# gamma-ray spectrum vs energy decay

I am going through An Introduction to Applied and Environmental Geophysics, a book by John Reynolds (2011), to better understand radiometrics and gamma-ray spectrometry. The author provides some tables about decaying as well as a plot of airborne gamma-ray spectrometry data.

Table 15.2A shows the Uranium decay series, while Table 15.2C shows the Thorium decay series. The plot shows the survey result of an airborne gamma-ray spectrometry.

As one can see in the plot, the author relates

• the Potassium series with peak at 1,46 MeV with corresponding element 40K
• the Uranium series (shown in Table 15.2A) with peak at 1,76 MeV with corresponding element 214Bi
• the Thorium series (shown in Table 15.2C) with peak at 2,82 MeV with corresponding element 208Tl

The problem is: for both Uranium and Thorium decay series shown, the decay values presented at the tables differ from the peaks at the plot. I would expect to see the Uranium series peak with a value of 3.272 MeV (214Bi decay value in Table 15.2A) instead of 1,76 MeV shown in the plot.

The same for Thorium series: if the corresponding decay energy of 208Tl is 5.001 MeV accordingly to Table 15.2C, why does the plot show a corresponding value of 2.82 MeV?