A small amount of ionizing radiation is always present in the surroundings. This radiation comes from many sources such as cosmic ray interactions with the atmosphere, radioactive minerals in the soil and rocks, Radon gas and fallout from nuclear weapons testing. To see what background sources could be identified with this setup, a gamma-ray spectrum was made with no source in front of the detector. This background radiation can be significant when measuring the spectra of weak radiation sources like most of the ones described here. The background spectrum was thus subtracted from most of the other spectra described so background readings would not interfere with the desired data.
The background gamma-ray spectrum is shown below. As with all the other spectra presented here, four 0.093" thick Plexiglass sheets were placed below the detector. This ensured that the background measurement would be consistent with that contributing to the other spectra.
A distinct peak can be seen at 1461 keV due to Potassium 40. A small percentage of all naturally occuring Potassium is in the form of K-40 so Potassium is slightly radioactive. It is present in the soil, in construction materials such as concrete and in the glass used to make the detector. These sources are probably the source of the peak seen here. The other labelled peaks in the spectrum are due to Radon daughter products even though a recent Radon test detected only 0.2 pCi/liter in the test area. This is lower than the "typical" outdoor reading of 0.4 pCi/liter and well below the "take action" level of 4 pCi/liter.
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