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

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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-

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T
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	 	 _— 	 —
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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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