The energy dependence of the relative yield of delayed neutrons and half-life of their precursor nuclei from 238 U fission by 14.2-17.9 MeV neutrons is measured. In addition, the detector-blocking effect, which is due to the exposure of the detector to intense fluxes of high-energy neutrons, and the effect of the secondary neutron source D(d, n) 3 He, a reaction which occurs when a T(d, n) 4 He based neutron source is used, are investigated experimentally and taken into account in the data analysis. The data are analyzed in terms of the average half-life of the precursor nuclei of the delayed neutrons. It is shown that the observed increase of with increasing primary-neutron energy in the energy range studied is predominantly due to the opening of emissive nuclear fission channels. The energy dependence of the average half-life of the delayed-neutron precursors can be described on the basis of the model of emissive fission of three nuclei that fission in this energy range -237 U * , 238 U * , and 239 U * .New designs of nuclear reactors, characterized by a complex neutron spectrum and fuel composition and the possibility of being used for transmuting wastes, are given a large role in the development of nuclear power. The operational safety and efficiency of these reactors impose greater requirements on the database for delayed neutrons. However, despite the efforts made to study the emission of delayed neutrons, substantial uncertainties still remain in the nuclear-physical data on delayed neutrons even for the main isotopes -235 U, 238 U, and 239 Pu.Until now, individual sets of the group parameters of delayed neutrons -the relative yield a i of the ith group of delayed neutrons and the half-life T i of the precursor nuclei belonging to the ith group -have been checked by comparing with the corresponding experimental data the dependences obtained for the reactivity from these data on the asymptotic period of the reactor. In [1], this approach was to show on the basis of experiments performed with the UARR research reactor (USA) that the group parameters for 235 U satisfactorily describe the kinetics of the reactor in which the experiments were performed [2]. Specifically, the data for 235 U from [2] make it possible to predict the reactivity of the system to within 3%. At the same time, it was shown that the group parameters for 235 U from the ENDF/B VI library [3] gave values for the reactivity in the range from 0.8β eff to -0.8β eff which are too low by 2-47%.The objective of the present work is to measure the energy dependence of the relative yield of delayed neutrons and of the half-life of their precursor nuclei from 238 U fission by 14.2-17.9 MeV neutrons. Attention is focused on describing the special features of an experiment where the reaction T(d, n) 4 He is used as the neutron source.Experimental Method. The method is based on cyclic irradiation of a fissile sample by neutrons from a target in an accelerator followed by measurement of the time dependence of the intensity of the delayed neutro...

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Analysis of the emission of delayed neutrons (the E-decay energy of the precursor nucleus of the delayed neutrons and the binding energy of a neutron in a daughter emitter-nucleus are compared) shows that approxunately 271 fission products can be treated as possible precursor nuclei [1]. Experimental investigations have yielded information about the probability of emission of delayed neutrons for 83 of the precursor nuclei as a whole and six in the isomeric state [2]. At the same time, the six-group representation of the characteristics of delayed neutrons, such as the relative yield, the half-life of their precursors, and the energy spectra, has proven itself well in many practical applications. The six-group representation of the characteristics was first introduced at the early stages of the research [3] and it is now known that it gives an average picture of the process of emission of delayed neutrons accompanying fission of heavy nuclei by neutrons. Nonetheless, analysis of the experimental data in the six-group representation is an effective tool for investigating the emission of delayed neutrons. The data obtained with this approach make it possible to analyze model representations of the mechanism of emission with different excitation energy of the fissioning nuclei. In addition, they are important for practical applications.In the present work, in contrast to previous measurements [1], we were able to decrease substantially the time between the end of irradiation and commencement of the delayed-nentron count. The measurements were performed on the apparatus based on the electrostatic accelerators at the Main Science Center of the Russian Federation --Physics and PowerEngineering Institute [5].The experimental method is based on cyclic irradiation of the fissioning sample by neutrons from an accelerator target followed by measurement of the decay of the delayed-neutron intensity of a 4r detector. The count rate as a function of time was measured using a multiscalar counter with a variable time channel. Measurements with different irradiation times of the sample and different counting times of the delayed neutrons were performed in the experiment. This procedure made it possible to increase the relative yield of delayed neutrons corresponding to definite groups of precursor nuclei in the integral curve of the decay of neutron activity and thereby to increase the reliability of the parameters determined.A block diagram of the apparatus is presented in Fig. 1. The reactions T(p, n)3He and D(d, n)3He served as sources of monoenergetic neutrons. The tritium and deuterium targets were irradiated by a beam of accelerated protons or deuterons in a KG-2.5 cascade generator. The energy distribution of the neutrons from the accelerator target was recorded with a 3He spectrometer. The main recording element of the delayed-neutron detector consisted of a standard SNM-11 counter with l~ enrichment to 80%, operating in a proportional mode at a voltage of 650 V. It consisted of an assembly of 30 SNM-11 counters, d~tribu...

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