In an idealised school dataset, a detector records counts in equal one-minute intervals: background alone 38; source with no absorber 578; paper 308; thick aluminium 143; thick lead 90.5. Geometry is fixed. Paper stops alpha, this aluminium stops beta, and neither appreciably attenuates gamma here. (a) Estimate the initial alpha, beta and gamma contributions. (b) Explain the lead result. (c) Give two reliability or safety precautions for a supervised demonstration.

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Alpha 270, beta 165, gamma 105 counts/min; lead halves the gamma contribution but does not eliminate it. Subtract successive appropriate readings to isolate what each absorber removes. Paper removes 578 − 308 = 270 counts/min, attributed to alpha under the stated idealisations. The extra loss after replacing paper with thick aluminium is 308 − 143 = 165 counts/min. The aluminium reading minus background gives 143 − 38 = 105 counts/min of gamma. With lead, the source contribution is 90.5 − 38 = 52.5 counts/min. This is half the original gamma contribution; penetrating gamma radiation is attenuated rather than completely blocked by a finite thickness. Radioactive counts fluctuate, so repeat equal-duration measurements and average, while keeping source–detector separation fixed. In a teacher-controlled demonstration minimise exposure time, maximise distance where practical and use suitable handling tools and shielding. Background must be measured, not assumed to be zero.

Interpreting absorber counts 10

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