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. The best method, however is to assess the available systems and establish as far as
20
practicable the main characteristics of the workplace fields in which the dosemeter is to be
worn, and if at all possible, carry out in-situ tests of available ADS.
21
5
1
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27
Published by the Health and Safety Executive
11/05
RR 385