Among the activities of EURADOS Working Group 2 formed by experts from several European countries is the harmonisation of individual monitoring as part of radiation protection of occupationally exposed persons. Here, we provide information about thermoluminescent detectors (TLDs) applied by the European dosimetric services and the dosimetric characteristics of dosemeters in which these detectors are applied. Among 91 services from 29 countries which responded to the EURADOS questionnaire, 61 apply dosemeters with TLDs for the determination of personal dose equivalent Hp(10) for photons and beta radiation, and 16 services use TLDs for neutron albedo dosemeters. Those most frequently used are standard lithium fluoride TLDs (mainly TLD-100, TLD-700, Polish MTS-N and MTS-7, Russian DTG-4), high-sensitive lithium fluoride (GR-200, MCP-N) and lithium borate TLDs. Some services use calcium sulphate and calcium fluoride detectors. For neutron dosimetry, most services apply pairs of LiF:Mg,Ti TLDs with 6Li and 7Li. The characteristics (energy response) of individual dosemeters are mainly related to the energy response of the detectors and filters applied. The construction of filters in dosemeters applied for measurements of Hp(10) and their energy response are also reviewe

Currivan, L.

Lopez, M. A.

Olko, P.

van Dijk, J. W. E.

Wernli, C.

RERO DOC Digital Library

THERMOLUMINESCENT DETECTORS APPLIED IN INDIVIDUALMONITORING OF RADIATION WORKERS IN EUROPE—A REVIEW BASED ON THE EURADOS QUESTIONNAIREP. Olko1,, L. Currivan2, J. W. E. van Dijk3, M. A. Lopez4 and C. Wernli51Institute of Nuclear Physics, PL 31 342 Krakow, Poland2RPII, Dublin 14, Ireland3NRG Radiation and Environment, 6800 ES Arnhem, The Netherlands4CIEMAT, 28040 Madrid, Spain5PSI, 5232 Villigen, SwitzerlandAmong the activities of EURADOS Working Group 2 formed by experts from several European countries is theharmonisation of individual monitoring as part of radiation protection of occupationally exposed persons. Here, weprovide information about thermoluminescent detectors (TLDs) applied by the European dosimetric services and thedosimetric characteristics of dosemeters in which these detectors are applied. Among 91 services from 29 countrieswhich responded to the EURADOS questionnaire, 61 apply dosemeters with TLDs for the determination of personal doseequivalent Hp(10) for photons and beta radiation, and 16 services use TLDs for neutron albedo dosemeters. Those mostfrequently used are standard lithium fluoride TLDs (mainly TLD-100, TLD-700, Polish MTS-N and MTS-7, RussianDTG-4), high-sensitive lithium fluoride (GR-200, MCP-N) and lithium borate TLDs. Some services use calcium sulphateand calcium fluoride detectors. For neutron dosimetry, most services apply pairs of LiF:Mg,Ti TLDs with 6Li and 7Li. Thecharacteristics (energy response) of individual dosemeters are mainly related to the energy response of the detectors and filtersapplied. The construction of filters in dosemeters applied for measurements of Hp(10) and their energy response are alsoreviewed.INTRODUCTIONIn the 1960s and 1970s individual dosimetrytechniques based on thermoluminescent detectors(TLDs) were intensively developed in manyEuropean countries and frequently implemented toroutine dosimetric services(1). TLD dosimetry hasbeen already commercialised in the 1970s by a fewcompanies worldwide, which offered complete dosi-metric systems including systems with automaticTLD readers. This led to certain unification ofequipment and techniques in the West Europeancountries but the radiation protection regulationswere created on a national basis. An attempt tounify the radiation protection regulations withinEuropean countries was undertaken in the frame-work of the EURATOM directive 23/96(2) by intro-ducing, e.g. common operational quantities, doselimits, etc. Up to 1990, dosimetric services in Centraland Eastern Europe developed in a certain degree ofisolation, resulting from the political division ofEurope and sometimes suffered owing to financialweakness. For these reasons many countries wereapplying their own systems of individual dosimetry,frequently based on self-developed TLDs. Somemodernisation of equipment, especially in servicesworking for Nuclear Power Plants, took placeafter the Chernobyl accident with internationalsupport and because of the technical co-operationprogramme of the International Atomic EnergyAgency (IAEA)(3). The studies on the status of thedosimetric services in Central and Eastern Europewere also undertaken in the frame of 5th FrameworkProgramme of European Commission(4).One of the aims of the European DosimetryGroup (EURADOS)(5), an organisation thatassociates institutions and scientists involved inresearch on dosimetry in Europe, is strengtheningco-operation and harmonisation of radiation protec-tion. In the year 2000 an overview of dosemeters anddosimetric services, within the European Union(EU) Member States and Switzerland, which wereable to estimate external radiation doses in the formof personal dose equivalent, Hp(d), was prepared bythe EURADOS Working Group and published in aspecial issue of Radiation Protection Dosimetry(6).Information for the compilation of that reportwas obtained from questionnaires sent to dosimetricservices. Over the years 2001–2004 this sameEURADOS questionnaire was distributed amongdosimetric services in the EU candidate countriesand other European countries, in order to obtaininformation on dosemeter design, photon dose cal-culation, background subtraction algorithms, energyand angle dependence of dosemeter response, calib-ration sources, performance and quality assurance.The full report providing the data obtained form thisquestionnaire was published in 2005 as a specialreport in Radiation Protection Dosimetry(7).Corresponding author: Pawel.Olko@ifj.edu.plRadiation Protection Dosimetry (2006), Vol. 120, No. 1–4, pp. 298–302doi:10.1093/rpd/nci538 Advance Access published on March 31, 2006 The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.orgThe aim of this paper is to summarise informationcollected from these questionnaires on types andcharacteristics of thermoluminescence dosemetersapplied by those services, which might be useful forthe harmonisation of radiation protection withinEurope. The energy response of individual dose-meters is mainly related to the energy response ofthe detectors, and algorithms, used to calculate dosefrom the reading of several detectors and filtersapplied. It will be demonstrated that most servicesin Europe, applying TLDs, are able to measure dosesin terms of personal dose equivalent, Hp(d). Thediscussion on the properties of the dosimetric sys-tems and the dosemeters design does not imply anycomparison of the quality of the systems. Therefore,in this paper, the dosimetry performance of the ser-vices listed is presented in anonymous form to avoidany comparison between particular dosimetricservices.DETECTORSOwing to historical and economical reasons, mostindividual dosimetric services in Europe using dose-meters based on TLDs are small and medium sizeservices, which operate typically for >10 000 radi-ation workers. Among 91 services from 29 countries,which responded to the EURADOS questionnairebetween 1997 and 2003, 61 services apply dosemeterswith TLDs for the determination of personal doseequivalent Hp(d) for photons and beta radiation,and 16 services use TLDs for neutron albedo dose-meters. The other services are applying dosemetersbased on dosimetric films, track etched detectors andradiophotoluminescent detectors. The list of appliedTLDs is given in Table 1. More than two-third of theservices are using lithium fluoride LiF:Mg,Tidetectors (mainly TLD-100, TLD-700, MTS-N andMTS-7) read in Harshaw/ThermoElectron andRADOS/Dosacus TL readers. The other most fre-quently applied detectors are Li2B4O7:Cu andCaSO4:Dy (used with Panasonic system), LiF:Mg,Cu,P and Li2B4O7:Mn,Si.In neutron albedo dosimetry, pairs of TLDs withdifferent sensitivity to thermal neutrons are applied,exploiting enhanced cross section for (n,a) reactionswith 6Li or 10B. Among 25 services, which providedneutron dosimetry service, 16 were applying pairs of7LiF/6LiF detectors, mainly TLD-700/TLD-600 andMTS-7/MTS-6. Among the 31 200 radiation workersmonitored with individual neutron dosemeters,20 000 workers were usually from the nuclear indus-try. Energy response of albedo dosemeters is poorfor energies above a few MeV and cannot be appliedfor higher neutron energies, e.g. around high-energyaccelerators.The photon energy response of a detector is cal-culated as the ratio of the detector signal measuredfor the given type of radiation and energy, to thesignal obtained after irradiation of the detector withthe same dose in tissue (or kerma in air) of referenceradiation, e.g. 137Cs gamma rays. The reason for thepopularity of lithium fluoride and lithium boratedetectors is their flat photon (X rays) energyresponse. To the first approximation the photonenergy response of the detector depends on thematerial composition, which determines the crosssections for photon interaction within the detector,as compared with tissue (air). The energy response ofa bare TLD can be estimated by its effective atomicnumber Zeff, calculated as a weighted average ofatomic numbers of all constituents of the detector.Table 1. List of TLDs applied in European individual dosimetric services, based on information collected form theEURADOS questionnaire.TLD type TLD name Producer No. of servicesLiF:Mg,Ti TLD-100, TLD-600, TLD-700 St Gobain (former Harshaw/Bicron NE) 23MTS-N, MTS-6, MTS-7(LiF-N, LiF-6, LiF-7 when distributedby RADOS, Finland named as)TLD Poland (former TLD Niewiadomski) 13DTG-4 Russia 1LiF Unspecified 6LiF:Mg,Cu,P MCP-N, MCP-Ns (thin layer detector) TLD Poland (former TLD Niewiadomski) 3GR-200 Beijing Shiying RadiationDetector Works, China3Li2B4O7:Mn,Si distributed by RADOS, Finland Tartu University, Estonia 5MgB4O7:Dy,Na Vinca, Serbia and Montenegro 1Li2B4O7:Cu,Ag,P Vinca, Serbia and Montenegro 1Li2B4O7:Cu Panasonic, Japan 8CaSO4:Tm Panasonic, Japan 8CaSO4:Dy Unspecified 3CaF2 Unspecified 1TLDS APPLIED IN INDIVIDUAL MONITORING OF RADIATION WORKERS299A substance is considered tissue equivalent withrespect to interactions with photons if its Zeff isclose to that of soft tissue, which is equal to 7.4.With Zeff ¼ 7.35 for lithium borate and Zeff ¼ 8.3for lithium fluoride the predicted energy response,normalised to air kerma at 662 keV (137Cs gammarays), should not be >30% for energies <30 keV(Figure 1). The measured energy response of LiF:Mg,Ti detectors is 10% higher owing to the vari-ation of TL efficiency for conversion of depositedenergy into TL light (LET effect). On the contrary inLiF:Mg,Cu,P detectors the measured response is upto 25% lower than predicted from the cross sections.Therefore, for the design of personal dosemeters oneshould apply only the experimentally determineddetector energy response.DOSEMETERS AND THEIR ENERGYRESPONSEA typical dosemeter used in individual monitoringconsists of a detector (or set of detectors) placedwithin the holder. The holder protects the detectorfrom disturbing environmental factors, corrects theenergy and angular response and assures the responseat the appropriate depth. An ideal dosemeter res-ponding in terms of Hp(d) should demonstrate read-ings proportional to dose (dose equivalent) to tissueat the given depth independently of the radiationenergy and proportional to the total dose at thesurface of the body, to take into account the con-tribution of backscattered radiation from the body.Therefore, the design of dosemeters has to take intoaccount mainly the energy response, thickness andphysical shape of the detector(s) and the composi-tion, thickness and shape of the filters applied.As the first guess, the thickness of the tissue-equivalent filter, for monitoring Hp(d), should cor-respond to the mg cm2 density value. This is usuallythe case for dosemeters with tissue-equivalent detect-ors. For holders with TLD-100/700 detectors thereported thickness of the filters for Hp(10) variedbetween 330 and 1100 mg cm2 and consist eitherof single material or, more frequently, of a combina-tion of two materials. The list of reported filtermaterials is given in Table 2. The users of RADOS/Dosacus systems with LiF and Li2B4O7 detectorsreported the thickness of Al filter between 241 and271 mg cm2 and the additional thickness of theplastic from 130 to 209 mg cm2. For CaSO4:Dydetectors high Zeff elements such as Pb and Cu areused as filters, to attenuate low-energy X rays and inthis way to compensate for the over response of bareCaSO4:Dy detectors. ForHp(0.07) in RADOS badgeno filter is used but most dosemeters apply foils(Mylar, PTFE or PP), with the thickness between3 and 42 mg cm2.The final value of dose is obtained from the signalmeasured with the TLD reader using a calculationprocedure (algorithm). Three types of procedurescan be considered as follows(1):1. No algorithm is used and Hp(d) is calculated byapplication of calibration factor.2. Combination of readings of different detectors isused to obtained the value of Hp(d).Figure 1. Photon energy response of LiF:Mg,Ti (MTS-N), LiF:Mg,Cu,P (MCP-N) and Li2B4O7:Cu TLDs. Lines presentthe ratio of mass energy absorption coefficients of TL material vs. air.P. OLKO ET AL.3003. Depending on the ratio of different detectorsa different formula is used to calculate Hp(d)(If-then algorithm)Among 45 services, which applied TLDs to deter-mine Hp(10) and responded to the questions con-cerning the algorithm, 28 use no algorithm, 12 usea combination of reading and 5 the if-then algo-rithm. For 15 services reporting the measurementsof Hp(0.07), 12 apply no algorithm and 3 a combina-tion of readings of different TLDs. Typically,services using tissue-equivalent detectors (LiF andLi2B4O7) apply no algorithms. For dosemeters withhigh Zeff detectors, the combination of reading orif-end algorithms are most frequently applied.The energy response of TL detectors, appropriatefilters, calibration procedure and algorithms of doseevaluation mostly influence the final energy responseof individual dosemeter for photons and beta parti-cles. In Figure 2 the measured response, Hp(10),normalised to the true value of Hp(10) is plotted asa function of energy for whole-body photon TLdosemeters for normal beam incidence. From 33services, which delivered infomation on Hp(10)response, only 5 were outside the 20% limit forX-ray energies>70 keV. For X-ray energies<70 keV12 services were outside the limit. These energyreponses are the results of type test experiments inwhich the dosemeters were exposed to monochro-matic or narrow spectrum radiation qualities all atthe same angle of incidence. Dosemeters with detect-ors far from tissue equivalent have to rely on ‘end-if’or ‘muliply detectors’ dose calculation algorithms.These systems may be very successful for monochro-matic or narrow spectrum qualities but fail in mixedradiation situations.Therefore, amulti-detector dose-meter with a very good response characteristic (asjudged from type tests) may in field conditionsbe inferior to dosemeters with only one detector(but which is approximately tissue equivalent), whichshow inferior type test response characteristics.SUMMARY AND CONCLUSIONSThis work forms part of the activities associated witha EURADOS Working Group 2 formed by expertsfrom several European countries involved in theprocess of harmonisation of individual monitoringTable 2. List of materials, reported in the EURADOSquestionnaire, as filters in individual dosemeters with TLDs.ABS PolyacrylbutadienestyreneAl AluminiumAlMylar Aluminised MylarCu CooperMylar Polyethylene teraphthalate by Du PontNylonPb LeadPC PolycarbonatePE PolyethylenePES PolyesterPlastic Not specifiedPMMA PolymethylmethacrylatePP PolypropylenePTFE PolytetrafluorethylenePVC PolivinyloTeflon Polytetrafluorethylene by Du Pont10 100 10000.00.20.40.60.81.01.21.41.61.82.0Hp(10) meas/Hp(10) trueEnergy / keVFigure 2. Response, Hp(10)measured/Hp(10)true as a function of energy for whole-body photon TL dosemeters for normalincidence.TLDS APPLIED IN INDIVIDUAL MONITORING OF RADIATION WORKERS301as part of the protection of occupationally exposedpersons. The aim of this paper was to provideinformation about TLDs applied by the Europeandosimetric services and the dosimetric characteristicsof dosemeters in which these detectors are applied.Owing to historical and economical reasons, mostindividual dosimetric services in Europe using dose-meters based on TLDs are small and medium sizeservices, which operate typically for <10 000 radi-ation workers. Sixty one of the total number of90 services, which responded, were applying TLDsfor radiation monitoring. The most frequently usedwere standard lithium fluoride TLDs (mainly TLD-100, TLD-700, MTS-N and MTS-7, DPG-4), high-sensitive lithium fluoride (GR-200, MCP-N) andlithium borate. For neutron dosimetry, most servicesapply pairs of LiF:Mg,Ti TLDs with 6Li/7Li. Themain aim of the harmonisation of radiation protec-tion systems within European states is to allow forreliable comparison and transfer of dosimetric datafor occupationally exposed people within Europeand in consequence, to facilitate the mobility ofradiation workers. A large number of dosimetricservices in Europe, using a great variety of dose-meters with different types of detectors, makes thistask difficult. Further harmonisation work is needed,towards periodical organisation of European inter-comparison of person dosemeters, and availablefor dosimetric services, which wish to participate.ACKNOWLEDGEMENTSThe authors would like to express their gratitudeto dosimetric services and country coordinators, inparticular Morkunas Boguciarskis, Vadim Chumak,Constantin Gendrutis, Metody Guelev, HelenaJanzekovic, Toomas Koop, Constantin Milu, DenisaNikodemova, Margit Osvay, Karla Petrova,Mirjana Prokic and Maria Ranogajec for their sub-stantial help in the distribution and collection of theEURADOS questionnaire.REFERENCES1. Herrmann, D., Kraus, W. and Will, W. Application ofthermoluminescent dosimeters in a centralized radiationprotection service. In: Proceedings of the Fourth Inter-national Conference on Luminescence Dosimetry,Krakow, 27–31 August 1974, Ed., Niewiadomski, T.,Institute of Nuclear Physics, Krakow, Poland Vol. 3,pp. 801–814 (1975).2. Council Directive 96/29/EURATOM of 13 May 1996.Basic safety standards for the protection of the health ofworkers and the general public against the dangers arisingfrom ionizing radiation. Official Journal of the EuropeanCommunities L159, 39 (1996).3. Gustafsson, M. and Griffith, R. V. IAEA activities in thefield of occupational radiation protection In: Intercom-parison for Individual Monitoring of External Exposurefrom Photon Radiation. International Atomic EnergyAgency, IAEA-Vienna 1999, IAEA TECDOC-1126,ISSN-1011-4289 pp. 4–7 (1999).4. Frasch, G., Petrova, K. and Schnuer, K. ESOREXProject - Trends and Developments in occupationalRadiation Exposure in Europe, 11 IRPA Congress,Madrid, 23–28 May 2004, Paper No. 5a3 available atwww.irpa11.org.5. Dietze, G. and Menzel, H. G. Harmonisation anddosimetric quality assurance in individual monitoring forexternal radiation. Radiat. Prot. Dosim. 89(1–2), 5–6(2000).6. van Dijk, J. W. E., Bordy, J. M., Vanhavere, T. F.,Wernli, C. and Zamani-Valasiadou, M. A catalogue ofdosemeters and dosimetric services within EU MemberStates and Switzerland able to estimate externalradiation doses as personal dose equivalent. Radiat.Prot. Dosim. 89(1–2), 53–105 (2000)7. Lopez Ponte, M. A., Castellain, C. M., Currivan, L.,VanDijk, J. W. E., Falk, R., Olko, P., and Wernli, C.A catalogue of dosemeters and dosimetric services withinEurope—an update. Radiat Prot Dosimetry 2004 112:45–68P. OLKO ET AL.302

Thermoluminescent detectors applied in individual monitoring of radiation workers in Europe—a review based on the EURADOS questionnaire

Abstract

Similar works

Full text

Available Versions

RERO DOC Digital Library