Twin Ionization Current Amplifiers and Ratio-meter Using Solid-state Modular Devices

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Abstract
The instrument was designed as part of a system for measuring radiation dose distributions with the X-ray from an 8 Mev linear accelerator specifically in mind. The system automatically scans the X-ray fields in a water phantom, at different depths perpendicular to the beam axis, and thus produces tables of transverse data which form the basis for computer calculation of single and multiple beam isodose curves (Orr, Laurie and Wakerley 1964, Hope and Walters 1964). The instrument has 2 small ionization chambers (0.4 cc), one for scanning the field and the other to function as a stationary reference. Errors due to beam fluctuations are then eliminated by taking the ratio of the 2 ionization currents. Modern solid-state modules (operational amplifiers and a multiplier/divider unit) are used throughout, resulting in a compact and easily operated instrument. The output from the ratio-meter is digitized and printed at each of a preselected matrix of points in the field. Many other uses are envisaged for this instrument which was designed to measure currents from 10-11 to 4 x 10-10 amp.

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