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 . ------- 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, ------- 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. ' ------- 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. ------- 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. ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- |