|Year : 2016 | Volume
| Issue : 2 | Page : 91-95
Response of CaSO4:Dy based thermo luminescence dosimeter badge to 137Cs and 60Co radiations and its implications on estimation of personal dose equivalent
C Sneha, Suresh M Pradhan, Kshama Srivastava, Ratna Pradeep
TLD-Personnel Monitoring and Services Section, Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CT and CRS, Anushaktinagar, Mumbai, Maharashtra, India
|Date of Web Publication||13-Sep-2016|
TLD-Personnel Monitoring and Services Section, Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CT and CRS, Anushaktinagar, Mumbai - 400 094, Maharashtra
Source of Support: None, Conflict of Interest: None
In view of an underestimation of personal dose equivalent due to 60 Co radiation when system is calibrated to 137 Cs, the effect of the atomic number of the filters used in the badge on the photon beams of 660 keV and 1250 keV was investigated. The low Z-high Z interface created between filters and discs was found to lead to discontinuity in dose at the surface of the disc. This effect was consistently observable due to the 0.8 mm thickness of the disc. It was concluded that under-estimation of dose due to 60 Co radiation could be avoided by either reducing the thickness of the dosimeter disc or by a slight modification in the design of the filters.
Keywords: Calibration, high Z phosphor, individual monitoring
|How to cite this article:|
Sneha C, Pradhan SM, Srivastava K, Pradeep R. Response of CaSO4:Dy based thermo luminescence dosimeter badge to 137Cs and 60Co radiations and its implications on estimation of personal dose equivalent. Radiat Prot Environ 2016;39:91-5
|How to cite this URL:|
Sneha C, Pradhan SM, Srivastava K, Pradeep R. Response of CaSO4:Dy based thermo luminescence dosimeter badge to 137Cs and 60Co radiations and its implications on estimation of personal dose equivalent. Radiat Prot Environ [serial online] 2016 [cited 2022 Jan 19];39:91-5. Available from: https://www.rpe.org.in/text.asp?2016/39/2/91/190392
| Introduction|| |
Personnel monitoring of radiation workers for X-, beta- and gamma-radiation in India is carried out using thermo luminescence dosimeter (TLD) badge system. This badge consists of a card with three identical CaSO4:Dy Teflon discs loaded in a cassette. The structure of the card and cassette is such that each disc of the card is covered by a different filtration. The first disc is covered, both back and front by a combined copper-aluminum filtration (named D1) which amounts to 1060 mg/cm 2. The second disc (D2) is covered, both back and front by plastic filter which amounts to 180 mg/cm 2 and the last disc (D3) which is covered by paper and plastic of 12 mg/cm 2.
Calibration of the badge to the quantity Hp(10) is carried out at 2 m distance on ISO water phantom as per procedures mentioned in ISO-4037. The reader is calibrated such that the net reading of the disc under metal filter (D1) when exposed to 137 Cs photons gives the personal dose equivalent. Routine calibration of personnel monitoring dosimeters is carried out by exposing TLD cards sandwiched between a pair of 3 mm polymethyl methacrylate (PMMA) buildup plates to a panoramic gamma radiation source of 660 keV energy (137 Cs) at 50 cm distance in free-in-air conditions. The TLD reader is calibrated to give a reading of 1 mSv when exposed to 0.87 mGy of air kerma for D1 in this case also. This calibration can be directly correlated to the calibration for the quantity Hp(10) with a 1:1 ratio for the two exposures and is used on a routine basis.
With this calibration, it is expected that the net reading of disc under metal filter should give the personal dose equivalent for higher energies up to photon energies of 1250 keV (60 Co) also. However, it has consistently been found that the estimated dose due to 60 Co radiations is always less than the true dose by a factor of about 10%. Although within acceptable limits, the consistency of the factor indicates a discrepancy worth investigation. Further, when irradiating with 60 Co there are variations in the ratios of the disc readings depending on whether the dosimeter has been irradiated in panoramic or collimated geometry. Since the ratio of the readings of the discs is necessary for use in a branching algorithm of dose evaluation, the relative readings of the discs have a lot of importance. An attempt has been made to understand these variations.
The available literature indicates that one reason for such discrepancies with 60 Co radiation is the effect of the atomic number Z of the filter. ANSI/HPS N13.11-2001 states “the sole use of 137 Cs to test for high energy photon performance has been mistakenly assumed to imply adequate performance for all high energy photons: However, this source does not introduce the effects that occur from photons with energies much above 1 MeV interacting with high atomic number filters found in many dosimeter designs.” Ogunleye et al. have found a marked Z dependence when irradiating stacks of LiF chips covered by build-up layers of different Z to 60 Co radiation. This pattern is primarily brought about by the Z-dependence of electron scattering. In low-Z media electrons projected forward by gamma ray interactions tend to keep going in that direction, whereas in high-Z media they are more often backscattered.
Although this appears to be the apparent reason for the variations occurring, an attempt is made to quantify these variations and also improve the design of the dosimeter including filters to avoid such variations.
| Materials, Methods and Observations|| |
The personnel monitoring badge in use in India consists of a TLD card with three identical CaSO4:Dy Teflon TLD Discs loaded in a cassette. The cassette has suitable metallic and plastic elements such that when the card is loaded in the cassette, one disc (D1) is sandwiched between metal filters, another (D2) between plastic filters and the third (D3) is left open. The metal filter is a combination of 1 mm aluminium and 1 mm copper discs with copper nearer the disc and the plastic is of 1.6 mm thickness.
It is expected that due to decreasing filtration from D1 to D3, the readings of the three discs should have an increasing tendency from D1 to D3 within statistical errors. However, deviations from this general expectation are found when calibrating the badge to 60 Co radiation as shown in [Table 1]. For irradiations carried out to a panoramic 60 Co radiation source, at 50 cm distance, the expected pattern of increasing readings from D1 to D3 is observed but is far greater than expected. The reason for such readings have been explained as due to electron contamination. Irradiations carried out at 2 m distance from a collimated 60 Co source show the opposite effect with D3 less than D1 and the disc D2 showing the maximum value. Since electronic equilibrium may not exist for a collimated source, ISO  has recommended the use of an additional buildup plate. Reports also exist on the distance of buildup plate from the dosimeter affecting the electronic equilibrium. Irradiations at both such conditions also show deviations in the relative readings of the three discs. All readings normalized to the reading under metal filter (D1) are shown in [Table 1].
|Table 1: Ratios of disc readings of TLD badge irradiated to 60Co radiation under various geometries. Readings are normalized to reading of disc under metal filter (D1). The values are based on averages of 4 readings|
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Electronic contamination can explain the readings of panoramic exposure. For collimated exposure without buildup, the lower reading at D3 could be due to an absence of electronic equilibrium. The increase seen due to addition of a buildup into the path of the beam makes it more evident. The readings at D1 and D2 are due to a combination of achievement of equilibrium and the effect of the filter. That the distance of the buildup also affects the equilibrium is also evident from the increase seen in the reading of D3 relative to D1.
To quantify the difference that may occur due to copper – dosimeter interface and the PMMA – dosimeter interface the experiment conducted by Ogunleye et al. was repeated. To remove any effect due to the presence of the back filter, stacks of four CaSO4:Dy Teflon discs were covered with nearly equal density thicknesses of copper, PMMA, aluminum and CaSO4:Dy Teflon tape. The material thicknesses and the density thicknesses are given in [Table 2]. These stacks were irradiated to 137 Cs and 60 Co radiations at 50 cm distance to a panoramic source in free-in-air geometry. The discs were read in a TL reader with a heating profile of rise from ambient to 300°C in 20 s and clamp at 300°C for 30 s. This profile closely matches the heating profile used for reading the personnel monitoring badges. The same experiment was also repeated for collimated exposure.
The relative TL response of stacks of discs exposed under various configurations is shown in [Figure 1],[Figure 2],[Figure 3].
|Figure 1: Relative TL response of stacks of discs covered with different filters exposed to panoramic 137Cs radiation at a distance of 50 cm. Readings are normalized to 1st disc of PMMA stack. (O – Copper, Δ – PMMA, × – Aluminium)|
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|Figure 3: Relative TL response of stacks of discs covered with different filters exposed to collimated 60Co radiation at a distance of 2 m. Readings are normalized to 1st disc of PMMA stack. (O – Copper, × – Aluminium, + – CaSO4:Dy Teflon , Δ – PMMA)|
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With irradiation to 137 Cs, a higher value for the disc immediately behind the PMMA filter and a slightly lower value for the disc immediately behind the copper filter can be observed as shown in [Figure 1]. This amounts to a net difference of 7% between readings under copper and PMMA filter. This fact could be significant when PMMA buildup is substituted for the cassette during routine calibration. However, given the inherent variation in disc readings, it is quite possible for the variation due to filter to go unobserved. All discs further down the stack show approximately the same readings with no difference due to the covering filter. A slight decrease down the stack can be seen due to attenuation of the beam.
The readings of discs exposed to 60 Co show a similar trend to discs exposed to 137 Cs radiation as shown in [Figure 2] but the TL responses appear to be more significantly dependent on the atomic number Z of the filter used. The difference between topmost discs under PMMA and copper filters is 15%. This effect is evident only in the topmost disc in the stack, although a slight increasing and decreasing trend in the 2nd and 3rd discs can be observed in stacks covered with copper filter and PMMA filter, respectively. The decrease can be expected due to attenuation; however, the increase seems to indicate that the effect of the filter persists further down the stack till the point where equilibrium is again attained in the material of the disc. In low-Z media, electrons projected forward by γ ray interactions tend to keep going in that direction, while in high-Z media, they are more often backscattered. At the interface of two different media therefore there is an inhomogeneity in dose deposition – a minimum occurring in the low Z side when incident direction is from the high Z to low Z, and a maximum occurring on the high Z side when incident direction is from low Z to high Z. With a copper filter, the TL disc has a lower value of dose and with a PMMA filter TL disc has higher value of dose. Further down the stack, as electrons are generated in the medium of the disc, equilibrium value of dose is gradually achieved.
In collimated irradiation, shown in [Figure 3], the difference in readings between the first disc under the PMMA and copper is 21% which is more than the 15% difference observed in panoramic irradiation. This difference between panoramic and collimated could be the effect of collimation due to which more electrons are incident normally at the interface of filter and disc. When the discs are covered with a filter made of the material of the discs themselves, the phenomena occurring due to interfaces of different Z should not occur. These values are close to those of aluminum filter indicating that aluminum could be a good filter material so far as Z is concerned.
The actual dosimeter used in service is sandwiched between a pair of identical filters both front and back. The cards during calibration are loaded so that filtration is present both in the front and back. Further, the direction of exposure is fixed and the direction of reading is also fixed. The side facing the radiation source during exposure will also be the side facing the photomultiplier tube in the reader. The other side is always the side facing the heater. Differences in TL counts when the two sides are reversed may occur due to possible temperature gradient in the 0.8 mm thick disc during heating and differences occurring in detection of light emitted from different layers of the disc. Therefore to see both the effect of the back filter and the heating of the disc in the reader, discs were exposed to 60 Co radiation with different combinations of filters. The discs were read both in the normal direction and also in the reverse direction to exposure. All readings are given in [Table 3].
|Table 3: Readings of discs exposed individually between different filters combinations to 60Co radiation. Readings are normalized to reading of disc under PMMA buildup|
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The results indicate that the back filter also affects the total reading although the effect is minimal. This can be seen with both copper and PMMA front filters. Copper filter on the back causes a higher dose deposition in the disc and PMMA in the back gives a lower dose deposition. A combination of PMMA and copper filters in front and back lead to similar effects and therefore there is not a very significant difference in readings, whether read normally or in the reverse direction. However, with only copper and PMMA filters, there is a significant difference in readings in both forward and reverse directions. This indicates that either the variation in the dose deposition is limited largely to the surface of the disc or the photomultiplier tube sees primarily the light from the surface of the disc facing it.
| Results and Discussion|| |
In view of the observation of variation between readings when read in the forward and reverse direction, similar experiments were carried out with 0.4 mm thick discs. The results are shown in [Table 4]. It can be seen that the variations in readings seen due to the effect of the filter are present in the thinner discs also but with reduced results. Therefore, this indicates that the thickness of the discs plays a significant role in the observed effect.
|Table 4: Readings of 0.4 mm discs exposed individually between different filters combinations to 60Co radiation. Readings are normalized to reading of disc under PMMA buildup|
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In view of the observed advantage of using an aluminum filter, a minor modification of the filter was attempted. The results are shown in [Table 5].
|Table 5: Readings of 0.8 mm discs exposed individually between different Aluminium and copper combinations to 60Co radiation. Readings are normalized to reading of disc with copper filter near disc|
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The results indicate that a slight modification of the filter combination by sandwiching the copper filter with aluminum may reduce the observed deviation in estimation of the dose due to 60 Co. Therefore, either by reducing the thickness of the TL disc or by a slight modification in the filter design the discrepancy observed in the estimation of the dose due to 60 Co could be avoided.
The observed variations in disc readings due to 60 Co radiation are specific to calibration irradiations alone. The disc readings of radiation workers who work with purely 60 Co sources such as operators of radio-therapy units do not show such highly specific patterns of readings. This may be due to the fact that the above studies all pertain to normal irradiations whereas during service irradiation of the discs may be expected from all directions and lower energies could also be expected due to scatter. The near equivalence observed for irradiations to 137 Cs at 2 m in the regular cassette on phantom and 50 cm in PMMA buildup free-in-air could be due to multiple factors such as phantom backscatter and differences in readings due to filter differences. The same may not be true for 60 Co and must not be assumed.
The difference in readings of the 0.8 mm and the 0.4 mm thick discs indicates that reduction in the disc thickness could significantly reduce the observed effects due to the filters. An alternative to reduction of disc thickness could be the replacement of the copper filter with an equivalent thickness of aluminum. Considering that this will increase the bulk of the cassette, an alternative design change could be a copper filter sandwiched between aluminum filters. This will not alter the filter design significantly, whereas the discrepancy observed with 60 Co radiation would be reduced.
| Conclusions|| |
Internationally,137 Cs is recommended as the preferable source for calibration of individual monitoring system. However, with calibration to this energy, the estimation of dose due to 60 Co radiation shows an underestimation, especially in laboratory level. This appears to be due to a combination of the thickness of the dosimeter disc and the arrangement of the filters in the cassette. This discrepancy can easily be removed either by reduction in the thickness of the disc or a slight modification in the design of the filters.
The authors are thankful to Dr. Pradeepkumar K.S., Asso Director, HS and E Group and Shri D. A. R. Babu, Head, RPAD for their encouragement in the work. Dr. M. M. Adtani and Dr. M. P. Chougaonkar are also gratefully acknowledged for their contribution in completion of the work.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
International Organisation for Standardisation. Calibration of area and personal dosimeters and the measurement of their response as a function of energy and angle of incidence. Part-3, ISO-4037-3. Geneva: ISO; 1999.
American National Standards Institute Inc. (ANSI). American National Standard for Dosimetry of Personal Dosimetry Performance Criteria for Testing. Report No. HPS N13.11-2001. 2001.
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[Figure 1], [Figure 1], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]