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Measurements of prompt-neutron lifetime by the Rossi-ct method in more than 30 different critical systems are reported. Most were assembled in Argonne's Zero Power Reactor III (ZPR-III). The Rossi-a method, which has been described in the literature,(l>4,5) represents a direct experimental determination of the ratio Akp/To, from which a value for To is obtained if an estimate of Akp is available. P(t) At = CAt + Ae^-i^i"^)* At + Be^-"'^''')* At , where the two amplitudes A and B were nearly equal. The larger alpha is interpreted as representing the time behavior of chain-related neutron pairs having no intervening events in the beryllium. The smaller alpha represents the time behavior of chain-related neutron pairs when at least one intervening event has occurred in the beryllium. When the terminating counter was placed in the beryllium, a single value for alpha was obtained which agreed with the smaller value found in the core. The disappearance of the fast decay in this case supports the hypothesis that the larger alpha is associated with the core composition. Further, an extension of Orndoff's theory gives a method, of determining the reactivity of the fast system independent of the reactivity contribution of a moderating reflector.

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