Fig. (32) Best upper limits on the admixture |U_e4|² of sterile neutrinos derived from measured β-ray spectra. All upper limits refer to the 95% CL except those for ⁶⁴Cu [186] and ³H [219] that were published at the 90% CL. The numbers denote the following radionuclides and their investigators: 1 – ³H [218], 2 – ³H [219], 3 – ¹⁸⁷Re [206], 4 – ³H [203], 5 – ³H [114], 6 – ⁶³Ni [204], 7 – ⁶³Ni [193], 8 – ³⁵S [194], 9 – ⁶⁴Cu [186], 10 – ²⁰F [189].