Learning outcomes
- Plan a fair comparison of absorber effects.
- Use corrected count rates.
- Identify alpha, beta or gamma from absorption data.
- Recognise random uncertainty and safety requirements.
6.1 Purpose of an absorption investigation
An absorption experiment measures how count rate changes when material is placed between a source and detector. It can compare absorber materials or thicknesses and help identify the emission. The source–detector distance must remain fixed because radiation spreads out and air itself can absorb alpha.
The investigation records count rate rather than claiming that every particle has the same range. Radioactive emission is random, and interactions in matter are statistical, so data should be repeated and averaged.
6.2 Basic method
Measure background count for a long interval. Place the sealed source at a fixed distance from the detector and record the count for a chosen time. Insert an absorber without moving source or detector, repeat the count and calculate corrected rates.
For beta, increase aluminium thickness in regular steps. Plot corrected count rate against thickness. When the rate approaches background, most beta particles have been absorbed. For gamma, a gradual decrease is expected rather than a sharp stopping thickness.
6.3 Identifying radiation
If paper causes the corrected count to fall almost to zero, alpha is present. If paper has little effect but a few millimetres of aluminium causes a major fall, beta is likely. If substantial radiation remains after aluminium and decreases only with thick dense material, gamma is present.
A mixed source can produce more than one stage in the graph. A large fall with aluminium followed by a slowly decreasing remainder could indicate beta plus gamma. Do not identify radiation from one uncertain reading alone.
6.4 Variables and uncertainty
Independent variable: absorber type or thickness. Dependent variable: corrected count rate. Control variables include source, detector, separation, alignment, counting time and apparatus settings. Repeat every reading, especially where counts are low.
Random uncertainty is large when only a few counts are collected. Background itself fluctuates, so corrected values near zero may occasionally be slightly negative after subtraction; this indicates measurement uncertainty, not a physically negative source activity.
6.5 Safety
Use only sealed educational sources under local rules. Minimise handling time, maximise distance, use a holder or tongs, point sources away from people, and return them immediately to shielded storage. Never open or modify a sealed source.
An exam plan should include both scientific controls and radiation precautions. Writing only “wear goggles” does not address the main radiation risk.
Worked examples
Identification from data
Corrected rates are 510 counts/min with no absorber, 500 with paper and 42 with 3 mm aluminium. The main radiation is beta.
Percentage reduction
Gamma count falls from 800 to 500 counts/min through lead. Reduction = 300/800 × 100% = 37.5%.
Practical focus
Investigation
Plan an absorber investigation from a supplied equipment list. Include background measurement, fixed geometry, repeated equal-time counts, mean corrected rate, a thickness table and a graph. State time, distance and shielding precautions.
Examination guidance
- Use corrected rather than raw count rate.
- Do not move the detector when inserting absorbers.
- Repeat counts because decay is random.
- A gamma count decreases gradually; it is not guaranteed to become exactly zero.
- Name a relevant safety precaution rather than a general laboratory phrase.
Check your understanding
- Why must source–detector distance remain constant?
- What result suggests beta radiation?
- Why may a corrected count near background be slightly negative?
Answers
- Changing distance changes the detected rate independently of absorption.
- Paper has little effect but aluminium causes a large reduction.
- Random fluctuations in the measured and background counts can make the subtraction slightly below zero.
