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A Clinical Servo System for Shifting the Range of Pion Beams

An energy absorbing instrument, designed for use with a linear accelerator producing a high intensity beam of negative pions is described. The system makes use of a variable column of energy absorbing fluid with depth controlled by a hydraulic servomechanism to conform to theoretically or experimentally derived sweep curves in the form of histograms or continuous functions. The dose versus depth curves which result from the vertical sweep of this instrument show excellent flattening over the stroke. The details of construction and control of a prototype "range-shifter" are described, as well as an innovative design for improving the form and function of the device. A unique method for command-function storage and interrupt control using programmable memories and microprocessor techniques is discussed which has proven satisfactory in applications involving human patients.