Cesium Deposition Research Articles

Estimation of the influence of the Chernobyl accident on the Baikal drainage basin

Abstract For the first time, contamination levels of cesium due to the Chernobyl accident in water of highland lakes (and soils from their watersheds) of the Lake Baikal drainage basin, have been measured. The reason for this study was that the main long-range transportation of Chernobyl accident products in an easterly direction took place at high altitudes (above 2 km). The amount of Chernobyl 137Cs, estimated on the basis of 134Cs content, varied in soils from 2 to 7% in the low-mountain areas, from 13 to 16% in the highlands and from 23 to 34% in the water of highland lakes. The estimated total deposition of cesium in the Baikal drainage basin was estimated to be 0.3% of the total Chernobyl emission.

Cs/GaAs(100) surface: Two-dimensional metal or Hubbard insulator?

The evolution of the characteristic electron energy loss spectra accompanying the deposition of cesium on GaAs(100) surfaces with various superstructural reconstructions is studied experimentally. It is shown that the appearance of loss peaks in the GaAs band gap for Cs coverages θCs>0.5 monolayers is due to resonances in the longitudinal and transverse polarizability of two-dimensional metallic clusters of adatoms.

Coadsorption of cesium and elemental sulfur on Ni(100) surfaces

Adsorption of cesium on clean Ni(100) at room temperature produces an hcp metallic structure at the completion of a single layer [0.3 monolayers (ML)]. Independently of the sequence of cesium and sulfur deposition, the latter is bound directly to the Ni(100) substrate giving a (2×2) and a c(2×2) structure similar to those of sulfur alone on clean Ni(100). Cesium deposition on preheated c(2×2)-S structure changes it successively to (2×2) and (3×3) by reducing the density of the sulfur structure which is contact with the nickel. The presence of sulfur causes a disordering and dimetallization of the well-ordered uniform cesium layer on clean Ni(100). Apparently these mutual effects of the coadsorbates are due to a strong Cs–S interaction. The presence of sulfur also increases the binding energy and the amount of cesium in multilayers. The binding energy of cesium on sulfated Ni(100) was found to be 2.7 eV as compared with 2.8 eV on MoS 2. There is no indication for the formation of any Cs–S compound.

Surface contamination of radiocesium measured and calculated in South Piemonte (Italy)

This work presents the results of the measurements of 137 Cs soil contamination in South Piemonte (Alessandria, Asti and Cuneo Provinces, Italy), undertaken in order to test the effectiveness of a simple wash-out and environmental decay model in forecasting cesium deposition and residence time in undisturbed soils. The relationship between observed soil contamination and soil characteristics (pH, texture and organic matter content) was also explored.

Optical Investigation of Submonolayer Phase Transitions of Cs on GaAs(001)

Optical surface sensitive techniques can probe phase transitions of cesium overlayers on GaAs(001) of less than one monolayer thickness. For cesium deposition at near liquid nitrogen temperature, surface diffusion of cesium is prohibited and there form disordered two-dimensional clusters. We consider two types of transitions: (i) Annealing to room temperature enables surface diffusion and induces an ordering of the overlayer. This transition is revealed from the Reflectance Anisotropy (RA) spectrum since, for the gallium-rich surface, the optical anisotropy is due to localized states, sensitive to the local potential created by the cesium overlayer. (ii) The formation of metallic clusters is revealed by photoreflectance spectroscopy. It is also found that this metallic phase is metastable and is destroyed by annealing to room temperature.

Electronic properties of cesium-covered GaN(0001) surfaces

Cesium was deposited on clean n-GaN(0001)-1 × 1 surfaces at 150 K using Cs dispensers. X-ray photoemission spectroscopy showed that the Cs-films grow layer by layer. Cesium-induced variations of electronic surface properties were observed using photoemission spectroscopy with monochromatized He I radiation (UPS) and a Kelvin probe (CPD). The UPS data recorded with clean GaN(0001) surfaces give their ionization energy as 6.8 ± 0.1 eV, so that their electron affinity measures 3.35 ± 0.1 eV. The work function of clean GaN(0001) surfaces was determined as 4 ± 0.2 eV. Cesium deposition first reduces the work function by up to 2.7 eV and then increases it until eventually a work function of 2 eV is reached. A simple point-charge model easily explains this decrease of the work function by the formation of surface dipoles. The latter value is characteristic of bulk cesium.

Investigation of the distribution of 137Cs from fallout in the soils of the city of Lund and the province of Skåne in Sweden

The vertical and horizontal behaviour of the distribution of 137Cs in soil taken from Skåne and Lund was determined using gamma spectroscopy on several bore cores. Samples were taken from 16 places in the province of Skåne and 16 sites in the town of Lund in the southern part of Sweden. The bore cores were sampled in a triangular pattern and sliced in 2–3-cm layers. The depth distribution of total 137Cs activity, from nuclear weapons fallout and the deposition after the Chernobyl accident, was found by fitting the experimental points to a modified exponential function, and the relaxation lengths found were in accordance with previous measurements at other sites in Sweden with a higher deposition of Chernobyl caesium. The horizontal variation was found to be about 20% for the Lund samples and about 40% in the Skåne samples, which is in accordance with results found for similar investigations on sites with smaller horizontal scales.

Interaction of Oxygen and Carbon Monoxide with CsAu Surfaces

The formation of cesium−gold alloys by the deposition of cesium on to a Au(100) surface has been investigated by both core-level (X-ray induced) and valence-level (He(II) radiation) spectroscopies. The stoichiometries of the alloy overlayer indicate the preferential segregation of cesium to the surface of the thinner films. Oxygen chemisorption generates three distinct oxygen species O2-(a), Oδ-(a), and O2δ-(a) characterized by their O(1s) binding energies and is accompanied by dealloying. With increasing oxygen exposure, the peroxo species is the major one present at 298 K. Dealloying is temperature dependent and is not complete at 80 K. Two of the oxygen states, Oδ-(a) and O2δ-(a), are reactive to carbon monoxide, forming carbonate species. Peroxo-type species are also formed readily by oxidation of pre-carbonated surfaces. The products formed from CO−O2 mixtures depend on the mixture composition: with a 3:1 (CO:O2) mixture at 298 K, O,2- O2δ-, and CO3 species are observed, while for very much richer C...

Enhanced field emission of diamondlike carbon films due to cesiation

A treatment that lowers the threshold field for field emission and increases the emission site density from a nominally n-type diamondlike carbon film is described. The film was deposited using an rf plasma of methane and nitrogen gases. The treatment involved deposition of cesium followed by a low temperature anneal. Field emission measurements were used to characterize the threshold field and emission site density before and after cesium treatment. Ultraviolet photoemission was used to study the effect of cesium on the work function. Dramatic improvements to field emission by cesiation cannot be generalized to all diamondlike samples, as similar treatment of a type IIb single-crystal (p-type) diamond did not produce as pronounced an improvement in turn-on field or emission site density.

2D cesium insulating layer deposited on Sb-precovered GaAs(110) surface

We present a study of Cs adatom deposition on an antimony-precovered GaAs(110) surface by means of ultraviolet photoemission and high resolution electron energy loss spectroscopy. The ordered Sb monolayer on III–V (110) represents an ideal bulk-like unrelaxed (1 × 1) substrate for alkali adsorption, useful to elucidate the role of different Cs adsorption sites and of the insulating properties of the 2D Cs layer. Cesium deposition on Sb-precovered GaAs (110) indicates the absence of clearly different adsorption sites, at variance with deposition on the bare GaAs(110) surface. One monolayer of Cs deposited on (1 × 1)- Sb GaAs(110) behaves as a semiconductor with a surface gap of 0.65 eV. The influence of the different bonding mechanism induced by the presence of an Sb intralayer does not affect the insulating properties of the 2D Cs layer.

Unpinned behavior of the Fermi level and photovoltage on p-(100) GaAs surface facilitated by deposition of cesium

The preparation technique of (100)GaAs surface with unpinned electronic properties is further developed in this work. It was proved that the unpinned behavior of the Fermi level and surface photovoltage on p-(100)GaAs surface was facilitated by the deposition of cesium. The surface preparation technique included removal of oxides in the solution of HCl in isopropanol and transfer to a UHV set-up in nitrogen atmosphere. The variations of band bending and surface photovoltage were measured by means of photoreflectance spectroscopy after anneals at successively increasing temperatures. Without Cs deposition, the band bending varied only by 50–100 meV as the annealing temperature increased from 300 to 600°C. Deposition of a monolayer of Cs, followed by annealing at 400°C, caused a pronounced decrease of band bending. The evolution of surface photovoltage, which depends both on band bending and on the rates of capture and recombination of the photogenerated carriers on the surface, was more diverse and sensitive to the details of surface treatment. Possible mechanisms of cesium influence on the electronic properties of the surface are discussed.

Valence-band electronic structure of MoS2 and Cs/MoS2(0002) studied by angle-resolved x-ray photoemission spectroscopy.

The angle dependence of the valence-band photoemission from the trigonal prismatic layered ${\mathrm{MoS}}_{2}$ shows both the forward-scattering features normally observed in core-level photoelectron diffraction and, in addition, the initial-state orbital character associated with partially occupied, nonbonding ${\mathit{Mo}}^{\mathit{IV}}$(4${\mathit{d}}_{{\mathit{z}}^{2}}$+4${\mathit{d}}_{{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}}$ +4${\mathit{d}}_{\mathit{xy}}$) orbitals near the top of the valence band. The difference in forward scattering between the Mo and S emitters is also used to assess relative contributions from the Mo and S atomic orbitals at specific binding energies within the valence band. Deposition of cesium (0.23 ML Cs with 1 ML equal to the Cs saturation coverage) onto the basal plane of ${\mathrm{MoS}}_{2}$ introduces a density of states at 1.25 eV above the top of the valence-band maximum. The intensity anisotropy for this Cs-induced valence level is interpreted via the angle dependence of the electric dipole matrix element as due to the initial-state orbital character at the bottom of the conduction band of the Cs/${\mathrm{MoS}}_{2}$ heterostructure. \textcopyright{} 1996 The American Physical Society.

Spin-dependent transmission of free electrons through ultrathin cobalt layers (invited)

We present an original technique to investigate spin-dependent electron interactions in ferromagnetic metals, and place it in the context of previous studies. Our technique is based on spin-polarized electron transmission through ultrathin, free-standing, metal foils. A longitudinally spin-polarized, quasimonoenergetic, free-electron beam impinges onto a ferromagnetic target consisting of a few atomic layers of cobalt sandwiched between gold layers, for an overall thickness of the order of 25 nm. It is remanently magnetized perpendicular to the film plane. The current transmitted through the foil is energy analyzed and its dependence on the relative orientation between the spin polarization of the primary beam and the magnetization direction of the cobalt layer is measured. The experiments are performed over a wide primary energy range, starting from the vacuum level of the target; the work function of the target can be lowered down to 2 eV by cesium deposition. We demonstrate a spin-filter effect, favoring the transmission of majority electrons. It is very large at low primary energy, when the electrons travel close to the 3d bands. Perspectives for compact and highly discriminative spin detectors are discussed.

Deposition of cesium and cobalt sorbed on zeolite in matrices of blast furnace slag

137Cs and60Co were sorbed from model solutions and waste water on chemically treated granular zeolite. The zeolite was incorporated into cement slurries based on blast furnace slag (BFS). The compressive and bending strength were measured after different times of hydration. The leaching tests were performed in water, base and acid solutions. The leachability was more pronounced only for samples in acidic solutions.

Directed cesium deposition into a large volume negative-ion source

A compact, safe system for directed cesium deposition into large volume sources is described. It consists of a small oven mounted on a feedthrough, which permits one to rotate and move the oven across the discharge chamber length or to remove it from the vacuum box to reload the oven. Special industrial pellets containing the cesium compound (30% of cesium chromate +70% titanium) were used for a pure cesium release. The pellets are insensitive to pollution by air and can provide the cesium release during several cycles of heating and cooling in long-term experiments. There was no ‘‘poisoning’’ impurities degassing (oxygen, water) during the standard oven operation. The Cs system was reliably operated for cesium deposition in multiampere negative-ion source experiments.

Negative ion production in large volume source with small deposition of cesium

Experimental data on the enhancement of D− (H−) negative ion production due to cesium injection into a large volume multiampere negative ion source (MANTIS) are described. The directed deposition of small cesium amounts (5–100 mg) from a compact, movable oven, placed into the central part of a MANTIS gas-discharge box was used. A calorimetrically measured D− beam with an intensity up to 1.6 A and an extracted current density up to 4.2 mA/cm2 (beam energy 25 kV) was obtained. Exactly 30 mg of cesium provides at least one month of source operation (1000 pulses with a discharge pulse duration of 4 s). The effect of cesium on NI enhancement was immediately displayed after the distributed Cs deposition, but it needed some ‘‘conditioning’’ of cesium by tens of discharge pulses (or by several hours ‘‘pause’’) in the case of a localized Cs deposition. No degradation of extraction-acceleration voltage holding on within the tested range of cesium injection was observed.

The effect of chemical interactions on the transport of caesium in the presence of boron

The effect of chemical interactions on the transport of caesium in circuits under laminar flow conditions in the presence of boron is studied by analysing deposition profile data from two experiments. The experiments were performed at AEA Technology, Winfrith (U.K.). Predictions of two transport computer codes, RAFT and VICTORIA, are compared with experimental data. Code calculations and experimental data show that in the presence of excess boron volatile caesium hydroxide converts to less-volatile caesium borate, thereby modifying caesium transport. In experiment one, where steam transported caesium hydroxide, both codes accurately predicted thermophoretic deposition and the position of a caesium hydroxide condensation peak. Thermo-dynamic data for caesium hydroxide species are compared and values for the effective caesium hydroxide vapour pressure are suggested. In experiment two, where steam transported boric acid and caesium, both codes underestimated caesium deposition. The discrepancy is partly due to uncertainties in release rates and in wall and gas temperature measurements. Code predictions are in agreement with the experimental observation that caesium hydroxide and boric acid reacted to form caesium borate. Transport codes with fixed speciation can be “turned” to either experiment but not to both simultaneously.

Electron transmission through ultra-thin metal layers and its spin dependence for magnetic structures

We present a new set of experiments in which the attenuation of a ‘monoenergetic’, possibly spin-polarized, free-electron beam is measured by direct transmission through an ultra-thin metal layer. The self-supported metal target is either a reference gold sample or a ferromagnetic structure. The overall thickness is of the order of 25 nm. The magnetic structure consists of a 1 nm thick cobalt film sandwiched between 21-2 nm thick gold layers, with perpendicular magnetization. Measurements are performed throughout a wide energy range, with incident electron energies 2–1000 eV above the Fermi level. The transmission of the gold layer is found to be substantially higher than that of the magnetic structure. In the latter case, at low energy, close to the clean surface vacuum level, we find that the majority spin electrons are more easily transmitted than the minority spin electrons. Cesium deposition on the exit side or on both sides of the target increases the overall transmitted current by almost an order of magnitude. In the case of the magnetic structure, this also increases the transmission spin asymmetry from 16 to about 40%. Such structures appear to be well-suited to the construction of convenient and compact spin-detectors.

Unpinned behavior of the electronic properties of a p-GaAs(Cs,O) surface at room temperature

The evolution of the surface band bending, photovoltage, and recombination rate is experimentally studied in situ by means of photoreflectance and photoluminescence techniques under deposition of cesium and oxygen on the surface of epitaxial p- and n-type GaAs layers at room temperature. The evolution of the band bending is explained in terms of Fermi level pinning by initial and adatom-induced donor-like and acceptor-like surface states. For the surface of p-type GaAs multiple reversible variations of the electronic properties are observed under alternate Cs and O 2 deposition. This unpinned behavior proves that adatom-induced surface states dominate over defect-induced states. Hysteresis in the dependences of photovoltage and photoluminescence intensity on the band bending shows that variations of the concentrations or cross sections of surface capture and recombination centers take place at the first cycles of Cs and O 2 deposition.

Evolution of electronic properties at the p-GaAs(Cs,O) surface during negative electron affinity state formation

The evolution of surface band bending and surface photovoltage was monitored in situ by photoreflectance spectroscopy during activation of the surface of epitaxial GaAs to the state of negative electron affinity by successive deposition of cesium and oxygen in a standard ‘‘yo-yo’’ technique. Considerable variations of the band bending (by approximately 0.3 eV) and surface photovoltage (by three orders of magnitude) were observed. It was found that the maximum of photoemission quantum yield corresponded to unexpectedly small value of the band bending φs=0.3 eV, as compared to widely accepted value of approximately 0.5 eV.