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Fission fragment mass distributions were measured in heavy-ion induced fissions using 238U target nucleus. The measured mass distributions changed drastically with incident energy. The results are explained by a change of the ratio between fusion and qasifission with nuclear orientation. A calculation based on a fluctuation dissipation model reproduced the mass distributions and their incident energy dependence. Fusion probability was determined in the analysis, and the values were consistent with those determined from the evaporation residue cross sections.
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 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.
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.
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.
Darstellung und Eigenschaften des Coenzymanalogen Nicotinamid-4-methyl-5-acetyl-imidazol-dinucleotid
(1970)
Kondensation des Quecksilbersalzes von 4-Methyl-5-acetyl-imidazol ** mit 1-Chlor-2.3.5-O-tribenzoyl-ribofuranose liefert das geschützte Ribosid 3. Zur Strukturaufklärung der Verbindung wurde 4-Methyl-5-acetyl-1-(β-D-0-2′.3′.5′-triacetyl-ribofuranosyl)-imidazol mit Methyljodid in das 3.4-Dimethyl-5-acetyl-1-(β-D-O-2′.3′.5′-triacetyl-ribofuranosyl)-imidazoliumjodid überführt und der Zuckerrest hydrolytisch gespalten. Das entstandene Imidazol-Derivat ist identisch mit 1.5-Dimethyl-4-acetyl-imidazol. 4-Methyl-5-acetyl-1- (β-D-ribofuranosyl) -imidazol wurde mit Aceton in das Isopropyliden-Derivat 4 überführt. Die Phosphorylierung zum Nucleosid-5′-phosphat (5) führten wir mit β-Cyanäthyl-phosphat durch. Durch Kondensation mit Nicotinamid-mononucleotid erhielten wir das Coenzymanaloge Nicotinamid-4-methyl-5-acetyl-imidazol-dinucleotid (6). Die Verbindung liegt im oxydierten Zustand in gefaltener Form vor. Das Fluoreszenz-Anregungsspektrum der Dihydroverbindung zeigt keine Energieübertragung vom nichtfunktionellen 4-Methyl-5-acetyl-imidazol-Teil auf den Dihydronicotinamid-Ring. Das Coenzymanaloge weist eine größere Michaelis- Konstante im Test mit Lactat-Dehydrogenase aus Schweineherz *** auf als das natürliche Nicotinamid-adenindinucleotid ***. Die maximale Umsatzzahl ist trotz der schwächeren Bindung vergrößert. Das unterschiedliche Verhalten des Coenzymanalogen 6 gegenüber NAD läßt, neben der π-Bindung des nichtfunktionellen Teils, eine polare Gruppe im aktiven Zentrum des Enzyms vermuten, die die Ausrichtung des Coenzyms im Coenzym-Enzym-Komplex bewirkt.
Dihydronicotinamid-4-methyl-5-acetyl-imidazol-dinucleotid bildet einen fluoreszierenden Komplex mit der Lactat-Dehydrogenase, der dem des NADH-LDH-Komplexes sehr ähnlich ist.
Die Synthese des Coenzymmodells Flavin-benzimidazol-dinucleotid * gelang durch Kondensation von Benzimidazolribotid-imidazolid 1 oder Benzimidazolribotid-guanidiniumamidat 2 mit Flavinmononucleotid. Das Coenzymmodell war enzymatisch nicht aktiv und bildete keinen Enzym-Coenzym-Komplex. Im Absorptionsspektrum konnte eine Extinktionszunahme nach der Spaltung der Pyrophosphatbrücke nur im Bereich von 260 mμ beobachtet werden. Das Molekül liegt daher vermutlich in einer gefalteten Form vor. Ein Komplex zwischen Flavin- und Benzimidazolteil konnte nicht nachgewiesen werden. Eine Fluoreszenzunterdrückung, die im FAD durch die Komplexbildung zwischen Flavin- und Adeninteil bedingt wird, wurde im FBD-Coenzymmodell nicht beobachtet.
Abrasion-ablation models and the empirical EPAX parametrization of projectile fragmentation are described. Their cross section predictions are compared to recent data of the fragmentation of secondary beams of neutron-rich, unstable 19,20,21O isotopes at beam energies near 600 MeV/nucleon as well as data for stable 17,18O beams.
A new technique developed for measuring nuclear reactions at low momentum transfer with stored beams in inverse kinematics was successfully used to study isoscalar giant resonances. The experiment was carried out at the experimental heavy-ion storage ring (ESR) at the GSI facility using a stored 58Ni beam at 100 MeV/u and an internal helium gas-jet target. In these measurements, inelastically scattered α-recoils at very forward center-of-mass angles (θcm ≤ 1.5°) were detected with a dedicated setup, including ultra-high vacuum compatible detectors. Experimental results indicate a dominant contribution of the isoscalar giant monopole resonance at this very forward angular range. It was found that the monopole contribution exhausts 79+12−11% of the energy-weighted sum rule (EWSR), which agrees with measurements performed in normal kinematics. This opens up the opportunity to investigate the giant resonances in a large domain of unstable and exotic nuclei in the near future. It is a fundamental milestone towards new nuclear reaction studies with stored ion beams.
The Coulomb Dissociation (CD) cross sections of the stable isotopes 92,94,100Mo and of the unstable isotope 93Mo were measured at the LAND/R3B setup at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. Experimental data on these isotopes may help to explain the problem of the underproduction of 92,94Mo and 96,98Ru in the models of p-process nucleosynthesis. The CD cross sections obtained for the stable Mo isotopes are in good agreement with experiments performed with real photons, thus validating the method of Coulomb Dissociation. The result for the reaction 93Mo(γ,n) is especially important since the corresponding cross section has not been measured before. A preliminary integral Coulomb Dissociation cross section of the 94Mo(γ,n) reaction is presented. Further analysis will complete the experimental database for the (γ,n) production chain of the p-isotopes of molybdenum.