Energy and direction distribution of neutrons in workplace fields: implication of the results from the EVIDOS project for the set-up of simulated workplace fields

Workplace neutron spectra from nuclear facilities obtained within the European project EVIDOS are compared with those of the simulated workplace fields CANEL and SIGMA and fields set-up with radionuclide sources at the PTB. Contributions of neutrons to ambient dose equivalent and personal dose equivalent are given in three energy intervals (for thermal, intermediate and fast neutrons) together with the corresponding direction distribution, characterised by three different types of distributions (isotropic, weakly directed and directed). The comparison shows that none of the simulated workplace fields investigated here can model all the characteristics of the fields observed at power reactors.     

Electronic neutron personal dosemeters: their performance in mixed radiation fields in nuclear power plants

This work describes spectral distributions of neutrons obtained as function of energy and direction at four workplace fields at the Krümmel reactor in Germany. Values of personal dose equivalent H(p)(10) and effective dose E are determined for different directions of a person's orientation in these fields and readings of personal neutron dosemeters--especially electronic dosemeters--are discussed with respect to H(p)(10) and E.     

Achievements in workplace neutron dosimetry in the last decade: lessons learned from the EVIDOS project

The availability of active neutron personal dosemeters has made real time monitoring of neutron doses possible. This has obvious benefits, but is only of any real assistance if the dose assessments made are of sufficient accuracy and reliability. Preliminary assessments of the performance of active neutron dosemeters can be made in calibration facilities, but these can never replicate the conditions under which the dosemeter is used in the workplace. Consequently, it is necessary to assess their performance in the workplace, which requires the field in the workplace to be fully characterised in terms of the energy and direction dependence of the fluence. This paper presents an overview of developments in workplace neutron dosimetry but concentrates on the outcomes of the EVIDOS project, which has made significant advances in the characterisation of workplace fields and the analysis of dosemeter responses in those fields.     

The influence of the energy distribution of workplace fields on neutron personal dosemeter reading

Variations in the energy dependence of response of neutron personal dosemeters cause systematic errors in the readings obtained in workplace fields. The magnitude of these errors has been determined theoretically by folding measured and calculated workplace energy distributions with dosemeter response functions, to determine the response of a given personal dosemeter in that field. These results have been analysed with consideration of the dosemeter response to various calibration spectra, and with reference to different workplaces. The dosemeters in the study are discussed in terms of the workplaces for which they can be suitably calibrated. Deficiencies in the published neutron energy distributions are identified.     

Compliance of electronic personal neutron dosemeters with the new International Standard IEC 61526

The recommendations and test requests for the dose equivalent response of personal neutron dosemeters formulated by the new International Standard IEC 61526 are summarised. In particular, IEC 61526 allows the use of broad fields if dosemeters do not fulfil the hard requirements using monoenergetic neutrons. Some broad fields which can work as a replacement field using ISO sources ((252)Cf, (252)Cf (D(2)O mod.), (241)Am-Be) and simulated workplace fields (CANEL and SIGMA) are described. This work shows the results of recent measurements of the personal dose equivalent response for the dosemeters Thermo Electron EPD-N2, Aloka PDM-313 and the prototype dosemeter PTB DOS-2002, and discusses their compliance with respect to the new IEC 61526 standard.     

Intercomparison on measurements of the quantity personal dose equivalent Hp(d) in mixed (neutron-gamma) fields--an IAEA project

Within its occupational radiation protection programme, the International Atomic Energy Agency (IAEA) initiated and funded an international intercomparison exercise of personal dosemeters to determine the quantity personal dose equivalent in mixed neutron-photon radiation. The objectives of the intercomparison are to assess the capabilities of the dosimetry services in measuring the quantity Hp(10) in mixed neutron-gamma fields; to assist IAEA member states in achieving sufficiently accurate dosimetry; and, if necessary, to provide guidelines for improvements (not simply a test of the performance of the existing dosimetry service). The intercomparison is directed to passive dosemeters to determine, in mixed neutron-gamma radiation fields, either these two components separately or the total personal dose equivalent. The intercomparison consists of two phases: Phase I--Type-test intercomparison: irradiation in selected calibration fields and results used to improve dosimetric procedures of participating laboratories, where needed. Phase II--Simulated workplace field intercomparison: irradiation in radiation fields similar to those in workplaces as a final check of performance. The exercise revealed clear deficiencies in the methodology used by several laboratories and necessitated a detailed analysis of the existing discrepancies. This papers summaries the finding and conclusions for radiation fields similar to those found in nuclear industry.     

Considerations in the design of an improved transportable neutron spectrometer

The Transportable Neutron Spectrometer (TNS) has been used by the Ministry of Defence for over 15 years to characterise neutron fields in workplace environments and provide local correction factors for both area and personal dosimeters. In light of advances in neutron spectrometry, a programme to evaluate and improve TNS has been initiated. This paper describes TNS, presents its operation in known radioisotope fields and in a reactor environment. Deficiencies in the operation of the instrument are highlighted, together with proposals for updating the response functions and spectrum unfolding methodologies.     

Characterization of neutron field in a NPP workplace

At the Krsko Nuclear Power Plant (NPP), albedo dosimeters are used for personal neutron dosimetry. Spectrometric measurements allow determination of reference dosimetric values of realistic neutron fields to be used for calibration of albedo dosimeters. The Laboratory for Neutron Metrology and Dosimetry from the Institute for Radiological Protection and Nuclear Safety (IRSN) was in charge of characterising neutron fields in the plant at two representative points with high neutron and gamma dose rate. Calibration of the dosimeters in the workplace used to be performed only by a spherical survey meter. Based on the reference dosimetric values, the Plant Dosimetry Laboratory has verified the response of albedo dosimeters.     

Determination of personal dose equivalents in accelerator radiation fields

Values for the dose equivalent are required for radiation protection purposes, but determination of such values can be quite difficult for high energy radiations. The accurate determination of personal dose equivalents in accelerator radiation fields requires the propel use of appropriate radiological quantities and units, knowledge of the dose equivalent response of the personal dosemeters used, measurement or calculation of the fluence spectrum in the workplace and the fluence spectrum of the reference radiation used to calibrate the dosemeters, in addition to knowledge of the appropriate fluence-to-dose equivalent conversion coefficients. This information can then be used to select the appropriate dosemeters, set up the optimum calibration conditions, or to establish correction factors that account for differences in the calibration and workplace fluence spectra. High energy neutrons account for a significant fraction of the dose equivalent received by workers at accelerator facilities, and this work discusses the procedures and methods needed to determine dose equivalent produced by neutrons in the vicinity of high energy particle accclerators.     

The response of the BTI bubble detectors in mixed gamma-neutron workplace fields

Bubble detectors have become a mature technology and are used as neutron dosemeters in a wide range of applications. At the SCK-CEN and Belgonucléaire they are used as official personal neutron dosemeter for the personnel. In the European Commission (EC) project of Evaluation of Individual Dosimetry in Mixed Neutron and Photon Radiation Fields (EVIDOS), a whole range of neutron dosemeters were irradiated in workplace fields in nuclear installations in Europe, including two types of bubble detectors. The responses of the bubble detectors are compared with the reference values determined using a directional spectrometer and a reference instrument to measure Hp(10).     

Improved characterisation of the HPA PADC neutron personal dosemeter

The fast neutron energy dependence of response of the HPA PADC neutron personal dosemeter has been measured from 144 keV to 19 MeV using monoenergetic neutron fields. Below 144 keV the relative energy and angle dependence of response have been determined using MCNP-4C2. New data from the SIGMA field at Cadarache, France, have been used to determine the appropriate scaling factor for the calculated response to thermal and intermediate energy neutrons. These newly determined response characteristics of the dosemeter are discussed with respect to its performance in the EVIDOS workplace field irradiations.     

Numerical and experimental results of the operational neutron dosemeter 'Saphydose-N'

Since 1993, the Institute for Radiological Protection and Nuclear Safety (IRSN) has lead, in association with Electricité de France (EDF), a R&D study of a neutron personal electronic dosemeter. This dosemeter, called 'Saphydose-N', is manufactured by the SAPHYMO company. This paper presents first the optimisation of some detector components using Monte Carlo calculations, and second the test of the manufactured Saphydose-N under radiation following the IEC 1323 standard's recommendations for active personal neutron dosemeters. The measurements with the manufactured dosemeter were performed on the one hand at PTB (Physikalisch-Technische Bundesanstalt) in mono-energetic neutron fields and, on the other hand at IRSN in neutron fields generated by a thermal facility (SIGMA), radionuclide ISO sources and a realistic spectrum (CANEL/T400). The manufactured dosemeter Saphydose-N was also tested during measurement campaigns of the European programme EVIDOS ('Evaluation of Individual Dosimetry in Mixed Neutron and Photon Radiation Fields') at different nuclear workplaces. The study showed that Saphydose-N complies with the recommendations of standard IEC 1323 and can be used at any workplace with no previous knowledge of the neutron field characteristics.     

EURADOS trial performance test for photon dosimetry

Within the framework of the EURADOS Action entitled Harmonisation and Dosimetric Quality Assurance in Individual Monitoring for External Radiation, trial performance tests for whole-body and extremity personal dosemeters were carried out. Photon, beta and neutron dosemeters were considered. This paper summarises the results of the whole-body photon dosemeter test. Twenty-six dosimetry services from all EU Member States and Switzerland participated. Twelve different radiation fields were used to simulate various workplace irradiation fields. Dose values from 0.4 mSv to 80 mSv were chosen. From 312 single results, 26 fell outside the limits of the trumpet curve and 32 were outside the range 1/1.5 to 1.5. Most outliers resulted from high energy R-F irradiations without electronic equilibrium. These fields are not routinely encountered by many of the participating dosimetry services. If the results for this field are excluded, most participating services satisfied the evaluation criteria.     

Characterisation of mixed neutron photon workplace fields at nuclear facilities by spectrometry (energy and direction) within the EVIDOS project

Within the EC project EVIDOS, 17 different mixed neutron-photon workplace fields at nuclear facilities (boiling water reactor, pressurised water reactor, research reactor, fuel processing, storage of spent fuel) were characterised using conventional Bonner sphere spectrometry and newly developed direction spectrometers. The results of the analysis, using Bayesian parameter estimation methods and different unfolding codes, some of them especially adapted to simultaneously unfold energy and direction distributions of the neutron fluence, showed that neutron spectra differed strongly at the different places, both in energy and direction distribution. The implication of the results for the determination of reference values for radiation protection quantities (ambient dose equivalent, personal dose equivalent and effective dose) and the related uncertainties are discussed.     

Evaluation of individual monitoring in mixed neutron/photon fields: mid-term results from the EVIDOS project

EVIDOS is an EC sponsored project that aims at an evaluation and improvement of radiation protection dosimetry in mixed neutron/photon fields. This is performed through spectrometric and dosimetric investigations during different measurement campaigns in representative workplaces of the nuclear industry. The performance of routine and, in particular, novel personal dosemeters and survey instruments is tested in selected workplace fields. Reference values for the dose equivalent quantities, H(*)(10) and H(p)(10) and the effective dose E, are determined using different spectrometers that provide the energy distribution of the neutron fluence and using newly developed devices that determine the energy and directional distribution of the neutron fluence. The EVIDOS project has passed the mid-term, and three measurement campaigns have been performed. This paper will give an overview and some new results from the third campaign that was held in Mol (Belgium), around the research reactor VENUS and in the MOX producing plant of Belgonucléaire.     

Performance of a personal neutron dosemeter based on direct ion storage at workplace fields in the nuclear industry

In the framework of the EVIDOS project, funded by the EC, measurements were carried out using dosemeters, based on ionisation chambers with direct ion storage (DIS-N), at several workplace fields, namely, at a fuel processing plant, a boiling and a pressurised water reactor, and near transport and storage casks. The measurements and results obtained with the DIS-N in these workplaces, which are representative for the nuclear industry, are described in this study. Different dosemeter configurations of converter and shielding materials were considered. The results are compared with values for personal dose equivalent which were assessed within the EVIDOS project by other partners. The advantages and limitations of the DIS-N dosemeter are discussed.     

Application of workplace correction factors to dosemeter results for the assessment of personal doses at nuclear facilities

Ratios of H(p)(10) and H*(10) were determined with reference instruments in a number of workplace fields within the nuclear industry and used to derive workplace-specific correction factors. When commercial survey meter results together with these factors were applied to the results of the locally used personal dosemeters their results improved and became within 0.7 and 1.7 of the reference values or better depending on the response of the survey meter. A similar result was obtained when a correction was determined with a prototype reference instrument for H(p)(10) after adjustment of its response. Commercially available survey instruments both for photon and neutron H*(10) measurements agreed with the reference instruments in most cases to within 0.5-1.5. Those conclusions are derived from results reported within the EC supported EVIDOS contract.     

Workplace monitoring of mixed neutron-photon radiation fields and its contribution to external dosimetry

Workplace monitoring is a common procedure for determining measures for routine radiation protection in a particular working environment. For mixed radiation fields consisting of neutrons and photons, it is of increased importance because it contributes to the improved accuracy of individual monitoring. An example is the determination of field-specific correction factors, which can be applied to the readings of personal dosemeters. This paper explains the general problems associated with individual dosimetry of neutron radiation, and describes the various options for workplace monitoring. These options cover a range from the elaborate field characterisation using transport calculations or spectrometers to the simpler approach using area monitors. Examples are given for workplaces in nuclear industry, at particle accelerators and at flight altitudes.     

Workplace characterisation in mixed neutron-gamma fields, specific requirements and available methods at high-energy accelerators

A good knowledge of the radiation field present outside the shielding of high-energy particle accelerators is very important to be able to select the type of detectors (active and/or passive) to be employed for area monitoring and the type of personal dosemeter required for estimating the doses received by individuals. Around high-energy electron and proton accelerators the radiation field is usually dominated by neutrons and photons, with minor contributions from other charged particles. Under certain circumstances, muon radiation in the forward beam direction may also be present. Neutron dosimetry and spectrometry are of primary importance to characterise the radiation field and thus to correctly evaluate personnel exposure. Starting from the beam parameters important for radiation monitoring, the paper first briefly reviews the stray radiation fields encountered around high-energy accelerators and then addresses the relevant techniques employed for their monitoring. Recent developments to increase the response of neutron measuring devices beyond 10-20 MeV are illustrated. Instruments should be correctly calibrated either in reference monoenergetic radiation fields or in a field similar to the field in which they are used (workplace calibration). The importance of the instrument calibration is discussed and available neutron calibration facilities are briefly reviewed.     

A method for evaluating personal dosemeters in workplace with neutron fields

Passive detectors, as albedo or track-etch, still dominate the field of neutron personal dosimetry, mainly due to their low-cost, high-reliability and elevated throughput. However, the recent appearance in the market of electronic personal dosemeters for neutrons presents a new option for personal dosimetry. In addition to passive detectors, electronic personal dosemeters necessitate correction factors, concerning their energy and angular response dependencies. This paper reports on the results of a method to evaluate personal dosemeters for workplace where neutrons are present. The approach here uses few instruments and does not necessitate a large mathematical workload. Qualitative information on the neutron energy spectrum is acquired using a simple spectrometer (Nprobe), reference values for H*(10) are derived from measurements with ambient detectors (Studsvik, Berthold and Harwell) and angular information is measured using personal dosemeters (electronic and bubbles dosemeters) disposed in different orientations on a slab phantom.