Dose equivalent response of a TLD badge--influence of body backscatter.

In personnel monitoring, operational quantities recommended by ICRU Publication No. 39 for photon radiation can be realized by calibrating dosimeters on a phantom and considering body backscatter photons by using established conversion factors. Personnel dosimeters used in this study are based on CaSO4:Dy Teflon thermoluminescence dosimeter discs (TLD) that have a highly photon energy-dependent response. Since body backscattered photons have lower energies than the incidence photons, methods for correcting for energy dependence of both the incident and body backscattered photons have to be developed. By using readouts of two TLD discs (one under a composite metal filter and the other without a metal filter) in an empirical relation valid at all energies, it is possible to correct for the effect of change in response from change in the photon energies. It was found that the new operational quantities recommended by ICRU could be estimated to within +/- 15% by a TLD badge design based on this method. Angular dependence limits for photons in accordance with the new international standards and a high beta dose-equivalent discrimination in the mixed fields of beta and low-energy x rays could also be achieved.

Kaolinosis: a radiological review.

A short account of the industrial process used to produce kaolin is given. The chest radiographs of 68 selected cases of kaolinosis were reviewed and the radiological appearances are presented. We also describe two cases of Caplan's syndrome occurring in kaolinosis.

X- and gamma-ray response of the TLD badge based on CaSO4:Dy Teflon TLD discs.

A TLD badge based on CaSO4: Dy Teflon TLD discs has been designed for personnel monitoring of radiation workers encountering X-rays of effective energies above 20 keV. The badge had two Teflon TLD discs, one of which is used bare (without any filter) and other which is used with a metal filter. By taking into account the TL readout of both of these discs, the problems of photon energy dependence as well as directional dependence of the badge have been reduced to a minimum. The uncertainty due to photon energy dependence above 29 KeV (eff.) was found to be within a value of +/- 15% (at an angle of incidence of 45 degrees). The directional dependence of the badge is within +/- 30% in the energy range from 35 KeV to 1.25 MeV. The badge is designed so as to provide approximate information on the effective photon energy of the X-ray exposures.

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).

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.

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.

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.

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.