O-LevelPhysicsNuclear Physics5054/11 · May/June 2015 · Q391 Mark
A source contains a radioactive material. Without the radioactive source present, a detector records a background count rate of 20 counts per minute. This source is placed in a fixed position near the detector. Initially a count rate of 520 per minute is recorded. What count rate is recorded after a time of two half-lifes of the radioactive source?
A125 counts per minute
B130 counts per minute
C135 counts per minute
D145 counts per minute
✓ Correct answer
D: 145 counts per minute
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