Radioactivity: Theory

Continuing the previous topic, let’s get into the theory itself.

The stability of nuclides is different depending on the number of protons and electrons that exist in the nucleus. In case of lighter nucleus (with low Z), great stability happens with nucleus with equal number of protons and neutrons; however in case of heavier nucleus with numbers of protons bigger than 23 are always unstable.

As we explained before, when an unstable nuclide turns into a more stable one, there may be emission of alpha, beta or gamma particles, right? So let’s start with the alpha decay.

Alpha Decay

In the alpha decay, there is of course emission of alpha particles, which are nuclei of Helium atoms (Z=2, A=4). When emitting an alpha particle, one nucleus loses two protons and two neutrons So, the Z decreases two units and the A decreases four units. The alpha decay can be symbolically represented by this way, where the X is the father nucleus and the Y is the son nucleus; one example of the real deal is the alpha decay of Radio-226, where are represented its respective nuclides, and exists conservation of the total charge and conservation of the nucleons in the reagents and in the products, which always exists in a radioactive decay.

Example:

Basically in the Alpha Decay, there are:

Emission of:

And when emitting an alpha particle, one nucleus loses two protons and two neutrons.

In the radioactive decay equations, where the respective nuclides of the elements are represented, exists conservation of the total charge and conservation of the nucleons in the reagents and in the products, which always exists in a radioactive decay.

But in which type of nuclides does the alpha emission happen? Well the alpha emission occurs mainly in heavier nuclides, in which the strong nuclear force is not capable of keeping the nucleons together. Thanks to the conservation of mass-energy, we can verify that the alpha decay is possible whenever the mass of the original nucleus is bigger than the sum of the masses of the nucleus that was created and of the nucleus of the Helium Atom.

Beta Decay

Now about Beta decay, it's a little bit more complicated. The beta decay occurs in nuclei that have an excessive or an insufficient number of neutrons to be stable. And there are two types of Beta decay: the beta minus decay and the beta plus decay.

Beta minus decay

In the beta minus decay, an unstable nucleus, with excess of neutrons, when emitting an electron and an anti-neutrin turns into another nucleus with an atomic number bigger in one unit and equal number of mass; the representation of this equation is expressed in the picture:

One example is the beta minus decay of bismuth-210, which is right below:

But where does the anti-neutrin come from? Well the emitted electron comes from the disintegration of a neutron, which is: Neutron ---> proton+electron+antineutrin, and then brings our anti-neutrin to the table. This is expressed on this picture:

The beta plus decay it’s the opposite; in the beta plus decay, an unstable electron, with very few numbers of neutrons, when emitting a positron e+ and a neutrin turns into another nucleus with an atomic number smaller in one unit and an equal number of mass; the representation of this equation is expressed right below:

One example is the beta plus decay of Sodium-23, which is expressed right below:

The emitted positron comes from the disintegration of a proton, which is Proton ---> neutron+positron+neutrin. The electron and the positron which participate in the beta minus and plus decays are made in the nucleus in the same way which a photon is created, that is, when an atom experiences a de-excitement.

Beta plus decay

Gamma Decay

And then for last but not least we have the Gamma Decay, the easiest decay to learn. In the gamma decay, a radioactive nucleus in an excited state decays to a state of lower energy emitting one or more photons. In this decay, unlike the others, there is no alteration in the atomic number (Z) and in the number of mass (A). The representation of the equation is expressed below.