Limits on the existence of heavy neutrinos in the range 50-1000 eV from the study of the 187Re beta decay

We analyzed the spectrum of the Re-187 beta decay, obtained with a cryogenic microcalorimeter, searching for heavy neutrinos in the mass range 50-1000 eV. No evidence has been found for them and the upper limits on the mixing angle with a zero-mass neutrino are reported. Upper limits of 9 x 10(-3) at 1000 eV/c(2), 1.2 x 10(-2) at 500 eV/c(2), 4.4 x 10(-2) at 200 eV/c(2), and 0.116 at 100 eV/c(2) at 95% C.L. have been obtained. These upper limits are a factor of 2 to 4 lower than the current limits reported in the literature.