A versatile integrated system for thermoluminescence and optically stimulated luminescence measurements

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 262 (2007) 348–356 www.elsevier.com/locate/nimb

A versatile integrated system for thermoluminescence and optically stimulated luminescence measurements M.S. Kulkarni *, D.R. Mishra, D.N. Sharma Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Received 25 January 2007; received in revised form 15 April 2007 Available online 25 May 2007

Abstract A versatile integrated reader system for TL and OSL measurements of phosphor materials has been described for luminescence research applications. The developed integrated reader system works either in TL or OSL or TL–OSL mode. In the OSL operation, besides the conventional CW-OSL, POSL and LM-OSL modes a novel non-linear OSL (NL-OSL) method has been incorporated in the reader system. The optical stimulation unit consists of four high power LEDs fitted in four channels and optically focused on the sample. Each of the LED is capable of delivering up to 80 mW/cm2 light power at the sample position. The LEDs with peak wavelength kp  470 nm and 530 nm and Dk  20 nm have been used for optical stimulation of the samples. A PID temperature controller has been used for generating and controlling user defined heating profiles for the TL measurements in the reader system. The reader system covers a wide dynamic dose range of 10 lGy to 103 Gy for TL/OSL measurements. The OSL grade a-Al2O3:C phosphor was used to test the reader system and investigate its impact on low dose assessment for personnel and environmental monitoring. The design concept of the reader system and the results of dose measurements are discussed.  2007 Elsevier B.V. All rights reserved. Keywords: Thermoluminescence; Optically stimulated luminescence; Al2O3:C; TL/OSL reader system

1. Introduction The thermoluminescence (TL) or optically stimulated luminescence (OSL) emission from an irradiated sample arises upon stimulation with heat or light respectively. During thermal/optical stimulation the trapped charges are released from metastable trap levels and subsequently recombine with charges of opposite polarity and in the process emit light. The minimum measurable dose and dynamic dose range of TL/OSL phosphor material depends considerably on the sensitivity of the measuring system. The development of sensitive TL and OSL reader systems for the study of phosphor materials is therefore equally important, as provision of such integrated readout system and associated instrumentation leads to optimization of various TL and OSL parameters to carry out *

Corresponding author. Tel.: +91 22 25595076; fax: +91 22 25505151. E-mail address: [email protected] (M.S. Kulkarni).

0168-583X/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.05.013

advance studies on phosphor materials for the radiation dosimetry applications [1–5]. Such advance reader systems would also be helpful for the basic TL and OSL research in the natural and synthetic phosphor materials. The programmable OSL reader systems [6–8] are in great demand in all the research and academic institutions for their applications in advanced radiation dosimetry research. However, such OSL measurement systems are not commonly available for the routine personnel and environmental radiation monitoring, medical and research applications unlike commercial TL reader systems. Therefore, most of the researchers assemble in-house reader systems to suit their specific requirements. Secondly, the automated OSL measurement systems commercially available for the OSL measurements are somewhat biased for their use in environmental dosimetry for dating of archaeological and geological samples. Such reader systems are very expensive and offer limited flexibility for researchers who may want to experiment with a variety of excitation

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sources at different light intensities and may require modification in the experimental setups accordingly. The most often used stimulation sources are blue, green and infrared LEDs in the OSL readers. A cluster of blue light emitting diodes (LEDs) has been reported to be an effective stimulating light source for OSL measurement of Al2O3:C [4,6]. The maximum light intensity of such LED cluster at the sample position was reported to be 100 mW/cm2 [9] and was found to be useful for research applications. In the recent past, there has been a considerable progress in the development of high intensity blue, green and infrared LEDs which can be used in the OSL reader systems as an alternative to expensive laser systems. A major concern in the design of OSL reader systems is to ensure that the light detection system should detect only the luminescence emitted from the sample without responding to the stimulating wavelength of incident light. Hence, suitable band pass filters are normally interposed between the sample and the photomultiplier tube (PMT) which totally block the stimulation wavelength and allow to pass only the emission wavelength from the sample. The OSL signals can be recorded using continuous wave OSL (CW-OSL), pulsed OSL (POSL) and linearly modulated OSL (LM-OSL) techniques. In CW-OSL mode, the stimulation intensity is kept constant (in time domain) throughout the readout and the decay of OSL signal with time is recorded. In POSL mode, a train of short pulses (<500 ns) of stimulation light are flashed on the sample and the emitted luminescence is synchronously detected in the period between the stimulation pulses [11,12]. OSL reader system having a facility of user defined variable pulse width for POSL measurements has also been reported by Danby et al. [10]. In this mode, the total luminescence signal measured between the desired pulses forms the POSL. However, stopping of stimulating light pulses, results in decay of OSL signal with a tail corresponding to the decay constant related to the luminescence lifetime of recombination centre of the material under study [11]. One of the advantages of the POSL technique is that, it requires less optical filtering to discriminate between the stimulation light and the emission light as compared to other OSL techniques. In the LM-OSL mode, the stimulating intensity is increased linearly as a function of time to obtain the OSL curve [13,14]. This leads to multiple OSL peaks corresponding to different photoionization cross-sections associated with various trap levels [15]. In addition to the linear modulation of OSL, other schemes can be imagined in which intensity could be modulated non-linearly. For example one can imagine an exponentially increasing stimulation intensity /(t) = /0 exp(t). This scheme can have advantages while emptying a range of traps with photoionization cross-sections that can differ by order of magnitude [5]. A new method of non-linear light modulation to obtain the OSL (NL-OSL) curve has been recently reported [16]. In this NL-OSL method, the stimulation intensity on the sample is modulated with a non-linear light profile with respect to time. The stimulat-

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ing light intensity at the sample position increases non-linearly as / = ctl; where / is the stimulation intensity seen by the sample and expressed in (photons/cm2/s), c is the parameter of NL-OSL stimulation and has dimensions of photons/cm2/s(l+1) and l can take values like 0.1, 0.5, 1, 2, etc. The value of l determines the power of light modulation in time domain, for example, l = 1 will produce linear light modulation (LM-OSL) and l = 2, parabolic non-linear light modulation, and so on. Hence l can be termed as time base power (TBP) of light modulation. The work presented here is focused on the indigenous approach to the development of an integrated and stand alone, highly sensitive TL and OSL measurement system. Although the present experimental setup is similar in design to the systems available commercially, it provides flexibility in terms of different stimulation sources with variable stimulation intensities, incorporation of CW-, LM-, NL- and pulsed-OSL options and elevated temperature OSL measurements in a single setup. A separate port has been provided for optical filter assembly to suit to the requirements in the experiments. The diverse design parameters covering the mechanical assembly, electronic hardware and software, optics, etc., highlighting the simplicity and flexibility of the systems for advance radiation dosimetry applications are discussed. 2. Instrument design A conceptual schematic block diagram of the instrument is shown in Fig. 1. The reader system consists of two parts. The first part is comprised of TL and OSL measuring instrument and the second part is a personal computer which controls the entire reader operations through an RS-232 serial interface. The main TL and OSL measuring instrument (reader assembly) is equipped with a light tight drawer assembly consisting of a 12 · 12 mm2 sample holder made up of 0.4 mm thick Kanthal strip which also acts as a heater planchet during TL measurements, power LED cluster based optical stimulation unit, a blue enhanced photodiode (OSD-5) to measure stimulating light intensity at the sample position, PMT for luminescence detection, optical filters to cut off the stimulation light from reaching the PMT and a 6 mm diameter reference light source (63Ni source coupled with plastic scintillator [20]). The operation of the reader system is controlled through a single chip Phillips 89C668 microcontroller based electronic circuitry comprising of a programmable dc high current driver for LED, a PID temperature controller based heater assembly for TL measurement, etc. The entire operation of the reader system is controlled by personal computer through assembly language software residing in the Phillips 89C668 microcontroller housed inside the reader assembly. The distance between PMT photocathode and planchet position was kept 4.5 cm. The cross-sectional view of the optical stimulation assembly and the drawer assembly are shown in Figs. 2(a) and 2(b). The system can be used for analysis of material up to 10 · 10 · 2 mm3 size.

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Fig. 1. A schematic block diagram of the TL/OSL instrument.

2.1. Optical stimulation assembly

Fig. 2(a). Schematic diagram of the stimulation light assembly.

The optical stimulation (photo excitation) of the samples is carried out by power LED clusters (Luxeon Blue LXHL-LB5C and Green LXHL-LM5C). These high power LEDs have a very high luminous intensity and brightness of conventional lighting that yields an output of 5.0 W at kp  470 nm or 530 nm and Dk  20 nm. The optical stimulation unit consists of four such LEDs placed in four channels facing each other at an angle of 30 with the vertical axis. Two oppositely placed channels have blue LEDs (kp  470 nm) while the other contains green LEDs (kp  530 nm). The user can select the blue or green light stimulation depending on the OSL samples under study

Fig. 2(b). The cross-sectional view of the drawer assembly.

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and the current is passed through the selected LEDs only. A specially designed heat sink is provided with each LED cluster and during the stimulation cold nitrogen gas is flushed continuously on it at a flow rate of 3 LPM to ensure that the junction temperature remains well within the tolerance limit. The top view of the drawer assembly is shown in Fig. 3. The complete drawer assembly showing the sample drawer, four stimulating light channels, the PMT housing is shown in Fig. 4. The operating life of the LED cluster (defined in terms of lumen maintenance – the percentage of initial light output remaining after a specified period of time) is 70% lumen maintenance at 50,000 hours of operation at a 700 mA forward current. As the junction temperature changes the wavelength

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characteristics of the LED; the LED forward current is restricted to 500 mA so that maximum junction temperature can never be exceeded. A copper heat sink is provided with each LED cluster as shown in Fig. 2(b) . GG435 optical color glass filters are placed on the blue and green LED clusters inside the channel to cut off the stimulating wavelength below 435 nm. A UG-1 optical color glass filter has been used across the PMT to prevent the stimulating radiation from reaching the PMT. However, based on the dominating luminescence emission (during TL and OSL) from the phosphor materials, appropriate UV band pass optical filters can be interposed between the sample and the PMT as per the requirement of the experiment. 2.2. Thermal stimulation assembly

Fig. 3. The top view of the optical stimulation assembly.

The thermal stimulation assembly consists of a high resistance, low mass alloy of Kanthal strip (12 · 12 mm2) with a Cr–Al (K-type) thermocouple spot welded on it as shown in Fig. 5. A PID temperature controller (Eurotherm 2416) controls the temperature of the heater strip within ±0.1 C using feedback from the thermocouple. The user can select the heating rates over a wide range (0.1–40 C/s) through the Eurotherm temperature controller. Alternatively, the heating profile can be generated through the 89C668 based software and can be fed to the analog set point of the temperature controller. This feature can be easily incorporated in the present system. In practice, our own experience [17,18] in designing the TL readers shows that heating of the TL sample up to temperature of 400 C with heating rates up to 10 C/s, is found to be adequate for routine TL research. However, for investigations involving studies of deep traps in certain TL phosphors, the reader system has been provided with variable clamping temperatures up to 650 C. 2.3. Light detection assembly The photon-counting module type EMI P25232 comprising of 9111B, 25 mm diameter end window PMT with blue–green sensitive bialkali photocathode having average dark count 13 cps is used in the reader system for light

Fig. 4. The drawer assembly used in the reader system showing the four stimulating light channels with gas cooling for the power LED clusters.

Fig. 5. The actual sample holder.

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detection and recording the OSL emitted by the samples. The photon-counting module is interfaced to computer through an RS-232 serial interface. The detection of light intensity of the stimulation wavelength per unit area at the sample position is the quantity of practical importance in the OSL measurements as it is used to determine the photoionization cross-section of the trap levels of the OSL sample. Hence, the stimulation light has to be characterized for its power, energy, stimulation intensity (/), rate of increase in stimulation intensity (c), etc. A UV enhanced photodiode (OSD-5) (as shown in Fig. 5) has been used in the reader system to determine the stimulation light intensity at the sample position for the given wavelength. The photodiode is operated in reverse bias as shown in Fig. 6. The light intensity at the sample position can be varied in the range of 0.02–100 mW/cm2 (with a percentage standard deviation of ±5%) by controlling the DC current through the LED cluster. The light intensity at the sample position is measured to be 100 mW/cm2 for a 500 mA dc current through the blue LED cluster.

2.5. Software The assembly software for the Phillips 89C668 was developed in C++ using Kiel cross compiler. The operation of the entire Reader system is controlled by the 64 Kbyte assembly software on command from the computer. Upon selection of a particular mode of operation (TL/OSL), the software generates the user defined voltage function in time domain. A basic internal delay of 250 ls has been generated using the interrupt overflow of programmable logic array (PLA). The track of the program cycle time and the set readout time are measured with respect to this internal timer. A windows based software package is developed in LabWindows/CVI (National Instruments) for use with integrated TL/OSL reader system. The Windows based software package provides a

START

2.4. The electronic circuitry The reader is based an a single chip microcontroller (Philips 89C668) and the basic hardware consists of five 12-bit DAC (MAX 539), a 12-bit ADC (MAX 1241) with multiplexed eight analogue channels, a 16-bit fast counter capable of counting up to 115 MHz frequency, MAX232 for serial transfer of data to the computer. The glitches in the output of the 12-bit DAC are removed by the sample-hold IC (LF398N). A second order Butter-worth filter across each deglitching circuit further removes the noise spikes and smoothens the voltage input to the LED current driver circuit. The assembly language software residing in 89C668 microcontroller monitors various circuits, generates various voltage profiles and time delays for the CWOSL, POSL, LM-OSL, NL-OSL and TL operations. After the readout is over, the readings are normalized and mean dose is calculated using the software.

Light Source (LS) Check

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Fig. 6. The photometer circuit based on OSD-5 photodiode using the FET input operational amplifier.

Fig. 7. Flow chart of the TL/OSL reader operations.

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versatile user friendly interface, minimizes operator intervention and ensures data integrity in case of any failure in hardware or software. The diagnostic software continuously monitors the vital circuits of the reader such as temperature of the heater strip, the intensity of the stimulation light source. The software also provides extensive facilities for data handling, analysis, dose estimation, trouble shooting, etc. When the system is switched on, the user can select the basic mode of operation; i.e. TL, OSL, or simultaneous TL–OSL mode. In TL mode the user can select the start and final clamping temperatures, different temperature profiles (linear, non-linear), etc. In the OSL mode, CW-OSL, LM-OSL and non-linear (NL-OSL) modes of operation are provided. Fig. 7 depicts the operation of the reader system under various modes in the form of a flowchart. In CW-OSL, user can set the stimulation intensity in the range 0.02–100 mW/cm2. In the LM-OSL mode, the user can select initial and final stimulation intensities and the ramp rate (rate of change) of stimulating intensity and the LM-OSL curve is recorded as OSL glow peak(s) depending upon different values of cross-sections associated with OSL trap levels in the sample under study. In the NL-OSL mode, in addition to the initial and final intensity, the values of c, and ‘l’, the time base power, need to be provided for generating non-linear stimulating intensity at the sample position. In the simultaneous TL/OSL mode, user can independently select the TL readout parameters (constant clamping/linear heating) and OSL readout parameters (such as CW/LM/NL, etc.) and activate them together, the combined TL and OSL signal is stored on the computer and plotted off-line on the monitor.

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The a-Al2O3:C samples (5 mm · 5 mm · 0.4 mm) prepared using post growth thermal impurification (PGTI) technique [19] are used to perform TL and OSL readouts on the reader system. The beta irradiations on the a-Al2O3:C samples were carried out using a calibrated 90 Sr/90Y beta source to different absorbed doses. Fig. 8 shows the TL glow curves of a-Al2O3:C samples for a test dose of 10 mGy for various heating rates in the range 2 K/s to 10 K/s using an UG1 filter across the PMT. The CW-OSL response of the a-Al2O3:C samples for a test dose of 10.0 mGy and stimulation intensity 40 mW/cm2 is shown in Fig. 9. A CW-OSL response of the a-Al2O3:C samples irradiated to 10 mGy absorbed dose (beta) for various stimulation intensities is shown in Fig. 10. The inset figure shows the normalized CW-OSL response which reflects the linear dependence of excitation rate f (= r/) with the stimulating intensity (/). Fig. 11 shows the dose vs OSL response of a-Al2O3:C samples (useful in the range of 10 lGy to 10 Gy) measured using the developed reader system. The reader system covers a wide dynamic range of 10 lGy to 103 Gy with a minimum detection limit of 10 lGy in the OSL mode. Fig. 12 shows the CW-OSL

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Time (s) Fig. 9. Typical CW-OSL response of the Al2O3:C samples for 10.0 mGy beta dose. The blue (470 nm) light stimulation intensity at the sample position 40 mW/cm2.

response of the a-Al2O3:C samples at elevated temperatures. As the temperature during stimulation is increased, the CW-OSL intensity increases up to a temperature of 140C. However, at temperatures 180 C, intensity decreases drastically due to effects of thermal decay and quenching [21]. This feature in the reader system is important for enhancing the OSL sensitivity at the optimized elevated temperature. In addition this will serve as a tool to study the effect of temperature on the OSL active traps, the temperature dependence of photoionization crosssection of OSL traps in phosphor materials and the thermal quenching [21] associated with some of the luminescent phosphors. Fig. 13 shows a typical LM-OSL peaks in a-Al2O3:C sample recorded on the reader system for two different

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Fig. 10. CW-OSL response of the a-Al2O3:C samples irradiated to 10.0 mGy 90Sr/90Y beta dose for various blue light (470 nm) stimulation intensities. The inset figure shows the normalized CW-OSL response.

linear ramp rates using 470 nm blue light stimulation. For a first order case, the information of LM-OSL peak intensity can be used to calculate the photoionization crosssection ‘r’ [13] using the relation sffiffiffiffiffi 1 tmax ¼ rc where tmax is the time required to attain peak value of intensity in the LM-OSL curve and ‘c’ is the linear modulation ramp rate having unit as number of photons/cm2/s2. The photoionization cross-section can also be measured for green light stimulation. Fig. 14 shows the NL-OSL curves recorded on the reader for the a-Al2O3:C sample with a

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Time (s) Fig. 12. CW-OSL response of single crystal a-Al2O3:C at various elevated temperatures for 10 mGy beta dose.

non-linear ramp rate of stimulation. Using stimulation rate higher than the linear, it is possible to acquire the OSL information (NL-OSL peak) at relatively lesser time. It can be seen from Figs. 13 and 14 that the NL-OSL curve is more symmetric (nearly Gaussian) as compared to LM-OSL peak. In an optimized NL-OSL readout mode, signal is acquired in a relatively shorter time as compared to LM-OSL and in order to conserve the area under the OSL curve, the peak height is much higher than in LM-OSL readout for the same absorbed dose. Thus, the signal to noise ratio in NL-OSL is expected to be higher than that in LM-OSL and CW-OSL modes of measurements. For the POSL measurement, the TL/OSL system offers flexibility of selecting pulse width from 5 ls to 1 ms with a

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Fig. 13. LM-OSL response for two different ramp rates for the Al2O3:C samples exposed to 10.0 mGy beta dose.

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Time (sec) Fig. 14. NL-OSL response of Al2O3:C samples for 100 mGy beta dose.

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Fig. 16. Decay of OSL signal for the repeated POSL readouts for a-Al2O3:C sample irradiated to 20 mGy beta dose.

4. Conclusion We have presented in this paper, the issues involved in the design and development of TL/OSL reader system. The introduction of such an integrated and versatile TL/ OSL reader will facilitate the availability of alternative low cost system for TL and OSL dose measurements in radiation dosimetry and advance research applications. The system has flexibility in selection of operational parameters for the measurement of TL and OSL with an additional NL-OSL mode of operation. It allows the use of different wavelengths of stimulation light and measurement of the value of stimulating intensity at the sample position and selection of optical filters with different cut off ranges.

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Time (ms) Fig. 15. A typical POSL recorded at room temperature on the a-Al2O3:C sample for a pulse width of 20 ls, pulse separation of 100 ls, frequency of 8 kHz for a duration of 450 ms. The sample was irradiated to 20 mGy of beta dose.

pulse repetition rate of 0.1 Hz to 10 kHz. A typical POSL readout of a-Al2O3:C sample for a pulse width of 20 ls and pulse separation of 100 ls for a duration of 450 ms is shown in Fig. 15. The POSL signal is recorded for a period of 550 ms after the termination of pulses. The luminescence lifetime (s) for the a-Al2O3:C samples was estimated to be 38 ms from the decay of POSL signal after cessation of the stimulation pulses. Fig. 16 shows decay of OSL signal for the repeated POSL readouts on a-Al2O3:C sample irradiated to 20 mGy absorbed dose from the 90Sr/90Y beta source.

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