Alpha, Beta and Gamma Radiation

What's in a smoke alarm?

Turn over many household smoke alarms and you will find a tiny label with a three-bladed symbol and the words americium-241. Inside is a speck of a radioactive substance that gives off alpha particles. They make the air in a small chamber conduct a little electricity. When smoke drifts in, the current drops, and the alarm sounds.

Is it dangerous to have a radioactive source on your ceiling? To answer, you need to know what alpha, beta and gamma radiation are, how far each can travel, and what stops them. That is this lesson.

(Never open or take apart a smoke alarm.)

Radioactive decay

In The Nucleus and Isotopes you saw that a nucleus with the wrong mix of protons and neutrons, or one that is too big, is unstable. Sooner or later it decays: it emits radiation and becomes a different nucleus. The original is the parent; the new one is the daughter. If the number of protons changes, the element changes. This is called transmutation.

In every nuclear equation, two things are conserved:

• the total mass number (top numbers add up the same on both sides), and
• the total charge, i.e., atomic number (bottom numbers add up the same on both sides).

There are three main types of radiation from decaying nuclei.

Alpha decay (α)

An alpha particle is a helium-4 nucleus: 2 protons and 2 neutrons, written ⁴₂He. Big, unstable nuclei often decay this way. The parent loses 4 from its mass number and 2 from its atomic number:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

Alpha particles are heavy (about 4 u) and have a charge of +2. They move at typically around 5% of the speed of light. Because they are big and highly charged, they slam into atoms along their path and ionize them very strongly, so they lose their energy quickly. They are stopped by a sheet of paper, a few centimetres of air, or the dead outer layer of your skin.

So: alpha sources outside the body are not very dangerous, but alpha emitters that get inside the body (breathed in, swallowed) are very harmful, because all that ionizing energy is dumped into living cells. This is why radon, a radioactive gas that seeps out of the ground into some basements, matters: when breathed in, it and its decay products give off alpha particles inside the lungs. Health Canada recommends that every home be tested for radon.

Beta decay (β)

In beta decay, a neutron inside the nucleus changes into a proton and an electron. The proton stays in the nucleus; the electron is shot out at high speed. That electron is the beta particle, written ⁰₋₁e (mass number 0, charge −1). (A tiny neutral particle called an antineutrino is also emitted; we usually leave it out of Grade 11 equations.)

The mass number stays the same and the atomic number goes up by 1:

¹⁴₆C → ¹⁴₇N + ⁰₋₁e (carbon-14 decay, used in carbon dating) ³₁H → ³₂He + ⁰₋₁e (tritium decay, which makes exit signs glow)

Beta particles are very light (about 1/1840 of a proton's mass), have a charge of −1, and can move at close to the speed of light. They penetrate further than alpha particles but are stopped by a few millimetres of aluminium or a thick piece of plastic. They ionize less strongly than alpha particles.

(Some nuclei with too many protons emit a positive electron, a positron, instead. This is called beta-plus decay and is used in PET medical scans.)

Gamma radiation (γ)

After an alpha or beta decay, the daughter nucleus is often left with extra energy, in an excited state. It gets rid of the energy by emitting a gamma ray: a photon of very high-energy electromagnetic radiation, like an X-ray but usually more energetic.

Gamma rays have no mass and no charge, so emitting one does not change A or Z. They travel at the speed of light. Because they have no charge, they interact with matter only occasionally, so they are the most penetrating type and the least ionizing per centimetre travelled. Several centimetres of lead or a thick wall of concrete greatly reduces them (nothing stops all of them).

Cobalt-60 is a beta emitter whose daughter, nickel-60, then gives off gamma rays. Those gamma rays are used to sterilize medical supplies and to treat some cancers.

Staying safe: time, distance, shielding

Ionizing radiation can break chemical bonds in cells and damage DNA. Large doses cause radiation sickness; smaller doses over time raise the risk of cancer. We are all exposed to a low level of background radiation all the time, from rocks and soil, radon, cosmic rays, and even the potassium in our own bodies. Medical X-rays and scans add some more. Workers who handle radioactive materials follow three rules to keep their dose as low as reasonably achievable:

• Time: spend as little time near a source as possible. Dose is proportional to time.
• Distance: stay as far away as possible. For a small source, the intensity drops with the square of the distance: twice as far gives one-quarter the intensity.
• Shielding: put the right material between you and the source: paper or plastic for alpha, aluminium or plastic for beta, lead or concrete for gamma.

Workers also wear dosimeters to track their total dose, and radioactive materials are labelled with the radiation warning symbol (the trefoil). Dose is measured in sieverts (Sv), usually millisieverts (mSv).