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A R Lakshmanan

Publications and source records attributed to A R Lakshmanan.

At least 19 recordsLinked to original sources
PubMed2006

Defect centres and thermoluminescence in CaSO4:Dy,Ag phosphor.

The defect centres formed in the TL phosphor CaSO4:Dy,Ag are studied using the technique of Electron Spin Resonance. The Ag co-doped phosphor exhibits three glow peaks around 130, 220 and 375 degrees C in contrast with the two glow peaks observed in the CaSO4:Dy phosphor at 130 and 220 degrees C at a gamma ray dose of 1Gy. ESR studies show that the additional peak at 375 degrees C correlates with a Ag2+ centre formed due to gamma irradiation and observable only below -170 degrees C. The Ag2+ centre is characterised by an axial g-tensor with principal values g(parallel) = 2.38 and g(perpendicular) = 2.41. ESR studies further indicate that the precursor to a centre observable at low temperature (-170 degrees C) appears to act as the recombination centre for the TL peak at 375 degrees C; this radical is characterised by the g-values g(parallel) = 2.0023 and g(perpendicular) = 2.0038 and is assigned to SO3- radical. It is observed that there is more incorporation of Ag in the CaSO4:Dy system as compared with that in pure CaSO4 system.

Artifacts↗
PubMed2002

Further studies on the new high sensitive CaSO4:Dy thermostimulated luminescence phosphor.

CaSO4:Dy phosphor prepared by a new recipe (denoted as N) is nearly 50% more sensitive than the presently used one (denoted as P). N consists of needle shaped crystals while P is mostly quadrilateral. In P most of the grains in as-grown condition are >75 microm in size while in N most of the grains are <75 microm. While the sensitivity of P increases with grain size, an exactly opposite trend is seen with N since higher sized grains (>105 microm) in N are agglomerates of particles and hence are opaque. The detection threshold of N (14.4 microGy) is nearly 4 times lower than that of P (54.2 microGy). The major glow peak(s) in both the phosphors occur in the 460-490 K (187-217 degrees C) region. But the low temperature peak near 390 K (117 degrees C) is very prominent in P while its presence is insignificant in N. The post-irradiation storage stability of N at approximately 30 degrees C was tested up to a period of 25 d and found to be better than that of P. The emission spectra of P and N are characteristic of Dy3+. In P, the 480 nm to 570 nm emission intensity ratio varies slightly with glow peak temperature, unlike that of N. The Dy concentration quenching effect in N is less serious than that in P. The intrinsic UV sensitivity of N is nearly a factor of 20 times lower than that of P.

Calcium Sulfate↗
PubMed1979

Thermal history and reusability of CaSO4: Dy Teflon TLD discs.

The TL sensitivity of CaCO4 : Dy Teflon TLD discs was found not to change after isothermal annealing for 1 h in the temperature range 220--400 degrees C. The reusability of these TLD discs after two separate annealing treatments (240 degrees C and 400 degrees C, each 1 h) were studied for 20 cycles of use. The former annealing treatment resulted in a 12% loss in TL sensitivity after 20 cycles, mainly due to the increase in optical density of these discs. A decrease in the maximum temperature from 300 to 250 degrees C while clamped for read-out in the TLD reader has prevented any change in the optical density of these discs, and their TL sensitivity did not show any change after 20 cycles. The darkening of PTFE-based TL dosemeters was attributed to the reorientation of molecules in PTFE when the temperature during readout exceeds 250 degrees C; isothermal annealing at 400 degrees C restored their original colour. Their TL sensitivity decreased gradually with the number of cycles (over 15% in 20 cycles) after the 400 degrees C anneal, mainly because of the loss in TL sensitivity of the CaSO4 : Dy phosphor itself. This study has demonstrated that under controlled conditions, the Teflon TL dosemeters can be reused a number of times without any change in TL sensitivity.

Calcium Sulfate↗
PubMed1979

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).

Borates↗
PubMed1978

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.

Calcium↗
PubMed1978

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.

Aluminum Oxide↗
PubMed1976

Ultraviolet response of CaSO4:Dy.

A simple method for measuring ultraviolet exposure using the thermoluminescent properties of CaSO4:Dy is presented in this paper. In general, three peaks at 145, 215 and 365 degrees C were observed by ultraviolet irradiation of virgin phosphor. The peak at 365 degrees C was found to be very small and that at 145 degrees C fades away within a week; no appreciable fading was observed for the 215 degrees C peak. Further, the peak at 215 degrees C was found to coincide with the gamma dosimetric peak and this peak was chosen for ultraviolet exposure measurements. A standard quartz pen-ray lamp was used for calibrating the phosphor both by peak height measurement and by integration. The dosimetric peak has been found to be linear from the detection threshold of 400 erg mm-2 to 4 X 10(4) erg mm-2. Beyond this exposure the phosphor behaved supralinearly to ultraviolet exposure up to the studied range of 5 X 10(5) erg mm-2. The phosphor exhibited strong dependence on photon energy. The TL sensitivity increases sharply by a factor of 300 as the wavelength is decreased from 250 to 200 nm. The response is negligibly small above 350 nm. Other important characteristics such as particle size dependence, fading characteristics, thickness dependence and re-usability are also presented. The ultraviolet sensitivity increased by a factor of 3 when the grain size range was changed from 105-210 mum. Self-shielding by a factor of 2 was observed when the sample thickness was increased from 10 to 72 mg cm-2.

Calcium↗