Campbell Scientific CS725 Snow Water Equivalency Sensor - Manual (Page 38)

CS725 Snow Water Equivalency Sensor
absorption and energy degradation by the snow of the gamma rays from the
ground is the basic information for the calculation of the SWE.
Each time a gamma ray is detected by the CS725, it is placed in its
corresponding bin in the energy histogram. This histogram is the basic data set
for the calculation of the SWEs. Since it is statistical in nature, the accuracy of
the SWE evaluation is proportional to the square root of the measurement time.
The CS725 builds each histogram for a period of 24 hours in order to achieve
the specified accuracy. An example of such a histogram is shown in FIGURE
13-1, Gamma histogram.
It should be noted that only selected portions of the histogram can be used to
evaluate the SWE. The reason is that the SWE is evaluated by measuring the
absorption by the snow cover of the natural radiation coming from the ground.
However, some of the measured radiation comes from above the snow cover.
This is the case of the cosmic radiation and the bismuth isotopes produced by
the radon decay chain. In short, the integrated counts under the
40 K and 208 Tl
peaks need to be evaluated after removing or accounting for all the other
contaminants.
FIGURE 13-1. Gamma histogram
30