Facilities for Europe - The Radiation Source ELBE

Peter Michel, Burkhard Kämpfer

The interaction of various forms of radiation with matter in atomic and subatomic dimensions as well as with tissues, cells and their components offers a wide range of new insights into their structures and functionalities. At the heart of ELBE (Electron Linear accelerator with high Brilliance and low Emittance) there is a superconducting linear accelerator consisting of two units which are cooled by liquid helium. It delivers a quasi-continuous electron beam of 5 to 40 MeV beam energy at beam currents up to 1 mA. This primary beam is particularly characterized by a low transverse emittance better than 10 mm mrad (even less than 1 mm mrad has been reached at low beam currents) and short pulses (typically 2 ps bunch length) with low energy spread and flexible temporal structure.

Due to these fairly dedicated properties a variety of secondary radiations are provided for experiments:

(i) Two Free Electron Lasers (FELs) with undulators of 27 mm and 100 mm period length deliver coherent radiation in the mid and far infrared. More precisely, the wavelength of the U27-FEL ranges from 4 to 20 µm while U100-FEL covers the range from 20 µm to 200 µm. Depending on the wavelength, several watts of optical power can be typically out-coupled. The infrared light beams are transported to several optical laboratories, where a broad range of different experiments are conducted. Main fields of research include semiconductor physics (ground state vibration population decay or experiments to determine the relaxation time of superlattices or self-assembled quantum dots), biophysics (IR-induced changes in thin DNA films), environmental and safety research and experiments on ellipsometry and nearfield microscopy. Further on, an additional transfer system directs the FEL light into the Dresden High Magnetic Field Laboratory (HLD) situated in a near-by building.

(ii) The high primary beam current allows generating intense secondary neutron beams, either in reactions with a rotating tungsten disc or a liquid lead target. The emerging neutron pulses carry the time structure of the primary electron beam making them well suited for time-of-flight experiments. Given this, neutron induced reactions aim at completing the data base for fusion reactor materials, transmutation of nuclear waste and certain steps in the astrophysical breeding processes of chemical elements.

(iii) By pair production from the intense gamma radiation field, positrons are produced in a stack of tungsten radiator foils. These will be extracted and delivered as a secondary beam for investigations in materials science.

(iv) The propagation of the well collimated primary electron beam through crystals, like diamond, generates channelling radiation, i.e. X-rays in the 5 to 100 keV range. While the process of X-ray production is a subject of research in its own right, X-rays are for instance used to investigate cell damage due to irradiation; however, these studies belong to the Biostructures and Radiation program and are addressed in the triennial report on life sciences at FZD.

(v) Irradiating a thin foil with the primary electron beam generates γ rays with energies up to 20 MeV. Exposing selected isotopes to the γ rays, their excitation and transformation into other isotopes in various reactions can be studied. The knowledge of such cross sections is important for understanding and modelling the cooking of chemical elements in explosive star phenomena. These investigations clearly complement those in item (ii). Further details can be studied by exposing isotopes directly to the electron beam.

(vi) Due to the excellent time structure and high intensity of the primary ELBE beam modern detector components with high time resolution below 100 ps and stability under high radiation load (10x particles/cm²) can be further developed.

The electron beam quality strongly depends on the source, i.e. the gun. In the first years of the operation of ELBE a thermionic gun was used. Research and development of a new gun is completed, and it is being installed. Table 1 displays the beam time statistics for the years until 2006.

Tab. 1: Beam time statistics for years until 2006.

Fig. 1: View from control room into the ELBE hall.

Fig.2: Scheme of the ELBE facility.

[1] Technology challenges for SRF guns as ERL sources in view of Rossendorf work, Janssen et al., NIM A 557, 80 – 86 (2006).

[2] RF Status of superconducting module development suitable for CWoperation: ELBE cryostats, J. Teichert et al., Proceedings of the 32nd Advanced ICFA Beam Dynamics Workshop on Energy Recovering Linacs ERL 2005.

[3] Test of the photocathode cooling system of the 3½ cell SRF gun, C. Staufenbiel et al., 441, 216 – 219 (2006).