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The aim of this work is experimental discovery and research of the neutron radiative beta-decay, where a new particle, the radiative gamma-quantum, is formed along with the expected decay products beta-electron, recoil proton and antineutrino. The discovery of this rare neutron decay mode was conducted through identification of triple coincidences events: simultaneous registration of beta electron, proton and radiative gamma-quantum. The ordinary neutron decay was registered by double coincidences of beta electron and recoil proton. The ratio of triple to double coincidences is connected with relative intensity of radiative neutron decay (branching ratio -B.R.) B.R. = (3.2±1.6)·10 -3 (with 99.7 % C.L. and in the gamma energy region greater than 35 keV ), which we are the first in the world to measure during the second and third cycles on FRMII ( TUM, Germany ) in Summer 2005 [1]. This value of B.R. is consistent with standard electroweak theory.

Results of the first experiment to search for the radiative decay mode of the free neutron are reported. The γ-spectrum was studied in the energy region from 35 keV to 100 keV in six Cs(Tl) scintillators, each set at an angle of 35° to, and shielded from, a central plastic scintillator electron detector. Triple coincidences were recorded with recoil protons detected in a micro-channel plate. A limit for the branching ratio BR < 6.9 × 10−3 (90 % confidence level) was obtained, which is greater that the theoretical prediction by not more than a few tenths of a percent.