Survey of Some Systematic Properties of the NuclearE1Transition Probability

A survey of the experimental data for $E1\mathrm{}$ transitions between levels below nucleon emission threshold in medium and heavy nuclei is presented. A general classification of these transitions is proposed and it is shown that, within most of the classes defined, the experimental "reduced" $E1\mathrm{}$ transition probabilities differ by less than a factor ${10}^4$. The data are compared with specific nuclear model predictions. Precise theoretical descriptions of the anomalous parity states involved in $E1\mathrm{}$ transitions exist in a few cases only. However it is often possible to link the observed behavior of $E1\mathrm{}$ transition rates as a function of different parameters, with qualitative model predictions.