Danny Johnson, age 11, of Indianapolis, Indiana, for his question:

What kind of atoms are in gamma rays?

Gamma rays are emitted by certain atoms, but they themselves are not atoms or even atomic particles. Uranium and other radioactive elements emit three main forms of radiation as their nuclei break apart. The first two are streams of high speed atomic particles. The third is gamma rays of energy with tremendous penetrating powers. These energetic gamma rays are the most deadly radiations emitted by radioactive substances.

Large, unstable atoms decay into smaller atoms on schedule. Uranium nuclei break down in stages according to a precise timetable and this gradual radioactivity ends with stable atoms of lead. At each stage of decay, the atomic nucleus emits a particle of matter and a quota of dynamic nuclear energy. At one stage, an alpha particle may be shed; at another stage, the lost particle may be a beta particle. Gamma rays of nuclear energy are emitted with either alpha or beta particles of matter. Gamma rays also are emitted when hydrogen atoms fuse to form atoms of helium.

Radioactivity taught us a lot about the structure of the atomic nucleus. We know that the tight fisted unit may contain more than 20 different particles and that these fragments of atomic matter are held together by fantastic nuclear energies. In nuclear fission, the nucleus emits some of its particles of matter plus some of the binding energy in the tight fisted package. In nuclear fusion, two hydrogen atoms fuse to form one helium atom. But the helium atom needs less binding energy than the two hydrogen nuclei. In both fission and fusion, surplus nuclear energy is emitted as gamma rays.

The alpha rays shed by uranium are streams of helium nuclei, with twice the positive charge of protons and four times their mass. These alpha particles can travel through a vacuum at 10,000 miles per hour. But they lose their penetrating speed after crossing a few centimeters of air. They can be stopped by a sheet of paper. Under certain conditions, each alpha particle picks up two electrons and becomes a harmless atom of stable helium.

Beta rays are streams of negatively charged electrons. Their penetrating power is 100 times greater than that of alpha particles because of their speeds. In a vacuum they almost match the speed of light. They can cross several meters of air or penetrate three centimeters of flesh.

Gamma rays are powerful emissions of electromagnetic energy, 100 times more penetrating than beta rays. Their wavelengths are about 100 times shorter than X rays and, like all electromagnetic energies, they travel at 186,000 miles per second. They can pierce through living flesh and it takes a lead shield ten centimeters thick to stop them. Though gamma rays contain no atoms or atomic particles of matter, they are the most dangerous of the three main nuclear radiations.

Radioactivity reveals the mysterious relationship of matter and energy. At this level of nuclear activity, fragments of matter can be converted into energy and we see that matter must be a sort of frozen form of energy. In both nuclear fission and fusion, portions of atomic matter are sacrificed when high energy gamma rays are emitted.