Dosimetric properties of an environmental H*(10) dosemeter based on radiophotoluminescence of silver doped phosphate glass

The objective of this study was to investigate the energy and angle dependence of the RPL dosimetry system in use at the Paul Scherrer Institute (GBFJ-01 dosimeter, FGD 660 reader, and CDEC-Easy algorithm) for the operational dose quantity H*(10) and compare the results with the requirements from the Swiss dosimetry regulation.

Available online 19 September 2020

1350-4487/© 2020 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Dosimetric properties of an environmental H*(10) dosemeter based on

radiophotoluminescence of silver doped phosphate glass

Frank Assenmacher, Elisa Musto, Eduardo G Yukihara*

Department of Radiation Safety and Security, Paul Scherrer Institute, Switzerland

A R T I C L E I N F O

Keywords:

Radiophotoluminescence

RPL

Environmental dosimetry

H*(10)

IEC 62387

Type testing

A B S T R A C T The objective of this study was to investigate the energy and angle dependence of the RPL dosimetry system in use at the Paul Scherrer Institute (GBFJ-01 dosimeter, FGD 660 reader, and CDEC-Easy algorithm) for the

operational dose quantity H*(10) and compare the results with the requirements from the Swiss dosimetry

regulation The energy response was determined for photon energies between 12 keV (N-15) and 1250 keV (60Co) and the angle response was determined for 65 keV (N-80) and 662 keV (137Cs) The data obtained in this study show that the RPL dosimeters satisfy the requirements of the Swiss dosimetry regulation for the energy dependence, but only partially satisfy the requirements for the angle dependence For directional irradiations around ±90◦the dosimeter response can deviate by more than the 20% allowed by the Swiss dosimetry regu-lation If a more realistic testing scenario for the 90◦irradiation is used, which includes full rotation of the dosimeter around its reference direction, the response is within 10% of the response at 0◦irradiation Full tests according to the IEC 62387 were not performed and, therefore, performance against this standard cannot be fully evaluated from this data Nevertheless, the data indicate that the asymmetric dosimeter design would have

difficulties satisfying the IEC 62387 standard for H*(10) for low energy photons and large angle irradiation, in

spite of the allowance of full rotation for the 90◦irradiations

1 Introduction

The radiophotoluminescence (RPL) of silver-doped phosphate glass

(P4O10:Ag+), which is the UV-excited photoluminescence from color

centers (Ag0, and Ag2+) created in the glass by ionizing radiation, is now

used commercially for personal dosimetry (Kurobori et al., 2010;

Miyamoto et al., 2010, 2011; Yamamoto et al., 2011) These color

centers are stable under optical excitation and do not fade over time,

making the readout non-destructive Annealing of the glass detectors (e

g 400 ◦C for 1 h) restores the original silver dopant (Ag+) concentration,

allowing the material to be used again with the same response (

Yama-moto et al., 2011)

The silver-doped phosphate glass is an integrating, passive solid-state

detector, which, combined with filters for ionizing radiation, a dose

calculation algorithm and automated readers, can be used for the

measurement of the personal dose equivalent quantities Hp(10) and

Hp(0.07), or of the environmental dose equivalent H*(10) (Burgkhardt

et al., 1990; Piesch and Burgkhardt, 1994; Juto, 2002; Ranogajec-Komor

et al., 2008) A modern RPL dosimetry system consisting of a dosimeter

badge containing a silver-phosphate glass and different filters, auto-mated reader and a dose calculation algorithm for personal and envi-ronmental dosimetry is now commercialized by CHIYODA TECHNOL CORP

In 2016 the Paul Scherrer Institute (PSI) adopted for individual monitoring of its staff the RPL system consisting of the GBFJ-01 dose-meter badge version and FGD-660 reader (CHIYODA TECHNOL CORP.)

This system was first used in 2008 at the Institut de Radioprotection et de

Sûret´e Nucl´eaire (IRSN, France) in their routine dosimetry service

(Hocine et al., 2011; Hocine, 2012) The dosemeter badge has di-mensions 61.0 × 30.0 × 8.0 mm3 and contains a RPL glass detector of dimensions 35.0 × 7.0 × 1.5 mm3 and filters made from aluminum, copper, tin, and two kinds of plastic materials, forming five differently filtered areas of the glass detector; for details, see Hocine et al (2011) The detector material FD-7 was produced by AGC TECHNO GLASS CO., LTD., Shizuoka, with a weight composition of 31.55% P, 51.16% O, 6.12% Al, 11% Na, and 0.17% Ag resulting in Ag+-doped phosphate glass (P4O10: Ag+) (Yamamoto et al., 2011) The FGD-660 reader mea-sures each glass detector in five distinct positions, corresponding to the

* Corresponding author Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland

E-mail address: Eduardo.Yukihara@psi.ch (E.G Yukihara)

Contents lists available at ScienceDirect Radiation Measurements journal homepage: http://www.elsevier.com/locate/radmeas

https://doi.org/10.1016/j.radmeas.2020.106468

Received 12 May 2020; Received in revised form 14 August 2020; Accepted 16 September 2020

five differently filtered areas in the dosemeter badge; see Huang and Hsu

(2011) for an illustration of the measurement principle The dosimetric

properties of this system for Hp(10) and Hp(0.07) were reported by

Assenmacher et al (2017) and demonstrated to satisfy the Swiss

dosimetry regulation for personal dosimeters

The updated Swiss dosimetry regulation in effect since 2018

intro-duced requirements for environmental dosimeters to be used in

Switzerland (Swiss Federal Council, 2017) Requirements for H*(10) are

specified for the measurement range (from 0.05 mSv up to 100 mSv),

linearity (<15% in the 0.1 mSv–100 mSv dose region), energy

dence (<30% for energies between 50 keV and 5 MeV), angle

depen-dence (<20% for energies > 50 keV), reproducibility (standard

deviation < 10%), and fading (<20% pro exposure period) Although

some of the dosimetric characteristics are not influenced by the quantity

of interest (e.g reproducibility, fading and linearity), compliance

regarding the energy and angle dependence must be independently

checked for H*(10), because of the different conversion factors from air

kerma to the operational quantity, reference irradiation conditions, and

dose calculation algorithm in comparison with those for Hp(10)

The objective of this study was, therefore, to investigate the energy

and angle dependence of the RPL dosimetry system in use at PSI for the

operational dose quantity H*(10) and compare the results with the

re-quirements from the Swiss dosimetry regulation The results are also

interpreted in terms of the IEC 62387 standard (IEC, 2020), but in this

case the tests were not comprehensive Because the entire system

cali-bration and RPL readout is independent on the operational quantity

calculated, tests such as dose range, reproducibility, and dose linearity,

already determined for Hp(10) (Assenmacher et al., 2017), were not

repeated The response of an H*(10) environmental type dosemeter to

beta radiation is not required to be verified according to the Swiss

dosimetry regulation or the IEC 62387 standard and, therefore, it was

not tested

2 Materials and methods

2.1 The RPL dosimetry system

The RPL dosimetry system investigated here is the same used by PSI’s

accredited personal dosimetry service and consists of the FD-7 RPL glass

(AGC TECHNO GLASS CO., LTD., Shizuoka), dosemeter badge type

GBFJ-01, reader type FGD-660, and dose calculation software CDEC-

Easy (CHIYODA TECHNOL CORP., Tokyo) (Assenmacher et al., 2017)

The same procedures used at PSI for personal dosimetry were also

adopted for the environmental dosimetry, which include annealing of

the RPL glass detectors at 370 ◦C for 10 min to reset the signal (the

complete cycle lasts about 2 h), and post-irradiation annealing at 100 ◦C

for 1 h to establish a complete RPL signal build-up (Assenmacher et al.,

2017) The temperature of 370 ◦C used at PSI was recommended by the

manufacturer and is slightly lower than the one mentioned in previous

publications (Yamamoto et al., 2011)

2.2 Irradiations

Photon irradiations according to the ISO norms 4037–1 (ISO, 1996)

and 4037–3 (ISO, 1999) were performed using 137Cs and 60Co sources

(OB20 irradiation device, Buchler GmbH, Braunschweig), or an

ISO-VOLT Titan TI320 X-ray device (GE General Electric Inspection

Tech-nologies) at PSI’s secondary standard dosimetry laboratory (SSDL) The

calibration is traceable to the primary standards of the

Physikalisch-Technische Bundesanstalt (PTB, Germany) The relative

uncertainties (coverage factor k = 2) of the irradiated dose values were

in the range from 3.4% to 3.9% for the OB20 device, and from 2.3% to

2.6% for the TI320, and include a 2% uncertainty of the conversion

factors from air kerma Ka to H*(10) (ISO, 1999) Typical

source-dosemeter distances were 2 m for the ISOVOLT TI320 device and

from 2 m to 6 m for the OB20 device

For positioning the dosemeters free in air, either Styrofoam™ (0.02 g/cm3) or card boards (sandwich type with a comb-like structure, 0.06 g/cm3) a few centimeter thick were used behind the dosemeters to support them; backscattering from these materials can be considered negligible Because the dosemeter badge construction is sufficient to

establishing secondary particle equilibrium for H*(10), the so-called

“build-up plate” was not used One experiment with Cs-137 irradia-tion with and without a 2 mm PMMA plate as build-up was performed to confirm this assumption

The irradiation conditions, photon energies, angles of incidence, and reference doses used in this study, are summarized in Table 1 and Table 2 For each condition typically 9–10 RPL dosemeters were irra-diated and measured, depending on the experiment In a few cases the number of dosemeters analyzed was reduced because of wrong irradi-ations The energy response was determined for photon energies be-tween 12 keV (N-15) and 1250 keV (60Co) and the angle response was determined for 65 keV (N-80) and 662 keV (137Cs)

When defining the irradiation angles and axes of rotation, the dosimeter symmetry was considered (Fig 1) The linear arrangement of the filters A to E in the GBFJ-01 dosemeter badge and the corresponding

measurement areas in the glass detector defines a unique axis H (resembling the horizontal dimension) of the dosemeter The rotation around the H-axis is symmetric in both directions, because the filters of

the badge surround the glass detector symmetrically

Perpendicular to the H-axis and to the reference direction of the

dosemeter (pointing towards the radiation field or the source), a second

axis V (vertical dimension) can be defined for which the rotation around the V-axis is not symmetric When rotating around the V-axis the glass

detector elements distinguished by their filters move towards, respec-tively away from the radiation source due to the rotation For a positive rotation +αV the detector element E surrounded by a tin filter, moves

closer to the source

Additionally, the measurement volume inside the detector, which is determined by the laser beam adjustment and the detection optics inside the FGD-660 reader, is closer to the front surface of the glass detector slab in order to increase the sensitivity to low energy beta radiation with low penetration depth in the detector material (Maki et al., 2016) Thus front-side and back-side are expected to show a differing response This effect was tested with the 180◦ irradiation and for the asymmetric rotation around the vertical axis only

The Swiss dosimetry regulation specifies that the environmental

dosimeter must have an angle dependence <20% for energy >50 keV

For most personal dosemeter designs, however, the 90◦can be prob-lematic because of the asymmetric filter design (the radiation may avoid the filter) or the detector positioning (one detector obstructing the field

of view of another detector), or both

The IEC 62387, on the other hand, proposes that, for the angle of

Table 1

ISO radiation qualities and mean gamma energies used at angle α =0◦and reference dose H*(10) = 10.0 mSv for

determining the energy dependency of the dosemeter response

Radiation Quality E mean [keV]

incidence α =90◦the dosemeter shall be rotated around its reference

direction during exposure If a continuous rotation during exposure is

not possible, the total dose shall be applied in eight identical fractions

from eight directions with an angle of 45◦in between, thus covering a

full circle

Therefore, in addition to the 90◦irradiation, a “full rotation”

irra-diation consisting of eight partial irrairra-diations as proposed in the IEC

62387 was also performed

2.3 Dose calculation and analysis

The doses were calculated using the linear algorithm (Juto, 2002)

implemented in the CDEC-Easy software using the option for

environ-mental dose calculation H*(10) Calibration of the dosimetry system is

based on dosemeters irradiated with 137Cs in the dose quantity air kerma

and is independent on the chosen calculation algorithm Here the H*(10)

values provided by the algorithm are considered as the indicated value

of the system The values include automatic corrections for reader

sensitivity and glass batch sensitivity The signal due to background

radiation accumulated in the laboratory since pre-dose measurement

(after annealing) were automatically subtracted by CDEC-Easy The

implementation details of the dose calculation algorithms are

intellec-tual property of CHIYODA TECHNOL CORP The reproducibility of the

dose measurements are typically <2% for doses above 0.5 mSv and for

Cs-137 irradiation (Assenmacher et al., 2017)

The energy and angle dependence are presented in terms of the

relative response, defined as R/R0, where R0 is the reference response, i

e., the ratio between the indicated value provided by the dosimetry

system and the conventional true value of the quantity (IEC, 2020)

The uncertainty in the indicated value G were estimated by dividing

the standard deviation of the results by the square root of the sample size

(standard deviation of the mean), multiplied by the corresponding tp(ν)

value for a 95.45% level of confidence (k = 2) from the t-distribution for

degrees of freedom ν, as in Table G.2 from the ISO/IEC Guide 99–3:2008 (ISO/IEC, 2008) The uncertainties u R in the responses R were calculated

by error propagation assuming an uncertainty of 3.9% (k = 2) for the Cs-137 irradiations and 2.6% (k = 2) for the X-ray irradiations (see

Section 2.2) The uncertainties in the relative response r = R/R0 were

estimated simply as u R /R0, since R and R0 are correlated (an over-estimation of the conventional true value will lead to an

over-estimation in both R and R0)

3 Results

3.1 H*(10) response

The response of the RPL glasses in terms of H*(10) is presented in

Table 3 One irradiation was performed with a 2 mm PMMA as build-up plate Firstly, the results confirm that the PMMA does not have an in-fluence on the indicated value of the RPL dosimeters Secondly, using the routine calibration procedure for the RPL system, the CDEC-Easy

algorithm seems to over-estimate the H*(10) values by 4–6%

This over-estimation is within the tolerance of the Swiss dosimetry regulations (±10%) Nevertheless, we should point out that the same

calibration is used by the RPL system to calculate both Hp(10) and H* (10) based on different algorithms In the case of Hp(10) measurements for RPL dosimeters irradiated on phantom, no over-estimation is observed (Assenmacher et al., 2017) Therefore, the user must decide whether or not to apply an additional correction to the RPL indicated

values for the H*(10)

3.2 Energy dependence

Fig 2 shows the H*(10) relative response to photon radiation

qual-ities with energy in the range from 12 keV to 1250 keV (ISO, 1999) Data

for the Hp(10) (Assenmacher et al., 2017) are also shown for comparison

The H*(10) relative response satisfies the Swiss dosimetry regulation requirement for the energy dependence, <30% for H*(10) in the range

50 keV ≤ E ≤ 5000 keV, at least up to 1250 keV Calibrated photon

radiation qualities at higher energies were not available at the time of the tests The data in Fig 2 shows that the relative response is also within the requirements between 0.71 and 1.67 of the IEC 62387

3.3 Angle dependence

Fig 3 shows the H*(10) relative response to the N-80 radiation quality for rotations around the H-axis (H-rotation) and the V-axis (V-

rotation) Fig 3 also indicates the requirements from the Swiss dosim-etry regulation (±20%) and the lower limit of the IEC 62387 The upper limit of the IEC 62387 is outside the plotting range

For rotations around the asymmetric V-axis (V-rotation), the

influ-ence of the detector asymmetry is noticeable in the data Furthermore,

Table 2

ISO radiation qualities, irradiation angles and reference doses used for

deter-mining the angular dependency of the dosemeter response

Radiation

Quality Rotation axis Angle

α [ ◦ ] H*(10) [mSv]

V − 120, − 105, − 90, − 75, − 60,

60, 75, 90, 105, 120

H − 75, − 60, 60, 75, 90

V or Hb 90 rotated 8 × 45 around the

reference direction 8.0 (8 × 1.0) or 16.0 (8 × 2.0)

aRotation direction V or H is not relevant here

b The axis is irrelevant, since the dosimeter is rotated 360◦around the

refer-ence direction of the dosemeter

Fig 1 Geometry and axes of the detector/dosemeter badge

Table 3

RPL indicated value and response irradiated using Cs-137 source at 0◦ The uncertainties are the combined uncertainties for a coverage factor k = 2

H*(10)(a) [mSv] n(b) G(c) [mSv] R(d)

5 (with PMMA) 3 5.30 ± 0.03 1.06 ± 0.04

(a) conventional true value for the irradiation, having an uncertainty of 3.9% (k = 2)

(b)number of dosimeters irradiated

(c)indicated value for the irradiated dosimeters and combined uncertainty

(d)calculated response and combined uncertainty

for irradiations at close to 90◦, the detected RPL signal changes rapidly

for a small angle change, since there is no filter covering the left and

right side of the glass detectors Also the penetration depth in the

phosphate glass along the long axis leads to smaller RPL signals from the

detector elements further away from the source and shielded by the bulk

mass of the glass detector in between As a result, for irradiation angles

around 90◦the RPL detectors do not satisfy the Swiss dosimetry

regu-lations, which requires the deviation between the measured and given

dose to be smaller than 20%

The relative response for H-rotation is relatively symmetric, but also

shows a pronounced drop around 90◦ The low penetration depth of N-

80 radiation with a mean photon energy of 65 keV leads to low RPL signals for the metal filtered detection elements of the RPL glass de-tector Even though the dosimeter construction is symmetric for rotation around the horizontal axis and the filters cover the bottom and top side

of the glass detectors, the RPL measurement position within the glass is shifted towards the front of the badge (Maki et al., 2016) Therefore, the signals for angles 90◦±α are not expected to be the same The relative response to the irradiation angles of 105◦and 120◦were not tested for

the horizontal rotation, as we expected the non-symmetrical V-rotation

to be the most problematic

If the full rotation proposed in the IEC 62387 is used for the 90◦ angle, the relative response is 0.93 ± 0.03 In this case, it is irrelevant if the rotation is around the horizontal or vertical axis

Fig 4 shows the angle dependence results for 137Cs, also for rotations

around the H-axis (H-rotation) and V-axis (V-rotation) Again, the

re-quirements from the Swiss dosimetry regulation and the lower limit of the IEC 62387 are indicated in the figure

The angular dependence of the response is much less pronounced for

137Cs radiation field due to the deeper penetration depth of the 662 keV

photon radiation Only for the V-rotation at +90◦there is a drop of the response below 0.8 Although 662 keV photons have a much higher penetration depth, it is the build-up of absorbed energy which is much lower at the surface than in deeper depth This is due to the secondary radiation transport processes after the absorption of a photon and transfer of energy to the electrons of the detector material This effect could possibly be reduced if a build-up plate had been used for the Cs-

137 irradiation, but this is not a realistic situation during actual deployment of the dosimeters in the field

As in the case of N-80, the drop in response at ± 90◦degree disap-pears when the full rotation proposed in the IEC 62387 is applied For the full rotation, the response is 0.90 ± 0.04

Fig 2 Relative response for the dose quantities H*(10) and Hp(10) as a

function of the average energy of the photon radiation field See Table 1 for

reference conditions Hp(10) data from (Assenmacher et al., 2017) The

re-quirements of the Swiss dosimetry regulations are outside the margins of

the graph

Fig 3 RPL dosimeter H*(10) relative response as a function of angle of

rota-tion around the H-axis (H-rotarota-tion) or around the V-axis (V-rotarota-tion) for

radi-ation quality N-80 Due to symmetry considerradi-ations, the data for − 90◦and

− 180◦are the same as for +90◦and +180◦for the H-rotation (i.e one

mea-surement only for each angle), and the data for ±180◦are the same for the H-

rotation and for the V-rotation “Full Rotation” means that the angle of

inci-dence was 90◦degrees, but the dosemeters were rotated around its reference

direction and irradiated in eight fractions (see Section 2.2) The upper limit of

the IEC 62387 is outside the plotting range

Fig 4 RPL dosimeter H*(10) relative response as a function of angle of

rota-tion around the H-axis (H-rotarota-tion) or around the V-axis (V-rotarota-tion) for 137Cs Due to symmetry considerations, the data for − 90◦and − 180◦are the same as for +90◦and +180◦ for the H-rotation (i.e one measurement only for each

angle), and the data for ±180◦are the same for the H-rotation and for the V-

rotation “Full Rotation” means that the angle of incidence was 90◦degrees, but the dosemeters were rotated around the reference direction and irradiated in eight fractions (see Section 2.2) The upper limit of the IEC 62387 is outside the plotting range

4 Discussion and conclusions

The data obtained in this study show that the RPL dosimeters satisfy

the requirements of the Swiss dosimetry regulation for the energy

dependence, but only partially satisfy the requirements for the angle

dependence For direction irradiations around ±90◦ the dosimeter’s

response can deviate by more than the 20% allowed by the Swiss

dosimetry regulation This result is expected, given the asymmetric

badge design

In practice, however, the natural background radiation is usually not

directional Therefore, the full rotation scenario proposed by the IEC

62387 for the response at α = ±90◦can be considered more realistic for

the natural background radiation If the full rotation is used for the ±90◦

irradiation, the RPL response does not deviate from the unity by more

than 10%

Moreover, the average photon energy of natural radiation

back-ground is usually higher than 600 keV at back-ground level around the Paul

Scherrer Institute, and thus the deviation in response with incident angle

at N-80 is not as relevant for the response to the natural background

radiation

Under the above considerations, the RPL dosimetry system

consist-ing of the GBFJ-01 dosimeter, FGD 660 reader, and CDEC-Easy

algo-rithm can be used for general environmental monitoring of natural

background radiation and area monitoring outside of buildings, if the

dosimetry system limitations are kept in mind A direct comparison

between existing environmental and area monitoring (e.g

thermolu-minescence dosimeters) and the RPL dosemeters should provide further

support for the adoption of RPL for these applications In cases where a

directed radiation field is to be monitored, irradiations with angles

around 90◦should be avoided

Full tests according to the IEC 62387 were not performed and,

therefore, performance against this standard cannot be fully evaluated

from this data For example, the IEC 62387 requires the angle

depen-dence for H*(10) to be evaluated with the three lowest energies in the

energy range, whereas here only the N-80 radiation quality was tested

Nevertheless, the data indicates that the asymmetric dosimeter design

would have difficulties satisfying the IEC 62387 standard for H*(10) for

low energy photons and large angle irradiation

Declaration of competing interest

The authors declare that they have no known competing financial

interests or personal relationships that could have appeared to influence

the work reported in this paper

Acknowledgements

This work was partially funded by the Swiss Federal Nuclear Safety Inspectorate ENSI, contracts no 100979 and CTR00491

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