[en] β-decay properties are usually determined by measuring, with high resolution Ge crystals, the intensity and energy of γ-rays emitted after the beta decay. In the case of large Q_β, or complex de-excitation pattern, it happens that some transitions are missed due to the low efficiency of Germanium detectors for high energy gammas or high multiplicity decay cascades. This leads to an overestimation of the high energy part of the β spectra and the anti-neutrino ones. This is called the Pandemonium effect. Some of the data present in nuclear databases suffer from the distortion caused by this effect. A way to avoid the Pandemonium effect is to use a Total Absorption Spectrometer (TAS), this detector is a 4π calorimeter constituted of one large or segmented crystals and with the particularity of having a cascade detection efficiency close to 100%. A TAS device has been built and calibrated with "1"3"7Cs, "6"0Co and "2"2"-"2"4Na sources. A test performed on "9"2Rb shows that we have a good agreement between the simulated spectrum and the experimental one. The use of TAS for β-decay properties will help to assess the decay heat (or residual power) of reactors

[en] To study the β-decay properties of some well known delayed neutron emitters an experiment was performed in 2009 at the IGISOL facility (University of Jyvaeskylae in Finland) using Total Absorption γ-ray Spectroscopy (TAGS) technique. The aim of these measurements is to obtain the full β-strength distribution below the neutron separation energy (S_n) and above the γ/neutron competition. This piece of information is a key parameter in nuclear technology applications as well as in nuclear astrophysics and nuclear structure. Preliminary results of the analysis show a significant γ-branching ratio above S_n and this surprising result is in contradiction with the general assumption of neutron emission domination above S_n

[en] In a nuclear reactor, the decay of fission fragments is at the origin of decay heat and antineutrino flux. These quantities are not well known while they are very important for reactor safety and for our understanding of neutrino physics. One reason for the discrepancies observed in the estimation of the decay heat and antineutrinos flux coming from reactors could be linked with the Pandemonium effect. New measurements have been performed at the JYFL facility of Jyvaeskylae with a Total Absorption Spectrometer (TAS) in order to circumvent this effect. An overview of the TAS technique and first results from the 2009 measurement campaign will be presented. (authors)