Introduction Zr-89 is a useful radionuclide for radiolabeling proteins and other molecules.1,2 There are many reports of cyclotron production of 89Zr by the 89Y (p,n) reaction. Most irradiations use thin metal backed deposits of Y and irradiation currents up to 100 µA or thicker amounts of Y or Y2O3 with ~ 20 µA irradiations.3,4 We are working to develop high specific activity 89Zr using a low energy 11 MeV cyclotron. We have found that target Y metal contains carrier Zr and higher specific activities are achieved with less Y. The goal of this work was to optimize yield while minimizing the amount of Y that was irradiated. Material and Methods All irradiations were done using a Siemens Eclipse 11 MeV proton cyclotron. Y foils were used for the experiments described here. Y2O3 was tried and abandoned due to lower yield and poor heat transfer. Yttrium metal foils from Alfa Aesar, ESPI Metals and Sigma Aldrich, 0.1 to 1 mm in thickness, were tested. Each foil was irradiated for 10 to 15 minutes. The targets to hold the Y foils were made of aluminum and were designed to fit within the “paper burn” unit of the Siemen’s Eclipse target station, allowing the Y target body to be easily inserted and removed from the system. Several Al targets of 2 cm diam. and 7.6 cm long were tested with the face of the targets from 11, 26 or 90o relative to the beam to vary watts cm−2 on the foil. The front of the foils was cooled by He convection and the foil backs by conduction to the Al target body. The target body was cooled by conduction to the water cooled Al sleeve of the target holder. Results and Conclusion The best target was two stacked, 0.25 mm thick, foils to stop beam. 92% of the 89Zr activity was in the front 0.25 mm Y foil. With the greatest slant we could irradiate up to 30 µA of beam on tar-get. However, the 13×30 mm dimensions of the foil was more mass (0.41 g) and lower specific activity than was desired. Redesign of the target gave a target 90o to the beam with 12×12 mm foils (0.15 g/foil) that were undamaged with up to 30 µA irradiation when two foils were used. This design has a reduction in beam at the edges of ~10%. With this design, a single Y foil, 0.25 mm thick sustained over 31 µA of beam and a peak power on target of 270 watts cm−2. The product was radionuclidically pure 89Zr after all 89mZr and small amounts of 13N produced from oxygen at the surface had decayed (TABLE 1). Our conclusion is that the optimum target is a single 0.25 mm thick Y foil to obtain the greatest specific activity at this proton energy. This produces 167 MBq of 89Zr at EOB with a 15 minute and 31 µA irradiation. We are continuing to redesign the clamp design to reduce losses at the edge of the beam.

This dissertation aims at designing a three-phase voltage source inverter based on the SiC devices and mainly the SiC-MOSFET. The designed inverter offers a possibility to drive the power inverter with a very high efficiency, which can reach up to 99% for 16 kW rated power. The design is dedicated to the electric vehicle application, and it aims at • Providing a comparative study on some of the current discrete SiC devices in terms of the total losses and the thermal conductivity. In addition, a behavioral study of the effective channel mobility with temperature variation in the SiC MOSFET will be investigated. • Designing a gate driver which fits with the driving requirements of the SiC-MOSFET and provides a trade-off between the switching losses and the EMI behavior. • Designing a three-phase voltage source inverter with 16 kW rated power; the design includes minimizing the inverter losses and extracts the EMI model of the power inverter by considering the effects of the parasitic parameters; moreover a short guideline for selecting the heat-sink based on the static network is introduced. • Proposing a new and simplified carried-based PWM, this will reduce the harmonics in the output waveforms and enhance the utilization of the DC-link voltage. • Proposing a new strategy for compensating the dead-time effect in carrier based-PWM and to find out the proper dead-time level in VSI based on SiC –MOSFET. • Designing faults diagnosis and protection circuits in order to protect the power inverter from the common faults; overcurrent, short-circuit, overvoltage, and overtemperature faults.

This work focuses on a better understanding of the behavior of organic light emitting devices (OLEDs) under intense electrical excitation. Attaining high exciton densities in organic semiconductors by electrical excitation is of special interest for the field of organic semiconductor lasers (OSLs). In these devices, the high singlet exciton density needed in the active layer to obtain population inversion is easily created by pulsed optical pumping, but direct electrical pumping has not been achieved yet. First, the steps necessary to achieve stable high current densities in organic semiconductors are discussed. After determining the optimal excitation scheme using single p-doped transport layers, the device complexity is increased up to full p-i-n OLEDs with their power dependent emission spectra. For this purpose, two exemplary emitter systems are chosen: the fluorescent laser dye 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped into Aluminum(III)bis (2-methyl-8-quinolinato)-4-phenylphenolate (Alq3) and the efficient phosphorescent emitter system N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-benzidine (alpha-NPD) doped by Iridium(III) bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate) (Ir(MDQ)2(acac)). For pulsed excitation using 50 ns pulses and a repetition rate of 1 kHz, single 100 nm thin p- and n-doped transport layers sustain current densities of over 6 kA/cm2. While the maximum current density decreases with increasing device thickness, the full OLEDs still sustain current densities beyond 800 A/cm2 and exhibit a continuously increasing emission intensity with increasing input power. Next, the time-resolved emission behavior of the singlet and triplet emitter device at high excitation densities is analyzed on the nanosecond scale. Here, the peak emission intensity of the phosphorescent emitter system is found to be more than eight times lower than for the singlet emitter system at comparable current densities. The triplet emitter system exhibits a slow rise of the EL after turn-on which prevents the usage of shorter pulses to enable higher current densities. The singlet emitter system, in contrast, exhibits a fast turn-on and reaches the maximum emission intensity within less than 20 ns. By several additional experiments including streak camera measurements and pump-probe experiments, the strong EL overshoot observed in the first few ns is successfully attributed to a reduced emission intensity in the steady state due to singlet-triplet annihilation. Hence, the separation of singlet emission and singlet-triplet quenching in time domain is demonstrated. At 550 A/cm2 and 10 ns pulse rise time, a peak luminance of 1.5E6 cd/m2 is recorded. Finally, the experimental results are validated by modeling the singlet and triplet population dynamics in the emission layer of the fluorescent system to explain the time-resolved emission characteristics. Using a set of rate equations for the polaron density and the singlet and triplet exciton densities, the overshoot in singlet exciton density at the device turn-on is attributed to the separation of singlet emission and triplet quenching in time domain. Furthermore, by fitting the experimental data, the triplet-triplet annihilation rate in the host guest system is shown to become exciton density dependent at sufficiently high excitation density. / Der Schwerpunkt dieser Arbeit liegt auf dem besseren Verständnis des Verhaltens von organischen Leuchtdioden (OLEDs) bei intensiver elektrischer Anregung. Das Erreichen hoher Exzitonendichten in organischen Halbleitern ist insbesondere für organische Halbleiterlaser (organic semiconductor lasers, OSLs) von Interesse. Hierbei werden die für die Inversion benötigten hohen Singulett Exzitonendichten zwar leicht mittels gepulstem optischen Anregen (Pumpen) erreicht, jedoch konnte eine elektrische Anregung bisher noch nicht realisiert werden. Der erste Abschnitt befasst sich mit dem Erreichen von hohen Stromdichten und den dazu nötigen Schritten. Nach dem Ermitteln des optimalen Anregungsschemas an p-dotierten Einzelschichten wird die Komplexität des Systems Schritt für Schritt bis zur kompletten p-i-n OLED erhöht. Hierfür wurden exemplarisch zwei verschiedene Emittersysteme ausgewählt: Aluminum(III)bis (2-methyl-8-quinolinato)-4-phenylphenolate (Alq3) dotiert mit dem fluoreszenten Laserfarbstoff 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) und der effiziente phosphoreszente Emitter Iridium(III)bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate) (Ir(MDQ)2(acac)) in einer N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-benzidine (alpha-NPD) Matrix.Bei gepulster Anregung mit einer Pulsweite von 50 ns und einer Wiederholungsrate von 1 kHz sind in 100 nm dünnen p- und n-dotierten Transportschichten Stromdichten von mehr als 6 kA/cm2 möglich. Der Maximalstrom sinkt mit zunehmender Gesamtschichtdicke ab. Die kompletten p-i-n OLEDs ermöglichen eine Stromdichte von über 800 kA/cm2 und weisen eine kontinuierlich mit der Stromdichte steigende Emissionsintensität auf. Anschließend wird die zeitlich aufgelöste Elektrolumineszenz der Singulett- und Triplett-Emitter OLEDs mit Nanosekunden-Auflösung untersucht. Die phosphoreszente OLED weist hierbei, im Vergleich zur fluoreszenten OLED bei vergleichbarer Stromdichte, eine mehr als achtmal geringere Emissionsintensität auf. Des Weiteren steigt die Emissionsintensität nur langsam an, die maximale Intensität wird erst nach 120 ns erreicht. Dies steht im Widerspruch zum Erreichen höherer Stromdichten mittels kürzerer Pulse. Die fluoreszente OLED hingegen zeigt ein schnelles Ansteigen der Emissionsintensität, die maximale Intensität wird nach weniger als 20 ns erreicht. Anhand von zusätzlichen Untersuchungen kann das beobachtete starke Überschießen der Elektrolumineszenz innerhalb der ersten Nanosekunden einer durch Singulett-Triplett Annihilation reduzierten Emission im Gleichgewichtszustand zugeordnet werden. Diese Experimente dokumentieren somit die zeitliche Trennung von Fluoreszenz und Singulett-Triplett Annihilation. Bei einer Stromdichte von 550 A/cm2 und 10 ns Flankenanstiegszeit wird eine maximale Lumineszenz von 1.5E6 cd/m2 gemessen. Der letzte Abschnitt befasst sich mit der Bestätigung der experimentellen Ergebnisse durch die Simulation der Dynamik von Singulett- und Triplett-Exzitonendichte in der Emissionsschicht. Mit Hilfe eines Satzes von gekoppelten Differenzialgleichungen für die Dichte der Polaronen, Singulett Exzitonen und Triplett Exzitonen lässt sich das Überschießen der Elektrolumineszenz der fluoreszenten OLED eindeutig der zeitlichen Trennung von Singulett Emission und Singulett-Triplett Annihilation zuordnen. Außerdem kann durch das Fitten der experimentellen Daten dargestellt werden, dass die Triplett-Triplett Annihilationsrate in dem untersuchten fluoreszenten Emittersystem bei ausreichend hohen Anregungsdichten eine starke Abhängigkeit von der Dichte der Triplett Exzitonen aufweist.

Die spanende Bearbeitung hat in den letzten Jahren und Jahrzehnten große Fortschritte in den Bereichen der Bearbeitungsgeschwindigkeiten erfahren und hat bereits Einzug in die industrielle Fertigung gehalten. Dies wurde durch Fortschritte in den Bereichen der hochtemperaturfesten Schneidstoffe ermöglicht und vorangetrieben. Für das wissenschaftliche Verständnis der beim Hochgeschwindigkeitsspanen ablaufenden Vorgänge werden exakte Kenntnisse über das mechanische Werkstoffverhalten bei sehr hohen Verformungsgeschwindigkeiten sowie hohen Temperaturen und hohen Verformungen vorausgesetzt. Die Ermittlung der relevanten Werkstoffeigenschaften für die beiden Stähle C45E und 40CrMnMo7 stand im Vordergrund dieser Arbeit. Für die Ermittlung der Werkstoffkenndaten wurden spezielle Prüfaufbauten verwendet, welche die gleichzeitige Ermittlung von Fließspannungen bei hohen Temperaturen, hohen Dehnungsgeschwindigkeiten und großen Formänderungen erlauben. Es wurde gezeigt, dass die Anwendung des Konzeptes der thermischen Aktivierung auch bei extremen Dehnungsgeschwindigkeiten für die beiden untersuchten Werkstoffe möglich ist. Eine Simulation des Spanbildungsvorganges mit den ermittelten Werkstoffmodellen zeigt, dass es möglich ist, sowohl den Prozess der Fließspan- als auch der Segmentspanbildung mit hoher Genauigkeit abzubilden.

Two new mesoporous metal–organic frameworks (DUT-75 and DUT-76) with exceptional ethene uptake were obtained using carbazole dicarboxylate based metal–organic polyhedra as supermolecular building blocks. The compounds have a total pore volume of 1.84 and 3.25 cm³ gˉ¹ and a specific BET surface area of 4081 and 6344 m² gˉ¹, respectively, and high gas uptake at room temperature and high pressure.

This work focuses on a better understanding of the behavior of organic light emitting devices (OLEDs) under intense electrical excitation. Attaining high exciton densities in organic semiconductors by electrical excitation is of special interest for the field of organic semiconductor lasers (OSLs). In these devices, the high singlet exciton density needed in the active layer to obtain population inversion is easily created by pulsed optical pumping, but direct electrical pumping has not been achieved yet. First, the steps necessary to achieve stable high current densities in organic semiconductors are discussed. After determining the optimal excitation scheme using single p-doped transport layers, the device complexity is increased up to full p-i-n OLEDs with their power dependent emission spectra. For this purpose, two exemplary emitter systems are chosen: the fluorescent laser dye 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped into Aluminum(III)bis (2-methyl-8-quinolinato)-4-phenylphenolate (Alq3) and the efficient phosphorescent emitter system N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-benzidine (alpha-NPD) doped by Iridium(III) bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate) (Ir(MDQ)2(acac)). For pulsed excitation using 50 ns pulses and a repetition rate of 1 kHz, single 100 nm thin p- and n-doped transport layers sustain current densities of over 6 kA/cm2. While the maximum current density decreases with increasing device thickness, the full OLEDs still sustain current densities beyond 800 A/cm2 and exhibit a continuously increasing emission intensity with increasing input power. Next, the time-resolved emission behavior of the singlet and triplet emitter device at high excitation densities is analyzed on the nanosecond scale. Here, the peak emission intensity of the phosphorescent emitter system is found to be more than eight times lower than for the singlet emitter system at comparable current densities. The triplet emitter system exhibits a slow rise of the EL after turn-on which prevents the usage of shorter pulses to enable higher current densities. The singlet emitter system, in contrast, exhibits a fast turn-on and reaches the maximum emission intensity within less than 20 ns. By several additional experiments including streak camera measurements and pump-probe experiments, the strong EL overshoot observed in the first few ns is successfully attributed to a reduced emission intensity in the steady state due to singlet-triplet annihilation. Hence, the separation of singlet emission and singlet-triplet quenching in time domain is demonstrated. At 550 A/cm2 and 10 ns pulse rise time, a peak luminance of 1.5E6 cd/m2 is recorded. Finally, the experimental results are validated by modeling the singlet and triplet population dynamics in the emission layer of the fluorescent system to explain the time-resolved emission characteristics. Using a set of rate equations for the polaron density and the singlet and triplet exciton densities, the overshoot in singlet exciton density at the device turn-on is attributed to the separation of singlet emission and triplet quenching in time domain. Furthermore, by fitting the experimental data, the triplet-triplet annihilation rate in the host guest system is shown to become exciton density dependent at sufficiently high excitation density. / Der Schwerpunkt dieser Arbeit liegt auf dem besseren Verständnis des Verhaltens von organischen Leuchtdioden (OLEDs) bei intensiver elektrischer Anregung. Das Erreichen hoher Exzitonendichten in organischen Halbleitern ist insbesondere für organische Halbleiterlaser (organic semiconductor lasers, OSLs) von Interesse. Hierbei werden die für die Inversion benötigten hohen Singulett Exzitonendichten zwar leicht mittels gepulstem optischen Anregen (Pumpen) erreicht, jedoch konnte eine elektrische Anregung bisher noch nicht realisiert werden. Der erste Abschnitt befasst sich mit dem Erreichen von hohen Stromdichten und den dazu nötigen Schritten. Nach dem Ermitteln des optimalen Anregungsschemas an p-dotierten Einzelschichten wird die Komplexität des Systems Schritt für Schritt bis zur kompletten p-i-n OLED erhöht. Hierfür wurden exemplarisch zwei verschiedene Emittersysteme ausgewählt: Aluminum(III)bis (2-methyl-8-quinolinato)-4-phenylphenolate (Alq3) dotiert mit dem fluoreszenten Laserfarbstoff 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) und der effiziente phosphoreszente Emitter Iridium(III)bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate) (Ir(MDQ)2(acac)) in einer N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-benzidine (alpha-NPD) Matrix.Bei gepulster Anregung mit einer Pulsweite von 50 ns und einer Wiederholungsrate von 1 kHz sind in 100 nm dünnen p- und n-dotierten Transportschichten Stromdichten von mehr als 6 kA/cm2 möglich. Der Maximalstrom sinkt mit zunehmender Gesamtschichtdicke ab. Die kompletten p-i-n OLEDs ermöglichen eine Stromdichte von über 800 kA/cm2 und weisen eine kontinuierlich mit der Stromdichte steigende Emissionsintensität auf. Anschließend wird die zeitlich aufgelöste Elektrolumineszenz der Singulett- und Triplett-Emitter OLEDs mit Nanosekunden-Auflösung untersucht. Die phosphoreszente OLED weist hierbei, im Vergleich zur fluoreszenten OLED bei vergleichbarer Stromdichte, eine mehr als achtmal geringere Emissionsintensität auf. Des Weiteren steigt die Emissionsintensität nur langsam an, die maximale Intensität wird erst nach 120 ns erreicht. Dies steht im Widerspruch zum Erreichen höherer Stromdichten mittels kürzerer Pulse. Die fluoreszente OLED hingegen zeigt ein schnelles Ansteigen der Emissionsintensität, die maximale Intensität wird nach weniger als 20 ns erreicht. Anhand von zusätzlichen Untersuchungen kann das beobachtete starke Überschießen der Elektrolumineszenz innerhalb der ersten Nanosekunden einer durch Singulett-Triplett Annihilation reduzierten Emission im Gleichgewichtszustand zugeordnet werden. Diese Experimente dokumentieren somit die zeitliche Trennung von Fluoreszenz und Singulett-Triplett Annihilation. Bei einer Stromdichte von 550 A/cm2 und 10 ns Flankenanstiegszeit wird eine maximale Lumineszenz von 1.5E6 cd/m2 gemessen. Der letzte Abschnitt befasst sich mit der Bestätigung der experimentellen Ergebnisse durch die Simulation der Dynamik von Singulett- und Triplett-Exzitonendichte in der Emissionsschicht. Mit Hilfe eines Satzes von gekoppelten Differenzialgleichungen für die Dichte der Polaronen, Singulett Exzitonen und Triplett Exzitonen lässt sich das Überschießen der Elektrolumineszenz der fluoreszenten OLED eindeutig der zeitlichen Trennung von Singulett Emission und Singulett-Triplett Annihilation zuordnen. Außerdem kann durch das Fitten der experimentellen Daten dargestellt werden, dass die Triplett-Triplett Annihilationsrate in dem untersuchten fluoreszenten Emittersystem bei ausreichend hohen Anregungsdichten eine starke Abhängigkeit von der Dichte der Triplett Exzitonen aufweist.

This dissertation aims at designing a three-phase voltage source inverter based on the SiC devices and mainly the SiC-MOSFET. The designed inverter offers a possibility to drive the power inverter with a very high efficiency, which can reach up to 99% for 16 kW rated power. The design is dedicated to the electric vehicle application, and it aims at • Providing a comparative study on some of the current discrete SiC devices in terms of the total losses and the thermal conductivity. In addition, a behavioral study of the effective channel mobility with temperature variation in the SiC MOSFET will be investigated. • Designing a gate driver which fits with the driving requirements of the SiC-MOSFET and provides a trade-off between the switching losses and the EMI behavior. • Designing a three-phase voltage source inverter with 16 kW rated power; the design includes minimizing the inverter losses and extracts the EMI model of the power inverter by considering the effects of the parasitic parameters; moreover a short guideline for selecting the heat-sink based on the static network is introduced. • Proposing a new and simplified carried-based PWM, this will reduce the harmonics in the output waveforms and enhance the utilization of the DC-link voltage. • Proposing a new strategy for compensating the dead-time effect in carrier based-PWM and to find out the proper dead-time level in VSI based on SiC –MOSFET. • Designing faults diagnosis and protection circuits in order to protect the power inverter from the common faults; overcurrent, short-circuit, overvoltage, and overtemperature faults.

Multiplication is one of the widely used arithmetic operations in a variety of applications, such as image/video processing and machine learning. FPGA vendors provide high-performance multipliers in the form of DSP blocks. These multipliers are not only limited in number and have fixed locations on FPGAs but can also create additional routing delays and may prove inefficient for smaller bit-width multiplications. Therefore, FPGA vendors additionally provide optimized soft IP cores for multiplication. However, in this work, we advocate that these soft multiplier IP cores for FPGAs still need better designs to provide high-performance and resource efficiency. Toward this, we present generic area-optimized, low-latency accurate, and approximate softcore multiplier architectures, which exploit the underlying architectural features of FPGAs, i.e., lookup table (LUT) structures and fast-carry chains to reduce the overall critical path delay (CPD) and resource utilization of multipliers. Compared to Xilinx multiplier LogiCORE IP, our proposed unsigned and signed accurate architecture provides up to 25% and 53% reduction in LUT utilization, respectively, for different sizes of multipliers. Moreover, with our unsigned approximate multiplier architectures, a reduction of up to 51% in the CPD can be achieved with an insignificant loss in output accuracy when compared with the LogiCORE IP. For illustration, we have deployed the proposed multiplier architecture in accelerators used in image and video applications, and evaluated them for area and performance gains. Our library of accurate and approximate multipliers is opensource and available online at https://cfaed.tu-dresden.de/pd-downloads to fuel further research and development in this area, facilitate reproducible research, and thereby enabling a new research direction for the FPGA community.

This paper aims to assess land cover change by high-resolution remote satellite images in Bo Trach district, Quang Binh province. The study used eCognition Developer 9.1 to classify SPOT and Sentinal-2 images of the study area. Objects on the images are characterized by values of Channels, including Red, Green, Blue, NIR, Brightness, NDVI, and RIV. Since then, maps of land cover status in the year of 2005, 2010, and 2017 were created with high accuracy 92.22%, 91.28%, 94.22%, respectively. Overlaid three land cover maps to develop the land cover change maps of two periods 2005-2010 (period 1) and 2010-2017 (period 2). The results show that there is a variation in the area of land cover types, especially agriculture and forest land. Of which, agrarian land increased by 7.7% in period 1 and 9.95% in period 2. Whereas, forest land decreased by 0.6% in period 1 and 1.5% in period 2. / Bài báo này nhằm mục đích đánh giá biến động sử dụng đất bằng viễn thám độ phân giải cao tại huyện Bố Trạch, tỉnh Quảng Bình. Nghiên cứu sử dụng phần mềm eCognition Developer 9.1để phân loại ảnh ảnh SPOT và Sentinal-2 trên địa bàn nghiên cứu. Các đặc trưng của đối tượng trên ảnh được xác định dựa trên giá trị độ sáng các Kênh 1, Kênh 2, Kênh 3, Kênh 4, giá trị độ sáng trung bình (Brightness), chỉ số khác biệt thực vật (NDVI) và tỷ số thực vật (RIV). Từ đó xây dựng được các bản đồ lớp phủ mặt đất các năm 2005, 2010, 2017 với độ chính xác lần lượt là 92,22%, 91,28%, 94.22%. Chồng ghép các bản đồ lớp phủ mặt đất, xây dựng bản đồ biến động sử dụng đất giữa hai thời kỳ 2005-2010 và 2010-2017. Kết quả nghiên cứu cho thấy có sự thay đổi giữa các loại hình lớp phủ gồm: đất nông nghiệp tăng khoảng 7,7% giai đoạn 1 và 9,95% giai đoạn 2. Đất lâm nghiệp giảm khoảng 0,6% giai đoạn 1 và 1,5% giai đoạn 2.

Random sampling is a well-known technique for approximate processing of large datasets. We introduce a set of algorithms for incremental maintenance of large random samples on secondary storage. We show that the sample maintenance cost can be reduced by refreshing the sample in a deferred manner. We introduce a novel type of log file which follows the intuition that only a “sample” of the operations on the base data has to be considered to maintain a random sample in a statistically correct way. Additionally, we develop a deferred refresh algorithm which updates the sample by using fast sequential disk access only, and which does not require any main memory. We conducted an extensive set of experiments and found, that our algorithms reduce maintenance cost by several orders of magnitude.