Establishing local workplace field correction factors for neutron personal dosemeters

The present personal neutron dosemeters still need local correction factors to be able to provide accuracy comparable with photon dosemeters. Characterisation of the local neutron field is an indispensable part of neutron dosimetry to obtain such correction factors. It is often overlooked that besides characterisation in the neutron energy also the directional distribution of neutrons plays a crucial part in this characterisation. The authors have done such characterisation in the energy and angle for four workplace fields in Paks NPP. For this a relatively simple approximation method was used using the Nprobe for the energy distribution and measurements on the six sides of the slab phantom with personal dosemeters for the directional distribution. This allowed one to estimate a reference neutron Hp(10) rate and to compare it with the response of several neutron personal dosemeters. In October 2011, a measurement campaign was set-up in Paks NPP, which is a VVER reactor type. The measurements were performed to estimate reference values for personal dose equivalent rates dHp(10)/dt and to evaluate the behaviour of several personal dosemeters at different locations inside the plant. Two locations in the pump room and two locations in the reactor hall were chosen. Instead of just assuming that the fluence is unidirectional or that the fluence is isotropic, an attempt was made to estimate the directional distribution of the neutron field using a relatively simple measurement procedure. A number of active and passive personal dosemeters were placed on the six faces of a slab phantom and the results were analysed to obtain partial fluences in several directions of incidence. This method has important limitations, but is relatively simple to perform. The results show that it is very important to include the directional distribution in the Hp(10) evaluation. When comparing Hp(10)/dt and H*(10)/dt, H*(10) can be considered a conservative value for Hp(10). In the pump room H*(10) is ∼2.5 times larger than Hp(10), while in the reactor hall this is about a factor of 4. As was already expected, none of the used personal dosemeters can measure the Hp(10) accurately. The track-etch detectors were the only to stay within a factor of 2, but show an underestimation. The other detectors overestimate the reference dose, but need an important local correction factor that can differ from one location to another. In the case of electronic personal neutron dosemeters the spread in correction factors is an argument for task-specific correction factors, in the case of passive dosemeters, it is potentially the largest source of uncertainty in the assessment of Hp(10). The correction factors presented in this study are only applicable in Paks NPP, since they are not only determined by their energy response, but also by the directional distribution of the neutron field. (authors)