Response of radiation protection dosemeters in mixed high-energy photon and electron radiation fields

The response of radiation protection dosemeters in terms of the phantom-related operational quantities Hp(10) and H'(10.0 degrees) was measured for personal and area monitoring systems in mixed high-energy electron and photon radiation fields with energies up to 7 MeV. Using mixed radiation fields composed of different fractions of charged particle and photon fluence, three conditions were produced at the point of measurement: charged particle equilibrium (CPE) (a), a lack (b) and an excess (c) of charged particles relative to the conditions of CPE. Personal and area dosemeters of different types were investigated under conditions (a)-(c). A large variability of the response of the different dosemeter types was observed. The results are presented and discussed.     

Determination of relevant parameters for the use of electronic dosemeters in pulsed fields of ionising radiation

Active electronic dosemeters using counting techniques are used for radioprotection purposes in pulsed radiation fields in X-ray diagnostics or therapy. The disadvantage of the limited maximum measurable dose rate becomes significant in these radiation fields and leads to some negative effects. In this study, a set of relevant parameters for a dosemeter is described, which can be used to decide whether it is applicable in a given radiation field or not. The determination of these relevant parameters-maximum measurable dose rate in the radiation pulse, dead time of the dosemeter, indication per counting event and measurement cycle time-is specified. The results of the first measurements on the determination of these parameters for an electronic personal dosemeter of the type Thermo Fisher Scientific EPD Mk2 are shown.     

Bonner sphere neutron spectrometry at spent fuel casks

For transport and interim storage of spent fuel elements from power reactors and vitrified highly active waste (HAW) from reprocessing, various types of casks are used. The radiation exposure of the personnel during transportation and storage of these casks is caused by mixed photon–neutron fields and, frequently, the neutron dose is predominant. In operational radiation protection, survey meters and even personal dosemeters with imperfect energy dependence of the dose-equivalent response are used, i.e. the fluence response of the devices does not match the fluence-to-dose equivalent conversion function. In order to achieve more accurate dosimetric information and to investigate the performance of dosemeters, spectrometric investigations of the neutron fields are necessary. Therefore, fluence spectra and dose rates were measured by means of a simple portable Bonner multisphere spectrometer (BSS). The paper describes briefly the experimental set-up and evaluation procedure. Measured spectra for different locations, types of casks and inventory are discussed. The spectra provide a basis to determine dose rates and other integral quantities with higher accuracy and for choosing suitable area monitors, respectively, to establish correction factors applied to the dosemeter reading.     

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.     

Current challenges in personal dosimetry at the US DOE Hanford site

An overview is presented of the dosimetry system, dose equivalent calculation methodology, and QA/QC practices used at the US Department of Energy Hanford site. It describes some of the problems encountered in accurately measuring dose equivalent quantities under a broad range of field conditions that do not necessarily correlate with laboratory calibration conditions and the approach taken to solve these problems. Personnel at Hanford are monitored with a combination of Harshaw model 8825 and 8816 thermoluminescence dosemeters and CR-39 etched track dosemeters. Extremities are monitored using the ICN MeasuRing loaded with a Harshaw XD740 chipstrate TLD. All dosemeters employ LiF:Mg,Ti elements that are read on-site with Harshaw model 8800 and 6600 TLD readers. CR-39 dosemeters are electrochemically etched in non-commercial etch chambers and counted with an automated track counting system developed by Pacific Northwest National Laboratory. Problems with over response of the 8825 with respect to Hp(0.07), under-response of the 8825 with respect to Hp(3), and over response of the 8825 with respect to Hp(10) in Hanford's 90Sr/90Y beta radiation fields are discussed. Approaches to measurement of the operational quantities for field conditions and algorithm solutions to the above problems are described. Methods used to calibrate the ring dosemeter for Hanford field conditions together with limitations of the ring dosemeter in measuring Hp(0.07) for extremities, particularly when covered with protective clothing, are also discussed.     

Intercomparison measurements of extremity dosemeters in beta and/or photon radiation fields

Beta dosimetry, especially at the extremities, is gaining in importance due to the increasing use of beta particle sources, e.g. in brachytherapy. The dosimetric properties of personal dosemeters to be worn on the extremities and capable of measuring the personal dose equivalent, Hp(0.07), in beta and/or photon radiation fields were investigated within the scope of intercomparison measurements organised by the PTB in two steps. The results were evaluated on the basis of recommendations from the German Commission on Radiological Protection (SSK). In the first step 10 types of dosemeter were investigated in beta particle fields in a range of mean energies from 0.06 MeV to 0.8 MeV. In the second step, five selected beta dosemeter types were exposed to beta particles and, in addition, to photons and to mixtures of both. Three dosemeters fulfill the requirements for the whole range of mean beta energy used for the intercomparison and meet the requirements for photon radiation from 8 keV to 662 keV.     

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.     

Photon energy response of luminescence dosemeters and its impact on assessment of Hp(10) and Hp(0.07) in mixed fields of varying energies of photons and beta radiation

X and gamma rays continue to remain the main contributors to the dose to humans. As these photons of varying energies are encountered in various applications, the study of photon energy response of a dosemeter is an important aspect to ensure the accuracy in dose measurement. Responses of dosemeters have to be experimentally established because for luminescence dosemeters, they depend not only on the effective atomic number (ratio of mass energy absorption coefficients of dosemeter and tissue) of the detector, but also considerably on the luminescence efficiency and the material surrounding the dosemeters. Metal filters are generally used for the compensation of energy dependence below 200 keV and/or to provide photon energy discrimination. It is noted that the contribution to Hp(0.07) could be measured more accurately than Hp(10). For the dosemeters exhibiting high photon energy-dependent response, estimation of the beta component of Hp(0.07) becomes very difficult in the mixed field of beta radiation and photons of energy less than 100 keV. Recent studies have shown that the thickness and the atomic number of metal filters not only affect the response below 200 keV but also cause a significant over-response for high energy (>6 MeV) photons often encountered in the environments of pressurised heavy water reactors and accelerators.     

Development of an algorithm for TLD badge system for dosimetry in the field of X and gamma radiation in terms of Hp(10)

In view of the introduction of International Commission on Radiation Units and Measurements operational quantities Hp(10) and Hp(0.07), defined for individual monitoring, it became necessary to develop an algorithm that gives direct response of the dosemeter in terms of the operational quantities. Hence, for this purpose and also to improve the accuracy in dose estimation especially in the mixed fields of X ray and gamma, an algorithm was developed based on higher-order polynomial fit of the data points generated from the dose-response of discs under different filter regions of the present TL dosemeter system for known delivered doses. Study on the response of the BARC TL dosemeter system based on CaSO(4):Dy Teflon thermoluminescence dosemeter discs in the mixed fields of X and gamma radiation was carried out to ensure that the accuracies are within the prescribed limits recommended by the international organisations. The prevalent algorithm, based on the ratios of the disc response under various filters regions of the dosemeter to pure photons, was tested for different proportion of two radiations in case of mixed field dosimetry. It was found that the accuracy for few fields is beyond the acceptable limit in case of prevalent algorithm. The new proposed algorithm was also tested in mixed fields of photon fields and to pure photon fields of varied angles. It was found that the response of the dosemeter in mixed fields of photons and its angular response are satisfactory. The new algorithm can be used to record and report the personal dose in terms of Hp(10) as per the international recommendation for the present TL dosemeter.     

Quality assurance of personal beta particle dosemeters used for individual monitoring of occupationally exposed persons

As a result of investigations and intercomparison measurements organised from 1996 to 1999 by PTB, several types of personal dosemeters, all based on TLD, were selected by the dosimetry services for the measurement of the personal dose equivalent H(p)(0.07) in beta and/or photon radiation fields. These dosemeters have now the status of legal personal beta partial-body dosemeters. Workplaces at which beta radiation might significantly contribute to the doses to the extremities are to be found today with increasing frequency in radiation therapy, radiation source production and nuclear power plants. Quality assurance for beta personal dosemeters is stipulated by guidelines for the official dosimetry service and is carried out by way of the intercomparison measurements organised periodically by the PTB. The results are evaluated based on the recommendations of the German Commission on Radiological Protection (SSK). The procedure of these intercomparison measurements will be explained in detail. The experience gained from three series of comparisons with seven types of fingerring dosemeters will be described and the results will be presented. The anonymity of the dosemeter types and of the participants in the intercomparison will be preserved.     

The right choice: extremity dosemeter for different radiation fields

Measurements of weakly penetrating radiation in personal dosimetry present problems in the design of suitable detectors and in the interpretation of their readings. For the measurement of the individual beta radiation dose, personal dosemeters for the fingers/tips are required. LiF:Mg,Cu,P is a promising thermoluminescent (TL) material which allows the production of thin detectors with sufficient sensitivity. Dosimetric properties of two different types of extremity dosemeters, designed to measure the personal dose equivalent Hp(0.07), have been compared: LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD700H). A type test for energy response for photon and beta radiation according to ISO 4037-3 and ISO-6980 was carried out and the results for both dosemeters were compared. Simultaneous measurements with both types of dosemeters were performed at workplaces, where radiopharmaceuticals containing different radioisotopes are prepared and applied. Practices in these fields are characterized by handling of high activities at very small distances between source and skin. The results from the comparison of the two-dosemeter types are presented and analysed with respect to different radiation fields. Experiments showed a satisfactory sensitivity for the thinner dosemeter (TLD 700H) for detecting beta radiation at protection levels and a good energy response.     

Hp(0.07) photon irradiations at Seibersdorf for the EURADOS extremity dosemeter intercomparison 2009

In August 2009, almost 1000 passive extremity dosemeters were irradiated at the Dosimetry Laboratory Seibersdorf as part of the EURADOS intercomparison IC2009. Forty-four European individual monitoring services participated, with a total of 59 dosimetry systems (46 finger ring, 4 finger tip and 9 wrist/ankle dosemeter systems). Additionally, finger-ring dosemeters from the Dosimetry Service Seibersdorf were irradiated in a non-competitive manner. Dosemeter irradiations on rod and pillar phantoms in four photon-radiation fields complying with the ISO standard 4037 were performed with personal dose equivalent values (H(p)(0.07)) ranging from 4 to 480 mSv. Traceability was established by using an air-kerma-calibrated monitor ionisation chamber together with the X-ray facility as well as a calibrated (137)Cs gamma radiation field with a collimated beam geometry. The ISO-tabulated conversion coefficients from air kerma free-in-air to H(p)(0.07) were applied, resulting in the main contribution to the expanded measurement uncertainties.     

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.     

Prediction analysis of dose equivalent responses of neutron dosemeters used at a MOX fuel facility

To predict how accurately neutron dosemeters can measure the neutron dose equivalent (rate) in MOX fuel fabrication facility work environments, the dose equivalent responses of neutron dosemeters were calculated by the spectral folding method. The dosemeters selected included two types of personal dosemeter, namely a thermoluminescent albedo neutron dosemeter and an electronic neutron dosemeter, three moderator-based neutron survey meters, and one special instrument called an H(p)(10) monitor. The calculations revealed the energy dependences of the responses expected within the entire range of neutron spectral variations observed in neutron fields at workplaces.     

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.     

Effect of background radiation on the lower limit of detection for extended dosemeter issue periods

An extension of dosemeter issue period brings significant economic and logistic benefits. Therefore, it is desirable to have an extended period as long as possible without significant loss of the quality of dose measurements. There are many studies devoted to the investigation of fading or reduction of the dose accumulated in dosemeters with time. However, this is one of many critical factors that need's to be taken into account when extending the dosemeter issue period. Background radiation is also a critical factor that needs to be appropriately accounted. In this report, a new approach has been suggested for evaluating the effect of background radiation on the lower limit of detection (LLD) of occupational radiation dose. This approach is based on the data collected from control dosemeters that are routinely used for subtraction of background radiation from occupational dose measurements. The results show that for LiF:Mg,Cu,P thermoluminescence dosemeters, variations in background radiation have a higher impact on the LLD than dose fading and the absolute value of background radiation. Although there is no significant dose fading in LiF:Mg,Cu,P for a dosemeter issue period up to 1 y, variations in background radiation during this period of time can significantly increase photon LLDs (up to 700 microSv) for workers operating in an environment of variable radiation background.     

Measuring cosmic-ray exposure in aircraft using real-time personal dosemeters

Aircrew exposure to radiation was measured on several long-haul flights using two small commercial electronic personal dosemeters: one was a photon dosemeter, the NRF20; the other was a neutron dosemeter, the NRY21-both manufactured by Fuji Electric Systems Co. Ltd. for radiation protection at nuclear facilities. Non-neutron doses were measured using the photon dosemeter, and neutron doses were measured using the neutron dosemeter. The measured non-neutron doses at commercial aviation altitudes agree with the EPCARD (European Program Package for the Calculation of Aviation Route Doses) dose calculation within a difference of 8 %. However, the recorded neutron doses were 5-15 times larger than the EPCARD calculation. These over-measurements are dependent on cut-off rigidities.     

Study and presentation of a fast neutron and photon dosemeter for area and criticality monitoring using radiophotoluminescent glass

This paper describes the results of a study performed on a mixed field neutron/gamma (n/gamma) area dosemeter incorporating radiophotoluminescent (RPL) glass detectors. RPL glass is known to be virtually insensitive to neutrons. The aim of the study was therefore to determine the neutron response of a dosemeter designed to combine n/gamma conversion with RPL detection capability. Monte Carlo calculations as well as measurements using monoenergetic beams and isotopic neutron sources showed this response to be constant, to within 30% in terms of H*(10), and independent of neutron energy from 250 keV to 10 MeV. For area monitoring, tests carried out in nuclear facilities (around PuO2 glove box and shipping casks containing PWR, MOX spent fuels or vitrified fission product) demonstrated that dosemeter response was accurate to within 15%, where the gamma component of the mixed n,gamma field remained below 1 MeV. When exposed in the Silene reactor simulating a criticality accident (10(17) fissions-liquid 235U--e.g. 1 Gy neutron and 1 Gy photon), the dosemeter exhibited good correlation with reference values and other measurement technologies (again to within 30%), for both neutron and gamma absorbed dose.     

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.     

Measurement of the fluence response of the GSI neutron ball dosemeter in the energy range from thermal to 19 MeV

At high-energy particle accelerators, area monitoring needs to be performed in a wide range of neutron energies. In principle, neutrons occur from thermal energies up to the energy of the accelerated ions, which is for the present GSI (Gesellschaft für Schwerionenforschung) accelerator facility approximately 1-2 GeV per nucleon. There are no passive dosemeters available, which are designed for the use at high-energy accelerators. At GSI, a neutron dosemeter was developed, which is suitable for the measurement of high-energy neutron radiation by the insertion of a lead layer around Thermoluminescence (TL) detection elements (pairs of TL 600/700) at the centre of the dosemeter. The design of the sphere was derived from the construction of the extended range rem-counters for the measurement of ambient dose equivalent H(10). In this work, the dosemeter fluence response was measured in the quasi-monoenergetic neutron fields of the accelerator facility of the PTB in Braunschweig and in the thermal neutron field of the GKSS research reactor FRG-1 in Geesthacht. For the accelerator measurements, the reactions (7)Li(p,n)(7)Be, (3)H(p,n)(3)He and (2)H(d,n)(3)He were used to produce neutron fields with energy peaks between 144 keV and 19 MeV. The measured fluence responses are 27% too low for thermal energies and show an agreement with approximately 14% for the accelerator produced neutron fields related to the computed fluence responses (MCNP, FLUKA calculations). The measured as well as the computed fluence responses of the dosemeter are compared with the corresponding conversion coefficients.