IT.R -10
-SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY
INTRALABORATORY TECHNICAL REPORT
July 28, 1966
EVALUATION OF THE E.G. & G. THERMOLUMINESCENT
DOSIMETRY SYSTEM
by
William Horn
A. Introduction
The E.G. & G. thermoluminescent dosimeter system was put in operation
at SWRHL during August 1965 and during the first four months of use
it became apparent that a great amount of experimental and evaluation
work would be required in order for us to become thoroughly familiar
with "he system and learn its characteristics.
Also during this time several minor malfunctions occurred with the
reader which were rectified by E. G. & G. at the Santa Barbara plant.
Numerous consultations were required with E. G. & G. engineers, and
it was resolved that the following main objectives had to be accomplished:
1. Perfect a system of reader calibration.
2. Establish a correction factor for each dosimeter.
3. Determine the internal background which is due to K-40 activity
within the dosimeter.
4. Determine the rate at which a known exposure will fade during
a given time.
5. Determine standard deviation from mean value at 67% and 90%
confidence level .
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B. description of Equipment
!. CaF2:Mn Thermolurninescent Dosimeter, Model TL-12
lj. G. & G. 's Model TL-12 Thermolurninescent Dosimeter consists
i'f two component parts - a detector (Model TL-32) and an energy
Compensating shield (Model TL-52).
'The detector consists of a layer of thermoluminescent CaF2:Mn
bonded to a helical heater element that is contained within an evac-
uated glass envelope. The aluminum-lead-tin shield that houses
the detector is designed to compensate for the detector over-response
in the low energy region of the gamma ray spectra.
Operation of the dosimeter is based on the thermoluminescent
properties of manganese-activated calcium fluoride. When the
CaF2 :Mn is exposed to ionizing radiation, valence electrons are
raised to higher metastable energy levels. This "stored" energy
is released in the form of light when the CaF2:Mn is heated by an
E.G. &G. instrument. The released energy, proportional to the
cumulative dose received, is converted into an electrical signal
and recorded by the strip chart recorder in the reader. After
readout the dosimeter may be reused.
2. Thermoluminescent Dosimeter Reader, Model TL-2B
The Model TL-2B is an integrated system designed to read out
E.G. &G. CaF2:Mn thermoluminescent dosimeters (TLD's) in
both standard and micro sizes. The reader accepts the TLD in
a light tight chamber, heats it with a regulated current, and converts
the emitted light energy into an electrical signal for display on the
built-in strip chart recorder. Two "read head" adapters are pro-
!
vided for inserting the different size dosimeters into the reader.
One adapter (TL-81) accommodates the vacuum tube type detector
and the other (TL-81A) the needle microthermoluminescent dosimeters,
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The TL/-2B comprises a recording photometer with power supplies
and control logic needed to read out the dosimeters. The control
iogic sequences the recorder operation, dosimeter heating cycle,
automatic ranging circuit, and status indicator to effect the chart
record of the dosimeter's light emission. The automatic ranging
circuit operates when the recorder pen reaches full scale and
changes reader sensitivity in decade steps over a total of six decades.
C. Procedures
The reader calibration technique was worked out and is described in
detail in the "TLD Evaluation and Reading Procedures", dated January 14,
1966,
In order to assure accurate exposures, a special calibration table on
which the dosimeters may be placed either at 50cm or 100cm from a
rotating source was fabricated.
Correction factors for the dosimeters -were established by exposing
each of the 400 dosimeters three times to doses verified by R -meter
readings as 400 mR + 1%. The three readouts were then averaged to
obtain the final correction factor.
The internal background build-up was determined by placing groups
of de-dosed dosimeters in locations of known background and also
placing them in a shield for varying time periods. The known environ-
mental background was then subtracted from the gross readout, the
remainder represented the internal background.
The fading rate was determined by exposing 36 groups of 11 dosimeters .
to known doses of 400 mR -f 1%, then storing each group together with
a group of 5 de-dosed dosimeters for time periods ranging from one day
to thirty days. '
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Eac > group was read out at the respective time period and the correc-
tion factor for each dosimeter was applied to the chart readout, resulting
in a corrected gross readout. Then the average total background build-
up f :'om the companion group of de-dosed dosimeters was subtracted
iron:: the corrected gross readout, resulting in a net readout. The dif-
fereiice between this net readout and the original exposure represented
the loss due to fading and was calculated in percent. Standard deviation
was calculated and is presented in Tables 1 and 2.
D. Results
Correction factors (multiplication) for the dosimeters vary from 0. 90
to i. 26.
.interaal background build-up was established to be a constant factor at
the rite of 0.7 mR -f 10%/day.
During this pahse of the work it became obvious that our original attempts
of trying to establish a factor for the ratio of the environmental back-
ground to the iiiternal background were misleading because the internal
background is a constant factor, while the environmental background
is variable. For instance, under conditions of very low environmental
background in a shield, the internal background is in the order of 6
times the environmental background, but with a normal outside environ-
mental background of 0.4 mR/day, the internal background is only 1.75
times the environmental background. Therefore, in order to measure
the actual environmental background and/or any exposures above back-
ground, the internal background at the rate of 0.7 mR/day must be
subtracted from the corrected gross readout figure.
The average fading rate has been determined as 5.4% up to 30 days.
Most of this fading seems to occur within the first five or six days,
leveling off thereafter.
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Avei age standard deviation over the 30 day period is 2. 6% at the 90%
confidence level.
E. Conclusions
The system, while it did require a great deal of time to learn its charac-
teristics, is very accurate and useful for our applications.
As anticipated at the beginning, this system will enable us to determine
environmental background and/or exposures above background, which
in most cases can not be done with film badges.
After extensive use during this six months period, it may be stated
that the reader is a reliable instrument and certainly does not require
any more care or maintenance than any other comparable electronic
instrument.
This report should not be considered final because additional data must
be obtained under different conditions.
All of the work to date was done using exposures of 400 mR, and must
be duplicated at other exposures, especially at lower ranges, in order
to determine if the present data -will remain valid for different exposures.
Also, during the summer season it must be determined if high environ-
mental temperatures will have an effect on fading. It is not anticipated
that this will be the case, but the possibility must be investigated.
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Table 1. TLD standard deviation.
Reading Group Storage
Date Number
First Experiment
1/25/66 1
1/28/66 2
2/01/66 3
2/21/66 4
Second Experiment
2/02/66 1
2/04/66 2
2/05/66 3
2/06/oc 4
2/07/66 5
2/08/66 6
2/10/66 7
2/11/66 8
2/13/66 9
2/15/66 10
2/18/66 11
2/21/66 12
3/02/66 13
Third Experiment
2/09/66 1
2/10/66 2
2/11/66 3
2/13/66 4
2/15/66 5
Fourth Experiment
3/02/66 1
3/03/66 2
3/04/66 3
3/05/66 4
3/06/66 5
3/07/66 6
(Days)
5
8
12
31
1
3
4
5
6
7
9
10
12
14
17
20
30
1
2
3
5
7
1
2
3
4
5
6
Gr os s
Me&n
Value
(mR)
389
395
406
410
396
387
386
375
391
389
401
402
394
393
399
404
398
i
392
375
385
384
391
391
388 '
386
385
393
396
67%
Confidence
Level
Deviation %
(mR)
4. 37
4. 52
5. 10
6.69
7.19
4. 70
4. 36
6.40
3.94
4. 52
6.36
5. 53
6.69
7.26
5.78
4. 29
7.45
4.80
8. 34
3.45
4.72
6.13
10.06
11. 39
8.93
13.75
7.02
20. 84
of M. V.
1. 12
1. 14
1.25
1.63
1.79
1. 21
1. 12
1.70
1.00
1.16
1. 58
1. 37
1.69
1.84
1.45
1.06
1.87
1. 22
2. 22
0.89
1. 22
1. 56
2. 57
2.93
2.31
3.56
1.78
5.26
90%
Confidence
Level
Deviation %
(mR) .
7.21
7.46
8.40
11.04
11.86
'7.75
7. 19
10. 55
6. 50
7.46
10.49
9.13
11.04
11.98
9. 54
7. 08
12. 29
7.92
13.76
5.69
7. 79
10. 12
16.61
18.79
14.74
22.68
11. 58
34. 39
of M. V..
1.85
1.88
2. 10
2.69
2.96
2.00
1.86
2.81
1.66
1.92
2.61
2. 27
2.80
3.04
2. 39
1.75
3.08
2.01
3.66
1,47
2.02
2. 58
4. 24
4.83
3.81
5.88
2.94
8.69
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Table 1. TLD standard deviation;. (Continued)
^ ~ Gross
Reading Group Storage
Date Number (Days) _.',
Value
(mR)
67%
Confidence
Level
Deviation %ofM.V.
(mR)
90%
Confidence
Level
Deviation % of'..M.V.
(mR)
Fourth Experiment
3/08/66
3/09/66
3/10/66
3/11/66
3/14/66
3/16/66
3/21/66
3/31/66
7
8
9
10
11
12
13
14
(Cont)
7
8
9
10
13
15
20
30
396
388
388
385
384
384
391
400
5.67
4.26
7.19
5. 57
4.88
5.25
4. 59
7.56
1.43
1.09
1.85
1.44
1.27
1.36
1. 17
1.88
9.35
7.03
11.87
9. 20
8.06
8.66
7. 57
12.47
2.36
1.81
3.05
2.38
2.09
2.25
1.93
3. 11
Fifth Experiment
3/28/66
3/29/66
3/30/66
1
2
3
5
6
7
383
395
389
9.66
10.73
11.73
2. 51
2.71
3.01
15.94
17.70
19. 36
4. 15
4. 48
4.97
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Table 2. Combined data from TLD evaluation.
DAYS IN STORAGE
FIRST EXPERIMENT 406mR
Corrected Gross Readout
Background
Net Read ing
Percent Fade
Std.Dev.at 907, C.L.
SECOND EXPERIMENT 400mR
Corrected Gross Readout
Background
Net Reading
Percent Fade
Std.Dev.at 907, C.L.
THIRD 'EXPERIMENT 402mR
Corrected Gross Readout
Background
Net Reading
Percen t Fade
Std.Dev.at 90% C.L. ,
FOURTH EXPERIMENT 404mR
Corrected Gross Readout
Background
Net Reading
Percent Fade
Std.Dev.at 907, C.L.
\
FIFTH. EXPERIMENT 402mR
Corrected Gross Readout
-Background
Net Readi.ng ", f
Percent Fade - -
Std.Dev.at 90% C.L.
Average Background/Day
Average Percent Fade
Average Std. Dev.
]
396.0
1 .3
394.7
•1 .3
3.0
392.1
1.1
391 .0
2.7
2.0
391 .4
0,9
390.5
3.3
4.2
1.1
2.4
3.1
2
375.2
2:i
373. 1
7.2
3.7
388.4
1.8
386.6
4.3
4.8
1.0
- 5.8
.4.3
3
392.0
3.0
389.0
3.2
1.7
387.0
4.0
383.0
4.25
2.0
384.5
3.0
381 .5
.5.1
1,5
385.8
2.8
383.0
5.2
3.8
1 . 1
4.6
2.2"
4
386.0
5.1
380.9
4.8
1.9
""*• ".
385.2
3.8
381.4
5.6
5.9
1 . 1
5.2
3.9
5
389.0
4.9
384.1
5.4
1 .9.
375.0
5.6
369.4
7.7
2.8
384.0
.4.8
379.2
5.7
2.0
392.8
4.2
388.6
3.8
2.9
383.4
4.4
379.0
5.7
4.2
0.95
5.7
.2.8
6
391.4
6.3
385.1
3.7
1.7
395.7
5.3
390.4
3.4
8.7
394.9
5.3
389.6
3.1
4.5
0.93
3.4
5.0
7
388.5
7.4
381.1
4.7
1 .9
391 . 1
7.3
383.8
4.5
2.6
395.5.
5.9
389.6
3.6
2.4
388.8
6.1
382.7
4.8
5.0
0.95
4.4
3.0
8
395.0
7.5
387.5
4.4
1.9
388.3
7.3
381.0
5.7
1.8
0.9
5.1
1.8
9
401.0
9.5
391.5
2.1
2.6
388.2
8.0
380.2
5.9
3.1
1.0
4.0
2.8
10
402 . 0
10.9
391.1
2.2
2.3
385.0
9.1
375.9
7.0
2.4
1.0
4.6 .
2.3
12
406.0
11.6
394.4
2.8
2.1
394.0
12.1
381 .9
4.5
2.8
\
1.0
3.7
2.5
13
.383.6
11.1
372.5
7.8
2.1
0.9
7.8
2.1
i
14
393.1
• 13.4
379.7
•5.1
3.0
1.0
5. 1
3,0
15
383.5
12.6
370.9
8.2
.2.3
• 0.8
8.2
2.3
17 ,
398.6
16.1
382.5
4.3
2.4
0.9
4 . 3
2.4
2.0
403.9
18.7
385.2
3.7
1.8
391.1
17.0
374. 1
7.4
1.9.
0.9
5.6
1.8
.30
398.0
27.9
370. 1
7.5
3.1
.400.4
25.9
374.5
7.3
3. 1
0.9
7,4
3.1.
.31
409.8
25.2
384.6
5.2
2.7
.~
0.8
5.2
2.7
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First Experiment - TLD Fading and Dispersion
QL
£
O INITIAL EXPOSURE
-. MEAN VALUE NET READ-OUT
9 FROM II DOSIMETERS.
A MEAN VALUE GROSS READ-OUT
FROM 5 DE-DOSED DOSIMETERS.
+—I—I—I—I—HH—I—I—I—I—I-
6
8 9 10 II
12
DAYS IN STORAGE
31
-------
Third Experiment - TLD Fading and Dispersion
O INITIAL EXPOSURE
* MEAN VALUE NET READ - OUT
w FROM II DOSIMETERS.
A MEAN VALUE GROSS READ - OUT
FROM 5 DE- DOSED DOSIMETERS.
1—I—I—I—I—I—I—I-
5
8 9 10
DAYS IN STORAGE
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Second Experiment - TLD Fading and Dispersion
30-.
20-•
10-.
0
O INITIAL EXPOSURE
A MEAN VALUE NET READ - OUT
v FROM II DOSIMETERS.
A MEAN VALUE GROSS READ - OUT
FROM 5 DE - DOSED DOSIMETERS.
I I I I I
10
12
14 17
DAYS IN STORAGE
20
30
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Fourth Experiment ~ TLD Fading and Dispersion
i—i—r—i—r
a:
30- •
20- •
io4-
o.
T t -t
f—i—i—r
T
T
r." !
II | II | II I I
I 23456 7 89 10
O INITIAL EXPOSURE
A MEAN VALUE NET READ - OUT
w FROM II DOSIMETERS.
A MEAN VALUE GROSS READ - OUT
A FROM 5 DE - DOSED DOSIMETERS.
I I—r^—-
13 (5
DAYS IN STORAGE
20
30
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Fifth Experiment - TLD Fading and Dispersion
410-
400*
390-
380-
370-
360-
350-
(T
20-
10-
0-
• • i i • i
•
^
. ^^^
T
i J
»
•
»
»
i i
'
s
L
•
k.
_— —
•
0 INITIAL EXPOSURE
* MEAN VALUE NET READ - OUT
w FROM II DOSIMETERS.
A MEAN VALUE GROSS READ - OUT
FROM 5 DE- DOSED DOSIMETERS.
-
-
-
-
-
:
-
m
1 23 4 5 6 7
DAYS IN STORAGE
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