Dose equivalent measurements in mixed and time varying radiation fields around high-energy accelerators

Measurements of ambient dose equivalent in stray radiation fields behind the shielding of high-energy accelerators are a challenging task. Several radiation components (photons, neutrons, charged particles, muons, etc.), spanning a wide range of energies, contribute to the total dose equivalent. The radiation fields are produced by beam losses interacting with structural material during the acceleration or at the ejection to experimental areas or other accelerators. The particle beam is usually not continuous but separated in "bunches" or pulses, which further complicates dose measurements at high-energy accelerators. An ideal dosimeter for operational radiation protection should measure dose equivalent for any composition of radiation components in the entire energy range even when the field is strongly pulsed. The objective of this work was to find out if an ionisation chamber operated as a "recombination chamber" and a TEPC instrument using the variance-covariance method ("Sievert Instrument") are capable of assessing the ambient dose equivalent in radiation fields with short pulses. The results are compared to those of a conventional TEPC instrument (HANDI-TEPC) and to the total dose equivalent values obtained from the suitable combination of readings from instruments currently used at CERN. These instruments are a combination of several detectors with different characteristics measuring individual dose fractions (i.e. an argon-filled chamber, a hydrogen-filled chamber and a rem ionisation chamber). The HANDI-TEPC is usually the preferred reference instrument in mixed radiation fields of unknown composition at CERN, but in strongly pulsed fields it fails because of pile-up effects. Although the mixed stray radiation fields differ strongly from those applied in standard calibration, the first step in using a radiation protection instrument is the intensive study and characterisation of the detector behaviour in reference fields of calibration sources. This forms the basis for improving the reliability and accuracy of measurements in unknown radiation fields. The next step is to test the detectors in mixed reference fields, which simulate workplace fields but are produced under laboratory conditions. At CERN, an opportunity for such a reference field is realized by the CERF-field (CERN-EU high-energy reference field). The field provides a mixed field of mainly neutrons ranging from thermal energies up to hundreds of GeV accompanied by photons and muons. At CERF the HANDI-TEPC was known to be suitable to be used as a reference instrument for the total dose equivalent. For this reason the results of the recombination chamber and Sievert Instrument were compared to those of the HANDI-TEPC for several reference locations and different beam intensities. In particular, the background radiation at CERF was studied, which has a non-negligible influence on the total dose equivalent and is strongly dependent on the reference location. At CERF, a very good agreement between the instruments was found. Finally the instruments were investigated in an unknown mixed stray radiation field with short pulses on the "PS-bridge". This PS-bridge is a location where 260 cm of concrete shield the beam line of the PS accelerator. The operation of the PS at CERN is composed of a supercycle of 16.8 s divided in 14 cycles of 1.2 s each. The duration of the particle pulse can be as short as 10 µs. In such a field the HANDI-TEPC was not able to resolve the single events within one particle pulse because of pile-up effects. The recombination chamber and the Sievert Instrument, however, appear to be capable of assessing the total dose equivalent correctly provided that the response is interpreted carefully by using appropriate corrections and calibration factors.