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An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines. Electron ionization Electron ionization Electron ionization is widely used in mass spectrometry, particularly for organic molecules. The gas phase reaction producing electron ionization is :M{} + e^- -> M^{+\bullet}{} + 2e^- where M is the atom or molecule being ionized, e^- is the electron, and M^{+\bullet} is the resulting ion. The electrons may be created by an arc discharge between a cathode and an anode. An electron beam ion source (EBIS) is used in atomic physics to produce highly charged ions by bombarding atoms with a powerful electron beam. Its principle of operation is shared by the electron beam ion trap. Electron capture ionization Electron capture ionization (ECI) is the ionization of a gas phase atom or molecule by attachment of an elect

A laboratory study of heterogeneous reactions relevant to the atmospheric boundary layer

Dominik Stadler

The present work deals with two subjects. The interaction of NO2 and HONO with different types of soot are examined in the first part whereas in the second part an experimental set-up is presented which has been built in order to measure the kinetics of the degradation of organic compounds by OH radicals. Both soot particles as well as NO2 are mainly produced by fossil fuel and biomass burning. The two species are therefore ubiquitous in the atmospheric boundary layer where they may react with each other. It has been shown that one possible product of this heterogeneous interaction is nitrous acid (HONO). HONO is an important trace gas in atmospheric chemistry because it is easily photolysed resulting in OH radical and NO. Thus, the photolysis of HONO significantly enhances photooxidation processes early in the morning. In the present study two different types of decane soot have been produced. It has been shown that the air/fuel ratio of the diffusion flame is a key parameter influencing the reactivity of soot towards NO2. Whereas soot from a rich flame ("grey" decane soot) leads to HONO with yields up to 100% upon interaction with NO2, only small amounts of HONO, but significant amounts of NO are formed in the presence of soot originating from a lean flame ("black" decane soot). A reaction mechanism has been developed showing that NO2 is initially converted to HONO by a redox reaction. HONO produced in this way is subsequently either quantitatively desorbed into the gas phase on "grey" decane soot or it decomposes to a large extent in a disproportionation reaction on "black" decane soot. The products of disproportionation are NO, which is instantaneously desorbed into the gas phase and NO2, which undergoes secondary reactions. In addition to the HONO producing pathway there is a reaction channel that leads to adsorption of NO2 for both types of soot which is irreversible on the time scale of our standard experiments. Saturation effects that are occurring on the time scale of our experiments are primarily caused by saturation of adsorption sites and not by the depletion of the reducing agent. It seems that the fraction of NO2 that is irreversibly adsorbed is blocking part of the surface sites where NO2 is initially adsorbed. Uptake coefficients γ take initial values of up to 0.1 for both types of soot. However, with increasing uptake of NO2 γ is decreasing. After a NO2 consumption of approximately 8×1013 molecule cm-2, which corresponds to 13% of a formal monolayer, γ drops to values of 3×10-7 for "grey" decane soot and 6×10-7 for "black" decane soot, respectively. Furthermore, our results show that the rate of initial uptake is the rate limiting step of the NO2/soot interaction mechanism. Experiments where "black" decane soot has been exposed to a flow of HONO revealed that it readily decomposes into NO and NO2. For HONO concentrations above 3.3×1011 molecule cm-3 a total product yield of 50%, whereof 40% NO and 10% NO2, has been observed. The initial uptake coefficient ( γ0) for HONO on "black" decane soot did not show clear saturation effects but varied randomly in the concentration range from 1.2×1012 to 5.9×1012 molecule cm-3. The average value measured at these concentrations was equal to 0.027. The strong interaction of HONO with "black" decane soot shows that soot is not only a potential source, but also a potential sink of HONO. On the other hand, no significant uptake could be observed when "grey" decane soot was exposed to HONO. This is consistent with the observation that NO2 is converted to HONO at yields of up to 100%. Acetylene soot is an additional type of soot that has been examined. The results for the initial uptake coefficients were basically the same as for "grey" and "black" decane soot, that is maximum values of γ0 of 0.1 for NO2 uptake decreasing with increasing concentrations of NO2. This similarity holds only for the first few seconds of interaction because acetylene soot saturates much faster than "grey" and "black" decane soot with continuous exposure. The acetylene samples are generally almost completely saturated after only three minutes of exposure to NO2. Soxhlet extractions of "grey" decane soot which have been performed using tetrahydrofuran as a solvent also produced HONO upon interaction with NO2. This shows that the active reactants are not part of the graphitic soot backbone but are extractable, rather polar compounds of the organic fraction of soot. This assumption is supported by the observation that the corresponding benzene extraction is not reactive in relation to NO2 uptake and HONO formation. In the second part of the present work an experimental set-up is described which has been built in order to measure the rate constants of the reaction of organic compounds with the OH radical in the gas phase using a relative rate technique at atmospheric pressure. The system may be called a "mini smog chamber" as the reaction cell has a volume of only 480 cm3. The system was validated by measuring the relative rate constants of the reactant pair cyclohexane/toluene. These measurements have been performed for temperatures ranging from 300 to 360 K and led to results which are in excellent agreement with literature data. However, the experimental set-up also had some important limitations. Biphenyl which has a boiling point of 529 K was the compound with the lowest volatility that could be examined in our reaction cell. Compounds with lower vapor pressures or compounds with polar groups adsorbed to a large extent onto the glass walls of the reaction cell even at temperatures of up to 373 K. Additional problems occurred in the closed ion source of the mass spectrometer leading to non linear and/or unstable MS signals. These phenomena were most probably caused by secondary ion-molecule reactions in the closed ion source.

EPFL2000

First operations with caesium of the negative ion source SPIDER

Riccardo Agnello, Michele Fadone

The negative-ion based neutral beam injector for heating and current drive of the ITER plasma (ITER HNB) is under development, at present focusing on the optimization of the full-scale plasma source in the SPIDER test stand. The production of H- or D- ions in the ion source is based on the low work function surfaces obtained by caesium evaporation. This paper describes the caesium conditioning procedure and the corresponding beam performances during the first operation of SPIDER with caesium. Technical solutions to overcome present limitations of the test stand are described. The influence of source parameters on the caesium effectiveness was investigated in short beam pulse operation; with total radio-frequency (RF) power of 400 kW and filling pressure below 0.4 Pa, and a limited number of extraction apertures, a negative ion current density of about 200 A m(-2) was extracted in hydrogen, with beam energy lower than 60 keV. Beam optics and beam uniformity were assessed thanks to the acceleration of isolated ion beamlets. A possible procedure to accelerate a uniform beam was demonstrated at low RF power. The results obtained in this first investigation provided key indications on the operation of one of the largest existing sources of accelerated negative hydrogen-like ions.

2022

Submicro- and Nano-Structured Porous Materials for Production of High-Intensity Exotic Radioactive Ion Beams

Sandrina Fernandes

ISOLDE, the CERN Isotope Separator On-line DEvice is a unique source of low energy beams of radioactive isotopes - atomic nuclei that have too many or too few neutrons to be stable. The facility is like a small 'chemical factory', giving the possibility of changing one element to another, by selecting the atomic mass of the required isotope beam in the mass separator, rather as the 'alchemists' once imagined. It produces a total of more than 1000 different isotopes from helium to radium, with half-lives down to milliseconds, by impinging a 1.4 GeV proton beam from the Proton Synchrotron Booster (PSB) onto special targets, yielding a wide variety of atomic fragments. Different components then extract the nuclei and separate them according to mass. The post-accelerator REX (Radioactive beam EXperiment) at ISOLDE accelerates the radioactive beams up to 3 MeV/u for many experiments. A wide international user radioactive ion beam (RIB) community investigates fundamental aspects of nuclear physics, particle and atomic physics, solid-state physics, materials science, astrophysics, biophysics and medicine. However, there are still a number of radioactive isotopes which are not accessible for experimental physics, either because it has been impossible to produce the element of interest, too low yields (number of isotopes per second measured in the beam line) are obtained or due to the higher post-acceleration energies required for the user's experiments. The aim of this thesis was to synthesize oxide and carbide porous materials, to be used as thick targets to increase the intensity of exotic RIBs. This was done by the evaluation of their radioactive isotope release properties and of their stability under irradiation conditions at high temperatures. This thesis is focused on the study of the chemical and physical processes occurring in the target system where the isotopes are produced, before these enter the ion-source system, are mass-separated and are sent to the user beam lines. The products formed in nuclear reactions between a proton beam and the target must, at a first step, diffuse from the interior of the target out to the surface and evaporate from it. Minimum delay times for the diffusion process can be realized by achieving the shortest diffusion lengths of highly-permeable, low-density, open-structure targets and by operating at the highest possible temperature so that release times are minimized and commensurate with the half-life of the isotopes of interest. Whereas until now target materials with micro-scale structures have been synthesized and used for RIB production, new porous submicro- and nano-structured materials offer the advantage of having lower diffusion activation energy of atoms and thus larger diffusion coefficient than the corresponding bulk counterpart, thanks to the increase of surface to volume ratio of these materials. The potential of this phenomenon for isotope mass separation online (ISOL) targets and many other applications is an evident drop of temperature to maintain a fast diffusion process, and higher release efficiency for the production of intense exotic isotope beams. In Chapter 1 the ISOL technique and the RIB production issues are introduced, followed by a review of recent progresses on target materials to increase yield production in the principal existing and next-generation ISOL and In-Flight facilities worldwide. Chapter 2 gives the theoretical fundamentals of diffusion and effusion in solid materials, and addresses specifically diffusion, effusion and release properties of ISOL targets. The study presented in Chapter 3 consists in the evaluation of release properties in different polycrystalline silicon carbide samples. The diffusion coefficients for Be, Na, Mg and F isotopes were extracted from off-line release measurements done at ISOLDE. The full characterization of the release time structure inherent to the physical and chemical processes of the ISOL RIB production on a submicrometric SiC target material (target SiC #334) used at ISOLDE, was made. This last target development resulted in improvement of magnesium short-lived isotopes yields. This was the first demonstration of the successful use of a submicrometric porous material as an ISOL target material, which delivered unprecedented 8x104 21Mg ions per µC. In Chapter 4, the synthesis of porous submicro- and nano-structured alumina to be used as an ISOL target, was followed by materials characterization in terms of microstructure and related with selected isotope release properties. Whenever possible, the release mechanisms were explained or proposed. The approach is relevant for the study of microstructure effects in other solid porous oxide and carbide targets, for optimization of their release performance. Chapter 5 shows the results of the TARPIPE experiment, set up within the design study task of the foreseen ISOL-type European facility called EURISOL, to investigate proton irradiation damage on typical and novel ISOL target materials under continuous proton irradiation at high temperature (> 1273 K). Materials such as SiC, Al2O3, Y2O3, C and Ta in a variety of forms were irradiated at the 72 MeV proton irradiation station of injector 1 at PSI (Paul Scherrer Institute). The effects of radiation damage on these materials were evaluated from microstructure and chemical analyses before and after irradiation by visual inspection, gamma spectroscopy, scanning electronic microscopy and electron dispersive X-ray analysis on a case-by-case basis. The moderate structure evolution of the porous oxide and carbide samples tested confirms that these materials are good candidates for high-power solid targets to be used at the existing and next generation high-power ISOL facilities, such as TRIUMF (TRI-University Meson Facility) and EURISOL (EURopean Isotope Separator On-line). In Chapter 6, the development and testing of a composite high-power alumina target for neon isotope production associated to a cold FEBIAD ion-source exposed to unprecedented proton beam intensities is reported. A 0.5 GeV proton beam from TRIUMF was used up to 25 µA, a factor 10 with respect to the present oxide targets in operation. The yields obtained were compared to the ones delivered by using a 70 µA proton beam in SiC targets at this facility. The target performance in terms of diffusion properties was evaluated. A general conclusion and discussion are given in Chapter 7. The release studies obtained with irradiation and ion implantation techniques on prospective SiC and Al2O3 matrices resulted in the identification of alternative target matrices for fast radioisotope release; this was confirmed experimentally from Na and Mg elements release efficiency determined online for a SiC target. A high-power oxide/metal composite target made of micrometric structural length scale Al2O3 pellets and Nb foils was operated online at unprecedented beam power intensities. Composite targets made of Nb foils and submicro- and nano-structured porous Al2O3 are expected to further increase yields of exotic isotopes. This ISOL target concept can be applied in the future for many other refractory insulating ceramics by using gradient microstructures and/or different ceramic/metallic combinations. Special care has to be taken, in order to minimize the stress induced by a mismatch of the coefficient of thermal expansion in the different ceramic layers and/or the underlying metallic substrate. Micro- and nano-technology fabrication techniques can be used to tailor pore and grain size distribution in ceramic and metallic complex-shaped composite materials and to shape these in compacts, monoliths and fenestrated structures. A balance between large surface area and large macropore volume for optimum target radioisotope release performance must be achieved. The resulting target material must in addition be stable at high temperatures, have good thermo-mechanical resistance and high thermal conductivity under irradiation for long irradiation time. The understanding of isotope release mechanisms in different solid materials, including the porous and dense fast-ion conductors will contribute to the production of exotic RIB at unprecedented high-intensity.

EPFL2010