Neutron Resonance Region Research Articles

Stimulated perturbation on the neutron flux distribution in the mutually-dependent source-to-absorber geometry

The complexity of the neutron transport phenomenon throws its shadows on every physical system wherever neutron is produced or absorbed. The Monte Carlo N-Particle Transport Code (MCNP) was used to investigate the flux perturbations in the neutron field caused by an absorber. The geometry of the present experiment was designed to reach a simulation of an isotopic neutron field. The neutron source was a 241 AmBe with the production physics of neutrons being dependent only on alpha-beryllium interaction and is independent of what happened to the neutron after it was generated. The geometries have been designed to get a volume of uniform neutron densities within a spherical volume of radius 15 cm in every neutron energy group up to 10 MeV. Absorbers of different dimensions were placed within the volume to investigate the field perturbation. Different neutron absorbers were used to correlate the phenomenon to the integral cross-section of the absorber. Flux density inside and outside the absorber samples was determined, while the spatial neutron flux distribution produced by the AmBe source without an absorber was taken as a reference. This study displayed that absorbers of various dimensions perturb the neutron field in a way that is dependent on the absorption and scattering cross-sections, particularly in the neutron resonance region. Unlike the simple picture of reducing the number density of neutrons, the perturbation was found to influence the moderation of neutrons in the medium, significantly above 1 MeV.

Fission cross-sections, prompt fission neutron and γ-ray emission in request for nuclear applications

In recent years JRC-IRMM has been investigating fission cross-sections of 240,242 Pu in the fast-neutron energy range relevant for innovative reactor systems and requested in the High Priority Request List (HPRL) of the OECD/Nuclear Energy Agency (NEA). In addition to that, prompt neutron multiplicities are being investigated for the major isotopes 235 U, 239 Pu in the neutron-resonance region using a newly developed scintillation detector array (SCINTIA) and an innovative modification of the Frisch-grid ionisation chamber for fission-fragment detection. These data are highly relevant for improved neutron data evaluation and requested by the OECD/Working Party on Evaluation Cooperation (WPEC). Thirdly, also prompt fission γ-ray emission is investigated using highly efficient lanthanide-halide detectors with superior timing resolution. Again, those data are requested in the HPRL for major actinides to solve open questions on an under-prediction of decay heat in nuclear reactors. The information on prompt fission neutron and γ-ray emission is crucial for benchmarking nuclear models to study the de-excitation process of neutron-rich fission fragments. Information on γ-ray emission probabilities is also useful in decommissioning exercises on damaged nuclear power plants like Fukushima Daiichi to which JRC-IRMM is contributing. The results on the 240,242 Pu fission cross section, 235 U prompt neutron multiplicity in the resonance region and correlations with fission fragments and prompt γ-ray emission for several isotopes will be presented and put into perspective.

Capture and fission with DANCE and NEUANCE

A summary of the current and future experimental program at DANCE is presented. Measurements of neutron capture cross sections are planned for many actinide isotopes with the goal to reduce the present uncertainties in nuclear data libraries. Detailed studies of capture gamma rays in the neutron resonance region will be performed in order to derive correlated data on the de-excitation of the compound nucleus. New approaches on how to remove the DANCE detector response from experimental data and retain the correlations between the cascade gamma rays are presented. Studies on 235U are focused on quantifying the population of short-lived isomeric states in 236U after neutron capture. For this purpose, a new neutron detector array NEUANCE is under construction. It will be installed in the central cavity of the DANCE array and enable the highly efficient tagging of fission and capture events. In addition, developments of fission fragment detectors are also underway to expand DANCE capabilities to measurements of fully correlated data on fission observables.

Levels in 168Er above 2 MeV and the onset of chaos.

Extremely high statistics \ensuremath{\gamma}-\ensuremath{\gamma} coincidence data for $^{168}\mathrm{Er}$ have been recorded following thermal neutron capture on $^{167}\mathrm{Er}$. The results alter considerably recent experimental and theoretical work on $^{168}\mathrm{Er}$ and affect conclusions concerning the quality of the K quantum number in the neutron resonance region: earlier claims that K remains good and that this energy region is nonchaotic are shown to be based in part on band assignments that need to be seriously reexamined.

On the K-dependence in the gamma-decay of neutron resonances in 168Er

The intensities of the K-allowed gamma transitions from neutron resonance states to low-lying states in 168Er are analyzed. The quasiparticle selection rule is taken into account. It is shown that the K-allowed gamma transitions satisfying the quasiparticle selection rule are much faster than the K-forbidden transitions. It may indicate that the K mixing in the neutron resonance region is incomplete.

Is there incomplete mixing of states with different K quantum numbers in the neutron resonance region?

A recent publication claimed incomplete mixing of states with different {ital K} quantum numbers in the neutron resonance region. We discuss the theoretical implications of such a claim and show that it leads to serious discrepancies with the statistical model. We, therefore, reexamine the experimental data on which such a claim is based. The totality of the evidence invalidates the claim that {ital K} mixing in the resonance region is incomplete.

Spin dependence of ḡn p and [formula omitted] for 239Pu(n, f) in the resonance region: The discrepancy between results above and below 1 eV

This work reports a data discrepancy concerning the spin dependence of \\ ̄ gn p and E ̄ K for 239Pu(n, f) in the neutron resonance region — a discrepancy between data measured above and below 1 eV. An attempt to rectify the discrepancy is made by an examination of some aspects of the data and by a measurement of E ̄ K for 235U(n, f).

Neutron Capture and Fission Cross Sections of Plutonium-241

Neutron capture and fission cross sections of 241Pu have been measured from 0.01 eV to 30 keV, and their ratio has been measured up to 250 keV. The cross sections were normalized at thermal-neutron energies (0.02 to 0.03 eV) to the ENDF/B-IV evaluation. The source of pulsed neutrons was the Oak Ridge Electron Linear Accelerator. The gamma-ray detector used to detect capture and fission events was the “total energy detector,” which is a low-efficiency detector whose average efficiency is forced to be proportional to the energy of the interacting gamma rays by weighting these events according to their pulse height in the scintillator. Fast-neutron scintillation detectors with pulse-shape discrimination were used to detect fission events. The shape of the neutron flux was measured relative to the 10B(n, α) cross section. The measurements are unique for 241Pu in that absorption and fission were determined directly and simultaneously over a wide neutron energy range rather than indirectly by inferring capture from separate fission and total cross-section measurements. The results indicate that the neutron resonance region of the ENDF/B-IV evaluation underestimates capture by a factor of ∼2. Above the resonance region (∼100 eV), there are no previous measurements of the differential capture cross section. These cross sections are important in plutonium-fueled reactors.

Anomalous radiative capture in the neutron resonance region: Analysis of the experimental data on electric dipole transitions

The formalism of the preceding paper 1) is used in a semi-quantitative attempt to analyze one of the most striking systematic features of low-energy neutron-capture γ-ray spectra, namely, the strong group of high energy transitions occuring in the spectra for nuclei below mass 208 and around mass 50. This is anomalous in the sense that it does not concur with our qualitative expectations from statistical theory. It is found that close to the giant resonance in the s-wave neutron strength function at A≈ 50 the anomaly appears to be associated with a selection rule operating in resonance capture while for lighter nuclei and for capture in some of the lead isotopes there is strong evidence that it is a consequence of “direct” or “potential” capture.