Development of electronic personal neutron dosemeters: a European cooperation

The European Commission recently sponsored an international research and development project, in which three 'active' personal neutron dosemeters were developed. Comparative irradiation experiments showed good results with respect to sensitivity and energy dependence of the response.     

Development of active environmental and personal neutron dosemeters

For neutron dosimetry in the radiation environment surrounding nuclear facilities, two types of environmental neutron dosemeters, the high-sensitivity rem counter and the high-sensitivity multi-moderator, the so-called Bonner ball, have been developed and the former is commercially available from Fuji Electric Co. By using these detectors, the cosmic ray neutrons at sea level have been sequentially measured for about 3 y to investigate the time variation of neutron spectrum and ambient dose equivalent influenced by cosmic and terrestrial effects. Our Bonner ball has also been selected as the neutron detector in the International Space Station and has already been used to measure neutrons in the US experimental module. The real time wide-range personal neutron dosemeter which uses two silicon semiconductor detectors has been developed for personal dosimetry and is commercially available from Fuji Electric Co. This dosemeter has good characteristics, fitted to the fluence-to-dose conversion factor in the energy range from thermal energies to several tens of mega-electron-volts and is now widely used in various nuclear facilities.     

Determination of the full response function of personal neutron dosemeters

The response of neutron dosemeters may be determined directly from measurements, provided a sufficiently large number of measurements in monoenergetic neutron fields covering the entire energy range of interest is available. In practice this is not feasible due to the lack of monoenergetic neutron fields in the thermal and intermediate energy region (i.e. energies <24 keV). To deal with this difficulty, we have developed a method which can take into account additional information about the response of the dosemeter. Our analysis makes use of two types of data, measurements made using monoenergetic neutron beams and measurements made in neutron fields with broad energy distributions. The dosemeter responses are described using a parametrised model, based on a minimum of assumptions: that they should fit the data within experimental uncertainties, and that they should remain close to a simple interpolation of the monoenergetic and thermal neutron field data.     

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.     

Study of the photon radiation performance of electronic personal dosemeters

Following the specifications and test methods given by international standards IEC-61526 and ISO 4037, the dosimetry department of the IPSN studied the photon radiation performance of seven recent electronic personal dosemeters: The personal dosimetric performance of each piece of equipment was tested with X and gamma radiation between 12 keV and 1.25 MeV.     

Ten years of personal neutron dosimetry with albedo dosemeters in The Netherlands

Since 1987, the dosimetry service of the Netherlands Energy Research Foundation (ECN) has been certified by the Dutch government to perform personal dosimetry, using thermoluminescence dosemeters (TLDs). Performing neutron personal dosimetry requires a rather large investment in readers, TLDs and personnel to operate the service. About 800 persons are subjected to routine neutron monitoring in The Netherlands and their annual neutron doses are a relatively small fraction (less than 10%) of the annual Hp(10). In general, the measured neutron dose values are low (on average 93% of the users receive an annual neutron dose <0.2 mSv). The collective annual (neutron) dose has tended to decrease since 1992, but incidentally high doses have been observed. Leaving these incidents out, the average collective annual neutron doses for the different users of neutron sources are about the same.     

Implementing new recommendations for calibrating personal dosemeters

This paper analyses the differences between the calibration procedures for personal dosemeters recommended by ICRU 47 and ISO 4037-3. The tissue equivalence of the PMMA and the ISO water slab phantoms are analysed by means of the Penelope Monte Carlo code for monoenergetic and filtered X ray photon beams and compared with the results of two other independent codes. The influence of the calibration method is also verified experimentally, both on a thermoluminescence and an electronic personal dosemeter. Good consistency between both calibration procedures is shown provided that a correction factor for backscatter differences between the PMMA and the ICRU phantom is introduced. The Monte Carlo simulation is used to determine this correction to a greater accuracy.     

Electronic personal neutron dosemeters for high energies: measurements, new developments and further needs

The results of measurements with neutron energies up to 60 MeV are shown for the personal neutron dosemeters Thermo Electron EPD-N2, ALOKA PDM-313 and the PTB prototype dosemeter DOS-2002. All dosemeters show dose equivalent responses that are about a factor of 10, too high at 60 MeV. A new prototype dosemeter-called DOS-2005-consisting of a detector with a thin effective layer of 6 microm has been set up at PTB. The dose equivalent response of this dosemeter and that of the newly developed dosemeter SAPHYDOSE-N was measured up to 19 MeV. Both dosemeters indicate a more flat response at high neutron energies. Further needs-optimisations, measurements and calculations-for use at high-energy accelerators and in space are discussed.     

Evaluation of two personal dosemeters in polyenergetic mono- and multi-directional neutron fields

The neutron dose-equivalent response of two commercially available electronic personal neutron dosemeters was studied in several laboratory-produced broad-spectrum neutron fields. Fluence-weighted mean energies ranged from 200 keV to 4 MeV; personal dose-equivalent rates ranged from 75 to 10 mSv h(-1); and angles of incidence were multidirectional, 0 degrees, 30 degrees and 60 degrees. Three of these fields have been shown previously to resemble ones found in CANDU (Canadian Deuterium Uranium is a registered trademark of the Atomic Energy of Canada Limited) power plant workplaces. Both dosemeters were found to perform reasonably well across the range of energy spectra and angles of incidence. One type of dosemeter displayed values of the personal dose equivalent that were, at worst, within a factor of approximately 2 of the reference values and, at best, within a few per cent of the reference values. The other type displayed values of the personal dose equivalent that were consistently within unity and 20% of the reference values. Although the radiological performance of one was found to be more accurate, this device was also found to be the less rugged of the two. Some of the data acquired in this work were compared with results previously published by others. There was consistency between these sets of data.     

Dose rate dependency of electronic personal dosemeters measuring X- and gamma-ray radiation

Three models of electronic personal dosemeters (EPDs)-Siemens Mk 2.3, Rados RAD-60S and Vertec Bleeper Sv-were irradiated with seven photon beam qualities: 60Co, 137Cs and the ISO narrow spectrum series X-ray qualities N-250, N-200, N-150, N-60 and N-20. The personal dose equivalent rates delivered to the devices varied between 0.002 and 0.25 mSv s(-1). Measurements were made with the EPDs mounted free-in-air as well as against Lucite and water phantoms. Results for all models of EPDs showed differences in personal dose equivalent energy response for different energies covered by this range of radiation qualities, with different models showing variations from 15 to 65%. In some cases, the personal dose equivalent rate response of these devices varied by a factor of 3 between irradiations at typical calibration dose rates and those normally encountered by nuclear energy workers.     

The use of passive personal neutron dosemeters to determine the neutron dose equivalent component of radiation fields in spacecraft

For the altitude range and inclination of the International Space Station (ISS), secondary neutrons can be a major contributor to dose equivalent inside a spacecraft. The exact proportion is very dependent on the amount of shielding of the primary galactic cosmic radiation and trapped particles, but is likely to lie in the range of 10-50%. Personal neutron dosemeters of simple design, processed using simple techniques developed for personal dosimetry, may be used to estimate this neutron component.     

Calibration of neutron albedo dosemeters

It is shown that by calibrating neutron albedo dosemeters under the proper conditions, two complicating effects will essentially cancel out, allowing accurate calibrations with no need for explicit corrections. The 'proper conditions' are: a large room (> or = 8 m on a side). use of a D2O moderated 252Cf source, and a source-to-phantom calibration distance of approximately 70 cm.     

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.     

Comparison test of electronic dosemeters

To assist with a planned purchase of electronic dosemeters by the Swiss Federal Office for Civil Protection, the calibration laboratory of the Paul Scherrer Institute performed tests on 11 types of electronic dosemeters manufactured by 10 European and American companies. The technical specifications for the World Trade Organisation (WTO) tendering procedure were largely in accord with the specifications of the international standard IEC 61526. First tests were performed with samples from each type of dosemeter. The reproducibility of a dose of 0.1 mSv generated with 137Cs radiation at a dose rate of 2.1 mSv.h-1 was found adequate for all tested dosemeter types. The response for environmental levels of radiation showed a large variation, indicating insufficient background correction of some dosmeters. A very high dose rate of 10 Sv.h-1 provoked faulty dose readings for more than half of the tested dosemeters. Dosemeter response for low-energy photon radiation was satisfactory for two of the tested dosemeter types. Four dosemeter types were selected for extended technical tests. Three samples of each of these dosemeter types were purchased. For drop and temperature tests the specifications of the WTO tendering procedure outranged the specifications of the IEC standard. Whereas even at a temperature of -25 degrees C the tested dosemeters functioned normally, drops from a height of 2 m onto a wooden surface rendered the samples of two dosemeter types inoperative.     

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.     

New concept of IEC standards for radiation protection dosemeters

The International Electrotechnical Commission (IEC) develops new standards for radiation protection dosemeters which follow a new concept. They are much more flexible in detail, but still ensure the same measurement quality. They are, for example, no longer specific for the detector type, but rather specific for the measurement task, e.g. for individual monitoring with active direct-reading instruments. Another example is that they are flexible with respect to the ranges of influence quantities. The conceptual changes are described in this paper, together with the advantages this new concept provides for manufacturers, users and legislators.     

Evaluating the uncertainty in measurement of occupational exposure with personal dosemeters

In the 1990 Recommendations of the ICRP it is stated that an uncertainty in a dose measured with a personal dosemeter under workplace conditions of a factor 1.5 in either direction 'will not be unusual'. In many documents similar to the EU Technical recommendations, the IAEA Safety Guides and papers in scientific journals, this statement is understood to be a basis for developing type-test criteria and criteria for the approval of dosimetric systems. The methods for evaluating the standard uncertainty as proposed in the above mentioned documents and in national and international standards use an approach that is based on the Law of Propagation of Uncertainties (LPU). This approach needs a number of assumptions, the validity of which cannot easily be verified for personal dosemeters. The current paper presents a numerical method based on Monte Carlo simulation for the calculation phase of the evaluation of uncertainties. The results of applying the method on the type-test data of the NRG TL-dosemeter indicate that the combined standard uncertainty estimated using the LPU approach might well not be realistic. The numerical method is simple and can be precisely formulated, making it suitable for being part of approval or accreditation procedures.     

IEC/ISO 62264国际标准探析

本文全面介绍了IEC/ISO 62264系列国际标准的整体情况;详细阐述了该标准的核心技术内容;在此基础上分析与探讨了该标准的主要贡献和价值;最后给出了针对该标准的展望与建议.