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Comparison of different dose algorithms used to evaluate a two element LiF:Mg,Ti TL personal dosemeter
Radiation Measurements 43 (2008) 571 – 575 www.elsevier.com/locate/radmeas
Comparison of different dose algorithms used to evaluate a two element LiF:Mg,Ti TL personal dosemeter H. Stadtmann ∗ , C. Hranitzky Austrian Research Centers, Radiation Safety and Applications, 2444 Seibersdorf, Austria
Abstract This paper presents the results of a comparison of four different dose algorithms used for evaluating a two element TL routine whole body dosemeter. Due to the energy response of LiF:Mg,Ti detectors a dose algorithm is applied to assess the relevant dose over the rated energy range with an acceptable energy response. The discussed dose algorithms are designed to calculate the dose in terms of three different dose quantities. The first two algorithms are used to evaluate the dose quantity photon dose equivalent Hx . This quantity was the legal quantity in Austria and Germany for the last decades. The other two algorithms are designed for reading out the dosemeter in terms of the dose quantities personal dose equivalents, Hp (10) and Hp (0.07). A huge number of individual dose values are recalculated and compared by statistical methods. In addition all dose values are analysed with respect to the ratio of the two detector readings which adds some spectral information to the dose values. This allows estimating the radiation quality (photon energy) for the different dose values. © 2008 Elsevier Ltd. All rights reserved. Keywords: Dose algorithms; LiF:Mg,Ti; TLD-100; Personal dosemeter
1. Introduction/scope
2. Dosemeter design
The scope of this paper is to present the results of a comparison of four different dose algorithms used for evaluating a two element thermoluminescent (TL) whole body dosemeter used in our large-scale routine Dosimetry Service Seibersdorf. The dose algorithms are designed to calculate the dose in terms of three different dose quantities Hx , Hp (10) and Hp (0.07) (respectively, the photon dose equivalent, and the personal dose equivalents at 10 and 0.07 mm depth). Although this paper describes a special dosemeter the emphasis of this communication is not the special dosemeter design itself but rather the description of the performance of the different dose algorithms and discuss how they differ in the reported dose values. An overview of complex nonlinear algorithms is given by Moscovitch (1993). The different dose algorithms were compared using basic mathematical procedures for simulated dose values, and using statistical methods for real measured results. For that purpose a huge number of individual measured dose values were recalculated and compared by statistical methods.
The beta-photon dosemeter investigated in this work consists of a commercial two element TL card and an in-house designed dosemeter badge (Fig. 1) used in our routine Dosimetry Service. The TL card contains two LiF:Mg,Ti (TLD-100) detectors. One TL detector is only covered with a thin Mylar foil, whereas the other TL detector is below a 2 mm aluminium filter. This dosemeter was originally designed for the measurement of the dose quantity photon dose equivalent Hx based on a free in air calibration. However it was already foreseen during the design phase to use the same dosemeter for the measurement of the operational quantities Hp (10) and additionally Hp (0.07). These quantities recommended by ICRU are based; however, on the calibration of the dosemeter on the ISO slab phantom applying dose conversion coefficients hpk given in ISO 4037-3 (1999). The main differences between Hx , Hp (10) and Hp (0.07) are their different energy and angular responses related to the basic field quantities such as air kerma free in air. Details on these quantities are given in Table 1. The differences can partially be considered by the application of appropriate mathematical algorithms for each dose quantity combining the two readings from each of the detectors.
∗ Corresponding author.
E-mail address: [email protected] (H. Stadtmann). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2008.01.033
572
H. Stadtmann, C. Hranitzky / Radiation Measurements 43 (2008) 571 – 575
3. Dose algorithms and energy response
1.6 1.4 Hx response
1.2 1.0 0.8 TL-window TL-filter Algorithm #1 Algorithm #2
0.6 0.4 0.2 0.0 10
100 photon energy (keV)
1000
Fig. 2. Relative energy response for photon dose equivalent (Hx ) for a free in air calibration. Both the values for the detector elements (TL) as well as for the dose results after application of the dose algorithms (#1, #2) are given. For energies > 60 keV the lines for “TL-filter” and “Algorithm #1” are overlapping!
3.0
2.5
Hp (10) response
Due to the energy response of the LiF:Mg,Ti detectors a dose algorithm is necessary to assess the relevant dose over the rated energy range with an acceptable energy response defined in the relevant Austrian standard ÖNORM 5237-1. Additional approval requirements used in Europe are summarised by Fantuzzi et al., 2001. This algorithms mathematically combine the readings of the two detector elements corrected for individual detector efficiency and reader sensitivity, TLi (i =window and filter) of the card and provide a calculated dose value Dalgorithm (TLw , TLf ) expressed in terms of the appropriate dose quantity. In Figs. 2–4 experimentally determined energy response curves normalised to photon radiation of 137 Cs (S–Cs) of both detector elements (TLw and TLf ) are given for the three different dose quantities. These measurements were carried out at the calibration laboratory Seibersdorf applying narrow spectrum photon radiation qualities (N-Series) according to ISO-4037-1 (1996). Only irradiations from the reference direction, perpendicular to the card are considered here. Both the definition of the applied dose quantity as well as the fact that measurements were performed free in air (for Hx ) or on the water slab phantom (for Hp (10) and Hp (0.07)) result in pronounced differences of the energy responses of the two detector elements especially in the lower energy region (E < 30 keV). The four different discussed dose algorithms are designed to measure doses for different dose quantities. A summary of these
TL-window
2.0
TL-filter Algorithm #3
1.5
1.0
0.5
0.0 10
Fig. 1. Whole body dosemeter of the routine Dosimetry Service Seibersdorf.
100 photon energy (keV)
1000
Fig. 3. Relative energy response for personal dose equivalent Hp (10) for a calibration on the water slab phantom. Response values for the element TL-window for energies E < 20 keV are truncated and not shown in the figure.
Table 1 Summary of different dose quantities used for the calibration of a personal whole body dosemeters Symbol
Quantity
Relation to basic field quantity
SI unit
Calibration of dosemeter
Ka X Hx Hp (10) Hp (0.07)
Air kerma Exposure Photon dose equivalent Personal dose equivalent Personal dose equivalent
= basic field quantity = basic field quantity Hx = 0.01 [Sv/R] · X Hp (10) = hpk (10, E, )Ka Hp (0.07) = hpk (0.07, E, )Ka
Gy C kg−1 Sv Sv Sv
Free in air Free in air Free in air On water slab phantom On water slab phantom
H. Stadtmann, C. Hranitzky / Radiation Measurements 43 (2008) 571 – 575
1
0.01
0.001
0.0001 0.8
4. Comparison of different dose algorithms The results of the dose values calculated by the different dose algorithms—both for simulated and measured detector element values TLi —were analysed and compared.
Algorithm #1 (2 radiations) Algorithm #1 (3 radiations) Algorithm #2 (2 radiations)
0.1 frequency
algorithms is given in Table 2. The first two algorithms are used to evaluate the dose quantity photon dose equivalent Hx . This quantity was the legal quantity in Austria and Germany for the last decades and is directly related to the quantity exposure X (Table 1). The other two algorithms are designed for reading out the whole dosemeter in terms of the operational dose quantities personal dose equivalents, Hp (10) and Hp (0.07). The normalised energy response of the dosemeter—after application of the corresponding dose algorithm is given in addition in Figs. 2–4. The recommended energy range as well as the limits of the energy response are summarised in Table 2.
573
0.9
1.0 1.1 linear deviation factor λ
1.2
1.3
Fig. 5. Frequency distribution of the simulated linear deviation factor for mixed radiation fields with two and three independent radiation qualities.
4.1. Simulated dose values In the first step only mathematical considerations were taken into account. Only linear dose algorithms allow—mathe1.2 1.6 1.0 dose ratio
Hp(0.07) response
1.4 1.2 1.0 0.8 0.6
0.2
0.4
0.0 10
100 photon energy (keV)
Hp(0.07) / Hx (#4 / #1) Hp(0.07) / Hx (#4 / #2) Hp(10) / Hx (#3 / #1) Hp(10) / Hx (#3 / #2)
0.6
TL-window TL-filter Algorithm #4
0.4
0.8
10
1000
Fig. 4. Relative energy response for personal dose equivalent Hp (0.07) for a calibration on the water slab phantom.
100 photon energy [keV]
Fig. 6. Ratio of dose values calculated by different algorithms for different dose quantities.
Table 2 Summary of the dose algorithms discussed in this paper Algorithm
Quantity
Math. algorithm
Energy response and energy range
Comment
#1
Hx
Hx = NTLf for TLw /TLf > 0.7 Hx = 0.8NTLw for TLw /TLf < 0.7
±15% 15–1300 keV
“If” algorithm with two linear branches Reference photon energy: 662 keV
#2
Hx
Hx = NTLw /fE (TLw /TLf )
±5%
Experimental interpolated energy response fE is used for correction. This algorithm is not used in routine
12–1300 keV #3
Hp (10)
Hp (10) = N[aTLw + (a − 1) TLf ] a = 0.71
±20% 20–1300 keV
Linear algorithm Reference photon energy: 662 keV
#4
Hp (0.07)
Hp (0.07) = N[bTLw + (b − 1) TLf ] b = 0.09
±20% 15–1300 keV
Linear algorithm Reference photon energy: 83 keV!!
TLi (i = window and filter), readings of the two card detector elements corrected for individual detector efficiency and reader sensitivity, N , calibration factor of the readout system, fE (RTL ), energy response of the “window position” detector with respect to the ratio of both detector elements (RTL = TLw /TLf ); in between the measured data interpolated values are calculated.
574
H. Stadtmann, C. Hranitzky / Radiation Measurements 43 (2008) 571 – 575
1.6 1 #4 frequency
1.4
TL-filter = TL-window All measured doses Hp(0.07) = 1 mSv (#4) Hp(10) = 1 mSv (#3) Hx = 1 mSv (#2) Hx = 1 mSv (#1)
1.2
0.01 0.001
#3 TL-filter
All dose values Hx > 1 mSv
0.1
#1 1
0.0001 0
0.2
0.8
0.4 0.6 0.8 TL-filter / TL-window
1
1.2
1 #2 #1 0.4 0.4
0.6
0.8
1
1.2
1.4
1.6
TL- window Fig. 7. Diagram of a range of single dose measurements with respect to the two detector readings (TLi ). The measuring points resulting in a calculated dose of 1 mSv (±1%) applying four different algorithms (#1, #2, #3, #4) for different dose quantities are marked with larger symbols. It must be noted that for Hp (10) and for Hp (0.07) different reference energies were chosen (Table 2). Therefore the intersection of the corresponding two lines is not in the area where TLw equals TLf .
matically—correct results in mixed radiation fields where a dosemeter is irradiated with different radiation qualities (photon energies) corresponding to different ratios of the TLi values. To analyse this a linear deviation factor was defined in Eq. (1). This deviation factor epresents the ratio between the only calculated dose value of a mixed radiation field and the sum of the single dose values of the corresponding individual irradiations. Both values should be equal for an ideal dose algorithm resulting in = 1. Dalgorithm ( i TLw , i TLf ) = i Dalgorithm (TLw , TLf ) ( = 1 for an ideal algorithm). (1) For algorithm #3 and #4 the value of is always 1, corresponding to a linear behaviour. Algorithm #1 and #2, however, are not linear (due to the application of ratios). To analyse this, the value of was simulated for a set of pairs and triples of randomly selected radiation qualities (the ratio between the readings of the two detector elements was randomly selected between 0 and 1). The resulting frequency distribution of is given in Fig. 5. Although a randomly selected radiation quality as described above does not reflect a typical work place field, the extrema of the distribution of ; however, show the possible deviation from 1 and are relevant for the estimation of the maximum mathematical error caused by the introduction of nonlinear algorithms. For both algorithms the most frequent deviation factor is around 1. A detailed analysis shows that the extreme values for algorithm #1 (< 0.95 and > 1.20) only occur for simulated
frequency
0.6
All dose values Hx > 1 mSv
0.1 0.01 0.001
0.0001 0
0.2
0.4
0.6
0.8
1
1.2
Hp (10) / Hx Fig. 8. Frequency distributions for two data sets: (a) Ratio TLf /TLw and (b) calculated doses Hp (10)/Hx .
radiation combinations where one of the element ratios RTL = TLf /TLw is near 0.7 (the value where algorithm #1 shows the discontinuity in Fig. 2). The frequency for extreme deviations; however, is quite low for both algorithms. Dose values calculated by different algorithms are compared in Fig. 6. There the ratios of these doses corresponding to different quantities are given. These dose ratios partly reflect the dose conversion coefficients given in ICRU Report 57 (1998). The pronounced backscatter component; however (maximum of the conversion coefficient for energies around 70 keV) is not visible due to the fact that the backscatter component is measured with the free in air calibrated Hx dosimeter as well. This effect, however, depends on the backscatter response of the tested dosemeter. 4.2. Measured dose values In the second step a huge number of real individual dose values evaluated from 1997 to 2006 were recalculated for all four algorithms and compared by statistical methods. A diagram of a number of single doses measurements with respect to the two detector readings (TLi ) is given in Fig. 7. Each single measuring point represents different calculated dose values depending which dose algorithm (and dose quantity) is applied. The relation between measured TLi -readings (x/y axis) and the resulting calculated doses D is marked in this figure as an example for D = 1 mSv. Linear (#3, #4) and nonlinear (#1, #2) algorithms can be identified very easily by the shape of the line. For the intersections of the marked functions (lines) the calculated dose values are identical for the respective algorithms.
H. Stadtmann, C. Hranitzky / Radiation Measurements 43 (2008) 571 – 575
The frequency distribution of the ratio RTL of the two detector readings (TLi ) summarised for two different sets of data is given in Fig. 8a. The unrestricted dose values (“all dose values”) including a high fraction of low background doses and a subset of doses exceeding 1 mSv are analysed and displayed in Fig. 8. For the latter the contribution of low photon energies (RTL < 0.7, ⇒ Eph < 45 keV) is pronounced about 20% of all values. This result is relevant for the assessment of typical work place fields of monitored people. The arithmetic mean of RTL for all dose values is 0.96. A similar shape shows the ratio RH of Hp (10) and Hx in Fig. 8b. The arithmetic mean of RH for all dose values is 0.94. For higher doses (> 1 mSv) only 7% of all Hx doses exceed the Hp (10) value by a factor of 1.5 (RH < 0.67). This result is relevant to estimate the shift of the dose distribution when changing to the new dose quantity Hp (10). 5. Discussion and conclusion This work proved that the described two element dosemeter can be evaluated in terms of three different dose quantities applying different algorithms with a satisfactory energy response. This feature allows to analyse the effect of changing the dose quantity from photon dose equivalent Hx to personal dose equivalent Hp (10) and Hp (0.07). The introduction of Hp (10), e.g. reduces the dose values in average by about 6%. For lower photon energies this effect is more pronounced as expected when considering the conversion coefficient from ICRU Report 57 (1998). The pronounced over response of Hp (10) for
575
energies around 70 keV; however, does not cause measured Hp (10) values in this energy range to be increased over Hx as explained. Although the relation between the described dose quantities is well defined the relation between measured dose values depends also on the dosemeter design especially on the response for backscatter radiation (from the person or phantom). That is why only measurements or detailed simulations considering the dosemeter design and the calibration procedure allow a detailed analysis of the effect of the change of the dose quantity on the dose distribution of the measured values. This paper does not include information from the angular response of the dosemeter. This, however, is foreseen to be carried out in a further step. References Fantuzzi, E., et al., 2001. Present status of approval procedures in the EU member states and Switzerland. Radiat. Prot. Dosimetry. 96, 73–80. ICRU. Report 57, 1998. Conversion coefficients for use in radiological protection against external radiation. ISO Report 4037-1, 1996. X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—part 1: radiation characteristics and production methods. ISO Report 4037-3, 1999. X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—part 3: calibration area and personal cosemeters as a function of energy and angle of incidence. Moscovitch, M., 1993. Dose algorithms for personal thermoluminescence dosimetry. Radiat. Prot. Dosimetry 47, 373–380. ÖNORM 5237-1, 1998. Strahlenschutzdosimeter—Personendosimeter zur Messung der Personendosis Hp (10) bzw. Hp (0.07).
Comparison of different dose algorithms used to evaluate a two element LiF:Mg,Ti TL personal dosemeter H. Stadtmann ∗ , C. Hranitzky Austrian Research Centers, Radiation Safety and Applications, 2444 Seibersdorf, Austria
Abstract This paper presents the results of a comparison of four different dose algorithms used for evaluating a two element TL routine whole body dosemeter. Due to the energy response of LiF:Mg,Ti detectors a dose algorithm is applied to assess the relevant dose over the rated energy range with an acceptable energy response. The discussed dose algorithms are designed to calculate the dose in terms of three different dose quantities. The first two algorithms are used to evaluate the dose quantity photon dose equivalent Hx . This quantity was the legal quantity in Austria and Germany for the last decades. The other two algorithms are designed for reading out the dosemeter in terms of the dose quantities personal dose equivalents, Hp (10) and Hp (0.07). A huge number of individual dose values are recalculated and compared by statistical methods. In addition all dose values are analysed with respect to the ratio of the two detector readings which adds some spectral information to the dose values. This allows estimating the radiation quality (photon energy) for the different dose values. © 2008 Elsevier Ltd. All rights reserved. Keywords: Dose algorithms; LiF:Mg,Ti; TLD-100; Personal dosemeter
1. Introduction/scope
2. Dosemeter design
The scope of this paper is to present the results of a comparison of four different dose algorithms used for evaluating a two element thermoluminescent (TL) whole body dosemeter used in our large-scale routine Dosimetry Service Seibersdorf. The dose algorithms are designed to calculate the dose in terms of three different dose quantities Hx , Hp (10) and Hp (0.07) (respectively, the photon dose equivalent, and the personal dose equivalents at 10 and 0.07 mm depth). Although this paper describes a special dosemeter the emphasis of this communication is not the special dosemeter design itself but rather the description of the performance of the different dose algorithms and discuss how they differ in the reported dose values. An overview of complex nonlinear algorithms is given by Moscovitch (1993). The different dose algorithms were compared using basic mathematical procedures for simulated dose values, and using statistical methods for real measured results. For that purpose a huge number of individual measured dose values were recalculated and compared by statistical methods.
The beta-photon dosemeter investigated in this work consists of a commercial two element TL card and an in-house designed dosemeter badge (Fig. 1) used in our routine Dosimetry Service. The TL card contains two LiF:Mg,Ti (TLD-100) detectors. One TL detector is only covered with a thin Mylar foil, whereas the other TL detector is below a 2 mm aluminium filter. This dosemeter was originally designed for the measurement of the dose quantity photon dose equivalent Hx based on a free in air calibration. However it was already foreseen during the design phase to use the same dosemeter for the measurement of the operational quantities Hp (10) and additionally Hp (0.07). These quantities recommended by ICRU are based; however, on the calibration of the dosemeter on the ISO slab phantom applying dose conversion coefficients hpk given in ISO 4037-3 (1999). The main differences between Hx , Hp (10) and Hp (0.07) are their different energy and angular responses related to the basic field quantities such as air kerma free in air. Details on these quantities are given in Table 1. The differences can partially be considered by the application of appropriate mathematical algorithms for each dose quantity combining the two readings from each of the detectors.
∗ Corresponding author.
E-mail address: [email protected] (H. Stadtmann). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2008.01.033
572
H. Stadtmann, C. Hranitzky / Radiation Measurements 43 (2008) 571 – 575
3. Dose algorithms and energy response
1.6 1.4 Hx response
1.2 1.0 0.8 TL-window TL-filter Algorithm #1 Algorithm #2
0.6 0.4 0.2 0.0 10
100 photon energy (keV)
1000
Fig. 2. Relative energy response for photon dose equivalent (Hx ) for a free in air calibration. Both the values for the detector elements (TL) as well as for the dose results after application of the dose algorithms (#1, #2) are given. For energies > 60 keV the lines for “TL-filter” and “Algorithm #1” are overlapping!
3.0
2.5
Hp (10) response
Due to the energy response of the LiF:Mg,Ti detectors a dose algorithm is necessary to assess the relevant dose over the rated energy range with an acceptable energy response defined in the relevant Austrian standard ÖNORM 5237-1. Additional approval requirements used in Europe are summarised by Fantuzzi et al., 2001. This algorithms mathematically combine the readings of the two detector elements corrected for individual detector efficiency and reader sensitivity, TLi (i =window and filter) of the card and provide a calculated dose value Dalgorithm (TLw , TLf ) expressed in terms of the appropriate dose quantity. In Figs. 2–4 experimentally determined energy response curves normalised to photon radiation of 137 Cs (S–Cs) of both detector elements (TLw and TLf ) are given for the three different dose quantities. These measurements were carried out at the calibration laboratory Seibersdorf applying narrow spectrum photon radiation qualities (N-Series) according to ISO-4037-1 (1996). Only irradiations from the reference direction, perpendicular to the card are considered here. Both the definition of the applied dose quantity as well as the fact that measurements were performed free in air (for Hx ) or on the water slab phantom (for Hp (10) and Hp (0.07)) result in pronounced differences of the energy responses of the two detector elements especially in the lower energy region (E < 30 keV). The four different discussed dose algorithms are designed to measure doses for different dose quantities. A summary of these
TL-window
2.0
TL-filter Algorithm #3
1.5
1.0
0.5
0.0 10
Fig. 1. Whole body dosemeter of the routine Dosimetry Service Seibersdorf.
100 photon energy (keV)
1000
Fig. 3. Relative energy response for personal dose equivalent Hp (10) for a calibration on the water slab phantom. Response values for the element TL-window for energies E < 20 keV are truncated and not shown in the figure.
Table 1 Summary of different dose quantities used for the calibration of a personal whole body dosemeters Symbol
Quantity
Relation to basic field quantity
SI unit
Calibration of dosemeter
Ka X Hx Hp (10) Hp (0.07)
Air kerma Exposure Photon dose equivalent Personal dose equivalent Personal dose equivalent
= basic field quantity = basic field quantity Hx = 0.01 [Sv/R] · X Hp (10) = hpk (10, E, )Ka Hp (0.07) = hpk (0.07, E, )Ka
Gy C kg−1 Sv Sv Sv
Free in air Free in air Free in air On water slab phantom On water slab phantom
H. Stadtmann, C. Hranitzky / Radiation Measurements 43 (2008) 571 – 575
1
0.01
0.001
0.0001 0.8
4. Comparison of different dose algorithms The results of the dose values calculated by the different dose algorithms—both for simulated and measured detector element values TLi —were analysed and compared.
Algorithm #1 (2 radiations) Algorithm #1 (3 radiations) Algorithm #2 (2 radiations)
0.1 frequency
algorithms is given in Table 2. The first two algorithms are used to evaluate the dose quantity photon dose equivalent Hx . This quantity was the legal quantity in Austria and Germany for the last decades and is directly related to the quantity exposure X (Table 1). The other two algorithms are designed for reading out the whole dosemeter in terms of the operational dose quantities personal dose equivalents, Hp (10) and Hp (0.07). The normalised energy response of the dosemeter—after application of the corresponding dose algorithm is given in addition in Figs. 2–4. The recommended energy range as well as the limits of the energy response are summarised in Table 2.
573
0.9
1.0 1.1 linear deviation factor λ
1.2
1.3
Fig. 5. Frequency distribution of the simulated linear deviation factor for mixed radiation fields with two and three independent radiation qualities.
4.1. Simulated dose values In the first step only mathematical considerations were taken into account. Only linear dose algorithms allow—mathe1.2 1.6 1.0 dose ratio
Hp(0.07) response
1.4 1.2 1.0 0.8 0.6
0.2
0.4
0.0 10
100 photon energy (keV)
Hp(0.07) / Hx (#4 / #1) Hp(0.07) / Hx (#4 / #2) Hp(10) / Hx (#3 / #1) Hp(10) / Hx (#3 / #2)
0.6
TL-window TL-filter Algorithm #4
0.4
0.8
10
1000
Fig. 4. Relative energy response for personal dose equivalent Hp (0.07) for a calibration on the water slab phantom.
100 photon energy [keV]
Fig. 6. Ratio of dose values calculated by different algorithms for different dose quantities.
Table 2 Summary of the dose algorithms discussed in this paper Algorithm
Quantity
Math. algorithm
Energy response and energy range
Comment
#1
Hx
Hx = NTLf for TLw /TLf > 0.7 Hx = 0.8NTLw for TLw /TLf < 0.7
±15% 15–1300 keV
“If” algorithm with two linear branches Reference photon energy: 662 keV
#2
Hx
Hx = NTLw /fE (TLw /TLf )
±5%
Experimental interpolated energy response fE is used for correction. This algorithm is not used in routine
12–1300 keV #3
Hp (10)
Hp (10) = N[aTLw + (a − 1) TLf ] a = 0.71
±20% 20–1300 keV
Linear algorithm Reference photon energy: 662 keV
#4
Hp (0.07)
Hp (0.07) = N[bTLw + (b − 1) TLf ] b = 0.09
±20% 15–1300 keV
Linear algorithm Reference photon energy: 83 keV!!
TLi (i = window and filter), readings of the two card detector elements corrected for individual detector efficiency and reader sensitivity, N , calibration factor of the readout system, fE (RTL ), energy response of the “window position” detector with respect to the ratio of both detector elements (RTL = TLw /TLf ); in between the measured data interpolated values are calculated.
574
H. Stadtmann, C. Hranitzky / Radiation Measurements 43 (2008) 571 – 575
1.6 1 #4 frequency
1.4
TL-filter = TL-window All measured doses Hp(0.07) = 1 mSv (#4) Hp(10) = 1 mSv (#3) Hx = 1 mSv (#2) Hx = 1 mSv (#1)
1.2
0.01 0.001
#3 TL-filter
All dose values Hx > 1 mSv
0.1
#1 1
0.0001 0
0.2
0.8
0.4 0.6 0.8 TL-filter / TL-window
1
1.2
1 #2 #1 0.4 0.4
0.6
0.8
1
1.2
1.4
1.6
TL- window Fig. 7. Diagram of a range of single dose measurements with respect to the two detector readings (TLi ). The measuring points resulting in a calculated dose of 1 mSv (±1%) applying four different algorithms (#1, #2, #3, #4) for different dose quantities are marked with larger symbols. It must be noted that for Hp (10) and for Hp (0.07) different reference energies were chosen (Table 2). Therefore the intersection of the corresponding two lines is not in the area where TLw equals TLf .
matically—correct results in mixed radiation fields where a dosemeter is irradiated with different radiation qualities (photon energies) corresponding to different ratios of the TLi values. To analyse this a linear deviation factor was defined in Eq. (1). This deviation factor epresents the ratio between the only calculated dose value of a mixed radiation field and the sum of the single dose values of the corresponding individual irradiations. Both values should be equal for an ideal dose algorithm resulting in = 1. Dalgorithm ( i TLw , i TLf ) = i Dalgorithm (TLw , TLf ) ( = 1 for an ideal algorithm). (1) For algorithm #3 and #4 the value of is always 1, corresponding to a linear behaviour. Algorithm #1 and #2, however, are not linear (due to the application of ratios). To analyse this, the value of was simulated for a set of pairs and triples of randomly selected radiation qualities (the ratio between the readings of the two detector elements was randomly selected between 0 and 1). The resulting frequency distribution of is given in Fig. 5. Although a randomly selected radiation quality as described above does not reflect a typical work place field, the extrema of the distribution of ; however, show the possible deviation from 1 and are relevant for the estimation of the maximum mathematical error caused by the introduction of nonlinear algorithms. For both algorithms the most frequent deviation factor is around 1. A detailed analysis shows that the extreme values for algorithm #1 (< 0.95 and > 1.20) only occur for simulated
frequency
0.6
All dose values Hx > 1 mSv
0.1 0.01 0.001
0.0001 0
0.2
0.4
0.6
0.8
1
1.2
Hp (10) / Hx Fig. 8. Frequency distributions for two data sets: (a) Ratio TLf /TLw and (b) calculated doses Hp (10)/Hx .
radiation combinations where one of the element ratios RTL = TLf /TLw is near 0.7 (the value where algorithm #1 shows the discontinuity in Fig. 2). The frequency for extreme deviations; however, is quite low for both algorithms. Dose values calculated by different algorithms are compared in Fig. 6. There the ratios of these doses corresponding to different quantities are given. These dose ratios partly reflect the dose conversion coefficients given in ICRU Report 57 (1998). The pronounced backscatter component; however (maximum of the conversion coefficient for energies around 70 keV) is not visible due to the fact that the backscatter component is measured with the free in air calibrated Hx dosimeter as well. This effect, however, depends on the backscatter response of the tested dosemeter. 4.2. Measured dose values In the second step a huge number of real individual dose values evaluated from 1997 to 2006 were recalculated for all four algorithms and compared by statistical methods. A diagram of a number of single doses measurements with respect to the two detector readings (TLi ) is given in Fig. 7. Each single measuring point represents different calculated dose values depending which dose algorithm (and dose quantity) is applied. The relation between measured TLi -readings (x/y axis) and the resulting calculated doses D is marked in this figure as an example for D = 1 mSv. Linear (#3, #4) and nonlinear (#1, #2) algorithms can be identified very easily by the shape of the line. For the intersections of the marked functions (lines) the calculated dose values are identical for the respective algorithms.
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The frequency distribution of the ratio RTL of the two detector readings (TLi ) summarised for two different sets of data is given in Fig. 8a. The unrestricted dose values (“all dose values”) including a high fraction of low background doses and a subset of doses exceeding 1 mSv are analysed and displayed in Fig. 8. For the latter the contribution of low photon energies (RTL < 0.7, ⇒ Eph < 45 keV) is pronounced about 20% of all values. This result is relevant for the assessment of typical work place fields of monitored people. The arithmetic mean of RTL for all dose values is 0.96. A similar shape shows the ratio RH of Hp (10) and Hx in Fig. 8b. The arithmetic mean of RH for all dose values is 0.94. For higher doses (> 1 mSv) only 7% of all Hx doses exceed the Hp (10) value by a factor of 1.5 (RH < 0.67). This result is relevant to estimate the shift of the dose distribution when changing to the new dose quantity Hp (10). 5. Discussion and conclusion This work proved that the described two element dosemeter can be evaluated in terms of three different dose quantities applying different algorithms with a satisfactory energy response. This feature allows to analyse the effect of changing the dose quantity from photon dose equivalent Hx to personal dose equivalent Hp (10) and Hp (0.07). The introduction of Hp (10), e.g. reduces the dose values in average by about 6%. For lower photon energies this effect is more pronounced as expected when considering the conversion coefficient from ICRU Report 57 (1998). The pronounced over response of Hp (10) for
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energies around 70 keV; however, does not cause measured Hp (10) values in this energy range to be increased over Hx as explained. Although the relation between the described dose quantities is well defined the relation between measured dose values depends also on the dosemeter design especially on the response for backscatter radiation (from the person or phantom). That is why only measurements or detailed simulations considering the dosemeter design and the calibration procedure allow a detailed analysis of the effect of the change of the dose quantity on the dose distribution of the measured values. This paper does not include information from the angular response of the dosemeter. This, however, is foreseen to be carried out in a further step. References Fantuzzi, E., et al., 2001. Present status of approval procedures in the EU member states and Switzerland. Radiat. Prot. Dosimetry. 96, 73–80. ICRU. Report 57, 1998. Conversion coefficients for use in radiological protection against external radiation. ISO Report 4037-1, 1996. X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—part 1: radiation characteristics and production methods. ISO Report 4037-3, 1999. X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—part 3: calibration area and personal cosemeters as a function of energy and angle of incidence. Moscovitch, M., 1993. Dose algorithms for personal thermoluminescence dosimetry. Radiat. Prot. Dosimetry 47, 373–380. ÖNORM 5237-1, 1998. Strahlenschutzdosimeter—Personendosimeter zur Messung der Personendosis Hp (10) bzw. Hp (0.07).