At IGISOL we have performed two trap-assisted TAS experiments related to the decay heat problem. Here we will discuss briefly the first experiment and its impact. The second experiment is presently in the analysis phase. It is worth noting that in the second experiment we have used for the first time a segmented BaF2 TAS detector which additionally provides information on the multiplicity of the gamma cascades following the beta decay. This extra information is useful for the analysis of the complex beta decay data.

The analysis of a TAS experiment is a lengthy procedure and requires several stages. The first phase requires a careful evaluation of the contaminants and distortions of the measured spectrum in order to determine d. Then the calibration of the experimental data in energy and width and a precise characterization of the TAS detector using Monte Carlo (MC) techniques is

required. For the MC simulations the GEANT4 code (Agostinelli, 2007) is used. In this phase a careful characterization of the setup is performed until measurements with conventional radioactive sources like 24Na, 137Cs and 60Co are very well reproduced by the MC code. Once this has been achieved, the response function of the detector to the decay of interest can be calculated. This requires the definition of the level scheme that may be populated in the decay (B or branching ratio matrix). To construct the branching ratio matrix we take into account known levels up to a certain excitation in the daughter (Ecut) and above that cut-off energy we use a statistical model to generate levels and their branchings. The information on the low-lying levels and their branchings is taken from conventional high resolution measurements, because this information is correct in general if available (known levels). For the statistical model we use a back shifted Fermi formula for the level density and gamma strength functions, which define the probabilities that gamma rays connect the different levels (known and unknown part). Once the B is determined, the R(B) is calculated from the MC responses of the detector to the different 7 and в transitions and 8 is solved. As part of the analysis, the cut-off energy, the accepted low-lying levels and the parameters of the statistical model are changed if necessary. The final result of the analysis is a feeding distribution, from which nuclear structure information can be obtained in the form of the beta strength distribution (Sp), and in the case of the decay heat application mean beta and gamma energies can be calculated (6 and 7).

The impact of our first experiment can be seen in Table 2, where the mean energies of the ENDF database are compared with the results of our measurements. From this table the relevance of performing experiments is also clear. Two nuclei (101Nb, 102Tc), that were suspected to suffer from the pandemonium effect did not, even though they have large Qp decay values. The remaining nuclei all suffered from the effect (see for example the large increase in the mean gamma energy and the reduction in the mean beta energy with respect to high resolution measurements for 104,105Tc). In Fig. 8 the results for the gamma component of 239Pu are presented. They are compared with ENDF (ENSDF, n. d.) before and after the inclusion of our new data (Sonzogni, n. d.). Similar conclusions have been obtained recently using the JEFF database (Mills, n. d.). The new TAS results were published in (Algora, 2010).

As a result of our measurements, a large part of the discrepancy pointed out by (Yoshida, 1999) in the 300-3000 s cooling interval and additionally the discrepancy at low energies has been solved within the ENDF database. The new data were also used to perform summation calculations for the gamma component of 235U. In this case the results were disappointing. Our new results had very little impact. This can be understood in terms of the cummulative fission yields of the nuclei in question. They sample approximatelly 33.8 % of the fission in 239Pu, but only 13.5 % in 235U. Additionnally from the 13.5 % in 235U, 101Nb, 102Tc amounts to 9.2 %, which does not bring a large change in the mean energies. This explains why our measurements to date can only represent a relative change of approximatelly 4.3 % in 235U, compared to the 22.6 % relative impact in 239Pu with respect to the earlier values of the ENDF database.