Search for particle-stable tetraneutrons in thermal fission of 235U

Background: The existence of a tetraneutron comprising four neutrons has long been debated.Purpose: Motivated by a recent observation of particle-stable tetraneutrons, we investigated potential particle -stable tetraneutron emission in thermal neutron-induced 235U fission using a nuclear research reactor.Methods: We performed gamma-ray spectroscopy for a 88SrCO3 sample irradiated in a reactor core. Stable 88Sr was expected to produce 91Sr by a tetraneutron-induced (4n, n) reaction; hence, observation of gamma rays followed by beta decay of 91Sr would indicate particle-stable tetraneutron emission.Results: The gamma-ray spectrum of an irradiated 88SrCO3 sample did not show any photopeak for 91Sr.Conclusion: The emission rate of particle-stable tetraneutrons, if they exist, is estimated to be lower than 8 x 10-7 per fission at the 95% confidence level, assuming the cross sections of reactions induced by hypothetical particle-stable tetraneutrons.