|Year : 2011 | Volume
| Issue : 1 | Page : 23-25
Adequacy of annealing duration in reducing the background counts of personnel monitoring TLD cards: A study
Kshama Srivastava1, Geetha Varadharajan1, MP Punekar1, P Ayappan2, MP Chougaonkar1
1 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CTCRS Building, Anushaktinagar, Mumbai, India
2 Avanttec Laboratory Pvt. Ltd, Chennai, India
|Date of Web Publication||17-Mar-2012|
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CTCRS Building, Anushaktinagar, Mumbai
Source of Support: None, Conflict of Interest: None
A case study was undertaken to resolve the problem of high background counts of annealed TLD cards at one of the TLD Laboratory observed during handling the over exposure report (OER) received from various accredited laboratories for Non-DAE institutions. In order to verify the appropriateness of annealing procedures adopted by the Laboratory, a detailed study was conducted using four sets of cards exposed to various dose levels. It was found that inadequate annealing time than the standardized protocol duration of 4h at 230°C leads to larger standard deviation and enhanced background, which may slowly build up with repeated use and result in rejection of the dosimeter from field use. Hence it is recommended that annealing procedure of TLD cards must be strictly followed as per standardized protocol.
Keywords: Personnel monitoring, TLD, annealing, residual TL
|How to cite this article:|
Srivastava K, Varadharajan G, Punekar M P, Ayappan P, Chougaonkar M P. Adequacy of annealing duration in reducing the background counts of personnel monitoring TLD cards: A study. Radiat Prot Environ 2011;34:23-5
|How to cite this URL:|
Srivastava K, Varadharajan G, Punekar M P, Ayappan P, Chougaonkar M P. Adequacy of annealing duration in reducing the background counts of personnel monitoring TLD cards: A study. Radiat Prot Environ [serial online] 2011 [cited 2019 Dec 10];34:23-5. Available from: http://www.rpe.org.in/text.asp?2011/34/1/23/93899
| 1. Introduction|| |
In India, presently personnel monitoring service is conducted using indigenously developed TLD system based on CaSO4:Dy embedded Teflon discs (Vohra et al., 1980). The dosimeter card consists of three TLD discs mechanically clipped in nickel plated aluminum card and put under different filter regions. Readout of the TLD cards is taken using semiautomatic TLD Badge Reader and during the first readout about 90% of the total TL is emitted out from the traps corresponding to dosimetric peak of 220°C. The remaining 10% can be read by the second readout, if and when needed. In order to remove residual TL from the deep traps and also to reset the distribution of defects/ trapping centers perturbed by the radiation, all cards are subjected to an annealing treatment at elevated temperature prior to their next use.
As per the standardized protocol, annealing is carried out in a hot air circulating oven at 230°C for 4 h (Pradhan et al., 2002). The annealing temperature at 230°C is found to be sufficient to remove the residual TL upto ~100 mSv of gamma dose and brings down the background counts to normal range of 40-100 μSv. However, for new cards, the background counts lie within range of 15-50 μSv for freshly annealed cards. Higher annealing temperature (>250°C) could lead to dislodging of disc from the clippings due to different heating characteristics of aluminium plate and Teflon (Lakshmanan et al., 1979). Also, the duration of 4 h is found to be adequate and any further increase in the annealing time does not result in significant reduction in the background counts. Even though the standardized annealing protocol at 230°C-4h, can reset the TL dosimeters for doses up to ~100 mSv, all the cards which receive dose ≥10 mSv (identified as OER case) are removed from field use as a precaution.
While handling of over exposure case reports (OER) received from different accredited laboratories for Non-DAE institutions (Varadharajan et al., 2008), it was observed that for one of the TLD laboratory, the background counts of OER cards were relatively higher by a factor of 2-3, in comparison to other laboratory data for similar dose levels. This difference in background was more significant for cases involving low energy X-ray dose ≥10 mSv, (in such cases disc 2 and 3 will receive TL output equivalent to 100 mSv or more, depending on energy). This high background was also evident in the glow curves of OER cards after test exposure of 2 mSv, which was attributed as residual TL by the laboratory. In order to verify the appropriateness of annealing procedures adopted by the laboratory, a detailed study was conducted.
| 2. Experimental|| |
As an initial measure, the laboratory was advised to check the calibration of annealing oven and provide the details of annealing procedure. The laboratory confirmed that oven was calibrated periodically at different temperatures within accuracy of 2°C and intimated that TLD cards are annealed at 230°C for 3 h. The annealing time was found to be less compared to the 4 h duration, generally followed by other laboratories. The effect of the change in annealing time was studied using three sets of cards namely A, B and C series, sent by the laboratory to BARC for this purpose.
The A-series cards had previously recorded overexposures at the laboratory in various dose ranges, whereas B and C series cards were exposed to very high doses in range of (190-1430 mSv) and read at the lab and at BARC, respectively. For A and B series cards, the lab provided the TL readings of the first and second readout, background readings and TL reading after exposing them to test dose of 2 mSv. Readout of these cards taken at BARC under three conditions: a) without any further annealing, b) after annealing at 230°C-3h and c) after re- annealing at 230°C-4h. A comparison was made for background readings of A, B & C series cards taken at BARC & TLD laboratory after two annealing durations.
The effect of change in annealing duration was also studied for the lower dose levels normally encountered in personnel monitoring. For this two groups of D-Series cards were exposed in the dose range 0.5 mSv-20 mSv and annealed at 230°C for 4h (as per the standard procedure) and at 230°C-3h (procedure followed at the TLD Laboratory), respectively. For each dose level 10 TLD cards were used and the TL readout of all cards were done on N 2 gas based semiautomatic TLD Badge Reader. The exercise was repeated for five cycles to confirm the reproducibility of result.
| 3. Results and Analysis|| |
[Table 1] gives the comparison of background readings of A- series cards (under open disc D3) taken at TLD Laboratory after annealing at 230°C-3h, and subsequently at BARC following annealing treatments at 230°C for 4h duration (i.e. total annealing time 7h). It shows that for all cards, background counts reduces further after subsequent 4h annealing and the reduction was significant for doses >100 mSv.
|Table 1: Background readings for A - series cards taken at TLD Laboratory and BARC|
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[Table 2] gives the background readings for B and C series cards exposed to very high dose levels. The B-series cards were read at TLD Laboratory after annealing at 230 o C-3h and C series cards at BARC after annealing as per standardized protocol at 230 o C-4 h. It shows that the background counts at dose levels >100 mSv, after annealing at 230 o C-3h, are less by factor of 0.16 to 0.34 compared to those after annealing at 230 o C-4h. From result of [Table 1] and [Table 2], it is evident that although the annealing treatment at 230 o C for 4h helps in reducing the background in the exposed (for X-ray doses>14.15 mSv and gamma ray doses in the range 191-01432 mSv) and but does not reduce the background counts in the desired range of 40-100 μSv. The problem of high background due to inadequate annealing time was not so severe when dose levels are upto 20 mSv (almost 99% of the personnel monitoring cards are subjected to such dose ranges). [Table 3] shows the background counts for two groups of D-series cards exposed to dose range of 0.5-20 mSv, and annealed at 230°C for durations of 3h and 4h respectively. It was observed that there is no appreciable difference in the background reading for the two annealing durations at such dose levels. However, it may be noted that in case of inadequate annealing time of 3h at 230°C, the standard deviation is always more compared to 4h annealing time.
|Table 2: Background Readings for B and C series cards taken at BARC and TLD Laboratory|
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|Table 3: Background for TLD Cards of D-series at BARC after annealing at 230°C for 3h and 4 h|
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This exercise was further repeated for 5 cycles (anneal - exposure- anneal) after annealing at 230°C for 3h and 4h and the coefficient of variation was evaluated. The results are plotted in [Figure 1] which indicates that the average coefficient of variation is 51.2% and 35.7% for annealing duration of 3h and 4h, respectively. This kind of variation in scatter, irrespective of low or high dose can play important role in the long term quality assurance of dosimeters and dose assessment. The enhanced background, which may slowly build up with repeated use may result in rejection of the dosimeter from field use. Therefore, it is necessary that all the TLD processing laboratories should follow the prescribed standard methodology for annealing at 230°C-4h, as given in the handbook (Pradhan et al., 2002).
|Figure 1: Plot of % standard deviation in background counts after annealing at 230°C for 3h and 4h durations for 5 cycles|
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| 4. Conclusions|| |
Subsequent to the reporting of high background counts after annealing at 230°C-3h, the adequacy of this annealing cycle was verified. It was observed that the background counts of dosimeters increase with the previously imparted dose in the range 190 mSv-1.4 Sv. Such an increase was less severe when annealing was carried out as per the prescribed annealing procedure of 230°C-4h. The increase in the background counts due to inadequate annealing was found to be not very significant for low dose levels upto 20 mSv.
However, in view of the occasional occurrence of OER cases and also for long term quality assurance of dosimeters, it is recommended that annealing procedure of TLD, as prescribed in the handbook, must be strictly followed.
| 5. Acknowledgement|| |
Authors are thankful to Dr. Y. S. Mayya, Head, Radiological Physics & Advisory Division and Dr. R. K. Kher, formerly Head, Personnel Dosimetry & Dose Registry Section, RPAD, for their constant support and encouragement in the above work.
| 6. References|| |
- Geetha Varadharajan, Srivastava Kshama, Kolambe D.H., Punekar M.P. and Kher R.K. (2008), Review and Analysis of Over Exposure Cases in the Non-DAE Institutions in India during 2004-2007, Radiat. Prot. and Environ., Vol. 31, No. 1-4, 396-398.
- Lakshmanan A. R., Popli K. L. and Bhatt R.C. (1979), Thermal History and reusability of CaSO 4 :Dy Teflon TLD discs, Phy. Med. Biol, Vol. 24, No. 6, 999-1008.
- Pradhan A.S., Adtani M.M., Varadharajan G., Bakshi A.K., Srivastava Kshama and Bihari R.R. (2002), Hand Book on the use of TLD Badge Based on CaSO 4 :Dy Teflon TLD Discs for Individual Monitoring, BARC/2002/E/025.
- Vohra, K. G., Bhatt, R. C., Chandra, B., Pradhan, A. S., Lakshmanan A. R. and Sastry S. S. (1980), A personnel dosimeter TLD badge based on CaSO4:Dy Teflon TLD discs, Health Phys. 38, 193-197.
[Table 1], [Table 2], [Table 3]