|Year : 2011 | Volume
| Issue : 1 | Page : 6-16
Thermoluminescence, optically stimulated luminescence and radiophotoluminescence dosimetry: An overall perspective
Bhuwan C Bhatt
C/o Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India
|Date of Web Publication||17-Mar-2012|
Bhuwan C Bhatt
C/o Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai
Source of Support: None, Conflict of Interest: None
Radiation dosimetric methods are used for the estimation of dose absorbed by radiation in a detector material. These methods are required for estimation of absorbed dose in various applications of radiation, such as personnel and environmental dosimetry, retrospective/ accident dosimetry and medical applications of radiation. The use of thermoluminescence (TL) as a method for radiation dosimetry of ionizing radiation has been established for many decades and has found many useful applications in various fields, such as personnel and environmental monitoring, medical dosimetry, archaeological and geological dating, space dosimetry. Several high sensitivity TL phosphor materials and thermoluminescent dosimeters (TLDs) are now commercially available in different physical forms. There are many commercial TLD systems which are being used for various dosimetric applications and even presently, TL is a major player in the field of radiation dosimetry, particularly in personnel dosimetry. In the last two decades an alternative technique, optically stimulated luminescence (OSL), has been developed, as the optical nature of the readout process does not involve problems of blackbody radiation and thermal quenching. Due to this and some other advantages OSL is also being used for various applications in radiation dosimetry, such as personnel and environmental dosimetry, retrospective/ accident dosimetry and medical dosimetry. The development of Al 2 O 3 :C TL/OSL phosphor by Akselrod et al. and later investigation of its suitability for personnel dosimetry using pulsed OSL (POSL) technique of stimulation by Akselrod and McKeever, resulted in the development of a personnel dosimetry system based on Al 2 O 3 :C OSL phosphor. Therefore, thrust of modern luminescence dosimetry development is more towards OSL. The main advantages of the small size optic fiber based OSL dosimeter over the currently available radiation detectors, such as TLD, used in clinical applications, are the capabilities of measuring both real-time dose rate (using radioluminescence, RL) and absorbed dose (using OSL). Although radiophotoluminscence (RPL) dosimeters were developed in parallel with TLD systems during 1960s, but high pre-dose and photon energy dependent detector material prevented major breakthrough of the glass dosimetry. Therefore, RPL glass dosimeters were used as an emergency dosimeter in accident situations. However, in mid 1980s introduction readout systems using a pulsed UV stimulation, in place of conventional mercury UV lamps, helped in reducing pre-dose by a factor of 100 (from mSv to a few μSv). Use of pulsed stimulation permits electronic discrimination of the signals from the pre-dose and absorbed dose on account of their different fluorescence decay times. This development resulted in the manufacture of improved RPL glass dosimeters and fully automatic RPL reader systems capable of measuring doses in the range 10μSv to 10Sv. In 2001, silver activated phosphate RPL glass dosimetry system has been introduced as the major personnel monitoring service in Japan marketed by Chiyoda Technol Corporation. Some of these developments in the field of TL, OSL and RPL dosimetry are reviewed.
Keywords: Radiation dosimetry, radiophotoluminscence, OSL dosimetry, TL dosimetry
|How to cite this article:|
Bhatt BC. Thermoluminescence, optically stimulated luminescence and radiophotoluminescence dosimetry: An overall perspective. Radiat Prot Environ 2011;34:6-16
|How to cite this URL:|
Bhatt BC. Thermoluminescence, optically stimulated luminescence and radiophotoluminescence dosimetry: An overall perspective. Radiat Prot Environ [serial online] 2011 [cited 2020 Sep 24];34:6-16. Available from: http://www.rpe.org.in/text.asp?2011/34/1/6/93897
| 1. Introduction|| |
Radiation dosimetric methods are used for the estimation of dose absorbed by radiation in a detector material using either thermoluminescence (TL) technique or optically stimulated luminescence (OSL) technique or radiophotoluminescence (RPL) or any other technique using passive solid state detectors. These dosimetric methods are required for estimation of absorbed doses in various applications of radiation, such as personnel and environmental dosimetry, retrospective/ accident dosimetry, medical applications of radiation and high LET dosimetry. The use of TL as a method for dosimetry of ionizing radiation has been established for many decades and has found many useful applications in various fields, such as personnel and environmental monitoring, medical dosimetry, archaeological and geological dating, space dosimetry (McKeever, 1985; McKeever et al., 1995; Kortov, 2007). There are many commercial TLD systems which are being used for various dosimetric applications and even presently, TL is a major player in the field of radiation dosimetry, particularly in personnel dosimetry (Cassata et al., 2002; Moscovitch et al., 2006; Jones and Stokes, 2011). However, thrust of modern luminescence dosimetry development is more towards optically stimulated luminescence (OSL) (McKeever, 2002; Botter-Jensen et al. 2003). OSL is a phenomenon in which an irradiated material when stimulated by an appropriate wavelength of light emits a light signal proportional to the absorbed radiation dose. The use of OSL for radiation dosimetry was first suggested in 1955 by Antonov-Romanovskii et al. (1956). It was later used by Braunlich et al. (1965) and Sanborn and Beard (1965). However, the use of OSL for various dosimetric applications in dosimetry started in mid 1990s. The main reason was non-availability of sensitive OSL phosphors till the development of Al 2 O 3 :C was reported by Akselrod et al. (1990) and later investigation of its suitability for personnel dosimetry using pulsed OSL (POSL) technique of stimulation by Akselrod and McKeever (1999). These led to the development of Landauer's Luxel TM personnel dosimetry system based on Al 2 O 3 :C OSL phosphor. Thus, the use of OSL as a personnel dosimetry technique started only in late 1990s. Since then OSL dosimeters are finding more and more applications in the field of personnel and environmental monitoring as well as in the field of medical physics (McKeever, 2004; Akselrod et al. 2007). However, the OSL has been popular technique for determination of environmental doses received by archaeological and geological materials in efforts to date those materials, following the introduction of the method in this area by Huntley et al. (1985). The main advantages of OSL technique are (McKeever and Moscovitch, 2003; Botter-Jensen et al., 2003):
- Sample heating is not required which also means that problems due to thermal quenching are removed;
- Optical nature of readout process also allows the use of low melting point dosimeter materials, namely, luminescence phosphors impregnated into a plastic matrix. Thus, robust dosimeters may be manufactured;
- The high sensitivity of OSL in some phosphors, such as Al 2 O 3 :C, also leads to advantages related to multiple readings (using POSL mode of readout) since it is often not necessary to stimulate all the trapped charge in order to read a sufficient luminescence signal, and
- Readout process can be made very fast (<1s) through adjustment of the stimulating light intensity leading to advantages associated with the rapid analysis of large number of dosimeters.
Radiophotoluminescent (RPL) glass dosimeters are integrating type solid state dosimeters, like TL and OSL dosimeters. The material used is silver-activated phosphate glass in the form of glass rods or square or rectangular glass plates. When this glass is exposed to radiation, stable luminescent centres (Ag 0 , Ag 2+ ) are created in the silver ions. Using ultraviolet excitation, the orange fluorescence emitted by the RPL glass is detected. The RPL signal is not erased during the readout and hence the dosimeter can be re-analysed many times.
Although radiophotoluminscence (RPL) dosimeters were developed in parallel with TLD systems during 1960s, but high pre-dose and photon energy dependent detector material prevented major breakthrough of the glass dosimetry. Therefore, RPL glass dosimeters were used as an emergency dosimeter in accident situations. LiF is one of the dosimetric materials which has been used as high-dose RPL dosimeter (Regulla, 1972; Miller and Endres, 1990).
However, in mid 1980s introduction of readout systems using a pulsed UV stimulation, in place of conventional mercury UV lamps, helped in reducing pre-dose by a factor of 100 (from mSv to a few μSv). Use of pulsed stimulation permits electronic discrimination of the signals from the pre-dose and absorbed dose on account of their different fluorescence decay times. This development resulted in the manufacture of improved RPL glass dosimeters and fully automatic RPL reader systems capable of measuring doses in the range 10μSv to 10Sv. In 2001, silver activated phosphate RPL glass dosimetry system has been introduced as the major personnel monitoring service in Japan marketed by Chiyoda Technol Corporation.
This article will endeavour to review the applications of TL, OSL and RPL techniques in radiation dosimetry.
| 2. Thermally and Optically Stimulated Luminescence Phenomena|| |
The absorption of energy from an ionizing radiation source by an insulating or semiconducting material causes the excitation of free electrons and holes and subsequent trapping of these electronic species at defects (trapping states) or metastable states within the material. The subsequent absorption of external energy by the metastable trapped charge results in the stimulated relaxation of the system back to its equilibrium condition. During the relaxation process recombination of the electronic charge occurs, and if the recombination is radiative, luminescence is emitted. [Figure 1] gives schematic representation of thermally and optically stimulated luminescence phenomena in an insulating or semiconducting material.
|Figure 1: Schematic representation of thermally and optically stimulated luminescence phenomena. Luminescence techniques (TL and OSL) monitor the charges as they undergo radiative recombination with charge of opposite sign. Photoluminescence (PL) and Radiophotoluminescence (RPL) are also indicated in the figure.|
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In TL the luminescence, emission is triggered by heating the material. OSL is a process in which an irradiated phosphor material when stimulated by an appropriate wavelength of light (UV, visible or infra-red) emits a light signal; the wavelength of the emitted light is characteristic of the OSL material. The optical energy required to release the charges from the traps depends on the depth of the trap. Thus, depending on the depth of the trap the optical stimulation energy may lie anywhere in the range infra-red to UV. The difference between TL and OSL is that in the latter optical stimulation is used instead of thermal stimulation to release the trapped charges. However, as in the case of TL, prolonged stimulation of the dosimeter depletes the concentration of trapped charges thereby zeroing the luminescent signal.
Radiophotoluminescence (RPL) is the luminescence stimulated by light from the defects which have been created by radiation, but which do not ionize during the optical stimulation (Perry, 1987; Botter-Jensen et al., 2003). As in TL and OSL, in RPL the light signal is proportional to the irradiation dose. The signal can be read any number of times from the irradiated material, however, resetting of the RPL signal can be done by a suitable heat treatment. Since photoluminescence (PL) and RPL processes do not involve transport of charge from one defect site to another (i.e. these are intraband transitions), such transitions do not affect the subsequent RPL signal, unless the excited state is thermally unstable at the temperature of RPL measurement.
| 3. TL, OSL and RPL Measurement Techniques|| |
3.1 Different readout modes for TL
The TL dosimeters can be read out either by using a linear temperature profile up to a temperature which is sufficient to cover a desired TL glow peak, or by using a clamped heating profile in which the final temperature can be clamped at a required fixed value, but in the beginning a high-initial heating rate may be used for the purpose of relatively faster readouts. Generally, peak height or area under the TL glow curve are used for dosimetric measurements.
3.2 Different readout modes for OSL
- The continuous wave OSL (CW-OSL) method in which the stimulation light intensity (φ) is kept constant and the OSL signal monitored continuously throughout the stimulation period.
- Linear modulation OSL (LM-OSL) method in which the stimulation intensity is ramped linearly (i.e. is linear modulation ramp rate) while OSL is measured. Non-linear stimulation of light intensity is also reported (Botter- Jensen et al., 2003; Mishra et al.,2008 and Bos and Wallinga, 2009) and
- Pulsed OSL (POSL) method in which the stimulation source is pulsed and the OSL is monitored only between the pulses.
The CW-OSL method has found particular popularity in retrospective dosimetry applications, particularly geological dating and accident dosimetry (Botter-Jensen et al.,2003).
3.3 RPL readout mode
Using pulsed ultraviolet (~365 nm) laser stimulation, the orange (~620 nm) luminescence emitted by the irradiated glass is detected using a RPL reader system.
| 4. Important Areas of Applications in Radiation Dosimetry|| |
- Personnel dosimetry
- Environmental dosimetry
- Retrospective dosimetry
- Dosimetry in medical applications of radiation
- Neutron dosimetry
- Space dosimetry
4.1 Personnel monitoring
The primary objective of personnel monitoring/individual monitoring is the measurement or assessment of radiation dose delivered to personnel during their occupational exposure (ICRP-60,1991). Examples include workers in nuclear industry, hospital radiotherapy technicians, workers in industrial radiography and high intensity gamma irradiators and naval personnel on nuclear powered vessels. In addition, the objectives of personnel monitoring for external exposures include the following (ICRP-60, 1991; ICRP-75, 1997):
- It is intended to provide information on the external radiation exposure of individuals working with radioactive materials and/or radiation producing devices;
- Personnel monitoring results provide information on routine exposures, assist in work planning, allow control of the workplace, and provide exposure information in accident situations;
- By means of such monitoring it is hoped to limit the exposure of such personnel to within prescribed limits, which are based on recommendation of international and national bodies;
- Assessment of effective dose and, if appropriate, equivalent doses for compliance purposes;
- In addition, these results assist those responsible for radiation safety in keeping exposures as low as reasonably achievable (ALARA).
Solid-state dosimeters used for personnel monitoring include TL, OSL and RPL dosimeters. There are about five million personnel dosimetry badges being used by radiation workers around the world (2002 data; Botter-Jensen et al., 2003), which include thermoluminescence dosimeters (TLDs), film badges, optical stimulated luminescence (OSL) dosimeters and radiophotoluminescence (RPL) dosimeters. Approximately 25% of the five million badges are OSL dosimeters, primarily based on pulsed OSL (POSL) from Al 2 O 3 :C. About 8.4% (4,20,000) of the total badges are RPL glass dosimeters, primarily based on relatively low-Z silver-activated phosphate glass dosimeters using the commercial system marketed by Chiyoda Technol Corporation, Japan. [Table 1] gives different personnel monitoring systems being used for individual monitoring of radiation workers.
|Table 1: Different TLD/OSLD/RPL dosimeter- based personnel monitoring (PM) systems being used for individual monitoring of radiation workers (Olko et al., 2006: Bhatt, 2006).|
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[Table 2] gives general characteristics of some commercially available thermoluminescent dosimeters relevant for personnel dosimetry, while the [Table 3] gives characteristics of some of the OSL phosphors relevant for their application in personnel and radiation dosimetry.
|Table 2: General characteristics of some commercially available thermoluminescent dosimeters relevant for personnel dosimetry.|
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|Table 3: Characteristics of some OSL materials relevant for personnel dosimetry (Yukihara and McKeever, 2011; Pradhan et al, 2008; Kulkarni et al., 2005)|
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| 4. OSL Based Personal Dosimetry Systems|| |
4.1.1 Landauer's Luxel personal dosimetry system
The OSL detector is a thin layer of Al 2 O 3 :C powder deposited onto a clear polyester film base. The size of the active area of the detector is 16.5 mm by 18.5 mm and the grain size of the Al 2 O 3 :C powder is in the range 20-90 μm. The powder layer is protected by a thin, clear polyester tape (Botter-Jensen et al., 2003).
A typical POSL measurement takes 350-1000 ms per reading, and each reading depletes the POSL signal by only a fraction of the stored information available. Thus, second, third, or more readings may be performed if required, in order to achieve true second dose readings from one dosimeter (Botter-Jensen et al., 2003, Yukihara and McKeever, 2011).
4.1.2 Landauer's InLight personal dosimetry system
A new development in OSL personal dosimetry is the InLight system, also by Landauer Inc. This is a bench-top, CW-OSL system specifically designed for personal dosimetry and uses bright LEDs (green) to stimulate the OSL. It is again based on Al 2 O 3 :C. Four Al 2 O 3 :C powder films are used as dosimeters. The use of LEDs results in longer readout times compared with POSL, but enables dose readings for very little depletion of the signal. It is reported that this system in turn leads to a different paradigm for personal dosimetry badge wearers-namely that the InLight badges are normally distributed to individuals for a full year, with periodic readings taken twice every month during normal operation. The stimulation power is such that the twice monthly readings deplete the signal by only approximately 10% over one year. InLight is designed for organizations that wish to perform their own dosimetry, whereas Luxel is designed for use by radiation dosimetry service providers (Botter-Jensen et al., 2003).
Sintered beryllium (BeO) ceramic has been explored for TL (Tochilin et al., 1969; Crase and Gammage, 1975) and OSL dosimetry (Bulur and Goksu, 1998). Recent investigations (Sommer and Henniger, 2006; Sommer et al. 2008) have shown that the OSL of BeO with blue (~455 nm) light stimulation is most effective and doses as low as 1μGy can be measured with linear response up to 5 Gy. TL glow curve of BeO shows three peaks at ~75, 220 and 340 o C for glow curve recorded up to 450 o C at a heating rate of 5K/s (Bulur and Goksu,1998). Detailed studies (Bulur and Goksu,1998; Bulur and Yeltik,2010 and Watanabe et al.,2010) show that OSL signal originates from the traps which become unstable near 340 o C. BeO is thus a useful material for radiation dosimetry due to its high OSL sensitivity and low effective atomic number (Z eff =7.14) similar to tissue (Z eff =7.42) and water (Z eff =7.51). Recently, a new OSL personnel dosimetry system based on BeO sintered chips has been developed in Germany (Sommer et al., 2010).
4.1.3 RPL glass based personnel dosimetry system
In 2001, silver-activated phosphate glass RPL dosimetry system, using pulsed UV laser excitation, was introduced as the major personnel monitoring system in Japan (Juto, 2002). The RPL glass dosimetry badge system has been designed to measure photons in the range 10 keV to 10 MeV and beta radiations in the range 300 keV to 3 MeV. In the year 2008, IRSN in France purchased the automatic glass dosimetry system of Chiyoda Technol Corporation, Japan for replacing the photographic film-based monitoring system (IRSN, France). RPL dosimeters are not expected show fading in room light condition. This in contrast to the OSL dosimeters which are stimulable by blue or green light, thus will show fading in room light. [Table 4] gives comparative advantages of TL, OSL and RPL dosimeters.
4.2 Environmental dosimetry
In the last three decades regulatory authorities in many countries have become more acutely aware of the increasing concern demonstrated by the public with regard to the potential environmental impact of "man-made" radiation exposure such as:
- controlled releases of gaseous radionuclides from nuclear power stations during day-to-day operations
- low-level waste disposal
- nuclear fuels reprocessing
- incidents of nuclear power station accidents, and
- activities connected with nuclear power industry.
In many countries TLD systems are in place near nuclear installations for the purpose of monitoring pre-operational levels (background levels) as well as levels above the natural background, which can be linked with the operation of these facilities. The capability of using the OSL response of Al 2 O 3 :C to measure the environmental photon radiation over short periods was tested by Botter-Jensen et al. (2003). The response of Al 2 O 3 :C compares well with that of high-pressure ionization chamber (HPIC). Even at low doses, the OSL uncertainties are small and the OSL measurements agree very well with those obtained with ionization chamber. It is reported (Ranogajec Komor et al., 2008) that the recently developed SC-1 flat RPL glass dosimeter by Chiyoda Technol Corporation, Japan fulfils the requirements of IEC-61066:2006 standard for personal and environmental dosimetry (IEC, 2006).
Optical fibers offer a unique capability for remote monitoring of radiation in difficult-to-access and/or hazardous locations, such as down wells to monitor ground water or inside of nuclear reactors and subsystems (Huston et al., 2001). Optical fiber sensors can be located in radiation hazardous areas and optically interrogated from a safe distance. A variety of remote optical fiber radiation dosimetry methods have been developed. Thus optical fiber dosimeters provide a unique opportunity to perform remote radiation dosimetry measurements under a variety of environmental conditions that preclude or limit the use of any other dosimetry techniques.
4.3 Retrospective accident dosimetry
In situations where no direct radiation monitoring data are available, luminescence dose reconstruction obtained using material from the immediate environment of population or from the persons in the vicinity of the incident/accident can be used to validate values obtained from computational techniques. TL technique has been successfully employed in dose evaluation in Hiroshima and Nagasaki Atomic bomb explosion sites in Japan (Ichikawa et al.,1966; Hoshi et al., 1989), radioactive fallout of nuclear tests at Nevada test sites in USA (Haskell et al., 1988) and radioactive fallout from the Chernobyl nuclear reactor accident (Hutt et al.,1993). The OSL technique has also been applied to retrospective dosimetry using environmental materials from the Chernobyl accident area (Bailiff et al., 2005; Banerjee et al., 2000).
OSL properties of memory chip modules (from telephone, ID and credit cards), ceramic resistors from mobile phones and other electronic components from personal electronic devices have shown interesting and promising results for their use in accident dosimetry (Goksu, 2003, Mathur et al., 2007; Inrig et al., 2008; Woda and Spottl., 2009; Woda et al., 2010). Recently, the European Radiation Dosimetry Group10 on Retrospective Dosimetry has published a "Review of Retrospective Dosimetry Techniques for External Ionizing Radiation Exposures" (Ainsbury et al., 2011).
Dental ceramics are of interest as a luminescence dosimeter because of their potential to provide a means of determining cumulative exposure to external gamma radiation arising from accidents. TL and OSL properties of dental ceramics have been studied in the dose range 100 mGy to 10 Gy (Bailiff et al., 2002; Veronese et al.,2010). Attempts to measure OSL from tooth enamel for accident dosimetry applications have been reported; this is being investigated with a view of development of an in-vivo dose assessment technique for medical triage following a radiological/nuclear accident or terrorist event (Yukihara et al., 2007; DeWitt et al., 2010).
4.4 Applications of TL, OSL and RPL phosphors in medical physics
One of the important applications of TL and OSL dosimetry has been in the field of medical physics for their application in radiodiagnosis, nuclear medicine and radiotherapy. Thermoluminescent dosimeters (TLDs) have become popular in these fields due to their high sensitivity, miniature size, tissue equivalence, high stability to environmental conditions, low TL fading, reusability, linear dose response and sufficient precision and accuracy. Integrating mode of TL phosphors, such as LiF:Mg,Ti, LiF:Mg,Cu,P, Al 2 O 3 :C, has been widely used for clinical dosimetric measurements- central axis depth-dose curves, in-phantom measurements, in-vivo dosimetry, surface doses, quality assuarance etc.- of high energy photon and electron beams(Kron, 1999). As per the published literature (Akselrod et al., 2007; Pradhan et al., 2008; Andersen et al. 2009; Yukihara et al., 2010) OSL dosimeters (OSLDs), such as Al 2 O 3 :C, can be used as passive dosimeters for various applications, including in-vivo dosimetry and charged particle high LET dosimetry, in medical physics in the same way as TLDs. High sensitivity, precise delivery of light during optical stimulation, fast readout times, simpler readers and easier automation are the main advantages of OSL in comparison with TLD. OSL allows for re-reads of the detector multiple times while maintaining the precision, and yet it still can be used as an erasable measurement technique. The use of Al 2 O 3 :C, along with the development of highly sensitive OSL measurement technique has led to its increasing applications in the field of medical physics in the dose range 10 μGy to 10 Gy.
A new radioluminescence (RL)/OSL fiber optics system using Al 2 O 3 :C fiber sensors, has high potential for in-vivo and in-vitro dosimetry in both radiation therapy and diagnostic mammography for real-time / on-line mode of measurements (Polf et al. 2002; Akselrod et al., 2007; Andersen et al., 2009). The main advantages of the small size optic fiber dosimeter over the currently available radiation detectors, such as TLD, used in clinical applications are the capabilities of measuring both real-time dose rate (from RL signal) and absorbed dose (from OSL signal). Another advantage is that these measurements can be performed remotely by using a fiber optics coupled to the detector systems. A variety of RL/OSL optical fiber probes has been tested using photon and soft X-ray beams at the different medical facilities.
Knezevic et al.(2010) report that GD-352M RPL glass dosimeter with FGD-1000 reader (Dose Ace system) developed for medical applications were investigated for uniformity/batch homogeneity, reproducibility, linearity, detection threshold, energy dependence in air and on phantom. Some characteristics were compared to two kinds of LiF;Cu,Mg,P TLDs: GR-200A (China) and TLD-100H (Harshaw). The characteristics investigated fulfil the requirements of the IEC 62387-1:2007 Standard for personal and environmental dosimetry (IEC, 2007). Nose et al.(2005) have reported in-vivo dosimetry of high-dose-rate brachytherapy for study carried out on 61 head and neck cancer patients using RPL glass dosimeters. New radiophotoluminescent glass rod dosimeters (RPL-GRDs) were also tested recently for possible mailed dosimetric services, and their performance was compared with LiF:Mg,Ti (Rah et al., 2009).
4.5 Neutron response of TL, OSL and RPL dosimeters
TL, OSL and RPL dosimeters are being used for the detection of X-, gamma and beta radiations, and these methods are well established, reliable and quite accurate. However, direct fast-neutron response of TL, OSL and RPL glass dosimeters is very small because of poor cross-section of the constituent elements to most of the nuclear reactions at high neutron energies. Al 2 O 3 :C has neutron sensitivity lower than 7 LiF:Mg,Ti (TLD-700) (Klemic et al.,1996).Thermal neutron sensitivity of RPL glass dosimeter is less than 1% when compared with its sensitivity to gamma radiation on mGy - to - mGy basis(Juto, 2010). Thus measurement of dose due to fast neutrons remains one of the challenging tasks in TL, OSL and RPL radiation dosimetry.
The luminescent materials that contain Li (e.g. LiF:Mg,Ti and LiF:Mg,Cu,P) or simultaneously Li and B (e.g. Li 2 B 4 O 7 :Mn and Li 2 B 4 O:Cu) are most sensitive to thermal neutrons. If these materials are enriched in 6 Li and 10 B isotopes, they become highly sensitive to thermal neutrons due to the reaction 6 Li(n,α) 3 H with a large cross-section (945b) and the reaction 10 B(n, α) 7 Li with cross-section 3840b. It should be noted that the above-mentioned detectors (e.g. LiF:Mg,Cu,P) also provide good sensitivity to gamma radiation, a wide useful range and very small fading. If the detector material LiF:Mg,Cu,P is enriched with 6 Li isotope, it may prove to be an efficient dosimeter for mixed (gamma and neutron) fields. The dosimetry of mixed (γ+n) fields is important at nuclear power plants.
Recently, Yukihara and co-workers (Yukihara et al., 2008; Mittani et al., 2007) have demonstrated the possibility of using neutron converter ( 6 LiF) incorporated into Al 2 O 3 :C dosimeters to produce new neutron-sensitive OSL dosimeters suitable for personnel monitoring. Based on their results, a new neutron-sensitive OSL composite material, identical to the one used in the Luxel TM and InLight TM dosimetry systems (Landauer Inc.) except for the smaller gamma sensitivity and enhanced neutron sensitivity, was produced and characterized. The gamma sensitivity of the new material is ~35% the sensitivity of the regular Al 2 O 3 :C material (Luxel TM ), due to reduced amount of Al 2 O 3 :C in its composition. The neutron sensitivity achieved in this new material is ~60% of the neutron sensitivity of LiF:Mg,Ti (TLD-600). The neutron dose measurement requires the use of the dosimeter in albedo configuration. Although the relative neutron to gamma sensitivity of the basic Al 2 O 3 :C detector was about one tenth of that of TLD-700, it was possible to enhance its neutron response by incorporating 6 LiF into the dosimeter.
4.6 Space dosimetry
The radiation environment in space is a complex mix of charged particles: medium and high-energy protons, electrons, alpha particles and high-energy heavy ions, over wide energy ranges and with varying fluxes (McKeever, 2002). Astronauts working in Low Earth Orbit (LEO) are exposed to a radiation level, which is about 100 times higher than the natural radiation level on Earth and will be further increased for travels to Mars (Bilski et al., 2010). Due to the variety of routine space crew activities on the International Space Station (ISS) and the Space Shuttle, there is a need to monitor the individual radiation exposures of the crew members for reliable estimation of radiation risk to them. TL and OSL detectors have good efficiency for LET of radiation <10 keV/μm, with decreasing efficiency, although non-zero, for higher LET (>10 keV/μm) radiation (Yukihara et al. 2004; Yukihara et al., 2006; Yukihara and McKeever, 2011). Therefore, no single dosimeter will provide absorbed dose estimates across the full LET spectrum of space radiation. Thus, combinations of dosimeters are required to estimate the dose equivalent due to space radiation. Currently, this is performed using passive detectors such as thermoluminescent detectors (TLDs) or optically stimulated luminescence dosimeters (OSLDs) and plastic nuclear track detectors (PNTDs). However, suitable correction needs to be applied for the response of TLDs or OSLDs for LET >10 keV/μm.
The National Council on Radiation Protection and Measurements (NCRP) recommended (NCRP Report 142, 2002) that OSL or TL detectors (i.e. Al 2 O 3 :C or LiF:Mg,Ti/ LiF:Mg,Cu,P), should be used as passive radiation detectors for the low linear energy transfer (LET) component of the space field (i.e. region of LET <10 keV/ mm and Q=1), while the high-LET component should be measured using plastic nuclear track detectors (PNTDs), such as CR-39 (i.e. region of LET >10 keV/μm and Q=Q(L)). CR-39 PNTDs are sensitive to high-LET radiation (>10keV/μm) and insensitive to low-LET radiation (<10 keV/μm). Therefore, the dose equivalent is defined as follows:
Where D OSLD/TLD is the absorbed dose measured by OSLD or TLD; D PNTD is the absorbed dose measured by PNTD and Q(L) is quality factor.
4.7 TL and OSL studies on nanophosphors
The processes which determine the efficiency of energy transfer to emission centers and radiative efficiency are expected to change in nano-crystalline material as compared to corresponding bulk phosphor material (Kortov, 2007). Recently, Blair et al.(2010) have investigated the radiation dosimetry properties of Al 2 O 3 nanopowders prepared by solution combustion synthesis(SCS). The potential advantages of the SCS method are reported to be: i) ease of production, ii) scalability and iii) ability to change the synthesis conditions (e.g. dopants, fuels, annealing procedure, etc.). However, TL (and OSL) sensitivities were 1/100 th of the Al 2 O 3 :C commercial phosphor (Blair et al., 2010). They comment that reduced TL (and OSL) sensitivity of nanophosphors may be due to fewer oxygen vacancies in the crystal lattice. Therefore, they suggest that future experiments in Al 2 O 3 system should concentrate on introducing dopants such as carbon to catalyze vacancy production in the host. Patil and co-workers (Patil, 2010) have used a slightly different synthesis method, using organic precursor method, for production of Al 2 O 3 :C phosphor. They have observed that its OSL sensitivity was about 20% of the Al 2 O 3 :C commercial phosphor, while TL sensitivity was considerably smaller. OSL decay of the synthesized phosphor was found to be faster than the Al 2 O 3 :C commercial phosphor. Efforts to improve OSL sensitivity are being pursued.
TL studies on CaSO 4 :Dy nanophosphors have been reported (Salah et al., 2006). Multiple peaks were observed in the case of nano-crystalline material. TL sensitivity of the nano-crystalline phosphor was found to be less than that of the micro-crystalline phosphor. But, the dose vs. TL response of nanophosphor does not saturate up to the studied dose of 10 4 Gy, whereas the response of the dosimetry peak in micro-crystalline material saturates at about 10 3 Gy. Nano-crystalline LiF:Mg,Cu,P phosphor on irradiation with 48 MeV 7 Li ion beam shows glow curve with a prominent peak at ~588K, besides a smaller peak at ~410K, however, irradiation with X-rays shows a drastic increase in the low temperature peak, while the high temperature peak nearly vanishes (Salah et al., 2008), the intensity of the former was found to increase with increasing the ion fluence. They conclude that nano-crystalline LiF:Mg,Cu,P phosphor may be useful for dosimetry of high LET beams as well as mixed fields.
| 5. Conclusions|| |
- Presently, TL, OSL and RPL dosimeters are being used for various applications in the field of radiation dosimetry.
- The optically stimulated luminescence (OSL) technique is gaining interest and importance due to its many useful characteristics for its application in different fields. The all-optical nature of OSL readout process allows the use of dosimeter samples in many different physical forms (chips, fibers, powders, thin films, etc). Al 2 O 3 :C is the most sensitive and versatile OSL dosimetry material.
- The main advantages of the small size optic fiber dosimeter over the currently available radiation detectors, such as TLD used in clinical applications, are the capabilities of measuring both real-time dose rate and absorbed dose.
- Development of many new OSL phosphors, such as Cu-doped, Eu-doped, Tb-doped phosphors, including Al 2 O 3 nanophosphors, which exhibit faster decay, is being reported. Fast decaying RL/OSL phosphors have important applications in medical dosimetry.
| 6. Acknowledgements|| |
The author is thankful to Dr. A.K Ghosh, Director HS & E Group, BARC and Dr. D. N Sharma, Associate Director, HS & E Group and Head RSSD for their encouragement and support. The author is also thankful to DST for the fellowship.
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[Table 1], [Table 2], [Table 3], [Table 4]