Optimal absorption thickness for gamma-scanning activation analysis

A method previously developed in this group for the determination of elemental profile distributions by measuring with a Pb collimator and again with a non-collimating Pb absorber of the same thickness present has been examined with the aim of describing the dependence of the resulting relative error on the thickness of the absorber. The results of calculations based on a detailed analysis of γ-absorption in Pb reproduce the experimental data satisfactorily and provide a guide for confining the relative error of measurement to within reasonable limits.     

A detection method for unified chromatography: Ion mobility monitoring

Ion mobility monitoring has been used for detection in gas, supercritical fluid, and liquid chromatography, illustrating its potential as a method of detection for unified chromatography. Applications presented include GC-IMD of dioxins in fly ash, SFC-IMD of vitamin E, and HPLC-IMD of alkylamines. Ion mobility spectra of several mixed supercritical fluid mobile phases are also presented. Use of methanol, acetonitrile, and dichloromethane as modifiers of supercritical carbon dioxide, and use of supercritical dichlorodifluoromethane and chlorodifluoromethane as mobile phases had little effect on the reactant ion pattern at the flow rates and concentrations used in this study. Only when acetone was used as a modifier of carbon dioxide did the positive reactant ions change significantly. No effect of modifiers or mobile phase was observed for the negative reactant ions.     

An efficient and scalable one-pot double Michael addition-Dieckmann condensation for the synthesis of 4,4-disubstituted cyclohexane beta-keto esters

A simple, scalable, and efficient one-pot methodology for the synthesis of 4,4-disubstituted cyclohexane beta-keto esters from benzylic nitriles or esters and methyl acrylate promoted by potassium tert-butoxide is described. The process relies on a tandem double Michael addition-Dieckmann condensation reaction, which results in the formation of three discrete carbon-carbon bonds in a single pot, including a quaternary center. The method allows for the convenient and rapid synthesis of a variety of 4-aryl-4-cyano-2-carbomethoxycyclohexanone and 4-aryl-2,4-biscarbomethoxycyclohexanone building blocks for use in natural products synthesis and medicinal chemistry.     

Thermal neutron fission cross sections in the range 302≧A≧223

A least squares method is used to fit a smooth curve to known thermal neutron fission cross section data for nuclei of evenN. By making use of fission barriers resulting from a modified liquid drop formalism coupled with the Strutinsky Normalization Procedure, the result is generalized and applied to the prediction within this framework of some 59 cross sections in the actinide and adjacent sub-actinide and trans-actinide regions, as well as to the prediction of an additional 37 cross sections in the superheavy region. In the first group is included a number of nuclides for which presently available cross section data are necessarily cited as upper limits. In the second group, interest is focussed on the predicted island of stability surrounding a neutron shell closure at 184 and a proton shell closure at 114. The results, while they are regarded as plausible estimates and not precise determinations, indicate that many cross sections in this region can be expected to undergo a pronounced reduction in magnitude relative to the limited number of estimates previously given in the literature.