Browsing by master's degree program "Master's Programme in Materials Research"

Monienerginen röntgensäde kokee kovenenemista aineessa. Yleisimmin röntgenpaneelien kalibroinnissa käytetty flat-field (FF)-korjaus on riittämätön säteen kovenemisen aiheuttamien kuvavääristymien korjaamiseen. Signal-to-thickness Calibration (STC)-menetelmässä kuvataan useita paksuuksia väliainetta ja luodaan pikselikohtainen sovitus säteen havaitun intensiteetin ja väliaineen välillä.Tämä sovitus ottaa sekä pikselikohtaiset vaste-erot, että säteen kovenemisen huomioon. Tässä tutkimuksessa arvioitiin STC-menetelmän kykyä parantaa kuvanlaatua verrattuna FF-korjattuun rekonstruktioon käyttäen kliinistä pään alueen kartiokeilatomografialaitetta. Kalibrointimateriaalina toimi akryylimuovi (PMMA). Kuvanlaatua arvioitiin kontrastin, kontrasti-kohina-suhteen, paikallisen kohinan ja cupping-ilmiön voimakkuuden sekä rengaskuvavääristymien arvioinnin avulla. Tutkimus avaa STC-menetelmän toimivuutta eri kudoskohteille hyväksikäyttäen fantomia ja kudosekvivalentteja materiaaleja. Tutkimuksessa tarkasteltiin sekä kovia että pehmeitä kudoksia. Kovina kudoksina käytettiin neljää eri konsentraatiota kalsiumhydroksiapatiittia (CaHA) sekä kolmea konsentraatiota jodia. Pehmeitä kudosekvivalentteja materiaaleja olivat aivot, imusolmukkeet, veri,rasvakudos, maksa, keuhko ja lihas. Fantomina toimi QRM Spectral Phantom II. Kun vertaillaan STC-korjattuja rekonstruktioita FF-korjattuihin, sekä kontrastin että kontrasti-kohina-suhteen havaittiin paranevan noin 27-30% aivo-, imusolmuke- ja lihaskudoksilla. Suurin kasvu tapahtui 80 kV:n putkijännitteellä. Ainoa kudosekvivalentti materiaali, jolle ei havaittu systemaattista kontrastin paranemista oli rasvakudos. Muissa pehmeissä kudoksissa kasvua tapahtui n.5%:sta 20%:iin. Koville kohtioille STC-menetelmän havaittiin myös kohentavan kontrastia n.4% - 9%. Cupping-ilmiön voimakkuuden heikkenemistä havaittiin systemaattisesti kaikilla kuvausparametreilla. Suurin heikkeneminen tapahtui kuitenkin intuitiivisesti matalimmalla putkijännitteellä 80 kV. Rengaskuvavääristymien voimakkuutta arvioitiin subjektiivisesti, eikä niiden havaittu heikkenevän merkittävästi. Tulokset antavat viitteitä kohenneesta pehmeän kudoksen kontrastista, joka on yksi suurimmista rajoittavista tekijöistä kartiokeilatietokonetomografiassa. Kudosten HU-arvoissa päästiin STC-korjatuilla rekonstruktoilla lähemmäksi todellisia kudosten HU-arvoja.

Several nuclear power plants in the European Union are approaching the ends of their originally planned lifetimes. Extensions to the lifetimes are made to secure the supply of nuclear power in the coming decades. To ensure the safe long-term operation of a nuclear power plant, the neutron-induced embrittlement of the reactor pressure vessel (RPV) must be assessed periodically. The embrittlement of RPV steel alloys is determined by measuring the ductile-to-brittle transition temperature (DBTT) and upper-shelf energy (USE) of the material. Traditionally, a destructive Charpy impact test is used to determine the DBTT and USE. This thesis contributes to the NOMAD project. The goal of the NOMAD project is to develop a tool that uses nondestructively measured parameters to estimate the DBTT and USE of RPV steel alloys. The NOMAD Database combines data measured using six nondestructive methods with destructively measured DBTT and USE data. Several non-irradiated and irradiated samples made out of four different steel alloys have been measured. As nondestructively measured parameters do not directly describe material embrittlement, their relationship with the DBTT and USE needs to be determined. A machine learning regression algorithm can be used to build a model that describes the relationship. In this thesis, six models are built using six different algorithms, and their use is studied in predicting the DBTT and USE based on the nondestructively measured parameters in the NOMAD Database. The models estimate the embrittlement with sufficient accuracy. All models predict the DBTT and USE based on unseen input data with mean absolute errors of approximately 20 °C and 10 J, respectively. Two of the models can be used to evaluate the importance of the nondestructively measured parameters. In the future, machine learning algorithms could be used to build a tool that uses nondestructively measured parameters to estimate the neutron-induced embrittlement of RPVs on site.

Ultrasonic transducers convert electric energy into mechanical energy at ultrasonic frequencies. High-power ultrasound is widely used in the industry and in laboratories e.g. in cleaning, sonochemistry and welding solutions. To be effective in these cases, a piezoelectric transducer must deliver maximal power to the medium. Most of these systems rely on having the power delivery maximized during long driving sequences where stable performance is critical. Power ultrasonic transducers are typically narrowband, featuring high Q-value, that are finely tuned to a specific resonance frequency. The resonance frequency can vary during driving due to temperature, mechanical loading and nonlinear effects. When the transducers resonance frequency changes, drastic changes in its impedance (resonance to anti-resonance) can lead quickly to damage or failure of the driving electronics or the transducers themselves. In this work we developed a multi-channel high-power ultrasonic system with a software-based resonance frequency tracking and driving frequency control. The implementation features a feedback loop to maximize power delivery during long driving sequences in an ultrasonic cleaning vessel. The achieved total real power increased from 6.5 kW to almost 10 kW in peak with our feedback loop. The feedback loop also protected the electronics and transducers from breaking due to heating and varying impedance.

Scanning probe microscopy (SPM), which includes atomic force microscopy (AFM), is an affordable and powerful tool for investigating surfaces. However, to ensure accuracy, a metrologically informed approach is required. In this thesis, a commercial Jupiter XR AFM was used for calibration experiments testing its reliability and accuracy. AFMs can further be improved by the use of active probes with added capabilities that allow for faster scanning speeds and other improvements. Efforts have been made to enable their broader use. At VTT MIKES, the commercial Jupiter XR AFM is being modified to use active probes capable of self-sensing and self-actuation. A simple AFM was first built as a test setup to better understand the probes and to split their integration into the Jupiter AFM into a step-by-step process where the mechanical, electronic, and software changes necessary could be separated into distinct parts. In this thesis, I describe experiments done to test the reliability of the commercial AFM, and the process of constructing and using the test setup AFM, after which I present some results obtained using them, and discuss the experiments and the integration process. Included is also an overview of metrology and some of the physics and other theory relevant to AFM, and a summary of the principles of AFM and its role in microscopy. This work is part of the MetExSPM project, which seeks to develop traceable high speed scanning probe microscopy, for example by achieving higher scanning speeds and larger scanning areas, while maintaining good resolution and metrologically traceable high accuracy. This will greatly increase the utility of SPMs, especially for industrial applications.

The NV⁻ center is an optically active defect consisting of a nitrogen atom next to a vacancy with a trapped electron. The defect is ideally suited for many quantum applications and thanks to its long coherence time at elevated temperatures, it can be used as quantum bits for quantum information processing even at room temperature. The defect is found in diamond, which is an exemplary host to a wide range of optically active defects due to its unique properties. As nitrogen is the most common impurity in diamond, NV centers occur naturally but at concentrations that are often deemed insufficient for applications. Thus, understanding the formation process of NV centers and how to efficiently produce them with high spatial resolution is of great interest. In this thesis, the formation of NV centers through irradiation has been studied both in the nuclear and electronic stopping power regimes with molecular dynamics. The analysis of the atomic configurations and displacement resulting from the irradiation revealed two different formation mechanisms, in which either a carbon vacancy is created next to a nitrogen atom or a nitrogen atom becomes mobile to be trapped by a vacancy. While the probability of NV formation from irradiation alone was shown to be low in the nuclear regime, two-temperature molecular dynamics simulations of swift heavy ions in the electronic regime showed the direct formation of NV-centers along the ion’s path. By producing NV centers along an almost one dimensional chain, swift heavy ions offer high spatial resolution in addition to high conversion rates from nitrogen to NV centers due to their high energies.

Omnidirectional microscopy (OM) is an emerging technology capable of enhancing the threedimensional (3D) microscopy widely applied in life sciences. In OM, precise position and orientation control are required for the sample. However, the current OM technology relies on destructive, mechanical methods to hold the samples, such as embedding samples in gel or attaching them to a needle to permit orientation control. A non-contacting alternative is the levitation of the sample. But, until now, the levitation methods have lacked orientation control. I enable omnidirectional access to the sample by introducing a method for acoustic levitation that provides precise orientation control. Such control around three axes of rotation permits imaging of the sample from any direction with a fixed camera and subsequent 3D shape reconstruction. The control of non-spherical particles is achieved using an asymmetric acoustic field created with a phase-controlled transducer array. The technology allows 3D imaging of delicate samples and their study in a time-lapse manner. I foresee that the described method is not only limited to microscopy and optical imaging, but is also compatible with automated sample handling, light-sheet microscopy, wall-less chemistry, and noncontacting tomography. I demonstrate the method by performing a surface reconstruction of three test samples and a biological sample. In addition, a simulation study and the levitation of test samples were used to characterize the levitation technique's performance. Both the shape reconstruction and orientation recovery were done by a computer vision based approach where the different images are stitched together. The results show the rotation stability and the wide angle range of the method.

Electroencephalography (EEG) is a non-invasive neurophysiological method for evaluating brain activity by measuring electrical potential at the scalp. The electrical potentials originate mainly from postsynaptic cortical currents created by neuronal activity. It is a valuable tool for both research and clinical practice. EEG can be used e.g. to diagnose epilepsy, focal brain disorders, brain death, and coma. Intermittent photic stimulation (IPS) is an important tool in clinical EEG. Healthcare professionals use it to induce epileptic activity in patients to help diagnose their conditions. In these tests, various IPS frequencies are used with eyes-closed, eyes-open, and eye-closure conditions. IPS test is listed in clinical practice guidelines in EEG globally, and it is mainly used to diagnose photosensitive epilepsy, i.e., to detect epilepsy-related abnormal sensitivity to flickering light. Magnetoencephalography (MEG) is a non-invasive neurophysiological method in which minute magnetic fields — produced by the same postsynaptic currents as in EEG — are measured with special superconductive sensors around the head. MEG is a valuable tool for research and clinical practice with increasing world-wide utilization. The main advantages of MEG over EEG are easier source modelling and higher resolution at cortical areas. IPS has not been introduced to MEG since the IPS stimulators used in EEG are not compatible with MEG. IPS in MEG could improve the analysis of IPS and provide better tools for diagnoses. Currently, data analysis of IPS is typically limited to healthcare professionals examining the visualization of the raw data while looking for induced epileptiform activites and lateralizing them. In this thesis, an MEG-compatible IPS stimulator is introduced and alternative ways of analyzing IPS data for both MEG and EEG are showcased. Although analysis methods were applied with decent signal-to-noise ratios, further research is needed—especially to compare responses between patients with epilepsy and healthy subjects.

Radiation therapy is one of the key treatments for cancer, utilizing ionizing radiation to destroy cancer cells. Proton therapy uses high-energy proton beams since protons have a favorable depth-dose curve. Clinical proton beams must meet strict quality standards in order to maximise the efficacy of the treatment while ensuring the patient safety. Real-time knowledge of the beam’s intensity profile is essential for an accurate beam delivery. While gas-filled ionization chambers have traditionally been used as the standard beam monitor, the swift development of the beam delivery techniques demands for more accurate beam monitors. Semiconductor detectors potentially offer more accurate and efficient alternative for ionization chambers. In this study, the feasibility of using a silicon pixel detector in proton beams was investigated. The detector was originally designed for tracking minimum ionizing particles at the CMS experiment at CERN. Two experiments — one with an alpha source and one in a proton beam — were carried out to characterize the detector. The response to protons with different intensities and energies was investigated more closely in the proton beam. The results show that the detector response to different proton energies agrees with theoretical expectations. The saturation of the pixels limits measuring the full energy of the protons, however measuring the full energy is not essential in beam profile measurements. The detector also has a linear response to the beam intensity, although, the counting efficiency of the detector should be improved with new readout electronics. With different readout electronics, the detector might be a viable option for the beam profile measurements in clinical proton beams.

Particle Induced X-ray Emission (PIXE) was originally introduced as an ion-beam analytical technique in Lund in the 1970s and has since then been part of the available techniques in many laboratories around the world. The external beam PIXE set-up is used in probing the annual tree rings. The goal is to see the effects of volcanic eruption activities from the perspectives of tree plants here in Finland. In the theory part, I tried to include the description of how volcanoes are formed and created with a bit of volcanic activity history, the growth metabolism mechanism in tree plants and characteristics x-ray productions. The two tree sample used for this experiment were gotten from two different regions of Finland. The first tree is a Pine tree from Parikkala(a small place near Savolinna) in the south-eastern part of Finland and the second tree is a Spruce tree from Pielavesi (place near Kuopio) in the central part of Finland. These samples were carefully prepared for ionisation. The collected spectra data were analysed in a software called PyMCA. PyMCA has been developed by the Software Group of the European Synchrotron Radiation Facility (ESRF). PyMCA is a ready to use and in many aspects state-of-the-art, set of applications implementing most of the needs of X-ray fluorescence data analysis. PyMCA is use to interpret X-ray fluorescence spectra from a diverse array of samples

Chiral assemblies of metal nanoparticles absorb and/or scatter left and right handed circularly polarized light with different intensities usually from the visible light spectral region. This difference in the absorption called circular dichroism (CD) and closely related anisotropy factor (g-factor), which is the CD spectrum normalized with the overall absorption, describe the optical activity of the chiral assemblies. The aim of this thesis was to study and optimize the different structural parameters affecting the g-factor of a chiral gold nanorod (AuNR) dimer to reach the highest possible value. The structure consisted of two AuNRs bound together with a DNA origami in a crossed fingers conformation. The properties studied were silver as a coating material of the AuNRs, the dimensions of the AuNRs, angle between the long axes of the AuNRs and the interparticle distance. The dimensions comparison was studied with different sized AuNRs, the angle was controlled by changing the DNA strands working as a bridge between the two bundles in the DNA origami and the distance between the AuNRs was controlled by the length of the thiol treated DNA strands used for the AuNR binding to the origami. The experiments showed that the best g-factor was achieved with 33×74 nm sized AuNRs with an angle of approximately 55° and an interparticle distance of 24nm. Optimized assembly made a notable increase in the g-factor from 0.05 to 0.12. This is a highest g-factor recorded in a AuNR dimer structure up to date and thus the assembly could be of great use in the chiral sensing field in the future.