Atomic Structure And Definitions Of Terms

In order to understand how xrays interact with matter, it is necessary to define terms used in the structure of the atom.

The atom consists of protons, neutrons and electrons, essentially. Protons and Neutrons comprise the nucleus of the atom. Protons, which have a positive charge and neutrons which carry no discernible charge, together equal the atomic mass of the atom or "A". Protons (+ charge) alone equal the "Z" number of the atom. In a stable atom the number of electrons (- charge) equal the number of protons thus effectively producing an overall neutral charge. If the balance or stability of the atom is changed by the removal or addition of an electron or a proton, the atom becomes a charged entity called an ion.

In nature, all atoms seek their lowest energy state or, in other words, seek to return to stability. This is fundamental to the production of x-rays or photons. In doing so, a series of events may occur.

Electrons orbit the nucleus in a series of shells or energy levels. The energy needed by the electron to remain in orbit is termed "binding energy". Those shells closest to the nucleus therefore are highest in binding energy while those levels furthest from the nucleus have less. It is important to note and remember that this binding energy is negative energy. In order to remove an electron from its orbit, one needs an energy equal to or greater than its binding energy. The binding energy of the electron is dependent upon the atomic number of the atom or "Z".

It is also important to remember that though an atom is a minute structure, the space between each electron is immense in proportion to the mass of each particle. In the event that an electron is removed from its orbit, thereby making the atom unstable, electrons will "fall" into the space vacated in the energy shell, releasing energy in the amount of difference between the two energy shells.

example: A K shell electron with a binding energy of 69.5 KeV is removed. The binding energy of the L shell (next energy level) electron is 12.1 KeV. Therefore, the excess energy of (-12.1)-(-69.5) or 57.4 KeV is given off as a photon or x-ray. It is important to remember that electron energy is NEGATIVE energy. [KeV is thousands of electron volts.] The K shell electron is therefore MORE negative in relation to the L shell electron.

Each energy shell requires a different number of electrons to fill it. Stray electrons may also be captured in order to achieve stability.

Electrons that are freed may in their turn react with other atoms. There are two major forms of interaction: Bremstrahlung or "braking" radiation which is the energy given up when a high speed electron passing close to a nucleus is deflected (change in direction) by the charge of the nucleus and slowed; and the other is known as characteristic radiation or interaction with another electron, which was previously described.

The photons (or X-rays) released in these reactions may then interact with matter. A photon has no charge and no mass. These are the interactions that diagnostic and therapeutic radiology use in producing films of patients and in the treatment of cancer.

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