Search for an admixture of heavy neutrinos in the beta-decay of Pu-241

The beta-spectrum of Pu-241 with the endpoint energy of 20.8 keV was measur ed in the energy range from 0.2 to 9.2 keV. The spectra were recorded durin g 5700 h with an electrostatic spectrometer set to a resolution of Delta E/ E = 0.011. Electron energy losses within the radioactive source were treate d by means of extensive Monte Carlo simulations of individual elastic and i nelastic electron collisions. The upper limit for the admixture of hypothet ical neutrinos with the rest masses between 14 and 17 keV/c(2) was derived to be less than 0.40% (the lowest limit being 0.10% for the 16 keV/c(2) mas s) at the 95% C.L., independently of any free phenomenological parameter.