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gree from Loyola College, Baltimore, e Johns Hopkins University, Mount niversity of Baltimore.

WILLIAM R. CORLISS is an atomic energy consultant and writer with 12 years of industrial experience including service as Director of

Advanced Programs for the Martin Company's Nuclear Division. Prior industrial connections were with the Flight Propulsion Laboratory of the General Electric Company and with Pratt & Whitney Aircraft Company. Mr. Corliss has B.S. and M.S. Degrees in Physics from Rensselaer Polytechnic Institute and the University of Colorado, respectively. He has taught at those two institutions and at the University of Wisconsin. n Systems for Space Flight (McGrawPlanetary Exploration (Van Nostrand isotopic Power Generation (Prenticerous articles and papers for technical n this series he has written Neutron Reactors in Small Packages, Direct -Nuclear Reactor Power in Space,

activity into electricity was demonstra first time on the desk of the President The device was the size of a grapefruit. and was capable of delivering 11,600 tricity for about 280 days. This is equi produced by nickel-cadmium batteri 700 pounds. It was called SNAP-3.

The U. S. Atomic Energy Commission had begun developing a series of these compact devices in 1956 to supply power for several space and terrestrial uses. The devices were all described by the general title: Systems for Nuclear Auxiliary Power. The initials form the word SNAP.

Two entirely different types of SNAP systems are being developed. Both convert heat into electricity. In one system, the heat is obtained from small nuclear reactors; in the other, from the decay of certain radioisotopes. This booklet discusses only the latter type, which are called radioisotope, or, more simply, isotope power generators. (See summary table, pages 20 and 21.) The other system is described in SNAP-Nuclear Reactor Power in Space, a companion booklet in this series.

To understand how radioisotope generators work, it may be helpful to review some basic principles.

Radioisotopes-Characteristics and Uses

The existence of isotopes was discovered about 1913 after nearly a decade of experimenting with naturally radioactive materials. Isotopes of a given element are atoms with the same number of protons and electrons but different numbers of neutrons. Because they have the same number of electrons, they are identical in chemical behavior. Because they have different numbers of neutrons, they differ in weight. Certain isotopes are unstable and undergo a process of decay during which they emit radiation. Such isotopes are called radioisotopes.

By the early 1930s scientists had learned that, by bombarding normally stable chemical elements with subatomic particles, using particle accelerators ("atom smashers"), radioactivity could be induced in the elements. That is, radioisotopes could be made artificially. Quantity production became feasible after the development of nuclear reactors during World War II. Neutrons, released in prodigious quantities in reactors, were well suited for bombarding target elements to produce radioisotopes.

Radioisotopes differ in the types of radiation they emit and in the rate at which they decay, or lose their radioac

Beginning 1 Half-life 2 Half-lives 28 Years 56 Years

of life

Radioactive decay pattern of str

Radioisotopes are used in four basi First, they are used as fixed sou make some change in a target mat cancerous tissue may be destroyed by

Second, radioisotopes are used, agai radiation, in measuring systems that about a target material by sensing penetrates it or is reflected from systems for measuring the thickness metal.