Neutron dosimetry with ion chamber-based DIS system

A new type of personal dosimeter has been developed by RADOS Technology Oy. This system is based on a novel detector type, an ionisation chamber with so-called Direct Ion Storage (DIS). Ionisation chambers are sensitive to both neutrons and photons. Therefore, the application of the DIS principle to neutron dosimetry requires a double-chamber system for separating photon from neutron dose components. A chamber with the highest-possible and a chamber with the lowest-possible neutron sensitivity must be used, and their signals have to be differentiated. The energy dependences for photons of the two chamber types have to be almost equal to obtain good photon discrimination. A prototype DIS neutron dosimeter has been built and tested. The ion chambers are positioned inside a hermetically-sealed housing measuring 35x40x7 mm3. The weight of a dosimeter is about 35 g. For the readout of the dosimeters two types of electronic readers were used, one being a table-top unit and the other a small pocket-size unit. Different wall materials for the ion chambers were studied. Prototype dosimeters with a high response for fast neutrons were built with tissue equivalent wall materials A-150 and polyethylene (PE) for detection of recoil protons. For the measurement of thermal neutrons A-150 containing different contents of boron nitride (BN) and PE containing LiNO3 were used. Teflon and graphite, which are materials with low interaction probabilities with neutron radiation, were used for the construction of dosimeters with low neutron sensitivity. The dosimeters were irradiated with various monoenergetic neutrons, neutron sources and field spectra. The responses for bare Cf-252 and moderated Cf-252 are about equal to the AmBe response except for detectors with a high content of BN and LiNO3, where the response increases for more thermalized spectra. The results show that the response of pure A-150 is nearly constant in the fast neutron energy range (1 to 10 MeV) but decreases for lower neutron energies. The responses of A-150 containing boron increase with lower energies due to the increasing fraction of thermalized albedo neutrons on the phantom. In Figure 1 the responses to monoenergetic neutrons are shown. The angle dependence of the neutron response of the combined detectors was measured at 0deg, 30deg and 60deg for various neutron sources, field spectra and monoenergetic neutrons. The results show that the neutron responses vary between ±30% for angles up to 60deg. For irradiations with neutron sources with about 3 mSv, the reproducibility was within 5-10%. Prototype units of complete personal neutron dosimeters are being developed. It is the aim to produce combined systems for photon, beta and neutron radiation for legal dosimetry. (author)