Relative yield of delayed neutrons and half-life of their precursor nuclei with fissioning of 239Pu by 14.2–17.9 MeV neutrons

Abstract

The energy dependence of the relative yield of delayed neutrons and of half-lives of their precursor nuclei with fissioning 239 Pu by 14.2-17.9 MeV neutrons is measured. The data obtained are analyzed in terms of the average half-life <T> of the precursor nuclei of the delayed neutrons. It is shown that the observed growth of <T> with increasing primary neutron energy in the energy range studied is due predominately to opening of the channels for the emission fission of nuclei. Comparing the data on the average half-life of the precursor nuclei of delayed neutrons obtained in the present work with those obtained by other authors showed that the anomalously high values of <T> are due to methodological features of the experiment which are related with the properties of the reaction T(d, n) 4 He used as a neutron source on electrostatic charged-particle accelerators. The data obtained are of great value for developing instrumentational nondestructive means for determining the nuclide composition of nuclear fuel based on the detection of delayed neutrons.The objective of the present work is to measure the energy dependence of the relative yield of delayed neutrons and the half-lives of their precursor nuclei when 239 Pu is fissioned by 14-18 MeV neutrons.Experimental Method. The experimental method is based on cyclic irradiation of a fissile sample by neutrons from an accelerator target followed by measurement of the time dependence of the intensity of the delayed neutrons in a 4π detector [1, 2]. The experimental arrangement made it possible to perform measurements with different sample irradiation times and delayed-neutron counting rates. This procedure made it possible to increase the relative contribution of definite groups of delayed neutrons to the total drop-off of neutron activity and thereby increase the reliability of the estimate of the relative yield of delayed neutrons and the half-lives of their presursor nuclei.The reaction T(d, n) 4 He served as the source of monoenergetic neutrons. A beam of accelerated deuterons with energy 0.5-2 MeV irradiated a solid tritium target on a molybdenum or copper substrate with a 1 mg/cm 2 thick titanium layer as the sorbent of tritium atoms. Neutron beams emitted at angles 0 and 90°with respect to the direction of the deuterons incident on the target were used in the measurements; this made it possible to investigate the temporal characteristics of the delayed neutrons in the neutron energy range 14.2-17.9 MeV. A long counter [3] and a fission chamber placed in direct proximity to the experimental sample at an angle of 0°with respect to the direction of motion of the deuterons served as monitors of the neutron flux from the target. The 3.61 g 239 Pu sample consisted of an assembly of six metal disks,~1 mm thick and 5.6 mm in diameter, of plutonium metal.The time dependences obtained in all cycles of a separate series of measurements and corresponding to the same parameters of the experiment were summed (Fig. 1). The total number of counts of the delayed-neutron detect...

Neutron energy dependence of delayed neutron yields and its assessments

“…It usually increases moderately up to incident neutron energy around E n = 3 MeV and decreases rapidly from about 4 MeV as the second chance fission occurs [4]. These phenomena were measured actively by the Obninsk group in the past twenty years [5][6][7][8][9]. From a theoretical point of view, Alexander and Krick studied the energy dependence by using a summation calculation about forty years ago [10], in which fission yields were calculated by Wahl's approach [11] and 47 precursors were considered.…”

Development of a Reference Database for Beta-Delayed Neutron Emission

“…However, in accelerator-based experiments a sample undergoes a number of cycles with the purpose of increasing the statistics. The general equation for the determination of the total delayed-neutron yields and temporal delayed-neutron characteristics (a i , T i ) on the basis of the delayed-neutron decay curve accumulated during n cycles is given by ( [222], [232], [233], [226], [234], [230])…”

“…, where c i , b i are constants. These constants are estimated for each DN group and then the obtained dependences a i have been used for the calculation of the relative abundances of all uranium isotopes including unmeasured uranium isotopes 234 U and 237 U. The results of this estimate are presented in Table XXXII.…”

“…The energy dependence of the average half-life of delayed-neutron precursors from neutron-induced fission of232 Th,237 Np,239 Pu,241 Am. The references of the data can be taken from the compilation by Spriggs and Campbell[248] except for the IPPE data[226,230,[232][233][234][252][253][254][255].…”