Examensarbeten för kandidatexamen // Bachelor Theses
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- PostChiral effective field theory with machine learning(2016) Aspman, Johannes; Ejbyfeldt, Emil; Kollmats, Anton; Leyman, Maximilian; Chalmers tekniska högskola / Institutionen för fysik (Chalmers); Chalmers University of Technology / Department of Physics (Chalmers)Machine learning is a method to develop computational algorithms for making predictions based on a limited set of observations or data. By training on a well selected set of data points it is in principle possible to emulate the underlying processes and make reliable predictions. In this thesis we explore the possibility of replacing computationally expensive solutions of the Schrödinger equation for atomic nuclei with a so-called Gaussian process (GP) that we train on a selected set of exact solutions. A GP represents a continuous distribution of functions defined by a mean and a covariance function. These processes are often used in machine learning since they can be made to emulate a wide range of data by choosing a suitable covariance function. This thesis aims to present a pilot study on how to use GPs to emulate the calculation of nuclear observables at low energies. The governing theory of the strong interaction, quantum chromodynamics, becomes non-perturbative at such energy-scales. Therefore an effective field theory, called chiral effective field theory (EFT), is used to describe the nucleon-nucleon interactions. The training points are selected using different sampling methods and the exact solutions for these points are calculated using the research code nsopt. After training at these points, GPs are used to mimic the behavior of nsopt for a new set of points called prediction points. In this way, results are generated for various cross sections for two-nucleon scattering and boundstate observables for light nuclei. We find that it is possible to reach a small relative error (sub-percent) between the simulator, i.e. nsopt, and the emulator, i.e. the GP, using relatively few training points. Although there seems to be no obvious problem for taking this method further, e.g. emulating heavier nuclei, we discuss some areas that need more critical attention. For example some observables were difficult to emulate with the current choice of covariance function. Therefore a more thorough study of different covariance functions is needed.
- PostSimulering av joner i time-of-flightmasspektrometer(2015) Dahl, Marcus; Carlsson, Nils; Azhirnian, Amin; Chalmers tekniska högskola / Institutionen för fysik (GU); Chalmers University of Technology / Department of Physics (GU)Den här rapporten detaljerar en generell bakgrund och teori för modern flygtidsmasspektrometri och använder det här ramverket för att kalibrera och optimera jondelen av en elektron-jon-korrelationsspektrometer vid namn FERMI-FELCO. Jondelen baserades på en trefälts Wiley-Mclaren modell. Optimeringen fokuserade på massupplösningen och uppsamlingseffektiviteten genom att bestämma optimala värden för spektrometerns längd och styrkan på de elektriska fälten som används för att accelerera jonerna. Arbetet utfördes enbart med numeriska simuleringar och analytiska metoder. För simulering användes i stor utsträckning mjukvaran SIMION, med målet att finna en uppsättning parametrar som gav en tillräcklig massupplösning utan att offra uppsamlingseffektivitet. Slutsatsen är att ett stort antal användbara lösningar existerar som alla resulterar i tillräcklig massupplösning och uppsamlingseffektivitet, men med begränsningar införda av den tillgängliga utrustningen krävs en avvägning av de två. De rekommenderade parametrarna för den specifika spektrometern FERMIFELCO presenteras i rapporten, samtidigt som den generella teorin och metoderna i arbetet kan användas för spektrometrar med liknande design.
- PostWhere is the Detection Limit?(2016) Eliasson, Mattias; Furufors, Anders; Glasell Johansson, Linnea; Lundqvist, Thomas; Roderus, Jens; Chalmers tekniska högskola / Institutionen för fysik (Chalmers); Chalmers University of Technology / Department of Physics (Chalmers)This report investigates the detection efficiency of the experimental setup LAND/R3B. This is done through Monte Carlo simulations. The simulation software geant4, via the wrapper program ggland, is used with the reconstruction tool land/r3b-tracker. Beams of different isotopes of carbon, boron and beryllium, produced by reactions of parent isotopes 16C, 17C and 18C, were simulated passing through the setup. The land/r3b-tracker reconstructs the nuclear masses and momentum 4-vectors of the ions from the data acquired by the detectors. The efficiency is determined assuming the nuclear masses follow a normal distribution. Events that fell inside the distribution were considered correctly reconstructed. The detection efficiency is the quotient of the correctly reconstructed events and the total number of simulated events. It was found that this efficiency depends on several variables, where the geometry of the detectors is one. The detector GFI2 (Groer FIberdetektor 2) was found to be the detector that has the lowest acceptance. The addition of transverse momentum to the ions was also found to affect the efficiency.