maximum) at the sample position. This allows the trading of intensity for resolution, depending on the scientific requirements. Efforts to enhance and develop suitable high‐resolution neutron detectors at an affordable price are also discussed. Finally, the parameters of the neutron guide and detectors were used to simulate diffraction from a large unit cell.
The effects of temperature and the presence of oxygen and reducing agent on the thermal stability of two naturally occurring labile folates were investigated Microbiological assay of samples heated from 49-100°C for various lengths of time revealed that the loss of S-methyltetrahydrofolic acid in aqueous solution followed first order kinetics with a low Arrhenius activation energy of 9.5 kcal/mole. Tetrahydrofolic acid was extremely labile during heating and the stability of both forms of folate was increased in the presence of ascorbate and nitrogen atmosphere.
24 32 ' ' 6 Figure 6. Adsorption of ENTMP on calcium fluoride crystals at equilibrium. Plots of r against [ENTMP].uration g = 0.4, 0.2, and 0.1 respectively. These values reflect the high adsorption affinity at low relative undersaturation in the presence of ENTMP. A similar dependence of the degree of inhibition with change in driving force has been observed for the influence of phosphonate on the dissolution rate of magnesium fluoride" and barium fluoride19 in aqueous solution. As noted 8 io ,~N T M P J mol c1 for the crystallization of gypsum by Van Rosmalen and coworkers,m the effectiveness of the HEDP as an inhibitor depends on the degree of supersaturation.In order to investigate the adsorption of phosphonate on the calcium fluoride crystals surface, adsorption equilibrium experiments were made for ENTMP on calcium fluoride crystals at u = 0. A typical adsorption isotherm is plotted in Figure 6. Assuming that the area occupied by an ENTMP molecule is 50 X m2, the fraction of the solid surface covered by adsorption molecules is only 6% at the plateau in Figure 6. At this ENTMP concentration, the dissolution rate is reduced by more than 92%. Adsorption affinity constant at equilibrium 5.2 X lo5 dm3 mol-' is in satisfactory agreement with the kinetic adsorption affinity constants los dm3 mol-'. The dissolution rate of magnesium fluoride was reduced by more than 80% when only 9% of the crystal surface was covered by HEDP molecule^.^' Moreover, the rate of crystallization may be reduced virtually to zero when only 5-7% of the crystal surface was covered by adsorbed inhibitor moIecules.2',22The emission spectroscopy/pulse radiolysis method of determining three-body ionic recombination rate constants in raregas-halogen source gas mixtures has been extended to systems where the emission is produced by both ionic and nonionic pathways. This has enabled recombination coefficient measurements to be done over a large pressure range for irradiated Kr/SF, and Xe/CFC13 gas mixtures. The rate constants measured for both these systems show the typical pressure dependence of an increase to a maximum value of -2.5 X IOl5 M-l s-l (-4 X IOd cm3 d), before the onset of the diffusion-controlled reaction. These values have been compared to the predictions of the Langevin-Harper diffusion-controlled and the Bates termolecular recombination models. The large discrepancies between theory and experiment have shown that other recombination processes dominate the ionic recombination.
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