Subtract background to find a corrected count rate
Use matched source-present and source-absent rates without treating background as zero.
Section 5.2.1 · Detection of radioactivity
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028. Use labelled nuclides, supplied count intervals and stated background conditions. Model drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Calculate count rates.
- Know what background radiation means.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How is a corrected source contribution estimated?
What you will learn
- Use matched source-present and source-absent rates without treating background as zero.
- Calculate and interpret a background-corrected detection rate with appropriate statistical limits.
Subtract rates with matching units
A source-present detector record includes the investigated signal plus background detections. An estimate of the source contribution is corrected rate = source-present rate − background rate, using comparable detector conditions and the same rate unit.
For 180 counts/min with the source present and 30 counts/min background, corrected rate is 150 counts/min = 2.5 counts/s. The background estimate is measured without the investigated source. The correction estimates the detected source contribution, not automatically the source’s full activity.
Retain statistical and apparatus limits
Use a representative background record and comparable geometry and settings. Repeated or longer reference observations can make a mean estimate more useful, but do not remove randomness. A single source-present count need not exceed a separate short background count because both fluctuate.
If subtraction produces a small negative estimate, it does not mean a source emits negative radiation. Report the statistical limitation and gather appropriate comparable evidence. Do not quietly convert every uncertain negative result into proof of zero physical source activity.
Pause and explain
A small negative corrected estimate arises from two short random records. What does it establish?
Worked example
Source-present rate is 180 counts/min and background is 30 counts/min. Find corrected rate in counts/min and counts/s.
Show the worked solution
- Both rates use counts/min.
- Subtract 30 from 180 to obtain 150.
- Divide by 60 for 2.5 counts/s.
Answer 150 counts/min; 2.5 counts/s.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Correct 180 counts/min using a background of 30 counts/min.
Give me a hint
Subtract comparable rates.
Compare my reasoning
- Gross rate is 180.
- Background is 30.
- Corrected rate is 150.
150 counts/min.
Look for these in your work
- Identify gross and reference rates.
- Keep the unit.
Correct 8 counts/s using 2 counts/s background.
Give me a hint
Retain the same rate unit throughout.
Compare my reasoning
- Gross rate is eight.
- Background rate is two.
- Their difference is six.
6 counts/s.
Look for these in your work
- Subtract in the correct order.
- Do not multiply the count rate by an unrelated time.
A 29 counts/min gross record and 31 counts/min background give −2 counts/min. Interpret the result.
Give me a hint
Consider statistical variation in separate observations.
Compare my reasoning
- The arithmetic estimate is −2.
- Physical source emission is not negative.
- The short records do not resolve a positive detected contribution reliably.
A −2 counts/min estimate reflects fluctuation limits, not negative radiation.
Look for these in your work
- Report the computed estimate.
- Explain its physical limitation.
Common mistakes
- Subtract background, not source rate from background.
- Use the same rate unit.
- Corrected detection rate is not automatically source activity.
What can you explain now?
Which condition measures the background reference?
Compare with the explanation
Without the investigated source
The source-absent observation estimates surrounding detections under comparable conditions.
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Make it stick
Calculate and interpret a background-corrected detection rate with appropriate statistical limits.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.