Measurement of neutron spectra and neutron doses at the Munich FRM II therapy beam with Bonner spheres

The neutron field at the FRM II neutron therapy was characterised by means of a Bonner sphere spectrometer. Inside the therapy room and the adjacent radiography room, gold foils were used in the center of the moderating polyethylene (PE) spheres of the spectrometer, to detect the moderated neutrons. The measured neutron spectra are dominated - as expected - by neutrons with an energy of about 1 MeV. The spectra were in quantitative agreement with those calculated by means of the MCNP code, by Breitkreutz and co-workers. Outside the therapy room, 3He proportional counters were used to detect the neutrons moderated by the PE spheres of the spectrometer. Here, the spectra were much softer and included thermal neutrons as major component. Based on these neutron spectra, the neutron ambient dose equivalent rate H.*(10) was calculated. The resulting H.*(10) values were between 0.92 and 1.68 μSv/h, depending on the position outside the shielding. These values were in close agreement to the one measured by means of the AMIRA tissue-equivalent proportional counter. It is concluded that the neutron spectra at the neutron therapy facility at the new FRM II research reactor at Garching, Germany, are now well characterised.