Browsing by discipline "Fysiikka"

Ion interaction with matter plays an important role in the modern silicon based micro- and nanoindustry. Ions accelerated to significant energies are able to penetrate into materials allowing for controlled tailoring of the materials' properties. However, it is extremely important to understand the nature of these interactions, and computer modelling is by far the most suitable technique for this purpose. The models used in ion irradiation software are either based on the binary collision approximation (BCA) or molecular dynamics (MD). The first mentioned is both the oldest and the most widely used one. There are three reasons for this: the simple idea, the fast calculation speeds, and the user-friendly graphical user interfaces distributed with the codes. However, there are still some pitfalls in accuracy compared to MD. MDRANGE, an ion range MD code, developed at the Accelerator Laboratory of the University of Helsinki, combines the accuracy of MD with the speed of the BCA. If the tool is given a graphical user interface, it would become more appealing to scientists not familiar with programming. Different methods and techniques for calculating the penetration depths and ranges of kinetic ions in solids are presented in this work. They are accompanied by an overview of the mathematics allowing them to be as physically accurate as possible, over reasonable computation times. For both BCA and MD, generally, the computationally most demanding part is the calculation of the interactions between two or more particles. These interactions are handled through evaluation of potential functions developed especially for different combinations of atoms. The graphical user interface developed in this work is meant as a robust setup tool for use with MDRANGE. The separation of parameters into different panels and the main functionality of the different parts are presented in detail. It is possible to generate the three mandatory input files (coords.in, elstop.in, and param.in) with the tool. Out of these three files, param.in is the file in main focus when the application is used. In addition to the generation of the three files, there are also functions included for investigating range calculation results in real time during simulations. During the last five decades, there has been a huge development of the simulation models intended for ion irradiation processes. Even though BCA models excel in speed, they are not able to compete with MD in simulating many-body interactions for atoms with kinetic energies lower than 1 keV. MDRANGE was developed as a bridge between the two models to allow for faster MD calculations, comparable to BCA calculations, while still taking into account the many-body interactions for ions with lower speeds. With the graphical user interface, developed in this work, it will become even more appealing to scientists not familiar with programming, but still in need of an ion range calculation software.

FINIX is a nuclear fission reactor fuel behaviour module developed at VTT Technical Research Centre of Finland since 2012. It has been simplified in comparison to the full-fledged fuel performance codes to improve its usability in coupled applications, by reducing the amount of required input information. While it has been designed to be coupled on a source-code level with other reactor core physics solvers, it can provide accurate results as a stand-alone solver as well. The corrosion that occurs on the interface between nuclear fuel rod cladding and reactor coolant is a limiting factor for the lifespan of a fuel rod. Of several corrosion phenomena, oxidation of the cladding has been studied widely. It is modelled in other fuel performance codes using semiempirical models based on several decades of experimental data. This work aims to implement cladding oxidation models in FINIX and validate them against reference data from experiments and the state-of-the-art fuel performance code FRAPCON-4.0. In addition to this, the models of cladding-coolant heat transfer and coolant conditions are updated alongside to improve the accuracy of the oxidation predictions in stand-alone simulations. The theory of the cladding oxidation, water coolant models and general structure of FINIX and reactor analysis will be studied and discussed. The results of the initially implemented cladding oxidation models contain large errors, which indicates that FINIX does not account for the axial temperature difference between the bottom and the top of the rod in the coolant. This was corrected with the updates to the coolant models, which calculate various properties of a water coolant based on International Association for the Properties of Water and Steam (IAWPS) industrial water correlations to solve the axial temperature increase in a bulk coolant. After these updates the predictions of cladding oxidation improved and the validity of the different oxidation models were further analyzed in the context of FINIX.

Electrical breakdowns occasionally occur near the first walls of fusion reactor chambers and the accelerating cavities of linear colliders, such as CLIC. These arcing events are localised plasma discharges, which are formed under high voltage electrical fields. Vacuum arcs cause various surface damage on the fusion reactor and linear accelerator structures. The surface damage, most significantly craters, have been studied experimentally, but the mechanism of the formation of this damage is still not clear. In this thesis we use the large-scale molecular dynamics simulations to study crater formation on Cu surface. We used ion irradiation to model the arcing events, where plasma ions are accelerated via the shield potential towards the metal surface. This ion irradiation causes multiple overlapping cascades in Cu surface, what can lead to the crater formation. The main goal was to be able to produce surface damage, which is identical to experimental results. Our results are divided to three categories. First we examined which are initial conditions needed to form experimental like craters. The electric field emission current accompanying the plasma discharge process, most likely, is to heat the sample locally to very high temperatures. Therefore we tested molten and solid structures at different temperatures, as well as different scenarios of cooling of the sample via electronic heat conduction. Second, we examined how different variables, such as the fluence of the ions, the energy flux or the potential model, affect on the crater shape. These results were compared with the experimental crater profiles in order to find out reasonable values. We also analysed how the volume of the produced crater depends on fluence. Our third part of investigation was not actually concentrated on the surface damage, but on dislocations and other damage under the surface. We again studied how different parameters affect on the results. We compared the simulations by calculating the number and ratio of non-FCC atoms in the bulk. The fluence dependency of the defects was studied as well.

X-ray computed tomography (CT) is widely used in medical imaging and materials science. In this imaging modality, cross-sectional images of a physical object are formed by taking numerous X-ray projections from different angles and then applying a reconstruction algorithm to the measured data. The cross-sectional slices can be used to form a three-dimensional model of the interior structure of the object. CT is a prime example of an inverse problem, in which the aim is to recover an unknown cause from a known effect. CT technology continues to develop, motivated by the desire for increased image quality and spatial resolution in reconstructions. In medical CT, reducing patient dose is a major goal. The branch of CT known as X-ray microtomography (micro-CT) produces reconstructions with spatial resolutions in the micrometer range. Micro-CT has been practiced at the University of Helsinki since 2008. The research projects are often interdisciplinary, combining physics with fields such as biosciences, paleontology, geology, geophysics, metallurgy and food technology. This thesis documents the design and construction of a new X-ray imaging system for computed tomography. The system is a cone beam micro-CT scanner intended for teaching and research in inverse problems and X-ray physics. The scanner consists of a molybdenum target X-ray tube, a sample manipulator, and a flat panel detector, and it is built inside a radiation shielding cabinet. Measurements were made for calibrating the measurement geometry and for testing reconstruction quality. Two-dimensional reconstructions of various samples were computed using the plane which passes through the X-ray point source and is perpendicular to the axis of rotation. This central plane of the cone beam reduces to fan beam geometry. All reconstructions were computed using the filtered backprojection (FBP) algorithm, which is the industry standard. Tomographic reconstructions of high quality were obtained from the measurements. The results show that the imaging system is well suited for CT and the study of reconstruction algorithms.

Interatomic potentials are used to describe the motion of the individual atoms in atomistic simulations. An accurate treatment of the interatomic forces in a system of atoms requires heavy quantum mechanical calculations, which are not computationally feasible in large-scale simulations. Interatomic potentials are computationally more efficient analytical functions used for calculating the potential energy of a system of atoms, allowing simulations of larger systems or longer time scales than in quantum mechanical simulations. The interatomic potential functions must be fitted to known properties of the material the potential describes. Developing a potential for a specific material typically involves fitting a number of parameters included in the functional form, against a database of important material properties, such as cohesive, structural, and elastic properties of the relevant crystal structures. In the Tersoff-Albe formalism, the fitting is performed with a coordination-based approach, where structures in a wide range of coordination numbers are used in the fitting process. Including many differently coordinated structures in the fitting database is important to get good transferability to structures not considered in the fitting process. In this thesis, we review different types of widely used interatomic potentials, and develop an iron-oxygen potential in the Tersoff-Albe formalism. We discuss the strengths and weaknesses of the developed potential, as well the challenges faced in the fitting process. The potential was showed to successfully predict the energetics of various oxygen-vacancy defect clusters in iron, and the basic properties of the common iron oxide wüstite. The potential might therefore mainly be applicable to atomistic simulations involving oxygen-based defects in solid iron, such as irradiation or diffusion simulations.

Diffuusiokuvantaminen perustuu magneettikuvauslaitteen avulla mitattuun vesimolekyylien satunnaiseen lämpöliikkeeseen. Pehmytkudoksessa vesimolekyyli diffuntoituu noin 17 mikrometrin matkan 50 millisekunnin aikana ja diffuusiokuvantaminen on ainoa kliininen kuvantamismenetelmä, joka pystyy rekisteröimään näin pientä liikettä ei-invasiivisesti. Tutkimalla, missä suunnissa diffuntoituminen on voimakasta, voidaan paikantaa esimerkiksi valkeasta aivoaineesta hermoratojen reittejä. Tämä edellyttää käytännössä vähintään 20 diffuusiosuunnan kuvaamista, joiden pohjalta lasketaan diffuusion suuntaa ja suuruutta kuvaava diffuusiotensori kuva-alkiokohtaisesti. Menetelmä edellyttää nopeaa kuvausaikaa, jotta fysiologiset virtaukset tai potilaan liike eivät häiritse tasoltaan huomattavasti heikomman lämpöliikkeen rekisteröintiä. Nopea kuvaus puolestaan asettaa laiteteknisiä vaatimuksia gradienttikentille, joita ei anatomisessa T1- tai T2-painotetussa kuvantamisessa esiinny. Gradienttikelojen on pystyttävä toimimaan äärirajoillansa koko kuvauksen ajan, jotta useat peräkkäiset rekisteröinnit eri diffuusiosuunnissa ovat mahdollisia. Optimoinnissa käyttäjä ei voi vaikuttaa laiteteknisiin ratkaisuihin, mutta kuvausparametrien variointi on mahdollista. Edellytyksenä mielekkäälle optimoinnille on kuitenkin valita vertailtavat suureet, joiden perusteella voidaan sanoa, mitkä testatuista vaihtoehdoista paransivat kuvanlaatua. Diffuusiotensorikuvantamiseen (DTI) on ehdotettu laadunvalvontaprotokollaa, joka huomioi kuvausmenetelmän laitetekniset haasteet. Kyseinen julkaisu on ainoa, joka ottaa kantaa useimpiin DTI:n ongelmakohtiin ja on siten luonteva lähtökohta DTI-optimoinnille. Julkaisun menetelmässä tutkittiin DTI-sekvenssin tuottamaa signaalikohinasuhdetta, kuvaussekvenssistä ja indusoituvista pyörrevirroista johtuvia erilaisia geometrisia vääristymiä sekä diffuusiotensorista johdettuja FA- ja MD-arvoja. Työn ensimmäisessä vaiheessa valittiin kliiniseen DTI-sekvenssiin pohjautuva referenssisekvenssi, jota varioitiin yksi kuvausparametri kerrallaan. Muunnellut parametrit olivat kaikuaika, rinnakkaiskuvantamiskerroin, k-avaruuden keräyslaajuus, päämagneettikentän tasoitusalue sekä diffuusiopainotuskerroin eli b-arvo. Varioituja sekvenssejä oli yhteensä 10, joiden pohjalta valittiin kuvanlaatuun myönteisesti vaikuttaneet parametrit työn toiseen vaiheeseen, missä referenssisekvenssiä varioitiin usean parametrin suhteen. Lopputuloksena todettiin, että lyhin mahdollinen kaikuaika 55 ms ja suurin mahdollinen k-avaruuden kartoitusalueparametrin arvo 0,780 kasvattivat signaalikohinasuhdetta 13 %. Rinnakkaiskuvantamiskertoimen kasvattaminen kahdesta kahteen ja puoleen pienensi geometrisia vääristymiä kvalitatiivisessa arviossa, mutta heikensi signaalikohinasuhdetta referenssisekvenssiin verrattuna suurimmillaan vain 5 %. Päämagneettikentän tasoitusalueen valinnalla tai b-arvon pienentämisellä tuhannesta kahdeksaansataan ei havaittu olevan merkittävää vaikutusta kuvanlaadulle fantomitutkimusessa. Tulokset eivät poikenneet teoreettisista ennusteista, mutta toisaalta laiteteknisistä rajoituksista johtuen optimointi ei voi perustua pelkästään teoreettiseen arvioon oikeista parametrien arvoista. Työssä esitettyä menetelmää on mahdollista jatkossa käyttää myös muiden diffuusiopainotettujen sekvenssien optimoinnissa.

Differentiaaliliikkuvuusspektrometri (Differential Mobility Particle Sizer; DMPS) -laitteistoa voidaan käyttää ilmakehän aerosolihiukkasten lukumääräkokojakauman mittaamiseen. DMPS-laitteisto koostuu impaktorista, kuivaajasta, bipolaarisesta diffuusiovaraajasta, differentiaaliliikkuvuusanalysaattorista (Differential Mobility Analyzer; DMA) ja kondensaatio- hiukkaslaskurista (Condensation Particle Counter; CPC). Tässä työssä verrataan DMPS-laitteistossa rinnakkain mittaavan muokatun A20 CPC:n ja TSI 3776 CPC:n laskentastatistiikkaa. Pienimmillä aerosolihiukkasilla on vaikutus ympäristöön ja terveyteen, minkä takia on kasvava tarve mitata tarkasti myös pienimpien hiukkasten kokojakaumaa. Aerosolihiukkasten lukumääräkokojakaumaan ja siitä johdettavien suureiden epävarmuuksia ei kuitenkaan tunneta vielä täysin. Työssä pyritään parantamaan perinteisen CPC:n laskentastatistiikkaa ja tutkimaan lukumääräkokojakauman sekä siitä johdettavien suureiden, kuten muodostumisnopeuden (Formation Rate; J) ja kasvunopeuden (Growth Rate; GR), epävarmuuksia. Perinteinen, ilman suojavirtausta toimiva, CPC voidaan muokata havaitsemaan jopa alle 3 nm hiukkasia kasvattamalla lämpötilaeroa saturaattorin ja kondenserin välillä ja muuttamalla aerosolivirtausta. Tässä työssä A20 CPC:n aerosolivirtaus optiikan läpi kasvatettiin 2.5 litraan minuutissa diffuusiosta johtuvien häviöiden minimoimiseksi ja laskentastatistiikan parantamiseksi. Verrattuna TSI 3776 CPC:hen muokatulla A20 CPC:llä on 50 kertaa suurempi aerosolivirtaus, joten voimme olettaa, että muokattu A20 mittaa TSI 3776 UCPC:hen verrattuna enemmän hiukkasia pienemmällä epävarmuudella. Muokatulla A20 CPC:llä on parempi laskentastatistiikka, jonka ansiosta kokojakauman laskennasta johtuva suhteellinen virhe on pienempi. Muokatulla A20 CPC:llä on TSI 3776 CPC:hen verrattuna 50 kertaa suurempi aerosolivirtaus ja se laskee keskimäärin 50 kertaa enemmän hiukkasia koko DMPS-laitteiston mittaamalla kokoalueella (1-40 nm). Muokatulla A20 CPC:llä laskettu GR on noin 60% suurempi pienimmillä (3-6 nm) hiukkasilla ja noin 3% suurempi 6-11 nm hiukkasilla. Myös J on noin 30% suurempi muokatulla A20 CPC:llä laskettuna 3-6 nm hiukkasille. CPC:n laskennasta johtuva epävarmuus on syytä huomioitava määritettäessä DMPS-mittauksen kokonaisvirhettä. Laskentastatistiikalla on merkitystä paitsi lukumääräkokojakaumaan, myös sen johdannaissuureisiin.

Gammaspektrometria on menetelmä gammasäteilyn havaitsemiseen ja sen analysoimiseen. Menetelmää sovelletaan Säteilyturvakeskuksen (STUK) gammalaboratoriossa, jossa mitataan ympäristönäytteiden aktiivisuutta ja radionuklidikoostumusta. Mittaustulosten analysoinnissa tarvitaan tehokkuuskalibrointia, jossa määritetään mitattavan lähteen emittoimien fotonien ja rekisteröityjen pulssien välinen suhde. Työssä selvitettiin EFFTRAN -nimisen ohjelman soveltuvuutta osaksi gammaspektrometristen mittausten analyysiprosessia. EFFTRAN perustuu Efficiency Transfer –menetelmään (ET), jossa analysoitavan näytteen piikkitehokkuuskäyrä johdetaan kokeellisesti määritetystä standardilähteen piikkitehokkuuskäyrästä. EFFTRAN ottaa kalibrointisiirroissa huomioon näytteen alkuainekoostumuksen, mikä vaikuttaa fotonien piikkitehokkuuteen alle 100 keVin energioilla. Työn ensimmäinen tavoite oli osoittaa, että EFFTRANin avulla saadaan yhteneviä tuloksia nykyisten laskentamenetelmien kanssa. Sen jälkeen kalibrointisiirtojen avulla laskettuja aktiivisuuspitoisuuksia verrattiin tunnettuihin referenssilähteiden aktiivisuuspitoisuuksiin. Viimeisessä vaiheessa tarkasteltiin alkuainekoostumuksen vaikutusta gamma-analyysien tuloksiin. EFFTRANin kalibrointisiirtojen avulla saatiin pääosin oikeita tuloksia, joten niitä voitaisiin jatkossa käyttää gamma-analyyseissa kokeellisten tehokkuuskalibrointien rinnalla.

Cellulose can be used as a renewable raw material for energy production. The utilization requires degradation of cellulose into glucose, which can be done with the aid of enzymatic hydrolysis. In this thesis, various x-ray methods were used to characterize sub-micrometer changes in microcrystalline cellulose during enzymatic hydrolysis to clarify the process and factors slowering it. The methods included wide-angle x-ray scattering (WAXS), small-angle x-ray scattering (SAXS) and x-ray microtomography. In addition, the samples were studied with transmission electron microscopy (TEM). The studied samples were hydrolyzed by enzymes of the Trichoderma reesei species for 6, 24, and 75 hours, which corresponded to 31 %, 58 %, and 68 % degrees of hydrolysis, respectively. Freeze-dried hydrolysis residues were measured with WAXS, SAXS and microtomography, whereas some of them were re-wetted for the wet SAXS and TEM measurements. The microtomography measurements showed a clear decrease in particle size in scale of tens of micrometers. In all the TEM pictures similar cylindrical and partly ramified structures were observed, independent of the hydrolysis time. The SAXS results were ambiguous and partly imprecise, but showed a change in the structure of wet samples in scale of 10-30 nm. According to the WAXS results, the degrees of crystallinity and the crystal sizes remained the same. The gained results support the assumption, that the cellulosic particles are hydrolyzed mostly on their surface, since the enzymes are unable to penetrate into the nanopores of wet cellulose. The hydrolysis therefore proceeds quickly in easily accessible particles and leaves the unaccesible particles almost untouched. The structural changes observed in the SAXS measurements might correspond to slight loosening of the microfibril aggregates, which was seen only in the wet samples because of their different pore structure.