Safety From External Radiation Hazard

Radiation Safety

• Radiation sources outside the body include α, β, X, γ, and neutrons, which can penetrate to sensitive organs.
• Alpha radiation is not typically considered an external radiation hazard since it cannot penetrate the outer skin layers.

ALARA Principle

• ALARA stands for "As Low As Reasonably Achievable," aiming to avoid unnecessary radiation exposure.
• Three basic protective rules in radiation safety:
  • Time: Minimize time spent near a radioactive source to reduce dose.
  • Distance: Maximize distance from the source to decrease dose rate.
  • Shielding: Use barriers to absorb radiation, depending on the type of radiation.

Distance and Inverse Square Law

• As distance from the source increases, the dose rate decreases.
• If distance from the source doubles, the dose rate reduces to 1/4 of its original value.
• If distance from the source triples, the dose rate reduces to 1/9 of its original value.

Shielding

• Shielding materials depend on the type of radiation:
  • Alpha radiation: Easily absorbed by thin sheets of paper.
  • Beta radiation: Shielded by low Z materials like Perspex (not lead) to avoid secondary x-rays.
  • Gamma and X-ray radiation: Attenuated exponentially by materials, with the dose rate decreasing with increasing shield thickness.
  • Neutron radiation: Materials with high hydrogen content (e.g., paraffin, water, concrete) and elements with high capture for thermal neutrons (e.g., boron, cadmium) are used.

Half-Value Layer (HVL)

• HVL is the thickness required to reduce radiation intensity to one half its incident value.
• High purity lead brick is ideal for X-ray and gamma ray shielding.

Neutron Shielding Materials

• Materials with high hydrogen content (e.g., paraffin, water, concrete) slow down incident neutrons through elastic scatter.
• Elements with high capture for thermal neutrons (e.g., boron, cadmium) are used to capture moderated/thermalized neutrons.
• Borated Polyethylene is a good shield for n-capture reactions, made from high-density polyethylene plastic with 5% boron content.