ABSTRACT
Target size analysis by radiation inactivation is now a well-established method to study structure-function relationships in biologically active macromolecules without prior purification or even solubilization. Recently, it was reported that a relatively low-dose-rate but commonly available gamma source such as the Gammacell 220 (Atomic Energy of Canada, Ltd.) can be used to carry out radiation inactivation experiments providing it is appropriately calibrated with enzymes of known radiation sensitivities (G. Beauregard and M. Potier (1982) Anal. Biochem. 122, 379-384). In this report, a tube rack designed to fit into the irradiation chamber of the Gammacell 220 which allows five experiments (at 30 tubes per experiment) to be carried out simultaneously with both standard and unknown samples is described. The dose rates delivered at different positions in the rack were determined by irradiating rat liver cytosolic neuraminidase, an enzyme of known radiation sensitivity. A better than 2.7% agreement was obtained between experimental dose rate and computed values from isodose curves previously published by other authors (O. A. Curzio and H. O. Quaranta (1982) Int. J. Appl. Radiat. Isot. 33, 1-3).
Subject(s)
Enzyme Activation/radiation effects , Gamma Rays/instrumentation , Radiation, Ionizing/instrumentation , Animals , Evaluation Studies as Topic , Liver/enzymology , Neuraminidase/radiation effects , Neuraminidase/standards , RatsABSTRACT
The most commonly used method of calibrating high-energy photon or electron beams consists in converting cavity ionization to dose by the application of the appropriate Clambda or CE multipled by the 60Co correction factor. The correct interpretation of calibration data for pulsed photon or electron beams requires a knowledge of the charge collection efficiencies of the ionization chambers used. The results are presented of efficiency measurements for both pulsed and continuous beams made with these chambers: 0.6-cm3 Farmer, 0.5-cm3 Spokas, 3-cm3 Shonka, 1-cm3 PTW, and 1-cm3 Memorial pancake. The dependence of collection efficiency on collection voltage, dose rate, and dose per pulse is demonstrated. These results are shown to agree with Boag's formulas for collection efficiency. Attention is drawn to the fact that several kinds of dosimeters provide only minimal collection voltages for efficient collection of charge at high dose rates, especially in Linac electron beams. It is recommended to check the collection efficiency of chambers which are to be used at high dose rates, and a simple method for this purpose is described.
