Alpha, beta, gamma compared

Problem

A radioactive source simultaneously emits $\alpha$, $\beta$ and $\gamma$ radiation. Explain why a simple sheet of paper is enough to stop the $\alpha$s, whereas the $\gamma$s pass through several centimetres of lead. What is needed instead to stop the $\beta$s? Justify your answer based on the physical nature of the three types of radiation.

Penetrating power of the three types of radiation: the $\alpha$s are stopped by paper, the $\beta$s by a few mm of aluminium, the $\gamma$s only by thicknesses of lead.

Solution

$\alpha$ radiation. These are helium nuclei ($2$ protons $+ 2$ neutrons): massive particles with double charge. They interact strongly with the electrons of matter and are therefore highly ionising: they deposit all their energy over a very short distance. A sheet of paper or the outer layer of skin is enough to stop them.

$\beta$ radiation. These are electrons (or positrons): much lighter than $\alpha$s and with single charge. They ionise less and penetrate further, but their charge still makes them stoppable. A few millimetres of aluminium (or a few metres of air) are needed to stop them.

$\gamma$ radiation. These are high-energy photons, electrically neutral. Having no charge, they interact little with matter and are not “slowed down” continuously: they can only be absorbed in single collisions. Appreciably attenuating them requires large thicknesses of dense material, such as several centimetres of lead or decimetres of concrete.

Summary. The increasing order of penetration $\alpha < \beta < \gamma$ reflects decreasing charge and mass, and hence decreasing ionising capacity.

$\ev{\alpha:\text{ paper};\ \beta:\text{ mm Al};\ \gamma:\text{ cm Pb}}$

Collegamenti

Argomenti: Fisica nucleare Concetti: Decadimento radioattivo