Messung des prompten gamma-Strahlungsspektrums aus der Spontanspaltung von 242Pu

Das prompte Gammatrahlungsspektrum spielt eine wichtige Rolle sowohl für die Dynamik der Kernspaltung, als auch in der Kernreaktortechnologie. In dieser Arbeit wurde das prompte Gammaspektrum aud der Spontanspaltung von 242Pu gemessen. Das Spektrum der prompten Spaltgammastrahlung wurde durch Lanthan-Bromid-Szintillatoren und High Purity Germanium (HPGe) Detektoren mit hoher Zeit- und Energieauflösung in Koinzidenz mit den Spontanspaltereignissen in einem Flugzeitexperiment nachgewiesen. Verwendet wurde dazu die am Helmholtz-Zentrum Dresden-Rossendorf entwickkelte 242Pu-Spaltionisationskammer. Die ermittelten Ergebnisse haben eine sehr viel geringere statistische Unsicherheit als vorherige Messungen. Das mit dem HPGe-Detektoren gemessene prompte Gammastrahlungsspektrum zeigt Linienstrukturen, die Rückschlüsse auf die zugrunde liegenden gamma-Übergänge in den Spaltfragmenten zulassen

Messung des prompten gamma-Strahlungsspektrums aus der Spontanspaltung von 242Pu

Das prompte Gammatrahlungsspektrum spielt eine wichtige Rolle sowohl für die Dynamik der Kernspaltung, als auch in der Kernreaktortechnologie. In dieser Arbeit wurde das prompte Gammaspektrum aud der Spontanspaltung von 242Pu gemessen. Das Spektrum der prompten Spaltgammastrahlung wurde durch Lanthan-Bromid-Szintillatoren und High Purity Germanium (HPGe) Detektoren mit hoher Zeit- und Energieauflösung in Koinzidenz mit den Spontanspaltereignissen in einem Flugzeitexperiment nachgewiesen. Verwendet wurde dazu die am Helmholtz-Zentrum Dresden-Rossendorf entwickkelte 242Pu-Spaltionisationskammer. Die ermittelten Ergebnisse haben eine sehr viel geringere statistische Unsicherheit als vorherige Messungen. Das mit dem HPGe-Detektoren gemessene prompte Gammastrahlungsspektrum zeigt Linienstrukturen, die Rückschlüsse auf die zugrunde liegenden gamma-Übergänge in den Spaltfragmenten zulassen

Messung des prompten gamma-Strahlungsspektrums aus der Spontanspaltung von 242Pu

Das prompte Gammatrahlungsspektrum spielt eine wichtige Rolle sowohl für die Dynamik der Kernspaltung, als auch in der Kernreaktortechnologie. In dieser Arbeit wurde das prompte Gammaspektrum aud der Spontanspaltung von 242Pu gemessen. Das Spektrum der prompten Spaltgammastrahlung wurde durch Lanthan-Bromid-Szintillatoren und High Purity Germanium (HPGe) Detektoren mit hoher Zeit- und Energieauflösung in Koinzidenz mit den Spontanspaltereignissen in einem Flugzeitexperiment nachgewiesen. Verwendet wurde dazu die am Helmholtz-Zentrum Dresden-Rossendorf entwickkelte 242Pu-Spaltionisationskammer. Die ermittelten Ergebnisse haben eine sehr viel geringere statistische Unsicherheit als vorherige Messungen. Das mit dem HPGe-Detektoren gemessene prompte Gammastrahlungsspektrum zeigt Linienstrukturen, die Rückschlüsse auf die zugrunde liegenden gamma-Übergänge in den Spaltfragmenten zulassen

Measurement of the 14N(n,p)14C cross section at the CERN n_TOF facility from sub-thermal energy to 800 keV

The 14N(n,p)14C reaction is of interest in neutron capture therapy, where nitrogen-related dose is the main component due to low-energy neutrons, and in astrophysics, where 14N acts as a neutron poison in the s-process. Several discrepancies remain between the existing data obtained in partial energy ranges: thermal energy, keV region and resonance region. Purpose: Measuring the 14N(n,p)14C cross section from thermal to the resonance region in a single measurement for the first time, including characterization of the first resonances, and providing calculations of Maxwellian averaged cross sections (MACS). Method: Time-of-flight technique. Experimental Area 2 (EAR-2) of the neutron time-of-flight (n_TOF) facility at CERN. 10B(n,α)7Li and 235U(n,f) reactions as references. Two detection systems running simultaneously, one on-beam and another off-beam. Description of the resonances with the R-matrix code sammy. Results: The cross section has been measured from sub-thermal energy to 800 keV resolving the two first resonances (at 492.7 and 644 keV). A thermal cross-section (1.809±0.045 b) lower than the two most recent measurements by slightly more than one standard deviation, but in line with the ENDF/B-VIII.0 and JEFF-3.3 evaluations has been obtained. A 1/v energy dependence of the cross section has been confirmed up to tens of keV neutron energy. The low energy tail of the first resonance at 492.7 keV is lower than suggested by evaluated values, while the overall resonance strength agrees with evaluations. Conclusions: Our measurement has allowed to determine the 14N(n,p) cross-section over a wide energy range for the first time. We have obtained cross-sections with high accuracy (2.5 %) from sub-thermal energy to 800 keV and used these data to calculate the MACS for kT = 5 to kT = 100 keV

Constraints on the dipole photon strength for the odd uranium isotopes

Background: The photon strength functions (PSFs) and nuclear level density (NLD) are key ingredients for calculation of the photon interaction with nuclei, in particular the reaction cross sections. These cross sections are important especially in nuclear astrophysics and in the development of advanced nuclear technologies. Purpose: The role of the scissors mode in the M1 PSF of (well-deformed) actinides was investigated by several experimental techniques. The analyses of different experiments result in significant differences, especially on the strength of the mode. The shape of the low-energy tail of the giant electric dipole resonance is uncertain as well. In particular, some works proposed a presence of the E1 pygmy resonance just above 7 MeV. Because of these inconsistencies additional information on PSFs in this region is of great interest. Methods: The γ-ray spectra from neutron-capture reactions on the 234U, 236 U, and 238 U nuclei have been measured with the total absorption calorimeter of the n_TOF facility at CERN. The background-corrected sum-energy and multi-step-cascade spectra were extracted for several isolated s-wave resonances up to about 140 eV. Results: The experimental spectra were compared to statistical model predictions coming from a large selection of models of photon strength functions and nuclear level density. No combination of PSF and NLD models from literature is able to globally describe our spectra. After extensive search we were able to find model combinations with modified generalized Lorentzian (MGLO) E1 PSF, which match the experimental spectra as well as the total radiative widths. Conclusions: The constant temperature energy dependence is favored for a NLD. The tail of giant electric dipole resonance is well described by the MGLO model of the E1 PSF with no hint of pygmy resonance. The M1 PSF must contain a very strong, relatively wide, and likely double-resonance scissors mode. The mode is responsible for about a half of the total radiative width of neutron resonances and significantly affects the radiative cross section

Study of the photon strength functions and level density in the gamma decay of the n + 234U reaction

The accurate calculations of neutron-induced reaction cross sections are relevant for many nuclear applications. The photon strength functions and nuclear level densities are essential inputs for such calculations. These quantities for 235U are studied using the measurement of the gamma de-excitation cascades in radiative capture on 234U with the Total Absorption Calorimeter at n_TOF at CERN. This segmented 4π gamma calorimeter is designed to detect gamma rays emitted from the nucleus with high efficiency. This experiment provides information on gamma multiplicity and gamma spectra that can be compared with numerical simulations. The code DICEBOXC is used to simulate the gamma cascades while GEANT4 is used for the simulation of the interaction of these gammas with the TAC materials. Available models and their parameters are being tested using the present data. Some preliminary results of this ongoing study are presented and discussed

Fission program at n_TOF

Since its start in 2001 the n_TOF collaboration developed a measurement program on fission, in view of advanced fuels in new generation reactors. A special effort was made on measurement of cross sections of actinides, exploiting the peculiarity of the n_TOF neutron beam which spans a huge energy domain, from the thermal region up to GeV. Moreover fission fragment angular distributions have also been measured. An overview of the cross section results achieved with different detectors is presented, including a discussion of the 237Np case where discrepancies showed up between different detector systems. The results on the anisotropy of the fission fragments and its implication on the mechanism of neutron absorption, and in applications, are also shown

New reaction rates for the destruction of 7Be during big bang nucleosynthesis measured at CERN/n_TOF and their implications on the cosmological lithium problem

New measurements of the 7Be(n,α)4He and 7Be(n,p)7Li reaction cross sections from thermal to keV neutron energies have been recently performed at CERN/n_TOF. Based on the new experimental results, astrophysical reaction rates have been derived for both reactions, including a proper evaluation of their uncertainties in the thermal energy range of interest for big bang nucleosynthesis studies. The new estimate of the 7Be destruction rate, based on these new results, yields a decrease of the predicted cosmological 7Li abundance insufficient to provide a viable solution to the cosmological lithium problem

Status and perspectives of the neutron time-of-flight facility n_TOF at CERN

Since the start of its operation in 2001, based on an idea of Prof. Carlo Rubbia [1], the neutron time of-flight facility of CERN, n_TOF, has become one of the most forefront neutron facilities in the world for wide-energy spectrum neutron cross section measurements. Thanks to the combination of excellent neutron energy resolution and high instantaneous neutron flux available in the two experimental areas, the second of which has been constructed in 2014, n_TOF is providing a wealth of new data on neutron-induced reactions of interest for nuclear astrophysics, advanced nuclear technologies and medical applications. The unique features of the facility will continue to be exploited in the future, to perform challenging new measurements addressing the still open issues and long-standing quests in the field of neutron physics. In this document the main characteristics of the n_TOF facility and their relevance for neutron studies in the different areas of research will be outlined, addressing the possible future contribution of n_TOF in the fields of nuclear astrophysics, nuclear technologies and medical applications. In addition, the future perspectives of the facility will be described including the upgrade of the spallation target, the setup of an imaging installation and the construction of a new irradiation area

Measurement of the energy-differential cross-section of the 12C(n,p)12B and 12C(n,d)11B reactions at the n_TOF facility at CERN

Although the 12C(n,p)12B and 12C(n,d)11B reactions are of interest in several fields of basic and applied Nuclear Physics the present knowledge of these two cross-sections is far from being accurate and reliable, with both evaluations and data showing sizable discrepancies. As part of the challenging n_TOF program on (n,cp) nuclear reactions study, the energy differential cross-sections of the 12C(n,p)12B and 12C(n,d)11 B reactions have been measured at CERN from the reaction thresholds up to 30 MeV neutron energy. Both measurements have been recently performed at the long flight-path (185 m) experimental area of the n_TOF facility at CERN using a pure (99.95%) rigid graphite target and two silicon telescopes. In this paper an overview of the experiment is presented together with a few preliminary results