A new summation method model of the reactor antineutrino energy spectrum is presented. It is updated with the most recent evaluated decay databases and with our Total Absorption Gammaray Spectroscopy measurements performed during the last decade. For the first time the spectral measurements from the Daya Bay experiment are compared with the detected antineutrino energy spectrum computed with the updated summation method without any renormalisation. The results exhibit a better agreement than is obtained with the Huber-Mueller model in the 2 to 5 MeV range, the region which dominates the detected flux. An unexpected systematic trend is found that the detected antineutrino flux computed with the summation model decreases with the inclusion of more Pandemonium free data. The detected flux obtained now lies only 1.9% above that detected in the Daya Bay experiment, a value that may be reduced with forthcoming new Pandemonium free data leaving less and less room to the reactor anomaly. Eventually, the new predictions of individual antineutrino spectra for the 235 U, 239 Pu, 241 Pu and 238 U are used to compute the dependence of the reactor antineutrino spectral shape on the fission fractions.

In this paper, we study the impact of the inclusion of the recently measured beta decay properties of the 102;104;105;106;107 Tc, 105 Mo, and 101 Nb nuclei in an updated calculation of the antineutrino energy spectra of the four fissible isotopes 235,238 U, and 239,241 Pu. These actinides are the main contributors to the fission processes in Pressurized Water Reactors. The beta feeding probabilities of the above-mentioned Tc, Mo and Nb isotopes have been found to play a major role in the γ component of the decay heat of 239 Pu, solving a large part of the γ discrepancy in the 4 to 3000 s range. They have been measured using the Total Absorption Technique (TAS), insensitive to the Pandemonium effect. The calculations are performed using the information available nowadays in the nuclear databases, summing all the contributions of the beta decay branches of the fission products. Our results provide a new prediction of the antineutrino energy spectra of 235 U, 239,241 Pu and in particular of 238 U for which no measurement has been published yet. We conclude that new TAS measurements are mandatory to improve the reliability of the predicted spectra.

The feeding probability of 102;104;105;106;107 Tc, 105 Mo, and 101 Nb nuclei, which are important contributors to the decay heat in nuclear reactors, has been measured using the total absorption technique. We have coupled for the first time a total absorption spectrometer to a Penning trap in order to obtain sources of very high isobaric purity. Our results solve a significant part of a long-standing discrepancy in the component of the decay heat for 239 Pu in the 4-3000 s range. DOI: 10.1103/PhysRevLett.105.202501 PACS numbers: 23.40.Às, 27.60.+j, 28.41.Fr, 29.30.Kv Nuclear reactors provide a significant fraction of the world's electricity. A burgeoning population and an associated growth in economic activity suggest that world demand will double by 2050. Until now, the bulk of this has come from the burning of fossil fuels. There is general concern that reserves of fossil fuels are limited and their burning damages the environment. In particular, it contributes to the emission of large amounts of CO 2 . In this context, nuclear power, based on the fission process, will be less damaging to the environment. Accordingly there is now a renaissance in the building of nuclear power stations around the world. Modern reactor designs, based on many years of operating experience, are much more efficient, more economical, and safer than earlier designs. Although the basic principles are well established, we still lack certain information, such as a knowledge of the decay properties of specific nuclei that are important contributors to the heating of the reactor during and after operation. The estimation and control of the heat emitted by the decay of fission products plays a key role in the safe operation of reactors. The primary aim of this work is to study the decay properties of specific nuclei that are important contributors to this source of heat.Approximately 8% of the total energy generated during the fission process is related to the energy released in the natural decay of fission products, and is commonly called decay heat [1]. Once the reactor is shut down, the energy released in radioactive decay provides the main source of heating. Hence, coolant needs to be maintained after termination of the neutron-induced fission process in a reactor, and the form and extent of this essential requirement needs to be specified on the basis of decay-heat summation calculations. Decay heat varies as a function of time after shutdown and can be determined theoretically from known nuclear data. Such computations are based on the inventory of nuclei created during the fission process and after reactor shutdown and their radioactive decay characteristics:where fðtÞ is the power function, E i is the mean decay energy of the ith nuclide ( , , and components), i is the decay constant of the ith nuclide, and N i ðtÞ is the number of nuclide i at cooling time t. These calculations require extensive libraries of cross sections, fission yields, and decay data. Obviously, an accurate assessment of the decay heat is highly relevant...

A novel method of deducing the deformation of the N=Z nucleus 76 Sr is presented. It is based on the comparison of the experimental Gamow-Teller strength distribution B(GT) from its β-decay with the results of QRPA calculations. This method confirms previous indications of the strong prolate deformation of this nucleus in a totally independent way. The measurement has been carried out with a large Total Absorption gamma Spectrometer, "Lucrecia", newly installed at CERN-ISOLDE.PACS numbers: 21.10. Pc, 23.40.Hc, 27.50.+e, 29.30.Kv, 29.40.Mc The shape of the atomic nucleus is conceptually one of the simplest of its macroscopic properties to visualise. However, it turns out to be one of the more difficult properties to measure. In general terms we now have a picture of how the nuclear shape varies across the Segrè Chart. Nuclei near to the closed shells are spherical. In contrast nuclei with the valence nucleons in between two shells have deformed shapes with axial symmetry and the extent of the quadrupole deformation is quite well described as being proportional to the product N p N n of the numbers of pairs of valence protons (N p ) and neutrons (N n ) [1]. This picture is underpinned by both the Shell and Mean Field models of nuclear structure. Experiment and theory concur that, as the N p N n parameterisation would suggest, nuclei rapidly deform as we add only a small number of valence nucleons to the magic numbers. Thus nuclei in the middle of the f 7/2 shell turn out to be deformed even although the numbers of valence nucleons are relatively small.Experimentally this picture is supported by a mass of independent observations: the strongly enhanced quadrupole transition rates between low-lying states, the strongly developed rotational bands built on low-lying states, and measurements of ground state quadrupole moments. Where we have evidence of the shapes of ground and excited states in the same nucleus they are, in general but not always, the same. It turns out that in some cases nuclear states with different shapes co-exist in the same nucleus [2].The nuclei with N≈Z and A≈70-80 are of particular interest in this context. Such nuclei enjoy a particular symmetry since the neutrons and protons are filling the same orbits. This, and the low single-particle level density, lead to rapid changes in deformation with the addition or subtraction of only a few nucleons. In terms of Mean Field models these rapid changes arise because of the proximity in energy of large energy gaps for protons and neutrons at Z,N=34 and 36 on the oblate and Z,N=38 on the prolate side of the Nilsson diagram. As a result Mean Field calculations predict the existence of several energy minima with quite different shapes in some of these nuclei [3,4]. Evidence of this co-existence has been found for instance in Se and Kr nuclei [5,6], and it is also predicted for the lightest Sr isotopes [7]. Thus it is of considerable interest to map out the deformation of the ground and excited states of nuclei in this region. This is easier said than do...