Design, calibration and tests of an extended-range Bonner sphere spectrometer

Stray radiation fields outside the shielding of hadron accelerators are of complex nature. They consist of a multiplicity of radiation components (neutrons, photons, electrons, pions, muons, ...) which extend over a wide range of energies. Since the dose equivalent in these mixed fields is mainly due to neutrons, neutron dosimetry is a particularly important task. The neutron energy in these fields ranges from thermal up to several hundreds of MeV, thus making dosimetry difficult. A well known instrument for measuring neutron energy distributions from thermal energies up to about E=10 MeV is the Bonner sphere spectrometer (BSS). It consists of a set of moderating spheres of different radii made of polyethylene, with a thermal neutron counter in the centre. Each detector (sphere plus counter) has a maximum response at a certain energy value depending on its size, but the overall response of the conventional BSS drops sharply between E=10-20 MeV. This thesis focuses on the development, the calibration and tests in various radiation fields of a new Bonner sphere spectrometer with an extended response function. First, two new Bonner spheres with a response up to E=2 GeV were developed and built as a complement to a conventional BSS. This was achieved by an extensive Monte-Carlo (MC) simulation study performed with FLUKA on possible moderating materials and combinations of these materials. In this study particular care was taken to reliably estimate the uncertainties on the calculated response functions due to variations of the sphere diameter, the moderator density and the moderator thickness. MC simulations also pointed out that the old sphere supports made of plastics increase the neutron scattering and may therefore influence the count rate of the Bonner spheres. Thus, new sphere supports made of aluminium were designed and constructed. The two new spheres were assembled with the conventional BSS into a new extended-range spectrometer. The new spectrometer was then calibrated at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany, using monoenergetic neutron beams of E=144 keV, 1.2 MeV, 5 MeV and 14.8 MeV. In the following it was also tested with an 241Am-Be source in the CERN calibration laboratory and at CERF (CERN-EU high-energy reference field). The CERF facility is very well characterized and provides a broad neutron field from thermal energies up to several hundreds of MeV with a large relative contribution of 10-100 MeV neutrons. The tests in both fields showed that the simulations agreed with the measured values to an accuracy of better than 20%. This is perfectly adequate for the use in radiation protection. In fact, the extended-range Bonner sphere spectrometer has not only demonstrated its advantages in routine measurements in this domain, but was also successfully employed in research work related to radiation protection. Thereby the superior performance of the new extended-range BSS as compared to the conventional one became clearly visible. In addition to the work on the Bonner sphere spectrometer, preliminary Monte-Carlo studies aiming at an upgrade of CERF were performed in this thesis. It was shown that an optimized shielding configuration at CERF could produce a radiation field with higher intensities, higher energies and a higher relative neutron fluence as compared to the present set-up. This new reference field is of interest for forthcoming measurements related to the space programme and also for testing and calibrating the Bonner spheres at energies higher than what was possible in this thesis.