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Each nucleus gets a random decay time from the real first-order law, so every run is a little different. Time is sped up enormously (or slowed down, for short half-lives) so one half-life takes a few seconds.
Real nuclides follow their measured decay modes and half-lives. A nucleus you place yourself follows the rule of thumb from the band of stability, which real nuclei do not always obey. The band is drawn from a simplified fit.
Your job
Drag a card onto a box, or tap a card and then tap the box's name. Cards are buttons, so Tab and Enter work too.
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Teaching model: a point source, inverse-square spreading, typical ranges (alpha about 4 cm of air) and half-value layers for cobalt-60 gamma rays (about 1.2 cm of lead). Background is about 0.5 counts per second.
The picture updates with the answer you type. The worked solution shows every unit cancelling (struck through in red), with given values and conversions flagged in gold.
Teaching models with rounded values. Not medical advice; follow your care team's and your local health authority's instructions.
Wrong picks are fine here. Each one tells you why.