Interaction with Matter
What happens when the invisible hits the visible?
The Bragg Peak & LET
Alpha particles (⁴He²⁺) are the "Mack Trucks" of the particle world. Due to their high mass (~7300 × me) and double positive charge, they interact continuously and violently with electrons in the medium.
They travel in a straight line, creating a dense column of ionization. As they slow down, their interaction cross-section increases (they spend more time near each electron), resulting in a massive energy dump at the very end of their path.
Linear Energy Transfer (LET)
Alphas have High LET. They transfer a huge amount of energy per unit distance (dE/dx). This makes them devastatingly destructive to biological tissue if internalized, creating unrepairable double-strand DNA breaks.
Scattering & Bremsstrahlung
Beta particles (electrons/positrons) are much lighter (1/7300 mass of alpha). When they hit atoms, they bounce around like ping-pong balls. Their path is tortuous and unpredictable.
Radiative Loss (Bremsstrahlung)
When a high-speed electron is deflected by a heavy nucleus (High Z), it brakes rapidly. Conservation of energy demands that this lost kinetic energy be emitted as a photon—an X-ray.
"Braking Radiation" increases with Energy (E) and Atomic Number (Z). Hence, we shield betas with Low-Z materials (plastic) first to minimize X-ray production.
The Invisible Ghost (Gamma)
Photons (Gamma/X-rays) have no mass and no charge. They play a game of chance. A photon travels until it hits "bingo"—a catastrophic interaction.
The Big Three Interactions
- PEPhotoelectric Effect (Low E, High Z) Total absorption. The photon kicks an inner-shell electron out. Dominant in Lead at low energies. σ ∝ Z5/E3.5
- CSCompton Scattering (Med E) Billiard ball collision. Photon hits an outer electron, scatters, and loses some energy. Dominant in human tissue.
- PPPair Production (High E > 1.02 MeV) Energy → Matter. The photon disappears near a nucleus, creating an electron (e⁻) and positron (e⁺).
The Exponential Law
Because gamma interaction is probabilistic, we never truly stop all radiation. We simply reduce the intensity (I) exponentially with thickness (x).
Half-Value Layer (HVL)
A practical engineer's unit. The thickness required to
cut intensity in half. HVL = 0.693 / μ
Apply this formula (I = I₀e-μx) to calculate safety requirements for real isotopes.
Virtual Shielding Lab
Theory is fine, but seeing is believing. In this virtual lab, you can fire all three types of radiation at different shielding materials.
Notice how Alpha is stopped by almost anything, while Gamma requires dense lead to be significantly attenuated.
Experiment
- Try stopping Beta particles with Paper vs. Plastic.
- Observe the random nature of Gamma interactions.
- See how plastic effectively shields charged particles.
- Compare penetration depths of different particles.