Tritium spike

Figure 2 Time history of the two dominant principal components of tritium in precipitation. Note that the northern component (red curve) is more spike-like and the southern component (blue curve) is more smeared out in time.

The decreased activity within the film tubes is due to physical diffusion of tritium out of the tube and not changes in efficiency from scintillator permeation of the tube wall (figure 2). Unspiked tubes progressively increase in tritium activity during incubation with spiked tubes. 

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