Upgrade of the Neutron Dose Measurement System at Bessy

Neutron radiation fields at synchrotron light sources are caused by bremsstrahlung from electron losses in accelerator components. Inside the enclosure and in transversal direction neutron and gamma radiation is of the same order of magnitude but high energy neutrons are much more penetrating. This causes outside the shielding neutron spectra with two broad maxima at about 1 MeV and 100 MeV. Standard Anderson-Braun or Leake neutron monitors measure thermalized neutrons in a proportional counter tube by nuclear reactions which limits the measurement range to neutron energies < 10 MeV. This implies two considerable systematic errors: Pulsed neutron beams causes dead-time losses due to the time structure of injections and the moderators are not sufficient to moderate high energy neutrons down to thermal energies. We determined and fixed these measurement errors by faster preamplifiers and by a more effective moderator developed by us, which expands the measurement range up to several GeV. Examples of the application at BESSY are presented.