The electromagnetic spectrum is the range of values defining the various forms of electromagnetic radiation.  Shown here is the spectrum, labelled according to the wavelength, ¥, of the radiation in a vacuum.  Also shown is the corresponding energy values for a photon of this vacuum wavelength.  Note that energy is inversely proportional to wavelength and thus the highest energy photons are in the gamma portion of the spectrum.

Alpha radiation is the result of alpha decay, a type of radioactive decay that occurs in radioactive species such as Uranium.  This radiation takes the form of a Helium nucleus, a comparatively heavy particle, ejected at a relatively low velocity (compared other forms of radiation).  Interaction with matter occurs through coulombic interactions between the incident alpha particles and electrons in the material being penetrated.  Because of the large charge and mass of these particles they are easily stopped over a short distance by most materials.  As such, this radiation is only able to penetrate the first few layers of our skin, causing little unrepairable damage.  These particles are, however, high energy, and can be extremely dangerous if emitted within a subject's body.  If one ingests a radioactive species the resulting alpha radiation can be extremely damaging to internal organs (which are much more negatively affected by the ionising radiation then skin which is regularly replaced by the body anyway). 

Beta radiation is the result of beta decay, a type of radioactive decay that occurs in radioactive species such as potassium-40.  Beta particles are high-speed electrons or positrons.  If the beta particle is an electron it is said to be ¥^-, where as a positron is said to be a ¥⁺ particle.  Interaction with matter occurs through coulombic interactions between the incident beta particles and electrons in the material being penetrated.  Beta particles are only 10% as ionising as alpha particles, and as such will travel ten times further in a material then an alpha particle.  A thin sheet of aluminium can easily stop beta radiation.  It should be noted that heavier elements are poor choices for shielding beta radiation as the radiation can interact with these materials and produce secondary gamma radiation. 

Gamma radiation can be emitted as a result of many different processes, and takes the form of a high energy photon with an energy corresponding to the ¥-ray portion of the electromagnetic spectrum.  This radiation commonly follows beta decay, with a gamma ray emitted during the de-excitation of an electron in the atom of interest.  A pair of characteristic gamma rays is also emitted during electron-positron annihilation.  This radiation may also be emitted following certain nuclear reactions or the absorption of a thermal neutron by a nucleus (these are called "neutron-capture gamma rays" and are important in understanding some background radiation spectrums).

X-rays are photons with energies corresponding to the x-ray portion of the electromagnetic spectrum.  The emission of an x-ray occurs when a fast electron (such as a ¥^- particle) interacts with matter. When a fast electron is slowed down in an electric field it sheds bremsstrahlung , or "breaking", radiation.  Another source of x-rays is the de-excitation of an excited atom.  This "characteristic" x-ray has an energy th at corresponds to the difference between the initial and final energy of the electron that is de-exciting.  Note that the magnitude of this energy difference dictates whether the photon emitted is an x-ray or a ¥-ray.

Neutron interactions with matter occur as collisions, called scattering events, or in capture events.  Because neutrons have no charge their interaction with matter is unlike that of alpha or beta radiation.  A neutron will collide with a nucleus of an atom in the material being penetrated, but because a nucleus is incredibly small (typically 10,000 times smaller then the electron cloud surrounding it) the probability of a collision is low, resulting in the high penetration distances associated with neutrons.  In the case of an elastic scattering event, a neutron collides with a nucleus it imparts a portion of its energy to the target nucleus, with collisions transferring less energy as the mass of the target nucleus increases.  This energised nucleus, called a recoil nucleus, will move throughout the material causing excitation and ionisation events.  In the case of inelastic scattering events, the neutron is absorbed by the target nucleus, with a gamma ray and a less energetic neutron emitted from the target.  After a neutron has lost a significant potion of its kinetic energy through scattering events it may be absorbed by a target nucleus in a capture event.  The probability of this event happening is inversely proportional to the energy of the neutron, and as such- low energy neutrons (called thermal neutrons) are most likely to initiate this type of event.  The result of this event is that the new atom has its mass number increased by one, and as such will undergo one of many possible nuclear events.  The result is often the emission of ionising radiation.  So, as we have seen, although neutrons are not directly ionising radiation, they often produce secondary events that produce various forms of ionising radiation.

Sieverts (Sv) are a unit of dosage used to describe an exposure to radioactivity in terms easily compared between different types of radiation.  A dose of one Sv is equal to the dose in units of Grays (where one Gray is equal to one joule of radiation energy absorbed per kilogram of tissue) multiplied by a weighting factor (W _R) specific to the type of radiation.

Radon is a naturally occurring element with symbol Rn and atomic number 86.  The heaviest of the noble gasses, it is radioactive, although chemically inert (in other words- non-reactive).  Colourless, it is a pervasive threat to indoor air quality, especially in poorly ventilated basements.  Radon is formed as a decay product of Radium (an alkaline earth metal with symbol Ra and atomic number 88) and is considered a serious health hazard- classified as a radiological poison and carcinogen.

Radiopac is the term for a material through-which electromagnetic radiation is impeded.  Some materials have a low radiopacity, meaning they allow most of the radiation through.  Soft tissues such as ligament muscle and fat are all considered to have very little radiopacity, and are generally invisible for the purposes of x-ray imaging.  Special contrasting agents are sometimes employed to counter this.  On the other hand, bones are highly radiopac making x-ray imaging efficient at determining the skeletal structure.

Electron Beams are streams of high-energy electrons that travel at relativistic speeds.  The effect on a target is identical to that of beta radiation.  The advantage to using an electron beam source is the beam is produced by accelerating an electron through an electric field rather then using a radioactive source to produce beta radiation.  More importantly, an electron beam source is electronic and can be turned off, unlike a radioactive source.  However, just like beta radiation, electron beams are poorly penetrating.  The advantage in using an electron beam for irradiation is a short treatment time compared to gamma and x-rays.

Pasteurisation refers to the use of heat to kill harmful organisms for the purposes of preventing spoilage.  The technique was invented by Louis Pasteur in the 1800's its most common use is on commercial milk products, although eggs, water, juice and beer are all products that can be pasteurised. 

Fumigation refers to the use of gaseous chemicals to kill organisms.  Fumigation is a common form of pest control for buildings as well as consumable goods.  The process is commonly used to prevent the transfer of exotic organisms across boarders in the import/export of fruits and vegetables.