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The aim of this work is to provide a precise and accurate measurement of the 238U(n,γ) reaction cross section in the energy region from 1 eV to 700 keV. This reaction is of fundamental importance for the design calculations of nuclear reactors, governing the behavior of the reactor core. In particular, fast reactors, which are experiencing a growing interest for their ability to burn radioactive waste, operate in the high energy region of the neutron spectrum. In this energy region most recent evaluations disagree due to inconsistencies in the existing measurements of up to 15%. In addition, the assessment of nuclear data uncertainty performed for innovative reactor systems shows that the uncertainty in the radiative capture cross section of 238U should be further reduced to 1–3% in the energy region from 20 eV to 25 keV. To this purpose, addressed by the Nuclear Energy Agency as a priority nuclear data need, complementary experiments, one at the GELINA and two at the n_TOF facility, were proposed and carried out within the 7th Framework Project ANDES of the European Commission. The results of one of these 238U(n,γ) measurements performed at the n_TOF CERN facility are presented in this work. The γ-ray cascade following the radiative neutron capture has been detected exploiting a setup of two C6D6 liquid scintillators. Resonance parameters obtained from this work are on average in excellent agreement with the ones reported in evaluated libraries. In the unresolved resonance region, this work yields a cross section in agreement with evaluated libraries up to 80 keV, while for higher energies our results are significantly higher.
New results are presented of the 234U neutron-induced fission cross section, obtained with high accuracy in the resonance region by means of two methods using the 235U(n,f) as reference. The recent evaluation of the 235U(n,f) obtained with SAMMY by L. C. Leal et al. (these Proceedings), based on previous n_TOF data [1], has been used to calculate the 234U(n,f) cross section through the 234U/235U ratio, being here compared with the results obtained by using the n_TOF neutron flux.
The 236U isotope plays an important role in nuclear systems, both for future and currently operating ones. The actual knowledge of the capture reaction of this isotope is satisfactory in the thermal region, but it is considered insufficient for Fast Reactor and ADS applications. For this reason the 236U(n, γ) reaction cross-section has been measured for the first time in the whole energy region from thermal energy up to 1 MeV at the n_TOF facility with two different detection systems: an array of C6D6 detectors, employing the total energy deposited method, and a FX1 total absorption calorimeter (TAC), made of 40 BaF2 crystals. The two n_TOF data sets agree with each other within the statistical uncertainty in the Resolved Resonance Region up to 800 eV, while sizable differences (up to ≃ 20%) are found relative to the current evaluated data libraries. Moreover two new resonances have been found in the n_TOF data. In the Unresolved Resonance Region up to 200 keV, the n_TOF results show a reasonable agreement with previous measurements and evaluated data.
The radiative capture cross section of 238U is very important for the developing of new reactor technologies and the safety of existing ones. Here the preliminary results of the 238U(n,γ) cross section measurement performed at n_TOF with C6D6 scintillation detectors are presented, paying particular attention to data reduction and background subtraction.
An important experimental program on Nuclear Astrophysics is being carried out at the n_TOF since several years, in order to address the still open issues in stellar and primordial nucleosynthesis. Several neutron capture reactions relevant to s-process nucleosynthesis have been measured so far, some of which on important branching point radioisotopes. Furthermore, the construction of a second experimental area has recently opened the way to challenging measurements of (n, charged particle) reactions on isotopes of short half-life. The Nuclear Astrophysics program of the n_TOF Collaboration is here described, with emphasis on recent results relevant for stellar nucleosynthesis, stellar neutron sources and primordial nucleosynthesis.
The 33S(n,α)30Si cross section measurement, using 10B(n,α) as reference, at the n_TOF Experimental Area 2 (EAR2) facility at CERN is presented. Data from 0.01 eV to 100 keV are provided and, for the first time, the cross section is measured in the range from 0.01 eV to 10 keV. These data may be used for a future evaluation of the cross section because present evaluations exhibit large discrepancies. The 33S(n,α)30Si reaction is of interest in medical physics because of its possible use as a cooperative target to boron in Neutron Capture Therapy (NCT).
The Cosmological Lithium Problem refers to the large discrepancy between the abundance of primordial 7Li predicted by the standard theory of Big Bang Nucleosynthesis and the value inferred from the so-called “Spite plateau” in halo stars. A possible explanation for this longstanding puzzle in Nuclear Astrophysics is related to the incorrect estimation of the destruction rate of 7Be, which is responsible for the production of 95% of primordial Lithium. While charged-particle induced reactions have mostly been ruled out, data on the 7Be(n,α) and 7Be(n,p) reactions are scarce or completely missing, so that a large uncertainty still affects the abundance of 7Li predicted by the standard theory of Big Bang Nucleosynthesis. Both reactions have been measured at the n_TOF facility at CERN, providing for the first time data in a wide neutron energy range.
73Ge(n, γ ) cross sections were measured at the neutron time-of-flight facility n_TOF at CERN up to neutron energies of 300 keV, providing for the first time experimental data above 8 keV. Results indicate that the stellar cross section at kT = 30 keV is 1.5 to 1.7 times higher than most theoretical predictions. The new cross sections result in a substantial decrease of 73Ge produced in stars, which would explain the low isotopic abundance of 73Ge in the solar system.
he study of the resonant structures in neutron-nucleus cross-sections, and therefore of the compound-nucleus reaction mechanism, requires spectroscopic measurements to determine with high accuracy the energy of the neutron interacting with the material under study.
To this purpose, the neutron time-of-flight facility n_TOF has been operating since 2001 at CERN. Its characteristics, such as the high intensity instantaneous neutron flux, the wide energy range from thermal to few GeV, and the very good energy resolution, are perfectly suited to perform high-quality measurements of neutron-induced reaction cross sections. The precise and accurate knowledge of these cross sections plays a fundamental role in nuclear technologies, nuclear astrophysics and nuclear physics.
Two different measuring stations are available at the n_TOF facility, called EAR1 and EAR2, with different characteristics of intensity of the neutron flux and energy resolution. These experimental areas, combined with advanced detection systems lead to a great flexibility in performing challenging measurement of high precision and accuracy, and allow the investigation isotopes with very low cross sections, or available only in small quantities, or with very high specific activity.
The characteristics and performances of the two experimental areas of the n_TOF facility will be presented, together with the most important measurements performed to date and their physics case. In addition, the significant upcoming measurements will be introduced.
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.
The accurate knowledge of the neutron-induced fission cross-sections of actinides and other isotopes involved in the nuclear fuel cycle is essential for the design of advanced nuclear systems, such as Generation-IV nuclear reactors. Such experimental data can also provide the necessary feedback for the adjustment of nuclear model parameters used in the evaluation process, resulting in the further development of nuclear fission models. In the present work, the 240Pu(n,f) cross-section was measured at CERN's n_TOF facility relative to the well-known 235U(n,f) cross section, over a wide range of neutron energies, from meV to almost MeV, using the time-of-flight technique and a set-up based on Micromegas detectors. This measurement was the first experiment to be performed at n_TOF's new experimental area (EAR-2), which offers a significantly higher neutron flux compared to the already existing experimental area (EAR-1). Preliminary results as well as the experimental procedure, including a description of the facility and the data handling and analysis, are presented.
We have measured the radiative neutron-capture cross section and the total neutron-induced cross section of one of the most important isotopes for the s process, the 25Mg. The measurements have been carried out at the neutron time-of-flight facilities n_TOF at CERN (Switzerland) and GELINA installed at the EC-JRC-IRMM (Belgium). The cross sections as a function of neutron energy have been measured up to approximately 300 keV, covering the energy region of interest to the s process. The data analysis is ongoing and preliminary results show the potential relevance for the s process.
Above 1 MeV of incident neutron energy the fission fragment angular distribution (FFAD) has generally a strong anisotropic behavior due to the combination of the incident orbital momentum and the intrinsic spin of the fissioning nucleus. This effect has to be taken into account for the efficiency estimation of devices used for fission cross section measurements. In addition it bears information on the spin deposition mechanism and on the structure of transitional states. We designed and constructed a detection device, based on Parallel Plate Avalanche Counters (PPAC), for measuring the fission fragment angular distributions of several isotopes, in particular 232Th. The measurement has been performed at n_TOF at CERN taking advantage of the very broad energy spectrum of the neutron beam. Fission events were recognized by back to back detection in coincidence in two position-sensitive detectors surrounding the targets. The detection efficiency, depending mostly on the stopping of fission fragments in backings and electrodes, has been computed with a Geant4 simulation and validated by the comparison to the measured case of 235U below 3 keV where the emission is isotropic. In the case of 232Th, the result is in good agreement with previous data below 10 MeV, with a good reproduction of the structures associated to vibrational states and the opening of second chance fission. In the 14 MeV region our data are much more accurate than previous ones which are broadly scattered.
Der ,Neue Mann' zwischen Familie und Beruf : Erkundungen bei Hans Fallada und Joseph Breitbach
(2014)
Falladas „Kleiner Mann“ gehört zu den Erfolgstiteln der späten Weimarer Republik und darf bis heute mit uneingeschränktem Publikumsinteresse rechnen. Empathisch, realitätsnah, detailreich, aber wenig politisch engagiert, rief die Darstellung der absinkenden Mittelschicht schon in der Weimarer Republik den Vorwurf der „linken Melancholie“ hervor, der neben Fallada auch Kästner traf. Noch 1970 diente diese Kategorie dem Literaturwissenschaftler Helmut Lethen als Kampf- und Abwehrbegriff gegen einen vorgeblich „weißen“ Sozialismus, dem er die Absicht sozialer Pazifizierung zuschrieb. Die Farbwahl deutet auf die Langlebigkeit des politisch-literarischen Topos hin, hatte doch Kurt Tucholsky schon 1926 die deutsche Sozialdemokratie in dem Gedicht „Feldfrüchte“ als „außen rot und innen weiß“ charakterisiert. Joseph Breitbach greift in seinen Erzählungen „Rot gegen Rot“ und „Das Radieschen“ von 1928/29 diese Metaphorik ebenso auf wie die mit ihr assoziierten widerstreitenden Positionen. „Rot gegen Rot“ lautet auch der Titel seines 1929 bei der Deutschen Verlagsanstalt erschienenen ersten Erzählbandes. Im Gegensatz zu den politischen Kampffronten sind diejenigen zwischen den Geschlechtern erst spät, dafür aber mit eindeutigem und nachhaltigem Interesse an der „Neuen Frau“ ins Blickfeld der Literaturwissenschaft geraten. Es scheint geradezu, als habe das Erlöschen der politischen Utopie, die sich zunächst in der Weimarer Republik, dann für zahlreiche, vor allem angloamerikanische, Forscher in der DDR verkörperte, die vorherigen Hoffnungen nun auf eine grundlegende Neudefinition des Geschlechterverhältnisses verlagert: Die „Neue Frau“ sollte im Privaten und Öffentlichen realisieren, was politisch eben nicht gelungen war. […] Parallel wäre zu wünschen, dass der Literarisierung des „Neuen Mannes“ die gleiche differenzierte Aufmerksamkeit zukäme - selbst wenn er massenmedial weit weniger präsent ist als seine Geschlechtsgenossinnen. Zeichen seiner Schwäche sind schon in den 20er Jahren unübersehbar. Der „Neue Mann“ ist zumeist – ob im wörtlichen oder übertragenen Sinne – der „Kleine Mann“.
Hoffmann unterschätzt und disqualifiziert hier - natürlich auch im Interesse der Aufwertung eigener künstlerisch-kritischer Produktion - die Beliebtheit des spät- und popularaufklärerischen Genres der Blindenheilung, das die handfest-praktische Gegenseite des hochambitionierten und spätestens seit Diderot auch philosophisch prominenten Phänomens darstellt. Als „hochbesetzte Technik des 17./18. Jahrhunderts“ oder gar als „Urszene der Aufklärung“ verbindet das Starstechen den Gewinn des Augenlichts mit dem potentiellen Verlust anderer, ersatzweise erworbener, oft aber auch stärker als normal ausdifferenzierter Fähigkeiten. So folgt zumindest auf die literarisierte Blindenheilung regelmäßig der Wunsch nach erneuter Blindheit m – oder der Abbruch der Narration, „als bedeute das Tageslicht das Ende der Fiktion“. Peter Utz belegte diesen Konnex bereits 1990 mit Beispielen von Jean Paul über Goethe bis zu Bonaventuras Nachtwachen, machte aber auch auf den Rat von dessen Erzählerinstanz aufmerksam, die mögliche Fortsetzung der Geschichte „als Material für trivialromantische Verwertung“ zu nutzen. Hoffmann hingegen entwertet – wahrscheinlich wider besseres Wissen – das philosophische wie narratologische Problempotential der Blindenheilung, indem er probehalber eine Reihe anderer Varianten durchspielt, vom Arterienverschluss (,Aneurisma') über den Wundbrand bis hin zur geglückten Amputation.
Explizite Notenzitate in der Schönen Literatur sind selten, weisen sie doch auf die grundsätzliche Fremdheit zwischen den beiden Medien hin. Diese wurde schon in der romantischen Musikliteratur ebenso oft beschworen wie zu widerlegen versucht. Musik sei die bessere Sprache, meinten zahlreiche romantische Autoren, niemals würde die verbale Sprache sie zu erreichen vermögen. Gleichwohl kämpften Wackenroder und Tieck, Heine und Grillparzer um jene poetische Sprache, die so viel mehr sein soll als die Prosa. So überrascht es nicht, wenn nur wenige Autoren sich der Herausforderung stellen, mit dem Notentext selbst auch die Grenzen ihres eigenen Mediums zu bezeichnen - und noch weniger nutzen sie das Notenzitat als Indiz ihrer poetologischen Grundüberzeugungen. Nicht zufällig sind Hans Henny Jahnn, Arthur Schnitzler und Ingeborg Bachmann die wichtigsten nach wie vor seltenen Zeugen einschlägiger medienkomparatistischer Betrachtungen.
Das Pathos, mit dem die Intellektuellen seit 200 Jahren als moralische Autoritäten angerufen oder als 'Verräter' oder 'Volksfeinde' mißachtet wurden, ist einer generellen Ernüchterung gewichen. "Ich möchte nicht, daß Männer für mich denken", schrieb Sibylle Berg 2009 in Literaturen. Mit Habermas, Luhmann und Adorno weiß sie nichts mehr anzufangen. Vorstellen kann sie sich als Intellektuelle allenfalls "Susan Sontag, Hannah Arendt oder Oriana Fallaci [... ] Alle nicht deutsch. Alle eine Mischung aus Publizistinnen und Philosophinnen. Und Frauen." Immerhin gesteht sie auch den in ihrer Sicht langweiligen und wirkungslosen männlichen Intellektuellen eines zu: Zwar sei es "möglicherweise egal, ob es sie gibt oder nicht, die Intellektuellen. Nur so langweilig ist es ohne sie. So grau und reduziert auf unsere Grundbedürfnisse: Fernsehen schauen, essen und meckern." Was hier eher leichtgewichtig als Kultur- und Patriarchatskritik daherkommt, findet sein positives Pendant im Konzept des 'nomadischen Intellektuellen', wie es Toni Tholen Enzensberger zuschreibt. Dessen Unterhaltungswert bleibt auch hier im Zentrum: Von besonderem Interesse sei "das Verfahren des Sich-Aussetzens", das das Ich zum Schauplatz innerer Erfahrung, nicht notwendigerweise aber prinzipienfester Selbstbehauptung mache. Damit aber ist jenes Moment des Posierens, Inszenierens und Skandalisierens ausgesprochen, das die intellektuelle Intervention von allem Anfang an begleitet hat - und vielleicht ihr zentrales Charakteristikum bildet. Zwischen Arbeitszimmer und Literatencafé, punktuellem Engagement und "Distanz gegenüber allen festen Positionen und ihren Inhabern", zwischen Bekenntnis und Verrat profiliert sich der/die Intellektuelle als "Paria und Privilegierter" gleichermaßen.
The neutron sensitivity of the C6D6 detector setup used at n_TOF facility for capture measurements has been studied by means of detailed GEANT4 simulations. A realistic software replica of the entire n_TOF experimental hall, including the neutron beam line, sample, detector supports and the walls of the experimental area has been implemented in the simulations. The simulations have been analyzed in the same manner as experimental data, in particular by applying the Pulse Height Weighting Technique. The simulations have been validated against a measurement of the neutron background performed with a natC sample, showing an excellent agreement above 1 keV. At lower energies, an additional component in the measured natC yield has been discovered, which prevents the use of natC data for neutron background estimates at neutron energies below a few hundred eV. The origin and time structure of the neutron background have been derived from the simulations. Examples of the neutron background for two different samples are demonstrating the important role of accurate simulations of the neutron background in capture cross-section measurements.
Ausgangsfragen: Was sind die grundlegenden methodisch-theoretischen Veränderungen des Faches in den vergangenen dreißig bis vierzig Jahren? Welchen Stellenwert haben Methoden und Theorien heute im Fach? In welchem Verhältnis steht die germanistische zur allgemeinen Methoden- und Theoriereflexion in den Geisteswissenschaften? Befinden wir uns am Ende des 'theoretischen Zeitalters'? Hat die Konzentration auf Methodik und Theorie das Fach der gebildeten Öffentlichkeit noch mehr als ohnehin entfremdet? Worin bestünden bzw. existieren überhaupt Alternativen zur Theorie? Etwa in Form einer Renaissance von Biographie oder Hermeneutik? Braucht das Fach überhaupt eine breite Debatte über seine theoretischen Horizonte und methodischen Standards? Oder sind andere Fragen vordringlicher, etwa eine stärkere Reflexion der gesellschaftlichen Aufgaben der Germanistik?