Dr. Arnd Junghans

Project leader
Nuclear Physics
Phone: +49 351 260 3589

Dr. Roland Beyer

Beam line scientist, radiation protection officer FWK
Nuclear Physics
Phone: +49 351 260 3281

Neutron spectral rate

The spectral rate of neutrons [n / (cm2 s MeV)] can be measured by detecting neutron-induced fission events in thin, homogeneous U-235 layers in a parallel-plate fission ionisation chamber. The typical spectral rate at nELBE is about 3·104 n /(cm2 s MeV) from 300 keV to 2 MeV as shown below. The neutron spectrum extends from below 100 keV to more than 10 MeV.

The neutron spectral rate at nELBE was measured using the H19 transfer instrument of PTB Braunschweig. It contains eight deposits of U-235 with a total mass 201.6(5) mg. The measurement time was 81 hours and no bremsstrahlung absorber was used.

nELBE neutron spectral rate

The emission peaks at 40, 89, 179, 254, 314, and 605 keV are from near threshold photo neutron emission in 208Pb (strong capture resonances of 207Pb) see [R. Beyer et al., NIM A 723 (2013) 151].

The neutron spectral rate is determined from a time-of-flight measurement. By setting a lower threshold on the pulse height of the fission chamber, only signals from bremsstrahlung and neutron-induced fission events can be selected. The time resolution of the fission chamber was determined from the width of the photo-fission peak to about 2.5 ns (FWHM). The typical rate of fission events is about 5 - 30 s-1 depending on the electron beam intensity, as the efficiency of the fission chamber is only about 1.5·10-5 . A constant background of room return neutrons is shown to be very low.

nELBE H19 time-of-flight spectrum

The time-of-flight is measured relative to the accelerator frequency. Bremsstrahlung induced fission events have a time-of-flight of about 20 ns (600 cm flight path). The first neutron-induced signals occur about 150 ns afterwards. The time-of-flight spectrum ends at about 2000-2500 ns.