Gamma spectrum: Question about cross sections If I have a gamma spectrum and I suspect that I should have an isotope, Fe-59 for example, present in the environment, I will then look here: 
http://nucleardata.nuclear.lu.se/toi/nuclide.asp?iZA=260059 
to see what spectral lines I might expect. As we can see in this case, there are high emission intensities at 1099KeV and 1291KeV. If I do not observe lines at these energies (or any of the others), can I then outright conclude that the isotope is not present?
It's worth clarifying that the reason I would expect this specific isotope to be present is because it's part of the decay chain of Cr-59, which I also expect to be present.
 A: The peaks at $E = 1099\:\mathrm{keV}$ ($P = 56.5\:\%$) and $E = 1292\:\mathrm{keV}$ ($P = 43.2\:\%$) are the most important gamma lines for Fe-59. If a significant Fe-59 activity is present in your sample and your detector is sufficiently sensitive in this energy range, you should be able to see both peaks. The intensity of both peaks might be reduced due to coincidence summing (so-called “summing out”), but the peaks would not completely disappear.
However, if you do not see the peaks in the spectrum, you cannot simply say that the isotope is not present.
First you should calculate the decision threshold. If the result of your measurement exceeds the decision threshold, you may conclude that the nuclide is present. Otherwise, the nuclide is assumed to be absent. Then you should calculate the corresponding detection limit, which is the smallest true activity that is detectable by your measuring method.
Thus, if the measured peak intensity is below the decision threshold, you can only guarantee that the activity in the sample is below the detection limit, but you cannot guarantee that the activity is zero.
