The nuclear-physics infrastructure behind PET scans

> Isotopes of a given element have the same number of protons (92 for uranium), but different numbers of neutrons (143 for U-235 and 146 for U-238), meaning they behave the same chemically but differ in mass or radiation emissions.

A fun but off-topic note: "behave the same chemically" is only approximately true. For heavy atoms like the ones discussed in the article, it's basically true. But for hydrogen, adding one neutron doubles its mass and you can get real effects on chemical reaction rates.

https://en.wikipedia.org/wiki/Kinetic_isotope_effect

And of course, an obligatory "In The Pipeline" link on how it's used in drug discovery: https://www.science.org/content/blog-post/isotopes-get-your-...