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L D Simpson

Publications and source records attributed to L D Simpson.

12 recordsLinked to original sources
PubMed1975

E.s.r. studies of halogenated pyrimidines in gamma-irradiated alkaline glasses.

The reactions of mobile electrons (em-) and oxygen radical anions (O--) with halogenated bases and nucleosides have been studies in gamma-irradiated alkaline glasses by e.s.r. and specific halogen-ion electrode techniques. It is shown that electrons react with halogenated uracil bases (XUr where X = Cl, Br. I but not F) by dissociative electron attachment to form uracil-5-yl radicals (U-) and halogen anions. The relative rates of reaction of em- with XUr decrease in the sequence BrUr greater than ClUr greater than FUr greater than IUr. Thermal annealing studies carried out on U- in H2O and D2O matrices support the hypothesis that U- in H2O hydrates across the 5-6 double bond in the temperature region 135 degrees-155 degrees K, and deuterates to a much smaller extent in D2O at temperatures above 155 degrees K. Studies on bromouridine and bromodeoxyurinde suggest that em- reacts with the base moieties to form U- type radicals which abstract H- from the sugar moieties of adjacent nucleosides.

Bromouracil↗
PubMedDate not supplied

Computer-compatible patient contour plotter.

The use of a compact, mobile, relatively inexpensive electromechanical patient-contouring mechanism is described. The system utilizes a Numonics Model 224 graphics digitizer suspended over the patient. The digital X and Y coordinates of the patient contour in either of multiple, parallel transverse, or sagittal planes are inherently accurate to within +/-0.25 mm over a total reading are of 60X90 cm. Display features include the interface to an analog 11-in.X17-in. X-Y plotter with a variable scale-down factor for those contours which would otherwise be larger than the plotting surface. The system is easy to use and may also be readily interfaced for on-line entry of patient contours to calculator- or minicomputer-based treatment-planning systems.

Computers↗
PubMedDate not supplied

Parallel-plate ionization chamber response in cobalt-60 irradiated transition zones.

The authors study the acceptance of a Capintec parallel-plate ionization chamber. The Capintec chamber is used for dose measurements in a lead and polystyrene slab phantom irradiated with cobalt-60 gamma rays. The authors define an enhancement ratio to quantify the dose measurements. The enhancement ratio equals the ratio of dose measured with the lead slab present to dose measured under equilibrium conditions in polystyrene at equal primary beam attenuation. The measured enhancement ratio at the exit side of the lead/polystyrene interface is 25% lower than the Monte Carlo predicted enhancement ratio. The authors propose that geometric acceptance limitations of the Capintec chamber to large-angle, low-energy electrons are the cause for this difference. A Monte Carlo simulation of the Capintec chamber acceptance confirms the hypothesis.

Cobalt Radioisotopes↗