Cauldron subsidence and subglacial floods

Ice cauldrons are depressions which form at the surface of ice sheets when an underlying subglacial lake empties. Notable examples of such cauldrons occur on the surface of the Vatnajökull ice cap in Iceland, and in particular are formed when subglacial volcanic eruptions occur. More generally, cauldrons will form when a subglacial lake empties during a jökulhlaup. The rate of subsidence of the ice surface is related to the rate at which the subglacial water empties from the lake. We use a viscous version of classical beam theory applied to the ice sheet to determine the relation between the subsidence rate and flood discharge, and we use the results to make inferences concerning ring fracture spacings in cauldrons, the consequent effect on flood discharge dynamics, and the likely nature of subsidence events in the Antarctic Ice Sheet

Electrostatic Analyzer for 1.5-Mev Protons

A system for analyzing the ion beam of an electrostatic generator is described. A weak magnetic field separates protons from heavier components and a 90° electrostatic deflection gives the required energy resolution. With the analyzer controlling the generator voltage, a proton beam of one microampere with an energy spread of the order of 300 volts in one million is obtained

Subglacial floods beneath ice sheets.

Subglacial floods (jökulhlaups) are well documented as occurring beneath present day glaciers and ice caps. In addition, it is known that massive floods have occurred from ice-dammed lakes proximal to the Laurentide ice sheet during the last ice age, and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago. We propose that drainage of lakes from beneath ice sheets will generally occur in a time-periodic fashion, and that such floods can be of severe magnitude. Such hydraulic eruptions are likely to have caused severe climatic disturbances in the past, and may well do so in the future

Multiple scattering of fast electrons

The scattering of electrons and positrons of energy from 5 to 17 Mev has been measured in lead foils of thickness 0.015 and 0.038 cm, in carbon laminae of thickness 0.132 and 0.381 cm, and in an aluminum foil of thickness 0.118 cm. The scattering has been shown to be in agreement with the multiple scattering theory of Williams in that the distribution in the product of the scattering angle times the energy of the scattered particle is Gaussian in form, and in that the mean scattering angle times energy is independent of certain geometrical aspects of the experimental method of measurement, and of the energy and sign of charge of the scattered particles. For carbon the experimental magnitude of the mean scattering is in satisfactory agreement with theory, but in aluminum and lead the experimental results are only 85 percent and 60 percent, respectively, of the theoretical predictions

Towards practical classical processing for the surface code: timing analysis

Topological quantum error correction codes have high thresholds and are well suited to physical implementation. The minimum weight perfect matching algorithm can be used to efficiently handle errors in such codes. We perform a timing analysis of our current implementation of the minimum weight perfect matching algorithm. Our implementation performs the classical processing associated with an nxn lattice of qubits realizing a square surface code storing a single logical qubit of information in a fault-tolerant manner. We empirically demonstrate that our implementation requires only O(n^2) average time per round of error correction for code distances ranging from 4 to 512 and a range of depolarizing error rates. We also describe tests we have performed to verify that it always obtains a true minimum weight perfect matching.Comment: 13 pages, 13 figures, version accepted for publicatio

Proton-induced noise in digicons

The Space Telescope, which carries four Digicons, will pass several times per day through a low-altitude portion of the radiation belt called the South Atlantic Anomaly. This is expected to create interference in what is otherwise anticipated to be a noise-free device. Two essential components of the Digicon, the semiconductor diode array and the UV transmitting window, generate noise when subjected to medium-energy proton radiation, a primary component of the belt. These trapped protons, having energies ranging from 2 to 400 Mev and fluences at the Digicon up to 4,000 P+/sec-sq cm, pass through both the window and the diode array, depositing energy in each. In order to evaluate the effect of these protons, engineering test models of Digicon tubes to be flown on the High Resolution Spectrograph were irradiated with low-flux monoenergetic proton beams at the University of Maryland cyclotron. Electron-hole pairs produced by the protons passing through the diodes or the surrounding bulk caused a background count rate. This is the result of holes diffusing over a distance of many diode spacings, causing counts to be triggered simultaneously in the output circuits of several adjacent diodes. Pulse-height spectra of these proton-induced counts indicate that most of the bulk-related counts overlap the single photoelectron peak. A geometrical model will be presented of the charge collection characteristics of the diode array that accounts for most of the observed effects

Fluorine-plus-proton reactions

The properties of certain energy levels in O16, F19, and Ne20 have been studied by observations on the alpha particles and inelastic protons from the bombardment of fluorine by protons. A high-resolution magnetic analysis of the alpha-particle groups to the 2+ and 1- levels in O16 from the F19(p, α)O16* reaction failed to reveal any doublet structure in these known levels. The angular distributions of the alpha-particle groups to these levels did not indicate degeneracy with a 2- level, nor did a search for new excited levels in O16 up to 8.7-Mev excitation reveal a 2- level. These results are not in agreement with the alpha-particle model of the O16 nucleus which predicts a 2- state close in energy to the 2+ state. Angular distributions of the alpha particles were measured at proton bombarding energies of 873, 935, 1290, 1355, and 1381 kev. The distributions at 1355 kev indicated that the corresponding Ne20 resonance level at 14.16 Mev has spin 2 and odd parity. The spin and parity assignments previously found for the other levels were confirmed. A study of the inelastic proton groups from the F19(p, p′)F19* reaction gave 108.8±0.8 and 196.0±1.4 kev for the excitation energies of the two lowest excited levels of F19. The cross sections at the 1431-kev resonance for these groups in the center-of-mass system were 0.187±0.015 barn for the first group and 0.007 ±0.002 barn for the second group. At 1381 kev the cross section was 0.0427±0.0040 barn for protons to the second excited level. Angular distributions of the proton groups were measured and, in conjunction with other studies made in this laboratory, resulted in spin and parity assignments of ½- and 3/2+ for the first and second excited states of F19, respectively

The s=1/2s=1/2 Antiferromagnetic Heisenberg Model on Fullerene-Type Symmetry Clusters

The $s_{i}={1/2}$ nearest neighbor antiferromagnetic Heisenberg model is considered for spins sitting on the vertices of clusters with the connectivity of fullerene molecules and a number of sites $n$ ranging from 24 to 32. Using the permutational and spin inversion symmetries of the Hamiltonian the low energy spectrum is calculated for all the irreducible representations of the symmetry group of each cluster. Frustration and connectivity result in non-trivial low energy properties, with the lowest excited states being singlets except for $n=28$. Same hexagon and same pentagon correlations are the most effective in the minimization of the energy, with the $n=32-D_{3h}$ symmetry cluster having an unusually strong singlet intra-pentagon correlation. The magnetization in a field shows no discontinuities unlike the icosahedral $I_h$ fullerene clusters, but only plateaux with the most pronounced for $n=28$. The spatial symmetry as well as the connectivity of the clusters appear to be important for the determination of their magnetic properties.Comment: Extended to include low energy spectra, correlation functions and magnetization data of clusters up to 32 site