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L A DeWerd

Publications and source records attributed to L A DeWerd.

41 records · Page 3Linked to original sources
PubMedDate not supplied

Thermal and scatter effects on the radiation sensitivity of well chambers used for high dose rate Ir-192 calibrations.

High dose rate (HDR) iridium sources must be calibrated regularly because of the short half-life of Ir-192. High dose rate sources can now be calibrated using a new well-type chamber that allows easy, reproducible source calibrations. The chamber includes a styrofoam insulator that surrounds the source in the well. A study of the radiation sensitivity of the well chamber exposed to an HDR Ir-192 source at two different activities (300 and 230 GBq) revealed that the sensitivity of the chamber varies by as much as 1.1% as the chamber is moved toward a scattering surface. Second, with the styrofoam insulator removed, the air temperature within the ion collecting volume increased during exposure, causing a gradual decrease in chamber sensitivity of 0.15% in 30 min. This temperature increase was caused by heat transfer from radiation emitted by the Ir-192 source, and diminished as the source decayed. However, with the styrofoam insulator around the central aluminum tube in the well, the source cannot heat the collecting volume and thus thermal equilibrium between the ion collecting volume and its environment is maintained throughout an exposure. The radiation sensitivity of the commercial well chamber was found to be constant for exposure times of 30 min.

Aluminum↗
PubMedDate not supplied

An electron density calibration phantom for CT-based treatment planning computers.

In order to correct for tissue heterogeneities on a voxel-by-voxel basis during CT-based treatment planning, the relationship between the correction factor (CF) and the CT number in Hounsfield units (HU) for the scanner in use must be established. Since the relationship between CF and electron density (rho e) of various materials is well documented, the rho e vs HU is required for direct computation of the correction factors by treatment planning computers. A CT phantom with 18 different tissue substitutes has been used to establish the rho e vs HU relationship. A description of the phantom and its contents is given and the calibration of the CT function of the planning computer is discussed.

Calibration↗
PubMedDate not supplied

Thermoluminescent characteristics of LiF:Mg,Ti from three manufacturers.

LiF:Mg,Ti thermoluminescent dosimeters (TLD's) were obtained from three manufacturers with the specification that they all lie within +/- 5% of the mean. All were chips of size 3 mm x 3 mm x 1 mm. The three sample groups were compared on the basis of their energy response, linearity of the thermoluminescent (TL) response with increasing exposure, sensitivity, and reproducibility of TL response. All TLD groups showed similar characteristics in energy response and linearity. This is to be expected since the base material and dopants were the same. However, the sensitivity varied among the three groups by as much as 40% and reproducibility measurements gave standard deviations ranging from 7% to 14% among the groups. Annealing procedures were identical in all cases, as were irradiation and readout conditions.

Fluorides↗