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SFB 634:  Nuclear Structure, Nuclear Astrophysics and Fundamental Experiments at Low Momentum Transfer at the Superconducting Darmstadt Electron Accelerator S-DALINAC

Subject Area Physics
Term from 2003 to 2015
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 5485852
 
Final Report Year 2015

Final Report Abstract

The Collaborative Research Center 634 “Nuclear Structure, Nuclear Astrophysics and Fundamental Experiments at Low Momentum Transfer at the Superconducting Darmstadt Electron Accelerator (SDALINAC)” addressed the scientific exploitation and technological advancement of the superconducting particle accelerator S-DALINAC set up at the Technische Universität Darmstadt. It accelerates electrons in strong electric fields oscillating with high frequency in superconducting microwave resonators and thereby provides energetic beams of accelerated electrons and photons. The latter are produced in bremsstrahlung processes of electrons in matter. The particle beams are sufficiently energetic to raise atomic nuclei to excited quantum states. Corresponding nuclear reactions can be observed with appropriate detector systems. Data obtained this way are prerequisites for scientific progress in intertwined research areas in nuclear physics and nuclear astrophysics. The CRC 634 was organized in five linked project areas that addressed scientific challenges from the research fields of (A) nuclear structure physics, (B) nuclear astrophysics, (C) fundamental experiments, (D) theoretical nuclear structure physics, and (E) accelerator development. Scientists from the disciplines of experimental nuclear physics, theoretical physics, and electrical engineering collaborated closely within the scope of the CRC 634. In doing so more than 400 scientific articles were published in refereed journals and 70 early-stage researchers have finished the doctoral theses based on the scientific research successfully conducted in the CRC. Among other things the scientists in the CRC 634 investigated the properties of heavy atomic nuclei, built up from many protons and neutrons, in oscillating electromagnetic fields with high oscillation frequencies. This was achieved by exposing the nuclei to intense beams of short-wavelength photons or to electromagnetic pulses in scattering reactions with electrons or other electrically charged particles. The data obtained within the CRC and their theoretical interpretations contributed considerably to the observation and the understanding of the so-called pygmy dipole resonance of atomic nuclei. It manifests itself in nuclei made out of more neutrons than protons in an excessive probability for reactions with comparatively low-energy photons and is understood as a partial density oscillation of neutrons versus protons within the atomic nucleus. The properties of the pygmy dipole resonance are sensitive to the local density dependence of the nuclear forces between protons and neutrons. The new insights contribute to a better understanding of extreme cosmic objects such as neutron stars or of the processes along the natural synthesis of heavy chemical elements in exploding stars such as supernovae. Scientists in the CRC 634 furthermore succeeded in observing a new radioactive decay phenomenon. Excited quantum states of atomic nuclei may eventually decay by simultaneous emission of two gamma quanta instead of a single one. This so-called competitive double-gamma nuclear decay has gone undetected since its theoretical postulation based on quantum-mechanical calculations some 85 years ago. It was identified in the CRC by using novel gamma-ray detectors with increased time-resolution capabilities that enabled the scientists to discriminate the simultaneous gamma emissions from the background. Theoretical analyses performed within the CRC offered a quantitative understanding of these observations. Further scientific results out of many obtained within the CRC 634 concerned, e.g., the development of new techniques for measuring fundamental properties of the proton, such as its charge radius and its electric and magnetic polarizabilities, or insight into chaotic properties of excited states of quantum systems by means of their simulation in superconducting microwave resonators with various shapes and properties. Theoretical methods for quantitatively understanding the properties of atomic nuclei based on the fundamental theory of quantum chromo dynamics (QCD) were considerably advanced within the CRC and also applied to an improved understanding of astrophysical processes in cosmic objects such as neutron stars or supernovae. A closely collaborating team of physicists and electrical engineers managed to technologically upgrade the S-DALINAC providing now electron beams of enhanced stability and improved energy spread. Within the scope of the CRC the S-DALINAC was advanced to the first particle accelerator in Germany capable of being operated in the mode of an energy recovery linac (ERL) in which the kinetic energy of the accelerated beam particles can be recovered to a large extent on their deceleration after experimental usage.

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