the exchange of the three Majorana neutrinos, and neutrino oscillation explanation of the solar and atmospheric neutrino data, which will be assumed to hold throughout this study, the observation of -decay [^4] can give unique information on the type of the neutrino mass spectrum and on the lightest neutrino mass, i.e., on the absolute scale of neutrino masses [@BGGKP99; @BPP1; @Paramm1; @WR00; @BPP2; @PPW; @WR0302; @PPSNO2bb; @bb0nuMassSpec1; @Weiler2001]. One of the important implications of the latest results of the solar neutrino experiments, notably of SNO, which show that $\tan^2\theta_{\odot} < 1$ and, e.g., $\cos2\theta_{\odot} \gtap 0.26$ at 99.73% C.L. [@SNO2], regard the predictions for $\!\!$. The fact that $\cos2\theta_{\odot}$ is significantly different from zero leads to [@PPSNO2bb] (see also [@BPP1; @PPW; @WR0302]) the existence of significant lower bounds on (exceeding 0.01 eV) in the cases of neutrino mass spectrum with inverted hierarchy (IH) and of the quasi-degenerate (QD) type, and of a stringent upper bound (smaller than 0.01 eV) if the neutrino mass spectrum is with normal hierarchy (NH). Using, e.g., the best fit values of , , $\cos2\theta_{\odot}$ and $\sin^2\theta$, obtained in the analyzes of the solar and atmospheric neutrino, and CHOOZ data in [@SNO2; @Gonza3nu], one finds respectively 