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The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process β-decay chains. These nuclei are attributed to the p and rp process.
For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections.
The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes.
We measured the Coulomb dissociation of 16O into 4He and 12C at the R3B setup in a first campaign within FAIR Phase 0 at GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. The goal was to improve the accuracy of the experimental data for the 12C(α,γ)16O fusion reaction and to reach lower center-ofmass energies than measured so far.
The experiment required beam intensities of 109 16O ions per second at an energy of 500 MeV/nucleon. The rare case of Coulomb breakup into 12C and 4He posed another challenge: The magnetic rigidities of the particles are so close because of the same mass-to-charge-number ratio A/Z = 2 for 16O, 12C and 4He. Hence, radical changes of the R3B setup were necessary. All detectors had slits to allow the passage of the unreacted 16O ions, while 4He and 12C would hit the detectors' active areas depending on the scattering angle and their relative energies. We developed and built detectors based on organic scintillators to track and identify the reaction products with sufficient precision.
Seitdem im Juli die Schiedsentscheidung über die Territorialkonflikte im südchinesischen Meer gefällt wurde, wird in Zeitungen und Blogs intensiv darüber diskutiert, wie diese Entscheidung einzuordnen ist und welche Folgen sich daraus ergeben. Das Schiedsgericht hat nicht über Fragen der Souveränität selbst entschieden, sondern über die rechtlichen Grundlagen, aus denen Souveränitätsansprüche abgeleitet werden können. In diesem Zusammenhang hatte das Gericht die interessante Frage zu klären, inwieweit die durch China angeführten „historischen Rechte“ geeignet sind, einen Gebietsanspruch zu begründen. Klar ist, dass der Schiedsspruch nicht geeignet ist, den Konflikt zu beenden. China hat von Beginn an deutlich gemacht, dass es das Verfahren weder anerkennen noch sich daran beteiligen würde und hat daher schließlich auch die Entscheidung als rechtwidrig abgelehnt. Die Funktion des Verfahrens ist daher auch weniger die Konfliktlösung, die es nicht leisten kann, als vielmehr das Herausarbeiten einer rechtlich gerechtfertigten Position....
Obgleich die Disziplin der internationalen Beziehungen global sein will, bleibt sie allzu häufig von westlichen Wissenschaftlern dominiert. Doch wie kann internationale Politik anders gedacht werden? Welche Perspektiven ergeben sich auch für westliche WissenschaftlerInnen aus einem Perspektivwechsel? Und welche Herausforderungen bringt dies mit sich?
Internationale Gerichte sollen Konflikte zwischen Staaten befrieden. Dass es dabei nicht immer nur um das Völkerrecht geht, zeigt der Streit zwischen den USA und dem Iran. Die gegenwärtige US-Regierung lehnt den Internationalen Gerichtshof als politisch gelenkt ab – und schadet sich damit vor allem selbst.
Das Recht nimmt keine zentrale Stellung ein in diesem Band zu "Asian Perspectives on the Paris Peace Conference and the Interwar Order, 1919–33", dies sei gleich zu Beginn dieser Rezension in einer rechtshistorischen Fachzeitschrift angemerkt. Was dieser Band allerdings bietet, sind äußerst vielschichtige und differenzierende Perspektiven auf einen Gegenstand, der in der Rechtsgeschichte bislang nicht nur, aber vor allem auf seine Bedeutung im europäischen Kontext hin erforscht wurde. ...
Humanitarismus und humanitäre Intervention müssen sich gleichermaßen die Frage nach Motivation und Rechtfertigung gefallen lassen. Sind Motive einer Intervention hinreichend humanitär, um von einer humanitären Intervention zu sprechen? Und im Rahmen welcher normativen Muster erscheinen sie als legitim? Zwei neue Bücher nähern sich dem Themenkomplex auf unterschiedliche Weise. ...
Jed Kronckes The Futility of Law and Development – China and the Dangers of Exporting American Law ist ein Buch über das rechtliche Sendungsbewusstsein der Vereinigten Staaten vom neunzehnten bis zur Mitte des zwanzigsten Jahrhunderts. Jürgen Osterhammel verwendete den Begriff des Sendungsbewusstseins, um den Kern der Zivilisierungsmissionen in dieser Zeit zu beschreiben. Der bestehe in der "Selbstbeauftragung damit, die eigenen Normen und Institutionen an andere heranzutragen oder gar ihre Übernahme mit mehr oder weniger sanftem Druck zu erzwingen". ...
In der gegenwärtigen Forschung zu Recht und Kolonialismus zeigt sich ein interessanter Gegensatz: Während das Recht vor allem als ein Instrument kolonialer Machtausübung verstanden wird, erscheinen Juristen, die nach einer Ausbildung an europäischen oder amerikanischen Universitäten in ihre Herkunftsländer zurückkehrten, als zentrale Akteure lokaler Unabhängigkeitsbewegungen, aus denen heraus neue Nationalstaaten entstanden. Dieser Gegensatz wird beispielhaft illustriert durch aktuelle Arbeiten von Turan Kayaoglu zum Legal Imperialism sowie Arnulf Becker-Lorca und seine Figur des Semi-Peripheral Jurist. Das Buch Asian Legal Revivals – Lawyers in the Shadow of Empire erschien 2010 und geht daher nicht direkt auf diese Arbeiten ein, dennoch ist es ein Beitrag dazu, beide Beobachtungen miteinander zu verbinden. Die Rechtssoziologen Yves Dezalay und Bryant G. Garth, die gemeinsam bereits sechs Bücher verfasst oder herausgegeben haben, untersuchen die Rolle von Juristen in der Herausbildung und Legitimation politischer Herrschaft in sieben asiatischen Ländern im 19. und 20. Jahrhundert. ...
Die geistes- und sozialwissenschaftlichen Disziplinen haben ihr Spektrum in der jüngeren Vergangenheit um globale Perspektiven erweitert. Auch für das Feld der Intellectual History liegt nun ein Sammelband von Samuel Moyn und Andrew Sartori vor, der die Frage des Globalen diskutiert und dabei viele Beobachtungen enthält, die über das Feld der Ideen- und Geistesgeschichte hinaus Beachtung verdienen. Von rechtshistorischem Interesse sind vor allem die Abschnitte zur Konzeption des Globalen und zur Übersetzung und Verankerung globaler normativer Muster. ...
The study of civilization is one of the core subjects of international legal history. This is no recent development. Jörg Fisch published his seminal work "Die Europäische Expansion und das Völkerrecht" in 1984, the same year in which Gerrit W. Gong presented his renowned "Standard of Civilization". Today, the more recent works by Martti Koskenniemi and Antony Anghie probably represent the most influential research in this field. What all these path breaking works have in common is that they discuss concepts of civilization in international law especially with regard to its function as providing justification narratives for the European/non-European unequal relations, in particular in the 19th century. ...
The relationship between past and present has been the subject of controversial debates in historical research time and again. In 2013, to give a prominent example, Philip Alston in a review essay discussed the issue of "Does the past matter?" with regard to a debate on the origins of human rights. The debate was dedicated to the controversial question of "[h]ow far back can we trace the genealogy of today’s international human rights system". In this review, I would like to rephrase this question to ask instead to what degree the present matters for historical writing. Other than in the work of Alston, this is not meant as a question on the contingency and path-dependence of history, but rather as a reflection on how historians describe and evaluate the past and what role knowledge of the present may have in this context. ...
Neuere Geschichten des Völkerrechts zeichnen sich dadurch aus, dass sie das Recht und dessen Wirksamkeit nicht losgelöst von sozialen und historischen Kontexten betrachten. In seinem beeindruckenden Buch "Frieden durch Recht?" über den Friedensschluss nach dem ersten Weltkrieg zeigt Marcus M. Payk (vgl. die Rezension in diesem Band), dass das Recht zwar über eine eigene Form und Logik verfügt, dessen Bindungswirkung aber nicht ohne dessen Kontexte verstanden werden kann. ...
In China war das europäische Völkerrecht bis zur Mitte des 19. Jahrhunderts weitgehend unbekannt. Abgesehen von einzelnen Verträgen aus dem 17. und 18. Jahrhundert gab es weder Vertragspraxis noch Völkerrechtswissenschaft. Dies änderte sich erst durch Chinas Kriege mit westlichen Mächten. Die »Barbaren« nutzten ihre militärische Überlegenheit gegenüber China, um durch erzwungene völkerrechtliche Friedensverträge Handelsinteressen zu verwirklichen und halbkoloniale Strukturen zu etablieren. Diese Friedensübereinkommen wurden und werden, als "ungleiche Verträge" überschrieben, vielfach in der wissenschaftlichen Literatur besprochen. ...
This policy note summarizes our assessment of financial sanctions against Russia. We see an increase in sanctions severity starting from (1) the widely discussed SWIFT exclusions, followed by (2) blocking of correspondent banking relationships with Russian banks, including the Central Bank, alongside secondary sanctions, and (3) a full blacklisting of the ‘real’ export-import flows underlying the financial transactions. We assess option (1) as being less impactful than often believed yet sending a strong signal of EU unity; option (2) as an effective way to isolate the Russian banking system, particularly if secondary sanctions are in place, to avoid workarounds. Option (3) represents possibly the most effective way to apply economic and financial pressure, interrupting trade relationships.
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.
The accuracy on neutron capture cross section of fissile isotopes must be improved for the design of future nuclear systems such as Gen-IV reactors and Accelerator Driven Systems. The High Priority Request List of the Nuclear Energy Agency, which lists the most important nuclear data requirements, includes also the neutron capture cross sections of fissile isotopes such as 233,235U and 239,241Pu. A specific experimental setup has been used at the CERN n_TOF facility for the measurement of the neutron capture cross section of 235U by a set of micromegas fission detectors placed inside a segmented BaF2 Total Absorption Calorimeter.
New neutron cross section measurements of minor actinides have been performed recently in order to reduce the uncertainties in the evaluated data, which is important for the design of advanced nuclear reactors and, in particular, for determining their performance in the transmutation of nuclear waste. We have measured the 241Am(n,γ) cross section at the n_TOF facility between 0.2 eV and 10 keV with a BaF2 Total Absorption Calorimeter, and the analysis of the measurement has been recently concluded. Our results are in reasonable agreement below 20 eV with the ones published by C. Lampoudis et al. in 2013, who reported a 22% larger capture cross section up to 110 eV compared to experimental and evaluated data published before. Our results also indicate that the 241Am(n,γ) cross section is underestimated in the present evaluated libraries between 20 eV and 2 keV by 25%, on average, and up to 35% for certain evaluations and energy ranges.
The study of neutron-induced reactions is of high relevance in a wide variety of fields, ranging from stellar nucleosynthesis and fundamental nuclear physics to applications of nuclear technology. In nuclear energy, high accuracy neutron data are needed for the development of Generation IV fast reactors and accelerator driven systems, these last aimed specifically at nuclear waste incineration, as well as for research on innovative fuel cycles. In this context, a high luminosity Neutron Time Of Flight facility, n_TOF, is operating at CERN since more than a decade, with the aim of providing new, high accuracy and high resolution neutron cross-sections. Thanks to the features of the neutron beam, a rich experimental program relevant to nuclear technology has been carried out so far. The program will be further expanded in the near future, thanks in particular to a new high-flux experimental area, now under construction.
The n_TOF facility operates at CERN with the aim of addressing the request of high accuracy nuclear data for advanced nuclear energy systems as well as for nuclear astrophysics. Thanks to the features of the neutron beam, important results have been obtained on neutron induced fission and capture cross sections of U, Pu and minor actinides. Recently the construction of another beam line has started; the new line will be complementary to the first one, allowing to further extend the experimental program foreseen for next measurement campaigns.
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.
High precision measurement of the radiative capture cross section of 238U at the n_TOF CERN facility
(2017)
The importance of improving the accuracy on the capture cross-section of 238U has been addressed by the Nuclear Energy Agency, since its uncertainty significantly affects the uncertainties of key design parameters for both fast and thermal nuclear reactors. Within the 7th framework programme ANDES of the European Commission three different measurements have been carried out with the aim of providing the 238U(n,γ) cross-section with an accuracy which varies from 1 to 5%, depending on the energy range. Hereby the final results of the measurement performed at the n_TOF CERN facility in a wide energy range from 1 eV to 700 keV will be presented.
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 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.
Neutron-induced fission cross sections of 238U and 235U are used as standards in the fast neutron region up to 200 MeV. A high accuracy of the standards is relevant to experimentally determine other neutron reaction cross sections. Therefore, the detection effciency should be corrected by using the angular distribution of the fission fragments (FFAD), which are barely known above 20 MeV. In addition, the angular distribution of the fragments produced in the fission of highly excited and deformed nuclei is an important observable to investigate the nuclear fission process.
In order to measure the FFAD of neutron-induced reactions, a fission detection setup based on parallel-plate avalanche counters (PPACs) has been developed and successfully used at the CERN-n_TOF facility. In this work, we present the preliminary results on the analysis of new 235U(n,f) and 238U(n,f) data in the extended energy range up to 200 MeV compared to the existing experimental data.
The neutron capture cross section of 58Ni was measured at the neutron time of flight facility n_TOF at CERN, from 27 meV to 400 keV neutron energy. Special care has been taken to identify all the possible sources of background, with the so-called neutron background obtained for the first time using high-precision GEANT4 simulations. The energy range up to 122 keV was treated as the resolved resonance region, where 51 resonances were identified and analyzed by a multilevel R-matrix code SAMMY. Above 122 keV the code SESH was used in analyzing the unresolved resonance region of the capture yield. Maxwellian averaged cross sections were calculated in the temperature range of kT = 5 – 100 keV, and their astrophysical implications were investigated.