The effects of strontium-90 on mice

On Sept. 19, 1958 there was published in Science a paper by Dr. Miriam P. Finkel of Argonne National Laboratory in which she communicated her observations on the effects of strontium-90 injected into mice on life expectancy and on incidence of tumors of bone and blood-forming tissues.(1) She discussed the question of whether or not the effects are proportional to the amount of injected strontium-90 at low doses, and reached the conclusion that it is likely that there is a threshold with value for man between 5 and 15 μc. (as compared with the present average value from fallout, about 0.0002 μc., and the predicted steady-state value from fallout for testing of nuclear weapons at the average rate for the past five years, about 0.02 μc.). Her paper ends with the sentence "In any case, the present contamination with strontium-90 from fallout is so very much lower than any of these levels that it is extremely unlikely to induce even one bone tumor or one case of leukemia.

Stress-gradient coupling in glacier flow: III. Exact longitudinal equilibrium equation

The "vertically" integrated, exact longitudinal stress-equilibrium equation of Budd (1970) is developed further in. such a way as to yield an equation that gives explicitly and exactly the contributions to the basal shear stress made by surface and bed slope, surface curvature, longitudinal stress deviators, and longitudinal stress gradients in a glacier flowing in plane strain over a bed of longitudinally varying slope. With this exact equation, questions raised by various approximate forms of the longitudinal equilibrium equation can be answered decisively, and the magnitude of errors in the approximations can be estimated. To first order, in the angle δ that describes fluctuations in the surface slope ɑ from its mean value, the exact equilibrium equation reduces to (1 + 2sin^2θ)τ_B = pghsinɑ + 2G + T + B + K where G and T are the well-known stress-deviator-gradient and "variational stress" terms, K is a "longitudinal curvature" term, and B is a "basal drag" term that contributes a resistance to sliding across basal hills and valleys. Except for T, these terms are expressed in simple form and evaluated for practical situations. The bed slope θ (relative to the mean slope) is not assumed to be small, which allows the effects of bedrock topography to be determined, particularly through their appearance in the B term

Glacier Surge Mechanism Based on Linked Cavity Configuration of the Basal Water Conduit System

Based on observations of the 1982–1983 surge of Variegated Glacier, Alaska, a model of the surge mechanism is developed in terms of a transition from the normal tunnel configuration of the basal water conduit system to a linked cavity configuration that tends to restrict the flow of water, resulting in increased basal water pressures that cause rapid basal sliding. The linked cavity system consists of basal cavities formed by ice-bedrock separation (cavitation), ∼1 m high and ∼10 m in horizontal dimensions, widely scattered over the glacier bed, and hydraulically linked by narrow connections where separation is minimal (separation gap ≲ 0.1 m). The narrow connections, called orifices, control the water flow through the conduit system; by throttling the flow through the large cavities, the orifices keep the water flux transmitted by the basal water system at normal levels even though the total cavity cross-sectional area (∼200 m^2) is much larger than that of a tunnel system (∼10 m^2). A physical model of the linked cavity system is formulated in terms of the dimensions of the “typical” cavity and orifice and the numbers of these across the glacier width. The model concentrates on the detailed configuration of the typical orifice and its response to basal water pressure and basal sliding, which determines the water flux carried by the system under given conditions. Configurations are worked out for two idealized orifice types, step orifices that form in the lee of downglacier-facing bedrock steps, and wave orifices that form on the lee slopes of quasisinusoidal bedrock waves and are similar to transverse “N channels.” The orifice configurations are obtained from the results of solutions of the basal-sliding-with-separation problem for an ice mass constituting of linear half-space of linear rheology, with nonlinearity introduced by making the viscosity stress-dependent on an intuitive basis. Modification of the orifice shapes by melting of the ice roof due to viscous heat dissipation in the flow of water through the orifices is treated in detail under the assumption of local heat transfer, which guarantees that the heating effects are not underestimated. This treatment brings to light a melting-stability parameter Ξ that provides a measure of the influence of viscous heating on orifice cavitation, similar but distinct for step and wave orifices. Orifice shapes and the amounts of roof meltback are determined by Ξ. When Ξ ≳ 1, so that the system is “viscous-heating-dominated,” the orifices are unstable against rapid growth in response to a modest increase in water pressure or in orifice size over their steady state values. This growth instability is somewhat similar to the jökulhlaup-type instability of tunnels, which are likewise heating-dominated. When Ξ ≲ 1, the orifices are stable against perturbations of modest to even large size. Stabilization is promoted by high sliding velocity ν, expressed in terms of a ν^(−½) and ν^(−1) dependence of Ξ for step and wave cavities. The relationships between basal water pressure and water flux transmitted by linked cavity models of step and wave orifice type are calculated for an empirical relation between water pressure and sliding velocity and for a particular, reasonable choice of system parameters. In all cases the flux is an increasing function of the water pressure, in contrast to the inverse flux-versus-pressure relation for tunnels. In consequence, a linked cavity system can exist stably as a system of many interconnected conduits distributed across the glacier bed, in contrast to a tunnel system, which must condense to one or at most a few main tunnels. The linked cavity model gives basal water pressures much higher than the tunnel model at water fluxes ≳1 m^(3/s) if the bed roughness features that generate the orifices have step heights or wave amplitudes less than about 0.1 m. The calculated basal water pressure of the particular linked cavity models evaluated is about 2 to 5 bars below ice overburden pressure for water fluxes in the range from about 2 to 20 m^(3/s), which matches reasonably the observed conditions in Variegated Glacier in surge; in contrast, the calculated water pressure for a single-tunnel model is about 14 to 17 bars below overburden over the same flux range. The contrast in water pressures for the two types of basal conduit system furnishes the basis for a surge mechanism involving transition from a tunnel system at low pressure to a linked cavity system at high pressure. The parameter Ξ is about 0.2 for the linked cavity models evaluated, meaning that they are stable but that a modest change in system parameters could produce instability. Unstable orifice growth results in the generation of tunnel segments, which may connect up in a cooperative fashion, leading to conversion of the linked cavity system to a tunnel system, with large decrease in water pressure and sliding velocity. This is what probably happens in surge termination. Glaciers for which Ξ ≲ 1 can go into surge, while those for which Ξ ≳ 1 cannot. Because Ξ varies as α^(3/2) (where α is surface slope), low values of Ξ are more probable for glaciers of low slope, and because slope correlates inversely with glacier length in general, the model predicts a direct correlation between glacier length and probability of surging; such a correlation is observed (Clarke et al., 1986). Because Ξ varies inversely with the basal shear stress τ, the increase of τ that takes place in the reservoir area in the buildup between surges causes a decrease in Ξ there, which, by reducing Ξ below the critical value ∼1, can allow surge initiation and the start of a new surge cycle. Transition to a linked cavity system without tunnels should occur spontaneously at low enough water flux, in agreement with observed surge initiation in winter

Ice. II. A proton-ordered form of ice

[No abstract

The crystal structure of zunyite

The structure proposed by Pauling for the rare aluminosilicate mineral zunyite (Al_(13)(OH)_(18)Si_5O_(20)Cl) has been confirmed and refined with the use of 163 hkO reflections and 409 hhl reflections obtained with Mo Kα radiation from single crystal Weissenberg photographs. The structure is isometric (T^2_d) and is built up of Si_5O_(16) groups of linked silicon tetrahedra combined with Al_(12)O_(16)(OH)_(30) groups of linked aluminum octahedra. Refinement is carried out independently for the hkO and hhl data, and the final reliability factors are 0·12 for both sets of data. Positional parameters are refined by the least-squares method, and isotropic temperature parameters for separate atoms are adjusted with the help of difference syntheses. The refined structure differs from the trial structure by distortion of coordination polyhedra in a fashion similar to the distortions in related structures. The interatomic distance Al-O of 1·80 ± 0·016 Å is derived for tetrahedrally coordinated aluminum. The averaged Si-O distance is 1·64 ± 0·01 Å. The arrangement of protons in the structure is deduced from structural arguments. The proposed arrangement requires the inclusion of at least two fluorine atoms per stoichiometric molecule, modifying the chemical formula to (OH, F)_(16)F_2Al_(13)Si_5O_(20)Cl and explaining the importance of fluorine in the formation of zunyite

Accuracy of atomic positions in the zunyite structure

The accuracy of positional parameters in the refined zunyite structure is estimated by four different statistical methods, including a comparison of two entirely independent refinements of the structure. The estimates show tolerable agreement, but disagree as to the importance of F_o measurement error in affecting the parameter error. Reliable estimates of ± 0.008 Å (standard deviation) for oxygen coordinates and ± 0.003 Å for silicon and aluminum coordinates are obtained

Structure of ice IV, a metastable highpressure phase

Ice IV, made metastably at pressures of about 4 to 5.5 kb, has a structure based on a rhombohedral unit cell of dimensions a_R = 760±1 pm, α = 70.1±0.2°, space group R3̄c, as observed by x‐ray diffraction at 1 atm, 110 K. The cell contains 12 water molecules of type 1, in general position, plus 4 of type 2, with O(2) in a special position on the threefold axis. The calculated density at 1 atm, 110 K is 1.272±0.005 g cm^(−3). Every molecule is linked by asymmetric H bonds to four others, the bonds forming a new type of tetrahedrally‐connected network. Molecules of type 1 are linked by O(1)⋅⋅⋅O(1′) bonds into puckered six‐rings of 3 symmetry, through the center of each of which passes an O(2)⋅⋅⋅O(2′) bond between a pair of type‐2 molecules, along the threefold axis. The six‐rings are linked laterally by type‐2 molecules to form puckered sheets that are topologically similar to such sheets in ice I, but are connected to one another in a very different and novel way. One quarter of the intersheet bonds connect not directly between adjacent sheets but remotely, from one sheet to the second nearest sheet, through holes in the intervening sheet. These remote connections are the O(2)⋅⋅⋅O(2′) bonds, passing through the O(1)‐type six‐rings. The sheets are stacked in a sequence based on ice Ic, modified by reversal of the puckering to form the remote connections and by internal distortion of the sheets to complete the remaining intersheet bonds. Of the four nonequivalent H bonded O⋅⋅⋅O distance in the structure, two (279 and 281±1pm) are only moderately lengthened relative to the bonds in ice I (275 pm), whereas the O(1)⋅⋅⋅O(1′) bond (288±1pm) and O(2)⋅⋅⋅O(2′) bond (292±1pm) are lengthened extraordinarily. This is caused by repulsion between O(1) and O(2) at nonbonded distances of 314 and 329 pm in the molecular cluster consisting of the O(1)‐type six‐ring threaded by the O(2)⋅⋅⋅O(2′) bond. The mean O⋅⋅⋅O bond distance of 283.3 pm, which is high relative to other ice structures except ice VII/VIII, reflects similarly the accommodation of a relatively large number (3.75 on average) of nonbonded neighbors around each molecule at relatively short distances of 310–330 pm. Bond bending in ice IV, as measured by deviation of the O⋅⋅⋅O⋅⋅⋅O bond angles from 109.5°, is relatively low compared to most other dense ice structures. All H bonds in ice IV except O(1)⋅⋅⋅O(1′) are required to be proton‐disordered by constraints of space‐group symmetry. The x‐ray structure‐factor data indicate that O(1)⋅⋅⋅O(1′) is probably also proton‐disordered. Ice IV is the only ice phase other than ice I and Ic to remain proton‐disordered on quenching to 77 K. The increased internal energy of ice IV relative to ice V, amounting to about 0.23 kJ mole^(−1), which underlies the metastability of ice IV in relation to ice V, can be explained structurally as a result of extra overlap and bond‐stretching energy in ice IV, partially compensated by extra bond‐bending energy in ice V. The structural relation between ice IV and ice I offers a possible explanation for the reduced barrier to nucleation of ice IV, as compared to ice V, in crystallizing from liquid water

Rheology of ice II and ice III from high-pressure extrusion

Rheological parameters for ice II and ice III, needed in tectonic models of the icy satellites of the major planets, are obtained from extrusion experiments and compared with the rheology of ice I at pressures ∼2 kbar and temperatures ∼240K. Ice II has a higher effective viscosity (by a factor ∼10) than ice I at similar stress levels, whereas ice III has a lower viscosity (by a factor ∼0.01). The Rheological contrasts among the ice phases are related to differences in the dielectric relaxation behavior and state of proton order/disorder in the structures in a way that sheds light on the nature of dislocation motion in ice. A striking transformation plasticity accompanies the ice I-III transition and could have large tectonic effects

Kamb Ice Stream flow history and surge potential

A basal zone, tens of meters thick, of debris-laden ice was observed in Kamb Ice Stream, West Antarctica, using a video camera lowered into boreholes made by hot-water drilling. The debris content varies, sometimes abruptly, forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction. In most parts, the concentration of debris is low, a few percent by weight, with particles, often mud clots, dispersed in a matrix of clear ice. The nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of Kamb Ice Stream including processes leading up to the termination of its streaming behavior and possible reactivation