Nuclei of an isotope of copper (Cu) each have 29 protons and 37 neutrons; this isotope is a β⁻ emitter. The energy spectrum of the β⁻ radiation emitted by a sample of this isotope is shown in Fig. 7.1.

Use Fig. 7.1 to explain why other particles apart from the β⁻ particles must be emitted during this decay.