Collaboration on Research and Development concerned with the Methodology and Data of Radiation Dosimetry (EURADOS = European Radiation Dosimetry Group)
The objectives are:
the stimulation of collaborative developments and research into methods and techniques for the evaluation of exposures to and risks of ionizing radiations;
the harmonization of collaborative developments and researching radiation exposures by means of intercomparisons, workshops, seminars and by active collaboration;
the collection and evaluation of physical data relevant to the assessment of the biological effects of ionizing radiations and to the assessment of the occupational and environmental exposures of the populations of the European Communities.
The research has concentrated on understanding the differences between different computation codes in the calculation of neutron spectra from radioactive sources used for calibration purposes. Progress was made in resolving difficulties in the codes used in conjunction with the Bonner sphere type of neutron spectrometer. Attention was also turned towards the problem of calculating the doses arising from the ground deposition of radioactivity from the Chernobyl accident. The problems of accurately specifying the detail and variation of the geometry of the human body and its organs for accurate dosimetry have begun to be discussed.
The Bonner sphere unfolding intercomparison has concentrated on the unfolding problem ignoring response function uncertainties by assuming these responses were known precisely.
The benchmark study on response functions for Bonner spheres concentrated on a particular set of spherical helium-3 detectors at the centre of moderating polyethylene spheres of different sizes.
Some of the work contributed to the numerical study of external radiation dose form caesium ground contamination. In calculations for a number of isotopes an agreement of about 2% was found.
In order to meet the increasing need for standardising phantoms for external and internal dosimetry a subgroup has been founded. First discussions of this subgroup concentrated on the shortcomings of the MIRD phantom and proposed the use of a voxel phantom. The MIRD phantom is essentially a standard for the mass and volume of man and his organs. It does not represent the range of human sizes and it does not include a representation of the form or age of organs. Its chemical composition is not specified and it is not representative of children.
The results of an intercomparison of dose equivalent meters based on microdosimetric techniques have been analyzed. The main conclusions drawn are as follows.
All tissue equivalent proportional counter (TEPC) based area dosimeters are in principle able to determine ambient dose equivalents in photon, neutron and mixed radiation fields.
The systems are able to determine a mean quality factor of an unknown neutron and photon field.
The ambient dose equivalent response decreases in monoenergetic neutron fields with decreasing neutron energy.
The decrease is in equal parts due to the deterioration of the TEPC as a linear energy transfer (LET) spectrometer and to differences in radiation transport processes in the detector wall and the ion cyclotron resonance unit (ICRU) sphere.
The dose equivalent responses of the different systems for the broad neutron energy spectrum of the californium-252 deuterated water (D20) radiation are in much better agreement than for monoenergetic neutrons.
The energy dependence of the dose equivalent responses of the TEPC dosimeters is opposite to that of moderator based systems.
The sensitivity of TEPC area dosimeters depends in a complex way on the radiation field and the counter geometry. However, by choosing the appropriate detector size and wall thickness, TEPCs can be made sufficiently sensitive for all operational conditions of practical interest. In particular, the sensitivity can be made as high as that of conventional moderator based dosimeters. The interpretation of measured data is facilitated if microdosimetric spectra are provided.
Several of the prototype instruments are suitable for operational health physics applications.
The conventional calibration procedures in terms of lineal energy or absorbed dose leads to a calibration in terms of kerma. Calibration in a neutron beam in terms of ambinet dose equivalent improves the accuracy. The application of a correction factor to conventionally calibrated counters appears to be a reasonable alternative.
The decrease of dose equivalent response with decreasing neutron energy will be less pronounced if sufficiently large TEPCs could be built which operate adequately at simulated diameters of 1 um or less.
For 24 keV neutrons, the dose equivalent response can be improved by increasing the wall thickness. This alone, however, is not sufficient to improve the overall dose equivalent response. Increasing the size of the counter appears to have an effect similar to increasing the wall thickness.
The dose equivalent response of the variance technique can be improved by a decrease of evaluation of the quality factor.
Further improvements of the dose equivalent response appear possible by modifying counter geometry, counter materials and evaluation procedure.
An intercomparison of response functions of Bonner spheres has revealed a probelm with thermal neutron group cross sections. Results of a joint neutron irradiation of etched track dosemeters have been analyzed and published. A joint study of the background and sensitivity characteristics of etch plastics has been completed.
This research has focussed on improvements in the dosimetry of radiations of low penetrating power into the body and in particular on the dosimetry relevant to the assessment of the radiation hazards to the skin. Theoretical and practical studies on aspects of beta ray dosimetry have been carried out and the practical problem areas in occupational situations have been identified. An important aspect has been the intercomparision of beta ray sources between the primary national laboratories in an effort to harmonise standards of measurement for these radiations.
Measurements have been performed of dose rates from an extended, 6 cm diameter, strontium-90 yttrium-90 source using an extrapolation chamber, in connection with a benchmark experiment testing the validity of computer programs for determining dose rates from beta sources. Results from comparative measurements, using extrapolation chambers, of dose rates from 4, equal 4 cm by 4 cm, promethium-147 sources at different laboratories have been evaluated and various uncertainty sources have been identified.
The collaboration with EULEP in evaluating data on biological effectiveness of low penetrating radiations and depth of sensitive layers in the skin has continued.
A document has been prepared on the review of survey instruments for the measurement of dose rates in mixed fields of beta and photon radiations. The planning of an international workshop on skin dosimetry has been completed.
Research has taken place in 2 areas. The first has been the use of ionisation chambers for the accurate dosimetry of mixed radiations, primarily in biological experiments. This had led to the recognition of some systematic uncertainties in measurements made with these instruments and progress towards their resolution. The second has been with the use of the detection of tracks produced by densely ionising particles in plastics as a method of individual dosimetry for neutron radiations. This has been marked by entirely successful joint irradiations of different types of dosimeters, including prototypes, in conjunction with groups from North America. This work has led to faster progress in identifying methods for dealing with the technical problems of this form of dosimetry. More recently attention has turned to the possibility of using electronic detectors for the dosimetry of neutron radiations.
A work programme has been established consisting of: basic physical data for ionization in gases; modelling and experimental work on discharge processes in proportional counters; and development of gas ionization devices for dosimetry.
Progress is being made toward the provision of comprehensive European databases for internal dosimetry. This is an important topic which has hitherto lacked a specifically European dimension.
An experimental programme for using stable isotope tracers to investigate the metabolism of elements of interest to radiological protection has been established. It has been suggested that metabolic studies should include stable isotopes of barium, caesium, cobalt, tellurium, molybdenum and strontium and that the intestinal absorption fraction (f1) is the metabolic parameter of prime concern. Both adults and children are included in the study. The study proposal for the registries is intended to cover collaboration between all Commission for the European Community countries. Confidentiality of the database will be of paramount importance to avoid the identification of personal information. The format will be modelled on the Uranium and Transuranium Registries in the United States of America so that derived information can be exchanged. The aim is to provide improved information for internal dose assessments for all purposes. An intercomparision of dose assessments has been organised including acute intakes of: strontium-90 and caesium-137 by inhalation, phosphorus-32 by injection, a wound contaminated by plutonium; inhalation of mixed actinide nitrate aerosol; and the chronic inhalation of plutonium-239 over 10 years. Initial assessment of the results gave promising agreement. Progess has been made in setting up a United Kingdom autopsy registry for cases of internal contamination with radionuclides and a basis for a European registry of internal dose assessment computer models has been investigated.
A task group has been established to produce improved respiratory tract models relating intakes of radionuclides by workers to organ doses and biassay measurements.
A proposal has been formulated for an intercomparison study of whole body monitoring by European laboratorie s using a person adventitiously contaminated internally with gamma emitting radionuclides.
The collaboration is implemented or organized by Working Groups on the following topics:
Working Group 2: Skin Dosimetry
The evaluation of dose to the skin, in particular from exposure to beta and low energy photon radiations, and the development of appropriate methods for its measurement and assessment.
Working Group 4: Numerical Dosimetry
To disseminate information about computer programs for numerical dosimetry.
Working Group 6: Assessment of Internal Dose
The preparation of guidance on the interpretation of monitoring data relating to internal exposures of radiation workers and the implementation of ICRP recommendations on this topics within Europe.
Working Group 7: Radiation Spectrometry in Working Environments
The determination of dose equivalent quantities in mixed photon and neutron fields is still a problem due to the strong energy dependence on the dose equivalent response of commonly used neutron monitors. Sufficient accuracy is only achieved if instruments or detector systems with spectrometric properties are employed.
Working Group 8: Development of Individual Dosemeters for External Penetrating Radiations
The objectives of the group would be to develop and improve techniques for the individual dosimetry of penetrating radiations.
Working Group 9: Criticality Accident Dosimetry
The objectives of the group are to provide training in the operation of criticality accident dosimetry systems and to improve the methods of interpretation in terms of relevant dosimetric quantities.
Working Group 10: Basic Physical Data and Characteristics of Radiation Protection Instruments
The collection and assessment of basic physical data relevant to the biological effects of ionizing radiations and to the development of instrumentation for dosimetry in radiation protection and radiobiology.