Browsing by discipline "Fysiikka"

Accelerator mass spectrometry (AMS) is a technique developed from mass spectrometry and it is able to measure single very rare isotopes from samples with detection capability down to one atom in 10^16. It uses an accelerator system to accelerate the atoms and molecules to break molecular bonds for precise single isotope detection. This thesis describes the optimization of University of Helsinki's AMS system to detect the rare radioactive isotope 14C from CO2 gas samples. Using AMS to detect radiocarbon is a precise and fast way to conduct radiocarbon dating with minimal sample sizes. Solid graphite samples have been in use before but as the ion source has been adopted to use also gaseous CO2 samples, optimizations must be made to maximize the carbon current and ionization efficiency for efficient 14C detection. Parameters optimized include cesium oven temperature, CO2 flow, carrier gas helium flow and their dependencies with each other. Both carbon current and ionization efficiency is looked at in the optimizations. The results are analyzed and discussed for further optimizations or actual measurements with gas. Ionization occurring in the ion source can be understood better with the results. Standard samples of CO2 were measured to determine the background and precision of the AMS system in gas use by comparing the results with literature. The current system was found to have tolerable background of 1.5% of the standard and the Fraction modern value of actual sample was 2.4% higher than values from literature. Ideas to improve background were discussed. A new theory of negative-ion formation in a cesium sputtering ion source by John S. Vogel is reviewed and taken into account in the discussion of optimization. Utilizing the theory, possible future upgrades to improve the ionization efficiency are presented such as cathode material choices to reduce competitive ionization and cesium excitation by laser.

Field emission from metal surfaces is an important phenomenon in modern technology, not least due to its role in the vacuum breakdowns limiting the gradient of the accelerating fields in the Compact Linear Collider being planned at CERN. Vacuum breakdowns are found to originate at locations on the surface of the accelerating structures where field emission is enhanced, making understanding field emission important for increasing the effectivity of the instrument. According to the standard Fowler–Nordheim theory of field emission, the work function of the surface and the geometric field enhancement are the two parameters which determine the field emission current, with the standard interpretation of experimental results focusing on the geometric field enhancement. The role of the work function, which can be significantly decreased near surface defects, is often overlooked. The aim of this work is to study the influence of atomic-scale defects on the work function and field emission characteristics of a copper (111) surface, and to verify the validity of the Fowler–Nordheim equation for the surfaces with defects. The metal surface potential barriers were determined using density functional theory with an image potential type term added manually to account for the long-range exchange and correlation interactions. The determined potential barriers were used in quantum transport calculations to compute the field emission current while taking into account the density of states. A Fowler–Nordheim plot analysis was done for the computed emission currents. The results show that, for the studied atomic-scale surface defects, the decreased work function of the surface is sufficient to explain the increased field emission current, while no effective geometric field enhancement was found. The validity of the Fowler–Nordheim equation for the studied systems was established, with only an approximately constant factor separating the computed currents from those predicted by the Fowler–Nordheim equation.

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.

Particle Induced X-ray Emission (PIXE) is a nondestructive Ion Beam Analysis (IBA) technique that can be used for identifying elements in a sample. In PIXE, the radiation emitted by electron state changes is measured, after which emissions are recorded as spectral peaks. Each element is then identified based on its unique spectral peak. PIXE analysis has been carried out using a 3 MeV proton beam generated with the aid of TAMIA 5 MV EGP-10-II tandem accelerator of the Department of Physics, University of Helsinki, at the Helsinki Accelerator Laboratory in Kumpula. The external PIXE measurement setup in the accelerator laboratory has been prepared to study nine coins from 18th to 20th centuries and from different countries (Russia, USSR, Romania, France, and Portugal). The coins have been irradiated in the external PIXE setup, in which x-rays have been then detected by an x-ray detector. Two different detectors have been employed in the measurements: a KETEK AXAS-D Silicon Drift Detector (SDD) for detecting x-rays of every coin, and a Canberra GUL0110 Ultra-Low Energy Germanium (Ultra-LEGe) detector for detecting x-rays of silver coins. After PIXE spectra have been obtained, PyMCA software has been used for the elemental analysis of the data. In the present study, various elements have been found from the measured PIXE spectra. In silver coins, the following 10 elements have been specified: Ag, Cu, As, Pb, Fe, Sb, Ni, Zn, Sn and Bi. In nickel plated steel coin observed elements are Fe, Ni, Co, and Cu. Copper-zinc-nickel alloy coins have been found to consist of Cu, Zn, Ni, Fe, and Mn. Copper-nickel alloy coins have been investigated to be made of Cu, Ni, Fe, and Mn. This study verifies that the external PIXE technique can be utilized as a practical tool to identify elements in metallurgical samples.

The feasibility of quantitatively measuring ultrasound in air with a Schlieren arrangement has been demonstrated before, but previous work demonstrating calibration of the system combined with computation to yield the 3D pressure field does not exist. The present work demonstrates the feasibility of this both in theory and practice, and characterizes the setup used to gain the results. Elementary ray optical and Schlieren theory is exhibited to support the claims. Derivation of ray optical equations related to quantitative Schlieren measurements are shown step by step to help understand the basics. A numerical example based on the theoretical results is then displayed: Synthetic Schlieren images are computed for a theoretical ultrasonic field using direct numerical integration, then the ultrasonic field is recovered from the Synthetic Schlieren images using the inverse Abel transform. Accuracy of the inverse transform is evaluated in presence of synthetic noise. The Schlieren arrangement, including the optics, optomechanics, and electronics, to produce the results is explained along with the stroboscopic use of the light source to freeze ultrasound in the photographs. Postprocessing methods such as background subtraction and median and Gaussian filtering are used. The repeatability and uncertainty of the calibration is examined by performing repeated calibration while translating or rotating the calibration targets. The ultrasound fields emitted by three transducers (100 kHz, 175 kHz, and 300 kHz) when driven by 5 cycle sine bursts at 400 Vpp are measured at two different points in time. The measured 3D pressure fields measured for each transducer are shown along with a line profile near the acoustic axis. Pressure amplitudes range near 1 kPa, which is near the acoustic pressure, are seen. Nonlinearity is seen in the waveforms as expected for such high pressures. Noise estimates from the numerical example suggest that the pressure amplitudes have an uncertainty of 10% due to noise in the photographs. Calibration experiments suggest that additional uncertainty of about 2% per degree of freedom (Z, X, rotation) is to be expected unless especial care is taken. The worst-case uncertainty is estimated to be 18%. Limitations and advantages of the method are discussed. As Schlieren is a non-contacting method it is advantageous over microphone measurements, which may affect the field they are measuring. As every photograph measures the whole field, no scanning of the measurement device is required, such as with a microphone or with an LDV. Suggestions to improve the measurement setup are provided.

Particle and nuclear physics experiments require state-of-the-art detector technologies in a pursuit to achieve high data collection efficiency, and thus ensuring reliable data recording from the particle collisions at the Large Hadron Collider (LHC) experiments of CERN. High demand for data to be used for precision analysis has led to the development of MicroPattern Gaseous Detector (MPGD) based structures: Gas Electron Multiplier (GEM) and MICROMEGAS (MM). A systematic study is conducted on charging-up behaviour in a two-stage amplification structure, consisting of a single GEM foil above a MM detector with 2D readout chamber. Charging-up effect arises in the detector system due to combined effects from polarization of dielectric surfaces and accumulation of charges on the dielectric surfaces of MM resistive strips under high external electric field. The internal fields created from charging-up of dielectric surfaces can lead to a change in the applied electric field and gain of the detector suffers. In this thesis, the instability of gain due to characteristic charging-up process in GEM and MM is observed for different event rates and humidity level in the detector fill gas (P-10 gas mixture). MM gain decreased with time due to charging-up of dielectric surfaces and an exponential drop of gain by ≈30% is detected in case of dry gas i.e. fill gas without moisture. By adding a small amount of water content into fill gas, the MM gain is observed to drop around 22-30%. Addition of 1320±280 ppmV water content into MM gas volume yielded a higher gain of about 10% compared to dry gas. In case of higher rate measurement, achieved by using GEM foil as a pre-amplification stage between drift and readout electrode, the gain saturates at 70%. For low rate measurement, the gain saturates at 68%. GEM gain is observed to increase slowly by 17% as the dielectric surfaces inside its holes charged-up gradually over time.

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.

In MRI, the diffusion of water molecules can be imaged using Diffusion Weighted Imaging (DWI). Increasing cellularity in tumor tissue causes the restriction of water diffusion to increase, which will show as a high contrast in DW-image. Due to this advantage, the DWI shows great promise as a biomarker for cancer treatment. However, currently the most commonly used DWI technique is an Echo-Planar-Imaging (EPI) based sequence which suffers greatly from geometric distortion. Therefore, it is not applicable for radiation therapy planning. Turbo-Spin-Echo (TSE) based DWI sequences are proposed due to their great geometric accuracy. However, Signal-to-Noise Ratio (SNR) of DWI-TSE is poor compared to DWI-EPI. The purpose of this work is to evaluate the image quality of different DWI sequences for the use of radiation therapy planning. The evaluation is done by comparing SNR, patient-induced susceptibility effect on geometric distortion, and Apparent Diffusion Coefficient (ADC) correctness of DWI-TSE sequences to the most commonly used DWI-EPI. The selected TSE based sequences are SPLICE and Alsop. The image quality comparison is also done between two radiation oncology products for MRI: Philips Ingenia 1.5T MR-RT and Elekta Unity 1.5T (MR-Linac). In addition, the aim is to see how Compressed SENSE (CS-SENSE) technique affects the image quality of SPLICE. The SNR comparison of DWI-EPI, Alsop, SPLICE, and SPLICE with Compressed SENSE were done both with a phantom and with a volunteer. Same results were observed from both studies. The DWI-EPI has significantly higher SNR than TSE-based DWI. However, the SPLICE achieved nearly square root of two times better SNR compared to the Alsop sequence. The CS-SENSE improved the SNR of SPLICE notably both in a volunteer and phantom studies. In volunteer studies, the SNR of SPLICE with CS-SENSE achieved 45% of the SNR of diagnostic DWI-EPI with Ingenia MR-RT setup, and only 27% with MR-Linac. Therefore, the image quality of diagnostic DWI-EPI can be reached with Ingenia MR-RT setup for SPLICE with CS-SENSE by optimizing the sequence. However, with MR-Linac the same image quality cannot be reached with acceptable acquisition time. The susceptibility-induced geometric distortion of DWI-EPI and SPLICE were analyzed using the same volunteer as in SNR measurements. As a result, the median distortion value for DWI-EPI was 1.5 mm in Ingenia 1.5T and 1.7 mm in MR-Linac 1.5T. Whereas, the median distortion of SPLICE was 0.02 mm in both systems. Therefore, the subject-induced susceptibility effect has an insignificant impact on the geometric accuracy of TSE-based DWI. Thus, DWI-TSE can be used for RTP in favor of geometric accuracy. The ADC value correctness of DWI-EPI, DWI-TSE, and SPLICE were measured using a standardized diffusion phantom. All ADC values of TSE-based DWI were within 14% of the reported literature values. Whereas, the maximum difference in DWI-EPI was 50%. The large variety in the ADC values of DWI-EPI were caused by geometric inaccuracy in the anterior side of the phantom. The minor changes in the ADC values of TSE-based DWI proves that the ADC values are reliable for clinical use.

Ioninesteet ovat suoloja, joilla on matala sulamislämpötila (alle noin 100 °C). Niillä on useita hyödyllisiä ominaisuuksia ja lukuisia mahdollisia sovelluksia. Tarkempi tieto ioninesteiden atomitason rakenteesta on kuitenkin tärkeää niiden ominaisuuksien ja mahdollisuuksien ymmärtämiseksi sekä sovellusten kehittämiseksi. Tässä työssä tutkittiin 1-3-dimetyyli-imidazoliumkloridia ([mmim]Cl), joka on molekyylimassaltaan kevyt prototyyppinen ionineste. Tässä tutkielmassa hyödynnettiin epäelastista röntgensirontaa uuden informaation saamiseksi. Epäelastisessa röntgensironnassa fotoni siroaa elektronisysteemistä luovuttaen sekä energiaa että liikemäärää. Fotonin epäelastista sirontaa kutsutaan Compton-sironnaksi, kun energian- ja liikemääränsiirto on suuri. Compton-sirontaa voidaan käyttää aineen atomi- ja molekyylitason rakenteen tutkimisessa, sillä Compton-sirontakokeissa määritettävä suure, Compton-profiili, on herkkä atomien välisen geometrian muutoksille. Mittaustulosten tulkinta on kuitenkin haastavaa ja laskennallisella mallintamisella on siinä suuri rooli. Tässä tutkielmassa laskettiin [mmim]Cl:n neste- ja kidefaasien isotrooppisten Compton-profiilien erotus (erotusprofiili). Tiettyjen oletusten ollessa voimassa Compton-profiili riippuu elektronien liikemäärätiheydestä, joten profiilit voidaan määrittää aineen perustilaa kuvaavien elektronirakennelaskujen avulla. Tässä tutkielmassa elektronirakennelaskuissa käytettiin Kohn-Sham-tiheysfunktionaaliteoriaa, periodisia reunaehtoja ja Gaussisia kantajoukkoja elektronitiloille. Lisäksi laskennan tarkkuuteen vaikuttavia tekijöitä arvioitiin. Liikemäärähilan tiheydellä sekä vaihto-korrelaatiofunktionaalin ja kantajoukon valinnalla havaittiin olevan suuri vaikutus laskettuun erotusprofiiliin. Nämä tekijät olivat selkeästi merkittävämpiä kuin nesterakenteiden äärellisestä määrästä johtuva tilastollinen epätarkkuus. Erotusprofiilin tulkitsemiseksi kiderakenteesta otettuun yhteen [mmim]Cl-ionipariin tehtiin muutoksia käytetyn nesterakenteen perusteella ja tarkasteltiin näiden muutosten vaikutusta Compton-profiiliin. Sekä molekylääristen ionien sisäisen rakenteen että ionien välisen geometrian muutosten havaittiin vaikuttavan merkittävästi laskettuun erotusprofiiliin. Tässä työssä esitetyt tulokset auttavat kokeellisen erotusprofiiliin tulkinnassa ja selittämisessä.