You can use radioactive sources to determine the density of something. The Cs-137 source emits photons when it decays (and betas but those don’t matter here). We know what energy those photons have as well as how many we should see in a second based on what we know about the age of the source itself. You can use that plus a bunch of fancy math to figure out how many photons you should count in a detector a certain distance away. Next we put something we want to know the density of in between the source and the detector and see how much lower our count is. For a lot of common materials we already have tables of what are called interaction coefficients. You use the right table and do some algebra with your measurements and you can figure out the density of the thing you want to measure. This is useful for knowing how well dirt is packed together, how dense rocks are, and it sometimes gets used in manufacturing for quality control.