> A CPM value means nothing without additional context
For CPM, what matters is "CPM of <WHAT>"
CPM just tells you the number of particle detection. It does not tell you the particle type (e.g. alpha, beta, gamma) nor the energy level (i.e. eV). Without context, it is meaningless.
As an example, I can confidently say you are getting over 100bn CPM right now. The reason it doesn't matter is that this is neutrinos and they're not interacting with you[0]. 1CPM or 1e20CPM, who cares. Conversely, 1 CPM can be deadly. You definitely don't want to be hit by a single ReV (10^27) proton (good luck producing that though). Context matters.
> This is why the Sievert exists as a unit.
Sievert is joule per kilogram. So energy divided per mass, much like pressure is force over area. But determining biological impact still takes interpretation. You have weight factors by particle types (e.g. alpha = 2x beta) and there is also weighting factor for internal/external dose and locations like soft tissue (e.g. higher weighting for dose at throat vs dose at hands).
This is why it is incredibly important to use caution when interpreting radiation values. If you don't have training in this it is incredibly easy to unknowingly make major errors. The little details can dramatically change the outcome. Context is critical.
I'm not here to tell you how to actually do the calculation (you'll need a lot more info), I'm here to tell you that it's not easy and you're likely doing it wrong. The experts are not dumb. You're just missing context and a first order approximation is nowhere near enough for an accurate conclusion. It's nuclear physics lol
It shouldn't need be said, but nuclear physics is, in fact, complicated.