Browsing by department "Fysiikan laitos"

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.

First-order phase transitions in the electroweak sector are an active subject of research as they contain ingredients for baryon number violation and gravitational-wave production. The electroweak phase transition in the Standard Model (SM) is of a crossover type, but first-order transitions are possible in scalar extensions of the SM, provided that interactions of the Higgs boson with the new particles are sufficiently strong. If such particles exist, they are expected to have observable signatures in future collider experiments. Conversely, studying the electroweak transition in theories beyond the SM can bring new insight on the cosmological implications of these models. Reliable estimates of the properties of the transition require non-perturbative approaches to quantum field theory due to infrared problems plaguing perturbative calculations at high temperatures. We discuss three-dimensional effective theories that are suitable for lattice simulations of the transition. These theories are constructed perturbatively by factorizing correlation functions so that contributions from light field modes driving the phase transition can be identified. Resummation of infrared divergences is naturally carried out in the construction procedure, and simulating the resulting effective theory on the lattice allows for a non-perturbative phase-transition study that is also free of infrared problems. Dimensionally-reduced theories can thus be used to probe the conditions under which perturbative treatments of the electroweak phase transition are valid. We apply the method to the SM augmented with a real $\text{SU}(2)$ triplet scalar and provide a detailed description of dimensional reduction of this model. Regions of a first-order transition in the parameter space are identified in the heavy triplet limit by the use of an effective theory for which lattice results are known. We provide a rough estimate for the accuracy of our results by considering higher-order operators that have been omitted from the effective theory and discuss future prospects for the three-dimensional approach.

Porous structures, such as foams make excellent thermal insulators. This happens because thermal transfer by conduction is hindered by the voids in the material. However, heat can still radiate through the material or just past the voids. Due to Stefan-Boltzmann law, heat transfer by radiation can be especially significant for large temperatures, and it follows that thermal transfer models that account for radiation may be necessary in some cases. Several existing models for radiative thermal transfer in porous materials, such as continuum models and Monte Carlo, have been used in the past. What many of these models tend to have in common, is that they are highly specific to the systems they were originally made for and require some rather limiting approximations. A more general method which would only require knowing the material and the geometry of the system would be useful. One candidate for such a method, discrete dipole approximation for periodic structures, was tested. In the discrete dipole approximation a structure is discretized into a collection of polarizable points, or dipoles and an incoming electromagnetic planewave is set to polarize it. This has the benefits that it accurately takes into account the target geometry and possible near field effects. It was found that this method is limited for high wavelength, by computation time and for small wavelengths by errors. The cause of the errors for small wavelengths was not entirely caused by the discretization and remains not fully understood.

The objective of this master s thesis was to better understand the impact of black carbon and its distribution in Northern Europe and the Arctic. To achieve the goal of the project, information on the observations relevant to black carbon (BC) pollution in Arctic dataset was collected. For the observational data all main BC measurement campaigns along with active satellite operations were collected. In this study, the BC concentration and deposition was estimated by the System Integrated Modelling of Atmospheric coMposition - a chemical transport model (CTM) SILAM. The model was driven with monitoring atmospheric composition and climate (MACCity), emission database for global atmospheric research hemispheric transport of air pollution (EDGAR-HTAP), and evaluating the climate and air quality impacts of short lived pollutants (ECLIPSE) emission inventories. For the computations, the year 2010 was chosen because of a better availability of data during that year. In the literature section, black carbon process in the atmosphere is explained along with its properties and characteristics. Furthermore, data description and data analysis is included which is followed by interpretation of model output on the seasonal deposition and concentration. As shown by the model-measurement comparison, the model basically captured the measured BC and organic carbon (OC) quite well for all emission inventories. However, the correlation coefficient for OC was still weak for most of the stations in Europe. The overall performance of BC for European stations is substantially better than in the Arctic areas. Deposition for BC and OC shows that the seasonal transport of BC from source regions is evident in the Arctic and near Arctic areas. Patterns of dry deposition is higher in winter period than in summer period. The SILAM model suggests winter period concentration of BC by MACCity and ECLIPSE inventory of 0.23 µg/m3 and 0.26 µg/m3 respectively for year 2010. This study provides a best performing setup for BC modeling , transport and deposition of BC in the Northern Europe and the Arctic despite the absence of ageing process. More observational data from Arctic stations would provide better result and model performance. Finally, the study gives an insight of the quality of existing emission inventories and the capabilities of reproducing seasonal deposition and concentration of BC to the Arctic.

Methods for mitigating climate change have been researched for decades. It has been established that most of greenhouse gases (GHG) such as carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) are due to anthropogenic activities. Significant amount of these gases, particularly N2O and CH4 are emitted due to agricultural practices. To boost food supply, nitrogenous fertilizers are the most common fertilizers used in farming, and as the global population will increase to 9 billion by 2050, it is crucial to keep pace with both the population growth and potential economic development. Applying biochar in soil has been regarded as a potential approach to combat climate change while boosting food supplies. In this study, the effect of two different biochars mixed with 15N-labelled ammonium nitrate ( either nitrate NH415NO3, or 15NH4NO3 ammonium labelled) fertilizers in growing Meroa Tetraploid Italian Ryegrasses (Lolium multiflorum) were investigated in Greenhouse Viikki Campus Greenhouse, University of Helsinki, Finland. We monitored the impact of biochars on the aboveground biomasses (g pot-1), roots biomasses (g pot-1), and total biomasses (gram/pot). Furthermore, retention of leachates (nutrient ions) such as atom percentage (at%) 15N-labelled ammonium ion (at% 15NH4-N), mg NH4-N, at% 15N -labelled nitrate ions (at% 15NO3-N %), and mg NO3-N were investigated as well. Lastly, GHG fluxes of N2O-N (ug Kg-1 soil hr-1), CH4-C (ug Kg-1 soil hr-1), CO2-C (mg Kg-1 soil hr-1) were also included in our research for a period of one month in comparison with control samples and fertilized control. Leachates were collected once a week, and gas samplings were measured twice each day for two days in a week. Gases sampled, and leachates collected were analysed, and the plants were detached to obtain the amount of aboveground biomasses (leaves), roots biomasses (roots) and total biomasses (leaves + roots). Our objective affirmed a more eco-friendlly model that biochars can reduce nitrogenous fertilizers applied with an increased agricultural produce. It was concluded that, the effects of different biochars on these mentioned attributes are distinguishable.

Since the inception of the AdS/CFT correspondence in 1997 there has been great interest in the holographic description of quantum gravity in terms of conformal field theory. Studying how classical gravity emerges in this framework helps us to understand the quantum foundations of general relativity. A fundamental concept is entanglement entropy which has a classical interpretation in terms of areas of minimal surfaces in general relativity, due to Ryu and Takayanagi. This thesis is a review on how Einstein's equations can be derived up to second order from the Ryu-Takayanagi formula in the context of AdS/CFT correspondence. It also serves as an introduction to entanglement entropy in quantum field theories and holography, while providing necessary mathematical ingredients to understand the derivation. We also review a related derivation, based on the entanglement equilibrium hypothesis, and discuss its extensions to higher order theories of gravity.

Kasvavan kaupungistumisen johdosta kaupunki-ilmaston tutkiminen on kasvanut selvästi viimeisten vuosikymmenien aikana. Yleisimmin tunnettu kaupungistumisen aiheuttama ilmiö on lämpösaarekeilmiö, jossa yöaikaan kaupungin ilmanlämpötila pysyttelee selvästi ympäröiviä maaseutuja korkeammalla. Tämä ja monet muut kaupungissa koetut ilmiöt ovat ihmisen aiheuttamia ja siksi on tärkeää selvittää, miten kasvava kaupungistuminen tulee jatkossa vaikuttamaan meidän jokapäiväiseen elämään, terveyteen ja ympäristöömme. Helsingin Kumpulassa sijaitseva SMEAR III (Station for Measuring Ecosystem-Atmosphere Relationships) kaupunkimittausasema sai syyskuussa 2010 uuden mittauspisteen Helsingin keskustaan Hotelli Tornin huipulle, missä mitataan mm. turbulenttista latentin ja havaittavan lämmön vuota. Tässä tutkielmassa tarkastellaan kaupunkialueen energiatasapainoa Helsingin keskustassa Tornissa tehtävien mittauksien avulla, sekä perehdytään energiatasapainon mallintamiseen yhdellä maailman pohjoisimmista kaupunkimittausasemista. Tutkielmassa käytetään mittauksia, jotka kattavat kokonaisen vuoden marraskuusta 2010 lokakuuhun 2011. Mitattuja lämmön turbulenttisia voita verrataan Surface Urban Energy and Water Balance Scheme -mallilla (SUEWS) mallinnettuihin voihin. Tarkoitus on selvittää sitä, kuinka hyvin malli toimii Helsingin keskustan hyvin tiheään rakennetulla kaupunkialueella. SUEWS:n lisäksi työssä käytetään Kormannin ja Meixnerin lähdealuemallia, jolla saadaan lasketuksi arvio mitattujen voiden lähdealueista. Saatujen tuloksien mukaan Helsingin keskustassa havaittavan lämmön vuo on monta kertaa latentin lämmön vuota suurempaa. Mallinnettaessa energiatasapainon komponentteja SUEWS-mallilla havaittavan lämmön vuon arvot olivat melko lähellä mitattuja arvoja, mutta latentin lämmön vuon tapauksessa mallinnetut arvot olivat liian pieniä. Ongelmia aiheutti saatavilla olevan energian ja kosteuden liian vähäinen määrä, jotka johtuivat mm. mallinnuksessa käytetystä liian suuresta albedosta ja maankäytön aineiston karkeasta resoluutiosta. Antropogeenisen lämmön vuon määrä Helsingissä oli mallinnuksen mukaan noin 20-50 W m-2:llä, joka vastaa lauhkean vyöhykkeen kaupunkien keskiarvoja. Varastotermin käyttäytyminen oli oikeanlaista, jossa päivällä energiaa sitoutuu rakenteisiin ja yöllä sitä vapautuu ilmaan turbulenttisina voina. Yhteenlasketut energian lähdetermit eivät kuitenkaan olleet tarpeeksi isoja, joka kävi hyvin selväksi energiatasapainosuhteen avulla tehdystä vertailusta, jossa suhteeksi saatiin 1,26.

Kärnkraftsproducenterna TVO och Fortum har beslutit att deponera kärnavfall i berggrunden i Olkiluoto. Kärnavfallet kommer att placeras i kopparkanistrar, vilka omringas av en buffert bestående av leran bentonit. Buffertens uppgifter är extremt viktiga för slutförvaringens säkerhet, då bufferten skall skydda kanistrarna från externa kontaminanter och seismiska rörelser samt hindra transporten av radioaktivt material. Bentonitens viktigaste komponent är lermineralen montmorillonit. Montmorillonit består av skivor vilka är en nm tjocka och flera hundratals nm breda. Dessa skivor bildar olika strukturer. I detta arbete studerades montmorillonitpartiklarnas former, samt deras andelar, i kolloidala lösningar. Som lösningsämnen användes olika natrium- och kalciumlösningar, vilka motsvarar möjliga omständigheter i berggrunden i Olkiluoto. Lösningarna studerades med lågvinkelröntgenspridning (SAXS) samt med anpassningar av en teoretisk modell utvecklad av Pizzey et al (2009). Med hjälp av dessa kunde partikelstorlekarna, samt andelarna av de olika partiklarna, bestämmas för varje lösning. I alla lösningar påträffades enskilda bentonitskivor samt bentonitskivor i staplar, d.v.s. varierande lager av bentonit och lösning. Stapelstorlekarna, samt deras andelar, berodde på lösningen. I kalciumlösningarna bildades stora, kompakta staplar. Avståndet mellan skivorna var i dessa c. 1 nm och staplarna kunde bestå av över 10 skivor. Andelen av dessa staplar ökade klart med kalciumkoncentrationen. I natriumlösningarna uppstod staplar bestående endast av två skivor. Avståndet mellan skivorna varierade mellan 6-10 nm beroende på natriumkoncentrationen. I blandningar av både kalcium- och natriumlösningar dominerades växelverkan mellan skivorna av kalciumjonerna, d.v.s. stapelstrukturerna bestod till största delen av den kompaktare strukturen.

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.

Interactions and mergers play important roles in the evolution of galaxies. In this Master's thesis I have studied some basic properties of merging galaxies using numerical simulations. I have created initial conditions for equal-mass disk galaxy mergers and run the simulations using the numerical simulation codes GADGET-3 and its recently updated version SPHGal. In this Thesis I have also reviewed the basic physics modeled by GADGET-3 and the differences between the astrophysical subresolution models in the standard GADGET-3 and SPHGal versions of the code. The aim of the Thesis was to see how different initial conditions affect the mergers of disk galaxies, and how the updated astrophysics affect the properties of the mergers. The simulations were run on the supercomputer Sisu and the supercluster Taito at CSC, the Finnish IT Center for Science. From the results of the simulations I have studied the effect of having a dark matter halo with either an NFW density profile or a Hernquist density profile. Four observed mergers were succesfully reproduced with NFW profiles using position and velocity maps, and then compared to their Hernquist profile analogues. By comparing the results of the mergers between galaxies with the two types of haloes, I found that the steeper gravitational potentials of the NFW haloes produced more prominent tidal tails than the Hernquist haloes. The more compact morphology of the disks within the Hernquist haloes before the final coalescence enabled also faster coalescence times in the Hernquist halo mergers. As expected, the NFW halo mergers experienced slower orbital decay, were more violent, experienced higher star formation rates and produced thus 2-17% more new stars than the Hernquist halo mergers. In the Thesis I have also studied the effect of the updated subresolution models on the star formation process in the merging galaxies. The star formation in the standard GADGET-3 is self-regulated, whereas the star formation in SPHGal is regulated by detailed feedback processes. The star formation rates vary both spatially and on much shorter time scales in the SPHGal compared to the analogous simulations with the standard GADGET-3. I analyzed also the metal yields in the merger remnants of the Antennae galaxies, and found a metallicity gradient within the inner 2 kpc from the center of mass of the remnant. A gradient was also present in the oxygen abundance of the remnants, following the fact that the metallicity was dominated by the abundance of oxygen. Gradients in the inner regions of the merger remnants are to be expected, since star formation is most active in the central regions of the remnant.