The W66 thermonuclear warhead was used on the Sprint anti-ballistic missile system, designed to be a short range interceptor to shoot down incoming ICBM warheads. The W66 was a low yield (details not declassified, but reportedly a few kilotons) thermonuclear bomb, which was the first Neutron bomb (technically, an Enhanced Radiation weapon) fielded anywhere in the world. It was designed to destroy or disable an incoming warhead using neutron flux, and to a lesser extent X-ray and blast effects. The W66 was 18 inches in maximum diameter and 35 inches long, with a weight of around 150 pounds.