Most experimental measurements of nuclear structure and decay-scheme data focus on the emission of discrete gamma-rays. This type of spectral measurement dominates fission-product decay-scheme studies because of the difficulties and scarcity of facilities throughout the world to measure the corresponding beta — transition energies and emission probabilities accurately. However, the inability to detect weak high-energy gamma-ray emissions satisfactorily has been noted by Hardy et al (1977), and this problem impacts on the P — decay data calculated from such measurements. A generalised assessment was made of a radionuclide with a complex decay scheme (labelled “pandemonium”): approximately 20% of the gamma-ray emissions above 1.7 MeV were estimated to remain undetected within the background, and would be omitted from the proposed decay scheme. Under these conditions, every complex p — decay scheme derived through gamma-ray studies must be regarded with some doubt — the recommended P—decay scheme may be inaccurate (Fig. 1), and might explain anomalies that sometimes occur between calculated and measured decay heat.

The P — decay of 87Br represents a good example of the problems faced by evaluators of complex decay schemes (Reich, 1987). This nuclide has a half-life of 55.7 sec, and an extremely thorough study by Raman et al (1983) has revealed the existence of 126 bound levels and 12 levels in the unbound region. Approximately 220 gamma rays were detected in a series of singles and coincidence measurements, and P-
emission probabilities were calculated from the measured gamma-ray intensities: 160 beta branches were defined, with evidence of broad resonance-like structure in the overall beta-strength distribution. These measurements have resulted in significant changes in the mean beta and gamma energies for this radionuclide. Unfortunately, few short-lived fission products with such complex decay schemes are likely to be studied in such a comprehensive manner. Many short-lived fission products are very poorly characterised because their gamma-ray spectra have been incompletely measured or remain undetermined; under these circumstances, theoretical models have been adopted to calculate mean beta and gamma energies (and half-lives).