Radiation penetration is a measure of how well a type of radiation passes through matter instead of being absorbed, reflected, or otherwise deflected. If you’re wondering which type of radiation is the most penetrating, the answer depends on what types of radiation you’re including and the nature of the matter.
- Gamma rays are the most penetrating type of common radiation from radioactive decay. Lead shielding blocks gamma radiation. X-rays are similarly penetrating.
- Energetic neutrons can penetrate the human body and even lead shielding, but a thick layer of water or concrete absorbs them.
- Overall, neutrinos are the most penetrating form of radiation. Neutrinos are energetic, nearly-massless particles that are nearly unstoppable. Billions pass through your body every second. Neutrinos pass through the Earth, stars, and entire galaxies, very rarely interacting with any matter.
Ionizing and Non-Ionizing Radiation
Mostly, questions about radiation penetration refer to ionizing radiation. Ionizing radiation is radiation with enough energy to ionize atoms, which changes and potentially damages matter. In contrast, non-ionizing radiation lacks the energy to ionize atoms, yet it still excites them to higher energy states. Some forms of non-ionizing radiation, like microwaves and radio waves, can penetrate the body. But, conductive metals like copper block the radiation. While non-ionizing radiation can cause harm, its penetrating power isn’t what most people want to know.
In contrast, ionizing radiation damages matter and causes cancer and possibly death. Knowing its penetrating power is important. But, less penetration does not necessarily make the radiation safer. Some forms of ionizing radiation make it no further than skin, yet interact with DNA and can cause tumors and cancer. Other forms of ionizing radiation get stopped somewhere inside the body and affect deeper tissues. Still other types of ionizing radiation penetrate the body and rarely interact with cells. Radiation with a high penetrating power also affects electronics and other devices.
Electromagnetic Radiation and Particle Radiation
Ionizing radiation is either electromagnetic radiation or particle radiation. Electromagnetic radiation is in the form of photons. In other words, it’s any radiation on the electromagnetic spectrum. The spectrum includes radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma ray light. Of these, ultraviolet, x-radiation, and gamma-radiation are forms of ionizing radiation. Gamma radiation has the most penetrating power. X-rays have comparable energy. Lead shielding or a thick layer of concrete stops most gamma rays and x-rays. However, when gamma rays or x-rays interact with matter, it’s usually bad news for the cells or machinery involved.
Particle radiation is any form of radiation that has mass. So, particle radiation includes alpha particles, beta particles, protons, neutrons, muons, other subatomic particles, cosmic rays, and neutrinos.
Alpha particles are the largest ionizing radiation particles. Each alpha particle is essentially a helium atom nucleus, with two protons and two neutrons. A sheet of paper or your skin stops alpha particles. They have a low penetration power both because of their size and net positive electrical charge.
Beta particles are energetic electrons and positrons. They are much less massive than alpha particles, so they penetrate further, yet carry a negative electrical charge and readily interact with matter. A sheet of aluminum foil, block of wood, or plastic bottle stops beta radiation.
Cosmic rays are mostly protons, which have a positive charge and largely stop in the Earth’s atmosphere. However, that interaction forms muons, which penetrate partially into the planet’s surface and deeply into the oceans.
Energetic neutrons have about the same mass as protons, so they are larger than beta particles. Unlike protons and beta particles, they have no net electrical charge. Neutrons can pass through paper, the human body, foil, and even lead radiation shielding. However, they interact with particles about the same size as themselves, so a hydrogen-rich layer of water or concrete absorbs most of them.
Neutrinos Are the Most Penetrating Radiation
Neutrinos are tiny particles with no electrical charge and almost no mass. They pass through your body, the Earth, the Sun, and for many light years with little chance of interacting with matter. This is because they travel so quickly (nearly the speed of light) and are so tiny that they fit between the spaces between particles of matter. While they are the most penetrating type of radiation, the very fact that they pass right through means they aren’t a threat to living organisms or other matter.
References
- Bellenir, Karen (2007). Cancer Sourcebook. Detroit, MI: Omnigraphics. ISBN 978-0-7808-0947-5.
- Fan, W.C.; et al. (1996). “Shielding considerations for satellite microelectronics”. IEEE Transactions on Nuclear Science. 43 (6): 2790–2796. doi:10.1109/23.556868
- Meggitt, Geoff (2008). Taming the Rays – A history of Radiation and Protection. ISBN 978-1-4092-4667-1.
- “Occupational Radiation Protection in Severe Accident Management“. The Organisation for Economic Co-operation and Development (OECD) and the Nuclear Energy Agency (NEA).