1517roughly the order of 1.5 Mev. The (gg/2) 2 state is depressed by somewhat more than the \p112) 2 state from the diagonal energy contribution, but the extra effects of configuration interaction should act principally in the (P112) 2 state, 6 adding the order of 0.5 Mev to its depression. The two states of the pi/2g9/2 configuration are depressed by a much smaller amount due to the poor overlap of the single particle states, perhaps by a few tenths of an Mev. In the limit of short-range forces, the 4-state is not shifted, 5 and the 5-is shifted downward by a small amount. One concludes that the occurrence of the 5-isomeric state (below the 4-) and its excitation energy are in accord with simple shell theory. Also, one expects that the spin 0 state of the (gQ/2) 2 configuration might lie below the 5-state, because of the much greater mutual interaction of two equivalent than of two inequivalent particles.A hypothetical level diagram is presented in the right side of Fig. 1, adjusted to fit what is now known experimentally. In addition to the 5-state discussed above, 2 and the new 0+ state just discovered, 3 there has been observed in the decay of 4 iNb 90 -• > 4oZr 90 a level at 2.23 Mev, 7 which appears not to be the 5state because of its rapid decay to the ground state and because the energy difference is outside experimental error. This state could be interpreted as the 2+ state of the (gg/2) 2 configuration. The small 0-2 separation (^0.5 Mev) of this configuration must then be attributed to the configuration interaction of the 0+ state. In a usual even-even nucleus, the 0-2 spacing is substantially increased by configuration interaction. In this nucleus it may be decreased because of the unusual circumstance that there is another 0+ state below the one of interest. According to the semiempirical formulas of Moszkowski, 8 the 5-to 2+ 70-kev E3 transition should be slower by a factor of about 100 than the 5-to 0+ 2.3-Mev E5 transition. The observation of the ground-state transition only is therefore reasonable.Regarding other nuclei in the neighborhood, we make two remarks. First, the nucleus 38Sr 50 88 does not show a double closed shell character. This may be attributed to the ease of exciting the pi/2 state and/or a substantial ^3/2-^1/2 mixing. Second, most odd-even nuclei in the region show a gwpi/2 energy difference small compared to the 0.9 Mev in 39Y50 89 . This is not necessarily in contradiction to a simple shell picture, since the other nuclei have multiparticle configurations. For example, in 4 iNb 5 o 91 the "£1/2" and "#9/2" states should more correctly be called g9/2 2 pi/2 and pi/2 2 g9/2, and these three-particle configurations should not have the same spacing as the single-particle levels.Aside from its interest as a double closed shell nucleus, Zr 90 calls to attention the way in which the beta-and gamma-decay selection rules can cause lowlying levels to go unobserved in conventional experiments. Shell theory predicts many low-lying states which have not been observed (see, e.g., reference ...

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The beta-spectra of the forbidden transitions of Y 91 , Sr 89 , Y 90 , Cs 137 , RaE, Au 198 , Re 186 , and P 32 have been measured with thin sources and high resolution. Forbidden type spectra are found for the first five. Au 198 , Re 186 , and P 32 yield spectra with the allowed shape. Evidence is advanced for the reliability of the nuclear shell structure model and for the validity of G.-T. selection rules. End points found are: Y 91 , 1.537±0.007 Mev; Sr 89 , 1.463±0.005 Mev; Y 90 , 2.180±0.007 Mev; Cs 137 , 0.51±0.01 Mev; Au 198 , 0.956±0.005 Mev; Re 186 , 1.063±0.006; and P 32 , 1.689±0.01 Mev.