Search for a neutrino with a mass of $(0.01$–$1.0)$ MeV in beta decays of $^{144}$Ce–$^{144}$Pr nuclei

Spectra of electrons from beta decays of $^{144}$Ce–$^{144}$Pr nuclei have been measured and analyzed in order to find a contribution from a heavy neutrino. For the neutrino with the mass $m_{\nu H}$ in the interval of $(150$–$350)$ keV, a new upper bound $|U_{eH}|^2 \leq (2-5)\times 10^{-3}$ at $90\%$ C.L. has been found for the mixing parameter.