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ORIGINAL ARTICLE
Year : 2014  |  Volume : 37  |  Issue : 2  |  Page : 106-111  

Design of prototype two element optically stimulated luminescence dosimeter badge for eye lens monitoring


1 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Technical Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
3 Health 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:
M S Kulkarni
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: 10.4103/0972-0464.147294

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  Abstract 

A prototype two element eye lens dosimeter badge based on highly sensitive α-Al 2 O 3 :C optically stimulated luminescence dosimeters (OSLDs) was designed and developed. The badge consists of a plastic card in which two thin α-Al 2 O 3 :C optically stimulated luminescence (OSL) discs are placed. The OSLDs in the plastic card (when inserted into plastic cassette) are covered with energy compensation filters made up of 0.3 mm thick Cu and 1.35 mm thick Teflon discs positioned symmetrically on both sides of the dosimeter. The OSLD badge is useful for monitoring doses from photons and beta particles. In this paper, theoretical studies using Monte Carlo method as well as using the analytical technique have been performed to study the energy response of the bare α-Al 2 O 3 :C based disc dosimeters. These dosimeter discs have been found to exhibit over-response by a factor of ~ 3.4 at ~ 33 keV photon energy, whereas, beyond 80 keV photon energy, the response is nearly energy independent. Studies have also been performed to find the energy response of the α-Al 2 O 3 :C disc dosimeters under different metal filters, viz., Al, Cu, Sn etc., and under various thicknesses of Teflon. From theoretical simulations, it has been found that 0.3 mm thick Cu is sufficient to correct the over-response in lower energy region within acceptable limits. Further, Teflon disc (DuPont, USA) having thickness of 1.35 mm is found to be the optimized choice as filter for the second dosimeter disc. It is worth mentioning that the ratio of the response of the OSL disc under Teflon to that under Cu filter indicates average energy of X-ray photons and same is used to correct the over-response as well as to estimate the quantity H p (3). Also for higher photon energy region, the readout of the dosimeter disc under Teflon filter directly measures the quantity H p (3). Same holds true for beta particles having maximum beta energy, Emax beyond 0.7 MeV.

Keywords: α-Al 2 O 3 :C, energy compensation filters, energy response, eye lens monitoring, Hp(3), Monte Carlo simulation, operational quantities, optically stimulated luminescence dosimeter


How to cite this article:
Kulkarni M S, Kumar M, Ratna P, Muthe K P, Biju K, Sunil C, Babu D, Sharma D N. Design of prototype two element optically stimulated luminescence dosimeter badge for eye lens monitoring . Radiat Prot Environ 2014;37:106-11

How to cite this URL:
Kulkarni M S, Kumar M, Ratna P, Muthe K P, Biju K, Sunil C, Babu D, Sharma D N. Design of prototype two element optically stimulated luminescence dosimeter badge for eye lens monitoring . Radiat Prot Environ [serial online] 2014 [cited 2019 May 19];37:106-11. Available from: http://www.rpe.org.in/text.asp?2014/37/2/106/147294


  Introduction Top


Ionizing radiations viz., beta particles, photons (X and gamma rays) and neutrons contribute to the dose received by the occupational radiation workers. For strongly penetrating radiation (i.e. photons >15 keV and neutrons of all energy), the quantity to be measured is personnel dose equivalent, H p (10), whereas for weakly penetrating radiations (i.e. beta particles and photons having energy <15 keV), H p (0.07) was defined. For monitoring the doses to the eye lens, the quantity personal dose equivalent H p (3) is defined as the dose equivalent in tissue at 3 mm depth in the phantom. [1],[2],[3] The beta particles and low energy photons (X-rays) are potential contributors towards the dose to the eye lens although the contribution from high energy photons and neutrons is not ruled out. This is due to fact that the beta particles and low energy photons have smaller range and deposit more dose at 3 mm depth compared to the dose deposition from high energy photons and neutrons at the same depth. Normally, beta particles having Emax ≥0.7 MeV contribute toward dose to the eye lens as the range of beta particles with Emax less than 0.7 MeV is less than 3 mm and do not pose any hazard to the eye lens. As per International Commission on Radiological Protection (ICRP), 103, in the past, the personal dose equivalent H p (3) was rarely used in practice and very few dosimeters were available for measuring these quantities. [4] This was because the exposure to the eye lens was sufficiently controlled by assessing H p (10) and H p (0.07) for complying with the annual dose limit of 150 mSv for the lens of the eye. This also follows from International Commission on Radiation Units and Measurements, which recommended that for beta sources, having Emax ≤3.50 MeV, the ratio of the absorbed dose to the skin around eyes to that at 3 mm depth is always ≥3.3. [5] This has led to the concept that the dose to skin around eyes is limiting and if the yearly dose limit for skin around eyes is followed, the dose limit for the lens of eye follows automatically, that is, the ratio of ICRP's dose limits (earlier recommendations of ICRP) for skin (500 mSv) and the lens of eye (150 mSv) will be ~ 3.33. Furthermore, as per the International Standards Organization report, [6] the dose determination for the lens of eye is not necessary for beta particles with Emax ≤3.50 MeV and photons having energy <10 keV, if the skin dose near the eyes does not exceed the dose limits. Therefore, determination of the dose for the lens of the eyes was only required in exceptional cases, that is, for beta radiation with Emax >3.50 MeV. [5],[6],[7]] The above facts have also formed the basis that there was no need to monitor the dose to the eye lens as long as whole body and skin dose limits recommended in ICRP 60 were followed. [1],[8] In view of this, monitoring of the doses to the lens of eye, (H p (3) was not directly performed and the whole body, H p (10) and skin doses, H p (0.07) were generally monitored. [5],[6],[7]]

In view of the recent ICRP recommendations (2011), in which the equivalent dose limit for the lens of the eye has been reduced from 150 mSv in a year to 20 mSv in a year (not exceeding 50 mSv in a single year) for occupational workers, [8] the eye lens dosimetry is likely to receive more attention than before in the coming years [1],[2],[3],[7],[9] and the eye lens monitoring may be essential in the situations where probability of receiving higher doses to the eye lens is not ruled out.

Worldwide status of eye lens dose monitoring

H p (3) monitoring may be essential in radiation fields where the low energy photons (X-rays) and/or beta particles (Emax > 0.7 MeV) can contribute towards the eye lens dose. The main concern is low energy X-rays used in interventional cardiology and radiology procedures where the average energy of X-ray is ~(25-50) keV and typical field size varies from 5 × 5 cm to 40 × 40 cm. It may be noted that eyes of the radiation workers are not likely to be exposed in the direct beam, but to the scattered photon radiations, and the scattering of photons is maximum around 50-70 keV. In institutions like hospitals having nuclear medicine facilities, occupational workers being exposed to beta sources having Emax > 0.7 MeV needs to be monitored for exposures to the eye lens. The field size associated with the beta sources are generally wider and may have maximum beta energy as high as 3.54 MeV ( 106 Ru/ 106 Rh). To ensure that the dose to the eye lens does not exceed the prescribed dose limits, various eye lens dosimeters have been designed and are being used internationally. [9],[10] The eye lens dosimeter badge available internationally consists of a single thermoluminescence (TL) dosimeter of LiF: Mg, Ti or LiF: Mg, Cu, P which is sealed in a plastic holder and have commonly been designed for photons. [10],[11] These dosimeters are generally worn with the head strap near the eye and the dose, the dosimeter receives is correlated to the eye dose. Such dosimeters are available from Rad Card-Germany/Poland, Public Health/NRPB, UK, Mirion Technologies, USA and Rotunda Scientific Technologies, USA.

In India, Bhabha Atomic Research Centre is the nodal agency who operates and accredit thermoluminescent dosimeter (TLD) Personnel Monitoring Services based on CaSO 4 :Dy Teflon embedded TLD badges. [11],[12] Chest and wrist TLD badges are in service for monitoring whole body and extremity dose. Presently, on a limited basis, CaSO 4 :Dy based Teflon embedded TLD badge (same as chest TLD badge) with headband is used for monitoring the dose to the skin around eyes and not to the lens of the eye. It may be noted that with this methodology there may be huge over-estimation for beta sources as the head badge measures dose to the skin around eyes rather than dose to the lens of the eye. Same may be applicable for photons especially having low energy that are prevalent at various radiation facilities. In addition, CaSO 4 :Dy Teflon discs (~0.8 mm thick) used in the personnel monitoring TLD badges exhibit severe energy-dependent response to beta particles as the dosimeter discs are thick, leading to additional errors in dose estimation. Also for photons, CaSO 4 :Dy Teflon embedded TLD badge exhibit high photon energy-dependent response. Therefore, this badge in the present form is not suitable for eye lens dosimetry and requires design modifications as well as algorithm corrections.

The OSL dosimeters based on α-Al 2 O 3 :C phosphor have found wider acceptance world over for personnel and environmental monitoring due to various advantages such as high sensitivity, multiple readout, better beta response (due to thin dosimeters), short time needed for processing etc. In view of the potential requirement for accurate measurement of eye lens dose, we have designed and fabricated an exclusive eye lens dosimeter badge using α-Al 2 O 3 :C phosphor dosimeters. The paper presents the design aspects of the prototype two element eye lens dosimeter badge based on OSL technique for its possible use by the occupational workers covering the medical, and nuclear facilities where there is potential for receiving dose to the lens of the eye. [14]


  Materials And Methods Top


Simulation studies for cassette design

The choice of the α-Al 2 O 3 :C detector material as an OSL phosphor was made based on the criteria applicable for selection of a phosphor for personnel monitoring applications as well as the ready availability of this phosphor [13] for its use as eye lens dosimeter. α-Al 2 O 3 :C OSL dosimeters (OSLDs) discs having thickness of 0.14 mm (25 mg/cm 2 ) and diameter ~ 7 mm, were prepared by sandwiching the dosimetric grade α-Al 2 O 3 :C (grain size 75-100 μm) between two thin transparent plastic sheets. Large number of such dosimeter discs were prepared as shown in [Figure 1]. It was intended that the designed eye lens dosimeter badge be versatile enough for monitoring occupational workers in medical, industrial and nuclear facilities.
Figure 1: Thin á-Al 2O3:C dosimeter discs (7 mm diameter)

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Photon energy response studies for α-Al2O3:C dosimeter

Theoretical studies using Monte Carlo method were performed to study the response of α-Al 2 O 3 :C OSLD's to photons. Monte Carlo simulations were carried out to obtain the relative dose response of the α-Al 2 O 3 :C dosimeter disc with respect to the dose to tissue using FLUKA Monte Carlo code. [15],[16] The dose response of the Al 2 O 3 detector was estimated by simulating mono-energetic parallel photon beams of varying energies; 10 keV to 1.5 MeV falling on the bare detector. The statistical error in the simulations was within 3%. In addition, analytical studies were also performed to find the energy response of α-Al 2 O 3 :C dosimeters.

Studies were also performed using analytical techniques by evaluating the response of α-Al 2 O 3 :C based OSLD's under different thickness of aluminum (Al), copper (Cu), tin (Sn) and Teflon. The metal filters were selected so as to estimate the response of the α-Al 2 O 3 :C OSLD's under various energy compensation filters. Studies were also performed to find the response of α-Al 2 O 3 :C OSLD's under various thicknesses of Teflon as a buildup material.

Response of 0.14 mm thick α-Al2O3:C OSL discs to beta particles

The beta energy loss per unit path length (dE/dx) is a strong function of its energy, with highest energy deposition near the end of the path. Typical TL dosimeters having thickness less than the range of the beta radiations act as dE/dx detectors as required by the definition of skin dose, whereas thick dosimeters act as total absorption detectors. Hence to make dE/dx independent of energy of beta particles, the detector thickness must be small as compared with the ranges of the electron energies of interest. [17],[18] It should also be noted that a thick dosimeter calibrated using high energy photon will yield a beta dose that is averaged over the thickness of the element. In view of this, a dosimeter likely to be used for measuring doses from beta particles should be as thin as possible. Since the presently used α-Al 2 O 3 :C discs have thickness of 0.14 mm, that is, 25 mg/cm 2 , they are expected to exhibit flat/energy independent response beyond maximum beta energy, Emax of 0.224 MeV. It is also worth mentioning that the beta particles having Emax ≥0.70 MeV have range >3 mm, the thin dosimeters of α-Al 2 O 3 :C exhibit energy independent response for all beta energies which are of concern in eye lens monitoring. This feature makes thin α-Al 2 O 3 :C discs a unique dosimeter as they are superior to 0.4 mm or 0.8 mm thick LiF or CaSO 4 :Dy based dosimeters. This is due to the reason that no response correction factors are required for α-Al 2 O 3 :C dosimeters as in the case of LiF or CaSO 4 :Dy based thick dosimeters.

Based upon above studies, two element dosimeter badge and cassette have been designed for use of α-Al 2 O 3 :C OSLD's for personnel monitoring applications and is suitable for monitoring doses from X and gamma rays and beta particles.


  Results and Discussion Top


The relative response of the bare detector along with different thickness of the Cu filters normalized to the dose to water is shown in the [Figure 2]a whereas [Figure 2]b presents the relative dose response of α-Al 2 O 3 :C detector with tin filters obtained using Monte Carlo simulations. Also, the effect of Teflon filter on the energy response of α-Al 2 O 3 :C obtained by Monte Carlo simulations is presented in [Figure 2]c. Results based on analytical calculations as shown in Figure 3a for bare disc, disc under 1.35 mm thick Teflon and 0.3 mm thick Cu filters. It may be noted from [Figure 2]b and [Figure 3]a that α-Al 2 O 3 :C OSL discs exhibits maximum response of 3.49 at photon energy of ~ 33 keV whereas for photon energies beyond 80 keV, the response is nearly independent of photon energy.
Figure 2: The relative energy response of á-Al2O3 detector with respect to water as a function of incident photon energy for various thicknesses of copper filter, (a) tin filter (b) and Teflon filter (c). The energy response of bare á-Al2O3 disc is also shown

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Figure 3: Optically stimulated luminescence (OSL) response of á-Al2O3:C OSL discs (a) 1: Bare, 2: Under 1.35 mm Teflon and 3: Under 0.3 mm Cu filters; (b) (OSLTeflon/OSLCu) 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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The studies were performed experimentally with 0.3 mm and 0.5 mm thick Cu filters, and it was found that 0.3 mm thick Cu is the optimized thickness as an energy compensation filter. It is also worth mentioning that the use of 0.3 mm thick Cu as an energy compensation filter transmits ~ 2% intensity for N-15 beam having energy ~ 10.7 keV whereas for N-40 beam having energy 33 keV, the transmission is ~ 22%. The use of 0.5 mm thick Cu, further attenuates these beams which hugely decreases the OSL response in low energy region, that is, for N-40 beam, the OSL response under 0.5 mm thick Cu filter is ~ 8% which was ~ 22% for 0.3 mm thick Cu. From the study, it follows that 0.3 mm thick Cu is suitable as a filter for making the response nearly acceptable as compared to 0.5 thick Cu filters and 0.3 mm thick Cu filter have been selected in the design of the OSLD badge.

Further in [Figure 3]b, the ratio of the OSL response of α-Al 2 O 3 :C dosimeter under 1.35 mm Teflon filter to that under 0.3 mm Cu filter is shown. The ratio of the response of OSL under Teflon to Cu filter is measure of the average energy of the photons. It may be noted that 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. These factors are important for the correction of the over-response and also to design dose estimation algorithm/protocols. 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 OSLD exhibit nearly energy independent response beyond 80 keV and response correction factors are not required. [Table 1] gives the over-response factor and the ratio of the response of OSL under Teflon to Cu filter for photons. From this, it follows that two-disc OSL badge (one under 0.3 mm Cu and second under 1.35 mm thick Teflon) is optimal for eye lens monitoring applications. The second filter viz., Teflon having thickness of 1.35 mm is ~ 300 mg/cm 2 and can be used in evaluation of Hp(3) for photon energies >80 keV, as well as for beta particles having energy >0.7 MeV. Based on these studies, the OSLD card, badge and cassette have been designed and are shown in [Figure 4]. The eye lens badge contains a 1.2 mm thick plastic card having two circular slot each 1.0 mm deep and 7 mm diameter [Figure 4]a. Two thin semi-transparent Al 2 O 3 :C dosimeter discs are fitted as shown in [Figure 4]b. This plastic card is inserted in a light tight black plastic cassette containing the energy compensation filters and provided with a headband [Figure 4]c. Experimental details of this badge are reported by Kumar et al. [14] in the same issue of the journal.
Table 1: Over - response factors and the ratio of the 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: (a) Two element á-Al2O3:C dosimeter card, (b) eye lens dosimeter badge (c) Proposed eye lens dosimetry badge showing the two element optically stimulated luminescence card along with plastic cassettee and energy compensating filters

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  Conclusions Top


A two-element eye lens OSL dosimeter badge using thin α-Al 2 O 3 :C discs has been designed using theoretical simulations supported by the experimental results. The dosimeter card contains two α-Al 2 O 3 :C OSL discs loaded in the cassette having two filter regions: (i) First region have 1.35 mm thick Teflon filter, (ii) second region have 0.3 mm Cu on both sides of the dosimeter. The OSLD badge is useful for monitoring doses from photons and beta particles. The ratio of the response of the disc under Teflon to that under Cu filter indicates the average energy of the X-rays and is used to correct the over-response. In this way, the readout of the disc under Teflon filter is correlated to H p (3) using appropriate calibration cum conversion factor. Beyond the photon energy of 80 keV, the response of α-Al 2 O 3 :C is energy independent, and readout of the disc under Teflon filter directly gives the dose to the lens of the eye. Same holds true for beta particles having maximum beta energy beyond 0.7 MeV.

 
  References Top

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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-Verlag, Heidelberg; 2013. p. 33.  Back to cited text no. 1
    
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Behrens R. On the operational quantity H (p)(3) for eye lens dosimetry. J Radiol Prot 2012;32:455-64.  Back to cited text no. 2
    
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Bhatt BC. The new dose limit for the lens of the eye and its implications. Radiat Prot Environ 2012;35:1-3.  Back to cited text no. 3
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The 2007 Recommendations of the International Commission on Radiological Protection. ICRP publication 103. Ann ICRP 2007;37:1-332.  Back to cited text no. 4
    
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Vanhavere F, Carinou E, Gualdrini G, Clairand I, Sans Merce M, Ginjaume M, et al. Measurements of eye lens doses in interventional radiology and cardiology: Final results of the ORAMED project. Radiat Meas 2011;46:1243-7.  Back to cited text no. 10
    
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Vohra KG, Bhatt RC, Chandra B, Pradhan AS, Lakshmanan AR, Shastry SS. A personal dosimeter TLD badge based on CaSO4: Dy teflon TLD discs. Health Phys 1980;38:193-7.  Back to cited text no. 11
    
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Kulkarni MS, Ratna P, Kannan S. A new PC based semi-automatic TLD badge reader system for personnel monitoring. In: Proceedings of International Radiation Protection Association Conference, Hiroshima, Japan; 14-19 May, 2000.  Back to cited text no. 12
    
13.
Mthue K P, Kulkarni M. S., et al. Large scale preparation of dosimetric grade Al2O3:C (unpublished).  Back to cited text no. 13
    
14.
Kumar M, Kulkarni MS, Ratna P, Bhatnagar A, Gaikwad N, Muthe KP, et al. 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.  Back to cited text no. 14
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Fasso A, Ferrari A, Ranft J, Sala PR. FLUKA: A Multi-Particle Transport Code. CERN-2005-10. INFN/TC_05/11, SLAC-R-773; 2005.  Back to cited text no. 15
    
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Battistoni G, Muraro S, Sala PR, Cerutti F, Ferrari A, Roesler S, et al. The FLUKA code: Description and benchmarking, proceedings of the hadronic shower simulation. Workshop 2006. Fermilab, 6-8 September 2006. AIP Conf Proc 2007;896:31-49.  Back to cited text no. 16
    
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Pinto TN, Cecatti SG, Gronchi CC, Caldas LV. Application of the OSL technique for beta dosimetry. Radiat Meas 2008;43:332-4.  Back to cited text no. 18
    


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