Excited states of 92,914996,98M and 94,96,98,l0O,lO2.R were studied by in-beam y-ray . spectroscopy The decays of levels populated by the (c,2n) reaction on even isotopes of Zr and Mo were studied by measurement of the y singles spectrum, beam-.'y'-delay, yy-coincidences (prompt and delayed), and y-ray angular distributions The most notable new feature of these experiments is the use, in-beam, of a sensitive coincidence technique, employing two Ge(Li) spectrometers in conjunction with a two-parameter incremental data-acquisition system, to establish the placement of weak transitions The level schemes are compared with previous studies of these nuclei A considerable number of new levels, predominantly of high spin (I > 6), were observedThe observed levels are analyzed in terms, of systematics and, where available, detailed shell-model calculations A general description in terms of excitations in the low-lying orbitals outside a 38-proton + 50-neutron core appears adequate to explain much of the data, particularly in those nuclei near the 50-neutron shell. The dataare also compared to recent macroscopic calculation of quasi-rotational bands, the VMI model.The observed spin distribution in the residual nuclei indicates a net loss of angular momentum in the emission of neutrons and/or unresolved y rays tWork performed under the auspices of the U S. Atomic Energy Commission -2- UCRL-19936 Calculations based on a microscopic model for neutron evaporation suggest that, with a moment-of-inertia parainetera'= 0.5 rigid' the angular-momentum loss can be accounted for entirely by theneutrons duced by a time-to-amplitude converter (TAC) started by the y-ray pulse and stopped by a (delayed) pulse generated at the upward zero-crossing point on the cyclotron RF voltage The time signal is further processed in two ways1. An "energy-walk" compensator corrects for the energy-dependent time shift introduced by the leading-edge discriminator 9 )2 Variations in the RF phase relative to the arrival of beam bunches at the target are compensated by a variable-baseline feedback loop, which stabilizes the position of the prompt y-ray peak in the time spectrum.The lower limit of detectable half-lives, 2 to 3 ns, is determined primarily by the time resolution of the detector-electronic system Next, isomeric transitions were sought by measuring 2-dimensional energy vs.time spectra, with 512 channels on the energy axis to cover intervals of 600 keV and 8 channels to cover the time interval during and between beam bunches.If isomeric transitions were observed, they were further studied in measurements of small energy intervals by employing more channels on the time axis.The estimated precision of our energy measurements for strong transitions, 0.1 to 0.2 keV,. is consistent with the observed agreement for crossovercascade relationships. However, the use of.external calibrations introduces additional, systematic uncertainties into the absolute energies. We feel that a standard error ±1 keV on all y rays is a reasonable estimate of th...

A γ-ray spectroscopy study of excited states in 26 Si has been performed using the 24 Mg( 3 He,n) reaction at a beam energy of 10 MeV. In particular, states have been studied above the proton threshold relevant for burning in the 25 Al(p,γ) 26 Si reaction in novae. This reaction influences the amount of 26 Al injected into the interstellar medium by novae, which contributes to the overall flux of cosmic gamma-ray emission from 26 Al observed in satellite missions. The present results point strongly to the existence of a 0 + state at an excitation energy of 5890 keV lying within the Gamow burning window, which raises questions about the existence and properties of another, higher-lying state/resonance reported in previous experimental work. The existence of two such states within this excitation energy region cannot be understood in the framework of sd shell-model calculations.

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