An alpha particle is made up of two protons and two neutrons bound together. This type of radiation has a positive charge. An alpha particle is sometimes represented using the chemical symbol He2+He2+, because it has the same structure as a Helium atom (two neutrons and two protons) ,but without the two electrons to balance the positive charge of the protons, hence the overall charge of +2. Alpha particles have a relatively low penetration power. Penetration power describes how easily the particles can pass through another material. Because alpha particles have a low penetration power, it means that even something as thin as a piece of paper, or the outside layer of the human skin, will absorb these particles so that they can't penetrate any further.

Alpha decay occurs in nuclei that contain too many protons, which results in strong repulsion forces between these positively charged particles. As a result of these repulsive forces, the nucleus emits an αα particle. This can be seen in the decay of Americium (Am) to Neptunium (Np).

Example:

95 241 Am → 93 237 Np + α particle 95 241 Am → 93 237 Np + α particle

Let's take a closer look at what has happened during this reaction. Americium (Z = 95; A = 241) undergoes αα decay and releases one alpha particle (i.e. 2 protons and 2 neutrons). The atom now has only 93 protons (Z = 93). On the periodic table, the element which has 93 protons (Z = 93) is called Neptunium. Therefore, the Americium atom has become a Neptunium atom. The atomic mass of the neptunium atom is 237 (A = 237) because 4 nucleons (2 protons and 2 neutrons) were emitted from the atom of Americium.

Figure 2

In nuclear physics, ββ decay is a type of radioactive decay in which a ββ particle (an electron or a positron) is emitted. In the case of electron emission, it is referred to as beta minus (ββ-), while in the case of a positron emission as beta plus (ββ+).

An electron and positron have identical physical characteristics except for opposite charge.

In certain types of radioactive nuclei that have too many neutrons, a neutron may be converted into a proton, an electron and another particle called a neutrino. The high energy electrons that are released in this way are the ββ - particles. This process can occur for an isolated neutron.

In ββ+ decay, energy is used to convert a proton into a neutron(n), a positron (e+) and a neutrino (ννe):

So, unlike ββ-, ββ+ decay cannot occur in isolation, because it requires energy, the mass of the neutron being greater than the mass of the proton. ββ+ decay can only happen inside nuclei when the value of the binding energy of the mother nucleus is less than that of the daughter nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron and a neutrino and into the kinetic energy of these particles.

The diagram below shows what happens during β-β- decay:

Figure 3: ββ decay in a hydrogen atom

During beta decay, the number of neutrons in the atom decreases by one, and the number of protons increases by one. Since the number of protons before and after the decay is different, the atom has changed into a different element. In Figure 3, Hydrogen has become Helium. The beta decay of the Hydrogen-3 atom can be represented as follows:

1 3 H → 2 3 He + β particle + ν ¯ 1 3 H → 2 3 He + β particle + ν ¯

Figure 4

When particles inside the nucleus collide during radioactive decay, energy is released. This energy can leave the nucleus in the form of waves of electromagnetic energy called gamma rays. Gamma radiation is part of the electromagnetic spectrum, just like visible light. However, unlike visible light, humans cannot see gamma rays because they are at a much higher frequency and a higher energy. Gamma radiation has no mass or charge. This type of radiation is able to penetrate most common substances, including metals. Only substance with high atomic masses (like lead) and high densities (like concrete or granite) are effective at absorbing gamma rays.

Gamma decay occurs if the nucleus is at a very high energy state. Since gamma rays are part of the electromagnetic spectrum, they can be thought of as waves or particles. Therefore in gamma decay, we can think of a ray or a particle (called a photon) being released. The atomic number and atomic mass remain unchanged.

Figure 5: γγ decay in a helium atom

Table 1 summarises and compares the three types of radioactive decay that have been discussed.