Lecture Notes: Radiation Detection and Gamma Ray Interactions

Introduction

• Utilizing cell phones as radiation detectors demonstrated via a real-world example using a cobalt-60 source.
• Cell phones can detect radiation due to interactions between gamma rays and the semiconductor in the camera.

Sources of Radiation

• Focus on potassium-40 decay, which can undergo electron capture or positron release, emitting a 1.461 MeV gamma ray.
• Examination of what happens when gamma rays encounter matter.

Gamma Ray Interactions

• Three main interactions:
  1. Photoelectric Effect: Gamma ray ejects an electron from the atom.
     • Involves energy balance: ( E_{\text{gamma}} = T + E_{\text{binding}} )
     • Produces a photo peak.
  2. Compton Scattering: Gamma ray scatters an electron and changes wavelength.
     • Described by energy and momentum conservation.
     • Results in a Compton edge on the energy spectrum.
  3. Pair Production: Creation of an electron-positron pair.
     • Occurs above 1.022 MeV, likelihood increases with photon energy.

Photoelectric Effect

• Demonstrated by Einstein; foundational in understanding photon interactions.
• Dependent on the work function and energy of incoming photons.

Compton Scattering

• Photon collides with an electron, changing energy and angle.
• Results in a wavelength shift; always increases (redshift).
• Conservation of energy and momentum used to derive scattering equations.

Pair Production

• Involves gamma rays creating electron-positron pairs.
• Positrons annihilate with electrons, emitting two 511 keV photons.
• Evident in detector as peaks in energy spectrum.
• Presence of 511 keV peak indicates pair production.

Spectrum Analysis and Detection

• Banana Spectrum: Used potassium-40 as an example.
• Photo Peak: Indicates a direct interaction of gamma rays with the detector.
• Compton Edge: Represents maximal energy transfer to electrons.
• 511 keV Peak: Detects positrons and annihilation events.

Detector Size and Efficiency

• Small vs. large detectors affect escape peaks (single and double escape peaks).
• Detector efficiency calculated based on known source activity.
• Efficiency impacts how many interactions are actually detected.

Practical Applications and Experiments

• Banana experiment: Used to explain the practical application of these concepts.
• Understanding how detectors work and how interactions are measured.

Summary

• Emphasis on understanding multi-step processes in gamma-ray interactions.
• Future focus on mathematical derivations of cross-sections and further analysis of interaction probabilities.