Browsing by discipline "Fysik"

Fuusioreaktiossa kaksi kevyttä ydintä yhtyy yhdeksi raskaammaksi ytimeksi ja samalla vapautuu energiaa. Fuusioreaktio tarvitsee tapahtuakseen hyvin korkean lämpötilan, minkä seurauksena aine on olomuodoltaan plasmaa. Esimerkiksi fuusioreaktoreissa käytettäväksi suunniteltu vedyn isotooppien deuteriumin ja tritiumin välinen reaktio vaatii tapahtuakseen plasman kuumentamista yli 100 miljoonan kelvinin lämpötiloihin. Tutkituin fuusioreaktorimalli on tokamak, jossa kuumaa plasmaa hallitaan toruksen muotoisessa kammiossa voimakkaiden magneettikenttien avulla. Plasmaa koossapitävästä magneettikentästä huolimatta plasmasta karkaa hiukkasia, jotka lopulta osuvat kammion pinnoille. Yksi tapa kammion pintoihin kohdistuvan lämpö- ja hiukkasvuon pienentämiseksi on suihkuttaa kammioon epäpuhtausatomeja tai -molekyylejä jäähdyttämään reunaplasmaa. Typpi on osoittautunut kiinnostavaksi vaihtoehdoksi tähän tehtävään. Typen kulkeutuminen ja kertyminen reaktorikammion sisällä vaatii kuitenkin vielä lisätutkimuksia. Typen harvinainen isotooppi 15N tarjoaa mahdollisuuden tutkia näitä kysymyksiä. Tyypillisesti tämä tehdään merkkiainekokeiden avulla, jolloin reaktorikammioon suihkutetaan valittua merkkiainetta tunnetuissa olosuhteissa ja kokeen jälkeen selvitetään merkkiaineen jakauma reaktorikammion pinnoilla. Tässä työssä keskityttiin seinätiiliin, jotka on irrotettu ASDEX Upgrade -fuusioreaktorista (AUG) vuosien 2010-2011 koekampanjan jälkeen. Kyseisen koekampanjan lopussa suoritettiin 15N-merkkiainekoe. Työssä tutkittiin tiilistä porattujen näytteiden 15N-pitoisuuksia lentoaika-rekyylianalyysilla (Time Of Flight Elastic Recoil Detection Analysis, TOF-ERDA), ydinreaktioanalyysilla (Nuclear Reaction Analysis, NRA) ja sekundääri-ionimassaspektrometrialla (Secondary Ion Mass Spectrometry, SIMS). Vertailun vuoksi tutkittiin myös 15N:llä implantoituja testinäytteitä. Tutkielman alkuosassa esitellään lyhyesti tokamak-fuusioreaktorin toimintaa, plasman vuorovaikutusta reaktorin seinämän kanssa, typen käyttöä fuusioreaktoreissa, merkkiainekokeita sekä käytetyt mittausmenetelmät. Tutkielma loppuosa keskittyy suoritettuihin mittauksiin, niiden analyysiin ja tuloksiin sekä johtopäätöksiin. Tulosten perusteella mittausmenetelmien välillä on merkittäviä eroja AUG-näytteiden kohdalla, kun taas implantoiduille näytteille erot menetelmien välillä ovat pienet. Erot johtuvat todennäköisesti AUG-näytteiden epätasaisesta pintarakenteesta, minkä seurauksena typen jakauma näytteiden pintakerroksissa vaihtelee. TOF-ERDA:lla tutkittiin näytteistä mahdollisimman sileää pintaa luotettavan analyysin onnistumiseksi. NRA-mittauksissa protonisuihku kohdistui näytteen keskelle suuremmalle pinta-alalle. Suureen alueeseen sisältyy myös karkeampia kohtia, joihin merkkiaineen kertyminen on sileää pintaa suurempaa. Tämän seurauksena NRA:lla saadaan selvästi suurempia tuloksia 15N:n pintatiheydelle kuin TOF-ERDA:lla. Kvadrupolimassaspektrometrissa ilmenneiden ongelmien vuoksi SIMS-mittauksia suoritettiin vain yksi, minkä vuoksi optimaalisten asetusten löytäminen 15N:n mittaamiseen SIMS:llä vaatii vielä lisätutkimuksia.

The study of air ions by applying air balance concept based on the Hyytiälä SMEAR II station measurement was performed in this work. Diurnal and seasonal variations in ion concentration and environmental ionizing radiation were studied by analysing data collected from long-term measurements. Total gamma radiation was the main source for ion production in the atmosphere, which can be attenuated by snow cover during winter periods. α and β emissions from radon decay process showed a share of about 20% in the production of total ion pairs, which were sensitive to variations in soil conditions. In general, more positive ions than the negative ones exist at ground level due to the earth electrode effect. Similar patterns were found in cluster ion concentration and the ion source rate derived from the total gamma radiation. On days with new particle formation (NPF), a relation was observed between cluster ion concentration, wind speed, temperature (T) as well as relative humidity (RH). A similar connection was also identified in ion source rate and ion production rate to T and RH. A high ion source rate derived from gamma dose rate was observed on non-event days and low on NPF days. The reversed case was found in the source rate derived from radon decay emissions. The ion production rate was typically higher on NPF event days than on non-event days. Two approaches were carried out in the determination of the ion production rate in the cluster size range by using an improved balance equation of air ions. The similar values obtained using these two approaches imply a balanced condition between ionizing source and the observed ion concentration. This suggests that measurement of air ions by the Balanced Scanning Mobility Analyser (BSMA) is likely to be reliable, though accurate parameterization for sub-0.8 nm ions is not available to the present knowledge. Moreover, the ion production rate and formation rate were found incomparable.

Annoksen ja pinta-alan tuloa (DAP, dose-area product) mittaavaa DAP-mittaria käytetään röntgendiagnostiikassa potilaan säteilyaltistuksen määritykseen. DAP-mittari on läpäisytyyppinen, tasomainen ionisaatiokammio, jolla voidaan mitata samanaikaisesti potilastutkimuksen kanssa. Röntgenlaitteessa oleva DAP-mittari tulisi kalibroida siten, että mittaustuloksena saadaan annoksen ja pinta-alan tulo potilaaseen kohdistuvassa säteilykeilassa. Mittareita voidaan kalibroida erilaisilla menetelmillä, mutta usein on tyydytty käyttämään mittarin valmistuksen yhteydessä tehtyä kalibrointia. Tässä työssä oli tarkoitus kehittää DAP-mittareille yhtenäinen ja toimiva kalibrointimenettely, jonka avulla mittaukset ovat jäljitettävissä kansainväliseen mittausjärjestelmään. Uudessa kalibrointimenettelyssä käyttöpaikalla suoritettava kalibrointi tehdään kalibroidulla DAP-mittarilla (vertailumittarilla), joka on säteilykeilassa samanaikaisesti kalibroitavan mittarin kanssa. Säteilyn käyttöpaikalla kalibroinnissa tarvittavat vertailumittarit kalibroidaan Säteilyturvakeskuksen (STUK) mittanormaalilaboratoriossa. Menetelmän kehittelyä varten DAP-mittareita tutkittiin laboratorioon rakennetulla mittausjärjestelyllä, jossa selvitettiin niiden toimintaa ja kalibrointiin vaikuttavia tekijöitä. Vertailumittarin kalibrointia varten tutkittiin kahta menetelmää, joissa todellinen annoksen ja pinta-alan tulo määritetään joko mittaamalla kalibroidulla DAP-mittarilla tai laskemalla ilmaan absorboituneen annoksen ja säteilykeilan poikkileikkauksen pinta-alan mitattujen arvojen tulo. Kahdella eri menetelmällä mitatut DAP-arvot poikkeavat useita prosentteja toisistaan. Aikaisempien tutkimuksien ja omien mittausten perusteella päätettiin, että vertailumittareiden kalibroinnissa käytetään mittanormaalina laboratorion kalibroitua DAP-mittaria. Kehitetyn menetelmän avulla mittanormaalilaboratoriossa kalibroitiin viisi vertailumittaria. Yhden vertailumittarin avulla kalibroitiin diagnostisten röntgenlaitteiden DAP-mittareita niiden omilla käyttöpaikoilla sairaalassa. Mittauksissa huomattiin, että tavanomainen paine- ja lämpötilakorjaus korjaa mittareiden näyttämää hieman liikaa. Siksi olosuhteiden vaihtelut vaikuttavat korjattuihin mittaustuloksiin ja kalibroinnin epävarmuuteen enemmän kuin aikaisemmin on arvioitu.

Graphene is the ultimately thin membrane composed of carbon atoms, for which future possibilities vary from desalinating sea water to fast electronics. When studying the properties of this material, molecular dynamics has proven to be a reliable way to simulate the effects of ion irradiation of graphene. As ion beam irradiation can be used to introduce defects into a membrane, it can also be used to add substitutional impurities and adatoms into the structure. In the first study introduced in this thesis, I presented results of doping graphene with boron and nitrogen. The most important message of this study was that doping of graphene with ion beam is possible and can be applied not only to bulk targets but also to a only one atomic layer thick sheet of carbon atoms. Another important result was that different defect types have characteristic energy ranges that differ from each other. Because of this, it is possible to control the defect types created during the irradiation by varying the ion energy. The optimum energy for creating a substitution for N ion is at about 50 eV (55%) and for B ion it is ca. 40% at about the same energy. Single vacancies are most probably created at an energy of about 125 eV for N (55%) and for B at ca. 180 eV (35%). For double vacancies, the maximum probabilities are roughly at 110 eV for N (16%) and at 70 eV for B (6%). The probabilities for adatoms are the highest at very small energies. A one atom thick graphene membrane is reportedly impermeable to standard gases. Hence, graphene's selectivity for gas molecules trying to pass through the membrane is determined only by the size of the defects and vacancies in the membrane. Gas separation using graphene membranes requires knowledge of the properties of defected graphene structures. In this thesis, I presented results of the accumulation of damage on graphene by ion irradiation using MD simulations. According to our results, graphene can withstand up to 35% vacancy concentrations without breakage of the material. Also, a simple model was introduced to predict the influence of the irradiation during the experiments. In addition to the specific results regarding ion irradiation manipulation of graphene, this work shows that MD is a valuable tool for material research, providing information on atomic scale rarely accessible for experimental research, e.g., during irradiation. Using realistic interatomic potentials MD provides a computational microscope helping to understand how materials behave at the atomic level.

As the data traffic, as well as the speed demands, increases, the mobile networks require means for economically fulfil these demands. The solution comes from the cloud. In order to move the processing to the cloud, it must be carefully dimensioned to know how much resources each situation requires. This means there must be a way to calculate from the traffic the virtual machines required and the hardware resources the virtual machines need, when the cloud infrastructure used is OpenStack. This thesis provides two methods for calculating the virtual machines from the traffic profile. The first one is based on performance testing of the virtual network functions and the second one is based on machine learning technique called multiple linear regression analysis. Furthermore in this work, approximation algorithms are being used in order to solve multidimensional variates of classical optimization problems such as bin packing problem and subset sum problem. These algorithms are used to dimension required resources from the virtual machines to hardware and vice versa. The algorithms are bundled to a program with a graphical user interface to make as user friendly as possible.

Freshwaters are a source of carbon to the atmosphere in the form of methane (CH4) and carbon dioxide (CO2). Global estimates of the freshwater contribution to the carbon budget are often based on a water boundary layer model (BLM) with gas transfer coefficient k calculated depending solely on wind speed. According to comparison studies, this model gives underestimated emissions and should not be used for more reliable results. A widely used flux measurement method over lakes is the floating chamber (FC) method. FC measures surface flux from a very small area of the lake, so it may not be representative of the whole ecosystem. Measurements are relatively cheap and easy, but also laborious and sporadic. Instead of measuring just a specific point on the lake, eddy covariance (EC) technique provides continuous flux measurements over a much larger source area (footprint). EC systems have been widely used over land areas, but are now growing their popularity in the lake community as well. The aim of this study was to compare EC, FC and BLM methods for CO2 and CH4 fluxes over a boreal lake. The measurements were made at a small dimictic Lake Kuivajärvi in Hyytiälä (Juupajoki, Southern Finland) during an intensive field campaign in September 2014. Manual FC measurements were done at four measurement spots in the EC footprint area 2-3 times a day for catching spatial and temporal variability. Gas transfer velocity for BLM was calculated according to three different parametrizations. Results indicate that BLM fluxes calculated based on water convection and wind driven turbulent gas exchange compare quite well with EC measurements while the model based solely on wind speed is a clear underestimate. FC measurements show about 1.7 times larger flux values than EC. The comparison is more clear for CH4 than CO2 fluxes. The greatest values of CH4 fluxes were measured near the shore, while CO2 flux did not show any spatial variability. After the lake started its autumn mixing, CH4 flux showed a diurnal variation with highest values measured during daytime. There was no diurnal variation before mixing. CO2 flux on the other hand showed diurnal variation only when calculated according to the BLM method.

Cytochrome bc1, also known as complex III, is the third enzyme of the electron transfer chain in cellular respiration, which is the main process generating energy in living cells. Complex III operates by oxidizing ubiquinol, and transferring two electrons to cytochrome c, while reducing ubiquinone. The electron transfer is coupled to proton translocation across the inner mitochondrial membrane. Thus, complex III contributes to generation of a proton electrochemical gradient, which is required for the function of ATP synthase. Cardiolipins (CLs), constituting up to 20 mol % of lipids in the inner mitochondrial membrane, have an important role in the structure and dynamics of the membrane, as well as in maintaining the correct function of the whole electron transfer chain. Cardiolipins are especially vulnerable to oxidation by reactive oxygen species (ROS) due to their dimeric structure with four doubly unsaturated acyl chains. Cytochrome bc1 is one of the main producers of ROS in mitochondria, increasing the exposure of tightly bound CLs to oxidation. Oxidative stress and CL oxidation have been associated with, for instance, programmed cell death and aging, and developing Alzheimer's and Parkinson's diseases. The objective of this thesis was to build a new computational model of cytochrome bc1 in a membrane, and to study the lipid interactions of complex III using atomistic molecular dynamics simulations. A model system with a high-resolution structure of complex III, embedded in a multicomponent bilayer mimicking the inner mitochondrial membrane was constructed. Four atomistic simulations of 1 μs each were performed to reveal possible cardiolipin binding sites and to examine the effects of CL oxidation on the complex. Altogether, eight CL binding sites on cytochrome bc1 were found, out of which two have not been suggested previously. The key residues of each binding site were listed, to compare with earlier results, and to identify the new binding sites in detail. In order to investigate the effects of CL oxidation, carboxylic acid and hydroperoxyl groups were attached to the acyl chains of three crystallographically resolved CLs. The oxidized region of the CL tails changed the nature of interactions with the protein and the surrounding water. As the tail was oxidized, the results showed an increase in the number of water molecules surrounding it. Additionally, the oxidized tails were found to affect the configuration of CL by bending the tail towards the lipid headgroup, or by reaching out to the water interface of the opposite leaflet. Normally, the acyl chains of CL mostly interact with the nonpolar residues of the protein. After oxidation, the number of polar and charged amino acids in the vicinity of the acyl chain increased.

Tässä työssä kehitettiin SQUID-pohjainen laitteisto helium-3:lla tehtäviä NMR-mittauksia varten ja suoritettiin mittauksia sekä nk. jatkuvan aallon (continous wave) NMR:llä että pulssitetun aallon (pulsed wave) menetelmällä. Helium-3:n korkean hinnan (n. 5000 euroa/litra) takia työssä käytettiin testitarkoituksiin NMR-materiaaleina myös fluoria sisältävää teflonia ja vetyä sisältävää jäätä. Laitteisto suunniteltiin ja rakennettiin Aalto-yliopiston O.V. Lounasmaa -laboratoriossa, nykyiseltä nimeltään Low Temperature Laboratory. NMR eli ydinmagneettinen resonanssi on ilmiö jossa ydinspinilliset atomiytimet sijoitetaan staattiseen magneettikenttään ja viritetään niitä ulkoisella sähkömagneettisella säteilyllä, jonka jälkeen niiden viritystila purkautuu vapauttaen NMR-signaalin. Tällä tavalla pystytään tutkimaan monia aineen eri ominaisuuksia. SQUID eli Superconducting Quantum Interference Detector taas on nimensä mukaisesti kvantti-interferenssiin perustuva laite, joka kykenee havaitsemaan erittäin pieniä magneettikenttiä. NMR:n yhteydessä se on tehokas esivahvistin, jonka avulla voidaan havaita hyvin pieniäkin signaaleja. Tässä työssä sillä on tarkoitus parantaa signaali-kohinasuhdetta verrattuna perinteisiin puolijohde-esivahvistimiin ja saada aikaan ilmaisin jolla voidaan mitata myös matalammilla taajuuksilla kuin tutkimusryhmällä on nykyisin käytössä. Suoritettujen mittausten perusteella laitteisto kykeni havaitsemaan NMR-signaalin jatkuvan aallon menetelmällä jokaisesta tutkitusta aineesta. Pulssitettuja mittauksia ei vielä toistaiseksi onnistuttu tekemään onnistuneesti johtuen heliumin pitkähköstä, n. 30 sekunnin, relaksaatioajasta joka teki pidemmistä mittaussarjoista vaikeita toteuttaa. Vastaavasti kahdella kiinteällä aineella, teflonilla ja jäällä, resonanssin leveys oli niin suuri että energian absorbointi pulsseilla näytteeseen olisi hankalaa ja tuottaisi signaaleja joiden pienuus tekisi niistä hankalasta havaittavia, joten näitä aineita tutkittiin tässä työssä vain jatkuvan aallon menetelmällä.

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.