1,721,079 research outputs found
Locating Energy Levels of Lanthanide Ions in Inorganic Ionic Compounds
No full textMany phosphors nowadays are lanthanide (Ln) activated semi-conductor and insulator materials due to their serviceable luminescent properties. Few years ago, an empirical model for semi-conductor and insulator materials with an Ln activator has been developed that not only explains the optical properties of phosphors but helps as well to find hitherto unknown materials with selected photoluminescence (PL), thermoluminescence (TL) or charge carrier trapping properties. Within the model, such properties are determined by the locations of the 4f and 5d energy levels of Ln dopant ions relative to the conduction band (CB) and the valence band (VB) of the host. One objective of this project was to localize the Ln 4f levels in different materials relative to the VB and the CB by means of temperature- (T-) dependent PL spectroscopy and (wavelength resolved) TL studies. Another objective was to study possible regularities when going from one type of compound to another one. In this context not only own measured data have been analyzed but also different models have been compared with each other which all aim at explaining the very same phenomenon. For the important class of compounds with formula ABO4:Ln3+ (A = rare earth; B = transition metal, P) it will be shown that the most important charge transfer (CT) energies can be approximated by using always the same approach.Radiation, Radionuclides & ReactorsApplied Science
Nonproportionality of inorganic scintillators
No full textA scintillator is a transparent material that emits a flash of light when it absorbs a ?-ray photon or an energetic particle. Scintillation crystals are widely used as spectroscopic detectors of ionizing radiation in nuclear science, space exploration, medical imaging, homeland security, etc. This thesis is about nonproportional response of inorganic scintillators to ionising radiation of different energies. Nonproportionality is the nonlinear dependence of the total light output of the scintillator on the detected amount of ionization energy, i.e., the emitted number of photons/MeV at 10 keV is not necessarily the same as at 100 keV or at 1000 keV. This dependence is due to a scintillation efficiency that depends on the density of the ionization track. The production of secondary electrons during slowing down of the primary electron inside the scintillator is a probabilistic process and may occur in different ways for the same absorbed energy. The dependence of the absolute light yield on the energy of secondary electrons and the probabilistic mechanism of their creation result in variability of the total number of photons produced inside the scintillator. This process leads to broadening of the photoabsorption peak in the pulse height spectrum measured by a scintillation detector.Radiation, Radionuclides & ReactorsApplied Science
Luminescence dating of storm-surge sediment
No full textGeological evidence of storm surges has the potential to provide vital information on storm-surge risk. Sediment from the coastal dunes of the Netherlands contains evidence of extreme floods that occurred before reliable measurements of water level began. For these sediments to be useful in flood-risk analysis, they need to be reliably dated. This thesis investigates the use of Luminescence dating for storm-surge sediment. Luminescence dating is a radiometric dating method, which uses tiny light signals emitted from mineral grains to estimate the time that the grains were deposited. The method is shown to be suitable for dating storm-surge sediment, and other types of flood deposits.Radiation, Radionuclides & ReactorsApplied Science
Aspects that govern the timing resolution of scintillation detectors
No full textA study of fast scintillating materials to be applied in future generations positron emission tomography (PET) scanners. The aspects that govern the timing resolution achievable with scintillating crystals were evaluated. This was studied experimentially, using very advanced high speed facilities like a 100 ps resolution pulsed X-ray generator, as well as by means of simulations using a homemade Monte Carlo based software package.Fundamental Aspects of Materials and EnergyApplied Science
Engineering the electronic structure of lanthanide based materials
No full textLanthanide based materials are used in everything from phosphors for light bulbs and LEDs, to scintillators for medical imaging purposes to magnets. They also show potential for spintronics and for increasing the efficiency of solar cells. Knowing the energy of the lanthanide 4f and 5d states is important for understanding the electronic structure and thus the optical properties of lanthanide based materials. In this thesis it is shown that an empirical model used to predict the electronic structure of lanthanide doped compounds can be used to predict the electronic structures of fully concentrated lanthanide based compounds. An example is then given that shows experimentally the systematic behaviour observed. It is also shown that it is possible to engineer the energy difference between the lanthanide 4f and 5d states in a lanthanide based compound. This may be done mechanically, by applying an external perturbation, such as by inducing stresses in an SmS thin film by polishing, or chemically, by alloying with a similar compound, in this case EuO with EuN, or by changing the stoichiometry of the compound, as in the case of Sm rich SmS.Radiation, Radionuclides & ReactorsApplied Science
New halide scintillators for gamma ray detection
No full textScintillators are used for the detection of ionizing radiation. Despite decades of intensive search and numerous compounds discovered, there is still a need for materials with improved properties. Recently, several new scintillators with excellent light yield, energy resolution, and proportionality have been discovered. Among them are Eu2+ doped SrI2 and CsBa2I5 first reported in Refs. [1, 2], and LaBr3:Ce improved by various co-dopants discovered in this thesis work. These materials were studied with the goal to understand the mechanism of the scintillation processes, and in order to improve the scintillation properties. The results of these studies are presented in this thesis. Table I summarizes the characteristics of the studied scintillators. Table I. Overview of inorganic scintillators for X-ray and ?-ray detection and their characteristics: density (?), effective atomic number Zeff, light yield, energy resolution (R), principal decay (?), and emission wavelength (?). Scintillator Co-dopant ? (g/cm3) Zeff Light yield (ph/MeV) R (% at 662 keV) ? (ns) ? (nm) LaBr3:5%Ce3+ - 5.3 46.9 76000 2.7 15 370 Li+ 78000 2.7 16 370 Na+ 73000 2.7 17 370 Mg2+ 73000 3.0 15 370 Ca2+ 71000 2.9 18+long 370 Sr2+ 78000 2.05** 18+long 380 Ba2+ 69000 3.7 17+long 370 Scintillator Eu2+ (%) SrI2 0 4.6[1] 50.3 43000 5.5 450 500-650 0.5 61000 3.7 700* 430 0.86 74000 3.3 800* 430 2 90000 2.8 1000* 430 5 85000 3.1 1200* 430 CsBa2I5 0 4.8[2] 54.1 22000 9.6 150 400-600 0.5 62000 5.1 700* 430 5 80000 2.3** 1200* 430 *for 1-3 mm thick samples at 295 K **measured with a super bialkali R6231-100 PMT In chapter 4, we for the first time demonstrate that several scintillation properties of LaBr3:5%Ce3+ can be significantly improved by Ca2+, Sr2+, or Ba2+ co-doping. Energy resolution of LaBr3:5%Ce,Sr shows record low values of 2% at 662 keV and 6.5% at 59.5 keV. The proportionality of the 8 keV- 1.33 MeV ?-ray response approaches the ideal response much closer as compared to standard LaBr3:5%Ce. The origin of these improvements is a better proportionality of the ~0.5-30 keV electron response. Ca2+, Sr2+, or Ba2+ co-doping creates charge carrier trapping centers in LaBr3:Ce. This results in multiple thermoluminescence glow peaks at temperatures below 350 K, ?s-long decay time components at 295 K, and decrease of the light yield at temperatures below 295 K. Li+, Na+, and Mg2+ co-dopants neither improve proportionality, nor create charge carrier traps. Chapter 5 concerns optical spectroscopy and decay time studies of LaBr3:5%Ce,Sr. Three different Ce3+ lattice sites are revealed. Ce3+ on an unperturbed site has the same optical properties as Ce3+ in standard LaBr3:Ce. The 4f?5d1 excitation and 5d1?4f emission bands of Ce3+ on the perturbed sites are red-shifted. The Ce3+ emission decay time in LaBr3:Ce,Sr is longer than in standard LaBr3:Ce. This is ascribed to a longer radiative lifetime of the excited state of Ce3+ on the perturbed sites and to self-absorption of Ce3+ emission. Lowering of the 5d1 level is attributed to a larger crystal field interaction on the perturbed sites. Two types of the crystal lattice point defects on the perturbed sites were proposed to explain the obtained results: the Br- vacancy and an interstitial site (0,0,z) occupation. A presence of Ce4+ ions as charge compensation centers for Sr2+ on La3+ sites was not confirmed. The studies presented in chapters 4 and 5 are a first attempt to explain the effects of co-dopants on the scintillation properties of LaBr3:Ce. Several issues require further investigation. First, we expect that by varying the co-doping concentration or by using different type of co-dopants the proportionality can be further engineered towards the ideal one. Second, the origin of the proportionality improvement remains unclear. Chapters 6 to 9 deal with Eu2+ doped scintillators. In chapters 6 and 7, the scintillation properties and self-absorption of SrI2:Eu are treated, and in chapter 9 those of CsBa2I5:Eu. Both scintillators possess very high light yield, excellent energy resolution and proportionality, but moderate density and Zeff. The decay time is strongly affected by self-absorption of Eu2+ emission. For example, the decay time constant lengthens from 400 ns for a 1 mm thick SrI2:5%Eu crystal at 78 K to 8 ?s for a 7.5 mm thick crystal at 600 K. The self-absorption red-shifts and narrows the Eu2+ emission peak. The light yield and energy resolution are strongly affected by temperature, sample size, and Eu concentration as well. To explain the obtained results, the self-absorption model was developed and applied to the experimental data. The self-absorption probability a and quantum efficiency ? of Eu2+ emission were derived from the model. a varies from 0.05 to 0.98 depending on temperature (78-600K), Eu concentration (0.5-5%), and sample thickness (1-7mm). ? was found to be 0.95 ± 0.05 for temperatures up to 600 K. The radiative lifetime of the Eu2+ 4f65d excited state is 350-400 ns in both SrI2:Eu and CsBa2I5:Eu. Worsening of light yield and energy resolution with increase of temperature and sample size can be attributed to self-absorption of Eu emission in combination with ?<1. Losses due to electron-hole recombination processes prior to excitation of Eu2+ and to poor optical crystal quality and light collection efficiency cannot be excluded. The analysis of energy resolution of SrI2:Eu in chapter 8 shows that it is also affected by the proportionality of the SrI2:Eu response. That proportionality varies with temperature, and it is optimal at 295 K. Chapters 6, 7, and 9 present a simple model of self-absorption that explains the obtained results fairly well. Nevertheless, several aspects require further research. First, undoped SrI2 and CsBa2I5 compounds both emit broad bands at 360-400 nm and 500-600 nm. The origin of these bands and the mechanisms of energy transfer from the host lattice to Eu2+ remain unclear. Second, for the application of large sized SrI2:Eu and CsBa2I5:Eu crystals one needs to reduce the effects of self-absorption. The search for methods other than the decrease of the sample size, Eu concentration, or temperature remains an important task. [1] N. J. Cherepy, G. Hull, A. D. Drobshoff, et al., Appl Phys Lett 92 (2008) 083508. [2] E. D. Bourret-Courchesne, G. Bizarri, R. Borade, et al., Nucl Instr Meth A 612 (2009) 138.Radiation, Radionuclides & ReactorsApplied Science
Deposition of luminescent thin films for solar energy applications
No full textPhotovoltaic devices are a widely available, long lasting means of generating sustainable energy. Unfortunately, the integration of such devices into society is to date still limited. This is in part due to the much less than optimal efficiency of conversion of sunlight to electricity, but also by the physical limitations of integrating photovoltaic devices into the built environment. In this thesis, efficiency enhancement of solar energy conversion by application of thin film luminescent spectral conversion materials is discussed on the one hand and on the other hand the development of thin film materials for luminescent solar concentration that lead to new means of building-integration of photovoltaic devices. Currently available reports on spectral shifting materials for efficiency enhancement of solar cells is mostly limited to fabricating luminescent powder materials that could possibly serve as spectral shifting materials and modelling the efficiency enhancement. The work that was discussed in this thesis has the common goal of developing experimental methods to make thin films by reactive magnetron sputtering that may be applied as efficiency-enhancing coatings for photovoltaic devices. While the fabrication process is still prone to various complications, magnetron sputtering has proved a viable technique for luminescent thin film deposition.Radiation, Radionuclides & ReactorsApplied Science
LaBr3 gamma?ray spectrometers for space applications
No full textLaBr3 has been developed into large volume scintillator detectors within an ESA and TU Delft programme during this thesis work. The programme, which aimed at the space applications of LaBr3, also led to extensive experiments within a collaborative framework which included representatives for all the development aspects, i.e., from crystal growers to scintillator materials researchers to final users. The research presented in this thesis is the result from my in-depth involvement in that programme. The thesis’s first achievement was the assessment of the radiation tolerance of LaBr3 in Chapter 2. In fact, BepiColombo’s payload must withstand proton fluence as high as 6 × 1010 protons/cm2 to guarantee successful scientific observations. It was found that LaBr3 exceeds that requirement without substantial deterioration of its performance. Such an ability, together with the relative low mass and power resources demand that is typical for scintillation detectors, has made LaBr3 the choice for the BepiColombo onboard gamma-ray spectrometer. LaBr3 detectors is a new technology, and experimental studies were necessary to support instrument design and optimization for space applications. The deterioration of performance with increased detector size has been closely monitored in Chapter 4. The limits of LaBr3 application were also studied and, with the use of synchrotron radiation, it was found that LaBr3 is a non-ideal scintillator for photon detection in the X-ray domain, below 100 keV. The same technique proved to be an effective tool for non-proportionality studies and extensively applied by TU Delft colleagues. Scintillation readout of LaBr3 pushes the PMT to its operational limits. Experimental campaigns were performed to study the response to high energy gamma-rays as reported in Chapter 5. This gave a unique opportunity to observe the correlation between gamma-ray energy and energy resolution up to 15 MeV. In addition, it offered the benchmark to verify in flight energy calibration capability. As demonstrated in Chapter 5 of this thesis, results are satisfactory, which makes LaBr3 applicable for the BepiColombo mission. However the results also suggest to initiate new programmes to develop alternative scintillation readout techniques as with silicon Photomultipliers and Silicon Drift Detectors. The major disadvantage of LaBr3 is a lack of sensitivity because of the intrinsic activity generated by 138La decays, in particular at 1.4 MeV, which is relevant for the detection of 40K. Full characterization of this activity in Chapter 6 led to the first experimental determination of the low-energy end of a second-order-unique-forbidden ? continuum. A deviation with the standard theoretical models on nuclear decay was found of which an explanation is not at hand yet. Development of alternative materials able to challenge LaBr3 for energy resolution and efficiency but with much reduced intrinsic activity is presently ongoing. A selection of recent results achieved with CeBr3 spectrometers were presented in Chapter 7, showing that CeBr3 detection sensitivity at 1.4 MeV is about 8 times higher compared to LaBr3.Radiation Science & TechnologyApplied Science
Lanthanide 4f-level location in AVO4:Ln3+ (A = La, Gd, Lu) crystals
No full textThe spectral properties of LaVO4, GdVO4 and LuVO4 crystals doped with Ce3+, Pr3+, Eu3+ or Tb3+ have been investigated in order to determine the position of the energy levels relative to the valence and conduction bands of the hosts along the trivalent and divalent lanthanide series. Pr3+ and Tb3+ ground state levels are positioned based on the electron transfer energy from those states to the conduction band, the so-called intervalence charge transfer (IVCT). This approach is compared with an alternative model that is based on electron transfer from the valence band to a lanthanide.RRR/Radiation, Radionuclides and ReactorsApplied Science
Lanthanide doped spectral conversion materials for solar cells
No full textRadiation, Radionuclides & ReactorsApplied Science
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