Differential Neutron Flux Research Articles

Regularized heuristic method for activation monitor neutron spectrum unfolding

This paper describes an activation foil measurement based neutron flux unfolding method utilizing a heuristic optimization approach. A regularization parameter based on the differential neutron flux was introduced into the optimization cost function to remove large nonphysical oscillations in the unfolded neutron energy spectrum. The procedure was completed for two example case studies ranging from thermal energies up to 20 MeV demonstrating recovery of neutron flux energy information on radiation environments where a model was partially missing or there was a discrepancy in a reaction channel. The unfolded results provided a distribution of unfolded spectra that conformed to the activation products but indicated degenerate solutions to the general activation problem. Additionally, increasing fidelity of initial estimate on the neutron spectrum reduced the degeneracies and lowered the uncertainty in the unfolded spectrum, even in a low-fidelity, physics-based estimate of the neutron spectrum. Epithermal neutron energies, where few nuclear reactions are available to inform on the neutron flux, were shown to be the most difficult region. This method can be extended to other heuristic optimization routines and offers an alternative option for traditional activation dosimetry neutron flux unfolding techniques while improving the characterization of the neutron flux in applications with poor initial models of the environment.

Measurement and analysis of neutron leakage spectrum in depleted Uranium-Polyethylene Mock-up

For the validation of neutron reaction data, the absolute neutron leakage spectra were measured on the analytical mockups of Fission Blanket Assembly (FBA) with Polyethylene (PE) to simulate reflector and water, in the energy range between 1 MeV and 16 MeV. A well characterized cylindrical BC501A scintillator detector was used in measurement. The assemblies were irradiated with D-T reaction neutrons. The pulse height spectra of recoiled proton in detector were recorded and unfolded with response calculated by O5S code. The uncertainties in the experimental results were 5.6%–7.9%. The measured spectra were compared with calculation with MCNP code and ENDF/BVI.5 library. The calculations agreed well with the experimental ones for the spectrum shape and the integrated neutron flux within experimental uncertainties. On the contrary, the discrepancy is found in differential neutron flux and also in the ratios of integrated neutron flux for FBA/PE Assembly to that for PE/FBA/PE Assembly.

Measured velocity spectra and neutron densities of the PF2 ultracold-neutron beam ports at the Institut Laue–Langevin

Ultracold neutrons (UCNs) are a useful tool for fundamental physics experiments. They can be used to probe the lifetime of free neutrons, search for new CP violating processes and exotic interactions beyond the Standard Model, perform Ramsey spectroscopy, and carry out neutron-optical interference experiments. All of these experiments require high neutron count rates for good statistics. For optimal exploitation of experimental beam time, these experiments need to be prepared and, at times, even simulated in advance. To this end, it is crucial to know the velocity-dependent UCN flux at each beam position. Knowing the absolute neutron flux also allows for an absolute calibration of previously gathered data. Using the same time-of-fight experimental setup, we have measured the differential neutron flux of three out of the four UCN beam ports at the PF2 instrument at Institut Laue–Langevin, Grenoble. These beam ports are commonly used for UCN flux experiments and proof-of-principle tests.

Validation of nickel isotopes neutron cross sections using nickel spherical benchmark

The fast leakage neutron spectra have been measured on spherical nickel benchmark assembly of diameter 50 cm. The 252Cf neutron source with approximate emission of 5.0·108 n/s was placed into the centre of the sphere. Fast neutron spectrum in the range of 1–10 MeV was measured in the distance of 1 m from the sphere centre by means of proton recoil method using scintillation stilbene crystal. The experimental data were compared to transport calculations based on several evaluated nuclear data libraries using MCNP6. MCNP6 was compared with SCALE/MONACO program using the same ENDF/B-VII.1 library which leads to different differential neutron flux results. Best experimental agreement with calculation in MCNP6 is achieved with ENDF/B-VII.1 library. Contrary, worst agreement is achieved with JENDL-4.0 and CENDL-3.1 libraries. Furthermore, cross section sensitivity analysis for elastic and inelastic scattering for both main nickel isotopes (58Ni, 60Ni) was performed. It was shown that 58Ni isotope has higher influence on the result than 60Ni isotope in the entire energy range under study. The highest influence has the elastic XS of 58Ni around energy of 1.5 MeV. The inelastic cross section (XS) of 58Ni dominates in the energies above 2 MeV where two percent rise due to the inelastic XS leads up to 3.3% decrease in the neutron flux.

Measurement of the differential neutron flux inside a lead shielding in a cryogenic experiment

The ROSEBUD collaboration measured the differential flux of fast neutrons inside a shielding of lead irradiated with a source of 252Cf using two scintillating bolometers of LiF and Al2O3 at 20 mK. We compare an unfolding method using a three parameter model for the fast component of the neutron flux with a multigroup method in which the energy interval of interest is divided into groups. Some issues regarding the neutron monitoring with LiF or Al2O3 in a cryogenic experiment searching for dark matter WIMPs are discussed.

Measurement of neutron flux and beam divergence at the cold neutron guide system of the new Munich research reactor FRM-II

A sophisticated neutron guide system has been installed at the new Munich neutron source FRM-II to transport neutrons from the D 2 cold neutron source to several instruments, which are situated in a separate neutron guide hall. The guide system takes advantage of supermirror coatings and includes a worldwide unique “twisted” guide for a desired phase space transformation of the neutron beam. During the initial reactor commissioning in summer 2004, the integral and differential neutron flux as well as the distribution of beam divergence at the exit of two representative and the twisted neutron guide were measured using time-of-flight spectroscopy and gold-foil activation. The experimental results can be compared to extensive simulation calculations based on MCNP and McStas. The investigated guides fulfill the expectations of providing high neutron fluxes and reveal good quality with respect to the reflective coatings and the installation precision.

Absolute Cross Sections for some Reactions Induced by Differential Neutron Flux

The measurements of activation cross sections for some reactions induced by a differential neutron flux of (D-T) compact neutrons generator were discussed. The comparison of measured cross sections with those averages calculated from the energy cross section curves found in the literature gives good agreement. The used thin targets were prepared under vacuum by both evaporation and sputtering techniques.

A New Statistical Calculation of Scattered Neutrons in an Infinite Slab

The differential neutron flux arising from clastic-scattering of an incident mono-energetic beam of fast neutrons, falling normally on an infinite slab of material has been treated analytically using solid geometry.

Investigation of D-T Differential Neutron Flux in Different Mediums

An experimental investigation of the absolute differential neutron flux of D-T compact neutron generator and spatial energy distributions within matter, using the threshold monitors technique have been carried out. Materials used were lead, glass, concrete, sea water and water, as well as air. Moreover, the neutron buildup factors within these media have been determined.

Prospekt-1 program for recovering the neutron spectra of nuclear reactors

on methods can now be taken as in principle solved [i], but the standardization of methods of making measurements and of processing the results is restricted to particular techniques for determining the neutron characteristics of individual energy groups [2]. The Prospekt-i method [3] is the first stage in the creation of an up-to-date program for recovering the spectra of nuclear reactors over a wide energy range as standardized for country-wide use. It provides a very detailed characterization of the neutron distribution, viz., the differential neutron flux density ~(E) as a function of the energy E (neutron spectrum) in the range 0.5 eV-18 MeV.

Traveling mode solution of transport-theory problems by inspection

Abstract The differential neutron flux in slab geometry can be represented as a linear combination of traveling eigenmodes that propagate with exponential decay as functions of distance. The operators that serve to analyze the flux into mode amplitudes and to synthesize the flux from the mode amplitudes are determined. The particular analyzers and synthesizers it is appropriate to use in any given situation depend on the boundary conditions. These operators have an interpretation independent of any particular problem. The fluxes of interest can be written down almost by inspection in terms of these operators.