Explain why, for the same kinetic energy, alpha particles travel a shorter distance in air than beta particles.
Answer
Alpha particles have a much larger mass and charge (+2e) than beta particles (≈−e); consequently they lose energy much faster through ionisation and excitation of air molecules, so they are stopped in a shorter path.
Step-by-step solution
- Note that the kinetic energy of both particles is the same, but the stopping power depends on charge and mass.
- Alpha particles carry charge and have a mass about 7300 times that of an electron; their large charge creates a strong Coulomb interaction with the atoms of air, producing many ionisations per unit length.
- Because they are heavy, alphas cannot be significantly deflected; they lose energy primarily by ionising, which quickly reduces their speed.
- Beta particles (electrons) have charge and much smaller mass, so they cause fewer ionisations per unit length and can travel farther before their kinetic energy is exhausted.
Common mistake: Attributing the short range only to the larger mass and ignoring the effect of the double charge of alphas.
Concept tested: Range of charged particles in matter
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