Well-type ionization chamber radiation detector for calibration of

Abstract

An ionization chamber radiation detector includes an outer shell, an inner well electrically connected to the outer shell to define an ionization chamber sensitive volume between them, and an ionization electrode that extends upwardly from a base into the space between the side walls of the well and shell. The shell and the walls of the well are grounded while the ionization electrode, which is electrically insulated from the shell, has a high voltage applied thereto. A guard electrode at the same voltage prevents electrical leakage. A sample positioner includes a thin walled guide tube which extends from a top cap down to a spacing disc. The spacing disc has an outer periphery which closely matches the interior of the well so that when the positioner is inserted into the wall the guide tube will be held at a desired position, preferably at the central axis of the well. A radiation source sample in a catheter is inserted through an opening in the top cap into the thin walled guide tube and down to a desired position within the tube, which precisely locates the source at a position which is preferably on the central axis of the detector. Close spacing between the source and the ionization chamber results in relatively high ionization current, allowing the radiation level to be measured based on the ionization current rather accumulated charge, and minimizing the effect of ambient background radiation.