Experimental Limit on the Probability of Neutral Pion Emission in Neutron-Induced Fission of 235U Nuclei.

Results of an experiment on the search for the emission of π⁰ mesons in neutron-induced fission of ²³⁵U nuclei based on the detection in coincidence of two γ-quanta from the π⁰-meson decay performed with the nuclear reactor developed at the Scientific-Research Nuclear Physics Institute at Tomsk Polytechnic University are described. The experimental setup comprised two Čerenkov spectrometers of total absorption for the detection and measurement of γ-quantum energy placed in two coaxial horizontal channels of the nuclear reactor. Water filters placed in the channels were used to protect the detectors from β-, γ-, and neutron fluxes. To reject the cosmic radiation, scintillation counters of large area, operating in anticoincidence mode, were placed above each spectrometer. The experimentally measured limitation on the probability of neutral pion emission in neutron-induced fission of nuclei was established ≤4.1·10^–11 with a 90% confidence level. The energy spectrum of γ-quanta from the active reactor zone was measured in the range 32–65 MeV. The parameters of the spectrum deviated from the results of theoretical calculations and from the experiment performed with the nuclear pulse reactor developed at the Joint Institute for Nuclear Research (Dubna). [ABSTRACT FROM AUTHOR]