Look back at Figure 15.12. You were asked what conclusion could be drawn from the observation that all the $\alpha  - $particle tracks were the same length. The answer is quite simple: it suggests that they all have the same initial kinetic energy. This should not surprise you, as they are all the result of the similar reactions in identical nuclei. However, when we look at the energies of $\beta  - $particles (both ${\beta ^ - }$ and ${\beta ^ + }$) the results are quite different, as shown by the graph in Figure 15.14.

You will notice that the energy of the $\beta  - $particles is measured in MeV (mega electronvolts). Alpha and beta particles move quickly; gamma photons travel at the speed of light. These types of radiation all have energy, but the energy of a single particle or photon is very small and far less than a joule. So we use another, much smaller unit of energy, the electronvolt, when considering the energy of individual particles or photons.
When an electron (with a charge of magnitude $1.60 \times {10^{ - 19}}C$) travels through a potential difference, energy is transferred. The energy change W is given by:

$W = QV = 1.60 \times {10^{ - 19}} \times 1 = 1.60 \times {10^{ - 19}}J$

One electronvolt ($1 eV$) is the energy transferred when an electron travels through a potential difference of one volt.
Therefore:

$1\,eV = 1.60 \times {10^{ - 19}}J$

There is more about the electronvolt and its use in energy calculations in Chapter 28.
The graph shows that the $\beta  - $particles have a wide range of energies. One of the great physicists of the early $20th$ century, Wolfgang Pauli, suggested that another particle carries off some of the kinetic energy.
This particle was not easy to detect–Pauli hypothesised its existence in 1930, but it was not detected until 1956. The particle has no charge and virtually no rest mass (much less than an electron) and barely interacts with matter at all. We now know that there is a steady stream of them given off by the Sun, some of which travel straight through the Earth without any interaction with it at all (which is why it’s difficult to detect them!) The particle was named the antineutrino, and is now known as the electron antineutrino. The particle given off when a positron is emitted is called the electron neutrino. The symbol used for the electron neutrino is the Greek letter (nu), and the electron antineutrino is (nu bar).