Roger Herbert, age 15, of Burlington, Vt., for his question:

WHAT EXACTLY IS A GAMMA RAY?

A gamma ray is an electromagnetic radiation of the same character as an x ray, but it has a much shorter wave length.

Members of the uranium radium series of radiactive elements give off gamma rays when they disintegrate to form a new elements. When a nucleus emits a gamma ray it remains unchanged. All that happens is that it loses a certain amount of energy.

But a nucleus gives off a very penetrating ray if it loses a very large amount of energy, say, 5 million electrons. A sheet of iron about one inch think will stop 50 percent of gamma rays of this energy. It takes about nine inches of water to equal this absorbing ability.

A heavy element such as lead is good for stopping gamma rays. About a half inch of lead equals one inch of iron in absorbing power.

Gamma rays lose energy by colliding with atoms in passing through water, air, lead, iron or any other materials. In these collisions, gamma rays make may electrons loose from their parent atoms. This process is called ionization because it changes the previously neutral atom or group of atoms into a charged particle called an ion.

High energy gamma rays may create matter by completely disappearing and forming a pair of electrons, one a positively charged positron and one a negatively charged ordinary electron, or negatron.

This creative process is the opposite of what happens when a positron unites with an ordinary electron. When these particles unite, they are destroyed and two gamma rays are formed.

When gamma rays pass through the body, they produce ionization in tissue. If too many bombard the tissue, it may be harmful to the cells.

Extremely small amounts of gamma rays bombard us from the natural radioactivity of the chairs we sit upon, the water we drink and the air we breathe. Only large amounts are dangerous.

Although gamma rays in large amounts may be harmful to the body, they can also.be of great benefit. They may be used to treat some cancers, tumors and skin afflictions.

With X ray machines, medical men use gamma rays to examine the body for broken bones or foreign objects and for signs of disease such as tuberculosis.

Sources of strong gamma rays, such as radium and radiocobalt, may be used to X ray iron castings for flaws. Penetrating gamma rays from a betatron may be used to inspect thick metal for flaws or cracks.

Scientists have also used gamma rays experimentally to preserve food and to volcanize rubber.

X rays were first produced by Wilhelm K. Roentgen in 1895. Roentgen was a German physicist who called the rays "X rays" because at first he did not understand what they were. X is a scientific symbol for the unknown.

But scientists did not discover Until some years after Roentgen's time that radioactive elements give off penetrating gamma rays.