Thermo-mechanical design of a photoneutron source for time-of-flight experiments

At the radiation source ELBE (Electron accelerator producing a quasi-continuous electron beam of high Brilliance and low Emittance) of the FZR a neutron source is being constructed. The electron beam with energies of up to 40 MeV and pulse frequencies of up to 13 MHz is converted into sub-ns neutron pulses by stopping the electrons in a heavy (high atomic number) radiator with a small volume. The neutrons are generated by bremsstrahlung photons through (gamma,n)-reactions. 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 escaping from the radiator and the secondary radiation are dumped to a large extent 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 to determine the volumetric heat generation in the liquid lead, in the channel wall and in the Al beam dump. Subsequent fluiddynamic and thermo-mechanic finite element analyses are performed to proof the mechanical integrity of the radiator channel. It could be shown that the equivalent plastic strain of the radiator channel can be kept sufficiently small, i.e. less than 1 %. Thermal analyses of the water cooled Al beam dump proved, that the maximum temperatures do not exceed 200 °C, thus a sufficient distance from the melting point is maintained.