Regge calculus from a new angle

In Regge calculus space time is usually approximated by a triangulation with flat simplices. We present a formulation using simplices with constant sectional curvature adjusted to the presence of a cosmological constant. As we will show such a formulation allows to replace the length variables by 3d or 4d dihedral angles as basic variables. Moreover we will introduce a first order formulation, which in contrast to using flat simplices, does not require any constraints. These considerations could be useful for the construction of quantum gravity models with a cosmological constant.Comment: 8 page

The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland

Experiments were performed during the period May–July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO42− were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 μm and greater than 0.01 μm were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42− is the dominant aerosol anion species, with Na+, NH4+, and Ca2+being the dominant cations. Impactor results indicate that MSA and SO42− have similar mass size distributions. Furthermore, MSA and SO42− have mass in both the accumulation and coarse modes. A limited number of samples for NH4+ indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/− 0.011 cm/s for NH4+ to 0.110 cm/s +/− 0.021 cm/s for Ca. SO42− dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor-estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles \u3e 0.5 μm in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles \u3c 0.5 μm are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42−serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3−, a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4+. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 26% and 32% for SO42− and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4+ and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals

Microcanonical statistics of black holes and bootstrap condition

The microcanonical statistics of the Schwarzschild black holes as well as the Reissner-Nordstr$\sf \ddot{o}$m black holes are analyzed. In both cases we set up the inequalities in the microcanonical density of states. These are then used to show that the most probable configuration in the gases of black holes is that one black hole acquires all of the mass and all of the charge at high energy limit. Thus the black holes obey the statistical bootstrap condition and, in contrast to the other investigation, we see that U(1) charge does not break the bootstrap property.Comment: 16 pages. late

Ultracold Neutron Production in a Pulsed Neutron Beam Line

We present the results of an Ultracold neutron (UCN) production experiment in a pulsed neutron beam line at the Los Alamos Neutron Scattering Center. The experimental apparatus allows for a comprehensive set of measurements of UCN production as a function of target temperature, incident neutron energy, target volume, and applied magnetic field. However, the low counting statistics of the UCN signal expected can be overwhelmed by the large background associated with the scattering of the primary cold neutron flux that is required for UCN production. We have developed a background subtraction technique that takes advantage of the very different time-of-flight profiles between the UCN and the cold neutrons, in the pulsed beam. Using the unique timing structure, we can reliably extract the UCN signal. Solid ortho-D$_2$ is used to calibrate UCN transmission through the apparatus, which is designed primarily for studies of UCN production in solid O$_2$. In addition to setting the overall detection efficiency in the apparatus, UCN production data using solid D$_2$ suggest that the UCN upscattering cross-section is smaller than previous estimates, indicating the deficiency of the incoherent approximation widely used to estimate inelastic cross-sections in the thermal and cold regimes

A next-generation inverse-geometry spallation-driven ultracold neutron source

The physics model of a next-generation spallation-driven high-current ultracold neutron (UCN) source capable of delivering an extracted UCN rate of around an-order-of-magnitude higher than the strongest proposed sources, and around three-orders-of-magnitude higher than existing sources, is presented. This UCN-current-optimized source would dramatically improve cutting-edge UCN measurements that are currently statistically limited. A novel "Inverse Geometry" design is used with 40 L of superfluid $^4$He (He-II), which acts as a converter of cold neutrons (CNs) to UCNs, cooled with state-of-the-art sub-cooled cryogenic technology to $\sim$1.6 K. Our design is optimized for a 100 W maximum heat load constraint on the He-II and its vessel. In our geometry, the spallation target is wrapped symmetrically around the UCN converter to permit raster scanning the proton beam over a relatively large volume of tungsten spallation target to reduce the demand on the cooling requirements, which makes it reasonable to assume that water edge-cooling only is sufficient. Our design is refined in several steps to reach $P_{UCN}=2.1\times10^9\,/$s under our other restriction of 1 MW maximum available proton beam power. We then study effects of the He-II scattering kernel as well as reductions in $P_{UCN}$ due to pressurization to reach $P_{UCN}=1.8\times10^9\,/$s. Finally, we provide a design for the UCN extraction system that takes into account the required He-II heat transport properties and implementation of a He-II containment foil that allows UCN transmission. We estimate a total useful UCN current from our source of $R_{use}=5\times10^8\,/$s from a 18 cm diameter guide 5 m from the source. Under a conservative "no return" approximation, this rate can produce an extracted density of $>1\times10^4\,/$cm$^3$ in $<$1000~L external experimental volumes with a $^{58}$Ni (335 neV) cut-off potential.Comment: Submitted to Journal of Applied Physic

Effective action and semiclassical limit of spin foam models

We define an effective action for spin foam models of quantum gravity by adapting the background field method from quantum field theory. We show that the Regge action is the leading term in the semi-classical expansion of the spin foam effective action if the vertex amplitude has the large-spin asymptotics which is proportional to an exponential function of the vertex Regge action. In the case of the known three-dimensional and four-dimensional spin foam models this amounts to modifying the vertex amplitude such that the exponential asymptotics is obtained. In particular, we show that the ELPR/FK model vertex amplitude can be modified such that the new model is finite and has the Einstein-Hilbert action as its classical limit. We also calculate the first-order and some of the second-order quantum corrections in the semi-classical expansion of the effective action.Comment: Improved presentation, 2 references added. 15 pages, no figure