Alpha particles consist of two neutrons and two protons (nucleus of He), have a positive charge of 2 and a velocity in air of approximately one-twentieth the speed of light.
Discovered by Ernest Rutherford in 1899 (Rutherford-Bohr planetary atom model) alpha particles became emitted by very large atoms in an unstable energy state (high atomic number mostly over 82, with a too low neutrons//protons ratio (e.g. <= 1.5)).
Through their relative slow travel speed, they get stopped by e.g. a thin sheet of paper or the outer layers of human skin or travel only inches; once stopped they pick up free electrons and become helium.
These alpha particles are only dangerous to humans when the alpha-emitting material is inhaled or ingested (it causes damage that may lead to cancer) or comes into contact with the lens of the eye, caused by their less penetrating properties.

Absorbers consist of material that stops ionizing radiation. For example, lead, steel and concrete attenuate x-rays. Alpha particles and most beta particles can be stopped or absorbed by a sheet of paper or thin metal.
The absorption depends on the atomic number, density, thickness, etc. of the used material.
The interactions between the radiation and the absorber are three major processes: photoelectric absorption, Compton scattering, and pair production.

See also Absorption.

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).

Activation is the production of radionuclides (instable atoms) by bombarding atomic nuclei (stable atoms) with radiation (e.g., photons, neutrons, alpha particles).
With the activation of an atom its decay starts.

See Neutron Activation, Neutron Activation Analysis.

The alpha decay is a corpuscular radiation. Two protons and two neutrons (a helium core) are emitted from the atomic nucleus. Because of the high biological effectiveness, the use of alpha particle radiation is not allowed in diagnostic nuclear medicine.