ther measurements were made during Operation Plumbbob (Reference $). The Plumbbob results indicated that a real-time link would not be feasible for any reasonable trequency or transmitter power. The plans for Shot Yucca then necessarily included means for data storage so that data transmission could take place after the ionization had cleared up. 1.3 THEORY An attractive procedure for measuring the energy spectrum of a neutron source is the time-of~flight method. It has been extensively applied in the past to cyclotron-produced, pulsed neutron beams and mechanically chopped reactor beams. Since the neutrons resulting from a nuclear detonation are all emitted in a relatively short time, the method is adaptable here also; but a relatively long path length is necessary, since many of the fission neutrons are very energetic. In addition, the extremely high particle flux available makes posaihle a detector system that need not count individual events, even at large distances from the source. Suppose the detector is a distance / from the neutron source, and at first let us neglect the influence of air surrounding the system. In this idealised case, since all the observed neutrons travel directly from source to detector, there is a aimple relation between time of arrival of a neutron (¢) and its energy (E): u t=] (#) Where: \!A2 . (1) M = neutron mass The detector records the number of neutrons reaching the detector per unit of time, nt). Then, if N(E) is the numberof neutrons per unit of energy emitted, the following simple relationahip holds, with E and ¢ connected by Equation 1: n(t) = N(E) EM? ( = ) (2) IM In the present experiment, | = 834 meters. Some typical neutron delay times are as follows (initial y rays arrive at ¢ = 2.78 usec): _E 10 Mev 4 2 1 40 kev t 19.0 psec 30.1 42.6 60.2 300 The interpretation of the actual experiment is, of course, complicated by two effects: interaction of neutrons with the air and variation of detector efficiency with energy. In 10