Performance of a personal neutron dosemeter based on direct ion storage at workplace fields in the nuclear industry

Abstract

In the framework of the EVIDOS project, funded by the EC, measurements were carried out using dosemeters, based on ionisation chambers with direct ion storage (DIS-N), at several workplace fields, namely, at a fuel processing plant, a boiling and a pressurised water reactor, and near transport and storage casks. The measurements and results obtained with the DIS-N in these workplaces, which are representative for the nuclear industry, are described in this study. Different dosemeter configurations of converter and shielding materials were considered. The results are compared with values for personal dose equivalent which were assessed within the EVIDOS project by other partners. The advantages and limitations of the DIS-N dosemeter are discussed. DOSEMETER DESIGN AND DOSE DETERMINATIONThe personal neutron dosemeter DIS-N, developed by RADOS, is based on ionisation chambers with direct ion storage and a double-chamber system which allows for differential readings to separate the neutron from the photon dose equivalent (1) . However, ionisation chambers are sensitive to both neutrons and photons. Therefore, application of the double-chamber principle requires that the photon energy sensitivities of the two chambers are almost equal and the neutron energy sensitivities of the two chambers are significantly different.Four different types of dosemeter were investigated within the EVIDOS project. The chamber with a high sensitivity for fast neutrons is build with tissue-equivalent wall material (A-150) or polyethylene (PE) for the detection of recoil protons. The response to thermal neutrons can be modified by adding boron nitride (BN) to A-150 or LiNO 3 to PE and by surrounding the dosemeter with additional 'boron-covers' (containing 40% B 4 C). Thermal neutrons can then be detected by the secondary charged particles of the (n,a) reaction with 10 B and 6 Li, respectively, while the boron-covers will reduce the thermal neutron flux.The chamber with a low sensitivity to neutrons was in all cases made up of Teflon (polytetraflouroethylene) containing 60% graphite. Owing to a 1-mm-thick tin shielding around the chambers, photon energies <100 keV may be neglected.The following combinations of wall materials and boron-covers of the chamber with a high-neutron response to fast neutrons were used: PE with 4% LiNO 3 . PE with 4% LiNO 3 and 1 mm B 4 C.A-150 with 1.25% BN. A-150 with 1.25% BN and 2 mm B 4 C.The measured quantities N 1 and N 2 (arbitrary units, a.u.) in both chambers 1 (high-neutron sensitivity) and 2 (low-neutron sensitivity) in a mixed neutron/ photon field are due to the sum of neutron-induced and photon-induced effects and can be written as follows:where H n and H g are the neutron and photon dose equivalent; S 1n and S 2n are the neutron sensitivities of chambers 1 and 2; and S 1g and S 2g are the photon sensitivities of chambers 1 and 2. The sensitivities S i are average sensitivities dependent on the angular and energy distribution of the neutron and photon fields. The neutron dose equivalent can then be ...

Summary of personal neutron dosemeter results obtained within the EVIDOS project

“…The DISN 4% device is the favoured one among the devices with different wall material and shieldings (8) . Although its neutron response is comparable to-or even better than-those of the electronic devices, its main drawback is that a photon reading has to be subtracted.…”