Will try to present these in more bite-sized portions.
crosspatch was looking for gamma emitters among the list of isotopes released at Fukushima. For each, wolframalpha answered “mode of decay: beta emission”. It never returned a hit of “gamma emission”.
So here’s the thing: gamma rays aren’t a mode of decay. The “mode of decay” is how a nucleus changes its proton/neutron ratio in seeking stability. Gamma rays aren’t a necessity, they’re a (common) by-product. Wolframalpha didn’t address whether beta-emitters also gave off gammas. Which is pitiful, really.
There are pure beta emitters: tritium, P-32, even Cs-137 (which I’d forgotten). But most beta emitters do emit gammas. Not necessarily with every decay, though.
And one beta emitter, mighty Cobalt-60, “Eater of Men’s Dose”, emits 2 gammas with every single decay. These, at 1.17 MeV and 1.33 MeV (mega electron-volts), are very powerful for decay gammas. In the commercial nuke field Co-60 is the largest single contributor to personnel dose–by far.
Are there pure “gamma” emitters, then? Not in decay. Again, radioactive decay is about the nucleus balancing protons and neutrons. Firing off a photon doesn’t change a thing in that regard.
But a nucleus can be in a “metastable” state. Which means…? Call it a state of excitation. Think of the entire nucleus “vibrating” with excess energy. Eventually, sooner or later, it will cast off that excess energy the way a plucked guitar string gives off a note.
(Which is a terrible analogy if extended any further, so just…don’t).
The most useful thing about the metastables comes in radiopharmacy. Since they emit only penetrating radiation, when taken into the body for diagnostic purposes you don’t hammer the patient’s insides with betas and alphas, which means….
Whoops, gotta run. Questions are always welcome!