Shots Across the Bow

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A Nuclear Power Primer: Part 2c: What is Radiation? Particles and Rays

So, we know that radioactive elements are elements with unstable nuclei that decay spontaneously, giving off the energy that once held the nucleus together. Now it's time to talk about the forms that energy takes.

There are four basic types of radiation,
  • alpha
  • beta
  • gamma
  • neutron


An alpha particle consists of a bundle of two protons and two neutrons. This gives it an atomic mass of 4, which makes it very massive as particles go. Also, since it has no associated electrons, it has a positive charge of 2, which makes it relatively highly charged. This combination of high mass and strong charge means that the alpha particle can do a lot of damage in human tissue, so we pay special attention to isotopes that emit alphas when they decay. On the other hand, the high mass and charge means that the alpha particle isn't very mobile. It will only move a few centimeters through the air, and can be stopped by a piece of paper.

Yep, plain old notebook paper can protect you from alpha radiation.

A beta particle looks a lot like an electron. It has the same mass, and the same negative charge, but it comes from the nucleus of the atom, not the electron cloud.

Earlier, we said that the nucleus contains only protons and neutrons, so where does this electron thing come from?

Glad you asked.

If we look very closely at a neutron, we find something unusual. A neutron can become a proton if it gives up an electron. (And an anti-neutrino, but let's not get too complicated here.) It adds up correctly when you think about it. A neutron has no charge, so, if it loses a negatively charged particle, the beta, it will have the same mass and will become positively charged. In other words, a proton. And, since the number of protons determines the type of element, beta decay transforms the element into a different one.

The beta particle can travel a little bit further than the alpha, and requires a bit more shielding to stop it. While they can't penetrate the dead cell layer of our skin, they can irradiate our eyes, but common plastic safety goggles takes care of that threat. However, like alphas, if they can get inside your body, whether by breathing them in or ingesting them, they can cause problems for us, so we use respirators with HEPA filters to prevent that.

The gamma ray is the odd man out. Unlike other forms of radiation, a gamma is not a particle, but a ray. The easiest way to think of it is as a photon of light. If you remember from your science classes, light behaves like a particle sometimes and like a wave sometimes, depending on how we look at it. A gamma ray does the same thing. Like light rays, gammas have specific frequencies, and energy levels. Unlike light, they aren't easily shielded. In fact, to reduce gamma radiation to 10% of it's initial level requires 4 inches of lead or 12 inches of steel.

Gamma radiation penetrates throughout your entire body, and while it won't let you transform into a giant green comic book superhero, it can cause significant damage to your tissues. This is the primary form of radiation that we deal with in the industry, and the only real way to control your exposure is to not be around it. We'll talk more about exposure control later.

The fourth class of radiation, neutron, occurs mostly around nuclear fission; there are relatively few isotopes that decay by emitting a neutron, and most of those actually decay through spontaneous fission. Neutrons are the vital link in the production of nuclear power as they create the chain reactions that allow us to get more power out of a reactor than we put in. We'll discuss the role of neutrons in power generation in a later topic.

The neutron has no charge and a mass of 1 AMU. When it is released during the fission process, it carries a lot of energy. Unlike the other forms of radiation discussed earlier, neutron radiation has the capability of activating previously stable elements, causing them to become radioactive themselves. Neutron radiation can also cause far more damage to living tissue than other types of radiation.

The best way to shield neutron radiation is water, or other hydrogen rich compounds.

So now, if we take everything we've covered, we have a working definition of nuclear radiation. It is the energy given off by the decay of an unstable atom in the form of particles or rays.

Our next topic will cover how radiation causes damage, how much is too much, and how we protect ourselves.
Posted by Rich
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