Decay Types

A smoke detector contains americium-241, a radioactive element that emits particles so weak a sheet of paper stops them. A nuclear reactor's spent fuel emits radiation that requires meters of concrete shielding. Both are "radioactive," but the difference in what they emit and how far it penetrates is the difference between a household device and a national security challenge.

Radioactive decay occurs when an unstable nucleus sheds excess energy by emitting particles or radiation. There are three primary types. Alpha decay emits a cluster of 2 protons and 2 neutrons (a helium nucleus). Alpha particles are heavy and slow; a few centimeters of air or a sheet of paper stops them. Dangerous only if inhaled or ingested. Beta decay emits a high-speed electron (or positron) from the nucleus. Beta particles penetrate further than alpha but are stopped by a few millimeters of aluminum or thick clothing. Gamma decay emits a high-energy photon with no mass. Gamma rays penetrate deeply and require dense shielding: lead, concrete, or meters of water.

Each decay type changes the nucleus differently. Alpha decay reduces the atomic number by 2 (the atom becomes a different element). Beta decay changes a neutron into a proton (or vice versa), shifting the atomic number by 1. Gamma decay releases energy without changing the element.

Worked Example

Uranium-238 undergoes alpha decay.

• Subtract 2 protons. U has 92 protons. 92 - 2 = 90 protons. Element 90 is thorium.
• Subtract 4 from mass number. 238 - 4 = 234.

What is the decay product?

Thorium-234. The uranium atom has become a completely different element by ejecting 2 protons and 2 neutrons. This is the first step in uranium-238's 14-step decay chain that eventually produces stable lead-206.

Understanding which type of radiation a material emits determines what shielding is needed and how hazardous the material is in different contexts.

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