| Whenever particle radiation encounters a matter one of three things can happen. |
| 1. The particle radiation may undergo surface reflection. Radiation which is reflected at the surface of a living substance, without interacting with the atoms and molecules of the which the substance is made is of no biological significance. |  Transparency Master |
| 2. The particle radiation may be transmitted completely through the substance it encounters. Radiation which passes through a living substance, without interacting with the atoms and molecules of which the substance is made is of no biological significance. |  Transparency Master |
| 3. The particle radiation may be totally or partially absorbed by the substance. The process of absorption transfers energy to the atoms and molecules of the substance. This may have a significant effect on living organisms. |  Transparency Master |
| Now..recall that matter is not really a continuous medium (as shown in the sketches above), but rather it is made of atoms and molecules which are mostly empty space filled by a small dense nucleus of neutrons and protons, surrounded by a "shell" of electrons in distinct and well defined energy levels. |
Deflection Transparency master |
Whenever particles interact with matter they transfer energy through collisions, in a process not unlike a car crash.
At low kinetic energies (less than the ionization potential of the atom) the incident particle is likely to be reflected, and in the process transfers a small amount of recoil energy to the whole atom. This energy manifests itself as a slight overall temperature increase in the substance. |
Ionization Transparency master |
If the kinetic energy of the (radiation) particle is relatively small, the collision is a subatomic "fender-bender".
For the neon atom in the illustration to the left, the impact is in excess of 21.564 eV ( the ionization potential of neon). In this instance an electron is ejected from the atom.
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Nuclear collision Transparency master |
If the kinetic energy of the particle is really large, (and for some cosmic ray collisions a single proton can hit another atom with the energy of a professionally pitched baseball....the energy density is astonishingly high), the result is the spectacular destruction of everything involved in the impact with hundreds of subatomic fragments flying in all directions. The impact fragments include a wide range of subatomic particles as well as radioactive fragments of the original nucleus. |
 Bubble chamber photograph of a neutron collision with a proton (hydrogen nucleus) |
The image to the left shows the collision of a neutron (whose trajectory is highlighted in red) which is moving from left to right through the bubble chamber. The neutron has zero electrical charge and therefore easily penetrates the atom's electron shell. At approximately mid-point the neutron collides with the nucleus of a hydrogen atom and produces a shower of charged sub-atomic particles. The collision fragments which are electrically charged move along curved trajectories because they are moving through an externally applied magnetic field. |
DATA SHEET on PARTICLE RADIATION