Bolometric Bounds on the Antineutrino Mass

High statistics calorimetric measurements of the β spectrum of ¹⁸⁷Re are being performed with arrays of silver perrhenate crystals operated at low temperature. After a substantial modification of the experimental setup, a new measurement with ten silver perrhenate microbolometers has been running since July 2002. The crystals have masses around 300 μg and their average FWHM energy resolution is of 28.3 eV at the β end point. The Kurie plot collected during 4485 h×mg effective running time has an end-point energy of 2466.1±0.8ₛₜₐₜ±1.5syst eV, while the half lifetime of the decay is found to be 43.2±0.2ₛₜₐₜ±0.1syst Gy. These values are the most precise obtained so far for ¹⁸⁷Re. The best fit value for m_νₑ² is 147±237ₛₜₐₜ±90syst eV², which corresponds to an upper limit for the electron antineutrino mass m_νₑ≤21.7 eV at 90% C.L.