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discovered_02_2013

research// The HZDR Research Magazine WWW.Hzdr.DE 26 27 their energetic ground state, they in turn emit particles or characteristic gamma rays. In photon-nucleus reactions, you can observe two phenomena. If the incident gamma quantum has a sufficiently high energy, a neutron may be ejected from the atomic nucleus, for example. This process, known as photo dissociation, is bound up with what is referred to as the giant electric dipole resonance (GDR). If the incident photons instead have a lower energy, scattering results that excites the atomic nucleus. "Whereas the giant electric dipole resonances for numerous nuclides had already been measured years ago, there is comparatively little knowledge about electromagnetic dipole excitations of the atomic nucleus in the case of lower energies lying below the threshold of photo dissociation," says Schwengner. Sensitive measuring instruments and techniques needed Experiments on photon scattering have been carried on for several years at the ELBE electron accelerator of HZDR. If the highly energetic electron beam from the ELBE electron source is fired at a metal foil, the beam will be slowed down. The high-energy bremsstrahlung radiation resulting from this is utilized to systematically investigate atomic nuclei in various regions of mass. But there are still unanswered questions about the distribution of energies and nature of the magnetic dipole excitations. This is because the electric radiation thoroughly obscures the magnetic radiation in the measured spectra, as Ronald Schwengner explains: "The magnetic dipole excitations have far lower intensities than the electric dipole excitations. You therefore need highly sensitive instruments and techniques to detect and unambiguously differentiate between magnetic and electric radiation." An experiment carried out at the High-Intensity Gamma-Ray Source HIGS in Durham has now made the breakthrough. While the gamma radiation from this source is very intense, more importantly it is polarized and almost monoenergetic. This polarization permits very precise differentiation between electric and magnetic dipole radiation. Data previously acquired at ELBE from bremsstrahlung experiments was used for calibrating the spectra measured at HIGS. Schwengner: "The combination of ELBE and HIGS proved to be ideal. The two institutions cover various aspects of the experimental techniques and the results can be combined very well." The outcome: an excitation spectrum of nuclide Zr-90 revealing an increased concentration of magnetic dipole excitations in a particular energy region. The intensity and location of the magnetic dipole energy uncover the fine structure of the magnetic dipole resonance and permit conclusions to be drawn about the properties of the atomic nucleus. On the one hand, the experiments at HZDR and in Durham produced an important increase in knowledge about the strength and structure of magnetic dipole excitations in atomic nuclei. On the other, they can also be incorporated into future calculations and simulations of nuclear reactions, because the probability that an atomic nucleus absorbs or emits a gamma quantum can be inferred from the distributions of the excitation strengths. The exact knowledge of the probability is important, for example, in describing neutron capture reactions. These reactions play a decisive role in nuclear astrophysics – where we are investigating the origin of heavy elements in stellar explosions – as well as in nuclear engineering, for instance in the transmutation of radionuclides with long half-lives. Ernest Rutherford would probably be excited about the breadth of detail that has been discovered about the atomic nucleus over the past century. Publications: G. Rusev et al., Phys. Rev. Lett. 110, 022503 (2013) R. Massarczyk et al., Phys. Rev. C 86, 014319 (2012) R. Schwengner et al., Phys. Rev. C 87, 024306 (2013) INCREASED ENERGY: At the HIGS facility, accelerated electrons circulate inside a storage ring and hit the incoming FEL beam at the collision point. The light particles are backscattered and, in the process, their energy is increased by a factor of a million into the gamma range. They then exit the ring and irradiate the sample that is to be examined inside the experimentation chamber. Diagram: Calvin R. Howell, Duke University Durham, North Carolina FEL mirror e-beam FEL mirror γ-ray beam e-beam from linac e-beam collision point free electron laser beam Contact _Institute of Radiation Physics at HZDR Dr. Ronald Schwengner r.schwengner@hzdr.de

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