By using the analytical superasymmetric fission model it is shown that all "stable" nuclei lighter than lead with Z > 40 are metastable relative to the spontaneous emission of nuclear clusters. An even-odd effect is included in the Zero point vibration energy. Half-lives in the range 1040-1050 s are obtained for Z > 62.The region of metastability against these new decay modes is extended beyond that for a decay and in some cases, in the competing region, the emission rates for nuclear clusters are larger than for a decay.During the last few years advances in studies of many nuclear decay modes have gained considerable interest. Recently, these have been reviewed by Hamilton et al. ' We have used (see Refs. 2-4, and references therein) several methods to show that nuclei heavier than a particles (A2 > 4) and lighter than fission fragments (A2 < 70) are spontaneously emitted from various parent nuclides ( A, Z ) leading to the daughters (A1,Z1). A review paper presenting our early work will be published el~ewhere.~ There is, already, experimental evidence concerning two of more than 140 new decay m o d e~:~.~ (1) 14C spontaneous emissiona-l2 from 223Ra andll from 222,224Ra and (2) 24Ne radioactivity13 of 232U and14 of 231Pa. The experimental data are in agreement with the halflives and the branching ratios relative to a decay calculatedS-',Is (see also Refs. 16 and 17) in the framework of the analytical superasymmetric fission model (ASAFM)3.'8 and with the branching ratios computed by Shi and Swiatecki19 using a proximity-plus-Coulomb potential.Up to now only the region of parent nuclides with Z > 82 have been investigated. The purpose of this paper is to extend the domain for nuclides lighter than lead, pointing out that all the so-called "stable" nuclides with atomic numbers Z > 40, are, in fact, metastable with respect to several new cluster decay modes.In order to estimate the half-lives, T' and T, relative to nuclear cluster emission we shall use ASAFM7 with two values of the Zero point vibration energy E,. This energy enters crucially the formula for the lifetime against cluster emission where the standard notations7 are used for the reduced mass, p , the potential interaction energy E ( r ) and E (R,) = E ( Rb) = Q'. We choose on the one hand, which leads the half-life T, regardless of the odd ( 0 ) or even ( e ) character of the neutron ( N I and proton ( Z ) numbers of the parent nuclide, and on the other hand, with (1.105, e-e I 0.947, e-o E; = E u x parent , 1.000, o-e leading to the half-life T', one can obtain better agreement for a decay of 380 emitters. Hence, T' and T are the halflives with or without the even-odd effect taken into acCount, respectively. A similar even-odd effect was observed20 for 14C radioactivity of Ra isotopesll and of 2 2 5 A~: an enhanced cluster emission rate from e-e nuclei, or equivalently a hindrance from o -e, e -0, and o -0 parents. The released energy, Q, is computed with the new version of the mass table.21 We do not consider the relatively small angular momentum c...

The great majority of the known nuclides with Z > 40, including the so-called stable nuclides, are metastable with respect to several modes of spontaneous superasymmetric splitting. A model extended from the fission theory of alpha decay allows one to estimate the lifetimes and the branching ratios relative to the alpha decay for these natural radioactivities. From a huge amount of systematic calculations it is concluded that the process should proceed with maximum intensity in the translead nuclei, where the minimum lifetime is obtained from parent-emitted heavy ion combinations leading to a magic ( 2 0 8~b ) or almost magic daughter nucleus. More thari 140 nuclides with atomic number smaller than 25 are possible candidates to be emitted from heavy nuclei, with half-lives in the range of

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