According to Weinberg and Wigner, Slotin was the first to propose the name dollar for the interval of reactivity between delayed and prompt criticality ; 0 is the point of self-sustaining chain reaction, a dollar is the point at which slowly released, delayed neutrons are no longer required to support chain reaction, and enters the domain called "prompt critical". Stable nuclear reactors operate between 0 and a dollar; excursions and nuclear explosives operate above a dollar. The hundredth part of a dollar is called a cent. When speaking of purely prompt critical events, some users refer to cents "over critical" as a relative unit.