Greenwood and co-workers have developed total absorption gamma-ray spectrometry (TAGS) in order to determine the intensity distributions of a wide range of P— emitting radionuclides (Greenwood et al, 1994; Helmer et al, 1994; Greenwood et al, 1996; Greenwood et al, 1997). The TAGS system consists of a NaI(Tl) detector with a deep axial counting well to allow almost complete summing of the gamma-ray cascades from the source. A Si detector is also located in the well so that P—particle- gated coincidence gamma rays can be measured in addition to singles gamma-ray spectra. Spontaneous fission of 252Cf generated the fission-product radionuclides, and a He gas-jet transported these nuclides to a mass separator. The selected fission — product mass fraction was collected on a tape for a pre-set time, before moving to the Si detector in the well of the TAGS system. Although only relative P—intensity distributions are obtained from the TAGS gamma-ray cascade-summed spectral data, the detector system can be used in 4ny-P — coincidence mode to obtain the P— branching intensity to the ground state; the measurement of this key parameter provides a simple means of converting the relative P—intensity distributions to absolute values.

The resulting singles spectra were compared with simulated spectra derived from the evaluated decay data to be found in Nuclear Data Sheets (Bhat, 1992): examples are shown in Figs. 26 and 27 for 138gCs and 138mCs, respectively. Measured and
calculated spectra for 138gCs are in good agreement below ~3200 keV, but the simulated spectrum exhibits serious deficiencies in the postulation of beta-particle emission probabilities above this energy. Additional pseudolevels have been introduced to the decay scheme in order to achieve the good fit shown in Fig. 28 (see also Table 11); these additions and increased populations to other high-energy levels increase the beta-particle emission probabilities above 3200 keV from 2.65% to 5.9%. Similar procedures can be applied to the spectrum of 138mCs (Fig. 29), with the addition of a considerable number of pseudolevels above 2500 keV as listed in Table 12.

The TAGS method of spectral measurement and analysis would appear to be an extremely powerful means of modifying inadequate decay-scheme data in a quantitative manner. This approach is very welcome, and should be encouraged further in order to eliminate serious gaps in our detailed knowledge of the discrete decay data of many important short-lived fission products.

NDS: Nuclear Data Sheets, 69(1993)69. P: placement of pseudolevel.

“Contribution to the total 138mCs decay is obtained by multiplying Pp by 0.19. NDS: Nuclear Data Sheets, 69(1993)69.

P: placement of pseudolevel.