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Gamma absorption is measured by comparing corrected count rates through different lead thicknesses. Keep geometry fixed so changing distance does not masquerade as attenuation.
What you'll be able to do
Under the centre’s supervised radioactive-source procedure, fix the source and detector positions and place measured lead absorbers between them. Record counts over a known interval for each thickness, including zero added absorber. Keep detector settings, alignment and source–detector separation unchanged.
Measure background independently over a sufficiently long interval and subtract its count rate from each observed rate. Never subtract counts measured over different durations without converting to rates. Handle lead and sources only through the approved equipment and procedures; the lesson supports, rather than replaces, practical supervision.
For a narrow beam at fixed photon energy, a simple attenuation model is I=I0e^(−μx). A graph of ln(I/Iref) against thickness x has gradient −μ. Half-value thickness is ln2/μ. This model is a useful analysis extension; the specified core practical itself requires investigating gamma absorption by lead.
Do not interpret the half-value thickness as a radioactive half-life: one varies absorber distance through material, the other elapsed decay time. Gamma attenuation is probabilistic rather than a sharp finite range. Different gamma energies and scattering geometries can give different effective attenuation.
Decay counts fluctuate randomly. For a sufficiently large raw count N, an approximate counting uncertainty is √N. Longer counting intervals reduce fractional statistical uncertainty; repeated short runs also reveal variation. At high thickness, subtracting background from two similar rates leaves a small signal with a large fractional uncertainty.
Keep the source–detector distance fixed when adding absorbers. Radiation scattered into the detector can alter the apparent attenuation; broad-beam geometry need not match the simplest narrow-beam model. A graph that flattens towards the uncorrected background is not evidence that lead stops absorbing radiation.
Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.
Equation recap
Common mistakes to avoid
Key takeaways
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.