Active neutron personal dosemeters - A review of current status

A review is presented of the technical feasibility and/or the availability of active neutron personal dosemeters, and the potential for development of such a dosemeter. The review consists of four parts. The first part is an introduction which considers the operational needs for an active neutron personal dosemeter. There does appear to be a need for an electronic neutron personal dosemeter, although it is not established that the demand for such a device is great. The relative magnitude of photon and neutron doses and dose rates in the nuclear industry has changed owing to the increased shielding of the photon component. Changes in fuel burnup and new fuel types have increased neutron exposure. Work on plant refurbishment or decommissioning will also increase neutron dose rates, and for these workplaces there may be sudden dose rate fluctuations in space or time. However, in the UK in 1996 for example, of 12,000 classified workers monitored for neutrons, there were only 100 recorded neutron doses of greater than 1 mSv. In the second part, the physics of personal neutron dosemeters is outlined and summaries are given of detection techniques. It is clear that the development of a practical electronic neutron dosemeter is difficult. Effort has been devoted by many research and development groups over several decades without complete success. The basic difficulty is the requirement to measure very small depositions of energy from neutron radiation which have to be recognised in the presence of photon and electron radiation. This basic difficulty is compounded by the present need to determine lower doses than hitherto. Brief summaries are given of techniques which are being applied to the development of an active neutron personal dosemeter, or are thought to be possible options for the future. The techniques considered are: low pressure (tissue-equivalent) proportional counters (TEPCs), thin semiconductors with event size analysis; two diode devices; multi-diode 'spectrometers'; detector arrays such as charge coupled detector devices; Li-6 or B-10 Sandwich spectrometers; radfets (field effect transistors); direct ion storage devices; memory devices; organic semiconductors; light pipes; GM tubes; organic films; advanced gas detectors; advanced silicon devices; and bubble detectors. Published papers up to mid-1998 have been considered. The third part summarises the performance and metrological requirements which have been proposed for neutron dosemeters by national and international standards and/or regulatory authorities. The fourth part of the review draws conclusions and suggests future action. Of devices presently being studied, it is considered that the more likely candidates for a successful outcome are, in the shea term, direct ion storage or bubble detectors, and in the longer term, charge coupled detectors, memory devices or optical fibres.