Probing shell structure and shape changes in neutron-rich sulfur isotopes through transient-field g-factor measurements on fast radioactive beams of 38S and 40S

The shell structure underlying shape changes in neutron-rich nuclei near N = 28 has been investigated by a novel application of the transient-field technique to measure the first-excited-state g factors in 38S and 40S produced as fast radioactive beams. There is a fine balance between proton and neutron contributions to the magnetic moments in both nuclei. The g factor of deformed 40S does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.     

Search for axionlike particles using a variable-baseline photon-regeneration technique

We report the first results of the GammeV experiment, a search for milli-eV mass particles with axionlike couplings to two photons. The search is performed using a "light shining through a wall" technique where incident photons oscillate into new weakly interacting particles that are able to pass through the wall and subsequently regenerate back into detectable photons. The oscillation baseline of the apparatus is variable, thus allowing probes of different values of particle mass. We find no excess of events above background and are able to constrain the two-photon couplings of possible new scalar (pseudoscalar) particles to be less than 3.1x10;(-7) GeV-1 (3.5x10;(-7) GeV-1) in the limit of massless particles.     

Updated triangular factors of the basis to maintain sparsity in the product form simplex method

In recent years triangular factorization of the basis has greatly enhanced the efficiency of linear programming inversion routines, leading to greater speed, accuracy and a sparser representation. This paper describes a new product form method for updating the triangular factors at each iteration of the simplex method which has proved extremely effective in reducing the rate of growth of the transformation (eta) files, thus reducing the amount of work per iteration and the frequency of re-inversion. We indicate some of the programming measures required to implement the method and give computational experience on real problems of up to 3500 rows.     

Molecular distribution of some commercial nonylphenol ethoxylates using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Nonylphenol ethoxylates (NPEs) belong to a group of nonionic surfactants that are collectively referred to as alkylphenol ethoxylates (APEs). APEs find widespread use in heavy-duty commercial and household cleaning formulations, shampoos, and industrial processing, i.e. textile manufacture. Their environmental impact depends on the molecular distribution and the extent of their biodegradation in municipal sewage systems, waterways and rivers. We have established two sample preparation methods that have enabled the determination of the molecular distributions of six commercial nonylphenol ethoxylates using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). In both methods, alpha-cyano-4-hydroxycinnamic acid, dissolved in acetonitrile/tetrahydrofuran, was used as the matrix. In one set of experiments, the NPEs were dissolved in an acetonitrile/tetrahydrofuran solvent system prior to mixing with the matrix solution, and the resulting MALDI-TOF mass spectra produced mostly sodiated molecules [M + Na](+). The NPEs, all having the formula 4-(C(9)H(19))-C(6)H(4)-(OCH(2)CH(2))(n)-OH, are Surfonic (R)N-95, N-100, N-102, N-120, N-150 and N-300. Surfonic N-95 and N-100 gave n values of 5-20; Surfonic N-102, N-120, N-150, and N-300 gave n values of 5-21, 5-22, 8-25 and 15-40, respectively. In order to develop a sample preparation method that could be used with less polar NPEs, we dissolved the NPEs (except N-300) in pentane prior to mixing with the matrix solution, and found that the MALDI spectral quality was unaffected by the solvent systems. Copyright 1999 John Wiley & Sons, Ltd.     

Formal optimization of some reduced linear programming problems

It is often possible (and profitable) to reduce or ‘Presolve’ linear programs. In particular, there are frequently constraints which force many of the variables to be at bound. Unfortunately, the solution found by the simplex method for such reduced models is not usually ‘formally’ optimal, in the sense that nonoptimal dual values may be present when the original problem is restored. Furthermore, the restored (full) problem is now totally degenerate, and may require many iterations to achieved formal optimality. We describe an efficient ‘Postsolve’ procedure for attaining the formal optimum solution, and give computational results.     

An integer programming approach to a class of combinatorial problems

In this paper, we discuss the solution of a class of modified quadratic assignment problems, with particular reference to an application involving decentralization of a large organization. The main emphasis is on the use of a standard branch and bound mathematical programming system (UMPIRE) and the problem manipulations required to carry this out efficiently.     

Evaluation of Combustion Mechanisms Using Global Uncertainty and Sensitivity Analyses: A Case Study for Low‐Temperature Dimethyl Ether Oxidation

A global uncertainty analysis is performed for three current mechanisms describing the low‐temperature oxidation of dimethyl ether (Aramco Mech 1.3, Metcalfe et al., Int J Chem Kinet 2013, 45, 638–675; Zheng et al., Proc Combust Inst 2005, 30, 1101–1109; Liu et al., Combust Flame 2013, 160, 2654–2668) with application to simulations of species concentrations (CH₂O, H₂O₂, CH₃OCHO) corresponding to existing data from an atmospheric pressure flow reactor and high‐pressure ignition delays. When incorporating uncertainties in reaction rates within a global sampling approach, the distributions of predicted targets can span several orders of magnitude. The experimental profiles, however, fall within the predictive uncertainty limits. A variance‐based sensitivity analysis is then undertaken using high dimensional model representations. The main contributions to predictive uncertainties come from the CH₃OCH₂ + O₂ system, with isomerization, propagation, chain‐branching, secondary OH formation, and peroxy–peroxy reactions all playing a role. The response surface describing the relationship between sampled reaction rates and predicted outputs is complex in all cases. Higher order interactions between parameters contribute significantly to output variance, and no single reaction channel dominates for any of the conditions studied. Sensitivity scatter plots illustrate that many different parameter combinations could lead to good agreement with specific sets of experimental data. The Aramco scheme is then updated based on data from a recent study by Eskola et al. (J Phys Chem A, in press), which presents quite different temperature and pressure dependencies for the rates of CH₃OCH₂O₂ → CH₂OCH₂O₂H and CH₂OCH₂O₂H → OH+2CH₂O compared with currently used values and includes well skipping channels. The updates from Eskola worsen the agreement with experiments when used in isolation. However, if the rate of the CH₂OCH₂O₂H + O₂ channel is subsequently reduced, very good agreement can be achieved. Owing to the complex nature of the response surface, the tuning of this channel remains speculative. Further detailed studies of the temperature and pressure dependence of the CH₃OCH₂O₂ + O₂, CH₂OCH₂O₂H + O₂ system are recommended to reduce uncertainties within current dimethyl ether mechanisms for low‐temperature conditions.     

Spectroscopy of exotic 121, 123, 125Ag produced in fragmentation reactions

Excited states in the neutron-rich ^{121, 123, 125}Ag were studied via the fragmentation of a ¹³⁶Xe beam at 120MeV/nucleon in a thick ⁹Be target. The levels in ¹²¹Ag were populated in the $ \beta$ decay of ¹²¹Pd while those assigned to ^{123, 125}Ag were identified via isomer spectroscopy. The transitions identified in ¹²¹Ag are consistent with the $ \gamma$ -rays reported in ^{117, 119}Ag . The newly observed transitions were placed in the level schemes of ^{123, 125}Ag based on the analysis of $ \gamma$ - $ \gamma$ coincidences and the systematics. We attribute the onset of isomerism in the ^{123, 125}Ag isotopes to the drop in energy of the negative-parity levels similar to the 5^- state in the even-even Cd cores. The proposed level scheme for ¹²⁵Ag is well described by the NuShellX shell model calculations.