|Year : 2011 | Volume
| Issue : 1 | Page : 29-31
Low temperature TL studies on indigenously developed and commercial α-AL 2 O 3 :C
Anuj Soni1, DR Mishra1, MS Kulkarni1, KP Muthe2, SK Gupta2, DN Sharma1
1 Radiation Safety Systems Division, Bhabha Atomic Research Center, Mumbai, India
2 Technical Physics Division, Bhabha Atomic Research Center, Mumbai, India
|Date of Web Publication||17-Mar-2012|
Radiation Safety Systems Division, Bhabha Atomic Research Center, Mumbai
Source of Support: None, Conflict of Interest: None
Commercially available and indigenously developed single crystal α-Al 2 O 3 :C (using Post growth Thermal Impurification (PGTI) technique), were studied for their thermoluminescence (TL) response in the temperature range from -100 o C to 60 o C. Both types of samples were found to possess similar pattern of TL peaks. In PGTI samples, a dominant TL peak occurred at 8 o C and commercial reference samples showed an intense TL peak at 12.5 o C. The results indicate that the nature of shallow traps formed using PGTI method is qualitatively analogous to those created by conventional method of growing single crystal α-Al 2 O 3 :C in vacuum in graphite ambience. The low temperature TL peaks are found to respond to blue light optical stimulation as well and therefore may play a crucial role in real time OSL radiation dosimetry in the regime of low temperature ambient conditions, especially with improved detection threshold.
Keywords: Al 2 O 3 :C, thermoluminescence dosimetry, PGTI, optical simulation
|How to cite this article:|
Soni A, Mishra D R, Kulkarni M S, Muthe K P, Gupta S K, Sharma D N. Low temperature TL studies on indigenously developed and commercial α-AL 2 O 3 :C. Radiat Prot Environ 2011;34:29-31
|How to cite this URL:|
Soni A, Mishra D R, Kulkarni M S, Muthe K P, Gupta S K, Sharma D N. Low temperature TL studies on indigenously developed and commercial α-AL 2 O 3 :C. Radiat Prot Environ [serial online] 2011 [cited 2020 Sep 24];34:29-31. Available from: http://www.rpe.org.in/text.asp?2011/34/1/29/93935
| 1. Introduction|| |
Thermoluminescence (TL) studies provide vital clues in connection with the mechanism of trap formation, concentration and nature of the traps as well as the modifications occurring in the nature traps due to variations in process parameters during preparation of the phosphor. For dosimetry generally, the dose measurement is carried out using TL peak which occurs at temperatures where shallow traps are ineffective and as such cannot contribute to the TL response. In OSL based dosimetry, however, the measurement is usually carried out at room temperature, where these shallow traps can significantly affect the OSL response. However, the TL measurement at low temperatures gives the useful information about such shallow traps which may be having life-time of the order of a few ms at room temperature. It has been observed that these traps exercise a decisive influence over the OSL response of samples as they lead to a delayed optically stimulated luminescence (DOSL), which results due to trapping of charges released from dosimetric traps, upon optical stimulation, at the shallow traps. There is likelihood that some of these low energy traps give rise to TL peaks below room temperature. Therefore study of TL peaks in this temperature range may add collateral information that may find application in engineering of dosimetrically pertinent defects which is an integral part of development of a sensitive OSL phosphor. The α-Al 2 O 3 :C OSL phosphor developed by different methods is expected to have the same type of low energy level defects along with the other dosimetric relevant defects as that in commercial phosphor. Therefore, monitoring the presence and intensity of low temperature peak will be good indicator for the development of dosimetry grade phosphor. Earlier investigators have reported TL peaks at about 260K and 310K, in addition to the main dosimetric peak at ~450K, for the Al 2 O 3 :C sample irradiated at 80K (3).
Anion defective α-Al 2 O 3 :C single crystals, commercially grown in Russia as TLD-500, are being incrementally used in personnel and environmental radiation dosimetry. It's popularity arises from it's high sensitivity for personnel and environmental application, low fading, a simple annealing regime and a convenient glow curve consisting of two clearly separated TL peaks - one occurring at ~200°C and another one at low temperature viz. at ~50°C. We have earlier shown that it is possible to engineer dosimetrically relevant defects in α-Al 2 O 3 crystals by annealing them in a composite ambience of vacuum and graphite using PGTI method (1) and it has been recently shown that by adopting a slightly different method of preparation, by changing the process parameters it is possible to selectively enhance low temperature peak (shallow traps) (2). In the present paper we have investigated and compared the TL response of commercial and indigenously developed α-Al 2 O 3 :C phosphor in the temperature domain (-100 o C to 60 o C), with the objective of recording TL peaks, if any, due to these shallow traps and their possible role in online OSL radiation dosimetry. For this, OSL was also recorded at low temperature.
| 2. Experimental|| |
The TL measurements were carried out in an indigenously built low temperature TL reader set up. A K-type thermocouple based PID circuit was used to control the temperature in the reader system. The reader comprises of a red-sensitive photomultiplier tube (Hamamatsu, R-669) having response in the range 300-700nm, for detecting the luminescence. The reader is connected to the PC through an RS-232 serial interface.
The commercially available α-Al 2 O 3 :C (TLD-500) powder from Landauer Inc., weighing 5 mg each was subjected to irradiation by UV light on the heater planchet held at (-100 o C) for 5 minutes so that the electrons excited from the F-center can be trapped at shallow traps (since these traps are stable at this low temperature). The F-center gets excited by 203 nm UV light and the excited state of F-center lies in the bottom of conduction band in α-Al 2 O 3 :C, thereby making the electron in the excited state mobile in the conduction band, which gets trapped in the low-temperature trap. The sample was heated from -100 o C to 60 o C in the reader system at different heating rates (0.1 to 0.3 K/s). The same procedure is used for the readout of indigenously developed α-Al 2 O 3 :C samples processed using PGTI technique. For checking the OSL sensitivity of the low temperature TL peak in Al 2 O 3 :C, the samples after the irradiation to UV light were exposed to intense blue light (470 nm) for 15 minutes and then it's TL from -100 o C to 60 o C was recorded in which only 55 o C peak was observed which is not OSL sensitive. The thermal trap-depth (E) of the TL peaks is determined using variable heating rate method as follows.
The condition of TL maxima is used for the evaluation of frequency factor S for first order kinetics as
The mean-life τ (in s) of TL traps for each peak for a given temperature T is determined by
For the second order kinetics following relation was used to evaluate S
[Figure 1] shows the TL glow curve of commercial α-Al 2 O 3 :C samples at different heating rates. The α-Al 2 O 3 :C shows TL peaks at 12.5 o C and 55 o C for 0.3 K/s heating rate and for readout up to 60 o C. [Figure 2] shows that for 0.3 K/s heating rate, the 12.5 o C peak occurs earlier (in time domain) as compared to that at low heating rate of 0.1K/s and the area under glow curve remains conserved for different heating rates. The 12.5 o C TL peak in commercially available Al 2 O 3 :C single crystals is found to have thermal trap depth of E=0.24 eV, which is much lesser than that of dosimetric trap (1.1eV), and half life ≈ 5 ms at room temperature, hence unstable at room temperature(RT) and does not show it's presence in TL measurements carried out at RT. The 55.5 o C TL peak is found to be sensitive in TL mode only but the 12.5 o C TL peak is found to be sensitive in OSL mode as well. Therefore, these shallow traps which become stable at low temperatures will also contribute to the total OSL signal used for OSL radiation dosimetry in low temperature ambient conditions, thus may improve the detection threshold, particularly in online OSL radiation dosimetry. Further this peak, being Observed in α-Al 2 O 3 :C developed by PGTI method [Figure 3] and [Figure 4], can be taken as one of the important signatures for the formation of α-Al 2 O 3 :C phosphor. The study also reveals that the defects formation in α-Al 2 O 3 single crystals, by processing in vacuum at 1500 o C in graphite ambience, are of same type as that in the commercial phosphor developed by melting of polycrystalline α-Al 2 O 3 in graphite ambience. [Table 1] shows the measured TL parameters on the processed and commercial α-Al 2 O 3 :C samples. The order of kinetics 'b' was determined using the method based on glow curve shape analysis using the geometrical shape factor μ g .
| 3. Results and Discussion
|Figure 1: TL Glow curve (TL-temp plot) of Commercial α -Al2O3:C at different heating rate showing shift in peak position in temperature scale|
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|Figure 2: TL Glow curve (TL-time plot) of commercial α -Al2O3:C at different heating rates showing shift of peak position in time scale|
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|Figure 3: TL Glow curve (TL-temp plot) of α -Al2O3:C developed using PGTI technique at different heating rates showing shift in peak position temperature scale|
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|Figure 4: TL Glow curve (TL-time plot) α -Al2O3:C developed using PGTI technique at different heating rates showing shift of peak position in time scale|
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| 4. Conclusions|| |
The low temperature TL studies on commercial and indigenously developed α-Al 2 O 3 :C samples reveal that thermal treatment (at 1500 o C) in vacuum in graphite ambience to α-Al 2 O 3 single crystals leads to the formation of same types of defects as those formed in case of commercial α-Al 2 O 3 :C single crystals. Hence the PGTI method in which the grown single crystal α-Al 2 O 3 is treated at 1500 o C, which is well below it's melting point, in reducing environment in graphite ambience, is equivalent to conventional method in which the crystal is grown from the melt, which involves a sophisticated technique. Further, as the low temperature peak is OSL sensitive it could play important role, particularly in online OSL radiation dosimetry, as it could help in improving detection threshold as both the low temperature peak (8.5 o C) as well as dosimetric peak (at 180 o C) will contribute to OSL signal for irradiations and measurements carried out in low temperature ambient conditions.
| 5. References|| |
- Kulkarni M.S., Mishra D.R., Muthe K.P., Singh Ajay, Roy M., Gupta S.K. and Kannan, S. (2005), Radiat. Meas. 39, 277.
- Mishra D.R., Kulkarni M.S., Muthe K.P., Thinaharan C., Roy M., Kulshreshtha S.K., Kannan S., Bhatt B.C., Gupta S.K. and Sharma D.N. (2007), Radiat. Meas., 42, 170-176.
- Mckeever S.W.S., Akselrod M.S., Colyott L.E., Larsen N.A., Polf J.C. and Whitley V. (1994), Radiat. Prot. Dosim., 84, 163.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]