Design of a photoneutron source for time-of-flight experiments at the radiation source ELBE

The new radiation source ELBE at Research Center Rossendorf uses the high brilliance electron beam from a superconducting LINAC to produce various secondary beams. Electron beam intensities of up to 1 mA at energies between 12 MeV and 40 MeV can be delivered with a wide variability in the electron pulse structure. The maximum pulse frequency is 13 MHz with a pulse width less than 10 ps. The small emittance of the electron beam permits the irradiation of very small volumes. These main beam parameters led to the idea to convert the intense picosecond electron pulses into sub-ns neutron pulses by stopping the electrons in a heavy (high atomic number) radiator and to produce neutrons by bremsstrahlung photons through (gamma,n)-reactions. In order to enable measurements of energy resolved neutron cross sections like (n,p), (n,alpha) and (n,f) with 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 determine neutron cross sections firstly for construction materials of fusion and fission reactors, for which it is important to select radiation hard materials, 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. In addition, the distribution of fragments can be analyzed which are produced by neutron-induced transmutation of long-lived radioactive nuclides. Furthermore experiments can be performed which address problems of nuclear astrophysics.
The energy deposition of the electron beam in the small neutron radiator is that high that any solid material would melt. Therefore, the neutron radiator consists of liquid lead flowing through a channel of 11.2×11.2 mm² cross section. From the thermal and mechanical point of view molybdenum turned out to be the most suited channel wall (thickness 0.5 mm) material. Depending on the electron energy and current up to 20 kW power will be deposited into a radiator volume of 3 cm³. This heating power is removed through the heat exchanger in the liquid lead circuit. Typical flow velocities of the lead are in the range of 2 m/s in the radiator section. The electrons that are not stopped in the radiator and the secondary radiation are dumped in an aluminum beam dump. To reduce the radiation background in the measuring direction, the neutrons are decoupled from the radiator at an angle of 90° with respect to the impinging electrons. Particle transport calculations were carried out using the Monte Carlo codes MCNP and FLUKA. These calculations predict a neutron source strength of 7.88e+12 and 2.67e+13 n/s for electron energies between 20 and 40 MeV. At the measuring place 362 cm away from the radiator, neutron fluxes of 1.7e+7 n/(cm² s) will be obtained. The mentioned time-of-flight distance allows for an energy resolution better than 1%. The maximum usable neutron energy is about 7 MeV.