Preliminary results on bubble detector as personal neutron dosemeter

The bubble detector is demonstrated as one of the best suitable neutron detectors for neutron dose rate measurements in the presence of high-intense gamma fields. Immobilisation of a volatile liquid in a superheated state and achieving uniform distribution of tiny superheated droplets were a practical challenge. A compact and reusable bubble detector with high neutron sensitivity has been developed at the Indira Gandhi Centre for Atomic Research by immobilising the superheated droplets in a suitable polymer matrix. Two types of bubble detectors have been successfully developed, one by incorporating isobutane for measuring fast neutron and another by incorporating Freon-12 for both fast and thermal neutron. The performance of the detector has been tested using 5 Ci Am-Be neutron source and the results are described.     

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.     

A prototype personal neutron dosemeter with one silicon diode

The performance of a personal neutron dosemeter with a single silicon diode using a linear combination of its pulse height information was studied. Its dosimetric behaviour in fields with neutrons of different energy and directional distribution is shown for neutron energies ranging from thermal to 100 MeV and for directions of incidence ranging from frontal to lateral. The dosemeter is photon-insensitive and its dose detection threshold is at about 20 microSv. The dosimetric characteristics are compared with those of commercial dosemeters based on silicon detectors.     

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.     

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.     

Present status of the personal neutron dosemeter based on direct ion storage

In this paper the present status of the Direct Ion Storage Neutron (DIS-N) prototype dosemeter (RADOS) is described. The separation of neutron from photon dose equivalent has been improved by adding tin shieldings. The neutron energy response has been changed by additional plastic covers containing 40% B4C in order to reduce the over-response to thermal neutrons. The responses of the dosemeters were determined for standard photon and neutron fields (monoenergetic neutrons, neutron sources and simulated workplace fields). Irradiations in real workplaces were also performed. The dependence of the neutron response on the angle of incidence was measured for different neutron sources.     

Introduction of a thermal response to the DSTL PADC personal neutron dosemeter

The response of the Defence Science and Technology Laboratory (DSTL) PADC personal neutron dosemeter is strongly dependent upon neutron energy, with a range of 300-500 tracks per cm2 per mSv for energies between 1 and 5 MeV. Below 1 MeV the response drops off sharply. This lack of sensitivity is undesirable when the dosemeter is employed with the softened fission spectra encountered in the workplace. In order to incorporate a thermal response, a polypropylene converter doped with LiF has been placed directly in front of the PADC elements. Tritons produced in the thermal neutron reaction 6Li (n,t)alpha at 2.7 MeV will then penetrate the PADC, leaving a trail of damage. The reaction rate within the converter has been calculated using MCNP for thermal neutrons and a range of higher energies, while transport of the tritons is modelled using the SRIM/TRIM package to determine the resultant track density and depth distribution. The modelling and experimental work have demonstrated that a concentration of 0.2% natural lithium by weight results in a track density in a thermal field comparable with that produced per unit personal dose equivalent by neutrons greater than 1 MeV in the standard dosemeter. Additional MCNP modelling has demonstrated that the dosemeters' albedo response to intermediate energy neutrons can be enhanced considerably by placing a boron-doped shield in front of the converter and increasing its lithium concentration.     

Bonner sphere neutron spectrometry at nuclear workplaces in the framework of the EVIDOS project

The Institute for Radiological protection and Nuclear Safety was engaged in the EC funded EVIDOS project to provide reference spectrometry data using its Bonner sphere system. The data were processed by means of two unfolding codes, NUBAY and GRAVEL, both provided by the Physikalisch-Technische Bundesanstalt. The NUBAY program, based on Bayesian parameter estimation methods, assumes a parameterised spectrum and provides posterior probability distributions for the parameters. The code GRAVEL, an iterative algorithm based on SAND-II, was used with various default spectra, among them the NUBAY solution. The BS measurements were used to establish the neutron fluence energy distributions and reference values for the neutron ambient dose equivalent. As this quantity depends strongly on the high energy neutrons, a sensitivity analysis was done by unfolding the BS data with GRAVEL using the NUBAY solution spectrum as default with various changes in the parameters of the high energy peak. This new method of analysing Bonner sphere data allowed the determination of reliable neutron spectra, as well as a very good estimate of the corresponding integral quantities with small associated uncertainties.     

Energy and angular dependence of active-type personal dosemeter for high-energy neutron

In order to develop an active-type personal dosemeter having suitable sensitivity to high-energy neutrons, the characteristic response of silicon surface barrier detector has been investigated experimentally and theoretically. An agreement of the shape of pulse-height distribution, its change with radiator thickness and the relative sensitivity was confirmed between the calculated and experimental results for 14.8-MeV neutrons. The angular dependence was estimated for other neutron energies, and found that the angular dependence decreased with the incident energy. The reason was also discussed with regard to the radiator thickness relative to maximum range of recoil protons.     

An active personal neutron dosemeter based on microdosimetric principles: CIME

Over the last few years IPSN has been developing a small, tissue equivalent proportional counter (TEPC) with multielement geometry for personal radiation protection monitoring. This paper presents the last prototype, which is insensitive to microphony, and the experimental results. Numerical modelling results using CERN codes are partly presented and allow an understanding of the nuclear and electrostatic physics involved in a TEPC.     

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.     

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.     

Performance of the electronic personal dosemeter for neutron 'Saphydose-N' at different workplaces of nuclear facilities

This paper mainly aims at presenting the measurements and the results obtained with the electronic personal neutron dosemeter Saphydose-N at different facilities. Three campaigns were led in the frame of the European contract EVIDOS ('Evaluation of Individual Dosimetry in Mixed Neutron and Photon Radiation Fields'). The first one consisted in the measurements at the IRSN French research laboratory in reference neutron fields generated by a thermal facility (SIGMA), radionuclide ISO sources ((241)AmBe; (252)Cf; (252)Cf(D(2)O)\Cd) and a realistic spectrum (CANEL/T400). The second one was performed at the Krümmel Nuclear Power Plant (Germany) close to the boiling water reactor and to a spent fuel transport cask. The third one was realised at Mol (Belgium), at the VENUS Research Reactor and at Belgonucléaire, a fuel processing factory.     

A prototype personal neutron dosemeter based on an ion chamber and direct ion storage

Ionisation chambers are sensitive to both neutrons and photons. In order to produce a neutron dosemeter based on an ion chamber a double-chamber system which allows for differential readings has to be built. The system consists of one chamber with high neutron sensitivity (e.g. A-150 or polyethylene with 10B or 6Li compounds) and one chamber with low neutron sensitivity (e.g. graphite or Teflon). Different combined dosemeter prototypes were produced and their responses for standard photon and neutron radiation fields, as well as various field spectra, were determined. The feasibility of neutron dosimetry with ion chambers and direct ion storage (DIS) electronics has been proved. The results obtained with prototype dosemeters indicate the system's promising potential for legal approval in the future. Apart from dosimetric properties, the advantages of the system are its small size and weight, easy readout and relatively low production cost.     

Development of a new electronic personal neutron dosemeter using a CMOS active pixel sensor

A CMOS active pixel sensor, originally designed for the tracking of minimum ionising charged particles in high-energy physics, has been recently used for the detection of fast neutrons. Data were taken at the IRSN Cadarache facility with a (241)Am-Be ISO source and a polyethylene radiator. A high-intrinsic efficiency (1.2 x 10(-3)) has been obtained. It is in good agreement with both calculations and a MCNPX Monte Carlo simulation. This experiment paves the way for a fully electronic personal neutron dosemeter.     

Neutron area survey instrument measurements in the EVIDOS project

Neutron survey instruments have been exposed at all the measurement locations used in the EVIDOS project. These results have an important impact in the interpretation of the results from the project, since operationally the survey instrument will be used for an initial assessment of and routine monitoring of the ambient dose equivalent dose rate. Additionally, since the response of these instruments is in some cases very well characterised, their systematic deviations from the reference quantities provide an important verification of the determination of those quantities.     

Evaluation of dose equivalent by the electronic personal dosemeter for neutron 'Saphydose-N' at different workplaces of nuclear facilities

This paper presents the results of measurements made with the electronic personal neutron Saphydose-N during the four campaigns of the European contract EVIDOS (EValuation of Individual DOSimetry in mixed neutron and photon radiation fields). These measurements were performed at Institute for Radiological Protection and Nuclear Safety (IRSN) in France (C0), at the Krümmel Nuclear Power Plant in Germany (C1), at the VENUS Research Reactor and the Belgonucléaire fuel processing plant in Belgium (C2) and at the Ringhals Nuclear Power Plant in Sweden (C3). The results for Saphydose-N are compared with reference values for dose equivalent.     

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.     

Development of advanced-type multi-functional electronic personal dosemeter

An advanced-type small, light, multi-functional electronic personal dosemeter has been developed using silicon semiconductor radiation detectors for dose management of workers at nuclear power plants and accelerator facilities. This dosemeter is 62 x 82 x 27 mm(3) in size and approximately 130 g in weight, which is capable of measuring personal gamma ray and neutron dose equivalents, Hp(10), simultaneously. The neutron dose equivalent can be obtained using two types of silicon semiconductors: a slow-neutron sensor (<1 MeV) and a fast-neutron sensor (>1 MeV). The slow neutron sensor is a 10 x 10 mm(2) p-type silicon on which a natural boron layer is deposited around an aluminium electrode. The fast neutron sensor is also a 10 x 10 mm(2) p-type silicon crystal on which an amorphous silicon hydride is deposited. The neutron energy response corresponding to the fluence-to-dose-equivalent conversion coefficient given by ICRP Publication 74 has been evaluated using a monoenergetic neutron source from 250 keV to 15 MeV at the Fast Neutron Laboratory of Tohoku University. As the result, the Hp(10) response to neutrons in the energy range of 250 keV and 4.4 MeV within +/-50% difference has been obtained.     

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.