An experimental setup is constructed to analyze the emission from a strontium-90 (Sr−90\text{Sr}-90Sr−90) source, which is a beta-minus (β−\beta^-β−) emitter.
The 5 mm thick aluminium sheet is completely removed from the gap, and the digital counter is reset to zero. A new count is recorded over a period of 1 minute1\text{ minute}1 minute.
Explain why this new count is significantly greater than 28.