NRPB - W25 Effect of the Energy Dependence of Response of Neutron Personal Dosemeters Routinely Used in the UK on the Accuracy of Dose Estimation
Authors:
R J Tanner, D J Thomas, D T Bartlett, L G Hager, N Horwood and G C Taylor
Publication date: September 2002
ISBN: 0-85951-496-X
Synopsis
A large set of neutron energy distributions was classified by workplace to provide sets of neutron fields to which workers in particular industries may be exposed. These were folded with personal dosemeter response characteristics to determine the systematic errors in values of neutron personal dose equivalent that those dosemeters would give in workplaces. Data for neutron effective dose equivalent recorded for UK classified workers have been taken from the Central Index of Dose Information (CIDI) reports for the years from 1990 to 2001, and related to the results from the folding process. Hence, it has been possible to determine the probable systematic errors that result from the use of the currently available neutron personal dosemeters.
The main conclusions that may be drawn from this work are as follows.
- In spontaneous fission or (alpha, n) fields there is no evidence for widespread over-reporting of neutron dose equivalent caused by the energy dependence of response of neutron personal dosemeters. Unless a highly inappropriate calibration response is used, there is no significant potential for such overestimates of dose equivalent.
- There is significant potential for underestimation of neutron personal dose equivalent, particularly if the dosemeter has no response below its fast neutron threshold.
- Above the fast neutron threshold, the energy of that threshold has more influence on the reading than the 'flatness' of the response function.
- The dependence of the response function on the direction distribution of the field will have a significant impact on the response in the workplace. The use of simple planar detectors calibrated for normal incidence may lead to underestimation of dose equivalent.
Consequently, in terms of the practical implications it is possible to state that there is no evidence of widespread restrictions on working practice caused by the energy dependence of response of neutron personal dosemeters; overestimates of dose will be infrequent. There is, however, significant potential for underestimates of dose because dosemeters fail to detect with adequate efficiency the whole energy range of workplace neutron spectra. The most significant influence that this may have is on the reporting of false negative doses rather than the underestimation of reported doses.
The study has been hampered by the limited availability of measured or calculated UK workplace energy distributions. Some workplaces are poorly covered by measured or calculated energy distributions that are in the public domain. Difficulties were also encountered in linking CIDI workplace categories to working practices.
Areas that require further study are listed below.
- Direction characteristics of workplace fields and the direction dependence of neutron personal dosemeter response: these are particularly important for a full assessment of the dosemeter response implications in terms of effective dose,
- Other aspects which can affect etched-track or nuclear emulsion neutron personal dosemeters: signal to noise ratio, background outliers, time dependence of response,
- The absence of large numbers of defence personnel from the CIDI tables: these are known to be one of the largest monitored groups,
- The dominance of the beta/gamma film badge in some CIDI categories: this dosemeter was outside the scope of this study, and is known to have significant flaws as a neutron personal dosemeter.
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nrpb w25 (PDF, 660 KB)Last reviewed: 29 July 2009
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