6LIF SANDWICH-TYPE DETECTORS FOR LOW-DOSE INDIVIDUAL MONITORING IN MIXED NEUTRON PHOTON FIELDS

ICRP Publication 60 recommends the reduction of the annual dose limit for occupational exposure from 50 to 20 mSv and a doubling of the qual ity factor for medium energy neutrons. If occupational doses are evalu ated every month (which is obligatory e.g. in Germany and in Poland), the individual neutron dosemeter will have to measure neutron doses in the range of 100 muSv. No commercially available, automatic individua l dosimetry monitoring system exists that fulfils this requirement. So me of the parameters which influence the evaluation of the neutron dos e from readings of TL dosemeters have been studied in order to decreas e the variance of the measured neutron signal. In mixed neutron-photon fields, clear separation of the neutron component from the total read ing depends also on the uncertainty of the gamma dose measurements. Wh ile the thermal albedo neutrons are absorbed mostly at the surface of the 6LiF detector, the reduction of the detector thickness results in a decrease of its photon sensitivity, while its neutron sensitivity is almost principally maintained. As a consequence, the uncertainty of g amma dose contributes with lower weight to the variance of the evaluat ed neutron signal. First tests of an optimised 200 mum thick sandwich detector and 0.9 mm thick standard LiF chips were made at low neutron and photon dose ranges using different readers, in order to determine the uncertainty versus dose for different neutron-photon combinations. The conditions under which the new sandwich type detectors may improv e albedo neutron dosimetry are demonstrated.