Slab Source Research Articles

Skin Dosimetry of Some Beta Sources using the VARSKIN2 Code

Introduction: Skin dose and depth dose distribution of some beta sources that can contaminate skin or protective clothing covering the skin have been calculated using VARSKIN2. Materials and Methods: In this research, absorbed dose to skin and depth dose from the skin surface have been calculated for 90Sr/90Y, 147Pm, 32P beta emitter radioisotopes by using input parameters: activity of source, cover thickness, cover density, air gap thickness, radiation time and different source geometries. Results: Absorbed dose variation was calculated for beta sources in different source forms, namely, point, 2-D disk, cylindrical, spherical and slab sources. The results show that a cylindrical source causes the minimum damage to skin cells. Therefore, this shape of source is the best with respect to radiation protection. Discussion and conclusion: The VARSKIN2 code is a very useful tool for skin dosimetry, as well being fast, accurate and user friendly. It can be used for dose optimization calculations especially for beta sources over human skin.

Application of the Monte Carlo method for the calibration of an in situ gamma spectrometer

A MCNP model was developed for the efficiency calibration of an in situ gamma ray spectrometry system based on a high purity germanium (HPGe) detector. The detector active crystal volume was adjusted semi-empirically against experimental measurements. Calculated full energy peak efficiency curves, over the photon energy range between 50 keV and 5 MeV, are presented for surface and slab source configurations. The effect of different collimator apertures and the contribution of different HPGe crystal regions in the detector response are also shown.

Chemistry of hydrothermal plumes above submarine volcanoes of the Mariana Arc

Fifty submarine volcanoes of the Mariana Arc, covering 1200 km from 13.5°N to 23.2°N, were surveyed for hydrothermal activity. Of these 50 volcanoes, eight showed limited evidence of hydrothermal activity, while another 10 volcanoes displayed intense, chemically rich, hydrothermal plumes that allow detailed chemical characterization and insight into the hydrothermal activity forming the fluids that generate the plumes. The most active volcanoes exhibit a wide range of CO2 to pH relationships, from the venting of acid‐rich fluids, to CO2‐rich fluids, to fluids rich in alkalinity and CO2. These pH‐CO2‐alkalinity relationships are partially responsible for the wide range of Fe:Mn (3 to 32) observed at the different volcanoes. This chemical heterogeneity is further manifest in a wide range of CO2:3He (3 × 109 to 55 × 109), indicating that for all but one, >80% of the CO2 venting from these volcanoes has a slab source. The helium, by contrast, has an upper mantle isotopic signature. The range of chemical conditions suggests that these volcanoes occupy various states of eruptive evolution ranging from ongoing magmatic activity to highly evolved hydrothermal systems.

Stable isotope compositions of serpentinite seamounts in the Mariana forearc: Serpentinization processes, fluid sources and sulfur metasomatism

The Mariana and Izu–Bonin arcs in the western Pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge. We present stable isotope (O, H, S, and C) data for serpentinites from Conical seamount in the Mariana forearc and S isotope data for Torishima seamount in the Izu–Bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge, and to investigate the transport of sulfur from the slab to the mantle wedge. Six serpentine mineral separates have a restricted range of δ18O (6.5–8.5‰). Antigorite separates have δD values of − 29.5‰ to − 45.5‰ that reflect serpentinization within the mantle wedge whereas chrysotile has low δD values (− 51.8‰ to − 84.0‰) as the result of re-equilibration with fluids at low temperatures. Fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300–375 °C. Two late cross-fiber chrysotile veins have higher δ18O values of 8.9‰ to 10.8‰ and formed at lower temperatures (as low as ∼100 °C). Aqueous fluids in equilibrium with serpentine at 300–375 °C had δ18O = 6.5–9‰ and δD = − 4‰ to − 26‰, consistent with sediment dehydration reactions at temperatures < 200 °C in the subducting slab rather than a basaltic slab source. Three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have δ18O = 16.7–24.5‰, consistent with the serpentinizing fluids at temperatures < 250 °C. δ13C values of 0.1–2.5‰ suggest a source in subducting carbonate sediments. The δ34S values of sulfide in serpentinites on Conical Seamount (− 6.7‰ to 9.8‰) result from metasomatism through variable reduction of aqueous sulfate (δ34S = 14‰) derived from slab sediments. Despite sulfur metasomatism, serpentinites have low sulfur contents (generally < 164 ppm) that reflect the highly depleted nature of the mantle wedge. The serpentinites are mostly enriched in 34S (median δ34Ssulfide = 4.5‰), consistent with a 34S-enriched mantle wedge as inferred from arc lavas.

Geochemistry of the NeoArchaean high-Mg basalts, boninites and adakites from the Kushtagi–Hungund greenstone belt of the Eastern Dharwar Craton (EDC); implications for the tectonic setting

Volcanic rocks of the Kushtagi–Hungund greenstone belt from the Eastern Dharwar Craton (EDC) show extreme diversity due to the presence of high-Mg, high Fe-, high Mg–Fe basalts, high Mg-dacites and andesites (boninitic affinities), and sodic–plagioclase–quartz rich felsic rocks (adakites with Na 2O (∼4–6 wt.%). The adakites ( Drummond and Defant, 1990; Martin, 1999) and high-Mg-dacites–andesites are being reported for the first time from greenstone belts of the Dharwar craton. Trace elements in these different types of volcanic rocks have distinct characteristics. For example, the basalts have (1) Al 2O 3/TiO 2 ∼16 (PM value of 17), (2) (La/Yb) N ∼1–3, (3) low order negative anomalies of Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti*, (4) Zr/Nb ∼4–10 and (5) Ce/Nb ∼3–4. Two samples of andesites having Mg-enrichment and Al 2O 3/TiO 2 ratios of 31–35 are interpreted to have boninitic affinities. Other dacites and andesites are characterized by (1) high-Mg (8–10 wt.%), (2) high Al 2O 3/TiO 2=22–27, (3) Zr/Nb=8–14, (4) (La/Yb) N ∼2–7, (5) negative Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti* anomalies and (6) Sc/Lu=67–172. The adakites are characterized by (1) a narrow range of SiO 2 (67–70 wt.%), (2) enriched Na 2O (4–6 wt.%) with very high Na 2O/K 2O (2.5–7.7), (3) Low Mg# (0.38–0.48), (4) high (La/Yb) N (13–31) with fractionated patterns, no Eu anomaly and (Gd/Yb) N =2–4, (5) very high Al 2O 3/TiO 2 (50–80) and (6) enriched Ni, Cr and Co (total 50 ppm). These adakites resemble 3.0 Ga tonalite–trondhjemite gneisses of the Dharwar craton, which are interpreted as products of partial melting of the basaltic slab. The available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting.

The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones

We studied the systematics of Cl, F and H 2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ∼400–4000 ppm and F ∼70–400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (∼6600–8600 ppm) and F (∼780–910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H 2O of basalt to andesite melt inclusions in plagioclase is estimated to range from ∼2 to ∼10 wt% H 2O. The Izu magmas should be undersaturated in H 2O and the halogens at their preferred levels of crystallization in the middle to lower crust (∼3 to ∼11 kbar, ∼820° to ∼1200°C). A substantial portion of the original H 2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H 2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H 2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94 ± 0.25 wt%, F = 990 ± 270 ppm and H 2O = 25 ± 7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (∼77–129%). Conversely, H 2O (∼13–22% recycled at arc) and F (∼4–6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H 2O, the calculated percentage of Cl and H 2O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (∼37,000 km), the global arc outflux of fluid-recycled Cl and H 2O at subduction zones amounts to Cl ∼2.9–3.8 × 10 12 g/yr and H 2O ∼0.7–1.0 × 10 14 g/yr, respectively—comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ∼0.3–0.4 × 10 12g/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H 2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H 2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (∼9.5), slab melting will still produce components with low Cl/F ratios (∼0.9), similar to those characteristic of the upper continental crust (Cl/F ∼0.3–0.9).

The systematics of boron isotopes in Izu arc front volcanic rocks

The process of fluid release from the subducting slab beneath the Izu arc volcanic front (Izu VF) was examined by measuring B concentrations and B isotope ratios in the Neogene fallout tephra (ODP Site 782A). Both were measured by secondary ion mass spectrometry, in a subset of matrix glasses and glassy plagioclase-hosted melt inclusions selected from material previously analyzed for major and trace elements (glasses) and radiogenic isotopes (Sr, Nd, Pb; bulk tephra). These tephra glasses have high B abundances (∼10–60 ppm) and heavy δ 11B values (+4.5‰ to +12.0‰), extending the previously reported range for Izu VF rocks (δ 11B, +7.0‰ to +7.3‰). The glasses show striking negative correlations of δ 11B with large ion lithophile element (LILE)/Nb ratios. These correlations cannot be explained by mixing two separate slab fluids, originating from the subducting sediment and the subducting basaltic crust, respectively (model A). Two alternative models (models B and C) are proposed. Model B proposes that the inverse correlations are inherited from altered oceanic crust (AOC), which shows a systematic decrease of B and LILE with increasing depth (from basaltic layer 2A to layer 3), paralleled by an increase in δ 11B (from ∼+1‰ to +10‰ to +24‰). In this model, the contribution of sedimentary B is insignificant (<4% of B in the Izu VF rocks). Model C explains the correlation as a mixture of a low-δ 11B (∼+1‰) ‘composite’ slab fluid (a mixture of metasediment- and metabasalt-derived fluids) with a metasomatized mantle wedge containing elevated B (∼1–2 ppm) and heavy δ 11B (∼+14‰). The mantle wedge was likely metasomatized by 11B-rich fluids beneath the outer forearc, and subsequently down dragged to arc front depths by the descending slab. Pb–B isotope systematics indicate that, at arc front depths, ∼53% of the B in the Izu VF is derived from the wedge. This implies that the heavy δ 11B values of Izu VF rocks are largely a result of fluid fractionation, and do not reflect variations in slab source provenance (i.e. subducting sediment vs. basaltic crust). Since the B content of the peridotite at the outer forearc (7–58 ppm B, mean 24±16 ppm) is much higher than beneath the arc front (∼1–2 ppm B), the hydrated mantle wedge must have released a B-rich fluid on its downward path. This ‘wedge flux’ can explain (1) the across-arc decrease in B and δ 11B (e.g. Izu, Kuriles), without requiring a progressive decrease in fluid flux from the subducting slab, and (2) the thermal structure of volcanic arcs, as reflected in the B and δ 11B variations of volcanic arc rocks.

Are the regional variations in Central American arc lavas due to differing basaltic versus peridotitic slab sources of fluids?

Central American arc volcanism shows strong regional trends in lava chemistry that result from differing slab contributions to arc melting. However, the mechanism that transfers slab-derived trace elements into the mantle wedge remains largely unknown. By using a dynamic model for mantle flow and fluid release, we model the fate of three different slab-fluid sources: sediment, ocean crust, and serpentinized mantle. In the open subarc system, sediments lose almost all their highly fluid mobile elements by ∼50 km depth, so other fluid sources are necessary to explain the slab signal in arc-lava compositions. The well-documented transition from lavas with a strong geochemical slab signature (i.e., high Ba/La ratios) found in Nicaragua to lavas with a weaker slab signature (i.e., low Ba/La ratios) erupted in Costa Rica seems easiest to produce by a higher fraction of serpentine-hosted fluids released from the deeply faulted, highly serpentinized lithosphere subducting beneath Nicaragua than from the less deeply faulted, thicker, amphibolitic oceanic-crust and oceanic-plateau lithosphere subducting beneath Costa Rica.

Eastbound sublithosphere mantle flow through the Caribbean gap and its relevance to the continental undertow hypothesis

Recent evidence indicates that beneath the Caribbean a tongue of sublithosphere mantle is flowing from the Pacific to the Atlantic, dragging the overlying lithosphere eastward: (i) Shear‐wave splitting results from beneath the Andean subduction zone and Venezuela suggest mantle flow eastward through the Caribbean. (ii) Volcanic chemistry in Central America indicates a slab source beneath Nicaragua, but a different source in Costa Rica, above the proposed Pacific outflow. (iii) An extinct volcanic arc accreted to the margins of the Caribbean swept eastward through the Caribbean gap between North &amp; South America. The 1982 ‘continental undertow’ model requires shallow‐mantle flow through the Caribbean gap from the Pacific to the Atlantic, if continents have deep roots and if shallow‐mantle flow beneath oceans is decoupled from convection at deeper levels. The new evidence from the Caribbean is thus compatible with the continental undertow model, and perhaps with other models involving decoupled shallow flow.

Electrode erosion and coating properties in pulsed air arc deosition of WC-based hard alloys

WC-based hard alloys were deposited from a slab source anode on to low carbon steel and pure Ti substrates using a pulsed air arc. The depositions were conducted with a series of pulses with current amplitudes of 75 A, 300 A, and 500 A, a pulse duration of 150 μs, and a pulse repetition rate of 100 Hz. The influence of the electrode material, discharge energy and deposition time on the electrode erosion rate, mass transfer direction, and the wear resistance and friction coefficient of the coatings was investigated. Anodic and cathodic mass change characteristics depended on both the anode and cathode materials. Using a steel cathode and a WC anode, depositions of the anode material always formed on the cathode. For the same WC anode but a Ti cathode, coatings only formed locally in the craters of the eroded cathode. The anodic mass loss when using the steel cathodes was about a factor of 3 larger than when using the Ti cathodes. The microhardness of the coated substrates was larger by factors of 5–12 than the uncoated Ti and low carbon steel substrates. The coatings increased the wear resistance of the steel and Ti substrates by factors of 4.3 and 1.4, respectively. The poorer performance of the coated Ti substrates is attributed to the intensive erosion of the Ti cathodes (compared to the steel cathodes) and the discontinuous nature of the coatings (i.e. only in the erosion caters).

The 9 June 94 Bolivian Deep Earthquake: An exceptional event in an extraordinary subduction zone

We investigate the physical setting of the Bolivian shock based on the history of the subducting Nazca plate, intraslab seismicity, deep seismic moment release, and seismic tomography. South America has two broad regions of reverse arc curvature. Subduction constrained to this unique geometry produces slab kinking contortions that may cause unusual slab thickening as they sink to the bottom of the transition zone and encounter resistance to penetration into the lower mantle. Such contortions are observed at intermediate depths “upstream” from the slab source regions of both the great 1970 Colombian and 1994 Bolivian events. Thickening helps explain how the Nazca slab accommodates large seismic source dimensions at depths of 625–650 km.

Dose-rate conversion factors for external exposure to photon emitters in soil.

Dose-rate conversion factors have been calculated for external exposure above ground to monoenergetic photon emitters in soil. These factors give external dose rates per unit source concentration in soil. The calculations are based on the point-kernel integration method and assume that the source concentration at any depth in soil is uniform over an infinite surface parallel to the ground plane. Dose-rate factors in air at a height of 1 m above ground are tabulated for discrete photon energies between 0.01 and 10 MeV and for source depths in soil between 0 and 300 cm. Application of the results for plane sources in soil to the calculation of photon dose rates from distributions of sources with depth in soil is described, and dose-rate factors are tabulated for the particular cases of uniform slab sources of finite thickness and sources which are exponentially distributed with depth. We also demonstrate how dose-rate factors in air for monoenergetic photon sources are used to estimate dose-rate factors for body organs of exposed individuals and for the spectrum of photons from radioactive decay. The calculations in this paper show that allowing for downward migration of radionuclides in soil can result in significant reductions in external dose compared with the usual assumption that radionuclides which are deposited on the ground surface remain there until removal by radioactive decay.

Subduction zone Calibration and teleseismic relocation of thrust zone events in the central Aleutian Islands

AbstractTeleseismically determined epicenters for thrust zone earthquakes in the central Aleutian Islands are systematically mislocated by about 40 km to the north relative to local network solutions. We attribute the mislocation to travel-time anomalies produced by the higher P-wave velocity in the subducting slab. These travel-time anomalies can be estimated by seismic ray tracing through a thermally modeled velocity structure for the slab. Observed teleseismic station residuals and slab travel-time anomalies computed from local network hypocenters, plotted as a function of event distance to the center of curvature of the island arc, show good agreement. This distance versus residual relationship can be used to validate the local network hypocenters and to constrain the length of subducting lithosphere. Computed slab source corrections, when incorporated in a standard location program, reduce teleseismic mislocations to 20 km; the addition of empirical station corrections reduces the mislocation to about 10 to 15 km. Depth and origin time estimates can be further improved through use of better station corrections, a local station within 2°, and depth phases.

Evaluation of self-absorption in 4 pi beta counting using spherical and slab sources

球状粒子線源および平板線源を用いて4πβ計測における線源の自己吸収量を評価した。測定した自己吸収量を1つの質量吸収係数を用いて表したところ, 測定値と良い一致が得られなかった。このため, β線の吸収を2つの質量吸収係数で表し, さらに, 平板線源におけるβ線透過距離に上限を設けることにより, 両線源に対する自己吸収量の測定値を良く説明することができた。また.平板線源における放射性核種の分布位置と自己吸収量の関係の測定から, 自己散乱により生じたβ線の自己吸収への寄与が大きいことが判明した。このため, 放射能濃度絶対測定用線源では, 放射性核種を表面に分布させることが, 自己吸収の改善の1方法になると考えられる.

Studies on the Manufacture of Low DP Pulps by Irradiation (Part 3)

Samples of dissolving pulps were irradiated by three types of irradiators-cobalt-60 slab source (125KCi), high energy (2MV) and low energy (0.5MV) electron accelerators-which were considered to be applicable as the industrial irradiators. Then, degree of polymerization (DP) at various points in the irradiated pulp samples were measured and from these values the absorbed dose distributions in the samples were calculated by means of the author's empirical equation which expresses the relationship between absorbed dose and extent of depolymerization.By reference to the absorbed dose distribution described above, we tried to establish the optimum irradiation methods for the manufacture of low DP pulps, taking into consideration of various viewpoints such as the uniformity of irradiation, the utilization efficiency of radiation, the production capacity etc., and the following conclusions were obtained.Cobalt-60 slab sources are inapplicable in all respects.High energy electron accelerators are expected to give the low DP pulps having desirable properties when applied with electron filters and/or both-sides irradiation technique. These irradiators, however, have some defects, that is capital investments per unit power become more or less higher and complicated handlings to pile up pulp sheets are required.For the low energy accelerators, it is possible to improve the uniformity of irradiation and the utilization efficiency of radiation by incorporating with application of back-scattering. For example, in the case of Pb-plates are adopted as the back-scattering materials, the ratio of maximum absorbed dose to minimum in pulp sheets amounts to 1.1 and the utilization efficiency of radiation reaches to 95%. Moreover, they can be installed at the ends of the common pulp machines, so that very low irradiation cost can be expected.