Fall-out

radioactive material deposited on the ground from the atmosphere. The source of the radioactive substances is nuclear explosions or escape from nuclear reactors whether through failure to adequately filter coolants or an accident. Radioactive particles settle on material in the air and, in the case of a nuclear explosion, large quantities of dust, soil etc. are sucked up into the central plume of fire and smoke to be deposited hundreds of kilometres from the source, having first risen high in the atmosphere.

Taken from Dictionary of Science

Backscatter

when radiation strikes a surface, some of the beam is REFLECTED back again towards the source. This is called backscatter.

Taken from Dictionary of Science

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In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror. Backscattering has important applications in astronomy, photography and medical ultrasonography.

Backscattering occurs in quite different physical situations. The incoming waves or particles can be deflected from their original direction by quite different mechanisms:

• Rayleigh scattering of electromagnetic waves from small particles, diffuse reflection from large particles, or Mie scattering in the intermediate case, causing alpenglow and gegenschein, and showing up in weather radar;

• inelastic collisions between electromagnetic waves and the transmitting medium (Brillouin scattering and Raman scattering, important in fiber optics, see below;

• elastic collisions between accelerated ions and a sample (Rutherford backscattering)

• Bragg diffraction from crystals, used in inelastic scattering experiments (neutron backscattering, X-ray backscattering spectroscopy);

• Compton scattering, used in Backscatter X-ray imaging.

Sometimes, the scattering is more or less isotropic, i. e. the incoming particles are scattered randomly in various directions, with no particular preference for backward scattering. In these cases, the term "backscattering" just designates the detector location chosen for some practical reasons:

• in X-ray imaging, backscattering means just the opposite of transmission imaging;

• in optical fibers, light can only propagate forward or backward. Forward Brillouin or Raman scattering would violate momentum conservation, so inelastic scattering in optical fibers cannot be anything else but backscattering;

• in inelastic neutron or X-ray spectroscopy, backscattering geometry is chosen because it optimizes the energy resolution;

• in astronomy, backscattered light is that which is reflected with a phase angle of less than 90°.

In other cases, the scattering intensity is enhanced in backward direction. This can have different reasons:

• In alpenglow, red light prevails because the blue part of the spectrum is depleted by Rayleigh scattering.

• In gegenschein, constructive interference might play a role (this needs verification).

• Coherent backscattering is observed in random media; for visible light most typically in suspensions like milk. Due to weak localization, enhanced multiple scattering is observed in back direction.

Taken from Wikipedia

Radiation

the giving out of energy from a source, which may be in the form of ELECTROMAGNETIC WAVES (radio, light, X-rays, infrared rays, etc.), particles or sound waves.

Taken from Dictionary of Science

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In physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body. Non-physicists often associate the word with ionizing radiation (e.g., as occurring in nuclear weapons, nuclear reactors, and radioactive substances), but it can also refer to electromagnetic radiation (i.e., radio waves, infrared light, visible light, ultraviolet light, and X-rays) which can also be ionizing radiation, to acoustic radiation, or to other more obscure processes. What makes it radiation is that the energy radiates (i.e., it travels outward in straight lines in all directions) from the source. This geometry naturally leads to a system of measurements and physical units that are equally applicable to all types of radiation. Some radiations can be hazardous.

Taken from Wikipedia