Calibration with double-pass measurements and computational peak finding

Abstract

A method of calibrating a gamma counter that makes two separate measurements over the peak of the standard source of I.sup.129 and a computational technique to obtain the center of that peak. A microcomputer controlling the instrument's operation changes the high voltage on the instrument's photomultiplier (PM) tube to find the setting which generally brings the I.sup.129 peak into the range of the preset pulse height analyzer (PHA) window. Each measurement in the first pass takes a relatively short period of time and looks for the arbitrarily defined end points of the peak. The microcomputer temporarily defines the midpoint of this range as the location of the I.sup.129 peak. In the second pass, the instrument takes longer measurements, for greater accuracy, at six points on either side of the temporary peak. The microcomputer then curve-fits a first line to the logarithms of the counts obtained at the six points on one side of the temporary peak and a second line to the six points on the other side of the peak. The intersection of the two lines then represents the actual center of the I.sup.129 peak. To bring another peak within the PHA's window, the instrument alters the voltage on the PM tube from the I.sup.129 peak just determined.