Minimal requirements for the maintenance of instrument calibrations shall consist of weekly performance checks with a suitable reference source and a complete recalibration semi-annually. Should the weekly performance check deviate from the value determined at the time of calibration by more than 4.0%, a complete recalibration of the instrument is required at that time.
This method involves three basic steps, namely photopeak integration, determination of the photopeak efficiency curve, and calculation of the activity of the specimen.
The method for the determination of the required photopeak area utilizes a Gaussian approximation for fitting the photopeak. A fixed fraction of the total number of photopeak counts can be obtained by taking the peak width, a
, at some fraction of the maximum, where the shape has been experimentally found to be very close to Gaussian, and multiplying by the counting rate of the peak channel, P
, after correction for any Compton and background contributions to the peak channel count rate. This background usually can be adequately determined by linear interpolation. This is illustrated in Figure 2.
Fig. 2 Typical Gamma-ray Spectrum Showing the Selection of the Peak Channel Counting Rate, P, after the Correction for Compton and Background Contributions.
The photopeak-curve shape is closest to a straight line at 0.606P, and the contribution of the fractional channels to a can be accurately estimated by interpolation. Calculate a by the equation:
in which c and d and also c¢ and d¢ are the single channel counting rates on either side of 0.606P, and D and D¢ are the channel numbers (locations) of d and d¢, respectively. The location of the required variables on the photopeak is illustrated in Figure 3.
From the known values for the counting rate in the peak channel of the photopeak, P, and the width of the peak at 0.606P, a, a calibrated fraction of the photopeak area is then obtained from the product, (aP).
To summarize the procedures involved in obtaining a calibrated fraction of a photopeak area using this method, the necessary steps or calculations are presented below in a stepwise manner:
(1) Subtract any Compton and background contributions from the photopeak to be measured.
(2) Determine the counting rate of the peak channel (maximum channel counting rate after subtracting Compton and background), P.
(3) Multiply P by 0.606, and locate the horizontal line corresponding to the peak width, a.
(4) Obtain the peak width, a, by inserting the values of variables (obtained as shown in the preceding figure) into the equation defining a.
(5) The desired calibrated fraction of the peak area is then equal to the product of a times P or F = aP, where F is a fractional area of the peak proportional to the emission rate of the source.
This method provides a quick and accurate means of determining the gamma-ray emission rate of sources while avoiding, to a large extent, subjective estimates of the detailed shape of the tails of the peaks. The error due to using the maximum channel counting rate, rather than the theoretical maximum or peak channel rate, is of the order of 1.0% if a is 6 or greater.