Neutrinos were hypothesised to conserve energy in beta decay

Atomic Structure & Nuclear Stability — AQA A-Level Physics

Electron neutrinos ($\nu_e$) and anti-electron neutrinos ($\bar{\nu}_e$) are subatomic particles with no chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). and negligible mass.
$\beta^{-}$ decay produces an anti-electron neutrino ($\bar{\nu}_e$).
$\beta^{+}$ decay produces an electron neutrino ($\nu_e$).

Evidence for the neutrino

Alpha particles have discrete energies (sharp spikes on an energyThe capacity to do work. Measured in joules (J). spectrum).
Beta particles have a continuous range of energies (smooth curve).
If only the beta particle were emitted, conservation of energyThe capacity to do work. Measured in joules (J).Energy cannot be created or destroyed, only transferred from one form to another. The total energyThe capacity to do work. Measured in joules (J). of a closed system remains constant. would require it to have a single fixed energy.
The continuous spectrum shows energy is shared between the beta particle and a neutrino (or anti-neutrino).
This was one of the first clues that neutrinos exist.

Examiner Tips and Tricks

The continuous energy spectrum of beta particles is the key piece of evidence for neutrinos.
Alpha particles have discrete energies because no neutrino is involved — all the energy goes to the alpha particle and the daughter nucleus.