Research Group Norbert Pietralla

Nuclear level densities are a central quantity in the description of statistical processes in atomic nuclei and have an important role in the modelling of nucleosynthesis.

Recently, a new experimental measurement method was developed and applied for the first time that allows to determine nuclear level densities in photonuclear reactions. For this, the method of nuclear self-absorption was employed. The experiment was conducted at the High Intensity Gamma-ray Source at Duke University, USA with the nuclide Sr-88.

This master thesis focuses on the analysis of the taken data with the objective to determine the nuclear level densities of Sr-88. Subsequently, existing phenomenological models and microscopic calculations shall be tested with the newly measured data. With this work, the student will contribute significantly to the research area for the investigation of statistical nuclear processes and provide potentially new results not existing before.

Inclusive electron scattering is an established method to study the structure of atomic nuclei in detail by measuring electromagnetic form factors. The existing experimental setup at the in-house electron spectrometer was recently extended by a gamma spectrometer consisting of six LaBr detectors. Here, electrons scattered from the atomic nucleus are measured simultaneously with emitted γ quanta – (e,e'γ) reactions – allowing so-called electron-gamma coincidence experiments. Recently, the first (e,e'γ) measurements worldwide in over 40 years were successfully performed to study the γ-decay behavior of the Ru-96 nucleus.

In order to be able to determine excitation probabilities of nuclear resonances in Ru-96 and their γ-decay behavior from the experimental data, we are looking for motivated students who would like to work intensively on the analysis of the data in the framework of theses. This includes, among other things, the calibration of the individual detector systems and the subsequent determination of reaction cross sections.

Scattering experiments are useful tools to investigate a large variety of nuclear structure phenomena. Inelastic proton scattering at extreme forward angles with relativistic protons are an excellent tool to study the dipole response of atomic nuclei. Nuclear resonances are excited via Coulomb excitation and hadronic interactions between the impinging protons and the nuclei. The dipole response and the associated dipole polarizability are important metrics to determine parameters of the equation of state of nuclear matter, which plays a central role in the description of neutron stars, among other things.

The data analyzed in this master's thesis are from measurements at the Research Center for Nuclear Physics in Osaka, Japan. The inelastically scattered protons were detected using the Grand Raiden magnetic spectrometer, allowing excitation spectra of 64Ni to be measured at various proton scattering angles. This master thesis focuses on the analysis of the excitation spectra of 64Ni. The goal is to experimentally determine the electric and magnetic strengths at excitation energies from 5 MeV to about 25 MeV to complete systematic studies of dipole polarizability in the nickel isotope chain.

The beam guidance is realized at an accelerator by using different beam guiding elements / magnets. A solenoid (lens) is characterized by its ability to focus the beam simultaneously in both planes. Typically, solenoids are used in low energy beamlines. Here, the design of the magnet as well as its alignment is of great importance, since the beam is very sensitive to disturbances in this range due to its still very low momentum. When using spin-polarized electrons, further boundary conditions to such a lens have to be fulfilled.

In this master thesis an optimized solenoid will be designed and simulated based on the existing lenses at S-DALINAC. The beam dynamics of the existing S DALINAC injector as well as the consideration of a generic optimized injector are also in focus. The designed lens will be built and measured and will be put into operation at the S DALINAC with beam.

The extraction beam guide of the S-DALINAC contains the high-energy scraper system. This has various slot systems that can be used to define the beam energy or to reduce the beam halo / background. The main slits – used for energy definition – have a current measurement. Via current changes on the two jaws of this system, an energy change of the beam can be detected with very fine resolution according to the very high dispersion set there. The extent to which energy changes can be transported from the accelerator hall to the extraction beamline depends on the beam dynamics set.

Within the scope of this master thesis, the current measurement on the energy-defining scraper system for stabilizing the beam energy will be set up and put into operation. Beam dynamics simulations and measurements will be used to investigate the differences between the different operating modes with the respective beam dynamics and their effects on the stabilization.

An Energy Recovery Linac (ERL) combines the possibility of a very high beam intensity with the very good beam quality of a linear accelerator. Possible applications or experiments for an ERL may have only minimal influence on the electron beam, since the electrons are transported further and decelerated after the interaction. A free-electron laser (FEL) would meet these requirements. The resulting radiation could either be used to perform experiments directly, or it could be brought into collision with the electron beam, for example. In the case of Compton backscattering, the photons thereby experience a significant boost in their energy and can be used as a very intense, monochromatic source for nuclear physics experiments.

Within the scope of this master thesis, a basic design for an FEL will be created, which will be used in an ERL. The beam dynamics before and after the FEL will be investigated as well as possible experimental setups.

For the experiments performed at S-DALINAC, the stability of the beam is of high importance. At the two electron scattering spectrometers QCLAM and LINTOTT the beam position and the beam phase should be monitored and corrected if necessary. For monitoring the beam position, a so-called OTR target would be applicable directly after the scattering target. The beam phase can be monitored non-destructively directly in front of the scattering target using a so-called high frequency monitor.

Within the scope of this master thesis, the systems for monitoring the beam position and phase will be set up and put into operation. In the process, stabilization will be set up with the aid of a closed-loop control system. Support of machine learning methods are also conceivable. Test measurements will allow a comparison between activated and non-activated beam stabilization.

In a recirculating accelerator, the beam quality, stability and, above all, the pulse blur can be optimized by cleverly choosing the phase with which the electron bunches hit the high-frequency accelerator and the correspondingly set energy-dependent time of flight in the recirculation (longitudinal dispersion). In this process, the arcs of the three recirculations on the S-DALINAC are adjusted to achieve the desired values of longitudinal dispersion. The up to four beams in the main accelerator are set in a defined way on the flank of the high-frequency field. During the simulations, given boundary conditions such as defined momentum ratios between the different paths have to be considered as well as the adjustment possibilities due to the given beam diagnostics.

Within the scope of this master thesis, phase landscapes shall be simulated, which represent the different adjustment possibilities of the phases and longitudinal dispersions. Results and experiences of the last beam settings will be used as additional information. The stability of the obtained settings will be investigated.

In laser Compton backscattering, a laser beam is scattered by an electron beam and experiences a significant increase in its energy. At S-DALINAC, such an experiment is set up in the straight line of the third recirculation. Electron energies of up to 100 MeV are present there, which collide with a laser with a photon energy of about 1 eV. The backscattered photons generated in the process then have energies of up to 180 keV. This setup will serve as a feasibility experiment in this form and will be used as an additional beam diagnostic for the electron beam.

In this master thesis, the laser beam transport between the location of the laser and the collision point with the electron beam will be designed and built. The required laser coupling chamber and interaction chamber with required diagnostics of laser and electron beam will also be designed.

Inclusive electron scattering is an established method to study the structure of atomic nuclei in detail by measuring electromagnetic form factors. The existing experimental setup at the in-house electron spectrometer was recently extended by a gamma spectrometer consisting of six LaBr detectors. Here, electrons scattered from the atomic nucleus are measured simultaneously with emitted γ quanta – (e,e'γ) reactions – allowing so-called electron-gamma coincidence experiments. Recently, the first (e,e'γ) measurements worldwide in over 40 years were successfully performed to study the γ-decay behavior of the Ru-96 nucleus.

In order to be able to determine excitation probabilities of nuclear resonances in Ru-96 and their γ-decay behavior from the experimental data, we are looking for motivated students who would like to work intensively on the analysis of the data in the framework of theses. This includes, among other things, the calibration of the individual detector systems and the subsequent determination of reaction cross sections.

In the method of nuclear resonance fluorescence, intense photon beams are used to induce photonuclear reactions. From the subsequent γ-decay of the excited states, conclusions can be drawn about the properties of the excited states or the structure of the nucleus.

Recently, an experiment was successfully performed at the High Intensity γ-ray Source (HIγS) at the prestigious Duke University in the USA to study the excitation and decay behavior of Mo-96. The focus of the experiment was on the low-energy flank of the giant dipole resonance, where the pygmy dipole resonance, which is not well understood until now, is believed to exist. The experiment succeeded in exciting a number of previously unknown nuclear states and observing the subsequent decay.

For this bachelor thesis, the individual states will be identified as well as their distribution as a function of excitation energy will be investigated. Furthermore, the photoabsorption process will be studied.

In nuclear photon scattering experiments, high-energy photon beams are used to excite nuclear states whose decay in the form of γ-rays can subsequently be measured with suitable detectors. In order to determine absolute measured quantities, it is necessary to know the intensity of the exciting photon beam.

Recently, an experiment was performed at the High Intensity γ-ray Source (HIγS) at the renowned Duke University in the USA. The facility generates a monoenergetic photon beam that was used to populate nuclear states at different excitation energies. The photon flux was measured in three different ways:

1) by Compton scattering of the beam at a well-defined scattering center, 2) by a uranium-238 fission chamber placed in the path of the beam, and 3) using a plastic scintillator also placed in the beam. All three measurement setups and methods provide information on the intensity of the photon beam.

In this bachelor thesis, the data from the three setups will be analyzed and compared qualitatively as well as quantitatively to determine the course of the photon flux for each series of measurements. The results of this work will thus allow the determination of absolute reaction cross sections.

Lanthanum bromide (LaBr) scintillation detectors, with high detection efficiency and a short signal decay time, are ideal for measuring γ-spectra in nuclear physics experiments, such as nuclear resonance fluorescence experiments.

To infer the energy of γ-quanta from the pulse height of a LaBr detector signal, its energy calibration must be precisely determined. The energy calibration is usually not constant, but depends, among other things, on the detector's count rates, i.e., the number of events the detector must process within a given time period; typically 10^5 events per second.

For a recent experiment conducted at the High Intensity γ-ray Source (HIγS) at the renowned Duke University in the USA, accurate energy calibration of the LaBr detectors used is of key importance in the analysis of the measured data. As part of the experiment, several series of measurements were performed with the nucleus C-12 to determine the rate dependence of the energy calibration of the LaBr detectors. As for this bachelor thesis, the C-12 data will be analyzed to qualitatively and quantitatively determine the observed rate dependence. The results of this work are essential for the accurate analysis of the recorded experimental data.

Photonuclear reactions play a central role in the study of the structure of atomic nuclei. For this purpose, one bombards a sample with an intense and high-energy photon beam and studies the nuclear reactions induced.

Recently, experiments on the nuclei Sm-154 and Ce-140 were successfully performed at the High Intensity γ-ray Source (HIγS) at the renowned Duke University in the USA. The aim is to investigate the decay properties of the so-called giant dipole resonance in more detail for the first time using photonuclear reactions.

In this bachelor thesis, the recorded γ-spectra will be evaluated for discrete transitions at low γ-energies. The results of this work will contribute to our understanding of the excitation and decay behavior of the giant dipole resonance.

Scattering experiments are useful tools to investigate a large variety of nuclear structure phenomena. Inelastic proton scattering at extreme forward angles with relativistic protons are an excellent tool to study the dipole response of atomic nuclei. Nuclear resonances are excited via Coulomb excitation and hadronic interactions between the impinging protons and the nuclei. This makes it possible, among other things, to measure the fine structure of the dipole response and to decompose it into its electric and magnetic components.

The data to be analyzed in this bachelor thesis were obtained from measurements at the Research Center for Nuclear Physics in Osaka, Japan. The inelastically scattered protons were detected by the magnetic spectrometer Grand Raiden allowing to measure excitation spectra of ⁶⁴Ni at different proton scattering angles. This bachelor thesis focuses on the analysis of the excitation spectra. The aim is to determine the excitation energy of individual nuclear resonances of ⁶⁴Ni. The results will be compared to data from photon scattering experiments and will allow for a separation between magnetic and electric excitations.

For the transport of particles in a beamline, the beamline must be evacuated. This is ensured by various pumps, so that in the case of the S-DALINAC pressure ranges from 10^-6 mbar up to 10^-9 mbar or even better prevail in the beamline. In the field of superconducting cavities, the pressure and the cleanliness (e.g. flow direction of particles) represent a very important criterion.

Within the scope of this bachelor thesis, a model of the complete vacuum system at S-DALINAC shall be created. With this model the current state shall be simulated and further optimized (number, type and positioning of pumps, used jet tube cross sections,…).

Each magnet in the beam guide has a defined position coming from the simulation. In reality, this positioning or even the alignment in space in general is only possible within certain tolerances. The magnetic fields in the magnets are also subject to certain tolerances.

Within the scope of this bachelor thesis, an exact investigation of the tolerances of the beam guiding elements is to be carried out with the help of simulations. On the basis of these simulations, it will be investigated how strongly which change of a beam guiding element affects the beam. The results obtained in this way will make a valuable contribution to the adjustment of the beam on the S-DALINAC.

An electron packet is described by a 6-dimensional phase space. The two components in the longitudinal direction (in the direction of flight) describe the length and momentum uncertainty of the electron packet. Shortly after the generation of the electron beam in the thermionic source of the S-DALINAC, a vertical diagnostic beamline provides the possibility to measure the length of the electron bunches with 250 keV kinetic energy. Up to this section there are further beam guiding elements which can influence the electron bunch length.

Within the scope of this bachelor thesis, systematic investigations of the dependence of different settings of the beam guiding on the electron bunch length shall be performed. The optimal settings for the acceleration in the superconducting injector will be verified.

The beam guidance of an accelerator can be divided into different sections such as arcs, straight lines, etc.. Depending on the requirements for the properties of the beam, various options are available. For example, particularly short electron bunches can be generated or the energy dependence of the beam position can be set to specific values.

Within the scope of this bachelor thesis, different setups of arcs are to be investigated and compared in terms of beam dynamics. The focus is on the use in a so-called energy-recovering accelerator (ERL – Energy Recovery Linac).

The method of nuclear resonance fluorescence is well suited for the investigation of quantum states in atomic nuclei. For this purpose, these quantum states – also called nuclear resonances – are excited by absorption of real photons. Subsequently, the γ-decay of the excited states is observed using γ-detectors (mainly high-purity germanium detectors).

A recent experiment at the High Intensity γ-ray Source (HIγS) at the prestigious Duke University in the USA is used to study magnetic and electric excitations in the isotope Mo-96

The goal of this Miniforschung is to determine, for the first time, resonance energies and parity quantum numbers of individual nuclear resonances in Mo-96 in order to separate magnetic and electric contributions in a model-independent manner.

Photonuclear reactions play an important role in the study of the structure of atomic nuclei. As one of the leading groups in this research field, we systematically investigate the probability for the absorption of photons by atomic nuclei. For this purpose, real photon beams are used to determine so-called photoabsorption cross sections.

In this Miniforschung, data from experiments at the High Intensity γ-ray Source (HIγS) at the renowned Duke University in the USA will be analyzed. The aim is to determine continuous photoabsorption cross sections as a function of the excitation energy of the nucleus Te-128.

The method of γ-spectroscopy, i.e. the investigation of the γ-radiation emitted by an atomic nucleus in a wide variety of reactions, allows far-reaching insights into the structure of atomic nuclei. For such investigations, among other things, the so-called detection efficiency of the corresponding γ-spectrometers must be calibrated. For this purpose, radioactive preparations with already precisely known γ-decay behavior and measured activity are usually used. A standard radioactive source used in γ-spectroscopy consists of Co-56, which decays via electron capture into the daughter nucleus Fe-56. Subsequently, γ-quanta of different energy and intensity are emitted, which are used to calibrate γ-spectrometers.

In this mini-research, the activity of a recently produced Co-56 source will be determined relative to a Co-60 source with already known activity. To do this, the γ-spectra of both sources will be measured with semiconductor detectors made of high-purity germanium and then analyzed.

The spectrometric analysis of electrons from scattering experiments, such as those performed at S-DALINAC, require the operation of dipole magnets with currents in the range of (150-300) A. In order to ensure sufficient removal of the heat generated, the dipole magnets are cooled with water. If the dipole magnets are not operated, the cooling capacity must be reduced to prevent condensation from forming inside the power supplies, which would cause corrosion and short circuits. For this purpose, a permanent temperature monitoring of the cooling water and a monitoring of the switching state of the power supply shall be implemented, which ensure that the cooling power is automatically readjusted according to the switching state of the power supply. The monitoring system shall be built with industry standard data acquisition modules and integrated and commissioned into the existing EPICS control system of the accelerator.

Learning Objectives: Digitizing analog signals from a temperature sensor, network-based readout of a device server, setting up an EPICS input/output controller, creating a graphical interface using Control-System Studio.

Prerequisite: Interest in measurement, control and regulation

The operation of the superconducting electron linear accelerator S-DALINAC leads to the fact that plant components are activated and thus must be treated as radioactive materials in the sense of the Radiation Protection Ordinance. In order for these plant components to be released back into the normal waste management cycle after their activity has subsided, a so-called release is required, the basis of which is a suitable measurement (clearance measurement) of the activity of these plant components. The aim of the mini-research is the construction and commissioning of a measuring station with the help of which the free measurements can be carried out. The challenge is to assemble, build and commission suitable modules for the envisaged sodium iodide detector. Then the detector system has to be calibrated and characterized. Finally, the first samples are to be measured.

Learning objectives: Digitization of analog signals from a gamma detector, operation of data acquisition modules of the NIM standard, operation of a sodium iodide detector, release procedures in radiation protection.

Prerequisites: Interest in measurement electronics for nuclear physics measurements.

A cavity resonator has a so-called quality. In simplified terms, this is a measure of how long an electromagnetic field oscillates in this cavity resonator after a single excitation. Superconducting cavity resonators (cavities) are characterized by very high Q-factors. One way to experimentally determine the Q is to measure the decay time of the excited field.

In this mini-research, the grades of cavities of the S-DALINAC will be measured and discussed. The grades of the cavities installed in the accelerator will be measured periodically so that degradations over time can be studied. Additional cavities can be measured and characterized in a vertical test cryostat.