AG Pietralla

Willkommen bei der Arbeitsgruppe von Prof. Pietralla

Prof. Dr. Norbert Pietralla
Prof. Dr. Norbert Pietralla

We conduct fundamental research on the areas of nuclear structure physics, quantum physics and accelerator physics. Atomic nuclei are fascinating quantum systems built up from protons and neutrons. Their properties are determined by the complex nuclear forces between the protons and neutrons and they are still not fully understood up to now.

In order to study the nuclear properties we excite them in nuclear reactions at particle-accelerator facilities and we detect their response with spectroscopic methods. At our Institut für Kernphysik at the TU Darmstadt we run the superconducting Darmstadt electron linear accelerator (S-DALINAC) and use it for our experiments.

On a regular basis we perform complementary experiments at external facilities, in particular at GSI (Darmstadt), at the REX-ISOLDE facility at CERN (Geneva, Switzerland), at the Laboratori Nazionale di Legnaro (Padova, Italy), at Argonne National Laboratory (ANL, Chicago, USA), at the High-Intensity Gamma-ray Source (HIGS, Duke Univ., USA), at iThembaLabs (Somerset, South Africa), at the Grand Raiden spectrometer at the RCNP of Osaka Univ. (Osaka, Japan) and at further facilities world-wide.

Furthermore, we are strongly engaged in the construction of the NUSTAR experiment at the FAIR project at Darmstadt. In particular, we are involved in the HISPEC/DESPEC experiment, the PRESPEC experiment and the European AGATA project for gamma-ray spectroscopy.

We constantly offer various research projects for PhD studies, for thesis work (Bachelor of Science / Master of Science) or for initial research work (Miniforschung / study projects).

Forschungsschwerpunkte

Beschleunigerphysik

The S–DALINAC consists of twelve superconducting 20 cell niobium cavities, operated at 2 K at a frequency of 2.9975 GHz. With a design accelerating gradient of 5 MV/m and a design quality factor of 3·109 in cw operation, the final energy of the machine is 130 MeV which is reached when the beam is recirculated twice. The first set of cavities was built in the 80’s at Interatom using low RRR material, so the observed performance regarding the gradient and the quality factor was rather poor. Accordingly, a second set of cavities was ordered in the 90’s made from RRR 300 material. These cavities, welded at Dornier, are used since then. All of them reach the design gradient, some exceeding it by more than 50 %. Unfortunately, the accelerator did not benefit from this improvement: Due to the limited refrigerator power of some 100 Watt and the rather low Q of the cavities (typically below 1·109) the cavities had to be operated below their maximum gradients. The final energy of the accelerator in cw operation therefore never exceeded 90 MeV. Consequently, reaching the design energy of 130 MeV is still a goal. Many measures towards achieving higher quality factors have been taken in the past laying the foundation of the SRF research conducted in house. Currently, the focus lies on the heat-treat cavities in our UHV furnace. The procedure, firing the cavity to 850 C aims to remove residual hydrogen known as a reason for a large degradation of the cavity quality factor, commonly referred to as Q-Disease. Additionally, improvements of the magnetic shielding inside the accelerator cryostat are performed, reducing the amount of magnetic flux freezing during cool-down.

The beam dynamics of the S-DALINAC is rather complicated. As a recirculation Linac, the machine is not a single Linac, nor a circular accelerator: it combines features of both types of machines in a microtron-like manner. Therefore, investigations of the longitudinal working point are conducted. From a theoretical model, a reduction of the energy spread is predicted and first signs of this existence have been observed many years before- a final prove is one of the major steps lying ahead.

In addition, several machine lattice upgrades are underway, requiring a detailed recalculation of the beam dynamics. One of this is the installation of a third recirculation to boost the accelerator energy, allowing to use the energy gain of the main linac another time, which would lead roughly to 120 MeV in cw operation. The installation of an additional recirculation has become possible recently as the old beam-line to the undulator (as part of the Darmstadt Free Electron Laser facility) has been de-commissioned in 2006. A preliminary design study undertaken so far showed the option to build the new recirculation in between the two existing ones. A considerable advantage of this layout is that most of the existing beam-line and magnets can be used again (even so the energy of the beam changes). Beside the 4 new bending magnets to form the additional recirculation path, only the separation and combining magnets at the beginning and the end of the recirculations have to be replaced.

An other upgrade, involving beam-dynamics issues is the installation of two scraper systems. The injector arc scraper system will remove the low energy tail of the beam injected into the main linac. Currently, the dynamics of the injector bending section is optimized to allow the installation of the recently designed collimator.

After three passes through the linac (housing 8 independently controlled accelerating cavities) a transversal scraping, combined with an additional longitudinal collimation will ensure the highest beam quality by removing any beam halo. The system proposed will be placed in the extraction beam line, the lattice and the dynamics of which has to be adapted. In addition, the longitudinal scraping can further reduce the energy spread of the beam at the cost of beam current. As the dispersion is maximized in this section, a more efficient energy collimation compared to the existing system can be assured, allowing an energy definition of as low as 10 keV being a key figure for several highly demanding experiments proposed within the CRC.

Remote control, data logging and active feedback loops are the essentials of a successful accelerator operation. Many years, the control system to do this was based on in-house developments. With the replacement of the low-level RF control, the transition to the EPICS system (developed in a worldwide collaboration of various accelerator institutes) was started for the slow data.

The controller boards are connected to a standard PC server via CAN bus. This interface is used to monitor and adjust all slowly varying (e. g. by user interaction) parameters of the control algorithm and the hardware. The microcontroller that connects the FPGA boards to the CAN bus runs Nut/OS, an open source real-time operating system providing cooperative multi-threading. The PC runs Linux and an EPICS IOC that is hooked up to the CAN bus via a device support that uses the Socket- CAN network stack included in recent Linux kernels (since2.6.25). SocketCAN provides access to the CAN bus via network devices (BSD sockets) which can be accessed by multiple applications at the same time (e. g. the IOC and a CAN sniffer). Furthermore it acts as an abstraction layer that makes the device support independent of a specific CAN card or hardware vendor (all all CAN cards having a SocketCAN driver are supported). All together the IOC currently provides several thousand records for all 16 RF channels.

The operator interface has been implemented with Synoptic Display Studio. Currently, the transition of the beam line magnet power supplies to an EPICS IOC is being programmed. One major issue is the implementation of the hardware turn-knobs being available for set-parameter variations within the in-house control system allowing the operator to adjust magnet parameters easily.

So far, data logging is achieved only for a limited set of parameters using a non-EPICS data base. Nevertheless, this self-made system provides a web interface, allowing easy access to the current and recent data. The transition of this system will be a future activity.

Kernstrukturphysik

The complex nuclear many-body system can assume under the complex nuclear forces collective nuclear quantum states to which many nucleous contribute in a coherent fashion. Such collective structures can exhibit astonishing regularities and simple pattern despite of the complex nuclear forces that are responsible for their formation.

Examples are collective vibrations of nuclei or rotational sequences of spatially deformed nuclear quantum states.

We study the properties of collective vibrations and rotations of nuclei with particular emphasis of their evolution as a function of the number of protons and neutrons forming the atomic nuclei. We conduct spectroscopic experiments to discover hitherto unknown properties of such collective nuclear states by using electron-scattering techniques at our in-house electron accelerator S-DALINAC or by using the method of gamma-ray spectroscopy at ion-accelerator facilities world-wide. We interpret our results in terms of phenomenological nuclear models that we develop further for an improved quantitative insight into the properties of atomic nuclei.

Students of the TU Darmstadt at all levels of their career are welcome to participate in our basic research, e.g., in the scope of initial research work (Miniforschung, for undergraduates starting from the 3rd semester), or for thesis studies for acquiring the academic degrees of Bachelor of Science or Master of Science and for first scientific publications. Current offers for concretely specified research works are given here.

Atomic nuclei are formed by a sufficiently small number (<300) of nucleons, i.e., protons and neutrons, such that a single nucleon can make a substantial contribution to the behaviour of the entire nucleus and to the properties of its excited states. This aspect is further enhanced by the fermionic quantum character of the nuclear system which leads to the formation of nuclear shell structure.

We investigate the role of single-particle effects on the properties of the nuclear quantum system in stable and in radioactive nuclei by using electron-scattering techniques at our in-house electron accelerator S-DALINAC or by using the method of gamma-ray spectroscopy at ion-accelerator facilities world-wide.

Our results contribute to an improved quantitative understanding of the nuclear quantum system.

We further contribute to the developments of improved experimental techniques for quantitative research on nuclear single-particle properties.

Students of the TU Darmstadt at all levels of their career are welcome to participate in our basic research, e.g., in the scope of initial research work (Miniforschung, for undergraduates starting from the 3rd semester), or for thesis studies for acquiring the academic degrees of Bachelor of Science or Master of Science and for first scientific publications. Current offers for concretely specified research works are given here.

The atomic nucleus formed by protons and neutrons represents the prime example of an isolated strongly-interacting two-fluid quantum system. Generic features are shell structure, collectivity and the isospin degree of freedom.

We are the world-leading group for experimental research on proton-neutron mixed-symmetry states (MSSs). The properties of MSSs and other isovector valence shell excitations are simultaneously sensitive to all three of these fundamental features of atomic nuclei and, hence, are a most appealing object of nuclear-structure research.

We have recently reviewed the status of the word-wide experimental information about MSSs of vibrational nuclei.

We study MSSs by using electron-scattering techniques at our in-house electron accelerator S-DALINAC or by using the method of gamma-ray spectroscopy at ion-accelerator facilities world-wide.

Students of the TU Darmstadt at all levels of their career are welcome to participate in our basic research, e.g., in the scope of initial research work (Miniforschung, for undergraduates starting from the 3rd semester), or for thesis studies for acquiring the academic degrees of Bachelor of Science or Master of Science and for first scientific publications. Current offers for concretely specified research works are given here.

Internationale Kollaborationen