Gamma Attenuation Experiment

This experiment kit allows you to measure gamma radiation attenuation, verify the exponential attenuation law, and understand shielding effectiveness using various materials.

📄 Download Workbook (DOCX)

1. Objective

To measure gamma radiation attenuation through various materials (Al, Fe, Pb, acrylic), verify the exponential attenuation law, and determine linear attenuation coefficients ($\mu$) to understand shielding effectiveness.

2. Theoretical Background

Gamma attenuation occurs via three primary mechanisms:

Photoelectric Effect – dominant at low energies, especially in high-Z materials.
Compton Scattering – dominant from 0.1–5 MeV, less Z-dependent.
Pair Production – dominant >1.022 MeV, increases with Z.

The exponential attenuation law:

$$I(x) = I_0 e^{-\mu x}$$

Where $\mu$ is the linear attenuation coefficient ($cm^{-1}$).

Important related terms:

Mass Attenuation Coefficient ($\mu/\rho$)
Half-Value Layer (HVL) $ = \frac{\ln(2)}{\mu} $
Tenth-Value Layer (TVL) $ = \frac{\ln(10)}{\mu} $

3. Materials & Equipment

Gamma source: Cs-137
NaI(Tl) detector with PMT and MCA, or GM counter
Shielding blocks: Aluminum, Iron, Lead, Acrylic (3–5 thicknesses each)
Ruler/caliper, stable stand, computer with MCA software

4. Procedure

Safety: Follow all radiation protection protocols.
Setup: Align the Cs-137 source, shielding, and detector in a fixed geometry.
Background: Measure the background count rate (CPM) for 5–10 minutes.
Initial Intensity ($I_0$): Measure the count rate without shielding and subtract the background count.
Attenuated Intensity ($I(x)$): Insert each material at a measured thickness, record gross counts, and subtract the background.
Data Recording: Record $ \ln(I_0/I(x)) $ for each thickness and material.
Quality Check: Ensure proper alignment and avoid gaps between stacked layers.

5. Data Analysis & Interpretation

Create a graph by plotting $ \ln(I_0/I(x)) $ vs. thickness for each material. The slope of the linear fit equals the linear attenuation coefficient ($\mu$).
Compare the experimental $\mu$ values with literature values and rank the materials based on their effectiveness.
Calculate the Half-Value Layer (HVL) and Tenth-Value Layer (TVL) and interpret their meaning for each material.
Discuss potential sources of error and their impact on the results.
Consider the practical applications of your findings, such as designing shielding for medical or industrial use.

6. Conclusion & Notes

Summarize your key findings, confirm the exponential behavior of gamma attenuation, and reflect on the material-dependent shielding and its real-world relevance.

Always follow safety protocols, and minimize exposure time when handling sources.
Maintain a constant source-detector distance throughout the experiment.
Aim for at least 10,000 counts per measurement for reliable statistics.