Browsing by master's degree program "Alkeishiukkasfysiikan ja astrofysikaalisten tieteiden maisteriohjelma (Particle Physics and Astrophysical Sciences)"

Aims. In this thesis, we review the history in studying the evolution path of magnetic white dwarfs and explain the longstanding questions and debates over their magnetic origin. We intend to find magnetic white dwarfs in various forms (isolated or with companions) from the spectral database of LAMOST DR7 to complement the current magnetic white dwarf catalogue. We then move on to compare our results with some commonly accepted scenarios regarding their magnetic origin. Methods. Low-resolution spectra is the main source in this study, we intend to locate signs of Zeeman-splitting in spectra of isolated white dwarfs, measure separations of substructures due to Zeeman-splitting and estimate magnetic field strength. Magnetic white dwarfs in binary or multiple systems are found by seeking signs of cyclotron radiation due to mass transfer and particle movement in magnetic fields. Photometric survey from Transiting Exoplanet Survey Satellite (TESS) was used to fold periodic light curves for targets of interest, in order to further study the nature of our candidates, especially the ones that are believed to coexist with companions. Results. We identified 31 isolated magnetic white dwarfs in the LAMOST DR7 database by the discovery of Zeeman-splitting components. Their estimated magnetic field strength ranging from below 1 mega gauss (MG) to a scale ten times larger. Two Polars/Intermediate Polars were found with both Zeeman-splitting components and broad Balmer emissions usually seen in cataclysmic variables. We also discovered two candidates of detached magnetic binaries. These systems are believed to be the progenitors of polars or intermediate polars. Despite their rarity, these candidates serve as vital hints in clarifying the ongoing debates concerning the magnetic field origins in white dwarfs.

Tutkielmani käsittelee kullan pysyvän isotoopin Au-197 tuottamista neutroniaktivaatiolla luonnollisesta elohopeanäytteestä. Kokeen kannalta pääasiallinen reaktiomenetelmä oli Hg-196 neutronikaappaus. Kyseinen transmutaatio suoritettiin myös kokeellisesti. Elohopeaa sisältävänä näytteenä käytettiin Ardentin valmistamia Futura Standard -hammasamalgaamikapseleita. Turvallisuussyistä kapselit olivat koejärjestelyssä alkuperäisessä purkissaan. Kapseleita oli kaikkiaan 50 kappaletta, ja jokaisessa oli 400 mg elohopeaa. Yhteensä näytteessä oli siis 20 grammaa elohopeaa. Näytettä säteilytettiin STUKin tiloissa 14 vuorokauden ajan kolmen AmBe-neutronilähteen avulla. Valmistajan ilmoittamat neutronituotot käytetyille lähteille ovat 2.0E+7 n/s, 2.1E+6 n/s ja 6.7E+5 n/s. Lähteiden ilmoitetut aktiivisuudet ovat vastaavasti 333 GBq, 37.0 GBq ja 11.1 GBq. Neutronien hidastamiseen käytettiin HDPE-tankoa. Säteilytyksen jälkeen näytettä mitattiin STUKin gammaspektrometrian laboratorion B6 p-HPGe BE5030 -germaniumilmaisimella, ja kullan synty voitiin todentaa spektristä löytyvien karakterististen gamma- ja röntgenpiikkien avulla. Koejärjestely onnistui, ja työni osoittaa, että kullan pysyvän isotoopin Au-197 valmistaminen luonnollisesta elohopeasta havaittavissa määrin on mahdollista toteuttaa melko yksinkertaisella koejärjestelyllä käyttäen varsin pienitehoisia neutronilähteitä. Varsinaisen kokeen lisäksi käsittelen työssäni myös kullanteon historiaa sekä aiheeseen liittyvää teoriaa.

The matter in neutron stars exist under extreme conditions, and the cores of these stars harbour densities unreachable in any laboratory setting. Therefore, this unique environment provides an exceptional opportunity to investigate high-density matter, described by the theory of Quantum Chromodynamics (QCD). This thesis centers on the exploration of twin stars, hypothetical compact objects that extend beyond the neutron star sequence. Originating from a first-order phase transition between hadronic matter and quark matter, our focus is on understanding the constraints on these phase transitions and their effect on the observable properties of twin stars. In our investigation of twin stars, we construct a large ensemble of possible equations of state featuring a strong first-order phase transition. We approximate the low- and high-density regions with polytropic form and connect them to chiral effective field theory results at nuclear densities and extrapolated perturbative QCD at high densities. The resulting equations of state are then subjected to astrophysical constraints obtained from high-mass pulsars and gravitational wave detections to verify their compatibility with observations. Within our simple study, we identify two distinct types of twin stars, each providing a clear signature in macroscopic observables. These solutions originate from separate regions in the parameter space, with both regions being relatively small. Twin stars in our approach generally obtain small maximum masses, while the part of the sequence corresponding to neutron stars extends to large radii, indicating that these solutions only marginally pass the astrophysical constraints. Finally, we find that all twin stars obtain sizable cores of quark matter.

Dark matter direct detection experiments still have found no evidence of the dark matter WIMPs. The search has therefore been expanded for lighter dark matter candidates. Light dark matter is nearly invisible to current detectors through the elastic nuclear recoils. This thesis is meant to provide understanding on the inelastic atomic scatterings, which are one good way to detect dark matter particles with mχ ∼ GeV. In this thesis we consider spin-independent scatterings. Inelastic scatterings are based on the fact that in an atom, electrons do not follow the motion of the recoil nucleus immediately, but instead it takes time. This results in a small probability of observable ionization or excitation of the atom. This is known as the Migdal effect. We will first study the theoretical framework of dark matter-nucleus scatterings, showing how to get the event rate and how it is factorized into the astrophysical, the particle physics and the target response part. Then we will move to the inelastic processes, Migdal and Bremsstrahlung effects, deriving their event rates. In the first, we try to detect ionized electrons. The latter one, the Bremsstrahlung, is a similar process to the Migdal, but there we try to detect photons emitted from the de-excitations of atoms excited in the inelastic recoils. We will also look into the Migdal in semiconductors. Because of the smaller gap for electron excitations in crystals, we find that the rate for the Migdal effect is much higher in semiconductors than in atomic targets, thus allowing the search for even lighter dark matter particles. The rate can be expressed in terms of the energy loss function of the target material.

The search for a profound connection between gravity and quantum mechanics has been a longstanding goal in theoretical physics. One such connection is known as the holographic principle, which suggests that the dynamics within a given region of spacetime can be fully described on its boundary surface. This concept led to the realization that string theory provides a lower-dimensional description that encapsulates essential aspects of spacetime. While the "AdS/CFT correspondence" exemplifies the success of this holographic theory, it was discovered soon after that the Universe has a positive cosmological constant, Λ. This immediately sparked interest in a potential correspondence centered around de Sitter (dS) space, which is also characterized by a positive cosmological constant. This thesis comprehensively explores the de Sitter/Conformal Field Theory (dS/CFT) correspondence from various perspectives, along with the unique challenges posed by the distinct nature of dS space. The original dS/CFT duality proposes that a two-dimensional Conformal Field Theory resides on the boundary of three-dimensional asymptotic dS space. However, the definition and interpretation of physical observables within the dS/CFT framework remain open questions. Therefore, the discussions in this thesis not only cover the original dS/CFT conjecture, but also extend into more recent advancements in the field. These advancements include a higher-spin dS/CFT duality, the relationship between string theory and dS space, and the intriguing proposal of an "elliptical" dS space. While the dS/CFT correspondence is still far from being well-defined, there have been extensive efforts devoted to shedding light on its intricate framework and exploring its potential applications. As the Universe may be evolving towards an approximately de Sitter phase, understanding the dS/CFT correspondence offers a unique opportunity for gaining fresh insights into the link between gravity and quantum field theory.

Ultra-low frequency (ULF) waves in the Pc4-Pc5, 2 – 25 mHz range have been observed to accelerate trapped 1 – 10 MeV electrons in the Earth’s radiation belts. This acceleration can lead to particle losses and injections that occur on timescales comparable to the particle drift periods. Current models rely on diffusion equations written in terms of Fokker-Planck equations and are not suitable for describing fast temporal variations in the distribution function. This thesis is a study of fast transport of equatorially trapped electrons in the radiation belts. We look at solutions for the time evolution of the linear part of the perturbed distribution function using both analytical and numerical methods. Based on this work we build a simple model of fast transport in the radiation belts using a spectral PDE framework called Dedalus. The resulting program is a computationally inexpensive, simple approach to modelling drift-periodic signatures on fast timescales. In this study we investigate the behavior of the distribution function in three systems: a simple system without a wave term, and systems with a single non-resonant and resonant ULF wave. The wave solutions are evaluated with magnetic field perturbations of different magnitudes. The Earth’s magnetic field is modelled with the Mead field. The numerical solution of the perturbed differential equation is studied for relativistic equatorially trapped electrons. Phase-mixing is found to happen regardless of field fluctuations or resonance. The non-resonant wave solution shows time-delayed, spatially localized structures in the equatorial plane forming in the presence of large magnetic field fluctuations. These transients are also seen in the analytical solution and provide a new theoretical explanation for the ubiquitous observation of drift echoes in the inner and outer radiation belts of the Earth (Li et al., 2024).

Mars is a rocky planet in the Solar System, fairly similar to the Earth. It is known for its red color and the theory that there has been liquid water and possibly life in Mars at some point in its history. Mars has been a target of study since space exploration began, with the first fly-by mission occuring in 1965. In the past, Martian climate could have supported a functioning hydrological system and Mars even could have had an ocean. This system would have been similar to the one happening in Antarctica due to cold weather. The Martian atmosphere is mostly composed of carbon dioxide, much thinner and colder compared to the Earth's atmosphere. Methane is a main point of interest due to it being a possible biosignature, a sign of life. Another interesting feature about the atmosphere are the dust devils that contibute to the climate by lifting dust into the air. The Martian soil is composed of rocks and dust containing toxic perchlorates. To colonize Mars, several requirements need to be met: the transportation and funding, designing the settlement efficiently with everything needed and determining how many people are necessary. The risk assessment to both people and equipment needs to be made and taken into account.

One of the most noticeable effects of solar–terrestrial physics is the aurora which regularly appears in the polar regions. This polar light is the result of the excitation of atmospheric species by charged particles originating from the solar wind and magnetosphere that enter the Earth’s atmosphere, which are called precipitating particles. We present the first results on auroral proton precipitation into the ionosphere using a global 3-dimensional simulation of near-Earth space plasma with the Vlasiator hybrid-Vlasov model, driven with a southward interplanetary magnetic field and steady solar wind parameters. The hybrid-Vlasov approach describes ions through their velocity distribution function in phase space (3-dimensional ordinary space and 3-dimensional velocity space), while electrons are represented by a massless charge-neutralizing fluid. Vlasiator is a global model describing the whole region of near-Earth space including the Earth’s magnetosphere (whole dayside and part of the magnetotail), the magnetosheath, as well as the foreshock region and some solar wind. The precipitating proton differential number fluxes for this run are determined from the proton phase-space density contained within the bounce loss-cone, which is set at a constant angle of 10 degrees everywhere. To determine the precipitation of particles at ionospheric altitudes (in this case a height of 110 km above the Earth’s surface), we trace magnetic field lines from the ionosphere to the inner boundary of the Vlasiator domain using the Tsyganenko model. With this, we obtain a magnetic local time–geomagnetic latitude map of differential number flux of precipitating protons in 9 energy bins between 0.5 and 50 keV. From the differential number flux, proton integral energy fluxes and mean energies can be obtained. The integral energy fluxes in the Vlasiator run are then compared to data of the Precipitation Electron/Proton Spectrometer (SSJ) instrument of the Defense Meteorological Satellite Program (DMSP) for several satellite overpasses during events with similar solar wind conditions as in the Vlasiator run. The SSJ instrument bins proton energies between 0.03 and 30 keV. Typical values of the total integral energy flux are between 5 · 10^6 and 5 · 10^7 keV cm−2 s−1 sr−1 in the cusp and between 1 · 10^6 and 3 · 10^7 keV cm−2 s−1 sr−1 in the evening sector for both Vlasiator and DMSP, although DMSP fluxes can locally be up to an order of magnitude higher. Additionally, global precipitation patterns in Vlasiator are compared to Ovation Prime, which is an empirical model based on data from DMSP which can be used to forecast precipitation of auroral electrons and protons. Although Ovation Prime shows a much wider cusp region compared to Vlasiator, both show similar maximum integral energy fluxes around 1 to 2 · 10^7 keV cm−2 s−1 sr−1 in the cusp region, and between 3 · 10^6 and 5 · 10^7 keV cm−2 s−1 sr−1 in the nightside oval.

Color confinement, the inability for free quarks to exist at normal temperatures and densities, is one of the most important properties of Quantum Chromodynamics, the quantum field theory (QFT) of the strong interaction. A simple representation of confinement can be obtained by considering a static (i.e. infinitely massive) quark-antiquark pair. The potential of the pair contains a term linear with respect to the separation. Therefore, separating the pair would require infinite energy meaning that the quarks are confined. The static potential can be related to the expectation value of a QFT operator called the Wilson loop. In the non-perturbative large coupling regime lattice field theory can be applied to estimate expectation values of observables. The idea of lattice field theory is to replace continuous spacetime with a lattice, where the fields are defined on the sites and links. The discretization of spacetime allows evaluating path integrals numerically using Monte Carlo methods. An alternate way of computing the Wilson loop expectation value is provided by holographic duality. It is an equivalence between a QFT in four-dimensional flat spacetime and a higher-dimensional theory of gravity in curved spacetime. The duality allows evaluation of hard, non-perturbative QFT calculations with easy classical computations on the gravity side. From the lattice data of the static potential, we can construct a holographic model that could produce those results in a process called bulk reconstruction. The constructed holographic model can then be used to compute other QFT quantities such as entanglement entropy. These quantities allow us to study confinement among other things. In this thesis, lattice field theory measurements of the static potential at different temperatures are presented. Then using a machine learning method, the corresponding holographic metrics are constructed from the lattice results. The lattice simulations have been done as a learning exercise and for the bulk reconstruction method, this thesis is a proof of concept, where no further computations are done using the constructed metrics. The lattice results are in good agreement with previous ones and the bulk reconstruction method seems to work as intended. In future works the method should be applied to a bigger dataset and other quantities should be computed with the constructed metrics.

In high energy physics the microscopic nature of our universe is studied. A common experimental way is to collide particles such as protons with almost the speed of light and study the fragments that fly away from the interaction point. The results are compared to existing theories and help to modify or create knowledge of our universe. However, our senses are not capable of directly measuring those tiny and fast fragments from the collisions. Therefore, we need dedicated devices called particle detectors. Several types of radiation detectors are known to us. Most of those utilize the fact that particles in such experiments are ionizing when traversing matter. The detectors we are interested in contain gas as the main interacting material for the ionizing radiation, called gaseous detectors. One of these is the Gas Electron Multiplier (GEM), which uses electric fields to make electrons drift through the holes of one or several foils producing a signal that can be detected. Gain and ion backflow are two properties that can be used to evaluate the effectiveness of the detector. The effect of the hole diameters of the foil on the gain and on the ion backflow is insufficiently known, however. In this thesis such measurements have been performed. For this purpose a GEM detector operating in proportional region was constructed. The detector contained a special foil with four quadrants in such a way that the diameters of the holes were different in each quadrant. The functionality of the detector was verified by measuring the leakage current, by using a multichannel analyzer, and by calculating the sum of all currents. The detector constructed passed all the tests. The diameters of the holes in the foil quadrants were estimated by calculating the amount of pixels from the foil image taken. The outer hole diameters were around 50 µm and inner hole diameters 40 µm in the quadrant with smallest holes. The corresponding diameters in the quadrant with largest holes were around 70 µm and 66 µm. The measurements were started by searching the proper values for the drift and the induction fields and for the GEM voltage range. A drift field of 2 kV/cm, an induction field of 7 kV/cm, and a voltage range from 400 V to 500 V were chosen. The results from the actual measurements were similar on both sides of the foil. The effective gain increased steadily along with the voltage being around 10-fold at 500 V compared to that at 400 V . The ion backflow, on the other hand, stayed constant or even slightly decreased. The results measured from the four quadrants differed clearly from each other. In the quadrant with the smallest holes the effective gain was about twice as high as in the quadrant with the largest holes. Respectively the ion backflow was about 30 % higher in the quadrant with small holes than in the quadrant with large holes.