Abstract
Experimental M1, E1, and E2 photon strengths have been compiled from ex- perimental data in the Evaluated Nuclear Structure Data File (ENSDF) and the Evaluated Gamma-ray Activation File (EGAF). Over 20,000 Weisskopf reduced transition proba- bilities were recovered from the ENSDF and EGAF databases. These transition strengths have been analyzed for their dependence on transition energies, initial and final level ener- gies, spin/parity dependence, and nuclear deformation. ENSDF BE1W values were found to increase exponentially with energy, possibly consistent with the Axel-Brink hypothesis, although considerable excess strength observed for transitions between 4-8 MeV. No sim- ilar energy dependence was observed in EGAF or ARC data. BM1W average values were nearly constant at all energies above 1 MeV with substantial excess strength below 1 MeV and between 4-8 MeV. BE2W values decreased exponentially by a factor of 1000 from 0 to 16 MeV. The distribution of ENSDF transition probabilities for all multipolarities could be described by a lognormal statistical distribution. BE1W, BM1W, and BE2W strengths all increased substantially for initial transition level energies between 4-8 MeV possibly due to dominance of spin-flip and Pygmy resonance transitions at those excitations. Anal- ysis of the average resonance capture data indicated no transition probability dependence on final level spins or energies between 0-3 MeV. The comparison of favored to unfa- vored transition probabilities for odd-A or odd-Z targets indicated only partial support for the expected branching intensity ratios with many unfavored transitions having nearly the same strength as favored ones. Average resonance capture BE2W transition strengths generally increased with greater deformation. Analysis of ARC data suggest that there is a large E2 admixture in M1 transitions with the mixing ratio ≈ 1:0. The ENSDF reduced transition strengths were considerably stronger than those derived from capture gamma ray data implying that those data are strongly biased by favored, unhindered transitions.
Highlights
The statistical model of the nucleus requires models of average level density and spin/parity dependence and the average photon strengths for M1, E1, and E2 transitions
Experimental M1, E1, and E2 photon strengths have been compiled from experimental data in the Evaluated Nuclear Structure Data File (ENSDF) and the Evaluated Gamma-ray Activation File (EGAF)
Average resonance capture BE2W transition strengths generally increased with greater deformation
Summary
The statistical model of the nucleus requires models of average level density and spin/parity dependence and the average photon strengths for M1, E1, and E2 transitions. Much recent discussion of the results of statistical model analysis of experimental data concerns the observation of nonstatistical fluctuations from the models which may be interpreted as nuclear structure effects This analysis is often stymied by the experimental difficulty determining whether their origin stems from the level density or the transition strength which are usually inseparable in the analysis. In this paper I will discuss the transition probability data for individual gamma ray transitions that is available from the Evaluated Nuclear Structure Data File (ENSDF) [11] and the Evaluated Gammaray Activation File (EGAF) [12] These data were measured independently of the level density and can provide a clearer picture of the gamma-ray strength functions with respect to transition energy, level excitation, and spin/parity
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