The use of high-energy linear electron accelerators (LINACs) for medical cancer treatments is widespread on an international scale. The associated bremsstrahlung X rays may produce neutrons as a result of subsequent photonuclear reactions with the different materials constituting the accelerator head. The generated neutron field is highly variable and depends strongly on the beam energy, on the accelerator shielding, on the flattering filter as well as on the movable collimators (jaws) design and on the irradiation field geometry. An estimate of this photoneutron component is, thus, of practical interest to quantify the radiological risk for the working staff and patients. Due to high frequency electromagnetic fields, and also to the presence of abundant leaked and scattered photons in these installations, measurements of the corresponding neutron fields by active dosemeters are extremely difficult. A modified version of the Bonner sphere system, based on passive gold activation detectors, has been used to perform neutron measurements at two points in a Varian 2,100C LINAC facility. A home-made unfolding procedure (CDM) has been utilised to determine the neutron spectra present at the measurement points. Results indicate that the giant dipole resonance process is the most adequate model to explain neutron production in the LINAC and that a thermal component is present at the measurement points.

The Institute for Radiological Protection and Nuclear Safety (IRSN) and the GFR, Universitat Autónoma de Barcelona (UAB) use Bonner spheres (BS) for neutron spectrometry at workplaces. The two systems, equipped with similar cylindrical 3He proportional counters, were simulated with the MCNP Monte-Carlo code to determine the response to neutrons of different energies for each polyethylene sphere. The BS systems were characterized at monoenergetic and thermal neutron fields. Measurements were performed at the Physikalisch-Technische Bundesanstalt (PTB) and at the National Physical Laboratory (NPL) standard laboratories, and with the newly characterized IRSN 'SIGMA' thermal neutron facility. The energy distribution of the reference neutron fluence was folded with the response functions for comparison purposes with the experimental data. In almost all cases related to monoenergetic neutrons, a good agreement between the experimental and the calculated count rates was found, and some discrepancies of a few per cent were observed in the thermal region.

The Grup de Física de les Radiacions (GFR) of the Universitat Autònoma de Barcelona (UAB), in collaboration with the Institute for Radiological Protection and Nuclear Safety (IRSN), has developed a passive Bonner sphere system (UAB-BSS), with gold foils as thermal neutron detectors, for application in pulsed neutron fields or in mixed neutron-photon fields with high photon intensities. In such fields, active devices suffer from saturation and dead-time effects. The MCNPX Monte-Carlo code has been used to determine the response to neutrons of different energies of each polyethylene sphere belonging to the BSS. The passive UAB-BSS system was characterised with the ISO (252)Cf reference source at the IRSN facilities. The energy distribution of the reference source neutron fluence was folded with the response functions for comparison with the experimental data. A good agreement between the experimental and calculated count rates was found.

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