Helium is the second most abundant element in our universe, and as such plays an important role not only on Earth, but also in sciences such as astronomy and astrophysics concerned with our whole universe. Helium-7 is different to the typical type of helium which is found in space. The normal, stable and most abundant type of helium is called helium-4, because the total number of nucleons is 4 (2 protons and 2 neutrons in nucleus).
Helium-7 is consequently considered a radioisotope. This is because it has 7 nucleons (2 protons and 5 neutrons) rather than the normal number of 4. Consequently, the nucleus of helium-7 is unstable and hence undergoes radioactive decay.
There are three main types of radioactive decay. Helium-7 has an excess of neutrons, and hence decays via beta-minus decay. This type of decay converts a neutron into a proton and electron. The produced proton stays in the nucleus whilst the electron is ejected and is said to be a beta minus particle.
Additionally, during beta minus decay, an antineutrino is released, and this is a small particle that conserves mass-energy. Energy in the form of gamma rays is also released. Consequently, although we say that helium-7 decays via beta decay, a far more accurate description would be that it decays via beta-minus decay emitting an antineutrino, electron (beta particle) and gamma rays.
Why does helium-7 undergo beta-minus decay rather than other forms of radioactive decay such as alpha decay? The answer to this is that in order to achieve stability, the helium nucleus needs to reduce its number of neutrons and increase the number of protons. This balances the opposing forces in the nucleus and hence prevents implosion. This is because protons, being positively charged will repel each other in the nucleus, whereas neutrons (and protons) will be attracted via a nuclear force. By undergoing beta-minus decay, helium-7 achieves stability.
In conclusion, beta particle emission is a type of radioactive decay that is carried out by a nucleus to achieve stability. Helium-7 undergoes beta-minus decay emitting a beta particle (electron), antineutrino and energy in the form of gamma rays. It is through understanding this, that we scientists can develop methods of utilising helium-7 in industry and medicine.