There are two kinds of beta decay: beta minus and beta plus decay. The differentiation depends on the charge of the emitted particle.
At the beta plus decay in the nucleus a proton changes to a neutron and emits a positron and a neutrino. The atom is after the decay a different element, but with the same number of particles in the nucleus.
At the beta minus decay in the nucleus a neutron changes to a proton and emits an electron and an antineutrino. As with the beta plus decay the atom changes to a different element but with the same number of particles in the nucleus.
Sometimes the electron capture is mentioned as a third kind of beta decay.
Beta decay is used for example in positron-electron tomography or in iodine-131 therapy.

See also Electron Capture.

Characterization of atoms by their nuclear properties, as the number of protons and the number of neutrons. The different nuclides of an element are its isotopes (equal proton number, but different neutron numbers). Isomers of this particular nuclide are equal in the proton and mass numbers, but differ in their energy content. Unstable nuclides which are radioactive are called radionuclides.

See also Isotope, Isomer and Decay.

[Beta Plus Decay] If an atom is unstable because there are too many protons in the nucleus, a proton is converted into a neutron and a positron is emitted. The atomic mass of the atom stays unchanged, but the number of protons increases by one, the number of neutrons decreases by one, which transforms the atom to a different element.

See also Beta Decay.

Ernest Rutherford and Niels Bohr developed in the early 20th century a solar system like model of the atoms, in which electrons orbit around the nucleus (protons and neutrons) held by electromagnetic forces (protons - electrons).
The nucleus is held together by a very strong but short distance nuclear force, attracting all nucleons. While the protons positive charges try pushing it apart, is it the balance between protons and neutrons which decide over an elements stability.
In their model the energy of orbiting electrons is quantized into fixed values. Electrons in outer orbits are more loosely bound than the ones at inner orbits and affect an atom's chemical properties.
Erwin Schrodinger and Werner Heisenberg developed probability functions which assigns the electrons to cloud like spaces instead of fixed orbits.

An accelerator uses electrostatic or electromagnetic fields to increase the kinetic energy of charged particles (see alpha particle, beta particle) in order to produce ionization or a nuclear reaction in a target.
Accelerators (see cyclotron, linear accelerator) are used for the production of radionuclides (see Fluorine-18, Molybdenum, Technetium-99m) or directly for radiation therapy. Accelerator-produced radioactive material (ARM) is any radioactive substance that is produced by a particle accelerator. The accelerators used for radiation therapy generate gamma rays (also called Bremsstrahlung) with continuous energy by collision of high energy electrons on materials with high density (also referred as 'high z' - chemical elements with a high atomic number (Z)).
Electron accelerators with energies above 10 MeV can also produce neutrons induced by photons in the accelerator head material (mainly caused by photo nuclear reaction).