Select radiation for thickness monitoring
Use penetration and absorption to choose a source and interpret a matched detector reading.
Extended only (Supplement); Physics 0625, 2026–2028. Use labelled nuclides, supplied count intervals and stated background conditions. Model drawings are not to scale. Written practice is self-checked.
Before you begin
- Compare radiation penetration.
- Calculate and compare count rates.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why is beta suitable for the supplied thin-paper model?
What you will learn
- Use penetration and absorption to choose a source and interpret a matched detector reading.
- Choose partial transmission and interpret thickness signals with matched operating conditions.
Choose for partial transmission
A thickness monitor places a source on one side of material and a detector on the other. In a supplied thin-paper model, beta is suitable because it is partly absorbed: enough reaches the detector, while thickness changes affect the count. Alpha is stopped too easily; gamma may transmit with too little change for the stated thin-material model.
Thicker or denser material may need more penetrating gamma. The best choice depends on material, thickness range, emission energy and detector conditions. No emission is automatically ideal for every sheet.
Read the feedback signal
With matched geometry, source, detector and background conditions, thicker material generally absorbs more of the chosen radiation, lowering the transmitted count rate. In a calibrated model with target 100 counts/s, 70 counts/s indicates greater absorption and a sheet thicker than the target.
The process controller should reduce thickness in that stated model, but the direction of a particular roller adjustment depends on the machine. A source with a suitable long half-life limits rapid baseline drift; source decay or changed detector geometry can otherwise mimic a thickness change.
Pause and explain
Does a lower count prove greater thickness if the source and geometry also changed?
Worked example
In the matched beta-paper model, the target is 100 counts/s and the reading is 70 counts/s. Interpret it.
Show the worked solution
- The reading is below the target.
- More beta is absorbed under the matched conditions.
- The sheet is thicker than the target.
Answer The sheet is too thick in this calibrated model.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Choose alpha beta or gamma for the supplied thin-paper model requiring partial absorption.
Give me a hint
Reject complete stopping and excessive transmission.
Compare my reasoning
- Alpha is readily stopped.
- Gamma may give too little stated thin-sheet absorption change.
- Beta provides useful partial transmission in this model.
Beta.
Look for these in your work
- Use the specified material range.
- Explain partial absorption.
A matched monitor target is 100 counts/s, while the reading is 70 counts/s. State the thickness correction needed.
Give me a hint
Compare the reading with the calibrated target.
Compare my reasoning
- The lower reading means greater absorption.
- The sheet is thicker than target.
- The process should reduce thickness.
Reduce the sheet thickness in the supplied model.
Look for these in your work
- State the interpretation.
- Avoid guessing a roller direction.
A source decays between measurements, while the sheet is unchanged. Can the lower reading be used alone as proof of a thicker sheet?
Give me a hint
Check whether source output stayed matched.
Compare my reasoning
- The source rate decreased.
- A lower signal can occur without thickness change.
- A matched or corrected baseline is needed.
No; source decay can mimic increased absorption.
Look for these in your work
- Name the confounding change.
- Require matched source conditions.
Common mistakes
- Choose partial transmission.
- Keep geometry and source conditions matched.
- Machine adjustment direction needs its own specification.
What can you explain now?
Which usually lowers transmitted count in the matched model?
Compare with the explanation
Greater thickness
More material absorbs more of the selected radiation.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Choose partial transmission and interpret thickness signals with matched operating conditions.
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.