Fission spectrum averaged cross-section of 94Zr(n, α)91Sr

The fission spectrum averaged cross-section of ⁹⁴Zr(n, α)⁹¹Sr reaction was obtained by comparing that of ²⁷Al(n, α)²⁴Na reaction as reference. Highly enriched ⁹⁴ZrO₂ and ²⁷Al samples were irradiated with fast neutrons having a fission-like spectrum above about 5.6 MeV. After the neutron irradiation, the produced ⁹¹Sr was chemically separated from the irradiated sample. Gamma-rays from ⁹¹Sr and ²⁴Na were measured with a Ge(Li) detector. The obtained value is 5.37 ± 0.43 μb assuming the averaged cross-section of the reference reaction to be 0.61 mb.     

The 232Th(n, f) cross-section for thermal and 2.44 MeV neutrons

The thermal neutron induced fission cross-section of ²³²Th was measured at an intense and very pure thermal neutron beam of the Grenoble High Flux Reactor, yielding a value of (3 ± 1) μbarn. For control purposes, the same targets were used at the VandeGraaff accelerator of the CBNM (Geel), where a ²³²Th (n, f) cross-section value of (0.12 ± 0.02) barn was obtained with 2.44 MeV neutrons, in agreement with other published data.     

A proposal of a benchmark for βeff, βeff/Λ, and Λ of thermal reactors fueled with slightly enriched uranium

A reactor noise approach has been successfully performed at the IPEN/MB-01 research reactor facility for the experimental determination of the delayed neutron parameters β_eff, β_eff/Λ, and Λ. In the measurement of the β_eff parameter, the reactor power, which is of fundamental importance, was obtained with a very high level of accuracy by a fuel rod scanning technique and a subsequent irradiation of a highly enriched ²³⁵U foil for the fission density normalization. The final measured values of β_eff and β_eff/Λ show very good agreement with independent measurements and can be recommended as benchmark values for thermal reactor applications because their uncertainties are much lower than the target accuracy recommended for β_eff calculations (|C-E|/E less than 3%). The theory/experiment comparisons reveal that only JENDL3.3 attends the target accuracy for β_eff calculations. This result fully supports the reduction of the ²³⁵U thermal yield as proposed by Okajima and Sakurai. The ENDF/B-VI.8 library and its revised version performed at LANL overpredict β_eff by as much as 7.2%. The newly released JEFF-3.1 library shows a discrepancy of 4.8% for β_eff. For β_eff/Λ, the deviations are relatively larger (more than 10%) for all libraries due to the underprediction of the prompt neutron generation time (Λ).     

Measurement of specific energy losses of individual fission fragments with a back-to-back δE−E detector system

The specific ionization of fission fragments of specified charge and $\text{E}\text{M}$ values in thermal neutron induced fission of ²³⁵U have been measured employing a back-to-back gridded ionization chamber ΔE-semiconductor detector E setup. The measurements were carried out for the case of a mixture of argon (95%) and methane (5%) gases at two gas pressures, 44 Torr and 270 Torr. The experimental data for the lower gas pressure were used to determine the effective charge parameter γ for the fragments at various velocities. These γ-values have been used to compute the fragment energy losses at the higher gas pressure and are compared with the measured energy loss values.     

“Point by Point” model calculation of prompt neutron emission data for Cm(SF) and Cm(SF)

This work is devoted to the modeling of the prompt neutron emission in the spontaneous fission of ^248,244Cm. The Point by Point model calculations of relevant quantities (like: the multi-parametric matrix ν(A, TKE), the fission fragment multiplicity as a function of fragment mass (usually named sawtooth) ν(A), fission fragment pair multiplicity as a function of total excitation energy ν_pair(TXE), total average prompt neutron multiplicity 〈ν_p〉 and spectra N(E), total average multiplicity as a function of the total kinetic energy 〈ν_p〉 (TKE) describe very well all existing experimental data.     

Systematic behaviour of the average parameters required for the Los Alamos model of prompt neutron emission

A large number of fissioning systems have been studied in the frame of the Los Alamos model which showed interesting regular behaviours of the average model parameters (the energy release in fission 〈E_r〉, the total kinetic energy of the fission fragments 〈TKE〉, the average neutron separation energy 〈S_n〉 from the fission fragments, the prompt gamma-ray energy 〈E_γ〉 and the level density parameter 〈a〉 parameterized as 〈C〉 = A/〈a〉, where A is the mass number of the fissioning nucleus)) as well as of other quantities in connection with the prompt fission neutron emission (such as the total average prompt neutron multiplicity at thermal incident energy, the total average fission fragment excitation energy leading to prompt neutron emission, the total average prompt fission energy deposition, the average excitation energy given to the fragments, and the average center-of-mass energy of prompt neutrons and so on) and to elaborate systematics of model parameters.     

The (He, tf) as a surrogate reaction to determine (n, f) cross sections in the 10-20 MeV energy range

The surrogate reaction ²³⁸U(³He, tf) is used to determine the ²³⁷Np(n, f) cross section indirectly over an equivalent neutron energy range from 10 to 20 MeV. A self-supporting ∼761 μg/cm² metallic ²³⁸U foil was bombarded with a 42 MeV ³He²⁺ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS) consisted of two 140 μm and one 1000 μm Micron S2 type silicon detectors. The ²³⁷Np(n, f) cross sections, determined indirectly, were compared with the ²³⁷Np(n, f) cross section data from direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets. Use of the (³He, tf) reaction as a surrogate to extract (n, f) cross sections in the 10-20 MeV equivalent neutron energy range is found to be suitable.     

Improved Los Alamos model applied to the neutron induced fission of 239Pu and 240Pu and to the spontaneous fission of Pu isotopes

The Los Alamos model with multiple fission chances upgraded with (a) the linear relation between the average prompt gamma ray energy and the average prompt neutron multiplicity and (b) the dependence of the average fission fragment kinetic energy on the incident neutron energy, is used for the n+²³⁹Pu and n+²⁴⁰Pu reactions, and also for the spontaneous fission of ^237–241Pu isotopes. In the case of ²⁴⁰Pu fissioning nucleus the variation of the average energy released versus the incident neutron energy is also taken into account. The calculated prompt fission neutron spectra and average prompt neutron multiplicity well represent the experimental data, proving a better predictive power of the improved Los Alamos model.     

The Westcott gf-factor for 239Pu and its temperature dependence

The Westcott g_f-factor, which is important for the interpretation of measurements with Maxwellian neutron spectra and reactor spectra, was calculated using our recent accurate neutron induced fission cross-section measurements. The value we obtained is g_f = 1.053 ± 0.003.We further calculated the temperature dependence of g_f from 0°C to 1000°C.     

Kinetics of precipitation of U4O9 from hyperstoichiometric UO2+x

For safe and reliable operation of fission reactors in space, the phase diagrams and reaction kinetics of systems used as nuclear fuels, such as U-O, U-N, U-C, are required. Diffraction allows identification of phases and their weight fractions as a function of temperature in situ, with a time resolution of the order of minutes. In this paper, we will provide results from a neutron diffraction experiment studying the U-O system. Using the neutron diffractometer HIPPO, the decomposition of UO_2+x into UO₂ and U₄O₉ as a function of temperature was investigated in situ. From the diffraction data, the participating phases could be identified as UO_2+x, UO₂ and U₄O_8.94 and no stoichiometric U₄O₉ was found. Results of the experiment were used to improve existing thermodynamic models. The presented techniques (i.e., neutron diffraction and thermodynamic modeling) are also applicable to the other systems mentioned above.     

Prompt fission neutron spectrum evaluation for 252Cf(SF) in the frame of the multi-modal fission model

In the present work, an attempt to improve the evaluation of the prompt fission neutron spectrum of ²⁵²Cf(SF) is made. The multi-modal fission concept is included into the Los Alamos model. A more generalized form of the fission fragment residual nuclear temperature distribution and a possible anisotropy effect of the prompt neutron emission in the center-of-mass system are taken into account, too. The multi-modal fission parameters entering the prompt fission neutron spectrum model are determined on the basis of the experimental data concerning the fission fragment total kinetic energy TKE(A) and mass distribution Y(A) measured at IRMM. The calculated prompt neutron spectrum is obtained in better agreement with the standard point-wise evaluation of Mannhart and compared to other evaluations made with different models.     

Improved Los Alamos model applied to the neutron induced fission of 235U and 237Np

The Los Alamos model, with multiple fission chances, was upgraded to include the linear relation between the average prompt gamma ray energy and the average prompt neutron multiplicity. A global, model-free parameterization of this relation versus the charge and mass number of fissioning nuclei is obtained. The average fission fragment kinetic energy dependence on the incident neutron energy is also taken into account. Employing this model for the n+²³⁵U and n+²³⁷Np reactions, the prompt fission neutron spectra, the average prompt neutron multiplicity and the average prompt gamma ray energy are obtained in very good agreement with the experimental data, proving a better predictive power.     

Technique for measuring angular correlations and g-factors of excited states with large multi-detector arrays: An application to neutron rich nuclei produced by the spontaneous fission of Cf

Triple coincidences between prompt γ-rays emitted in the spontaneous fission of ²⁵²Cf were measured with Gammasphere. These data are used to measure the angular correlation of cascades of γ-rays from excited states of neutron rich fission fragments stopped in an unmagnetized iron foil. The hyperfine fields in the iron lattice cause attenuations of the angular correlations between γ-rays emitted from the excited states which have sufficiently long lifetimes. This attenuation is measured and used to calculate the g-factors of excited states in many neutron rich nuclei.     

The 10B(n, αγ)7Li cross-section between 0.1 and 2.2 MeV

The shape of the excitation function for the standard neutron reaction ¹⁰B(n, αγ)⁷Li in the energy range from 0.1 to 2.2 MeV has been determined relative to the angular distribution of the neutron source reaction T(p, n)³He at 1.6, 2.3, and 3.0 MeV proton energy. The 478 keV γ's produced in a 3-mm thick boron carbide sample were observed in a Ge(Li)-detector. The time-of-flight method was used to discriminate against events from neutrons of degraded energy.     

Cross section measurements of the 23Na(n, 2n)22Na, 58Ni(n, 2n)57Ni and 115In(n,n′)115mIn reactions

Cross sections for two high energy threshold reactions ²³Na(n, 2n)²²Na and ⁵⁸Ni(n, 2n)⁵⁷Ni were measured by the activation method in the neutron energy range from 14 to 18 MeV. Inelastic scattering cross section for ¹¹⁵In was measured in the threshold region, i.e. from 0.5 to 1.3 MeV. The results of measurements are compared with scarce and divergent earlier data.     

Activation cross sections for Os(n,p)Re, Os(n,p)Re and Os(n,n′)Os reactions from 13.5 to 14.8 MeV

Cross sections for (n,p) and (n,n′) reactions have been measured on osmium isotopes at the neutron energies from 13.5 to 14.8 MeV using the activation technique in combination with high-resolution gamma-ray spectroscopy. Neutrons were produced via the ³H(d,n)⁴He reaction using solid TiT. The neutron fluences were determined using the monitor reaction ⁹³Nb(n,2n)^92mNb. Data are reported for the following reactions: ¹⁹⁰Os(n,p)^190mRe, ¹⁹⁰Os(n,p)^190gRe, ¹⁹⁰Os(n,p)¹⁹⁰Re, ¹⁸⁸Os(n,p)¹⁸⁸Re and ¹⁹⁰Os(n,n′)^190mOs. Nuclear model calculations using the code HFTT, which employs the Hauser-Feshbach (statistical model) and exciton model (precompound effects) formalisms, were undertaken to describe the formation of the products. The cross sections were discussed and compared with experimental data found in the literature, with values of model calculations including the pre-equilibrium contribution, and with evaluation data of JEFF-3.1/A.     

Approach for the fission fragment total kinetic energy TKE(A) calculation: Application to prompt neutron emission models

A simple approach for the calculation of the fission fragment total kinetic energy, TKE(A), based on the electrostatic repulsion between the fragments connected by a neck in the pre-scission configuration is described. The calculated TKE(A) is obtained in good agreement with the experimental data for many fissioning systems, such as ^233,235U(n_th, f), ²³⁹Pu(n_th, f), ²³⁷Np(n, f), ²⁴²Pu(SF), with minor adjustment of only one parameter. Due to the fact that the present approach can provide with enough trust TKE(A) distributions for fissioning systems for which experimental TKE(A) data do not exist, the possibilities to use the refined Point by Point model of prompt neutron emission can be considerably extended.     

0.04–4.0 Mev neutron fission cross section of Pu240

A detailed investigation of the Pu²⁴⁰ nucleus cross section is of interest with regard to an experimental verification of theoretical concepts of the energy dependence of fission probability as well as regarding possible uses of Pu²⁴⁰ as a nuclear fuel in fast neutron reactors.We measured the energy dependence of the fast neutron fission cross section of Pu²⁴⁰ for neutrons with the energy E_n = 0.04–4.0 Mev. The T(p, n)He³ reaction served as the neutron source. The Pu²⁴⁰ fission cross section in the plateau region (1–4 Mev) amounts to ~ 1.6 barn and is equal to only one-half of this value for the neutron energy E_n 0.7 Mev. A sharp decrease in the fission cross section value occurs as E_n decreases to 0.3 Mev; for a further decrease in E_n, the cross section value drops less sharply, and it remains practically constant (~ 0.065 barn) for 0.04 < E_n < 0.15 Mev. The correlation between the irregularities in fission cross section values and the levels of Pu²⁴⁰ nuclei which correspond to inelastic scattering channels is discussed.The authors extend their heartfeltthanks to A. I. Leiptmskii and I. I. Bondarenko for their helpfulness and interest in the work, to L. N. Usachev for the discussion of the results, to Yu. I. Baranov and N. E. Tokmantseva for their help in measurements, and to V. A. Romanov, G. A. Strigin, and Yu. I. Parfenov, who kept the accelerator in good running order.     

Uncertainty evaluation of calculated and measured kinetics parameters of Tehran Research Reactor

Effective delayed neutron fraction β_eff and neutron generation time Λ are important factors in reactor physics calculation and transient analysis. In the first stage of this research, these kinetics parameters have been calculated for two states of Tehran Research Reactor (TRR), i.e. cold (fuel, clad and coolant temperature 20 °C) and hot (fuel, clad and coolant temperature 65, 49 and 44 °C, respectively) states using MTR_PC computer code. The ratio of (β_eff)_i/(β_eff)_core plays an important role in reactivity accident analysis codes. This parameter and its contribution to effective delayed neutron fraction from each nucleus have been calculated in cold and hot reactor states. Uncertainty of effective delayed neutron fraction is evaluated in terms of following four quantities; basic delayed neutron constants, delayed neutron spectra, energy dependence of delayed neutron yield (ν_d) and fission cross-section of ²³⁵U and ²³⁸U. In the second stage, these parameters have been measured with an experimental method based on Inhour equation. The calculated and measured values are in good agreement. Relative Percent Errors (RPEs) are 2.8% for β_eff and 5.7% for Λ in the cold state.     

<Superscript>243</Superscript>Am neutron fission cross section and resonance parameters

The SVZ-100 lead moderation time neutron spectrometer at the Institute of Nuclear Physics of the Russian Academy of Sciences was used to measure the fission cross section for ²⁴³Am in the neutron energy range E_n = 0.3 eV – 10 keV. The resonance fission integrals and the area and fission width of the resolved resonances were calculated. The properties of the intermediate-structure resonances were evaluated. The results were compared with existing data and recommended evaluations.