C.4.2 Coincident Photon Summing Correction

When another gamma ray or x-ray is emitted in cascade with the gamma ray being measured, in many cases a multiplicative coincidence summing correction C must be applied to the net full-energy-peak count rate if the sample-to-detector distance is 10 cm or less. Coincident summing correction factors for the primary gamma rays of Co-60 and Y-88 are approximately 1.09, 1.04, and 1.01 for a 65 cm3 detector at 1 cm, 4 cm, and 10 cm sample-to-detector distances, respectively. The data for cascade-summing corrections for some major nuclides can be found in the literature(3).

Similarly, when a weak gamma ray occurs in a decay scheme as an alternate decay mode to two strong cascade gamma rays with energies that total to that of the weak gamma ray, a negative correction would be applied to the weak gamma ray. However, the correction factors may be negligible for most of the radionuclides observed in nuclear power plants.

C.4.3 Sample Geometry Calibration

The gamma-ray full energy peak efficiency is sensitive to the following counting geometry factors: (1) source-to-detector distance; (2) physical form of the source (gas, liquid, solid); and (3) size and shape of the source or source container (point source, filter papers of various sizes, charcoal cartridge, liquid bottles of various sizes, gas vials of various sizes, Marinelli beakers of various sizes). For most accurate results, the source to be measured must duplicate, as closely as possible, the calibration standards in all aspects. If this is not practical, appropriate corrections must be determined and applied. The methods of determining geometric correction factors and the preparation of standard source for various geometric calibration are described in Appendix D.

C.5 RESOLUTION OF A COMPOSITE DECAY CURVE

For a mixture of several independent activities, the result of plotting log A versus t is always a curve concave upward (convex toward the original). This curvature results because the shorter-lived components become relatively less significant as time passes. In fact, after sufficient time, the longest-lived activity will entirely predominate, and its half-life may be read from this late portion of the decay curve. Now, if this last portion, which



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