Qucosa – Hemholtz-Zentrum Dresden-Rossendorf
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Wasserstoffbrennen in der Sonne: Die 12C(p,γ)13N-Reaktion und die Radiofrequenz-Ionenquelle für den Felsenkeller-Beschleuniger
Full text linkDie Reaktion 12C(p,γ)13N bestimmt die Rate des Bethe-Weizsäcker-Zyklus in der anfänglichen Entwicklungsphase von Sternen und am äußeren Rand der Sonne. Eine genaue Kenntnis der Reaktionsrate ist somit für die Entwicklung von stellaren Modellen erforderlich. Über das Verhältnis der Raten von den Protoneneinfangreaktionen von 12C und 13C kann außerdem das entsprechende Isotopenverhältnis in Sternen bestimmt werden. Eine Revision der Rate von 12C(p,γ)13N könnte damit einen unerwartet hohen Isotopenanteil von 13C erklären, der in verschiedenen Meteoriteneinschlüssen gemessen wurde und mit den existierenden stellaren Modellen nicht hinreichend in Konsistenz gebracht werden kann.
Für den S-Faktor der Reaktion existieren im Energiebereich unterhalb von 190 keV nur Messdaten aus den 1950er Jahren. Bei der Untersuchung von ähnlichen Reaktionen des Wasserstoffbrennens wurden die mit der verwendeten Messtechnik erlangten Messdaten durch moderne Experimente teilweise um einen Faktor zwei oder höher revidiert.
Ziel der gegenwärtigen Arbeit war das Messen von S-Faktor-Werten in einem weiten Energiebereich von 130 keV bis 450 keV zur Überprüfung der alten Messdaten und um eine zukünftige präzisere Extrapolation zu astrophysikalisch relevanten Energien hin zu ermöglichen. Dabei wurde eine Messung in inverser Kinematik, eine Methode, für die bisher keine publizierten Daten zu der Reaktion existieren, am HZDR 3 MV Tandetron Beschleuniger durchgeführt mit TiH2-Proben, die mit 12C2+-Ionen bestrahlt wurden. Die Reaktion wurde mittels Gammaspektrometrie ausgewertet und die Proben durch die Methode der Nuklearen Resonanz-Reaktionsanalyse charakterisiert.The reaction 12C(p,γ)13N determines the rate of the Bethe-Weizsäcker cycle in the initial development phase of stars and near the surface of the Sun. An exact knowledge of the reaction rate is thus required for the development of precise stellar models. In addition, the ratio of the rates of the proton capture reactions of 12C and 13C is used to determine the corresponding isotopic ratio in stars. A revision of the rate of 12C(p,γ)13N might help to explain an unexpectedly high isotopic abundance of 13C, which was measured in presolar grains and cannot be sufficiently explained with the existing stellar models.
For the S-factor of 12C(p,γ)13N in an energy range below 190 keV, the only existing data were measured in the 1950s. For similar reactions of hydrogen burning, data obtained with these measuring techniques were revised by a factor of two or higher by modern experiments.
The aim of the present thesis was to measure S-factor data in a wide energy range from 130 keV to 450 keV in order to verify the old data and to allow a more precise extrapolation towards astrophysically relevant energies in the future. A measurement in inverse kinematics, a method for which no published data on the reaction exist, was performed at the HZDR 3 MV Tandetron accelerator with a 12C2+ ion beam and the use of TiH2 targets. Gamma spectroscopy was used to measure the yield and the targets were characterized with nuclear resonant reaction analysis (NRRA)
Measuring sub-femtosecond temporal structures in multi-ten kiloampere electron beams
Full text linkIn laser wakefield acceleration, an ultra-short high-intensity laser pulse excites a plasma wave, which can sustain accelerating electric fields of several hundred GV/m.
This scheme advances a novel concept for compact and less expensive electron accelerators, which can be hosted in a typical university size laboratory. Furthermore, laser wakefield accelerators (LWFA) feature unique electron bunch characteristics, namely micrometer size with duration ranging from several fs to tens of fs. Precise knowledge of the longitudinal profile of such ultra-short electron bunches is essential for the design of future table-top X-ray light-sources and remains a big challenge due to the resolution limit of existing diagnostic techniques. Spectral measurement of broadband coherent and incoherent transition radiation (TR) produced when electron bunches passing through a metal foil is a promising way to analyze longitudinal characteristics of these bunches. Due to the limited reproducibility of the electron source this measurement highly requires single-shot capability.
An ultra-broadband spectrometer combines the TR spectrum in UV/NIR (200-1000 nm), NIR (0.9-1.7 µm) and mid-IR (1.6-12 µm). A high spectral sensitivity, dynamic bandwidth and spectral resolution are realized by three optimized dispersion and detection systems integrated into a single-shot spectrometer. A complete characterization and calibration of the spectrometer have been done concerning wavelengths, relative spectral sensitivities, and absolute photometric sensitivities, also taking into account for the light polarization. The TR spectrometer is able to characterize electron bunches with charges as low as 1pC and can resolve time-scales of 0.4 fs. Electron bunches up to 16 fs (rms width) can be reconstructed from their TR spectrum. In the presented work, the self-truncated ionization induced injection (STII) scheme has been explored to study the relevant beam parameters especially its longitudinal bunch profile and the resulting peak current
Testing RPC Performance with Discharges Ignited by UV Laser Pulses: Precise measurement of gas parameters in approximately realistic RPC configurations
Full text linkThis thesis is devoted in two associated topics: a unique laser facility for researches of gaseous detectors; the investigations of Resistive Plate Chamber (RPC) detectors and the measurement of gas parameters in a realistic condition of timing RPC.
A pulsed UV laser test facility has been assembled in HZDR. The focus of pico-second laser pulses is placed in a specific position in a gaseous detector sample to produce laser plasma, where free electrons are generated in ionizations with well defined number, micro-meter spatial accuracy in a volume of micro-meter scale. It provides a method, independent from accelerators, to make investigations with gaseous detectors in a laboratory.
Samples of RPC detectors are designed and assembled for experiments with the laser test facility. Methods are developed to acquire the waveforms of electron avalanches for different drift lengths and to obtain the key gas parameters: the effective Townsend coefficient and the electron drift velocity. We have succeeded in the direct measurement of gas parameters at the field strength of timing RPC under atmospheric pressure for the first time in experimental conditions.
The research has obtained different achievements. The laser test facility is proven to be qualified for the measurement of gas parameters, and has a potential to contribute to the eco-gas research for future RPC. The possible measurement range of electric field of gas parameter at atmospheric pressure is extended by a factor of two, from the range of trigger RPC to timing RPC. The results of experiments have revealed some fundamental mechanisms, which will extend the understanding of RPC performance and electron avalanche process
The study and development of pulsed high-field magnets for application in laser-plasma physics
Full text linkThe thesis at hand addresses design, characterization and experimental testing of pulsed high-field magnets for utilization in the field of laser-plasma physics. The central task was to establish a technology platform that allows to manipulate laser-driven ion sources in a way that the accelerated ions can be used in complex application studies, e.g. radiobiological cell or tumor irradiation.
Laser-driven ion acceleration in the regime of target normal sheath acceleration (TNSA) offers the unique opportunity to accelerate particles to kinetic energies of few 10MeV on the micrometer scale. The generated bunches are short, intense, show broad exponentially decaying energy spectra and high divergence. In order to efficiently use the generated particles, it is crucial to gain control over their divergence directly after their production. For most applications it additionally is favorable to reduce the energy spread of the beam. This work shows that the developed pulsed high-field magnets, so-called solenoids (cylindrical magnets), can efficiently capture, transport and focus laser-accelerated protons. The chromaticity of the magnetic lens thereby provides for energy selection.
Three prototype solenoids, adapted to fit different application scenarios, and associated current pulse drivers have been developed. The magnets generate fields of several 10 T. Pulse durations are of the order of one millisecond and thus the fields can be considered as quasi-static for laser-plasma interaction processes taking place on the ps- to ns-scale. Their high field strength in combination with abandoning magnetic cores make the solenoids compact and light-weight.
The presented experiments focus on a solenoid magnet designed for the capture of divergent laser-driven ion beams. They have been carried out at the 6MV tandetron accelerator and the laser acceleration source Draco of Helmholtz-Zentrum Dresden – Rossendorf as well as at the PHELIX laser of GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt
Untersuchungen zu den Wechselwirkungen zwischen unter Tage lebenden Mikroorganismen mit Uran und deren Einfluss auf das Migrationsverhalten von Uran in gefluteten Urangruben und Spektroskopische Bestimmung der Bindungsform (Speziation) trivalenter Actinide/Lanthanide in Biofluiden des menschlichen Gastrointestinaltraltes und im Blut
Full text linkTeil A:
Es wurde gezeigt, dass das Transportverhalten von Uran in der Umwelt und an den ehemaligen Uranabbaustätten stark von der Anwesenheit und Aktivität natürlich vorkommender Mikroorganismen abhängt. Die Untersuchungen zeigten, dass die Isolate eine hohe Toleranz gegenüber Uran aufweisen und in der Lage sind, relativ hohe Mengen an Uran zu immobilisieren und aus der umgebenden Lösung zu entfernen. Durch anaerobe Versuche konnte gezeigt werden, dass die mikrobielle Reduktion von Uran(VI) allein durch die Zugabe von 10 mM Glycerin bei zukünftigen Anwendungen als in situ Biosanierungsapplikationen genutzt werden könnte. Die Ergebnisse dieser Arbeit konnten die Wechselwirkungsmechanismen zwischen natürlich vorkommenden Mikroorganismen und Uran im Detail beschreiben und neue Zusammenhänge zwischen aktivem und inaktivem Stoffwechsel der Mikroorganismen zeigen. Zusammenfassend können diese einen wertvollen Beitrag zur Entwicklung von Biosanierungsansätzen für die Behandlung von Radionuklid-kontaminierten Standorten aus der ehemaligen Bergbauindustrie leisten.
Teil B:
Im Speichel dominiert neben einem kleinen Bindungsanteil an dem Enzym alpha-Amylase die Komplexierung mit anorganischen Liganden, im Magen dominiert aufgrund des sauren pH-Wertes das Eu- bzw. Cm-Aquo-Ion, und im Darm dominiert neben anorganischen Komplexen die Bindung der Metallionen an das Glycoprotein Mucin. Die starke Komplexfähigkeit von Mucin gegenüber dreiwertigen f-Elementen könnte die Absorption dieser im menschlichen Körper unterdrücken und deren Exkretion fördern. Die Ergebnisse dieser Arbeit geben neue Einblicke in das biochemische Verhalten dreiwertiger f-Elemente und können zudem zur Einschätzung von Gesundheitsrisiken nach der Inkorporation von Radionukliden und der Entwicklung von Dekontaminationstherapien beitragen
Wissenschaftlich-technische Berichte / Helmholtz-Zentrum Dresden - Rossendorf / HZDR
No full textUnregelmäßig erscheinende Berichte über wissenschaftliche Ergebnisse aus den einzelnen Forschungsbereichen der zum HZDR gehörenden Institute
Investigation of decommissioned reactor pressure vessels of the nuclear power plant Greifswald
Full text linkThe investigation of reactor pressure vessel (RPV) material from the decommissioned Greifswald nuclear power plant representing the first generation of Russian-type WWER-440/V-230 reactors offers the opportunity to evaluate the real toughness response. The Greifswald RPVs of 4 units represent different material conditions as follows:
• Irradiated (Unit 4),
• irradiated and recovery annealed (Units 2 and 3), and
• irradiated, recovery annealed and re-irradiated (Unit1).
The recovery annealing of the RPV was performed at a temperature of 475° for about 152 hours and included a region covering ±0.70 m above and below the core beltline welding seam.
Material samples of a diameter of 119 mm called trepans were extracted from the RPV walls. The research program is focused on the characterisation of the RPV steels (base and weld metal) across the thickness of the RPV wall. This report presents test results measured on the trepans from the beltline welding seam No. SN0.1.4. and forged base metal ring No. 0.3.1. of the Units 1 2 and 4 RPVs. The key part of the testing is focussed on the determination of the reference temperature T0 of the Master Curve (MC) approach following the ASTM standard E1921 to determine the facture toughness, and how it degrades under neutron irradiation and is recovered by thermal annealing. Other than that the mentioned test results include Charpy-V and tensile test results. Following results have been determined:
• The mitigation of the neutron embrittlement of the weld and base metal by recovery annealing could be confirmed.
• KJc values of the weld metals generally followed the course of the MC though with a large scatter.
• There was a large variation in the T0 values evaluated across the thickness of the multilayered welding seams.
• The T0 measured on T-S oriented SE(B) specimens from different thickness locations of the welding seams strongly depended on the intrinsic structure along the crack front.
• The reference temperature RT0 determined according to the “Unified Procedure for Lifetime Assessment of Components and Piping in WWER NPPs - VERLIFE” and the fracture toughness lower bound curve based thereon are applicable on the investigated weld metals.
• A strong scatter of the fracture toughness KJc values of the recovery annealed and re-irradiated and the irradiated base metal of Unit 1 and 4, respectively is observed with clearly more than 2% of the values below the MC for 2% fracture probability. The application of the multimodal MC-based approach was more suitable and described the temperature dependence of the KJc values in a satisfactory manner.
• It was demonstrated that T0 evaluated according to the SINTAP MC extension represented the brittle fraction of the data sets and is therefore suitable for the nonhomogeneous base metal.
• The efficiency of the large-scale thermal annealing of the Greifswald WWER 440/V230 Unit 1 and 2 RPVs could be confirmed
Optimal beam loading in a nanocoulomb-class laser wakefield accelerator
Full text linkLaser plasma wakefield accelerators have seen tremendous progress in the last years, now capable of producing electron beams in the GeV energy range. The inherent few-femtoseconds short bunch duration of these accelerators leads to ultra-high peak-currents. Reducing the energy spread found in these accelerators, while scaling their output to hundreds of kiloampere peak current would stimulate the next generation of radiation sources covering high-field THz, high-brightness X-ray and -ray sources, compact free-electron lasers and laboratory-size beam-driven plasma accelerators. At such high currents, an accelerator operates in the beam loaded regime where the accelerating field is strongly modified by the self-fields of the injected bunch, potentially deteriorating key beam parameters. However, if appropriately controlled, the beam loading effect can be employed to improve the accelerator’s performance, specifically to reduce the energy spread.
In this thesis the beam-loading effect is systematically studied at a quasi-monoenergetic nanocoulomb-class laser wakefield accelerator. For this purpose, a tailored scheme of the self-truncated ionisation injection process is introduced for the non-linear bubble regime. This scheme facilitates stable and tunable injection of high-charge electron bunches within a short and limited time-frame, ensuring low energy spread right after injection. Employing a three millimetres gas-jet acceleration medium and a moderate 150 TW short pulse laser system as driver, unprecedented charges of up to 0.5 nC within a quasi-monoenergetic peak and energies of ~0.5 GeV are achieved. Studying the beam loading mechanism, it is demonstrated that at the optimal loading condition, i.e. at a specific amount of injected charge, performance of the accelerator is optimised with a minimisation of the energy spread. At a relative energy spread of only 15%, the associated peak current is around 10 kA, while scaling this scheme to operate with a petawatt driver laser promises peak-currents up to 100 kA
Dosimetry of Highly Pulsed Radiation Fields
Full text linkSynchrocyclotrons and laser based particle accelerators, developed with the goal to enable more compact particle therapy facilities, may bring highly pulsed radiation field to external beam radiation therapy. In addition, such highly pulsed fields may be desirable due to their potential clinical benefits regarding better healthy tissue sparing or improved gating for moving tumors. However, they pose new challenges for dosimetry, the corner stone of any application of ionizing radiation.
These challenges affect both clinical and radiation protection dosimetry. Air-filled ionization chambers, which dominate clinical dosimetry, face the problem of increased signal loss due to volume recombination when a highly pulsed field liberates a large amount of charge in a short time in the chamber. While well established descriptions exist for this volume recombination for the moderately pulsed fields in current use (Boag's formulas), the assumptions on which those descriptions are based will most likely not hold in the prospective, highly pulsed fields of future accelerators. Furthermore, ambient dose rate meters used in radiation protection dosimetry as survey meters or fixed installations are generally only tested for continuous fields, casting doubt on their suitability to measure pulsed fields.
This thesis investigated both these aspects of dosimetry - clinical as well as radiation protection - to enable the medical application of highly pulsed radiation fields. For a comprehensive understanding, experimental investigations were coupled with theoretical considerations and developments.
Pulsed fields, varying in both dose-per-pulse and pulse duration over a wide range, were generated with the ELBE research accelerator, providing a 20 MeV pulsed electron beam. Ionization chambers for clinical dosimetry were investigated using this electron beam directly, with an aluminium Faraday cup providing the reference measurement. Whereas the dose rate meters were irradiated in the photon field generated from stopping the electron beam in the Faraday cup. In those measurements, the reference was calculated from the ionization chamber, then serving a an electron beam monitor, cross-calibrated to the photon field with thermoluminescent dosimeters.
Three dose rate meters based on different operating principles were investigated, covering a large portion of the operating principles used in radiation protection: the ionization chamber based RamION, the proportional counter LB 1236-H10 and the scintillation detector AD-b. Regarding clinical dosimetry, measurements of two prominent ionization chamber geometries, plane-parallel (Advanced Markus chamber) and thimble type (PinPoint chamber), were performed. In addition to common air-filled chambers, chambers filled with pure nitrogen and two non-polar liquids, tetramethylsilane and isooctane, were investigated.
In conjunction with the experiments, a numerical solution of the charge liberation, transport, and recombination processes in the ionization chamber was developed to calculate the volume recombination independent of the assumptions necessary to derive Boag's formulas. Most importantly, the influence of the liberated charges in the ionization chamber on the electric field, which is neglected in Boag's formulas, is included in the developed calculation.
Out of the three investigated dose rate meters only the RamION could be identified as an instrument truly capable of measuring a pulsed field. The AD-b performed below expectations (principally, a scintillator is not limited in detecting pulsed radiation), which was attributed to the signal processing, emphasizing the problem of a typical black-box signal processing in commercial instruments. The LB 1236-H10, on the other hand, performed as expected of a counting detector. While this supports the recent effort to formalize these expectations and standardize testing for counting dosimeters in DIN IEC/TS 62743, it also highlights the insufficiency of counting detectors for highly pulsed fields in general and shows the need for additional normative work to establish requirements for dose rate meters not based on a counting signal (such as the RamION), for which no framework currently exists. With these results recognized by the German radiation protection commission (SSK) the first steps towards such a framework are taken.
The investigation of the ionization chambers used in radiation therapy showed severe discrepancies between Boag's formulas and the experimentally observed volume recombination. Boag's formulas describe volume recombination truly correctly only in the two liquid-filled chambers. All the gas-filled chambers required the use of effective parameters, resulting in values for those parameters with little to no relation to their original meaning. Even this approach, however, failed in the case of the Advanced Markus chamber for collection voltages ≥ 300 V and beyond a dose-per-pulse of about 100 mGy.
The developed numerical model enabled a much better calculation of volume recombination and allowed the identification of the root of the differences to Boag's formulas as the influence of the liberated charges on the electric field. Increased positive space charge due to increased dose-per-pulse slows the collection and reduces the fraction of fast, free electrons, which are unaffected by volume recombination. The resultant increase in the fraction of charge undergoing volume recombination, in addition to the increase in the total amount of charge, results in an increase in volume recombination with dose-per-pulse that is impossible to describe with Boag's formulas. It is particularly relevant in the case of high electric fields and small electrode distances, where the free electron fraction is large. In addition, the numerical calculation allows for arbitrary pulse durations, while Boag's formulas apply only to very short pulses.
In general, the numerical calculation worked well for plane-parallel chambers, including those filled with the very diverse media of liquids, nitrogen and air. Despite its increased complexity, the thimble geometry could be implemented as well, although, in the case of the PinPoint chamber, some discrepancies to the experimental data remained, probably due to the required geometrical approximations.
A possible future development of the numerical calculation would be an improved description of the voltage dependence of the volume recombination. At the moment it requires characterizing a chamber at each desired collection voltage, which could be eliminated by an improved modeling of the volume recombination's dependence on collection voltage. Nevertheless, the developed numerical calculation presents a marked improvement over Boag's formulas to describe the dose-per-pulse dependence and pulse duration dependence of volume recombination in ionization chambers, in principle enabling the application of ionization chambers in the absolute dosimetry of highly pulsed fields