Academic Conference on Natural Science for Master and Ph. D students From Cambodia Laos Vietnam
The thermoluminescence characteristics of LiF(Mg,Cu,P) after exposing the alpha radiation
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The thermoluminescence characteristics of LiF(Mg,Cu,P) after exposing the alpha radiationNguyen Quang Mien1* and Trinh Nang Chung11 Vietnam Institute of Archaeology Abstract The LiF(Mg,Cu,P) is one of the thermoluminescence materials, it has been used commonly in investigation on natural radiation dose and control safety of nuclear radiation. In our paper, the procedure of making the thermoluminescence dosimeter (TLD) of LiF(Mg,Cu,P) have been performed. These dosimeters were exposed by alpha radiations and measured by the equipment RGD-3A TLD reader in the laboratory, Institute Of Archaeology (Vietnam). The thermoluminescence characteristics of this material have been estimated. The results obtained have shown this is suitable method for evaluation of natural radiation dose and control of nuclear radiation. Key words: thermoluminescence dosimeter, RGD-3A. INTRODUCTION Thermoluminescence material of series LiF(Mg,Cu,P) has extreme low detectable threshold [1], and is near or tissue equivalent. Its unique features make it an ideal environmental, personal and medical dosimeter. Especially in the radioactive field of low background, it behaves better than other kind of dosimeters for the natural radioactivity dose monitoring. The details of the mechanism by which thermoluminescence is produced in any given mineral are not well understood, and in general it is only for crystals grown in the laboratory with strict control of impurities that these details can be elucidated. However, the main features of them be discussed, usefully in term of a simple model a convenient way to present the process is shown in Figure 1. A trap is characterized by the energy E which an electron must acquire from the lattice vibrations in order to escape from it and diffuses around the crystal; while diffusing it is described as being "in the conduction band". The traps and centers provided by defects are represented as intermediate detached from these as well. Thus, thermoluminescence process consists of three items: Ionization due to exposure of crystal to nuclear radiation, with trapping of electrons and holes at defects T and L respectively. Storage during antiquity: the lifetime of the electrons in the traps needs to be much longer than the age span of the sample in order that leakage be negligible. This lifetime is determined by the depth E of the trap below the conduction band and for dating purposes we are interested in those deep enough (~1.5 eV) for lifetime to be the order of a million years or more. To observe thermoluminescence the sample is heated and there is a certain temperature at which the thermal vibration of the crystal lattice causes eviction. Some of these evicted electrons reach luminescent centers, and if so, light is emitted in the process of recombining at these centers. Alternatively, the electron may recombine at a non luminescent centre (a 'killer' centre) or be captured by a deeper trap. 
Figure 1. Energy level scheme of thermoluminescence EXPERIMENTAL 2.1. Establishing the TL dosimeters a) Measuring alpha radiation (Group A dosimeters) The problem in adapting any of the beta thermoluminescence dosimetry methods is that because alpha particles have such a short range the sample needs to be in direct contact with the phosphor. Dosimeter measures alpha radiation has ultra-thin plastic film, the fine phosphor grains of LiF(Mg,Cu.P) is deposited on 0.8 cm diameter of plastics ultra-thin film, it is about 0.5m thick. The alpha dosimeters have also a top covering of this plastic film. The unit measuring alpha radiation is shown in the Figure 2. The alpha dosimeters are embedded in a glass container of powdered pottery. The remaining one dosimeter is embedded in either quartz of CaSO4 (both low radioactivity) to measure environmental radiation. Containers are sealed and stored in the dark for at least one month. 
Figure 2. Preparation the alpha dosimeter At the end of the storage period, the dosimeters are removed and measured by RGD-3A TLD Reader at the Laboratory of Archaeological Institute (Hanoi, Vietnam). b) Measuring beta radiation (Group B dosimeters). In measuring the beta dose, it is important that the capsule wall is thick enough to prevent alpha particles from reaching the phosphor. Unit which measures the beta dose is shown in the Figure 2. Thus, the phosphor grains are contained in a thin-walled narrow tube of plastic which is inserted into powdered sample. The internal diameter of plastic tube is 1.1 mm; thick wall of plastic tube is 1.0 mm ; the length of plastic tube is about 20mm, the mass of phosphor, series of LiF(Mg,Cu,P) is about 20mg, The dosimeters are embedded in a plastic container of powdered pottery with 40 mm diameter and 60 mm height (Figure 3). 
Figure 3. Scheme of distribution unit measuring beta dosimeter The plastic tube contained phosphor is sealed by applying pressure with hot pliers. This tube is immersed in the powdered sample. The sample needs to be pressed into container firmly enough for it to be effectively infinite as far as beta are concerned. Alpha particles are prevented from reaching the phosphor by the 0,5mm thick wall of the plastic tube. Typically, the storage time is long enough for an accurately measurable level of thermoluminescence to be reached, it is about 30 days. For this dimensions, the advantage that the ratio between the phosphor dose and the dose within the sample is higher; the ratio is 0.51, being less than unity because there is attenuation of beta particles in the wall of plastic tube. Also the small size of the container makes this unit easier to shield and hence on both accounts the background is relatively less important. Thus, the TL of Group B dosimeters derive from beta and environmental radiation only while Group A dosimeters produce TL resulting from additional alpha particle dose. However, since these dosimeters the TL induced by alpha irradiation can be obtained by the equation: G = (GA -GB) where: G is the average TL by alpha radiation in the sample GA is the average TL of Group A dosimeters, and GB is the average TL of Group A dosimeters. Although not rigorously correct, it is assumed in the calculation that the phosphor thickness is small compared to the alpha particle range. The alpha dose-rate (D) is: D = G / (..t) where: is the TLD beta sensitivity t is the storage time and is the ratio of alpha to beta sensitivity (0.15 for this phosphor). The beta dose - rate, D can also be obtained from the measurement by the equation: D = (GB - GC) / (.t) where GC is the average TL of the environmental dosimeters. 2.2) Procedure of measuring by RGD-3A TLD reader a) Operating conditions Heating rate: 6oC/sec Pre-heating:135oC for 6 sec Readout: 240oC for 10 sec Annealing temperature for reuse 240 2oC for 10 min, do not surpass 245oC
The working principle diagram of apparatus for TL is shown as Figure 3. When light is incident upon the photocathode there is emission of electrons by photoelectric material, with which the photocathode is coated (see Figure 4). These electrons are attracted to the second dynode and because of the coasting on the dynode an average of between two and three electrons is emitted for each electron striking it. Thus at the anode there is a succession of electrical pulses. The anode pulses are amplified and led to a rate meter, the output of which is proportional to the thermolouminescence intensity - the rate of arrival of photons at the photocathode. This output is fed to the Y axis of an X-Y recorder. 
Figure 4. The working principle scheme of TL Reader The RGD-3A TLD Reader is apparatus designed for measuring TL detectors after exposed to nuclear radiations in the powdered samples (Figure 5). The sensitivity of RGD-3 TLD reader can be calibrated with a reference light source inside the instrument which shows a constant intensity. A semi conductor refrigerator is applied to keep the PM-tube at a constant temperature to avoid the influence from environment so as to reduce noise and to keep stability of reader performances. 
Figure 5. RGD-3A TLD Reader in Laboratory (Hanoi) This apparatus can be used in measuring the personal and environmental accumulative radiation dose with various types of TL detectors and studying feature of TL material. With heating tray: 6 x 6 mm, diameter; 10 mm, R 2x 13 mm with available shapes of square, circle, rod, film and powder. rESULTS AND DISCUSSION. a) The gamma dosimeters exposed from gamma radiation resource (cobalt-60) within the doses of 5mGy, 10mGy, 15mGy and 25mGy. The thermoluminescence spectrum of LiF(Mg,Cu,P) measured by RGD-3A TLD reader, is shown as Figure 6. In the Fig. 6, the illustrated glow curves are correspondingly: M4: 25mGy M2: 15mGy M2: 10 mGy M1: 5 mGy 
Figure 6. Thermoluminescence spectrum of LiF(Mg,Cu,P) from the RGD-3a TLD reader The main dosimetric peaks of LiF (Mg,Cu,P) glow curve between 150OC and 230OC is used for dosimetry. The peak temperature is high enough so as not to be affected by room temperature and still low enough so as not to interfere with black body emission from the heating planchet. b) The experimental thermoluminescence spectrums of LiF (Mg,Cu,P) after exposed to nuclear radiations of powdered pottery samples are shown as the Figure 6. In the Figure 7, the glow-curves for LiF(Mg,Cu,P) have a larger peak expanding from 170OC to 220OC. While, the glow-curves of LiF(Mg,Cu,P) dosimeters were exposed by gamma resource (cobalt-60) have two peaks (see Figure 6). 
Figure 7. Thermoluminescence of LiF (Mg,Cu,P) dosimeters exposed by powdered pottery So, strictly to calculate the value TL intensive we have to separate the TL peak suitable for alpha radiation. In this experiment we divided the calibration glow-curves gamma ray into two peaks by program Origin, Figure 8 [8]. We are performed the comparison the obtained dose with calculated dose to the natural radioactive components in samples. Natural radioactive concentration (U, Th, K) in pottery have determined by gamma spectrometry GMP – 100. 
Figure 8. Scheme separation TL peak in calibration After Aitken (1985), if the results of radioactive analysis are expressed in element concentrations (Bq/Kg), then it is convenient to have the annual dose data (mGy/yr). The dose–rate results are shown in Table 1. Table 1. Comparison alpha dosimeters with dose from U, Th components
The data obtained in the Table 1, which illustrated the thermoluminescene dose agreement with dose calculating from the radioactive components, primarily. IV. ConclusionS We have demonstrated that it is possible to measure internal alpha dose rates pottery using ultra-thin TLD and have found the results to be in reasonable agreement with gamma spectrometry. The TLD method has the advantages of low cost and convenience. In addition, some correction for water content and disequilibria is accomplished. References Aitken M.J 1985. Thermolumenescence Dating. Oxford University Press (England). Aitken M.J. 1979. The alpha particle response of fluorite. Ancient TL, No 2, p:2-5. Washington University Mortlock A.J 1979. A convenient dosimeter for measuring the environmental radiation dose rate as it applies to thermoluminescence dating. Ancient TL, No 2, p:5-8. Washington University. McKeever S.W 2000. Thermoluminescence of Solid. Cambridge University Press Nguyen Quang Mien, Bui Van Loat, Le Hong Khiem. Kinectic Characteristics of the Thermoluminescence of Li(Cu, Mg,P).Báo cáo hội nghị vật lý chất rắn toàn quốc lần thứ IV, Núi cốc 5-7/11/2003. Vĩnh Hào, Lê Hồng Khiêm. Mô phỏng đường cong nhiệt huỳnh quang. Báo cáo hội nghị vật lý chất rắn toàn quốc lần thứ IV, Núi cốc 5-7/11/2003, Wang Weida 1985. Ultrathin TLD measurement of Alpha Dose-rate and Comparison with Alpha counting. Ancient-TL, p:2-5. Washington University. Hoàng Đức Tâm 1999. Nghiên cứu xác định liều bức xạ bêta hằng năm trong mẫu gốm bằng vật liệu LiF:Mg,Cu,P. Luận văn thạc sĩ vật lý. Đại học sư phạm Tp. Hồ Chí Minh. Wang Weida 1985. Ultrathin TLD measurement of Alpha Dose-rate and Comparison with Alpha counting. Journal of TL ancient: Grzegorz Adamiec., Martin Aitken 1998. Dose-rate conversion factors: update. Ancient TL vol 16, No.2, p: 37-50. Michael C.T, Zacharias N, Maniatis Y, Dimotikali D 1997. A new technique (foil technique) for measuring the natural dose in TL dating and applcatiion in the dating of a mortar containing ceramic fragments. Ancient TL vol 15, No 2-3, p:36-42. Каталог: activities -> clv -> clv10 activities -> Phiếu Ghi Danh (xin điền mỗi người một phiếu) activities -> Dương Ngọc Tùng(*), Vũ Hữu Khánh tải về 13.82 Mb. Chia sẻ với bạn bè của bạn:
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