E.3.13—Background radiation

Separate sample and background

A detector count rate can include decays from the sample plus background radiation. The measured rate is $R_{measured}=R_{sample}+R_{background}$.

Subtract before analysing

Estimate the background count rate with the source absent or from the long-time plateau, then calculate $R_{net}=R_{measured}-R_{background}$. Use the net rate for half-life comparisons.

Worked example — subtract, decay, restore

A detector reads $260\,\mathrm{Bq}$ with a $20\,\mathrm{Bq}$ background. The initial net rate is $240\,\mathrm{Bq}$. After four half-lives it is $240/16=15\,\mathrm{Bq}$, so the detector reads $15+20=35\,\mathrm{Bq}$.

Interpret a non-zero limit

If the measured rate approaches a non-zero constant, the remaining signal may be background radiation or a systematic detector contribution. The sample activity itself may have continued toward zero.

Common trap

Do not fit a half-life directly to a count rate that still contains background; the offset distorts the decay curve.