Bhuwan Chandra

A comparison of thermal neutron and gamma ray sensitivities of common TLD materials.

The thermal neutron responses in the range 1010 to 2*1012 n cm-2 and the gamma -ray sensitivities of common thermoluminescent materials (CaSO4:Dy, CaSO4:Tm, CaF2:Dy, CaF2:Mn, Li2B4O7:Mn, LiF (TLD-700, TLD-100, TLD-600), Mg2SiO4:Tb and BeO) have been studied in detail and the results compared with those published elsewhere.

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.

Mixed field dosimetry with CaSO4:Dy

The thermal neutron response and gamma -ray sensitivity of CaSO4:Dy as a function of Dy concentration were studied in detail. CaSO4 with 0.2% Dy has maximum gamma -ray sensitivity. The thermal neutron response of CaSO4:Dy increases rapidly with concentration beyond 0.1% Dy. CaSO4:0.05% Dy (or Tm) has high gamma -ray sensitivity coupled with low thermal neutron response and hence is useful for gamma -ray dosimetry in mixed fields. A mixture of LiF TLD-100 and TLD-700 was arrived at for rem equivalent dosimetric purposes in a mixed field consisting of thermal and intermediate energy neutrons and gamma -rays. Mixtures of CaSO4:Dy with various lithium based materials were also selected for the same purpose. The thermal neutron response and glow peak temperature of CaSO4:(Dy, 6Li) were studied as a function of 6Li concentration.

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.

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).

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.

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.

The development of thin CaSO4:Dy Teflon TL dosimeters for beta dosimetry in personnel monitoring

Abstract This paper describes the development of thin (0.1 and 0.2 mm) CaSO4:Dy Teflon dosimeters thermally bonded to thick (0.7 and 0.6 mm respectively) Teflon bases for application to β-dosimetry in personnel monitoring. The composite dosimeters (13 mm dia. and 0.8 mm thick) were found quite rigid and they do not deform during their readout. The TL response of the 0.1 mm TL dosimeter was found to be β-energy independent in the energy range 0.76–2.25 MeV. The TL response of the 0.2 mm dosimeter, however, decreased with decreasing β-energy in this range. The minimum detectable γ-dose with 0.1 mm TL dosimeters was found to be 0.13 mGy. Hence, due to their energy independence, mechanical strength, TL efficiency, and capability in low dose measurements, the 0.1 mm CaSO4:Dy Teflon dosimeters thermally bonded to 0.7 mm Teflon base were found suitable for β-dosimetry in personnel monitoring.