The Radiation Source ELBE

At the Forschungszentrum Rossendorf (FZR) a major new installation is approaching its completion - the ''Strahlungsquelle ELBE''. This radiation source will use the high brilliance electron beam from a linac with superconducting rf-cavities to produce various secondary beams for experiments in nuclear, solid-state, environmental and bio-medical physics as well as in various other fields. To present a comprehensive status report of this important and innovative project, the present Annual Report combines contributions made to it by several groups and departments of the FZR. The Departments ''Experimental Facilities and Information Technology'' and ''New Accelerators'' as well as the ELBE-beam-diagnostics-group have carried out numerous developments within the ELBE project; the most important are described mainly in the beginning of this Report. The subsequent contributions from groups of the IKH are more strongly focused on the production and future use of the different kinds of secondary radiation.

Physics with IR-FEL's and their radiation

The high brilliance electron beam produced by a super-conducting linac like ELBE allows the investigation of various processes which produce high quality electromagnetic radiation in various energy (i.e.) ranges. An especially illustrative and theoretically clear example for the generation of radiation is the Free-Electron-Laser (FEL). With the electron energy range available at ELBE coherent infrared radiation from 300 mm (corresponding to 1 THz) to 3 mm (equivalent to 0.4 eV) can be produced in short pulses of a few ps and with a large repetition rate finally allowing tens of watts average intensity. At the long wavelength side the limitation results mainly from the difficulties in handling the strong diffraction effects. Various theoretical and numerical studies have been performed on this problem and on the resulting IR-beam properties; laboratory experiments to test some of these calculations are presently being set up. For the short wavelengths the quality of the electron pulses and the undulator field is crucial. Our newly perfected pulsed-wire field scan allows to cross-check the detailed field maps obtained with Hall-probes and it will be used to document 'in situ' future variations in magnetic field strength. Research making use of IR- radiation in the 10 mm-range has been performed at existing FEL facilities and with Fourier- transform (FTIR)-spectroscopy methods. A FTIR-spectrometer operable in conjunction with a reflection-optic microscope is newly available at the institute to be used in conjunction with the IR-studies on biomedical probes to be performed at the FEL in the next years by scientists from the IKH.

X-rays, Bremsstrahlung Photons and Neutrons

Quasi-monochromatic X-rays can be generated from the fast electrons either by using the channeling process in single crystals (esp. diamond) or by Compton scattering of intense laser light from the electrons. For both processes a very high brilliance e-beam has to be realized which requires a beam transport system allowing for the reduction of unwanted emittance growth effects. At ELBE a decrease of the transverse emittance beyond present values is expected from improvements of the e-gun and finally the use of a photo cathode gun directly coupled to a superconducting cavity. Space charge effects can be minimized by enlarging the repetition rate from 13 up to 260 MHz - at constant average beam intensity. Studies will be performed to enhance the yield of channeling X-rays by resonant variation of the crystal parameters using a variable frequency ultra-sound field; additionally an improvement of the X-ray-to-bremsstrahlung-ratio will be strived for by inserting polycristalline carbon segments between radiator and probe. To promote the radiation damage studies planned by the IKH for the X-ray beam a cell-laboratory has been set up in the ELBE building.
In the case of MeV-bremsstrahlung to be used for nuclear spectroscopy a reduction of the background seen by the high resolution Ge-detectors will be aimed for by installing: (a) a vacuum tube to transport the beam from the radiator to the nuclear target, (b) a graded collimator made from pure Al, a material with low neutron production yield, (c) an extended photon beam stop formed by a hydrocarbon surrounded by Pb, (d) anti-coincidence shields from BGO around the detectors and (e) a coincidence circuit suppressing radiation delayed with respect to the e-bunches. First experiments on this beam-line will study the dipolar response of Mo-nuclei to photons of 5 to 15 MeV in dependence of their neutron number. Even more neutron-rich isotopes in this mass range will be accessible for spectroscopy as products from photon-induced fission in the future; such studies also deliver information of importance for the detailed understanding of the stellar synthesis of the heavier chemical elements.
The fast neutrons produced from the ps-e-bunches available at ELBE can be tagged such that a rather good energy resolution is obtained already after a few meter of flight path. The research envisaged here includes the investigation of fast neutrons with matter, especially with material for or from fission or fusion reactors. Also novel studies in the field of nuclear spectroscopy or nuclear astrophysics may become possible.

Collaborations

Within the FZR a close collaboration was established with the two departments responsible for the installation and operation of ELBE (i.e. Experimental Facilities / Information Technology and New Accelerators / ELBE-beam-transport and -diagnostics). Intense contacts also exist with the other Rossendorf institutes planning to use ELBE:

• The Institute of Ion Beam Physics & Materials Research and the IKH are jointly installing equipment in the optics laboratories including a fs-laser system which will be synchronized to the ELBE e-bunches thus allowing studies of the temporal e-bunch structure as well as investigations of ultra-fast processes involving IR-pulses from the FEL.
• The Institute of Radiochemistry and the IKH are jointly working on improving the sensitivity of IR-spectroscopical methods by replacing conventional thermal sources by the intense FEL- radiation; first exploratory studies in this field have been started at the CLIO-FEL at Orsay.
• The Institute of Safety Research has joined the DFG-project on the ''Interaction of fast neutrons with matter, esp. with materials for and from fusion and fission reactors'', which also includes groups from IKH as well as from Dresden University.

Various collaborative efforts have been or are being formed to make use of the various novel research possibilities offered by ELBE and its secondary beams. As a typical example the EU-project THz-Bridge is mentioned here; it brings together scientists from IKH, from Frankfurt and Stuttgart universities and from four other EU-countries in a study on the interaction of far-IR and THz radiation with living cells.  IKH 06/27/01 © E. Grosse