Neutron total cross section measurements of gold and tantalum at the nELBE photoneutron source

Neutron total cross sections of $^{197}$Au and $^\text{nat}$Ta have been measured at the nELBE photoneutron source in the energy range from 0.1 - 10 MeV with a statistical uncertainty of up to 2 % and a total systematic uncertainty of 1 %. This facility is optimized for the fast neutron energy range and combines an excellent time structure of the neutron pulses (electron bunch width 5 ps) with a short flight path of 7 m. Because of the low instantaneous neutron flux transmission measurements of neutron total cross sections are possible, that exhibit very different beam and background conditions than found at other neutron sources.Comment: article (18 pages, 10 figures, 2 tables) with attached data tables (13 pages

Inelastic scattering of fast neutrons from 56

The inelastic scattering of fast neutrons on 56Fe was investigated in different manners at the neutron time-of-flight facility nELBE. The scattering cross section was determined via the measurement of the γ-ray production and by means of a kinematically complete double time-of-flight method. In a further measurement the γ-ray angular distribution was determined to correct the measured cross sections for anisotropy. The resulting inelastic scattering cross section determined from the photo production cross sections is in very good agreement with evaluations and previous measurements. In contrast, the result of the double time-of-flight measurement is about 10% lower than these data, giving a hint to neutron-γ-ray angular correlations in the process of inelastic neutron scattering

Determination of binary fission-fragment yields in the reaction 251Cf(nth, f) and Verification of nuclear reaction theory predictions of fission-fragment distributions in the reaction 238U(n, f)

Neutron-induced fission has been studied at different excitation energies of the compound nucleus by measurements on the two fissioning systems, 252Cf* and 239U*. For the first time, the light fission fragment yields from the reaction 251Cf(nth, f) have been measured with high resolution. This experiment was performed with the recoil mass spectrometer LOHENGRIN at ILL in Grenoble, France. When the results from this work, where the compound nucleus is at thermal excitation, are compared to the spontaneous fission of 252Cf, enhanced emission yields as well as an increased mean kinetic energy is observed around A = 115. This suggests the existence of an additional super-deformed fission mode in 252Cf. The reaction 238U(n, f) was studied using the 2E-technique with a double Frisch grid ionization chamber. Fission fragment mass, energy and angular distributions were determined for incident neutron energies between 0.9 and 2.0 MeV. The experiments were performed at the Van de Graaff accelerator of IRMM in Geel, Belgium. This is the first measurement of the mass distribution for incident neutron energies around 0.9 MeV. The motivation for studying 238U(n, f) was to verify theoretical predictions of the mass distribution at the vibrational resonance in the fission cross section at 0.9 MeV. However, the predicted changes in fission fragment distributions could not be confirmed. A precise modelling of the fission process for the minor actinides becomes very important for future generation IV and accelerator driven nuclear reactors. Since fission fragment distributions depend on the excitation of the fissioning system, so does the number of delayed neutrons, which are one of the safety parameters in a reactor

Determination of binary fission-fragment yields in the reaction 251Cf(nth, f) and Verification of nuclear reaction theory predictions of fission-fragment distributions in the reaction 238U(n, f)

Neutron-induced fission has been studied at different excitation energies of the compound nucleus by measurements on the two fissioning systems, 252Cf* and 239U*. For the first time, the light fission fragment yields from the reaction 251Cf(nth, f) have been measured with high resolution. This experiment was performed with the recoil mass spectrometer LOHENGRIN at ILL in Grenoble, France. When the results from this work, where the compound nucleus is at thermal excitation, are compared to the spontaneous fission of 252Cf, enhanced emission yields as well as an increased mean kinetic energy is observed around A = 115. This suggests the existence of an additional super-deformed fission mode in 252Cf. The reaction 238U(n, f) was studied using the 2E-technique with a double Frisch grid ionization chamber. Fission fragment mass, energy and angular distributions were determined for incident neutron energies between 0.9 and 2.0 MeV. The experiments were performed at the Van de Graaff accelerator of IRMM in Geel, Belgium. This is the first measurement of the mass distribution for incident neutron energies around 0.9 MeV. The motivation for studying 238U(n, f) was to verify theoretical predictions of the mass distribution at the vibrational resonance in the fission cross section at 0.9 MeV. However, the predicted changes in fission fragment distributions could not be confirmed. A precise modelling of the fission process for the minor actinides becomes very important for future generation IV and accelerator driven nuclear reactors. Since fission fragment distributions depend on the excitation of the fissioning system, so does the number of delayed neutrons, which are one of the safety parameters in a reactor

Properties of the Reaction 238U(n,f) at the Vibrational Resonances

Recent fission cross-section calculations for the reaction 238U(n, f), based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by dA = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy En = 0.9 MeV. This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on 238U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from En = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at En = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around En = 1.2 and 1.6 MeV was observed at En = 0.9 MeV as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at En = 1.8 MeV. At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.JRC.D.5-Neutron physic

Binary Fission-Fragment Yields from the Reaction 251Cf(nth,f)

The interpretation of fission-fragment properties in terms of so-called fission modes has been successfully applied in the actinide region to describe mass yield and total kinetic energy distributions as a function of incident neutron energy. There, the asymmetric standard I (S1) and standard II (S2) modes as well as the symmetric superlong (SL) mode have been used. In the case of spontaneous fission of 252Cf the number of traditional fission modes is not sufficient to properly describe the experimentally obtained fission-fragment distributions. Additional theoretically obtained fission modes have to be included into the analysis to improve the description of the data. In order to achieve experimental confirmation of the number of fission modes neutron-induced fission of 252Cf*, using 251Cf as target material, has been investigated at thermal excitation. The experiment has been performed at the recoil mass-separator LOHENGRIN of the ILL, Grenoble. Light post-neutron fission-fragment kinetic energy distributions were measured for A = 80 to 124 for the first time ever, and relative emission yields together with mean kinetic energies as a function of A have been determined. Due to electric field instabilities of LOHENGRIN special corrections had to be applied to the raw data. Data analysis and final experimental results will be present.JRC.D.5-Neutron physic

First results on the neutron-induced fission cross-section of 231Pa for incident neutron energies En > 17 MeV

First results on the neutron-induced fission cross-section of 231Pa for incident neutron energies En > 17 MeV are presented. The experiments were carried out with quasi mono-energetic neutrons produced in the reaction T(d, n)4He. Corrections for low-energy neutron background produced in this reaction at incident deuteron energies Ed > 2 MeV are taken into account and based on experimental data obtained by two different techniques. Despite the relatively large error bars at the higher neutron energies, the new cross-section values meet the accuracy requirements set by the IAEA and will allow to remove the hitherto existing large spread between different previously published data. Recent cross-section calculations describe well the new experimental results, which are in consistency with cross-section values obtained in a particle-transfer reaction at excitation energies corresponding to neutron energies En < 10 MeV.JRC.D.4-Nuclear physic

Recent Results on the Neutron-Induced Fission Cross-Section of 231Pa

Our studies of neutron-induced reactions for advanced nuclear applications have recently been extended to the nuclide 231Pa. In accordance with the heavier isotope 233Pa, which we reported about previously, 231Pa is supposed to play an important role for future reactors involving the thorium-uranium fuel cycle. Thus, 231Pa belongs to the isotopes that were pointed out by the IAEA to be investigated with highest priority. Although the neutron-induced fission cross-section has been measured before, the compiled experimental results as well as data in existing evaluated data files exhibit quite some differences. For this reason we have performed new experiments with a quasi mono-energetic neutron beam, provided by the Van de Graaff accelerator at IRMM. Recent results, covering the neutron energy range from 0.8 to 20 MeV, will be presented.JRC.D.5-Neutron physic

Inelastic scattering of fast neutrons from

The inelastic scattering of fast neutrons on 56Fe was investigated in different manners at the neutron time-of-flight facility nELBE. The scattering cross section was determined via the measurement of the γ-ray production and by means of a kinematically complete double time-of-flight method. In a further measurement the γ-ray angular distribution was determined to correct the measured cross sections for anisotropy. The resulting inelastic scattering cross section determined from the photo production cross sections is in very good agreement with evaluations and previous measurements. In contrast, the result of the double time-of-flight measurement is about 10% lower than these data, giving a hint to neutron-γ-ray angular correlations in the process of inelastic neutron scattering

Inelastic scattering of fast neutrons from 56Fe

The inelastic scattering of fast neutrons on 56Fe was investigated in different manners at the neutron time-of-flight facility nELBE. The scattering cross section was determined via the measurement of the γ-ray production and by means of a kinematically complete double time-of-flight method. In a further measurement the γ-ray angular distribution was determined to correct the measured cross sections for anisotropy. The resulting inelastic scattering cross section determined from the photo production cross sections is in very good agreement with evaluations and previous measurements. In contrast, the result of the double time-of-flight measurement is about 10% lower than these data, giving a hint to neutron-γ-ray angular correlations in the process of inelastic neutron scattering