Definitions

Radioactivity studies the spontanuous transformations of unstable 
nuclei; that is their spontanuous nuclear reactions. 

In 1896, Becquerel, Antoine-Henri underlined the natural radioactivity 
of the Uranium. He observed that photographic plates have been fogged 
when exposed to radiation from a uranium salt.

Radiaoactive nuclei decay into another nucleus more stable, plus one or 
more light particles.

In general, nuclei heavier than lead, and many isotopes of lighter nuclei,
have a finite probability of decaying spontaneously into another nucleus 
plus one or more lighter particles. 

The decay products may be:

1. alpha-particle

An alpha-particle (α) that contains two protons and two neutrons. It 
is the stable nucleus of a helium atom. The related process is written 
as folows:
(A,Z)X → (A-4, Z-2)Y + (4,2)He

Example:
(238,92)U → (234,90)Th + (4,2)He

2. Beta(electron)-particle

A nucleus with more neutrons than it can maintain in stability may 
decay by emission of an electron from the nucleus. This is the 
beta-decay (β-). Along this process, a neutron is 
converted to a proton:

 n → p + e-

These electrons may emerge with a kinetic energy 
of up to several MeV. The process of β- is:

(A,Z)X → (A, Z+1)Y + e-

Example:
(14,6)C → (14,7)N + e-

3. Beta(positron)-particle

The third process β+ is encountred only for 
the artificial nuclei, where a proton is converted to a 
neutron plus a positron:

 p → n + e+

The process of β+ is:

(A,Z)X → (A, Z-1)Y + e+

Example:
(74,33)As → (74,32)Ge + e+