Chapter 25

• The four main types of radiation are due to alpha particles, electrons (negatively charged beta particles or positively charged positrons), gamma rays, and x-rays.
• Alpha particles are helium nuclei (consisting of two protons and two neutrons), with a charge of +2, that are ejected from the nucleus of an atom.
• Beta particle decay occurs when a neutron in the nucleus of an element is effectively transformed into a proton and an electron, which is ejected.
• Gamma-ray emission is not a primary process except in rare instances, but it occurs in combination with alpha, beta, or positron emission or electron capture. Whenever the ejected particle does not utilize all the available energy for decay, the excess energy is released by the nucleus as photon or gamma-ray emission coincident with the ejection of the particle.
• Ionizing radiation, by definition, loses energy when passing through matter by producing ion pairs (an electron and a positively charged atom residue).
• Radiation may deposit energy directly in DNA (direct effect) or may ionize other molecules closely associated with DNA, hydrogen, or oxygen, to form free radicals that can damage DNA (indirect effect).

The four main types of radiation are due to alpha particles, electrons (negatively charged beta particles or positively charged positrons), gamma rays, and x-rays. An atom can decay to a product element through the loss of a heavy (mass = 4) charged (+2) alpha particle (He2+) that consists of two protons and two neutrons. The alpha particle is ejected from the nucleus with energy depending on the element. After it loses its energy, it is a stable helium atom. An atom can decay by loss of a negatively or positively charged electron (e, a beta particle, or e+, a positron). Gamma radiation results when the nucleus releases excess energy, usually after an alpha, beta, or positron transition. X-rays occur whenever an inner-shell orbital electron is removed and rearrangement of the atomic electrons results, with the release of the element's characteristic x-ray energy.

### Alpha Particles

Alpha particles are helium nuclei (consisting of two protons and two neutrons), with a charge of +2, that are ejected from the nucleus of an atom. When an alpha particle loses energy, slows to the velocity of a gas atom, and acquires two electrons from the vast sea of free electrons present in most media, it becomes part of the normal background helium in the environment. The formula for alpha decay is:

where Z is the atomic number and A the atomic weight.

The energy available in this decay is Qα and is equal to the mass difference of the parent and the two products. The energy is shared among the particles and the gamma ray if one is present.

An example of alpha decay is given by the natural radionuclide radium (226Ra):

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