Bob House, age 17, of Phoenix, Ariz., for his question:

WHEN WAS THE FIRST BETATRON BUILT?

A betron is a machine for accelerating electrons to high speed so that they can be used for nuclear physics experiments. Its inventor, a scientist named Donald Kerst, built the first betatron in 1940 at the University of Illinois.

The betatron has a circular tube called a doughnut. The tube contains a vacuum and is said to be evacuted. This tube is placed between the poles of a powerful electromagnet. An electron gun shoots electrons into the doughnut.

Changes in the magnetic field make the electrons gain energy. This causes them to be pushed along at a steadily increasing rate of speed. They whirl round and round inside the doughnut, held in a circular path by the magnetic field.

When the electrons reach their highest energy, they are made to collide with the nuclei of the atoms in a small tungsten target. These collisions produce powerful X rays and gamma rays.

In the huge 300 million electron volt betatron, electrons travel in a doughnut 13 feet in diameter. Here, in a little less than five  one thousandths of a second, they make 140,000 revolutions and travel 850 miles.

Smaller, portable 22 million electron volt betatrons produce beams of electrons and X rays for industry and medicine.