Spectrum and yield of prompt232Th fission neutrons

Abstract

investigations of these spectra has increased on the other. However, the scattering of data and the errors in these data are large, which is connected with the difficulties such measurements involve. Therefore, further measurements of spectra of fission neutrons at different energies are necessary, both for determining the adequacy of theory for calculating fission neutron spectra and for calculating nuclear power plants, the request for which figures in the list of the required data [I]. We measured the spectra of emission neutrons from 2~2Th nuclei in the energy range of secondary neutrons from the detector threshold (0.6 MeV) to E = 8 MeV. The emission spectrum in the energy range above the initial energy of bombarding neutrons contains only neutrons generated by fission. The spectra of emission neutrons were measured by using the flight time method and a spectrometer based on an EGP-10M accelerator operating under pulse conditions. A gaseous tritium target served as the neutron source (T (p, n)3He reaction). The initial energy of the neutrons incident to the specimen was equal to 1.45 + 0.05 MeV. The design and the characteristics of the target have been described in detail in [2]. The mean proton current to the target was equal to 1 ~A at a pulse repetition frequency of 5 MHz and a proton pulse duration of ~i nsec. The specimen, made of metallic thorium in the form of a hollow cylinder with an outside diameter of 4.5 cm, an inside diameter of 4 cm, and a length of 5 cm, was placed at an angle of 0 ~ to the direction of the proton beam at a distance of 16.8 cm from the center of the gaseous tritium target. The specimen contained 0.737 mole 232Th nuclei. The experimental geometry has been described in [3]. Neutrons were recorded by means of a scintillation detector, located at a distance of 198 cm from the center of the specimen. The detector, which consisted of a stilbene crystal (diameter, 6.3 cm; height, 3.9 cm) in contact with an FEU-30 photomultiplier, was placed in a protective structure, which was described in [3]. The absolute efficiency of neutron recording by the detector was determined by comparing the experimental and the theoretical spectra of prompt neutrons of spontaneous 252Cf fission. The Maxwellian distribution with T = 1.42 MeV was used as the theoretical spectrum; in the energy range above 6 MeV, a correction accounting for the deviation of the spectrum of 2S2Cf fission neutrons from the Maxwellian distribution was introduced [4]. The time resolution of the spectrometer was equal to 3 nsec at the half-height of the peak of gamma-quanta from the target. The channel width of the time analyzer was equal to 0.503 nsec, the integral nonlinearity was equal to 0.4%, while the differential nonlinearity amounted to 0.6%. Another scintillation detector, similar to the main one, was used as the neutron flux monitor. The monitor was used for measuring the time spectrum of neutrons from the target for checking the accelerator operation. The monitor was placed at an angle of 45 deg with respect to the direction of the proton beam. The yield of neutrons from the target was also monitored by an all-wave detector. The block diagram and the operating principle of the electronic equipment of the spectrometer are described in [5]. The spectra of emission neutrons from 232Th were measured at angles of 90 and 150 ~ at the 1.45-MeV energy of neutrons incident to the specimen. The investigations consisted in measuring the neutron spectra with the specimen (the effect with the background) and the background without the specimen for a fixed number of protons that have reached the target. At