Darmstadt superconducting electron linear accelerator

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The superconducting Darmstadt electron linear accelerator ( English superconducting Darmstadt electron linear accelerator , S-DALINAC ) is a particle accelerator for electrons at the Institute of Nuclear Physics of the Technical University of Darmstadt . It was designed and built as the successor to a pulsed normal conducting linear accelerator ("DALINAC"), which was in operation from the 1960s to the 1980s, by Achim Richter's research group and in its current layout in 1991 taken. It is used for basic research experiments.

Working principle

The S-DALINAC provides electron beams from a few mega electron volts (MeV) up to approx. 90 MeV; it was originally designed for a maximum energy of 130 MeV. The high frequency of the accelerator structures - from niobium existing cavity resonators , at the temperature of about 2  K superconductive are - is 3 GHz. The superconducting structures allow continuous operation with average electrical field gradients of typically 5 MV / m, that is, the kinetic energy of the electrons increases by approx. 5 MeV per meter of acceleration distance. The cavity resonators of the S-DALINAC consist of 20 cells. The average electron beam currents are variable from a few nanoampere up to 60 microampere (at the so-called injector linac) or 20 µA behind the so-called main accelerator.

The S-DALINAC has a thermionic electron gun for generating an unpolarized beam and, since 2011, a source of spin-polarized electrons based on the photoelectric effect . The sources emit electrons with 250 keV or 100 keV kinetic energy through electrostatic high voltage . The electrons are then accelerated to an energy of up to 10 MeV in the so-called injector linac. For higher energies, the beam can be shot into the so-called main accelerator, which can increase the kinetic energy by up to 40 MeV. Since the main accelerator can be run through up to three times ("recirculation"), the maximum energy is calculated to be approx. 130 MeV. However, these values ​​are typically not achieved because the cooling capacity of the helium liquefier is insufficient.

Experimental facilities and key results

Various experiments to investigate atomic nuclei and protons are carried out at S-DALINAC . Brake radiation is used at the experiment station behind the injector to excite or split atomic nuclei . Behind the main accelerator, bremsstrahlung can be used to study the proton and to study nuclear excitations with energy-marked photons (“photon dredger”). Further work deals with inelastic electron scattering , which provides information about the processes that take place when the atomic nucleus is excited.

At the S-DALINAC and its predecessor, the DALINAC, some groundbreaking discoveries in nuclear physics were made:

  • At DALINAC, Peter Brix's group determined core radii using elastic electron scattering.
  • The giant quadrupole resonance was observed for the first time at the DALINAC.
  • Excitations from changing the spin of individual particles at shell closures have been studied, e.g. B. in the calcium isotope chain.
  • In 1983 Achim Richter and co-workers succeeded in identifying a collective magnetic nuclear excitation which can be traced back to the orbital movement of protons and neutrons, the so-called scissors mode; the study of related core stimuli continues to this day.
  • In electron scattering at backward angles, indications of the elasticity of nuclear matter were found through the discovery of the so-called M2 twist mode.
  • Coincidence experiments, in which a particle released after excitation of the nucleus is detected at the same time as the scattered electron, were carried out for various questions. Recently z. B. measured the form factor for the breakup of the deuteron and interpreted the results with regard to the primordial nucleosynthesis of nuclear astrophysics .
  • In the last few years, experiments have been carried out at the bremsstrahlung measuring station to investigate nuclear excitations below the giant dipole resonance in the area of pygmy resonance or to investigate the photo-dissociation of nuclei. The results of the latter experiments are of interest for understanding the formation of neutron-poor heavy atomic nuclei in explosive astrophysical environments.
  • Operation and (continuous further) development of the S-DALINAC are mainly carried out in the context of study and doctoral theses. The results are incorporated into numerous theses and dissertations.
  • In the 1990s, the S-DALINAC was the driver for the first free-electron laser in Germany. In addition, experiments were carried out to generate narrow - band X - rays in the interaction of the electron beam with crystals .

financing

The construction of the S-DALINAC was funded in the 1980s and 1990s by the then Federal Ministry for Research and Technology. Since the 1990s, further development and experiments at S-DALINAC have been funded by the German Research Foundation (DFG). Currently, the work on S-DALINAC is part of the Collaborative Research Center 634 of the DFG (duration 2003–2015). The basic equipment for operation and the infrastructure costs are mainly borne by the State of Hesse via the Technical University of Darmstadt .

Web links

Individual evidence

  1. ^ A. Richter: Operational experience at the S-DALINAC. (PDF; 2.8 MB) In: Proc. EPAC 1996, Sitges / Barcelona. P. 110.
  2. ^ Y. Poltoratska et al .: Status and recent developments at the polarized-electron injector of the superconducting Darmstadt electron linear accelerator S-DALINAC . In: Journal of Physics: Conference Series . tape 298 , 2011, pp. 012002 , doi : 10.1088 / 1742-6596 / 298/1/012002 .
  3. ^ R. Pitthan, Th. Walcher: Inelastic electron scattering in the giant resonance region of La, Ce and Pr . In: Physics Letters B . tape 36 , no. 6 , 1971, p. 563-564 , doi : 10.1016 / 0370-2693 (71) 90090-6 .
  4. ↑ W. Steffen, H.-D. Gräf, W. Gross, D. Meuer, A. Richter, E. Spamer, O. Titze, W. Knüpfer: Backward-angle high-resolution inelastic electron scattering on 40, 42, 44, 48 Ca and observation of a very strong magnetic dipole ground-state transition in 48 approx . In: Physics Letters B . tape 95 , no. 1 , 1980, p. 23-26 , doi : 10.1016 / 0370-2693 (80) 90390-1 .
  5. ^ D. Bohle, A. Richter, W. Steffen, AEL Dieperink, N. Lo Iudice, F. Palumbo, O. Scholten: New magnetic dipole excitation mode studied in the heavy deformed nucleus 156 Gd by inelastic electron scattering . In: Physics Letters B . tape 137 , no. 1-2 , 1984, pp. 27-31 , doi : 10.1016 / 0370-2693 (84) 91099-2 .
  6. ↑ P. von Neumann-Cosel et al .: Spin and Orbital Magnetic Quadrupole Resonances in 48 Ca and 90 Zr from 180 ° Electron Scattering . In: Physical Review Letters . tape 82 , no. 6 , 1999, p. 1105-1108 , doi : 10.1103 / PhysRevLett.82.1105 .
  7. ↑ N. Ryezayeva et al .: Measurement of the Reaction 2 H (e, e ') at 180 ° Close to the Deuteron Breakup Threshold . In: Physical Review Letters . tape 100 , no. 17 , 2008, p. 172501 , doi : 10.1103 / PhysRevLett.100.172501 .
  8. ↑ z. BA Zilges, S. Volz, M. Babilon, T. Hartmann, P. Mohr, K. Vogt: Concentration of electric dipole strength below the neutron separation energy in N = 82 nuclei . In: Physics Letters B . tape 542 , no. 1–2 , 2002, pp. 43-48 , doi : 10.1016 / S0370-2693 (02) 02309-2 .
  9. ^ P. Mohr, K. Vogt, M. Babilon, J. Enders, T. Hartmann, C. Hutter, T. Rauscher, S. Volz, A. Zilges: Experimental simulation of a stellar photon bath by bremsstrahl: the astrophysical γ -process . In: Physics Letters B . tape 488 , no. 2 , 2000, pp. 127-130 , doi : 10.1016 / S0370-2693 (00) 00862-5 .
  10. ^ M. Brunken et al .: First lasing of the Darmstadt cw free electron laser . In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment . tape 429 , no. 1–3 , 1999, pp. 21-26 , doi : 10.1016 / S0168-9002 (99) 00060-1 .