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ORIGINAL ARTICLE
Year : 2014  |  Volume : 37  |  Issue : 2  |  Page : 89-94  

Studies on α-Al 2 O 3 :C based optically stimulated luminescence badge for eye lens monitoring applications


1 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
3 Technical Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
4 Health Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Web Publication18-Dec-2014

Correspondence Address:
Munish Kumar
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai 400 085, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.147286

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  Abstract 

The prototype two element eye-lens dosimeter badge based on indigenously developed α-Al 2 O 3 :C optically stimulated luminescence dosimeter was investigated comprehensively for its suitability for eye-lens monitoring applications. The badge is calibrated to measure the eye-lens dose in terms of H p (3). The minimum measurable dose using the eye-lens dosimeter badge is observed to be ~ 35 μSv. This prototype eye-lens dosimeter badge was found to be suitable for measuring doses from X-rays, beta and gamma radiations to the eye-lens. The satisfactory performance of the prototype two element eye-lens dosimeter badge along with its attractive features such as multiple readout, less processing time, very good beta response uniquely position it for monitoring the eye-lens dose are presented.

Keywords: α-Al 2 O 3 :C, energy response to photons and beta particles, eye-lens monitoring, minimum measurable dose and dosimeter calibration, optically stimulated luminescence


How to cite this article:
Kumar M, Kulkarni M S, Ratna P, Bhatnagar A, Gaikwad N, Muthe K P, Tripathi S M, Sharma S D, Babu D, Sharma D N. Studies on α-Al 2 O 3 :C based optically stimulated luminescence badge for eye lens monitoring applications . Radiat Prot Environ 2014;37:89-94

How to cite this URL:
Kumar M, Kulkarni M S, Ratna P, Bhatnagar A, Gaikwad N, Muthe K P, Tripathi S M, Sharma S D, Babu D, Sharma D N. Studies on α-Al 2 O 3 :C based optically stimulated luminescence badge for eye lens monitoring applications . Radiat Prot Environ [serial online] 2014 [cited 2019 Jun 18];37:89-94. Available from: http://www.rpe.org.in/text.asp?2014/37/2/89/147286


  Introduction Top


Recently, International Commission on Radiological Protection recommended a seven-fold reduction in dose limit for lens of the eye from 150 mSv to 20 mSv, averaged over a defined period of 5 years, with no single year exceeding 50 mSv. [1] In the wake of this major reduction in the dose limit, a prototype two element eye-lens dosimeter badge was designed and developed in Bhabha Atomic Research Centre (BARC) based on α-Al 2 O 3 :C optically stimulated luminescence phosphor dosimeters (OSLD) by Kulkarni et al.[2] This paper reports the comprehensive details about the design of the eye-lens dosimeter badge, including the results of the energy response of α-Al 2 O 3 :C (bare and under Teflon and metal filters viz. Cu, Sn). This badge is suitable for monitoring doses from X, and gamma radiations in the energy range 15 keV to 3 MeV and beta radiation having energy in the range 0.7-3.54 MeV and is suitable for monitoring eye-lens doses in interventional procedures and radiation facilities of nuclear power program where there may be a possibility of receiving dose to the eye-lens. [3],[4],[5],[6]

The present paper addresses various dosimetric aspects of the eye-lens dosimeter badge which includes the energy response (bare as well as under energy compensation filters used in the badge), minimum measurable dose (MMD), possibility of multiple readouts, sensitivity (counts/mGy) of the dosimeter badge, etc. Details about performing calibration in terms of H p (3) using cylindrical water-filled phantom are also discussed in the present study. The α-Al 2 O 3 :C based dosimeter badge, using the optically stimulated luminescence (OSL) reader system for the measurements, has been characterized comprehensively. The data have been used to validate the results obtained using theoretical simulations on the two-element α-Al 2 O 3 :C based eye-lens dosimeter badge.


  Experimental details Top


Bhabha Atomic Research Centre developed dosimetric grade α-Al 2 O 3 :C phosphor having grain size 75-100 μm was sandwiched between two thin transparent plastic sheets and dosimeter discs having diameter 7 mm and thickness 0.14 mm were prepared. [2] A large number of such dosimeter discs was prepared, and the discs were optically bleached using blue light stimulation. [7] Background OSL measurements of all the discs were carried out using in-house developed OSLD badge reader system. [8],[9] The α-Al 2 O 3 :C dosimeter discs having response within ±5% were selected for their use in the two element OSLD badge. The two element card is loaded in the light-tight cassette having two filter regions viz. first one is made up of 1.35 mm thick Teflon disc, and the second one in the form of 0.3 mm Cu disc on both sides of the dosimeter [2] and is depicted in [Figure 1]. The 0.3 mm Cu filter is used to correct the over-response at lower photon energies, whereas the second filter that is, 1.35 mm thick Teflon is useful while evaluating the H p (3) response.
Figure 1: Two element á-Al2O3:C based optically stimulated luminescence dosimeter badge for eye-lens monitoring applications

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For the readout of OSLD cards, the OSL reader system was operated in a continuous wave OSL (CW-OSL) mode at 20 mW/cm 2 with blue (470 nm) light stimulation. [8] Two high-power light emitting diodes (LEDs) (blue, ~470 nm) placed at an angle of 45° with respect to the sample holder are used as stimulation light source in the reader system. A GG-435 color glass filter was fixed in front of the blue LEDs to cut off the stimulation wavelengths below 435 nm. A UG-1 color glass filter is placed in front of the PMT (Electron Tube, 9125B ET Enterprises Ltd., United Kingdom) to prevent the scattered stimulating light from reaching the PMT.

Methodology used to study energy response of the badge

Studies were performed to measure the energy response of bare α-Al 2 O 3 :C OSL discs to photons in the energy range 10.7 keV to 1.25 MeV. Irradiations were performed using X-ray machine (model YXLON MG325, YXLON International, Germany) in narrow beam geometry at a distance of 2 m from the focal spot of X-rays. The dosimeters were kept at a height of 1.5 m from the ground to reduce the scattering effect. Narrow beams used in above study were N-15, N-40, N-80, N-100, N-120, N-150, N-200 and N-250 and had average energy ~10.7 keV, 33 keV, 63 keV, 82 keV, 100 keV, 123 keV, 167 keV and 213 keV respectively. Free in air irradiations were performed for each of these beam energies using a set of eight OSL dosimeters to a dose of 4.38 mGy (500 mR). In addition, the irradiations were also carried out for the same dose using 241 Am (60 keV), 137 Cs (662 keV) and 60 Co (1.25 MeV) photon sources. During irradiations, build-up of 3 mm and 5 mm was provided using Perspex sheets for 137 Cs (662 keV) and 60 Co (1.25 MeV) photon sources, respectively, for achieving electronic equilibrium. Similar studies were also carried out on the eye-lens dosimeter badge.

Studies with 90 Sr/ 90 Y beta source

Bare α-Al 2 O 3 :C OSL discs were also irradiated to beta dose of 10 mGy using calibrated 90 Sr/ 90 Y source, and response was normalized with respect to the response at 137 Cs photon energy. Similar study was performed for eye-lens dosimeter badge.

Minimum measurable dose and multiple readout studies

The background contribution arising due to the dark current, scattered stimulation light and its dependence on stimulation power was accounted for the OSLD badge reader system prior to the evaluation of MMD using the eye-lens dosimeter badge. As per definition, MMD is defined as the OSL counts (in μGy or μSv) equivalent to 3σ value, where σ is the standard deviation associated with the set of freshly bleached dosimeters. [10] The value of σ is dependent on the reader system as well as on the dosimeter. The optically bleached eye-lens dosimeter badges were readout on OSL reader system. The stimulation intensity was fixed at 20 mW/cm 2 and the CW-OSL curves were recorded for the duration of 10 s with 0.1 s acquisition time. In addition, the possibility of multiple readout in OSL was also studied for various dose ranges spanning from 1 mGy to 10 mGy.


  Results and discussion Top


The dark current, background counts and CW-OSL curves recorded on α-Al 2 O 3 :C dosimeters for 0.2 mGy, 0.5 mGy and 1 mGy using BARC OSL reader system are shown in [Figure 2]. The energy response (free in the air) of the α-Al 2 O 3 :C OSL discs for photons under 0.3 mm thick Cu filter and the ratio of the response under Teflon to Cu filter is shown in [Figure 3]. It may be noted from [Figure 3]a that α-Al 2 O 3 :C OSL discs exhibit maximum response of ~3.50 at photon energy of ~33 keV whereas for photon energies beyond 80 keV, the response is nearly independent of energy. It is also worth mentioning that the use of 0.3 mm Cu as an energy compensation filter transmits ~2% intensity for N-15 beam (average energy ~10.7 keV) whereas for N-40 beam (average energy ~33 keV), the transmission is ~22%. The ratio of the response of OSL under Teflon to Cu filter [Figure 3]b is a measure of the average energy of the photons and is used to correct the over-response exhibited by α-Al 2 O 3 :C discs in lower photon energy region. The ratio of the response of OSL under Teflon to Cu filter varies from 25 at 10.7 keV to 1.3 at 80 keV. Beyond 80 keV, the ratio of the response of OSL under Teflon to Cu filter is < 1.3 and energy discrimination is not possible. It is also important to note that α-Al 2 O 3 :C dosimeters exhibit energy independent response beyond 80 keV and do not require response correction factors. The readout of the OSL disc below the Teflon filter can be directly correlated to H p (3) for photons (>80 keV), whereas for photon energy (<80 keV) the response correction is required which is obtained using the ratio of the readout of the disc under Teflon filter to that under metal filters.(H p (3) response is measured below thickness of 3 mm of unit density plastic material. 1.35 mm Teflon (density 2.2 g/cm 3 ) thickness is equivalent to 3 mm of unit density plastic material).

The response of a dosimeter to beta particles is mainly decided by the thickness of the dosimeter disc. Since the presently used α-Al 2 O 3 :C discs have thickness of 0.14 mm (~25 mg cm -2 ), these discs are expected to exhibit energy independent response beyond maximum beta energy (Emax ) of about 0.224 MeV. The response of the 0.14 mm thick α-Al 2 O 3 :C discs to various beta energies in the energy range from 0.224 to 3.54 MeV is shown in [Figure 4]. It may be noted that response correction factors are not required for beta particles that are relevant for eye-lens monitoring. Measurements were also performed by covering OSL discs with 1.35 mm (~300 mg/cm 2 ) thick Teflon filter and it was found that the response at 300 mg/cm 2 was ~ 48% when compared with the response at ~ 10 mg/cm 2 for irradiations performed with 90 Sr/ 90 Y beta source. In fact, OSL 10 /OSL 300 response ratio was found to be 2.09 ± 0.19 for 90 Sr/ 90 Y beta source. These measurements showed that the readout of the disc under Teflon filter (absorbed dose at 3 mm depth) represents the dose to the lens of eye and approximates H p (3) response.
Figure 2: Typical dark current, background counts and continuous wave optically stimulated luminescence (OSL) curves for 20 mR, 50 mR and 100 mR for Bhabha Atomic Research Centre OSL reader system

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Figure 3: (a) Experimental energy response (free in air) of the á-Al2O3:C optically stimulated luminescence (OSL) discs for photons. The response (free in air) of á-Al2O3:C OSL discs for photons under 0.3 mm thick Cu filter is also shown. (b) Ratio of OSL response of á-Al2O3:C OSL discs with 1.35 mm Teflon filter to that under 0.3 mm Cu filter

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Figure 4: Response of á-Al2O :C optically stimulated luminescence discs for various beta energies

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[Figure 5]a depicts the stimulation background as a function of stimulation power for the BARC OSLD badge reader system. [Figure 5]b shows the dose versus counts for α-Al 2 O 3 :C dosimeter discs in the range 1-10 mGy. The variation of background counts [Figure 5]c follows Gaussian/normal distribution. [11]
Figure 5: (a) Background as a function of stimulation power for Bhabha Atomic Research Centre optically stimulated luminescence reader system. (b) Dose versus counts for á-Al2O3:C dosimeter discs. The doses of 1 mGy, 2 mGy, 5 mGy and 10 mGy were delivered using 137Cs photons. (c) Variation of background counts which follows Gaussian/normal distribution

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The MMD was found to be ~ 35 μGy. In addition to above, the multiple readout studies for α-Al 2 O 3 :C dosimeter discs were performed for doses of 1 mGy, 2 mGy, 5 mGy and 10 mGy; the results are shown in [Figure 6]a. [Figure 6]b, shows the variation of MMD with readout number. It was found that the MMD increases with the readout number. From 6a, it follows that with the current readout protocol (recording of CW-OSL curve at stimulation intensity of 20 mW/cm 2 for a period of 10 s/disc with data acquisition every 0.1 s), it may be possible to perform multiple readout up to 5-10 times for doses incurred up to ~ 10 mGy. It is worth mentioning that the reader system covers a useful dose range of 50 μGy to 10 Gy with minimum detection limit of ~ 35 μGy. The reproducibility of various measurements was found to be within ± 6% where D-is the mean OSL response for a given set of dosimeters exhibiting a standard deviation of σ.
Figure 6: (a) Variation of optically stimulated luminescence (OSL) counts with readout number for á-Al2O3:C dosimeter discs. The stimulation intensity during continuous wave OSL recording was 20 mW/cm2. (b) Variation of minimum measurable dose with the readout number for á-Al2O3:C dosimeter discs

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Evaluation of H p (3) using α-Al 2 O 3 :C based two element optically stimulated luminescence dosimeter badge

As the eye-lens dosimeter must be worn at the center of the forehead (near to eyes), an appropriate phantom must be used to calibrate the eye-lens dosimeters and to estimate the quantity, H p (3). The use of phantom during calibration is also necessary to simulate the attenuation and scattering properties of the human head. As the human head is nearly cylindrical having typical head volume of ~ 6300 cc and hence a cylindrical phantom having diameter of 20 cm and height of 20 cm is reasonable. In view of this, studies on water filled cylindrical phantom were performed (volume = 6280 cc). The phantom and associated conversion coefficients have already been recommended by the ORAMED project task force dealing with eye-lens dosimetry. [6]

After measuring the air kerma at the point of irradiation of the dosimeter, the dose delivered to eye-lens (mSv) dosimeter in terms of H p (3) is air kerma (mGy) × conversion coefficient. The response of the dosimeter on phantom is related to the above value. If the net OSL response of the dosimeter disc under Teflon filter is R, then H p (3) calibration factor for that reader system is evaluated as:

H p (3) mSv = OSL response of the dosimeter disc under Teflon filter (R) × calibration factor and Calibration factor = H p (3) in mSv/OSL response of the dosimeter disc under Teflon filter. In this study it was found that, H p (3) value of 1 mSv corresponds to ~7310 counts for 137 Cs irradiation of eye-lens dosimeter badge. Similarly H p (3) can be estimated using the ratio of the response of the OSL under Teflon to Cu filter for low energy photons.


  Conclusions Top


α-Al 2 O 3 :C OSL discs exhibit maximum response of ~ 3.50 at photon energy of ~ 33 keV whereas for photon energies beyond 80 keV, the response is nearly independent of photon energy. The ratio of the response of the OSL dosimeters under Teflon to Cu filter varies from 25 at 10.7 keV to 1.3 at 80 keV. The readout of the OSL disc below the Teflon filter can be directly correlated to H p (3) for photons energies higher than 80 keV whereas for photon energy < 80 keV response correction is required. Measurements performed by covering OSL discs with 1.35 mm (~300 mg/cm 2 ) thick Teflon filter showed that the response at 300 mg/cm 2 was ~ 48% when compared with the response at ~ 10 mg/cm 2 . The MMD was found to be ~35 μSv and the reproducibility of the measurements was found to be within ± 6%. Using CW-OSL method (for 10 s readout time at 20 mW/cm 2 stimulation intensity) it is possible to perform multiple readouts up to 5 times for doses ~1 mSv whereas for doses around 10 mSv, multiple readouts up to 10 times are possible. The badge can cover energy range of 10 keV to 1.25 MeV for photons and 0.7-3.54 MeV for beta radiation. The OSL based RHS, eye lens dosimeter system thus can become the prime system for monitoring the dose to the eye-lens of occupational radiation workers.


  Acknowledgments Top


The authors are thankful to their colleagues of RSS, RP and AD, Health Physicists/RSO's of Dhruva, Cirus, RMC, TMH, Hinduja and Jupiter hospitals for their valuable cooperation during course of this work. Authors are also thankful to Dr. K. Biju, Dr. C. Sunil, Shri Kamaldeep, Shri A. A. Shanbhag and Shri Sunil K. Yadav for many fruitful discussions.

 
  References Top

1.
ICRP. International Commission on Radiological Protection. Statement on Tissue Reactions . Canada: ICRP 4825-3093-1464; 2011.  Back to cited text no. 1
    
2.
Kulkarni MS, Kumar M, Ratna P, Muthe KP, Biju K, Sunil C, et al. Design of a New Two Element OSLD Badge for Eye Lens Monitoring. Radiation Protection and Environment (appears elsewhere in the same issue). 2014;37:106-111.  Back to cited text no. 2
    
3.
Kumar M. Eye lens dosimetry using thermally (TL) and optically stimulated luminescence (OSL) based dosimeters: Design and Dosimetric Concerns, Proceedings of the Theme Meeting on Recent Advances in Solid State Dosimetry, Organized by Luminescence Society of India (Maha. Chap.), Mumbai, 22, 2013.  Back to cited text no. 3
    
4.
Dietze G. Radiobiology and radiation dosimetry for the lens of the eye. In: Mattsson S, Hoeschen C, editors. Radiation Protection in Nuclear Medicine. Germany: Springer; 2013. p. 33.  Back to cited text no. 4
    
5.
Gualdrini G, Mariotti F, Wach S, Bilski P, Denoziere M, Daures J, et al. A new cylindrical phantom for eye lens dosimetry development. Radiat Meas 2011;46:1231.  Back to cited text no. 5
    
6.
Vanhavere F, Carinou E, Gualdrini G, Clairand I , Sans Merce M, Ginjaume M, et al. ORAMED: Optimization of Radiation Protection of Medical Staff. European Radiation Dosimetry Group e. V. EURADOS Report 2012-02. Braunschweig, April; 2012.  Back to cited text no. 6
    
7.
Paliwal L, Rawat NS, Ratna P, Khanderao RN, Gaonkar UP, Kulkarni MS. Optical bleaching setup for fast resetting of large number of OSLD cards. Proceedings IARPNC-2012. Mangalore University; March 15-17, 2012.  Back to cited text no. 7
    
8.
Ratna P, Gaonkar UP, Vinay M, Kulkarni MS. A new programmable semi automatic OSLD badge reader for personal and environmental monitoring applications. Proceedings IARPNC-2012. Mangalore University; March 15-17, 2012.  Back to cited text no. 8
    
9.
Kulkarni MS. Development of optically stimulated luminescence technology for personnel monitoring applications at BARC. Proceedings IARPNC-2012. Mangalore University; March 15-17, 2012.  Back to cited text no. 9
    
10.
Chougaonkar MP, Kumar M, Bhatt BC. Testing of phosphors for their use in radiation dosimetry: Detailed procedure and protocol. Int J Lumin Appl 2012;l: 197-221.  Back to cited text no. 10
    
11.
Chougaonkar MP. Takale RA, Shetty PG, Mayya YS, Puranik VD. Performance characteristics of CaSO4: Dy based indigenous thermoluminescenct dosimeters for environmental radiation monitoring, BARC Report, BARC/2008/E007; 2007.  Back to cited text no. 11
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]



 

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