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Estimation of TLD dose measurement uncertainties and thresholds at the Radiation
Protection Service
Du Toit Volschenk
SABS
Purpose
To give an overview of some of the factors that cause uncertainties in TLD dose measurement
To quickly describe how lower limits of detection and measurement uncertainties can be determined conventionally and to give the results of such measurements at the RPS, in order to give an impression of the status of the RPS TLD system in this regard
To describe a method of computer simulation to estimate uncertainties in reported dose due to the dose algorithm used at the RPS
Topics of Discussion
General considerations for uncertainty estimation of a TLD system
Lower limits of detection Uncertainty budget Computer simulation to estimate uncertainties due to the
dose algorithm used
General considerations for uncertainty estimation of a TLD system
Calibrations of dosimetry systems and tests to determine uncertainties are done under laboratory conditions
Dosimeters are used under field conditions, the specifics of which is not often known (e.g. angle of incidence, geometry of irradiation, movement of wearer, position of dosimeter on body)
The method(s) used to determine uncertainties in reported dose must take as many of the unknown factors into consideration as possible
Uncertainties in reported dose differ from radiation type to radiation type as well as for magnitude of given dose
Lower limits of detection: Definitions
Detection threshold LC (also known as: decision limit, critical level)
– That dose level above which measured doses can be regarded (in this case 95% confidence) as being not of the background population (i.e. definitely a dose)
– Lc = tnsb
tn is the student t factor (one-sided) for n measurements and the required confidence level
sb is the standard deviation of the background population
Lower limits of detection: Definitions (Continued)
Lower limit of detection LD (also known as detection limit)
– The lowest dose that can be detected with a specified level of confidence (in this case 95% confidence)
– Does not specify the precision of the minimum dose, only the probability that it will be reported greater as zero
– LD = 2(tnsb + tm22sm
2Db) / (1 - tm2sm
2)
tn is the student t factor (one sided) for n measurements of the background
sn is the standard deviation of the background population
tm is the student t factor (one-sided) for m measurements at a dose level significantly higher than the required lower limit of detection
sm is the relative standard deviation at the above dose level
Db is the average of the background readings
Lower limits of detection: Definitions (Continued)
Determination limit LQ
– The lowest dose that can be measured with a given precision
k is the inverse of the desired maximum relative standard deviation (and therefore related to the precision of the measurement)
sm is the relative standard deviation of the measured doses at a level significantly higher than the required lower limit of detection
Db is the average of the background readings
sb is the standard deviation of the background population
Confidence level = 1,96 / k
LQ
k s D k s D k s k s
k s
m b m b b m
m
2 2 4 4 2 2 2 2 2
2 2
1
1
12[ ( )]
Lower limits of detection and determination levels(Determination levels for +/- 30% uncertainty at 95% confidence)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
LI 0.029
MF
G 0.051
Am
0.06
MF
I 0.07
Cs 0.662
Co 1.25
Tl 0.24
Sr 0.57
Radiation type and energy (MeV)
LD
an
d L
Q (
mS
v)
Ld (Deep)
Ld (Skin)
LQ (Deep)
LQ (Skin)
Ld (Deep) corresponds to a precision of on average +/- 56% at the 95% confidence level
Ld (Skin) corresponds to a precision of on average +/- 78% at the 95% confidence level
Lc (Deep)
Lc (Skin)
Uncertainty budget: General
The basic question here is “If a dose is reported, what is the 95% confidence interval for that dose?”
The confidence interval for reported dose should be calculated for each type (energy) of radiation, as well as for dose magnitude ranges
An upper (maximum) acceptable limit should be established based on internationally acceptable criteria
Uncertainty budget: Criteria
Interpretation of ICRP35 and ICRP60 requirements (based on limits of 20 mSv/a and 500 mSv/a for body and skin dose respectively)
Dose range(mSv)
Deep Skin
Uncertaintylevel (95%confidence)
Lowerbound
Upperbound
Bound on S
> 4 > 100 1,5 - 33% + 50% |S| < 0,212
4 100 2 - 50% + 100% |S| < 0,383
Uncertainty budget: calculation and results
Examples of random uncertainties:
inhomogeneity of detectors, reader sensitivity, reader background, etc. Examples of systematic uncertainties:
energy dependence, directional dependence, non-linearity, fading, light-exposure, calibration errors, etc.
“Rough” results (to be used as an indication only of the system as a whole):
– SDeep = 0,21
– SSkin = 0,27
S r s
s
2 2 ,where
is the standard deviation of random uncertainties, and
is the standard deviation of systematic uncertaintiesr
Computer simulation: uncertainties due to dose algorithm - Background
Question:
– Why look at uncertainties due to the dose algorithm separately and not do an uncertainty budget study only?
Some of the answers:
– The dose algorithm uses the ratios of the four elements of a dosimeter to determine the energy of incident irradiation and then do a conversion to dose.
– Small differences in the element values of different dosimeters may lead to different incident irradiation types being identified even though the dosimeters were exposed to the same type and magnitude of dose.
– Some parts of the dose algorithm may be prone to causing larger uncertainties than others and they must be identified
– In short: to design an accurate algorithm and to verify that existing algorithms are accurate and do minimize errors due to misidentification
Computer simulation: uncertainties due to dose algorithm - Background (continued)
Question: Why a computer simulation? Answers:
– To process many dosimeter readings manually through the dose algorithm will be very time consuming and error prone
– Many dosimeters will have to be irradiated to test the various branches of the algorithm with an acceptable degree of accuracy, which will be time consuming and costly
– To analytically determine the uncertainties inherent to the algorithm will be difficult because of the many branches in the algorithm and their interdependencies
– New and changed algorithms can be tested relatively painlessly by means of a computer simulation and problem areas so identified
Photons
C > 1.5
C < 21
D > 0.98
E >= 0.40and
C > 7.0
C > 10and
A > 13
C > 5and
A > 10
D > 1.55
GammaItype = 11Hs = E4Hd = E4
LGItype = 13Hs = 2.01*E2Hd = 0.87*E2
LG = GammaItype = 16XG = -0.3873+0.08195*C+0.002234*C^2FG = 1/(1+XG)FX = 1 - FGHs = 1.85*FX*E2 + FG*E2Hd = 0.80*FX*E2 + FG*E2
A > 13.5
LI + GammaItype = 15XG = -2.487 + 0.7897*C - 0.02831*C^2FG = 1/(1 + XG)FX = 1 - FGHs = 1.52*FX*E2 + FG*E2Hd = 1.10*FX*E2 + FG*E2
LIItype = 14Hs = 1.62*E2Hd = 1.13*E2
MFGItype = 18Hs = 1.43*E2Hd = 1.32*E2
LKItype = 17Hs = 1.37*E2Hd = 1.14*E2
MFIItype = 19Hs = 1.39*E2Hd = 1.34*E2
MIDItype = 22Hs = 1.18*E2Hd = 1.11*E2
LG 20 keVLI 29 keVLK 39 keVMFC 36 keVMFG 51 keVMFI 70 keVMID ~120 keV
FUNEL
FUNEL
FUNEL
FUNEL
A = E3/E2B = E1/E2C = E3/E4D = E4/E2E = E1/E4F = (E2-E4)/(E1-E4)
PanasonicAlgorithm1994-08-19Page 3 of 4
Yes
Yes
Yes
YesYes
Yes
Yes
Yes
E1 or E2 Ei = (Ei/ECFi - BgLi - Resbg1)/BCF*LiFF - BAYes
Ei = (Ei/ECFi - BgCa - Resbg2)/BCF*CaFF - BA
No
For i=1 to 4Ei=Ei+BA/4Tag = 1
Any Ei<-6Any Ei<-6For i=1 to 4Ei=Ei+BA/4Tag = 2
Any E<-6 Any Ei>=50Itype = 30Tag = 4
Tag = 3i = 1
i<=4 Ei<=0
i = i + 1
Ei = 1
All Ei <= ETR Itype = 30
E1 < 30Do checks and ifpositive, flagdosimeter with ** onEdit List (Possiblydamaged)
E2 > 10^5 orE3 > 10^7 orE4 > 10^7
Hd = E2Hs = E2Itype = 26Flag dosimeter with <= onEdit List. Requires manualintervention
Yes Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
No
No
No
No
Yes
No
No No
No
Next Page
Stop
Stop
Stop
Stop
PanasonicAlgorithm1994-08-19Page 1 of 4
FUNEL
Tot = E1 + E2 + E3 + E4Emax = Max( E1,E2,E3,E4 )
E2 > Max(E1,E3,E4)
E4 > Max(E1,E2,E3)
E1 < Min(E2,E3,E4)
E3 < Min(E1,E2,E4)
E1/Tot < 0.03
E2/Tot < 0.015
Itype = 9Hs = EmaxHd = Emax
T1 = E2 / Max(E1,E3,E4)
T1 = E4 / Max(E1,E2,E3)
T1 = E1 / Min(E2,E3,E4)
T1 = E3 / Min(E1,E2,E4)
T1 > 1.3 + 5/E2
T1 > 1.2 + 5/E4
T1 < 0.6 - 5/E1
T1 < 0.8 - 5/E3
Stop
Stop
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
PanasonicAlgorithm1994-08-19Page 4 of 4(Hs>=20) or (Hd>=20) Stop
Yes
Flag dosimeter with<= on Edit List.Requires manualintervention
A = E3/E2B = E1/E2C = E3/E4D = E4/E2E = E1/E4F = (E2-E4)/(E1-E4)
B>8 or ( A<15 and E>=1.5 )
Photons
Yes
Gamma = E4
F>NPand
( E2 - E4 ) > 25
Gamma + Betaor
Gamma + Neutron
No
Beta + Gamma
Neutron + GammaBFs = 2.92 - 2.96*FBFs = Min(Max(0.95 ,BFs),2.92)
BFe = 0.0
F > 0.25
BFe = 0.12
BETAs = BFs*( E1 - E4 )BETAe = BFe*( E1 - E4 )Itype = 12Hs = BETAs + GammaHd = Gamma
NF = 4.86Neutron = NF*( E2 - E4 )Itype = 28Hs = E4Hd = E4
FUNELFUNEL
Yes
YesNo
No
PanasonicAlgorithm1994-08-19Page 2 of 4
Do checks and if positive,flag dosimeter with <= onEdit List. Requires manualintervention
Computer simulation: uncertainties due to dose algorithm - Background (continued)
An example of part of an algorithmPhotons
C > 1.5
C < 21
D > 0.98
E >= 0.40and
C > 7.0
C > 10and
A > 13
C > 5and
A > 10
D > 1.55
GammaItype = 11Hs = E4Hd = E4
LGItype = 13Hs = 2.01*E2Hd = 0.87*E2
LG = GammaItype = 16XG = -0.3873+0.08195*C+0.002234*C^2FG = 1/(1+XG)FX = 1 - FGHs = 1.85*FX*E2 + FG*E2Hd = 0.80*FX*E2 + FG*E2
A > 13.5
LI + GammaItype = 15XG = -2.487 + 0.7897*C - 0.02831*C^2FG = 1/(1 + XG)FX = 1 - FGHs = 1.52*FX*E2 + FG*E2Hd = 1.10*FX*E2 + FG*E2
LIItype = 14Hs = 1.62*E2Hd = 1.13*E2
MFGItype = 18Hs = 1.43*E2Hd = 1.32*E2
LKItype = 17Hs = 1.37*E2Hd = 1.14*E2
MFIItype = 19Hs = 1.39*E2Hd = 1.34*E2
MIDItype = 22Hs = 1.18*E2Hd = 1.11*E2
LG 20 keVLI 29 keVLK 39 keVMFC 36 keVMFG 51 keVMFI 70 keVMID ~120 keV
FUNEL
FUNEL
FUNEL
FUNEL
A = E3/E2B = E1/E2C = E3/E4D = E4/E2E = E1/E4F = (E2-E4)/(E1-E4)
PanasonicAlgorithm1994-08-19Page 3 of 4
Yes
Yes
Yes
YesYes
Yes
Yes
Yes
Computer simulation: uncertainties due to dose algorithm - Specifics
Purpose:– To estimate the uncertainties in reported dose of the RPS TLD system for
doses above the determination limits and for various types of radiation Method:
– Irradiate a small group of dosimeters (16 or less) to a certain dose and type of radiation
– Use the element responses of the group and a student t data generator to generate a very large set of test data for input into the dose algorithm (E i = f(t)s + xMean)
– Generate a frequency distribution of the dose values output of the algorithm– Generate frequency distributions of the radiation types identified by the
algorithm– Calculate confidence intervals (indicating the uncertainties) for the deep and
skin dose
Computer simulation: uncertainties due to dose algorithm - Results (continued)
Deep DoseSimulated data (outer limits)
Measured data from small groups (inner limits)
-50
-40
-30
-20
-10
0
10
20
30
40
50
LG
80
0.0
2
LI1
50
0.0
29
LI1
00
0 0
.02
9
MF
G2
40
0.0
51
Am
30
0 0
.06
Am
10
00
0.0
6
MF
I28
0 0
.07
MF
I10
00
0.0
7
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs3
00
0.6
62
Cs4
00
0.6
62
Cs5
00
0.6
62
Cs1
00
0 0
.66
2
Cs4
00
0 0
.66
2
Co
20
0 1
.25
Tl1
00
0.2
4
Tl2
00
0.2
4
Tl1
00
0 0
.24
Sr1
00
0.5
7
Sr1
00
0.5
7
Sr2
00
0.5
7
Sr1
00
0 0
.57
Radiation Type and Energy (MeV)
95
% C
on
fid
en
ce
in
terv
al (%
)
Computer simulation: uncertainties due to dose algorithm - Results (Continued)
Confidence intervals for deep dose
0.0
5.0
10.0
15.0
20.0
25.0
LG
80
0.0
2
LI1
50
0.0
29
LI1
00
0 0
.02
9
MF
G2
40
0.0
51
Am
30
0 0
.06
Am
10
00
0.0
6
MF
I28
0 0
.07
MF
I10
00
0.0
7
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs3
00
0.6
62
Cs4
00
0.6
62
Cs5
00
0.6
62
Cs1
00
0 0
.66
2
Cs4
00
0 0
.66
2
Co
20
0 1
.25
Tl1
00
0.2
4
Tl2
00
0.2
4
Tl1
00
0 0
.24
Sr1
00
0.5
7
Sr1
00
0.5
7
Sr2
00
0.5
7
Sr1
00
0 0
.57
Radiation type and energy (MeV)
Siz
e o
f c
on
fid
en
ce
in
terv
al (%
)
Small groups
Simulation
Computer simulation: uncertainties due to dose algorithm - Results (Continued)
Skin DoseSimulated data (outer limits)
Measured data from small groups (inner limits)
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
140
LG
80
0.0
2
LI1
50
0.0
29
LI1
00
0 0
.02
9
MF
G2
40
0.0
51
Am
30
0 0
.06
Am
10
00
0.0
6
MF
I28
0 0
.07
MF
I10
00
0.0
7
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs3
00
0.6
62
Cs4
00
0.6
62
Cs5
00
0.6
62
Cs1
00
0 0
.66
2
Cs4
00
0 0
.66
2
Co
20
0 1
.25
Tl1
00
0.2
4
Tl2
00
0.2
4
Tl1
00
0 0
.24
Sr1
00
0.5
7
Sr1
00
0.5
7
Sr2
00
0.5
7
Sr1
00
0 0
.57
Radiation Type and Energy (MeV)
95
% C
on
fid
en
ce
in
terv
al (%
)
Computer simulation: uncertainties due to dose algorithm - Results (Continued)
Confidence intervals for skin dose
0
10
20
30
40
50
60
70
80
90
LG
80
0.0
2
LI1
50
0.0
29
LI1
00
0 0
.02
9
MF
G2
40
0.0
51
Am
30
0 0
.06
Am
10
00
0.0
6
MF
I28
0 0
.07
MF
I10
00
0.0
7
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs2
00
0.6
62
Cs3
00
0.6
62
Cs4
00
0.6
62
Cs5
00
0.6
62
Cs1
00
0 0
.66
2
Cs4
00
0 0
.66
2
Co
20
0 1
.25
Tl1
00
0.2
4
Tl2
00
0.2
4
Tl1
00
0 0
.24
Sr1
00
0.5
7
Sr1
00
0.5
7
Sr2
00
0.5
7
Sr1
00
0 0
.57
Radiation type and energy (MeV)
Siz
e o
f c
on
fid
en
ce
in
terv
al (%
)
Small groups
Simulation
Computer simulation: uncertainties due to dose algorithm - Results (Final)
The sizes of the 95% confidence intervals calculated from the measured data of the small groups are in some cases unrealistically small
For deep dose the 95% confidence intervals calculated from the simulated data are all smaller than 25%
For skin dose the 95% confidence intervals for photon radiation are all less than 30%, but there is definitely a problem for beta radiation (In part due to mis-identification of the radiation type but primarily because of the high spread in response values of the small groups.)
Computer simulation: uncertainties due to dose algorithm - Conclusions
The computer simulation yields results comparable with the “rough” results of the uncertainty budget, although it must be kept in mind that no input data for different angles of incidence were used
The simulation method can be used effectively to test existing dose algorithms, and to design new algorithms and changes to existing algorithms
The simulation method can be used to identify problem areas in a dose algorithm
The uncertainty in reported dose of a TLD system can be estimated relatively painlessly and quickly from small suitable input data sets by effectively treating the TLD system as a “black box” into which data are fed and which delivers calculated doses and their associated confidence intervals
The simulation method can be used without extensive knowledge of and experience with statistical data analysis