Detection of light neutron nuclei in the alpha-particle-induced fission of $^{238}$U by the activation method with $^{27}$Al

Light nuclear-stable multineutrons among products of the fission of $^{238}$U nuclei that is induced by $62$-MeV alpha particles have been searched by the activation method with a $^{27}$Al sample. These multineutrons have been detected by characteristic gamma rays emitted by the nuclei from the beta-decay chain $^{28}\text{Mg}\to{}^{28}\text{Al}\to{}^{28}\text{Si}$. The $^{28}$Mg parent nucleus can be formed in the $^{27}\text{Al} + {}^xn\to {}^{28}\text{Mg}+p(x-2)n$ process. The gamma-ray spectra of the irradiated sample exhibit lines of $1342$- and $1779$-keV photons accompanying the beta decay of the $^{28}$Mg and $^{28}$Al nuclei, respectively. The decrease in the activity corresponds within the measurement accuracy with the half-life $T_{1/2}\sim21\,$h of $^{28}$Mg, which certainly indicates the detection of nuclear-stable multineutrons ${}^xn$ with $x\geq6$.