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A photo-neutron source for time-of-flight measurements at the radiation source ELBE
Altstadt, E.; Beckert, C.; Freiesleben, H.; Galindo, V.; Grosse, E.; Junghans, A. R.; Klug, J.; Naumann, B.; Schneider, S.; Schlenk, R.; Wagner, A.; Weiss, F.-P.;
The radiation source ELBE (Electron Linear accelerator with high Brilliance and low Emittance) at Forschungszentrum Rossendorf uses the high brilliance electron beam from a superconducting LINAC to produce various secondary beams. Electron beam intensities of up to Ie- = 1 mA at energies up to Ee- = 40 MeV can be delivered with a pulse width of less than 10 ps.
With these parameters the electron beam allows to generate sub-ns neutron pulses by stopping the electrons in a heavy (high atomic number) radiator and producing neutrons by bremsstrahlung photons through (gamma,n)-reactions. In order to enable measurements of energy resolved neutron cross sections like (n,gamma), (n,n'gamma), (n,p), (n,alpha), and (n,f) at a time-of-flight arrangement with a short flight path of only a few meters it is necessary to keep the volume of the radiator for neutron production as small as possible to avoid multiple scattering of the emerging neutrons, which would broaden the neutron pulses. It is the primary physics objective of this neutron source to measure neutron cross sections firstly for construction materials of fusion and fission reactors, for which it is important to select materials with low activation cross sections, and secondly for the handling of waste from such reactors, especially in order to find processes which transmute long-lived radioactive nuclides into short-lived and finally stable ones. Furthermore experiments can be performed which address problems of nuclear astrophysics.
The power deposition of the electron beam in the small neutron radiator volume of 1 cm³ reaches up to 25 kW. This is such a high power density that any solid high Z number material would melt. Therefore, the neutron radiator consists of liquid lead circulated by an electromagnetic pump. The heating power introduced by the electrons is removed through the heat exchanger in the liquid lead circuit. Typical flow velocities of the lead are between 1 m/s and 5 m/s in the radiator section. From the thermal and mechanical point of view, molybdenum turned out to be the most suited target wall material in the region where the electrons impinge on the neutron radiator.
To reduce the radiation background at the measurement position, the neutrons are decoupled from the radiator at an angle of about 90° with respect to the impinging electrons. Particle transport calculations using the Monte Carlo codes MCNP and FLUKA predict a neutron source strength in the range of 7.9E12 n/s to 2.7E13 n/s for electron energies between Ee- = 20 and 40 MeV. At the measuring place 3.9 m away from the radiator, a neutron flux of about 1.5E7 n/(cm² s) will be obtained. The short beam pulses allow for a neutron energy resolution of better than 1 % for neutron energies between En = 50 keV and 5 MeV. The usable energies range up to about 10 MeV.
Keywords: Pulsed photo-neutron source, Bremsstrahlung, Time-of-flight cross section measurement, Monte Carlo simulation, Thermo-mechanical analysis, Liquid lead radiator
  • Annals of Nuclear Energy 34(2007), 36-50

Publ.-Id: 8858 - Permalink