The stability and spontaneous decay of naturally occurring atomic nuclei have been much studied ever since Becquerel discovered natural radioactivity in 1896. In 1960, proton-rich nuclei with an odd or an even atomic number Z were predicted to decay through one- and two-proton radioactivity, respectively. The experimental observation of one-proton radioactivity was first reported in 1982, and two-proton radioactivity has now also been detected by experimentally studying the decay properties of 45Fe (refs 3, 4) and 54Zn (ref. 5). Here we report proton-proton correlations observed during the radioactive decay of a spinning long-lived state of the lightest known isotope of silver, 94Ag, which is known to undergo one-proton decay. We infer from these correlations that the long-lived state must also decay through simultaneous two-proton emission, making 94Ag the first nucleus to exhibit one- as well as two-proton radioactivity. We attribute the two-proton emission behaviour and the unexpectedly large probability for this decay mechanism to a very large deformation of the parent nucleus into a prolate (cigar-like) shape, which facilitates emission of protons either from the same or from opposite ends of the 'cigar'.

The compounds Cs12[VIV 18O42(H2O)]·14H2O (1a), K12[VIV 18O42(H2O)]·16H2O (1b), Rb12[VIV 18O42(H2O)]·19H2O (1c), K9[H3VIV 18O42(H2O)]·14H2O·4N2H4 (1d), K11[H2VIV 18O42(Cl)]·13H2O·2N2H4 (2a), K9[H4VIV 18O42(Br)]·14H2O·4N2H4 (2b), K9[H4VIV 18O42(I)]·14H2O·4N2H4 (2c), K10[H3VIV 18O42(Br)]·13H2O·0.5N2H4 (2d), K9[H4VIV 18O42(NO2)]·14 H2O·4N2H4 (3), Cs11[H2VIV 18O42(SH)]·12H2O (4), K10[HVIV 16VV 2O42(Cl)]·16H2O (5a), Cs9[H2VIV 16VV 2O42(Br)]·12H2O (5b), K10[HVIV 16VV 2O42(Br)]·16H2O (5c), Cs9[H2VIV 16VV 2O42(I)]·12H2O (5d), K10[HVIV 16VV 2O42(I)]·16H2O (5e), K10[HVIV 16VV 2O42(HCOO)]·15H2O (6), (NEt4)5[VIV 10VV 8O42(I)] (7), and Na6[H7VIV 16VV 2O42(VO4)]·21H2O (8) have been characterized by elemental analyses, redox titrations, IR and EPR spectroscopy, and magnetic susceptibility measurements, as well as by single crystal X-ray structure analyses. The anions containas a common structural featurean approximately spherical {V18O42} shell, which can act as a unique host system for neutral and anionic guests. Remarkably, the electron population of these clusters candepending on the specific reaction conditionsbe varied in a wide range between 10 and 18 3d electrons without changing the overall structure. The related magnetic properties in connection with the phenomenon of electron-dependent spin organization on a spherical cluster shell have been discussed in detail.

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The levels in 74 » 76 Kr were studied with in-beam y-spectroscopy techniques and the j3 + decay of 7G Rb. The energies of the 2 t + states in 74,76 Kr deviate from smooth behavior compared with the higher spin levels. The yrast cascade B (E2)'s are highly collective. The 74 » 76 Kr ground states have unusually large deformation. The origin of this deformation and of shape coexistence in this region is described in terms of the protons driving the deformationo PACS numbers: 23.20. Ck, 23.20.Lv, 27.50. + e The 0 2 + energies have a deep minimum in 7O,72Q G an( j 72,74g e anc j are near or below the 2 X + energies. 1 These and other data have led to various suggestions of shape coexistence in these nuclei, where the low-lying 0 2 + states are more deformed than the ground states. 1 " 4 However, questions have been raised about shape coexistence and deformation in these nuclei, in part because well-developed, deformed bands built on the 0 2 + states in 70,72 Ge are not seen. 1 * 3 In this paper we suggest that the origin of shape coexistence for iV«38 nuclei is related to the number of protons which delicately controls whether a deformed shape or near-spherical shape is lowest in nuclei in this region. Our 74 » 76 Kr data give evidence that their ground states have remarkably large deformation.The origin of strong deformation and shape coexistence in this region can be attributed to the gaps in the single-particle spectrum seen in Fig. 1514

Owing to the higher electron density on the oxygen atom, the protonation of V"-0-V1" centers is more reasonable than that of V'v-O-Vv centers. Furthermore, twelve V"' and six Vv centers can be unequivocally localized by an exponentially weighted summation of all V -0 bond lengths of each V atom (cf. [4]; the bond lengths for the Vv atom of the central VO, tetrahedron deviate from those of the other Vv centers since they are in error owing to the disorder). The anion [V/~V~0,,(OH)9]sQ differs from ''[V~"V~~O,,(OH)]""" prepared by Johnson (but not published), (cf. [6,7]), above all in the structure, the number of 0 and H atoms, and the V'"/VV

Although the inclusion of molecules in oxidic solids, e.g. zeolites, has been known for some time and finds wide application, the first soluble oxide inclusion complex has only recently been obtained in the form of ~i 2 0 , 2 ( C H , C N ) ] 4~.~' a 1 Moreover, very little is known about other discrete inorganic host-guest systems. Mixed valence V1v/Vv-O and V"-O clusters are, according to the results of our investigations, remarkable regarding their molecular and electronic structure, since, on the one hand, V"-and Vv-centers are present in almost any desired ratio, and, on the other, the coordination of the vanadium can be widely varied."] The fomiation of "hollow spheres" with unusual "cryptand properties" and interesting topology is possible by the linking of VO, pyramids. The spherical cluster shells {V,,042) 1 and {V,s036} 2, which deserve special mention here because of their topological properties, have in the compounds 1 a, b and 2a, b (2c is included only for the discussion of the type of bonding) the Cs9[H,Vi80,,(X)] . 12H,O (NMe,),[V,,O,,(Y)] -4 H 2 0 Li,[V,,O,,(CO,)]~ ca. 3 9 H 2 0 2c

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We investigated the magnetic properties of two polyoxovanadate anions, [Vi2Ass040(HC02)]3-(I) and [VIZA~8040(HC02)]5-(11). Both I and I1 comprise four localized vanadium(1V) ions while two vanadium(1V) ions in I and four in I1 are delocalized on eight sites, the remaining ions formally being vanadium(V). The temperature dependence of the magnetic susceptibility for the two compounds was fitted with a model which takes into account an antiferromagnetic coupling between the localized vanadium( IV) ions. In I1 the delocalized vanadium( IV) ions are strongly coupled to give a ground S = 0 level by exchange and delocalization effects, while in I the delocalized vanadium(1V) must be considered to be ferromagnetically coupled to the localized ones. The magnetic behavior correlates with the geometry and the topology of the clusters. EPR spectra were recorded both at X-band frequency and with a high-field spectrometer using a far-infrared laser. The spectra of I and I1 show opposite behaviors at low and high fields, presumably due to field-dependent relaxation effects.