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

The β-spectrum of 241Pu with the endpoint energy of 20.8 keV was measured in the energy range from 0.2 to 9.2 keV. The spectra were recorded during 5700 h with an electrostatic spectrometer set to a resolution of ΔE/E=0.011. Electron energy losses within the radioactive source were treated by means of extensive Monte Carlo simulations of individual elastic and inelastic electron collisions. The upper limit for the admixture of hypothetical neutrinos with the rest masses between 14 and 17 keV/c2 was derived to be less than 0.40% (the lowest limit being 0.10% for the 16 keV/c2 mass) at the 95% C.L., independently of any free phenomenological parameter. (author)