Beta particle

Some beta particles pass through the paper to a detector, while some are absorbed. If the paper becomes thicker, fewer particles reach the detector. Alpha would be stopped too easily, whereas gamma would penetrate so readily that a small thickness change might have too little effect on the count-rate.

In this context, a beta particle means a beta-minus particle: a fast-moving electron emitted from the nucleus.

An unstable nucleus may emit an alpha particle, a group of two protons and two neutrons. This is the same combination of particles as a helium nucleus. It may instead emit a beta particle: in the beta-minus decay considered here, this is a fast-moving electron produced during the nuclear change.

The OCR specification includes four possible emissions: alpha particles, beta particles, neutrons and gamma rays. Each carries a characteristic mass number and charge, so each affects the nucleus differently.