R.C. Bhatt

Gamma radiation induced sensitization in CaSO4:Dy TLD phosphor

Abstract High pre gamma exposures (> 100 R) and low post-annealing temperatures ( 600 °C) both sensitization and RTL were found to disappear. A model is postulated to explain the observed phenomena on the basis of competition between the sensitization traps and the RTL traps, in trapping the charge carriers generated during the test exposure. As a consequence of the appearance of new TL glow peaks in the temperature region of 200 to 300 °C, virgin phosphor was found to exhibit supralinear response above 100 R test-exposure. The sensitized phosphor was however found to show extended linearity up to 3 × 103 R. The dependence of the TL sensitivity of virgin CaSO4 :Dy on the annealing temperature in the range 300 to 700 °C, and its memory effect for gamma exposures in the range 500 to 107 R after a high temperature post-annealing treatment were also studied.

Mechanism of nonlinearity in the response characteristics of thermoluminescent dosimeters

A new model is proposed for explaining the nonlinearity in the response characteristics of three common thermoluminescent phosphors (CaSO4:Dy, CaF2:Dy and Mg2SiO4:Tb). The increased trapping of charge carriers in the dosimetry traps during irradiation, due to a reduction in trapping efficiency of deep traps caused by radiation damage, was found responsible for the supralinearity in TL response of dosimetry peaks and sublinearity in TL response of high temperature peaks of these phosphors at high gamma ray doses. The dose versus TL response characteristics of deep traps in CaSO4:Dy was found to vary drastically from batch to batch, and in CaSO4:Dy, the sensitisation observed after a gamma ray dose of 103 Gy and annealing at 400 degrees C was found useful in low dose measurements.

Thermoluminescence studies in LiF dosimeter irradiated at 77K and their implications on theoretical models

In LiF:Mg, Ti dosimeters, gamma -irradiation at low temperature (77K) reduces the TL intensities of peaks 7 (260 degrees C) and 10 (450 degrees C) drastically whereas the TL intensity of peak 5 (200 degrees C) remains nearly the same as that after irradiation at room temperature (RT). This implies that during irradiation, interstitial migration is involved in the formation of traps giving rise to peaks 7 and 10. The generic nature of peaks 7 (Z2) and 10 (Z3) traps and their (Z2 to or from Z3) trap conversion are further demonstrated. Since diffusion processes are inhibited, F and Z centres are not formed efficiently in LiF dosimeters irradiated at 77K. Studies on supralinearity, sensitisation, thermal history, optical absorption, optical bleaching, phototransfer and LET dependence following irradiation at RT and at 77K imply that four different processes (competing nonluminescent centre, track interaction during read-out, Z centre and mobile interstitial) are operating simultaneously to varying extents during the trapping/luminescent stage of different TL glow peaks. The photon emission mechanisms of these peaks are discussed. The claim that supralinearity of peak 5 is related to Z3 production is disproved. Further studies on Z centre identification in LiF:Mg, Ti are indicated.

On the role of Z centres and competing nonluminescent centres in the sensitisation and supralinearity mechanism of LiF TLD-100 phosphor

On the basis of a competing nonluminescent centre model, an attempt has been made to explain the dependence of supralinearity on glow-peak temperature and LET of the radiation, and enhanced TL sensitivity at elevated Tirr. Thermal annealing near 300 degrees C or 254 nm optical bleaching treatment (which are reported to result in the creation of Z3 centres) not only reduces peak 5 but also reduces peak 7 TL intensity. After a high gamma ray dose, the sample annealed at 400 degrees C for 1 h in the dark exhibits intense photo-TL at peak 5 but not at peak 7, thereby confirming that the presence of peak 10 is necessary for the formation of peak 7 and not of peak 5. All these data tend to suggest that (i) the TL peaks 7 and 10 are due to the thermal ionisation of Z2 and Z3 centres respectively, and (ii) a trap conversion between these two centres is taking place with a mechanism similar to that proposed by Nink and Kos (1976).

Ultraviolet-induced thermoluminescence and phosphorescence in Mg2SiO4:Tb

Among the common TLD materials studied (Mg2SiO4:Tb, Al2O3:Si, Ti, CaSO4:Tm, CaSO4:Dy, Li2B4O7:Mn and LiF TLD-100), Mg2SiO4:Tb was found to have the highest intrinsic TL sensitivity to UV radiation (wavelength 253.7 nm). The TL response of the dosimetric peak (approximately 200 degrees C) of virgin Mg2SiO4:Tb was studied as a function of UV exposure in the range 10--10(4) J m-2. The UV-induced TL was not affected by room-light and no appreciable fading was observed up to 20 days after irradiation indicating that this is a promising TL phosphor for UV dosimetry. The UV sensitivity was found to increase with increasing preliminary gamma exposure above 10(-2)Ckg-1 (after a post-irradiation anneal at 300 degrees C for 1 h). The degree of sensitisation was found proportional to the intensity of the residual TL peak at 450 degrees C. A study of the TL response of the sensitised sample as a function of UV test exposure has demonstrated the transfer of charge carriers from the deep traps to the dosimetry traps. Both virgin and gamma-exposed Mg2SiO4:Tb were found to exhigit phosphorescence decay at room temperature after UV stimulation. The intensity of the UV stimulated phosphorescence was found to increase with the gamma exposure above 10(-2) C kg-1.

Graphite-mixed CaSO4:Dy Teflon TLD discs for beta dosimetry

The thickness of commonly employed thermoluminescent dosemeters (TLDS) is responsible for the problem of energy dependence in beta dose estimation in situations where this thickness is comparable to the range of beta particles in the TLD material. The effective thickness of solid TLD discs can, however, be reduced by use of some additives which would reduce their transparency to the TL light. The incorporation of various amounts of graphite powder into 0-8 mm thick CaSO4: Dy Teflon TLD discs has been investigated. This has considerably reduced their energy dependence for beta rays with Emax values ranging from 0-4 MeV to 2-27 MeV. Addition of graphite also results in sharp glow curves thereby minimizing the heating time during TL measurements. Significantly, graphite was also found to reduce the non-radiation-induced TL signal and this helps in low dose measurements.

A thermoluminescent fast-neutron dosemeter based on pellets of CaSO4:Dy mixed with sulphur

The fast neutron activation reaction 32S(n, p) 32P in CaSO4: Dy has been used in the measurements of fast neutron dose by employing a post-irradiation TL accumulation method. In order to increase the efficiency of the method CaSO4: Dy powder was mixed with sulphur powder in various proportions from which pellets weighing each were made. After neutron irradiation these pellets were each burnt in an aluminium planchette and the phosphor residue with 32P beta activity on it was allowed to undergo self-irradiation for TL accumulation. The fast neutron efficiency of the system employing 10 pellets of 0.1% CaSO4: Dy was found to be about 100 times that of bare CaSO4: Dy powder.

High-level gamma-ray dosimetry using common TLD phosphors

Thermoluminescence of high temperature peaks, ultraviolet radiation (253.7 nm) induced thermoluminescence and phosphorescence, damage effects and optical density changes in a few common TLD phosphors namely CaSO4 : Tm, CaSO4 : Dy, LiF TLD-100 and Li2B4O7 : Mn were studied from the point of view of measuring high level gamma doses (10(2)--10(6) Gy).

Effect of dysprosium concentration on the TL characteristics of CaSO4:Dy phosphor

Abstract The glow curve structure of CaSO 4 :Dy phosphor was studied in the dose range of (0.2−3.3)×10 5 Gy and it was found to be Dy-concentration-dependent in this range. The samples with high Dy-concentration (2.0 mol %) exhibit extended linearity and reduced supralinearity as compared to the normal concentration dosimetric samples (0.05 mol % Dy). In contrast to the results reported in literature, no difference in the TL fading was observed for these phosphors during a storage period of one month.

Dependence of thermoluminescence sensitivity on temperature of irradiation in LiF (TLD-100) phosphor

A strong dependence of the intensity of the thermoluminescence (TL) output of peaks 5 (208 degrees C), 7 (266 degrees C) and 10 (422 degrees C) on temperature (Tirr) (in the range 25-295 degrees C) at which the phosphor samples were irradiated was observed in LiF (TLD-100) in the dose range covered by the authors (1.7*102-2.5*103 Gy). Below the dose level of 1.26*103 Gy, the intensity of peak 10 was 35-75% higher for high Tirr compared with the room-temperature (25 degrees C) irradiation. Above 1.26*103 Gy, a reversal was observed in which the decrement in TL intensity of peak 10 increased with Tirr. In general, the TL intensities of peaks 5 and 7 decrease with increasing Tirr. The effect of Titt on the sensitisation of peaks 5 and 7 was also studied. The sensitisation of peak 7 depends on the intensity of peak 10. The sensitisation of peak 5 at high Tirr is more than that at 25 degrees C for pre-doses below 1.26*103 Gy and a reversal is then observed from this dose onwards. The authors explain increase of TL intensity and S/S0 at high temperatures as due to increased elimination of competitors to luminescent centres. The decrease in the intensity of TL and S/S0 with increasing Tirr is explained as due to the increased radiation damage to luminescent centres.