Wai Chak 發問於 科學及數學其他 - 科學 · 1 十年前

# penetrating abilities of ionising radiation

Explain the reason for the different penetrating abilities of the different types of ionising radiation through paper, aluminium and lead

### 1 個解答

• Carson
Lv 6
1 十年前
最愛解答

Penetrating abilities : Alpha particles < Beta particles < Gamma rays

Reason : Energy / Mass : Alpha is smallest, Beta is medium, and Gamma have virtually no mass, so it is greatest.

Shielding abilities : paper < aluminium < lead

Reason : Density of paper is the least, aluminium is medium, and lead is the highest.

Alpha particles can be stopped by paper.

Beta particles cannot be stopped by paper, but can be stopped by aluminium plate.

Gamma rays cannot be stopped by paper and aluminium plate, but can be stopped only by thick lead plate.

Alpha particles have a typical kinetic energy of 5 MeV (that is ≈0.13% of their total energy, i.e. 110 TJ/kg) and a speed of 15,000 km/s. This corresponds to a speed of around 0.05c. Because of their relatively large mass, +2 charge and relatively low velocity, they are very likely to interact with other atoms and lose their energy, so they are effectively absorbed within a few centimeters of air. As any heavy charged particle, alpha particles lose their energy within a very short distance in dense media. So with a piece of paper (cardboard be better) with enough density, alpha particles can be shielded and absorbed.

Though beta decay is in general a slower process than gamma or alpha decay, beta particles can penetrate hundreds of times farther than alpha particles.

Shielding for gamma rays requires large amounts of mass. The material used for shielding takes into account that gamma rays are better absorbed by materials with high atomic number and high density. Also, the higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically illustrated by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example, gamma rays that require 1 cm (0.4 inches) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 6 cm (2½ inches) of concrete or 9 cm (3½ inches) of packed dirt.