Ah, but is your sample still live enough to be "cryptographic grade" random? Is the hardware that measures the source and the software that reports it subject to any periodicity that you don't know about but your attackers might?
(Some) People who study this often get lost down the rabbit hole and come out thinking the universe is deterministic.
Any distribution with a sufficient amount of entropy can be turned into "cryptographic-grade" randomness source using randomness extractors [1]. These work independently of any outside factors that might be trying to sneak signal (e.g. periodicity) into the noise -- as long as you can prove there's sufficient entropy to start with, you're good to go.
Low-intensity radiation is random enough, but it's slow: your device is necessarily twiddling thumbs between a detected event and the next, and entropy is mostly proportional to the number of events (for example, almost n bits from what of 2^n identical units is hit by the next particle).
Or, it's what one of my ex-NSA buddies told me: we almost never break the encryption, we break the implementation, because that's where the errors are.
The same can assuredly apply to capturing entropy.
