|Year : 2013 | Volume
| Issue : 4 | Page : 146-149
Environmental monitoring using LiMgPO4:Tb, B based optically stimulated luminescence dosimeter
SN Menon1, Sonal Kadam1, Bhushan Dhabekar1, AK Singh1, MP Chougaonkar1, D. A. R. Babu1, AK Patra2
1 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
2 Environmental Survey Laboratories, Kakrapar Atomic Power Station, Gujarat, India
|Date of Web Publication||8-Oct-2014|
S N Menon
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra
Source of Support: None, Conflict of Interest: None
A new optically stimulated luminescence based environmental dosimeter (EnOSLD) was developed using LiMgPO 4 :Tb, B phosphor (LMP). The dosimeters were deployed along with the conventional thermoluminescent based environmental thermoluminescent detectors (EnTLD) for a period of six quarters in the environs of a nuclear power plant in India. The dose estimated by the EnOSLDs was compared with that of the dose estimated by EnTLDs. The mean ratio of the doses measured by thermoluminescent detector to that measured by optically stimulated luminescence dosimeters was found to be 1.04 ± 0.11. The results show that LMP based OSLDs can be used as environmental dosimeter.
Keywords: Dose, environmental dosimeter, LiMgPO 4 :Tb, B, optically stimulated luminescence
|How to cite this article:|
Menon S N, Kadam S, Dhabekar B, Singh A K, Chougaonkar M P, Babu D, Patra A K. Environmental monitoring using LiMgPO4:Tb, B based optically stimulated luminescence dosimeter. Radiat Prot Environ 2013;36:146-9
|How to cite this URL:|
Menon S N, Kadam S, Dhabekar B, Singh A K, Chougaonkar M P, Babu D, Patra A K. Environmental monitoring using LiMgPO4:Tb, B based optically stimulated luminescence dosimeter. Radiat Prot Environ [serial online] 2013 [cited 2019 Nov 11];36:146-9. Available from: http://www.rpe.org.in/text.asp?2013/36/4/146/142383
| Introduction|| |
Thermoluminescent detectors (TLD) are widely used for monitoring of ionizing radiation in the environment. Environmental monitoring is being performed around the world using thermoluminescent detectors, such as CaSO 4 :Dy, CaF 2 , LiF: Mg, Ti, and LiF: Mg, Cu, P. Monitoring of environmental radiation levels - preoperational background as well as postoperational radiation levels - in the environment of nuclear power plants (NPPs) in India is carried out using CaSO 4 :Dy based TLDs. The database thus generated helps assuring the public at large that there are no releases in the public domain. There are 20 NPPs at six sites in India with a total generating capacity of 4780 MWe. The environs of the nuclear facilities are constantly monitored by local networks of ground radiation detectors and thermoluminescence dosimeters.
Optically stimulated luminescence (OSL) has emerged as a popular dosimetric technique during the past 10 years, after the development of crystalline α-Al 2 O 3 :C phosphor. OSL based dosimeter is being used increasingly because it has various advantages over thermoluminescent (TL) based dosimeter such as faster and multiple readout, absence (no role) of thermal quenching, high sensitivity, possible use of phosphor in plastic binders etc., A1 2 O 3 :C was proposed as a sensitive OSL dosimeter (OSLD) for rapid assessment of environmental dose rates.  Environment dose measurement using the Al 2 O 3 :C based OSL dosimeter was carried out in Japan.  OSL from Al 2 O 3 :C was used for retrospective assessment of environmental dose rates.  Al 2 O 3 :C based OSL "dot" dosemeters made by Landauer ® was tested for its feasibility for environmental monitoring.  An environmental OSLD using BeO for emergency response was developed by Woda et al. 
In an effort to find alternative OSL phosphor to alumina (α-Al 2 O 3 :C), several other OSL materials have been synthesized. Such as LiMgPO 4 :Tb, B (LMP),  KBr: Eu,  MgO: Tb,  LiAlO 2 :Tb/Ce, , Y 3 Al 5 O 12 :C  etc., Dhabekar et al.  observed that the OSL versus dose response of LMP is linear from 1 mGy to 1 kGy, there is a loss of about 16% of the OSL signal within 4 days after which the intensity gets stabilized and the minimum detectable dose (MDD) was 20 μ Gy (dose corresponding to 3σ of the background). The phosphor possesses other impressive OSL dosimetric properties and hence could be an alternative to α-Al 2 O 3 :C and has the potential to be used in personnel, environmental, medical and high dose dosimetry (food irradiation). Further optimization of phosphor/reader system has brought down the MDD to 1 μGy. The synthesis technique of LMP is simple, cost effective with readily available raw material and is amenable to large-scale production of the phosphor.
The favorable dosimetric properties and high sensitivity of LMP along with the fact that apart from Al 2 O 3 :C no other OSL sensitive phosphors are recommended for the use in environmental monitoring prompted us to study the feasibility of using dosimeters made of LMP for environmental monitoring.
In this paper, we report the results of the environmental monitoring carried out in the environs of Kakrapar Atomic Power Station (KAPS) in India using LMP based environmental OSLDs. One of the authors had previous experience in environmental monitoring using environmental thermoluminescent detectors (EnTLDs) in KAPS. This was the reason for choosing KAPS. These results are compared with the dose measured using conventional TL based environmental dosimeters in the same environment. The data generated under this work will be of immense help to the operators as well as regulators in deciding the usability of LMP based EnOSLDs in the environmental radiation monitoring.
| Materials and methods|| |
Environmental gamma radiation monitoring was carried out using EnOSLD based on LMP. These dosimeters are specially designed for environmental gamma radiation by BARC and have two discs made from LMP using poly tetra fluoro ethylene as a binder and covered with 1 mm copper filters from both sides thereby making OSL response energy independent. The relative energy response curve with respect to Cs-137 gamma energy for the bare LMP discs and with 1 mm copper filter is shown in [Figure 1].
The OSLD cards were prepared by fixing the two discs with the wire clamps and were enclosed in a black plastic pouch to protect it from light and placed in a plastic cassette so that the discs were properly covered with the copper filters.  The cassettes were sealed in polythene envelope and deployed in the field. The EnOSLDs were handled in the dark room during the reading of the cards. [Figure 2] shows a typical environmental OSLD assembly. Same design was employed to make EnTLDS except that the discs made using CaSO 4 :Dy. EnTLDS were also placed along with the EnOSLDs at different monitoring locations in KAPS. [Table 1] shows the comparison of the properties of EnTLDs and EnOSLDs.
A total of about 30 pairs of EnTLDs and EnOSLDs were deployed on a quarterly basis. Gamma monitoring was carried out for six quarters. After 3 months (one-quarter) the exposed dosimeters were replaced with the fresh ones and the exposed dosimeters were shipped back to the laboratory. The transit dose, during shipment of the dosimeters to and from the site, has been determined by mailing a set of dosimeters to the site, which is mailed back to the laboratory immediately. The monitoring procedure ensures that a TLD/OSLD is always present in each location at any point of time. Storage lead pots are maintained in both the laboratories and used for preserving the TLDs and OSLDs when not in use.
Thermoluminescent measurements were performed using a Nucleonix made computerized TLD badge reader (TL-1010) and OSL measurements were carried out using an automatic Riso TL/OSL-DA-15 reader system.  Spot reading of the ambient radiation levels has been obtained using the scintillometer every time while keeping fresh dosimeters. The scintillometer consists of NaI (TL) crystal coupled with a PMT. The dosimeters were analyzed and dose levels were arrived at using appropriate calibration curve. The calibration of the instruments was carried out by exposing the sample dosimeters using a well-calibrated Cs-137 source.
| Results and discussion|| |
As seen in [Figure 1], the maximum response of 4.3 occurs for the photon energy in the region of 60 keV for Teflon based disc of LMP. This implies that proper energy compensation filters should be incorporated in the OSLDs that use LMP. Natural gamma radiation spectrum is known to have a peak around 70 keV. An environmental dosimeter should have energy independent response at least above 50 keV.  International Electrotechnical Commission (IEC) has prescribed an energy response between −29% to +67% in the energy range 80 keV to 1.25 MeV. The figure also shows the energy response curve for the discs under 1 mm copper filters. A copper filter of 1 mm thickness covering the discs on both the sides was found to be optimum to reduce the energy dependence to the levels as suggested by IEC. The energy response of EnTLD is ±20% for 30 keV to 3 MeV. 
It is evident from the [Table 1] that the properties of LMP based OSLDs are quite comparable with those of EnTLDs. However, it is worth noticing the fact that the MDD of LMP based OSLDs is much lower that of CaSO4:Dy based EnTLDs. This will be a distinct advantage for accurate measurements of environmental radiation levels which are in the microgray levels over the period of 3 months.
Environmental thermoluminescent detector is widely used for environmental monitoring at various NPPs in India. It is also being used to determine the preoperational baseline radiation levels around the proposed NPP sites in India. Intercomparison studies of any environmental dosimeter with the CaSO 4 :Dy based EnTLDs will give confidence in the performance of the new system.
Gamma monitoring was carried out for six quarters viz. Q2 (April to June), Q3 (July to September), Q4 (October to December) during 2012 and Q1, Q2, and Q3 during 2013. The ratios of the quarterly doses measured using EnTLD to that of measured using EnOSLD were found to be: 1.11 ± 0.2 for Q2-12, 1.08 ± 0.23 for Q3-12, 0.98 ± 0.17 for Q4-12, 0.92 ± 0.22 for Q1-13, 1.10 ± 0.15 for Q2-13 and 1.03 ± 0.20 for Q3-13. It is seen that the dose measured using EnOSLDs and EnTLDs was within ± 10% during all the quarters. The maximum and minimum ratios are observed to be 1.11 and 0.92 respectively against expected value of 1.00.
[Table 2] shows the annualized dose measured using EnTLDs and EnOSLDs. The annualized dose was arrived at using the dose measured at respective locations during each quarter. The individual quarterly doses were added and scaled to yield the annualized dose. Table also shows the ratio of the doses measured by EnTLD to that measured by EnOSLDs. The mean ratio was found to be 1.04 ± 0.11. The results showed good agreement between the two dosimeters.
|Table 2: Comparison of annualized dose measured using EnTLDs and EnOSLDs|
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[Figure 3] shows the correlation between the dose measured by EnTLDs and EnOSLDs. The slope of the fit between the doses estimated by EnTLDs and LMP based EnOSLDs was found to be 0.96 although ideally it should have been unity. A good correlation between the doses measured by the two dosimeters shows that the LMP based EnOSLDs can be used as environmental dosimeters.
|Figure 3: Co-relation between the annualized dose arrived at using environmental thermoluminescent detector and environmental optically stimulated luminescence dosimeter|
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| Conclusions|| |
LiMgPO 4 :Tb, B based environmental OSLDs were developed and deployed in the environs of an NPP along with conventional TL based dosimeters in order to assess the feasibility of using the OSLDs for environmental monitoring. This exercise was carried out for a period of six quarters. The results showed good agreement between the two dosimeters. A good correlation between the doses measured by the two dosimeters shows that the LMP based OSLDs can be used as environmental dosimeters. The MDD of LMP based OSLDs is much lower that of CaSO4:Dy based EnTLDs. This will be an advantage for accurate measurements of environmental radiation levels which are in the microgray levels over the period of 3 months.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]
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