Considerable progress has been achieved in neutron dosimetry within the framework of this project.
Bonner sphere spectrometers proved to be very valuable instruments for specifying neutron fields at workplaces. Provided that the response functions are carefully determined by MCNP calculation and experimental calibration, the spectral neutron fluence and related dosimetric quantities can be determined for reference purposes, with uncertainties of less than 10% and 15% for the integral fluence and the ambient dose equivalent respectively.
A complete set of fluence-to-dose (equivalent) conversion functions has been calculated and evaluated taking into account recent recommendations of IRCP and ICRU. Application of these factors to neutron spectra encountered at workplaces in nuclear facilities results in an increase of about 30% if the ambient dose equivalent H*(10) is compared with the maximum dose equivalent Hmade according to the former ICRP recommendation (publication 21).
This trend can easily be calculated using the new catalogue with the comprehensive set of neutron spectra and conversion functions. Versatile programs handling this catalogue allow the neutron fields to be categorised, for example according to their mean dosimetric quantities, and the properties of instruments in these fields to be inspected, provided the response functions are known. Since the program package requires only standard PC hardware the system should become a valuable tool for the practitioners in radiation protection service.
The imperfect response of the neutron survey meters most commonly used make a field calibration necessary if significant over- or underreadings are to be avoided. Since reference measurements at the workplace may be very difficult, for example due to safety regulations, it is advisable to perform routine calibrations in the laboratory. Besides standard neutron fields based on radionuclide neutron sources, various accelerator-based neutron fields may be used, in particular those replicating the rather soft neutron spectra encountered at workplaces in nuclear facilities. Standardisation, however, requires greater experience as regards the field properties and the calibration procedures applicable.
The objective of this collaborative project is to produce in the laboratory a few well characterized neutron fields that replicate typical spectral neutron fluence distributions encountered in radiation protection practice. These fields are needed for the calibration of neutron area and personnel dosemeters which do not generally have the energy response required to determine dose equivalent quantities and hence often give wrong answers even if functioning correctly.Before any attempt can be made to produce these ''realistic'' fields, considerable preliminary work is needed to identify and characterise the type of neutron fields to which radiation workers are exposed. For this reason a series of measurements has already been performed over recent years, but these investigations must be continued at sites and facilities where risks due to neutron radiation are to be expected. It is our intention to gather together existing information from all sources worldwide, and to present this in a unified format. 0nly measurements and calculations which are sufficiently well-documented for their reliability to be evaluated will be included. The catalogue, which also includes dose equivalent to fluence conversion functions ( partially still to be calculated on the basis of the new ICRP recommendations ) and response functions of personnel and area dosemeters, will continously be updated and dose-equivalent values and detector responses for the spectra will be evaluated bY means of a generalized program already developed.Provided a few representative reference fields can be deduced, the next step will involve the use of general neutron-photon transport codes to design and optimise moderator assemblies with accelerator based or radionuclide neutron sources, which simulate these basic spectral distributions in the laboratory. As an example, one assembly has been realised at the 14 Mev neutron facility in Cadarache and was recently used for an intercomparison of spectrometers. The collaborating laboratories will take part in the scientific program in accordance with annex II of this proposal. This includes improvements of the spectrometric systems, further measurements in the field to be included in the catalogue, the evaluation of the catalogue, the development of a user guided expert system, the calculation of the dosimetric quantities and the realisation of realistic fields for calibration purposes.
Funding SchemeCSC - Cost-sharing contracts
TW11 0LW Teddington