Recent developments in accelerator physics have led to new challenges for radiation protection dosimetry. As is well known, the ambient dose equivalent indicated by area monitors in high-energy neutron fields behind shielding is often unreliable. Therefore, it is desirable to measure the spectrum and to do "in-field" calibrations using the spectral information as reference. For this purpose, the PTB NEMUS (an extended-range Bonner sphere spectrometer) was modified with a new active thermal neutron detector based on silver activation capable of measuring in intense pulsed fields with high energies and fluence rates.