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Types of ionizing radiation: alpha, beta, gamma, x-rays and neutrons

Ionizing radiation has enough energy to strip electrons from atoms. The kind of radiation decides how far it travels, what stops it and whether it is mainly an external or internal hazard.

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Study text drafted from public-domain US government sources. Practice questions are reviewed before publication.

Charged particles: alpha and beta

An alpha particle is a helium-4 nucleus: two protons and two neutrons with a +2 charge. It interacts so strongly that a sheet of paper or the dead outer layer of skin stops it, so alpha emitters are mainly a hazard when inhaled, swallowed or taken in through a wound. A beta particle is an electron (or positron) emitted from a nucleus. Betas travel up to a few meters in air and are stopped by a few millimeters of plastic or aluminum. Shield strong beta sources with low-atomic-number material first, because stopping betas in lead produces more bremsstrahlung x-rays.

Photons and neutrons

Gamma rays and x-rays are both photons; gamma rays come from the nucleus and x-rays from electron processes. Being uncharged, photons interact only occasionally and can pass through the body, so they need dense shields such as lead, steel or concrete, and their intensity falls off gradually rather than stopping at a fixed range.

Neutrons are also uncharged. They are released by fission and by some special sources such as americium-beryllium. They slow down best in hydrogen-rich materials such as water, polyethylene and concrete, and once slow they are captured, sometimes making the capturing material radioactive (activation). Ordinary gamma irradiation does not make things radioactive.

Non-ionizing radiation — radio waves, microwaves, visible light — lacks the energy to ionize atoms and is not what radiation-protection rules for radioactive materials address.

In a reactor all of these appear together: fission releases neutrons and gamma rays, fission products emit beta and gamma radiation as they decay, and heavy actinides in spent fuel emit alpha particles. That is why reactor shielding combines water or concrete for neutrons with dense material for gamma rays, and why contamination control focuses on keeping alpha and beta emitters out of the body.

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Practice questions for this section are still in review and will appear here once they have been checked.

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Frequently asked questions

Which type of radiation is the most dangerous?
It depends on where the source is. Outside the body, penetrating gamma and neutron radiation matter most; inside the body, alpha emitters deposit their energy most densely and receive the highest weighting.
What stops gamma rays?
Nothing stops them completely; dense materials such as lead, steel and concrete reduce the intensity exponentially. Each half-value layer halves it.
Can radiation make me radioactive?
Gamma, x-ray, beta and alpha exposure from outside does not. Neutron exposure can activate materials, and contamination (radioactive material on or in the body) keeps emitting until removed or decayed.
What is bremsstrahlung?
X-rays produced when fast electrons are slowed by atomic nuclei. The yield rises with atomic number, which is why beta shields start with plastic or aluminum.

Sources

  1. U.S. NRC, Radiation Basics www.nrc.gov
  2. U.S. EPA, Radiation Basics www.epa.gov
  3. DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory, DOE-HDBK-1019/1-93 and DOE-HDBK-1019/2-93
  4. DOE Handbook: Radiological Control Technician Training, DOE-HDBK-1122-2009
  5. U.S. NRC Glossary (NRC Basic References) www.nrc.gov

Explanations on this page are original. Cited US government works are in the public domain; they do not endorse this site.