Adequacy of external dosimetry methods and suitability of personal dosemeters
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Adequacy of external dosimetry methods and suitability of personal dosemeters
HSE Health & Safety Executive Adequacy of external dosimetry methods and suitability of personal dosemeters for workplace radiation fields Prepared by the Health Protection Agency for the Health and Safety Executive 2005 RESEARCH REPORT 385 HSE Health & Safety Executive Adequacy of external dosimetry methods and suitability of personal dosemeters for workplace radiation fields D T Bartlett & R J Tanner Health Protection Agency Radiation Protection Division Chilton Oxon OX11 0RQ Under the Ionising Radiations Regulations 1999 and the Radiation (Emergency Preparedness and Public Information) Regulations 2001 (REPPIR), HSE approves dosimetry services to carry out a number of specified functions. There are a number of dosemeter types and dosimetry methods by which ionising radiation doses can be assessed. However, not all types of dosemeter/dosimetry methods are adequate or suitable for the different types of radiation fields or conditions. The main duty holder under IRR99 is the employer and it is their responsibility to appoint a dosimetry service which is suitable for their work. In addition, HSE assessors must consider aspects of adequacy when assessing dosimetry services. There are many different industries that make use of approved dosimetry services. The approved dosimetry services only provide information on the types of dosemeters which they supply. Similarly manufacturers of dosemeters and dosimetry systems only provide type test data on the dosemeters they manufacture, for specific radiation types, energies and environmental conditions. The criteria chosen vary with the manufacturer, making it difficult for dosimetry services and employers to assess the suitability of the different types of dosemeters for the types of radiations and radiation field conditions in the workplace. This report gives information on dosemeters/dosimetry methods currently available in the UK and information on workplace fields. This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy. HSE BOOKS © Crown copyright 2005 First published 2005 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of the copyright owner. Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected] ii ACKNOWLEDGEMENTS The authors should like to thank their many colleagues who have contributed information or ideas to this report, but in particular to members of the EURADOS Working Groups on “Harmonization of Individual Monitoring in Europe”. iii iv CONTENTS 1 INTRODUCTION ............................................................................................................. 1 2 WHOLE BODY DOSIMETRY METHODS AND AVAILABLE DOSEMETER TYPES ........................................................................................................... 2 2.1 GENERAL ............................................................................................................................ 2 2.2 WHOLE BODY DOSEMETERS TO DETERMINE HP(10) AND HP(0.07) FOR PHOTONS AND ELECTRONS..................................................................................................... 2 2.3 WHOLE BODY DOSEMETERS TO DETERMINE HP(10) FOR NEUTRONS ... 10 3 WORKPLACE FIELDS............................................................................................. 15 3.1 3.2 3.3 GENERAL .......................................................................................................................... 15 PHOTON AND ELECTRON FIELDS .......................................................................... 16 NEUTRON FIELDS AND REQUIREMENTS OF DOSIMETRY SYSTEMS ...... 18 4 CHOICE OF PERSONAL DOSEMETER ...................................................20 5 REFERENCES .................................................................................................................. 22 v vi EXECUTIVE SUMMARY Under the Ionising Radiations Regulations 1999 and the Radiation (Emergency Preparedness and Public Information) Regulations 2001 (REPPIR), HSE approves dosimetry services to carry out a number of specified functions. There are a number of dosemeter types and dosimetry methods by which ionising radiation doses can be assessed. However, not all types of dosemeter/dosimetry methods are adequate or suitable for the different types of radiation fields or conditions. There are many different industries that make use of approved dosimetry services. Dosimetry services only provide information on the types of dosemeters which they supply. Similarly manufacturers of dosemeters and dosimetry systems only provide type test data on the dosemeters they manufacture, for specific radiation types, energies and environmental conditions. The criteria chosen vary with the manufacturer, making it difficult for dosimetry services and employers to assess the suitability of the different types of dosemeters for the types of radiations and radiation field conditions in the workplace. The determination of energy and direction distributions of workplace fields is not a simple matter, requiring sophisticated measurement methods and analysis. The fields will usually comprise direct and scattered components resulting in broad energy and direction distributions. In addition the fields may be significantly spatially non-uniform, leading to non uniform exposure of the body. It is then difficult to make appropriate assessments of Hp(10) and Hp(0.07), and of effective dose when this is required. There is not much published information on photon and electron workplace fields, but a reasonably large number of published papers on neutron fields. The reason for this is that, in general, there are designs of photon and electron dosemeters available which are able to determine Hp(10) and Hp(0.07) within acceptable limits for the range of energies and directions present in workplaces, whereas this is not the case for neutron fields. For neutron workplace fields it is always necessary to have at least some information on the energy and directions characteristics of workplaces. The designs of dosemeters and dosimetry systems have developed along with changes in applications. Recent tests in the UK and in Europe of photon and electron whole body dosemeters indicated that most current designs have acceptable energy and angle dependences of Hp(10) and Hp(0.07) response characteristics for large regions of the entire range of particle energies likely to be encountered in the workplace. Nevertheless, some unexpected inadequacies were found in a study of the suitability of personal dosemeters for a particular set of workplaces. This latter study showed the value of investigating the performance of dosemeters and ADS in the actual workplace. For several of the available neutrons dosemeters and dosimetry systems, less than adequate performance was found in recent international inter-comparisons, which included simulated workplace fields. However, it is still considered that a choice of neutron dosemeter should be possible such that regions of inadequate dosemeter response characteristics are in energy regions where there is not a significant contribution to total Hp(10). The best method of choosing a suitable personal dosemeter is to base the choice on an assessment of the dosimetric and other characteristics of the available systems combined with an assessment, as far as practicable, of the main characteristics of the workplace fields in which the dosemeter is to be worn, with, if at all possible, in-situ tests. vii viii 1 INTRODUCTION Under regulation 35 of the Ionising Radiations Regulations 1999 (IRR99)1 and regulation 14 of the Radiation (Emergency Preparedness and Public Information) Regulations 2001 (REPPIR)2, HSE approves dosimetry services to carry out a number of functions specified in those regulations. In order to ensure that HSE is achieving its stated goals of consistency and transparency, the HSE process of dosimetry service approval is currently undergoing review. There are a number of dosemeter types and dosimetry methods by which ionising radiation doses can be assessed. However, not all types of dosemeter/dosimetry methods are adequate or suitable for the different types of radiation fields or conditions. The main duty holder under IRR99 is the employer and it is their responsibility to appoint a dosimetry service which is suitable for their work. In addition, HSE assessors must consider aspects of adequacy when assessing dosimetry services. There is a diversity of industries that make use of approved dosimetry services, and the dosimetry services only provide information on the types of dosemeters which they supply. Similarly manufacturers of dosemeters only provide type test data on the dosemeters they manufacture, for specific radiation types, energies and environmental conditions. The criteria chosen vary with the manufacturer, making it difficult for dosimetry services and employers to assess the suitability of the different types of dosemeters for the types of radiations and radiation field conditions in the workplace. There is no readily available comprehensive documentation on suitability of dosemeters or dosimetry methods or intercomparison of their suitability in the radiation fields encountered in the workplace. Consideration of the properties of existing dosemeters and dosimetry methods for different types of radiation and workplace fields will enable HSE to provide guidance to employers and HSE assessors on the suitability of dosemeters and dosimetry methods. This report provides information on dosemeters/dosimetry methods currently available in the UK, to assist in the appropriate choice for different radiation work (and radiation energies and environmental conditions) in order to improve the ability of employers to make appropriate decisions, and also to aid assessors (and thus speed up the assessment process); to inform the policy making process with respect to HSE’s dosimetry approvals programme; and to inform the review of the system for approval of dosimetry services. 1 2 WHOLE BODY DOSIMETRY METHODS AND AVAILABLE DOSEMETER TYPES 2.1 GENERAL The quantities to be determined are Hp(10) and Hp(0.07) in order to control, respectively, effective dose and dose to the skin1,3,4,5,6,7. In some instances an estimate of Hp(3) may be required for the control of equivalent dose to the lens of the eye, and this is normally derived from determinations of Hp(10) and Hp(0.07). For the assessment of effective dose (whole body dose), it is assumed that the dosemeter is worn on a part of the trunk that is representative of the most highly exposed part. There may, of course, be difficulties in assessing which part of the trunk is the most highly exposed, and for fields which are significantly spatially non-uniform, it may be difficult to relate Hp(10) to effective dose, and to relate Hp(0.07) to average dose to skin if this is required. In some cases, more than one dosemeter may need to be worn for the control of effective dose or dose to skin. There are particular considerations where lead aprons are worn. Personal dosemeters worn on the body are intended to determine personal dose equivalent, Hp(10) and Hp(0.07) close to the wearing position, and, in general, should be sensitive to radiation backscattered from the body. Dosemeters are type tested and routinely calibrated in terms of Hp(10) defined in, and calculated for a phantom of ICRU four-element tissue of the same size and shape as that on which the dosemeter is fixed for testing, generally the 30 by 30 by 15 cm ISO water phantom. It is implicit in this procedure that the energy and angle dependence of response characteristics determined in terms of the phantom quantity are adequately similar to those in terms of Hp(10) and Hp(0.07) in the body had the dosemeter been tested on the body8,9. 2.2 2.2.1 WHOLE BODY DOSEMETERS TO DETERMINE HP(10) AND HP(0.07) FOR PHOTONS AND ELECTRONS General In Figure 1(data taken from [10]), the ratios of dose to some commonly used detector materials to dose to tissue are shown as a function of photon energy. Not shown are values of the ratios for the film emulsion: this ratio increases to a maximum value of about 30 at about 50 keV. For materials such as lithium fluoride and lithium borate which have energy absorption characteristics similar to those of tissue, the reading (output) of a thin detector with minimal thickness of covering material (a few µm) will be proportional to Hp(0.07), and a detector with a covering of material of thickness equivalent to 10 mm of tissue, proportional to Hp(10), reasonably independent of photon energy. The energy dependence of the Hp(10) response characteristics can be improved, particularly in the case of lithium fluoride, by the choice of the covering material (the filter). The filter material can be chosen to correct for the imperfect tissue equivalence of the detector material, and also to take account of differences in the responses of detectors from those based purely on energy absorption. These differences are the result of energy deposition distribution effects on relative thermoluminescence efficiency, 2 and depend, for example, on the type of lithium fluoride (in particular which dopants and in what concentrations - see below). Further improvements in the Hp(10) and Hp(0.07) response characteristics of some of these types of detectors can be achieved by using several detectors with different filters and combining the output signals by a simple linear equation (algorithm) or, in some cases, by a more complicated logic pathway in the reader computer. The use of domed filters can improve the angle dependence of response. For detector materials with energy absorption characteristics which depart significantly from those of tissue, such as calcium sulphate and most notably film emulsion, higher atomic number (high Z) filter materials (metals) are necessary and the readings of several detectors or detector areas have to be combined to determine both Hp(10) and Hp(0.07). The combination of the several output signals are optimized to obtain the best Hp(10) and Hp(0.07) response characteristics, but it can prove difficult to obtain a satisfactory Hp(10) and Hp(0.07) angle dependence of response where high Z filter materials are used. For further general information on dosemeter design considerations, see, for example, [11], [12] and [13] for more detailed information, plus the other references cited in section 2.2.2. 16 12 137 Cs 14 Tissue Normalized to Ratio of Absorbed Dose in Detector Material to Absorbed Dose in 18 Lithium Fluoride 10 Calcium Sulphate Calcium Fluoride Lithium Borate 8 Aluminium oxide 6 4 2 0 10 100 1000 10000 Photon Ene rgy (ke V) Figure 1 Ratios of absorbed dose in detector material to absorbed dose to tissue, normalized to 137Cs. 2.2.2 Dosimetry methods Photographic emulsion (film badge) Photographic emulsions consist of microscopic crystals (grains) of silver halide, usually silver bromide, dispersed in gelatin. The grain size ranges from typically 0.2 Pm in nuclear emulsions up to 2 Pm in some radiology films. The emulsion is coated as a thin layer on one 3 or both sides of a cellulose acetate or polyester film. For a grain to be developable, a number of silver ions must be reduced to elemental silver by the passage of a proton or heavier charged particle, or secondary electrons through the grain. Photographic development is the process of amplification in which, by preferential reduction, the few silver atoms present in developable grains are increased in number by a factor of 107 or more, converting the whole grains to elemental silver and making them visible under a microscope. Radiation exposure measurements can be made by densitometry, after calibration. The reduction process is enhanced by the presence of sensitizers in the grain, for example iodine and sulphur. For photon and electron radiation measurements the film needs to placed in a holder that incorporates filters. Unfiltered, the Hp(10) or Hp(0.07) response at a energy of a few tens of keV is about 30 times that at several hundred keV. The simplest design has just one filter, lead or tin/lead, and the dosemeter response is the addition of the normalized filtered and unfiltered densities. More complicated designs allow the discrimination of low energy electrons from incident photons, and the correction of the high photon energy response. Additionally, a cadmium ‘filter’ can be included for thermal and epithermal neutron detection by (n,Ȗ) reactions in the cadmium and detection of the cascade of photons. Further information can be found in [14] and [15] and references therein. Thermo(radio)luminescence detectors (TLD) The basic mechanism of thermoluminescence is that when energy from ionising radiation is absorbed in a thermoluminescent (TL) material, usually a crystalline inorganic solid, part of that energy causes electrons to become trapped at defects in the crystalline structure. When the TL material is later heated, the electrons escape from the traps and return to a lower energy state, accompanied by the release of photons of visible light. The amount of light emitted by a TL material is proportional to the dose absorbed by the material since the last reading. The plot of light emitted against temperature (the glowcurve) normally has several peaks. The number of peaks, their relative magnitude and shape, depend on the TL material, heating rate, and previous heat treatment (annealing). The most commonly used TL material is lithium fluoride doped with magnesium and titanium impurities (LiF:Mg,Ti). There are 6 or 7 (depending on method of classification) main peaks in the glowcurve; peak 5 being the main dosimetry peak. Before use, the conventional anneal treatment is 400 oC for 1 hour followed by either a slow cool to 80oC and a hold at this temperature for 24 hours, or a slow cool to 100oC and a hold at this temperature for 2 hours. However, there are a number of variations on this for routine repeat use of detectors. The purpose of annealing is to establish, and re-establish a stable concentration of defect centres in the material, and to completely empty the traps. LiF:Mg,Ti comes in three forms with different isotopic abundances of lithium, namely natural abundance (92.5% 7Li and 7.5% 6Li); enriched in 6Li (95.5%); enriched in 7Li (99.5%). Since 6Li has a large cross-section for neutron interaction, via the reaction 6 Li(n,D)3H, it is much more sensitive to thermal neutrons and epithermal (below about 1keV. Other commonly used materials are lithium borate, doped with manganese or copper (Li2B4O7:Mn or Li2B4O7:Cu), aluminium oxide doped with carbon (Al2O3:C), calcium sulphate doped with dysprosium and thulium (CaSO4:Dy) and (CaSO4:Tm), and high sensitivity lithium fluoride doped with magnesium, copper and phosphorus (LiF:Mg,Cu,P). 4 Further information can be found in [16] and [17], and in the many publications in the proceedings of the series of International Conferences on Solid State Dosimetry, in particular the latest published proceedings18. Optically stimulated (radio)luminescence (OSL) In optically stimulated (radio)luminescence (OSL), the method of signal storage and materials used are similar to those for TL, (most commonly Al2O3:C). However, the read method is different: the signal which is proportional to the radiation dose is obtained by stimulating electron transitions between trapping centres or to the ground state (valence band) using light from a laser. The laser stimulation can be continuous or pulsed. Further information can be found in references [18], [19] and [20]. Active (electronic) personal dosemeters (APDs) (including direct ion storage (DIS) Most electronic (active) personal dosemeters detect photons and electrons by means of one or more silicon semiconductors, with different radiators/filters and/or detector thresholds, and with algorithms to combine the detector output signals, or by means of an energy compensated Geiger-Müller tube. In addition, there are solid-state detectors that combine an ionization chamber with a semiconductor. Specifically, the direct ion storage dosemeter is based on coupling a gas-filled ion chamber with a semiconductor non-volatile memory cell. These are compact integrating devices that can be read out periodically and used to estimate accumulated doses over periods of several hours to at least a year. Further information can be found in [21], [22], [23], [24] and [25]. 2.2.3 General dosimetric characteristics of dosimetry methods and of dosimetry systems available in the UK General dosimetric characteristics of dosimetry methods These are shown in summary form in Table 1 (information taken from the references cited in section 2.2.2). Dosimetric characteristics of dosimetry systems available in the UK These are shown in summary form in Table 2 (information supplied by ADs and dosemeter/dosimetry system manufacturers). 5 Detector Film emulsion Li F:Mg,Ti Film badge T.L Detector has reading approximately proportional to photon tissue dose from 10kev to 10Mev and greater, with maximum of 50% deviation (over response) at about 30kev relative to 137Cs. For Hp(10), plastic filters of thickness of about 1 gm/cm2 can give good response characteristics for all energies, subject to the over-response stated. The use of metallic filters can give improved energy dependence of response or provide a less bulky design but this can adversely affect the angle dependence of response. The use of a simple plastic filter ensures the good Hp(10) response for both photons and electrons above 2Mev. For the determination of Hp(0.07), minimal covering of the detectors is needed, in particular for electrons (beta particles). The electron energy dependence of response becomes dependant on the detector thickness below energies of about a few hundred keV. The angle dependence of electron response is dependent on both detectors thickness and the design of the opening in the holders above the detector. See note 1on the use of algorithms. Using standard badge and algorithm, Hp(10) and Hp (0.7) within a factor of 2 from approximately 20keV to 6/7 MeV, 0˚ to 60˚. Energy and angle dependence of response characteristics 6 The detector shows linearity of Hp(10) and Hp (0.7) response up to 10 Sv. The detection limit is 100 to 200 µSv The detector shows linearity of Hp(10) and Hp (0.7) response up to a few Sv, the exact threshold and degree of supralinearity is dependent on the LET of the energy. deposition. The material has shown no experimental evidence of dose rate dependence of response. Generally the detection limit is a few tens of µSv. Linearity, dose rate dependence, detection limit Some neutron sensitivity. With cadmium filter, the film badge can be used to measure thermal and epithermal neutrons. If neutrons are present need to use 7LiF Effect of other radiation type Little effect of range of temperatures and relative humidities encountered in normal working conditions. Fading of about 5% in a year, but this depends on the pre-use treatment and read cycle, note also the possibility of ‘sensitivity transfer’. When used incorporated into a PTFE matrix, account needs to be taken of exposure to visible light. Otherwise only small response to visible light, but does have response to UV. There can be tribothermoluminescence effects for fine powder. Can be susceptible to temperature and humidity Effect of other influence quantities General dosimetric characteristics of detectors used to determine Hp(10) and Hp(0.07) from exposure to external photons Method Table 1 1 See notes OSL TL Al2O3:C Li2B4O7: Cu Without filtration, the material shows a maximum deviation from a tissue dose response of about a factor of 4 at about 30 keV. High sensitivity. 7 Linear Hp(10) and Hp (0.7) response up to 30 Sv. No dose rate dependence. Detection limit of a few µSv. Linear Hp(10) and Hp (0.7) response up to a few Sv. No dose rate dependence. Detection limit of a few tens to 100 µSv. Lithium borate has an effective atomic number close to tissue and therefore a good tissue dose response. However its sensitivity is only about 1/10 that of LiF:Mg,Ti. Li2B4O7: Mn CaSO4: Tm Linear Hp(10) and Hp (0.70 response up to 30 Sv. No dose rate dependence reported. Detection limit of a few µSv. Calcium Sulphate has a relatively high effective atomic number. This gives rise to a non-ideal energy dependence of response with a maximum deviation for unfiltered material of about a factor of 1at about 30keV. With appropriate filters used in conjunction with an algorithm, the maximum deviation can be restricted to about 20% from 17keV up to 3Mev. CaSO4 is approximately 50 times more sensitive than LiF:Mg,Ti CaSO4 : Dy TL The detector shows linearity of Hp(10) and Hp(0.07) response up to tens of Sv. No dose rate dependence; detection limit of a few µSv Better energy dependence of tissue dose response than LiF:Mg, Ti – maximum deviation of ± 20% (under-response at about 200keV), therefore easier to design good dosemeter for both Hp(10) and Hp (0.07). Same general design considerations as for Li F:Mg, Ti, including use of algorithms. Li F:Mg, Cu, P T.L. Little neutron sensitivity If neutrons present need to use 7 Li210B4O7 Little neutron sensitivity As LiF:Mg, Ti Little fading. Strongly affected by light. Reasonable fading characteristics, about 30% in a year. CaSO4 shows greater thermal fading (5.to 30% in 6 months) and shows optical fading. Little effect of temperature and humidity. Fading of less than 5% in a year, dependent on pre-use treatment. Little visible light sensitivity and also lower UV sensitivity than L:F:Mg, Ti 5 4 3 2 G.M tube – compensated APD 7. 5. 6. 4. 3. 2. 1. With compensation by metallic filters, GM tube detectors show a maximum deviation of photon energy dependence of response of about ± 20% for the energy range from about 50kev to 3 MeV. Without filtration, the detector shows a maximum deviation of tissue dose response of about 5 at 30 keV. But in a simple configuration of two or three detectors and simple circuitry, a maximum, deviation of ± 20% over the photon range 15 kev to 9 MeV. The lower energy limit is determined mainly by the electronic noise threshold. Direct ion storage detectors are, in effect, small ionization chambers where energy dependence of response is determined by the walls, and other surrounding materials. Maximum deviation of 15% in range from 15 kev to 9 MeV. Linear Hp(10) and Hp (0.7) response up to 10 Sv. Detection limit of 1 µSv. Linear Hp(10) and Hp (0.7) response up to > 16 Sv. Detection limit of 1 µSv. For commercial design with MOSFETs for higher doses, linear Hp(10) and Hp (0.7) response up to 40 Sv. Detection limit of 1 µSv. Little neutron sensitivity Little neutron sensitivity Little neutron sensitivity Generally environmentally robust. Generally environmentally robust, but note possible effects of impact and electromagnetic fields. Generally environmentally robust. 8 LiF:Mg, Ti has a complex trap structure and pre – and post – irradiation heat treatments are important to ensure reproducibility, limit fading and reduce ‘sensitivity transfer’. Note also that there may be grain effects for some preparations of material. The use of algorithms can lead to difficulties. If the algorithms are set up to give correct response characteristics for approximately mono-energetic calibration (photon) fields, in some cases poorer results are obtained for broad fields, or for fields without electron equilibrium. LiF:Mg, Cu, P has a simpler trap structure than LiF:Mg, Ti, and therefore reduced complications of heat treatment. However the high sensitivity can be affected if read or anneal temperatures are taken above about 240°C. There can be problems with dosemeter design, in particular the design of metals filters to ensure good angle dependence of response. Sometimes CaSo4 detectors are used in conjunction with Li2B4O7 detectors, the latter for long-term dose assessment for record keeping purposes. The good tissue dose response characteristics have encouraged the use of this material in specialist applications. Its use in personal dosimetry is not widespread owing to its lower sensitivity and environmental vulnerability. Li2B4O7 : Cu, Ag, P and Mg B4O7 : Dy, Na are also available and occasionally used. This material can also be used as a T.L detector, but usually only for environmental measurement applications. Note OSL readout is non-destructive. DIS detectors are also available as APDs. Dosemeters using detectors of this type can have different lower photon energy thresholds; some are not able to meet performance requirements below 50keV. Silicon photodiode APD Notes Air ionization chamber DIS 7 6 Thermo Electron EPD Mk1 Global Dosimetry Solutions TLD (LiF:Mg,Ti ; both natural and isotopic ) Thermo Electron Harshaw TLD (LiF:Mg,Ti ; both natural and isotopic ) Thermo Electron Harshaw TLD (LiF:Mg, Cu, P; both natural and isotopic) NRPB TLD (7LiF:Mg, Ti) Panasonic TLD (Li2 B4 O7:Cu; both natural and isotopic; CaSO4:Tm ) Landauer OSL Luxel® Al2O3:C) Film badge Global Dosimetry Solutions Dosemeter Type/ Dosimetry System Film badge NRPB/UKAEA Design Hp(10): from 15 keV to 6/7 MeV, 0˚ to 60˚ ±30% Hp(0.07): from 10keV to 1.5 MeV, , 0˚ to 60˚ ±30% Hp(10): from 15 keV to 6/7 MeV, 0˚ to 60˚ ±25% Hp(0.07): from 15 keV to 6/7 MeV, , 0˚ to 60˚ ±25% Hp(10): from 15 keV to 6/7 MeV, 0˚ to 60˚ -10% to +40% Hp(0.07): from 10 keV to 1.5 MeV, 0˚ to 60˚ 10% to +60% Hp(10): from 17 keV to 6/7MeV, normal incidence ±30%; for 241 Am, 0˚ to 75˚, ±50% Hp(0.07): from 20 keV to 6/7 MeV normal incidence±30% Photon energy and angle dependence of response Hp(10): from 20keV to 6/7MeV, 0˚ to 60˚, -50% to +80%; Hp(0.07): from 10keV to 6/7 MeV, 0˚ to 60˚, ±50% Hp(10): from 15keV to 3MeV, 0˚ to 60˚, ±20% Hp(0.07) : from 15keV to 3 MeV, 0˚ to 60˚, ±20%; NVLAP accredited Hp(10): from 15 keV to > 10 MeV, 0˚ to 60˚, -20% to +30%; Hp(0.07): from 15 keV to > 10 MeV, 0˚ to 60˚, 20% to +40%; NVLAP accredited Hp(10): from 15 keV to 6/7 MeV, 0˚ to 60˚ ±20% Hp(0.07): from 10keV to 1.5 MeV, 0˚ to 60˚ ±20% Hp(10) and Hp(0.07): from 15 keV to 6/7 MeV, 0˚ to 60˚, ±20% NVLAP accredited and DOELAP approved Low fading (<5% fading per year) generally environmentally robust About 5% fading per year, generally environmentally robust 50 µSv to 10 Sv 10 µSv to 10 Sv 100 µSv to 8 Sv Hp(0.07): normal incidence, 204 Tl -20%; 90Sr/90Y +10% Hp(0.07): normal incidence, 147 Tl -30%; 90Sr/90Y -10% 9 1 µSv to >16 Sv About 5% fading per year, generally environmentally robust 100 µSv to 10 Sv Hp(0.07): 204Tl , 90Sr/90Y, DU: ±20%; NVLAP accredited and DOELAP approved Hp(0.07): normal incidence, 204 Tl ±25%; 90Sr/90Y ±10% Hp(0.07): from about 250 keV to 1.5 MeV Eȕ,mean ±30%, 90Sr/90Y 0˚ to 55˚, ±30% About 5% fading per year, generally environmentally robust 10 µSv to 10 Sv Hp(0.07): 204Tl , 90Sr/90Y, normal incidence, ±20% May need to consider impact and strong electromagnetic fields About 5% fading per year, generally environmentally robust 10 µSv to 10 Sv photons, 30 µSv to 10 Sv electrons Temperature stable- minimal fading; effects of light sensitivity prevented by encapsulation Can be susceptible to temperature and humidity Can be susceptible to temperature and humidity 100 µSv to 10 Sv 100 µSv to 5 Sv Environmental effects Dose range Hp(0.07) > 150 keV NVLAP accredited Hp(0.07): 90Sr/90Y NVLAP accredited Electron energy and angle dependence of response Hp(0.07): 204Tl , 90Sr/90Y, but no detailed dependence of response available Table 2 Summary of Performance Characteristics of Photon and Electron Dosemeters used by UK ADS (available manufacturers’ or ADS’ data) 2.3 2.3.1 WHOLE BODY DOSEMETERS TO DETERMINE HP(10) FOR NEUTRONS General For neutrons, normally only Hp(10) is assessed. The determination of Hp(10) is considered sufficient for the control of effective dose and localized skin exposure. For the assessment of Hp(0.07) for the control of neutron dose to extremities, the use of dosemeters calibrated in terms of Hp(10) should be adequate7, with the exclusion of some designs of albedo dosemeters. There is a large energy dependence of effective dose and Hp(10) per unit fluence. In the energy range from thermal up to a few keV, effective dose and Hp(10) are dominated by capture reactions on hydrogen (producing a 2.2 MeV photon) and nitrogen (producing a short range proton) of neutrons incident on, and scattered and moderated by, the large receptor (body or calibration phantom). At higher energies, tens of keV to 10 MeV, the main interaction is elastic scattering on hydrogen producing recoil protons. For the higher energy range, for reasons linked to proton ranges, dosemeters do not in general attempt to mimic the quantity. That is, designs are not based on a tissue equivalent detector with a 10 mm covering of tissue equivalent material as is the case for some photon and electron dosemeters, but by a combination of proton radiator, detector material and processing and readout in such a way that the energy and angle dependence of reading matches the Hp(10) dependence. Personal dosemeters to determine Hp(10) for neutrons are in general based on capture reactions for the lower energy region26, capture reactions detecting incident and/or moderated and scattered ‘albedo’ neutrons27,28,29 for the lower, or both lower and higher energy region, and recoil protons for the higher energy region30. The determination of Hp(10) is sometimes done separately for the two energy ranges, but can be combined by suitable design of dosemeter.. A detailed discussion of approaches to the determination of Hp(10) for neutrons can be found in [7]. 2.3.2 Dosimetry methods Etched tracks in plastics Plastic etched track detectors detect charged particles by means of etchable damage to the detector structure. The damage trail in a material, which constitutes a charged particle track, is a result of local deposition of energy during the passage of the particle. The damage is generally permanent but may be partly repaired or may be modified over time, influenced by factors such as temperature, humidity and the local presence of oxygen or other gases. The particle tracks may be rendered visible under an optical microscope by etching with a suitable solvent, or enlarged so that they are visible to the naked eye by electrochemical etching. Automated track counting can be accomplished by several methods. Present day plastic etched track detectors all use PADC (poly allyl diglycol carbonate, often referred to by its tradename CR-39™). Depending on the etch process used neutrons can be detected from about 100 keV up to hundreds of MeV via the secondary charged particles generated. Protons of energies from about 50 keV up to 10 to 20 MeV may be detected, and recoil nuclei produced by neutrons of energies above about 1.5 MeV. The addition of a converter layer for the neutron energy region from thermal to a few keV (via the capture reaction on nitrogen, in nylon for example, or a 6Li compound) produces a dosemeter with acceptable energy dependence of response for most practical purposes. When combined with a thermal to a few keV detection element, PADC etched track detectors are suitable for most neutron workplace fields. However, in fields where there are significant personal dose equivalent contributions in the energy region between a few keV and a few hundred keV, a 10 correction or suitable normalization may need to be applied. For dosemeter designs with single planar detectors, the dosemeter will have a poor response for neutrons of energy between 50 keV and a few MeV incident at oblique angles. However, in many workplaces, the relative positions of sources and workers, together with worker movements, will reduce the effect of any deficiency in the angle dependence of response, and both the energy and angle dependence of response in the workplace fields can be incorporated in a normalization factor. Where this is not a practical option, multielement designs with better angle response characteristics may be required. The non-radiation induced signal (background) and the detection level are important parameters. The plastic quality may be variable, with large variations in background from sheet to sheet within a batch, and across sheets. Quality control testing becomes paramount. The detection level depends critically on the total number of tracks counted, that is on both the sensitivity and area read. Further information can be found in review papers [31] and [32] and in reference [7]. ‘Albedo’ dosemeters using thermoluminescence detectors (TLDs) TLD materials containing 6Li or 10B can be used to detect incident and backscattered low-energy neutrons and/or incident fast neutrons moderated by, and backscattered from the wearer's body. These materials are much more sensitive to thermal and epithermal neutrons than to fast neutrons or photons, due to the reactions 6Li(n,D)3H and 10B(n,D)7Li.whose interaction cross section is inversely proportional to velocity. The sensitivity of a TL material to thermal and epithermal neutrons can be further enhanced by using materials enriched in these isotopes. The materials most often used for thermal and epithermal neutron detection are enriched lithium fluoride (6LiF:Mg,Ti and 6 LiF:Mg,Cu,P), natural lithium borate (Li2B4O7:Mn) and enriched lithium borate (6Li210B4O7:Mn). To determine the net signal due to neutrons in a mixed field, pairs of detectors are used: one that is sensitive to thermal neutrons, one that is not. For example, a 6LiF:Mg,Ti detector may be paired with a 7LiF:Mg,Ti detector with similar sensitivity to photons and fast neutrons, but with almost no sensitivity to thermal neutrons. The difference in the readings of the two detectors is the thermal and epithermal neutron reading. In fields where the neutron to gamma dose rate ratio is low, the relative difference in the two readings will be small and subject to a large statistical uncertainty. This technique is then not effective for separating the thermal and epithermal neutron component of the signal from the photon component. By suitable design of dosemeter holder, including shielding of incident thermal and epithermal neutrons, an almost constant Hp(10) and Hp(0.07) response characteristic from thermal energies up to a few keV can be obtained. A workplace field correction factor must be applied- effectively the ratio of total dose equivalent to that up to a few keV neutron energy. The angle dependence of the Hp(10) response of a TL albedo neutron dosemeter is generally good. The detection limit for an albedo dosemeter using LiF:Mg,Ti is in the range 20-100 PSv. The response increases linearly with increasing dose, to doses well above normal protection levels. The TL material 6LiF:Mg,Cu,P, which is 10-30 times more sensitive to photons than 6LiF:Mg,Ti, is only about four times more sensitive to thermal neutrons. This results in a decrease in the relative difference in photon and neutron signals, which increases uncertainty in determining the neutron component of a mixed field. Nonetheless, it provides a lower neutron dose detection threshold. Further information can be found in [7], [29] and [33]. Superheated emulsions (bubble detectors) Superheated emulsion neutron detectors comprise small droplets of a superheated liquid (i.e., a liquid at a temperature above its normal boiling point) suspended in a viscous medium. Secondary charged particles generated by a neutron depositing energy in a droplet may cause localized evaporation. A small vapour bubble is formed and expands by vaporizing adjoining liquid. If sufficient energy has 11 been transferred, the bubble will exceed a critical radius and all of the liquid in the droplet will be vaporized producing a long-lived bubble. The neutron sensitivity depends on the atomic composition of the superheated droplets, the number of droplets, and the temperature and pressure. The droplets consist of halocarbons and hydrocarbons. Detectors containing chlorine are sensitive to thermal and epithermal neutrons via the 35Cl(n,p)35S reaction. The sensitivity of a detector is set during manufacture by controlling the number of droplets it contains. Detectors have been made with sensitivities as high as 10 bubbles PSv-1. The pressure sensitivity provides a convenient “on-off” switch for the detector: when sufficient pressure is applied to the host medium of a bubble detector, the latter becomes completely insensitive to neutrons. The device is “turned on” by simply releasing the pressure. The temperature dependence is severe, and can be as much 5%/qC. In the devices referred to as bubble damage detectors (BDDs), the medium is a stiff polymer gel that is sufficiently viscous so that the bubbles remain in the location where they were formed. Counting can be done by eye, for small numbers of bubbles, or by automated techniques. Current versions of this detector are temperature compensated. With compensation, the temperature variation is reduced to within about ±20% over 15-40ºC, relative to the response at 20ºC. In the devices referred to as superheated drop detectors (SDDs), a less viscous, aqueous gel is used, which allows the bubbles to rise to the surface after they have formed. The gas that accumulates on top of the medium displaces a piston within a graduated cylinder. The distance that the piston moves is a measure of the neutron dose. The Hp(10) response of superheated emulsion detectors is reasonably constant above about 100 keV, but falls rapidly below this. Some types have a thermal and epithermal response (from capture reactions). The detector sensitivity typically ranges from 1-10 bubbles per PSv and the detection limit is typically a few PSv. If the bubbles are counted by eye, the maximum number that can be reliably counted is in the range 50 to 100. If automated counting techniques are used, up to about 500 bubbles can be counted. Passive bubble detectors do not appear to have any dose-rate dependence. Superheated emulsion detectors are particularly suited for measurement of neutron dose equivalent with high sensitivity in mixed fields with a large photon component. The immediate indication of neutron exposure provided by bubble detectors is a useful feature for neutron dose control in areas of high neutron dose rates. Further information can be found in [34], [35], [36] and [37]. Active neutron personal dosemeters Personal neutron dosemeters employing semiconductors detect charged particles generated in neutron-induced nuclear reactions in the semiconductor itself (generally not practicable for energies below about 1 MeV, unless organic semiconductors become available for this purpose) and/or charged particles generated in specially selected converter layers mounted close to the detector. For the energy range up to a few keV, capture reactions can be utilized in converter materials such as 6LiF or 10B. For higher energies, this type of converter layer can also be used for the detection of moderated and backscattered neutrons, with full or partial shielding for incident thermal and epithermal neutrons. To detect incident neutrons of energy above a few tens of keV, hydrogenous converters are generally used. Such converters can be used as layers upon, or incorporated into charged particle detectors. In principle, the relatively large energy deposition by the secondary charged particles allows discrimination against intrinsic noise and photon events. The general problems of designing a full-range neutron dosemeter have been discussed in section 2.3.1 above. Some particular aspects for electronic devices are mention below. In order for semiconductor detectors to be used to detect neutrons above a few tens of keV via recoil protons, several problems have to be solved which have a strong influence on the energy dependence 12 of response and photon discrimination. Firstly, the range of low-energy (recoil) protons is short and thus any insensitive surface layer (dead layer) between converter and sensitive layer of the detector will reduce the response at neutron energies below a few hundred keV. Secondly, where the neutron energy, and therefore the average energy of the recoil protons being counted, is low, the energy in each pulse is similar to that deposited by interactions of photons and also approaches noise levels for typical detectors. Thirdly, it is difficult to obtain adequate sensitivity. Relatively large areas of semiconductor are therefore needed and this may increase noise and photon discrimination problems. Fourth is the problem of photon discrimination. Whereas the measurement of low-energy neutrons with converters employing an exothermal reaction such as 6LiF(n,t)D does not exhibit any problems because of the high energies of the secondary charged particles, there are problems with discrimination against photons for the counting of recoil protons. Two-diode devices are used to subtract the photon component with paired detectors only one of which has a hydrogenous converter layer. With solid state detectors such as p-n junctions this allows the detection threshold for fast neutrons via recoil protons to be reduced to about 200 keV which would otherwise be at about 500 keV, using just an electronic threshold. Photon discrimination is also possible, in principle, via pulse shape analysis which, however, requires sophisticated electronics. Full-range devices can use several. semiconductor or surface barrier detectors, some of them covered with different layers of 10B or 6Li, and with or without cadmium or boron shields to absorb incident thermal neutrons, and with or without polyethylene converters of different thicknesses. The difficulty remains the fast neutron sensitivity, particularly in the range between 30 keV and 1 MeV. Unfolding of the resulting pulse-height spectra at each of the several detectors can result in an improved energy dependence of the response. At this time, although there are several commercial devices available, all have some drawbacks, and none are used in routine radiation protection. Further information, including descriptions of other approaches, can be found in [38], [39], [40], [41], [42], [43]. 2.3.3 Dosimetric characteristics of neutron dosimetry systems available in the UK These are shown in summary form in Table 3. A summary of the information on systems available in the UK and EU can be found in [32]. 13 Neutron energy range(s) Thermal to a few keV, extended to fast neutron region with field-specific correction factor About 200 keV upwards. Can be extended to cover thermal and epithermal region with 6 Li(n,t) radiator From 150 keV to 10 MeV, higher energies with specific neutron calibration factor Thermal to a few keV and 100 keV upwards Two options, 50 keV to 15 MeV with or without thermal and epithermal detection element. (Harvey Autoscan 60 etched track dosimetry system Global Dosimetry Solutions Autoscan 60 etched track dosimetry system track TLD Albedo design) NRPB etched dosimetry system Landauer etched track dosimetry system and Few data, probably -100% +50% for 50 keV to 15 MeV, and better for broad energy distributions, NVLAP and DOELAP approved From thermal to a few keV ±50%; from about 100 keV to 20 MeV ±50%; better for broad energy distribution NVLAP accredited DOELAP approved ± 20% From about 200 keV to 15 MeV ± 50% Neutron energy dependence of Hp(10) response Thermal to a few keV ± 20% 14 Thermal and epithermal ± 20%; from about 50 keV to 15MeV expected –100% to +50% up to 60°, better for rotational isotropy, but few data available Thermal to a few keV ± 30% < 60°; from 150 keV to 20 MeV ± 50% < 60°, better for rotational isotropy 0° to 30°: ± 10%; 30° to 60°: ± 30%; 60° to 85°: ± 70% 500 keV to 15 MeV ± 50% up to 60°, better for rotational isotropy, can be improved by use of 3 element dosemeter. Neutron angle dependence of Hp(10) response Few data, but expected to be good 200 µSv to a few tens of mSv 100 µSv to few tens of mSv, extended to a few Sv using chemical etching 200 µSv to 50 mSv Few hundred µSv to few tens of mSv, extended to a few Sv using Al filtered area. Few tens of µSv to few tens of Sv Dose range No fading. About 30% fading per year. Also some increase in background signal About 30% fading per year, correction applied About 30% fading per year, but also sensitivity changes and some increase in background signal. About 5% per year fading. Generally environmentally robust. Environmental effects Summary of performance characteristics of neutron dosemeters used by UK ADSs (manufacturers’ or ADSs’ data) Dosemeter Type Table 3 3 WORKPLACE FIELDS 3.1 GENERAL 3.1.1 Characterization of workplace fields There are several reasons for making measurements of the energy and direction distributions of workplace fields. One reason is to assess the relationship of Hp(10) and Hp(0.07) to effective dose or skin dose. (For example, at photon energies less than 100 keV there is a progressive overestimate by Hp(10) of effective dose (E), reaching, for the anterior-posterior (AP) direction, a factor of five at 20 keV). A second reason is the consideration of the performance of practical designs of personal dosemeter in estimating Hp(10) and Hp(0.07). The third reason, is to assess the suitability or adequacy of the dosemeter performance requirements. In summary, good knowledge of workplace fields (i.e. data on energy and direction distributions, dose rates, worker orientation and occupancy factors), is useful in order to assess the dosimetric model used to determine Hp(10) and Hp(0.07); predict the performance of practical, non ideal dosemeters; optimize the design of dosemeters; frame the dosemeter performance requirements sensibly; and assist the retrospective interpretation of dosemeter readings if required. The detailed determination of the energy and direction distributions requires specialized equipment and specialists to use it. The measurements can be lengthy and therefore expensive. In some instances limited additional information on workplace fields can be sufficient to enable the choice of suitable personal dosemeter. For instance, procedures can be used to identify areas where there is a strong low energy component which may lie below, for example, the threshold of an electronic personal dosemeter. Similarly it is possible to search for radiation incident at unexpected angles, by using lead shielding around a Geiger Müller detector to collimate the response to a few tens of degrees. Further information of the characterization of workplace fields may be found in a special issue of Radiation Protection Dosimetry “Neutron and photon spectrometry techniques for radiation protection”44, and in [7]. 3.1.2 Field calibration Practical considerations may well result in the use of a dosemeter with some deficiencies of response characteristics. Equipment needed for the measurement of the energy and direction distribution of the workplace field is expensive, the analysis time consuming, and the results often difficult to apply to an individual worker. The most effective procedure may be the inter-comparison on phantoms of specialized devices which give a better determination of Hp(10) and Hp(0.07), but are not suitable for routine use, and the preferred practical dosemeter, a so-called field calibration. Overnight or over weekend exposures can often be employed to allow the accumulation of sufficient dose well above the measurement threshold. Multiple dosemeters can be used on the same phantom to mimic rotation of the worker. 3.1.3 Use of algorithms If a dosemeter has only one detector to determine each or either of Hp(10) and Hp(0.07), and meets performance criteria for narrow spectra or mono-energetic radiation fields, in general, the dosemeter will be appropriate to all fields, mixed and wide energy, within the range of energies and angles investigated. The situation is more complicated if a dosemeter has more than one detector. In such cases a dose calculating algorithm is required to combine the reading from each detector in order to produce a measured dose value. The simplest is the 15 linear combination of the detector readings. Another method uses linear programming. For these two linear methods the situation is almost as simple as for a dosemeter with one detector. A type test with narrow spectra covering the anticipated energy range is sufficient to establish whether the dosemeter is appropriate. Algorithms which rely on the ratio of readings from several of the detectors in the dosemeter are more difficult to test, particularly those that use branching programmes. Strictly, the performance of such dosemeters can only be assumed for the radiation qualities used in the testing process45. Performance in workplace fields may be disappointing, as the algorithm may have been designed, quite deliberately, to generate good results in established testing programmes rather than to operate well in environments with wide energy and direction distributions. Hence it is important to test such dosemeters using energy and direction distributions spectra typical of workplace fields46,47. 3.2 PHOTON AND ELECTRON FIELDS In practical situations personal dosemeters are required to estimate the quantity of interest with reasonable accuracy for the workplace photon radiation field, which, in principle, may be distributed over all angles of incidence, and over the energy range 10 keV to 6/7 MeV. This is generally taken as meaning that the dosemeter indication is within a factor of 1.5 of the conventional true value48. In a complicated situation with well shielded sources and multiple scattering it is not possible to predict either the energy or direction distribution of radiation incident at the location of the worker. In principle, a suitable dosemeter for such situations has to be capable of responding within acceptable limits to the full range of possible energies and for all angles of incidence. For photons of energies greater than, respectively, 60 keV and 2 MeV, Hp(0.07) and Hp(10), have contributions from both photons and secondary electrons incident on the body. The dosemeter must also be able to accurately include both contributions. For example, if a design of dosemeter does not have a thickness of covering over the Hp(10) element equivalent to 10 mm electron range in tissue, in radiation fields with a significant photon component of energy greater than 2 MeV, significant errors in the estimation of Hp(10) can occur if there is not secondary electron equilibrium.. A summary of the range of energies in the more usual workplace photon fields is shown in Table 4. Further information is given in [44], [49] and [50]. 16 Table 4 Examples of energy ranges for some commonly encountered photon and electron workplace fields Field/source Photon/electron energy ranges Radiopharmaceuticals, manufacture and use Generally only low energy photons and electrons 147 Electrons plus photons Eȕ,max.: 225 keV Photons 20 to 120 keV. Industrial beta thickness gauges, for example, 85Kr 90 Sr/90Y Electrons plus photons Eȕ,max.: 687 keV Electrons plus photons Eȕ,max.: 2.274 MeV Photons: 10 to a few 100 keV. 30 to a few hundred keV 20 to 150 keV Pm Contaminated waste Interventional radiology General diagnostic radiology Industrial radiography Photons plus secondary electrons Photons plus secondary electrons Photons plus secondary electrons 20 to 150 keV Photons plus secondary electrons Photons plus secondary electrons Photons plus secondary electrons 50 to 700 keV Nuclear fuel cycle Electrons, photons plus secondary electrons Nuclear power reactors Research facilities Photons plus secondary electrons Photons plus secondary electrons Electrons from 60 keV to a few MeV plus photons from 17 keV to a few MeV 30 keV to 6/7 MeV Industrial sterilization facilities Medical linacs 100 keV to 1.3 MeV 100 keV to 20 MeV 100 keV to > 1 GeV. 17 Comments Very dependent on shielding, probably only concern for dose to extremities, and possibly eye dose Very dependent on shielding, probably only concern for dose to extremities, and possibly eye dose; possible photon contribution. Very dependent on shielding; note possible bremsstrahlung contribution Very dependent on shielding, note probable bremsstrahlung contribution Dependent on scatter and shielding Dependent on scatter and shielding Dependent on scatter and shielding at location of radiographers Dependent on scatter and shielding Dependent on scatter and shielding Dependent on scatter and shielding at location of radiographers Large range of energies Secondary electron equilibrium not always present Very dependent on shielding/secondary particles 3.3 NEUTRON FIELDS AND REQUIREMENTS OF DOSIMETRY SYSTEMS Neutron fields in workplaces in the nuclear fuel cycle, in nuclear power generation, and in areas near medical accelerators or where radionuclide sources are used, span energies from thermal to 20 MeV. For workplaces near high energy accelerators and cosmic radiation fields at aircraft altitudes the energies extend up to many GeV. In most workplace fields where any annual doses of significance are received, there is a thermal and epithermal component of neutron fluence, and a fast component. The relative magnitude of the two components varies from fields with a large thermal and epithermal neutron component outside the thick shielding of power reactors (‘soft’ energy distributions), to almost completely fast neutrons near some reprocessing lines and near unshielded radionuclide sources (‘hard’ distributions). What is almost universally true, is that there is little dose equivalent between a few keV and 50 keV. However, there is often a significant contribution between 50 keV and 500 keV, and it is the energy region which gives rise to the greatest difficulty in neutron detection and photon discrimination. Table 5 shows examples of measured (plus one calculated- the MOX field) neutron energy distributions at various workplaces broadly representative of those in the nuclear industry and source manufacture and use in the UK32,51,52,53. There are likely to be particular fields where a correction to the reading of the dosimetry system used, may need to be included for the contribution to total Hp(10) from the component between 5 and 50 keV. Further information on neutron workplace field are given in two recently compiled catalogues of measured and calculated energy distributions [54] and [55], and in [44], [51] and [56]. 18 Table 5 Hp(10,AP) Dose fractions within energy bands for various workplace spectra Type Bare Source Thermal (<0.4eV) 0.00 0.4eV to 5keV 0.00 5 keV to 50keV 0.00 50keV to 100keV 0.00 100keV to 300keV 0.01 300keV to 20 MeV 0.99 Am-Be in glove box 252 Cf in bunker Source production 0.04 0.02 0.01 0.02 0.04 0.87 Source production /use 0.00 0.01 0.00 0.01 0.04 0.94 Trawsfynydd - filter gallery Calder Hall – control room Ringhals – A GCR 0.21 0.07 0.02 0.05 0.17 0.48 GCR 0.55 0.22 0.03 0.04 0.08 0.08 Westinghouse PWR 0.11 0.20 0.06 0.08 0.21 0.34 Fuel Pin Assembly – little shielding Pu finishing plant – little shielding BNFL MOX Site 2 Pos 1 CLAB D Fuel production 0.01 0.01 0.01 0.01 0.03 0.93 Fuel processing 0.01 0.01 0.01 0.02 0.15 0.80 MOX production 0.04 0.02 0.01 0.02 0.07 0.84 Fuel flask 0.03 0.14 0.04 0.07 0.20 0.52 Field 241 Am-Be 241 19 4 CHOICE OF PERSONAL DOSEMETER In general, the choice of personal dosemeter and dosimetry system by an employer should be made in consultation with his radiation protection adviser(s) and, where appropriate, the health physics staff. The consultation should include discussions of the characteristics of the radiation fields in the workplaces, the most appropriate wear position, issue period etc. The choice of dosemeter for use in a particular set of radiation field parameters may permit, or require, a normalization factor to be applied in order to minimize the deviation of the dosemeter relative Hp(10) response of the range of radiation energies and directions to be encountered in the workplace, or to minimize the deviation of the estimation of effective dose. Where an assessed dose received exceeds a relevant dose limit or investigation level, the employer may ask the ADS, in conjunction with the RPA, to take account of information on the wearing position, the response characteristics of the dosemeter, and the characteristics of the workplace field, in any reassessment to provide the best estimate of effective dose. The determination of energy and direction distributions of workplace fields is not a simple matter, requiring sophisticated measurement methods and analysis. The fields will usually comprise direct and scattered components resulting in broad energy and direction distributions. In addition the fields may be significantly spatially non-uniform, leading to non uniform exposure of the body. It is then difficult to make appropriate assessments of Hp(10) and Hp(0.07), and of effective dose when this is required. There is not much published information on photon and electron workplace fields, but a reasonably large number of published papers on neutron fields. The reason for this is that, in general, there are designs of photon and electron dosemeters available which are able to determine Hp(10) and Hp(0.07) within acceptable limits for the range of energies and directions present in workplaces, whereas this is not the case for neutron fields. For neutron workplace fields it is always necessary to have at least some information on the energy and directions characteristics of workplaces. Nevertheless, it can still be useful to have information on photon and electron workplace fields. The designs of dosemeters and dosimetry systems have developed along with changes in applications. Recent tests of photon and electron whole body dosemeters by EURADOS, including irradiations in simulated workplace fields46 and by HSE57, indicated that most current designs have acceptable energy and angle dependences of Hp(10) and Hp(0.07) response characteristics for large regions of the entire range of particle energies likely to be encountered in the workplace. Nevertheless, some unexpected inadequacies were found in a study of the suitability of personal dosemeters for a particular set of workplaces58, This latter study showed the value of investigating the performance of dosemeters and ADS in the actual workplace. For several of the available neutrons dosemeters and dosimetry systems, less than adequate performance was found in both the EURADOS study46 and in a recent IAEA inter comparison59 which included simulated workplace fields. However, it is still considered that a choice of neutron dosemeter should be possible such that regions of inadequate dosemeter response characteristics are in energy regions where there is not a significant contribution to total Hp(10). An EC funded study is investigating neutron personal dosemeter performance in actual neutron/photon workplaces60, and the results will be of much interest. It has been proposed that regular periodic EURADOS organized international intercomparisons take place for both photon/electron and neutron dosimetry systems. Such a programme might contribute significantly to the process of choosing a suitable dosimetry system. 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