A fast-neutron spectrometer of advanced design

Fast neutron spectrometer combines helium filled proportional counters with solid-state detectors to achieve the properties of high efficiency, good resolution, rapid response, and effective gamma ray rejection

The Geometry of Axisymmetric Ideal Fluid Flows with Swirl

The sectional curvature of the volume preserving diffeomorphism group of a Riemannian manifold $M$ can give information about the stability of inviscid, incompressible fluid flows on $M$. We demonstrate that the submanifold of the volumorphism group of the solid flat torus generated by axisymmetric fluid flows with swirl, denoted by $\mathcal{D}_{\mu,E}(M)$, has positive sectional curvature in every section containing the field $X = u(r)\partial_\theta$ iff $\partial_r(ru^2)>0$. This is in sharp contrast to the situation on $\mathcal{D}_{\mu}(M)$, where only Killing fields $X$ have nonnegative sectional curvature in all sections containing it. We also show that this criterion guarantees the existence of conjugate points on $\mathcal{D}_{\mu,E}(M)$ along the geodesic defined by $X$.Comment: 8 page

The geometry of whips

In this paper we study geometric aspects of the space of arcs parametrized by unit speed in the $L^2$ metric. Physically this corresponds to the motion of a whip, and it also arises in studying shape recognition. The geodesic equation is the nonlinear, nonlocal wave equation $\eta_{tt} = \partial_s(\sigma \eta_s)$, with $\lvert \eta_s\rvert\equiv 1$ and $\sigma$ given by $\sigma_{ss}- \lvert \eta_{ss}\rvert^2 \sigma = -\lvert \eta_{st}\rvert^2$, with boundary conditions $\sigma(t,1)=\sigma(t,-1)=0$ and $\eta(t,0)=0$. We prove that the space of arcs is a submanifold of the space of all curves, that the orthogonal projection exists but is not smooth, and as a consequence we get a Riemannian exponential map that it continuous and even differentiable but not $C^1$. This is related to the fact that the curvature is positive but unbounded above, so that there are conjugate points at arbitrarily short times along any geodesic. We also compare this metric to an $L^2$ metric introduced by Michor and Mumford for shape recognition on the homogeneous space $\text{Imm}(I, \mathbb{R}^2)/\mathcal{D}(I)$ of immersed curves modulo reparametrizations; we show it has some similar properties (such as nonnegative but unbounded curvature and a nonsmooth exponential map), but that the $L^2$ metric on the arc space yields a genuine Riemannian distance.Comment: 24 page

Immigration, wages, and compositional amenities

Economists are often puzzled by the stronger public opposition to immigration than trade, since the two policies have symmetric effects on wages. Unlike trade, however, immigration changes the composition of the local population, imposing potential externalities on natives. While previous studies have focused on fiscal spillovers, a broader class of externalities arise because people value the ‘compositional amenities’ associated with the characteristics of their neighbors and co-workers. In this paper we present a new method for quantifying the relative importance of these amenities in shaping attitudes toward immigration. We use data for 21 countries in the 2002 European Social Survey, which included a series of questions on the economic and social impacts of immigration, as well as on the desirability of increasing or reducing immigrant inflows. We find that individual attitudes toward immigration policy reflect a combination of concerns over conventional economic impacts (i.e., on wages and taxes) and compositional amenities, with substantially more weight on composition effects. Most of the difference in attitudes to immigration between more and less educated natives is attributable to heightened concerns over compositional amenities among the less-educated