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Radiation from particle annihilation

How much energy is created when the particle-antiparticle annihilate? Does it release a large amount of radiation? If so, about how many rads?

Heather


Dear Heather,

I am Don Cossairt, a physicist, and I am also the Associate Head for Radiation Protection in Fermilab's Environment, Safety, and Health Section.

The Fermilab Public Affairs Office forwarded to me your question about particle-antiparticle annihilation that I am happy to try to answer your question and I since I do not know much about your background knowledge in science, I will probably explain some aspects of this topic that you may already understand. As you apparently already know, the matter we encounter in everyday life is nearly all matter, rather than antimatter. However, scientists now believe that for every particle that can be found in nature, an antiparticle exists. The simplest example that illustrates the principles involved is that of the electron and positron. Electrons are found in ordinary atoms and, of course, moving electrons in various devices such as electrical lines, telecommunications networks, and television and radio form much of the technological basis for modern living.

The electron has a rest energy of 0.511 million electron volts. One million electron volts is called one "MeV" for short. This energy of 0.511 MeV is released if all the mass of the electron is converted to energy by means of Albert Einstein's famous formula E = mc**2 [or "E equals the mass (m) times the square of the speed of light (c)), an equation that is commonly seen even on T-shirts]. In different units, the mass of the electron amounts to 9.11 E-31 kilograms (in scientific notation or, in words, 9.11 times ten to the negative 31 power).

Some atomic nuclei are naturally radioactive and when some of these decay, they emit the antiparticle of the electron called the positron. This happens in nature all of the time. A positron has the same mass as an electron, but opposite electric charge.

While these positrons may be fast moving at first eventually they encounter matter and slow down. Soon, they will encounter an electron in an atom and annihilate with it. In doing so, the total rest energy of BOTH particles, 2 times 0.511 MeV = 1.022 MeV is released. This release most commonly occurs by producing two particles of light called photons or gamma rays. These photons are emitted in opposite directions and each have an energy of 0.511 MeV. (The reason TWO photons are produced may seem a bit complicated, but two are needed to conserve both momentum and energy. Briefly, since the electron and the positron were at rest, the total system had no momentum. Thus the available energy of 1.022 MeV is accounted for by the emission of the two photons while zero total momentum is achieved by having the two photons going off in opposite directions.)

You are quite correct in recognizing that this energy appears as radiation. After all, these photons are essentially the same as high energy x-rays or other photon radiation used in medical procedures, for example. Also, this type of annihilation radiation is routinely seen and measured in connection with work with radioactive materials, natural or man-made.

You also appear to be aware that radiation hazards are routinely quantified in units called "rads". If someone (or even something) is exposed to ionizing radiation at the level of one rad, that means that a certain amount of energy called 100 ergs (in metric units) is deposited in each gram of their tissue (or material, if not a person).

People on earth are exposed to about 1/10 of a rad per year from natural sources in the ground (from naturally radioactive elements) and from outer space (from the sun and from cosmic rays). It turns out that it takes about 5 billion 0.511 MeV photons per square centimeter of tissue to result in a dose of one rad. Thus, if a person found themselves 1 foot (30 centimeters) away from a point source made up of material containing annihilating electrons and positrons, to receive one rad of dose, one would have to witness 57 trillion annihilations (5.7 followed by 13 zeros preceding the decimal point).

Other particles also have been observed to experience annihilation with their antimatter counterparts. Some of these processes are more complicated but all of them release radiation and the basic principles involved are the same. I hope this helps. It is always fun communicating with people who are curious about science. You can contact me by e-mail if something is not clear. You may also call me at (620)840-3465.

Don Cossairt
Fermilab

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last modified 6/29/2001   physicsquestions@fnal.gov