Pearson International A Level Physics Core Practical 15: Investigate gamma absorption by lead
Pearson IAL Physics practical notes on investigate gamma absorption by lead.
This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.
Aim and scientific principle
Measure count rate through absorber thicknesses, subtract background and model attenuation statistically.
Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.
Apparatus and setup
Use sealed school source, Geiger-Muller tube and counter, lead absorbers, ruler or calipers, fixed stands, timer and tongs. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.
Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.
Method
- Measure background count for a sufficiently long interval with the source absent or shielded as directed.
- Fix source-detector geometry and record counts for equal times at several absorber thicknesses.
- Repeat counts where practical and subtract background rates, preserving counting uncertainty.
- Plot corrected count rate against thickness and use a semilog treatment for exponential attenuation if required.
Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.
Variables and controls
- Independent variable: lead thickness.
- Dependent variable: background-corrected count rate.
- Important controls:
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
Start the topic quiz