As mentioned earlier, to calculate the mean energies the feeding probability to the different levels populated in the decay is needed (Eqs. 6, 7). Now we can understand why TAS measurements can have an impact and should be applied to decay heat studies. If we have a beta decay that suffers from the pandemonium effect, the mean gamma energies will be underestimated and the beta energies will be overestimated. This is shown schematically in Fig. 4. Since the application of the TAS technique is the only way to avoid the pandemonium effect, this technique should be used for studying decays that are important for the decay heat problem.

Our interest in the decay heat topic was triggered by the work of Yoshida and coworkers (Yoshida, 1999). At the begining of the 1990s one of the most successful data bases for summation calculations was the JNDC-V2 (Japanese Nuclear Data Committee version 2) database. In this database the gross theory of beta decay (Takahashi & Yamada, 1969) was used to supplement experimental data that might suffer from the pandemonium effect. For example in (Tasaka, 1988) it is mentioned that the mean energies of many decays having a beta

decay Q value larger that 5 MeV were supplemented by theoretical values calculated using the gross theory. Even though the database in general worked well for the gamma component of the 239 Pu decay heat, there was a significant remaining discrepancy in the 300-3000 s cooling time. Yoshida studied the possible causes for the discrepancy and proposed several possible explanations. The most plausible was the possibility that the decay energies of some nuclides with half-lives in the range of 300-3000 s or with precursors with similar half-lives suffered from the pandemonium effect. After a careful evaluation of possible candidates he proposed that the decays of 102,104,105 Tc should be measured with the total absorption technique.

In the process of defining priorities for the TAS measurements related to decay heat, contact with specialists in the field was established and a series of meetings were held under the auspices of the International Atomic Energy Agency (IAEA). As a result of these meetings a list of nuclei that should be measured with the total absorption technique was defined (Nichols, 2007). This list included the Yoshida cases (102,104,105Tc) and some additional nuclei (in total 37). The nuclei were identified based on their contribution to the decay heat in different fuels, and in order to reduce the discrepancies between the major international databases (JENDL (Shibata, 2011), JEFF (Kellett, 2009), ENDF (Chadwick, 2006)). Additionally, this list included some cases that deserve to be measured for other reasons. For example there are TAS measurements that were performed in the past by Greenwood and coworkers (Greenwood, 1992). These TAS measurements were analyzed using different procedures from those used nowadays. It is important to verify the results and compare them with new measurements using different analysis techniques to look for systematic uncertainties. Similarly the measurements of (Rudstam, 1990), obtained from direct spectral measurements can be checked against TAS measurements (Tain & Algora, 2006).