Welfare Reform when Recipients are Forward-Looking

Welfare Reform, Time Limit, Dynamic Programming

The Dynamics of Prenatal WIC Participation and the Role of Past Participation

The special Supplemental Nutrition Program for Women, Infants, and Children (WIC) provides food vouchers, nutritional counseling, and health care referrals to low-income pregnant and breastfeeding women and their young children. This paper uses non-parametric and parametric duration models to study the factors that influence the timing of prenatal WIC participation among eligible women. The estimates show that black and Hispanic women, women with low levels of education, women who participate in other welfare programs and who have no insurance, and women who are overweight participate in WIC earlier than others. Thus, there appears to be negative selection on observables across a number of dimensions. The model is extended to allow past WIC participation to influence current WIC participation, and past participation is found to be the most important determinant of current participation. This result suggests that policies reach out to women who have not received WIC before may be especially beneficial. Finally, policy simulations show that recent policy changes requiring income documentation while also making recipients of other programs automatically eligible for WIC have off-setting effects with a small net increase in participation.Duration Analysis, WIC Program

Lunar geological field investigations

Apollo lunar geology investigation

Effect of dipole polarizability on positron binding by strongly polar molecules

A model for positron binding to polar molecules is considered by combining the dipole potential outside the molecule with a strongly repulsive core of a given radius. Using existing experimental data on binding energies leads to unphysically small core radii for all of the molecules studied. This suggests that electron-positron correlations neglected in the simple model play a large role in determining the binding energy. We account for these by including polarization potential via perturbation theory and non-perturbatively. The perturbative model makes reliable predictions of binding energies for a range of polar organic molecules and hydrogen cyanide. The model also agrees with the linear dependence of the binding energies on the polarizability inferred from the experimental data [Danielson et al 2009 J. Phys. B: At. Mol. Opt. Phys. 42 235203]. The effective core radii, however, remain unphysically small for most molecules. Treating molecular polarization non-perturbatively leads to physically meaningful core radii for all of the molecules studied and enables even more accurate predictions of binding energies to be made for nearly all of the molecules considered.Comment: 26 pages, 18 figure

Contribution of street food to dietary intake of habitual urban consumers: a cross-sectional study in Kampala city, Uganda

Background: Street food has continued to be a popular food source in the urban settings of developing countries and is proving to be a vital urban dietary source. However, its dietary contribution among urban populations is yet to be comprehensively understood. Aim: To assess how street food contributes to the dietary intake of habitual street food consumers. Methods: We conducted a community-based cross-sectional study among habitual street food consumers in Kampala city. We defined habitual intake as consumption of a serving of any street food for ≥2 days/week regardless of the food group and number of times it was consumed in a particular day. Questionnaires were used to capture quantitative data on sociodemographic characteristics, anthropometry, 24-hour diet intake and 2-month street food consumption frequency. The Nutritics® diet analysis software version 4.3 and STATA version 13.0 were used for nutrient and statistical analyses respectively. Results: Street food contributed considerably to the daily intake of fat (49.1%), sodium (38.4%) and calcium (36.5%) and least towards the daily intake of vitamin A (11.3%). The majority of consumers opted for street food at breakfast (50%) whereas lunch and snacks featured the least for overall street food inclusion (all 20%). Overall, men demonstrated more dietary intake and inclusion at meals from street food than women. Conclusions: This study indicates a significant contribution of street food for urban consumers but men derive more benefit than women in terms of nutrient intake and inclusion of street food in meals

Calculations of positron binding and annihilation in polyatomic molecules

A model-potential approach to calculating positron-molecule binding energies and annihilation rates is developed. Unlike existing ab initio calculations, which have mostly been applied to strongly polar molecules, the present methodology can be applied to both strongly polar and weakly polar or nonpolar systems. The electrostatic potential of the molecule is calculated at the Hartree-Fock level, and a model potential that describes short-range correlations and long-range polarization of the electron cloud by the positron is then added. The Schrodinger equation for a positron moving in this effective potential is solved to obtain the binding energy. The model potential contains a single adjustable parameter for each type of atom present in the molecule. The wave function of the positron bound state may be used to compute the rate of electron-positron annihilation from the bound state. As a first application, we investigate positron binding and annihilation for the hydrogen cyanide (HCN) molecule. Results for the binding energy are found to be in accord with existing calculations, and we predict the rate of annihilation from the bound state to be $\Gamma=0.1$--$0.2 \times 10^9~\text{s}^{-1}$.Comment: 13 pages, 6 figures, accepted by J. Chem. Phy

van der Waals coefficients for positronium interactions with atoms

The random-phase approximation with exchange (RPAE) is used with a $B$-spline basis to compute dynamic dipole polarizabilities of noble-gas atoms and several other closed-shell atoms (Be, Mg, Ca, Zn, Sr, Cd, and Ba). From these, values of the van der Waals $C_6$ constants for positronium interactions with these atoms are determined and compared with existing data. Our best predictions of $C_6$ for Ps--noble-gas pairs are expected to be accurate to within 1%, and to within a few per cent for the alkaline earths. We also used accurate dynamic dipole polarizabilities from the literature to compute the $C_6$ coefficients for the alkali-metal atoms. Implications of increased $C_6$ values for Ps scattering from more polarizable atoms are discussed.Comment: 6 pages, submitted to Physical Review

Many-body theory for positronium-atom interactions

A many-body-theory approach has been developed to study positronium-atom interactions. As first applications, we calculate the elastic scattering and momentum-transfer cross sections and the pickoff annihilation rate $^1Z_\text{eff}$ for Ps collisions with He and Ne. The cross section for He is in agreement with previous coupled-state calculations, and the momentum-transfer cross section for Ne agrees with available experimental data. $^1Z_\text{eff}$ is found to be 0.13 and 0.26 for He and Ne, respectively, in excellent agreement with the measured values.Comment: Accepted by Phys. Rev. Lett. (V2 contains update to text and Figs. 3 and 5. V3 contains further discussion on the calculation of pickoff annihilation rates.

Geology of Hadley Rille

The regional setting, external and internal shape, and materials of Hadley Rille near the Apollo 15 landing site are described. The petrography presented includes lithologies, regolith, talus, and outcrops. The stratigraphy exposed on the rille wall is also considered