Year
2010
Abstract
The differential die-away analysis (DDAA) is a technique applied for the detection of special nuclear materials embedded in a hydrogenous surroundings, carried in trans- portable cargoes [1] - [5]. Its application requires the use of a pulsed neutron generator, which constitutes some constraints in ?eld applications. Recently, it was suggested that similarly to the case of reactivity measurement methods, where pulsed measurements can be replaced by the measurement of temporal correlations (Rossi-alpha method) with a stationary random source, the DDAA method can also be converted into a type of Rossi- alpha measurement of fast neutrons, where the pulsed source can be replaced by the in- herent source of neutrons (spontaneous ?ssions) in the sample [6]. The method was called the differential die-away self-interrogation (DDSI) technique. In [6], it was assumed that the dependence of the temporal correlations of the detector counts of fast neutrons at two different time points has the same dependence on the time lag t as that of the detector counts at time t in the traditional DDAA method with an interrogating pulse emitted at t = 0. The goal of the present paper it to give a rigorous derivation of the DDSI formula. The calculations show that the temporal dependence of the detection rate of fast neutrons at time t + t , following a triggering detection at time t indeed has a form of the sum of two exponentials, just as in the traditional DDAA method, but the coef?cients of the two terms are different. The correct DDSI formula is given and some possible further applications of the method are discussed.