Nuclear reaction | EPFL Graph Search

In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. If a nucleus interacts with another nucleus or particle and they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclear scattering, rather than a nuclear reaction. In principle, a reaction can involve more than two particles colliding, but because the probability of three or more nuclei to meet at the same time at the same place is much less than for two nuclei, such an event is exceptionally rare (see triple alpha process for an example very close to a three-body nuclear reaction). The term "nuclear reaction" may refer either to a change in a nuclide induced by collision with another particle or to a spontaneous change of a nuclide without collision.

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

Development of a novel microreactor-based calorimeter for the study of fast exothermal reactions in liquid phase

Marie-Agnès Schneider

Following several tragic chemical accidents that have occurred in recent years, some directives concerning chemical process safety have been issued. The Seveso directive, for example, requires a precise description of the consequences of a possible chemical accident assuming a worst-case scenario. However, to build such scenarios and, hence, make chemical processes safe, profound knowledge of the kinetic and thermal parameters of the reactions in question is necessary. These parameters are usually determined by calorimetric methods. However, in the existing commercial calorimeters the characterization of fast, exothermal reactions raises several problems. Indeed, on the one hand, it is difficult to maintain the isothermal conditions required to determine the kinetics simply and precisely. On the other hand, the calorimeter should be able to measure the heat flow generated as soon as the reactants are brought into contact. It should be able to do this without the need for a period of thermal equilibration that would perturb the measurements, and without any limitations due to the mixing of the reactants. The aim of this work is to develop a calorimetric method that is particularly adapted to studying fast exothermal reactions. The proposed system combines a microreactor with a commercially available microcalorimeter. In the first part of this project a flexible technique for constructing microreactors was chosen. The technique had to be precise, reproducible and inexpensive. The geometry of the microreactor had to fit into the cavity of the commercially available microcalorimeter. The technique chosen was the silk-screen printing of a thick film dielectric. This method makes it possible to build quickly and at low cost numerous microreactors that can be regarded as disposables. The geometry of the reaction channel can be varied and can be adapted to the type of reaction. Once the microreactors were built, the degree of mixing obtained in the microchannels was estimated first by simulation and then by experiment. The flow in the reaction channel was found to be purely laminar and the mixing time corresponded to the time for radial diffusion. Due to the small size of the channels, the mixing time was found to be adequate and not limiting for the characterization of fast reactions. The microreactor was then inserted into the cavity of the commercial calorimeter. The resulting microsystem was calibrated using the neutralization reaction of sulphuric acid with NaOH. This system was further modified to optimize the thermal transfer between the reaction channel and the sensor of the microcalorimeter and to increase its thermal efficiency. Finally, an electrical preheating system of the incoming liquids was put in place and tested. Once the quality of the thermal signal had been optimized, kinetic studies of chemical reactions could be undertaken. First, a model reaction was studied in order to validate the results obtained with the microsystem and to avoid the risk of systematic errors. In the second stage, a previously unknown fast exothermal reaction was characterized. The heat flows measured during the reaction reached 160'000 W·kg-1 but the conditions, however, remained completely isothermal. The global kinetics of this reaction as well as its activation energy were determined.

EPFL2004

Stability and physical properties of alkylsilane and alkylphosphonate self-assembled monolayer (SAM) films on metal oxide substrates characterized at the micro- and nanoscale

Enamul Hoque

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.

EPFL2007