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From the half-lives and atomic masses (the little numbers that determine how many grams a mole weighs), they can calculate the specific activity of each sample.

• ²³⁵U: 7.99 × 10⁴ Bq/g
• ²³⁹Pu: 2.29 × 10⁹ Bq/g
• ²²⁵Ra: 1.44 × 10¹⁵ Bq/g

Yeah, Simon's sample is 600000x more active than Theodore’s, which is a further 3000x more active than Alvin’s. Even though Simon’s sample produces mostly β particles (which are generally about 10 times less destructive), he is clearly the worst here.

Multiply that by the number of grams in the sample and you get the activity of each sample in becquerels.

Now just use a chipmunk body model and estimated distance from each sample to calculate the absorbed dose in grays (not to be confused with equivalent dose measured in sieverts). 70% lethal dose over 30 days is 10~12 Gy for mice so chipmunks should have it similar but take into account that they weigh around 100 g.