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The spent fuel of current nuclear reactors contains fissile plutonium isotopes that can be combined with 238U to make mixed oxide (MOX) fuel. In this way the Pu from spent fuel is used in a new reactor cycle, contributing to the long-term sustainability of nuclear energy. The use of MOX fuels in thermal and fast reactors requires accurate capture and fission cross sections. For the particular case of 242Pu, the previous neutron capture cross section measurements were made in the 70's, providing an uncertainty of about 35% in the keV region. In this context, the Nuclear Energy Agency recommends in its “High Priority Request List” and its report WPEC-26 that the capture cross section of 242Pu should be measured with an accuracy of at least 7–12% in the neutron energy range between 500 eV and 500 keV. This work presents a brief description of the measurement performed at n_TOF-EAR1, the data reduction process and the first ToF capture measurement on this isotope in the last 40 years, providing preliminary individual resonance parameters beyond the current energy limits in the evaluations, as well as a preliminary set of average resonance parameters.
The CERN n_TOF neutron beam facility is characterized by a very high instantaneous neutron flux, excellent TOF resolution at the 185 m long flight path (EAR-1), low intrinsic background and coverage of a wide range of neutron energies, from thermal to a few GeV. These characteristics provide a unique possibility to perform high-accuracy measurements of neutron-induced reaction cross-sections and angular distributions of interest for fundamental and applied Nuclear Physics. Since 2001, the n_TOF Collaboration has collected a wealth of high quality nuclear data relevant for nuclear astrophysics, nuclear reactor technology, nuclear medicine, etc. The overall efficiency of the experimental program and the range of possible measurements has been expanded with the construction of a second experimental area (EAR-2), located 20 m on the vertical of the n_TOF spallation target. This upgrade, which benefits from a neutron flux 30 times higher than in EAR-1, provides a substantial extension in measurement capabilities, opening the possibility to collect data on neutron cross-section of isotopes with short half-lives or available in very small amounts. This contribution will outline the main characteristics of the n_TOF facility, with special emphasis on the new experimental area. In particular, we will discuss the innovative features of the EAR-2 neutron beam that make possible to perform very challenging measurements on short-lived radioisotopes or sub-mg samples, out of reach up to now at other neutron facilities around the world. Finally, the future perspectives of the facility will be presented.
The neutron capture cross section of several key unstable isotopes acting as branching points in the s-process are crucial for stellar nucleosynthesis studies, but they are very challenging to measure due to the difficult production of sufficient sample material, the high activity of the resulting samples, and the actual (n,γ) measurement, for which high neutron fluxes and effective background rejection capabilities are required. As part of a new program to measure some of these important branching points, radioactive targets of 147Pm and 171Tm have been produced by irradiation of stable isotopes at the ILL high flux reactor. Neutron capture on 146Nd and 170Er at the reactor was followed by beta decay and the resulting matrix was purified via radiochemical separation at PSI. The radioactive targets have been used for time-of-flight measurements at the CERN n_TOF facility using the 19 and 185 m beam lines during 2014 and 2015. The capture cascades were detected using a set of four C6D6 scintillators, allowing to observe the associated neutron capture resonances. The results presented in this work are the first ever determination of the resonance capture cross section of 147Pm and 171Tm. Activation experiments on the same 147Pm and 171Tm targets with a high-intensity 30 keV quasi-Maxwellian flux of neutrons will be performed using the SARAF accelerator and the Liquid-Lithium Target (LiLiT) in order to extract the corresponding Maxwellian Average Cross Section (MACS). The status of these experiments and preliminary results will be presented and discussed as well.
Seit Längerem ist ein beträchtlicher Rückgang der Vorkommen des Katzenpfötchens (Antennaria dioica) zu beobachten, welcher in ihren verschiedenen Lebensräumen unterschiedlich stark abzulaufen und durch die Zweihäusigkeit der Art begünstigt zu werden scheint. Im Jahr 2019 wurden ausgewählte, unterschiedlich große Vorkommen auf silikatischen und karbonatischen Ausgangssubstraten in Nordost-Hessen floristisch, pflanzensoziologisch und populationsbiologisch erfasst und hinsichtlich vorliegender Unterschiede von Populationskennzeichen, Gesellschafts- und Lebensraumunterschieden dokumentiert. Des Weiteren wurden die Mitglieder der Botanischen Vereinigung für Naturschutz in Hessen (BVNH) zur Mitteilung ihnen bekannter Vorkommen aufgerufen und die Nennung der Art in historischen Florenwerken recherchiert. Die Ergebnisse zur historischen Verbreitung von A. dioica in Hessen zeigen, dass die Art offenbar bis Mitte des 19. Jahrhunderts nicht nur im Offenland (vor allem Hutungen, Heiden), sondern ebenso regelmäßig in lichten Wäldern (vor allem in halb-offenen Hutewäldern) anzutreffen war. In der Auswertung der meisten erfassten populationsbiologischen Kennzeichen von Vorkommen mit geklumptem und verstreutem Wuchsmuster konnten keine statistischen Zusammenhänge identifiziert werden, jedoch bestand zwischen der Anzahl generativer Sprosse von A. dioica als Populationskennzeichen und der Krautschichthöhe als Vegetationsstrukturparameter ein negativer statistischer Zusammenhang. Unter Berücksichtigung des angetroffenen Wuchsmusters von A. dioica (geklumpt oder verstreut im Bestand wachsend) wurden Unterschiede in vielen Vegetationsstrukturparametern zwischen diesen Gruppen festgestellt, welche auf nachteilige Wuchsbedingungen für die Art bei ausgebildetem geklumpten Wuchsmuster hindeuten. Die erfassten A.-dioica-Vorkommen wuchsen entweder in Beständen der Nardetalia strictae oder Festuco-Brometea. Vor dem Hintergrund der starken Trockenheit der Jahre 2018 und 2019 wurde mit diesem Beitrag auch der deutliche Einfluss von Trockenstress auf die Vitalität von A. dioica belegt.
Neutron-induced reaction cross sections are important for a wide variety of research fields ranging from the study of nuclear level densities, nucleosynthesis to applications of nuclear technology like design, and criticality and safety assessment of existing and future nuclear reactors, radiation dosimetry, medical applications, nuclear waste transmutation, accelerator-driven systems and fuel cycle investigations. Simulations and calculations of nuclear technology applications largely rely on evaluated nuclear data libraries. The evaluations in these libraries are based both on experimental data and theoretical models. CERN’s neutron time-of-flight facility n_TOF has produced a considerable amount of experimental data since it has become fully operational with the start of its scientific measurement programme in 2001. While for a long period a single measurement station (EAR1) located at 185 m from the neutron production target was available, the construction of a second beam line at 20 m (EAR2) in 2014 has substantially increased the measurement capabilities of the facility. An outline of the experimental nuclear data activities at n_TOF will be presented.