Field calibration of fission track detectors within moderators in a high-energy field

At the Paul Scherrer Institut, measurements of ambient dose equivalent due to neutrons in stray radiation fields behind thick shielding are routinely done with fission track detectors in the centre of moderators. The fission track detector consists of a 10 μm thin polycarbonate detector foil (Makrofol KG) attached to two radiator disks made of uranium-aluminium alloy in a plastic holder. The moderator spheres have a diameter of 25 cm. The stray radiation fields behind shielding are produced by beam losses interacting with structural material during the acceleration or at the ejection to experimental areas. Naturally, the neutron spectrum depends strongly on the type and energy of the accelerator as well as the shielding material. The objective of this work was to determine field calibration factors for the detectors in a high-energy reference field. An example of such a reference field is the CERF-field (CERN EU high-energy reference field) at CERN, where neutrons from fractions of eV to about 1 GeV contribute to the ambient dose equivalent. The advantage of the CERF-field is that it simulates a workplace field behind thick shielding but is produced under laboratory conditions. Measurements were carried out at two reference positions and different beam intensities. Moreover, the determined field calibration factors were compared to calibration factors obtained with a 241Am-Be and a 252Cf radionuclide source. The comparison showed that the use of field calibration factors is essential, because calibration factors achieved with radionuclide sources underestimate the ambient dose equivalent due to neutrons in high-energy fields by more than a factor 2. (author)