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Name: Charles
Status: educator
Grade: 6-8
Location: MO
Country: N/A
Date: 4/1/2005


Question:
In class we are studying nuclear radiation. When discussing alpha particles being composed of 2 protons and 2 neutrons (actually the nucleus of a helium atom), a student asked me "if an alpha particle is truly the nucleus of a helium atom, why isn't helium considered a radioactive element or substance?"


Replies:
Charles,

A radioactive element sends energy away, carried in the motion of a radioactive particle or ray. A Helium atom does not send its nucleus away from itself.

Ken Mellendorf
Math, Science, Engineering
Illinois Central College


A radioactive substance is one that EMITS some sort of "elementary" particle (electron, proton, neutron, or an atomic nuclear fragment such as an alpha particle), or some high energy electromagnetic radiation (X-rays, gamma rays, etc.). With rare exception (electron capture) the particle or radiation emanates from the atomic nucleus. This distinguishes radioactivity from atomic processes such as absorption, emission, fluorescence, or ionization where the radiation/electron originates from the surrounding electrons rather than from the atomic nucleus itself.

Since elemental helium does not emit any particles or radiation, it is not classified as "radioactive".

One additional comment: The student's question is a good one. It shows perception about the topic and is by no measure a trivial question.

Vince Calder


"Radiation" and "Radioactive" basically refer to what things do, not what they "are made of".

"Radiation" in this context is a particle with high enough energy (speed) to do some damage to molecules it hits. It can be made of a photon, electron, proton, helium nucleus, or any other nucleus or particle. It is often called "ionizing radiation" to specify that it at least has enough energy to bump some electrons out of place, and in living tissue some chemical reactions may follow which are considered degradation. We should specify a word for a higher degree of damage, such as "displacing radiation", which refers to things which can bump whole atoms out of their molecules instantly. That is obviously damaging.

A helium atom is usually just sitting there, with no net energy, no speed, no damaging abilities. If you take one and accelerate it to 10 MeV or more, it would definitely be called radiation. Conversely, after an alpha particle has spent all its energy plowing through air or any other medium, it slows to a stop and picks up a couple of electrons from its new surroundings, and then we call it a helium atom.

A "Radioactive substance" is a substance which emits ionizing radiation from its nuclei, which "makes radiation". Normal helium atoms emit nothing, and make no radiation. Radioactive radium metal, on the other hand, emits these helium nuclei with high energy; it makes radiation, so it is called radioactive.

Analogies:

Radiation is kind of like light, fleeting. Radioactive refers a substance with stored energy, like a battery that can power the light bulb making light.

Radioactive atoms are tiny bombs. Radiation is their explosion, the fragments which fly out.

I hope that clarifies it for your students-

Jim Swenson


Radioactive means unstable -- eventually to decay and emit radiation or particles. Alpha particles are one possible product of radioactive decay, and they might be emitted in an excited state, from which they will very quickly decay by emitting gamma radiation (extremely high-frequency light -- higher than x rays). But once they have reached their lowest energy state, they are very stable.

Tim Mooney



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