Probability Of Neutron Absorption Research Articles

Optical and radiation shielding studies on tellurite glass system containing ZnO and Na2O

Three TeO2-ZnO-Na2O ternary glass samples with chemical formula 80TeO2-xZnO-(20-x)Na2O (where x = 20, 10, and 0 mol% represents TeZnNa1, TeZnNa2, and TeZnNa3 respectively) were investigated for their optical and radiation shielding attributes. The calculated optical parameters namely structure molar refractivity (RMol), molar polarizability (αMol.), reflection loss (RLoss) and optical transmission (TOpt.) all vary with the chemical content of the glasses. The FLUKA Monte Carlo code and XCOM software was used to estimate mass attenuation coefficient MAC of the glasses. Furthermore, the fast neutron absorption probability ΣR was estimated through standard expressions. The MAC values varies between 0.03796 and 0.16191 for TeZnNa1, 0.03793–0.16206 for TeZnNa2 and 0.03791–0.16221 cm2/g for TeZnNa3 as photon energy changes from 284 keV to 2.506 MeV. Analysis of other photon shielding parameters such as effective atomic number and mean free path showed that TeZnNa3 is the best shield among the glasses. The ΣR of the glasses also changed from 0.0942 to 0.1117 cm−1 as Na2O content increased from 0 to 20 ml%. The photon and fast neutron removal cross section of the glasses compared relatively well with standard radiation shields. The glasses are hence, can be used for radiation absorption purposes.

Methodology and algorithm to correct the thermal neutron porosity for the effect of rare elements and rock minerals with high neutron absorption probability

The thermal neutron porosity is routinely acquired in almost every well. When combined with the density, gamma ray and resistivity logs, the basic petrophysical parameters of a reservoir are evaluated. The design of the thermal neutron tool is simple, but its interpretation is complex and affected by the formation constituents. The most challenging situation occurs when the formation contains elements with high absorption probability of the thermal neutrons. The existence of such elements changes the neutron transport parameters and results in a false increase in the measured porosity. The problem is reported by many users throughout the years. In 1993, higher thermal neutron porosity is reported due to the existence of an iron-rich mineral, Siderite, in the Nazzazat and Baharia formations in Egypt. Siderite and all iron-rich minerals have high thermal neutrons absorption probability. Recently, in 2018, high thermal neutron porosity in Unayzah field in Saudi Arabia is also reported due to the existence of few parts per million of gadolinium. Gadolinium is a rare element that has high probability of thermal neutron absorption. Currently, none of the existing commercial petrophysics software(s) have modules to correct the thermal neutron porosity for such effects. This represents a challenge to the petrophysicists to properly calculate the actual reservoir porosity. In this paper, the effects of the rare elements and other minerals with high thermal neutron absorption probability on the thermal neutron porosity are discussed, and a correction methodology is developed and tested. The methodology is based on integrating the tool design and the physics of the neutron transport to perform the correction. The details of the correction steps and the correction algorithm are included, tested and applied in two fields.

Efficiency calculation of the nMCP with 10B doping based on mathematical models

The nMCP (Neutron sensitive microchannel plate) combined with advanced readout electronics is widely used in energy selective neutron imaging because of its good spatial and timing resolution. Neutron detection efficiency is a crucial parameter for the nMCP. In this paper, a mathematical model based on the oblique cylindrical channel and elliptical pore was established to calculate the neutron absorption probability, the escape probability of charged particles and overall detection efficiency of nMCP and analyze the effects of neutron incident position, pore diameter, wall thickness and bias angle. It was shown that when the doping concentration of the nMCP was 10 mol%, the thickness of nMCP was 0.6 mm, the detection efficiency could reach maximum value, about 24% for thermal neutrons if the pore diameter was 6 μm, the wall thickness was 2 μm and the bias angle was 3 or 6°. The calculated results are of great significance for evaluating the detection efficiency of the nMCP. In a subsequent companion paper, the mathematical model would be extended to the case of the spatial resolution and detection efficiency optimization of the coating nMCP.

A stochastic process approach to multilayer neutron detectors

AbstractThe sparsity of the isotope Helium‐3, ongoing since 2009, has initiated a new generation of neutron detectors. One particularly promising development line for detectors is the multilayer gaseous detector. In this paper, a stochastic process approach is used to determine the neutron energy from the additional data afforded by the multilayer nature of these novel detectors. The data from a multilayer detector consist of counts of the number of absorbed neutrons along the sequence of the detector's layers, in which the neutron absorption probability is unknown. We study the maximum likelihood estimator for the intensity and absorption probability and show its consistency and asymptotic normality, as the number of incoming neutrons goes to infinity. We combine these results with known results on the relation between the absorption probability and the wavelength to derive an estimator of the wavelength and to show its consistency and asymptotic normality.

Geant4 Analysis of a Thermal Neutron Real-Time Imaging System

Thermal neutron imaging is a technique for nondestructive testing providing complementary information to X-ray imaging for a wide range of applications in science and engineering. Advancement of electronic imaging systems makes it possible to obtain neutron radiographs in real time. This method requires a scintillator to convert neutrons to optical photons and a charge-coupled device (CCD) camera to detect those photons. Alongside, a well collimated beam which reduces geometrical blurriness, the use of a thin scintillator can improve the spatial resolution significantly. A representative scintillator that has been applied widely for thermal neutron imaging is 6LiF:ZnS (Ag). In this paper, a multiphysics simulation approach for designing thermal neutron imaging system is investigated. The Geant4 code is used to investigate the performance of a thermal neutron imaging system starting with a neutron source and including the production of charged particles and optical photons in the scintillator and their transport for image formation in the detector. The simulation geometry includes the neutron beam collimator and sapphire filter. The 6LiF:ZnS (Ag) scintillator is modeled along with a pixelated detector for image recording. The spatial resolution of the system was obtained as the thickness of the scintillator screen was varied between 50 and $400~\mu \text{m}$ . The results of the simulation were compared to experimental results, including measurements performed using the PULSTAR nuclear reactor imaging beam, showing good agreement. Using the established model, further examination showed that the resolution contribution of the scintillator screen is correlated with its thickness and the range of the neutron absorption reaction products (i.e., the alpha and triton particles). Consequently, thinner screens exhibit improved spatial resolution. However, this will compromise detection efficiency due to the reduced probability of neutron absorption.

Trigger Efficiency of a ZnS:6LiF Scintillation Neutron Detector Readout With a SiPM

The efficiency of detecting signals from neutron absorption events (trigger efficiency) in detectors using ZnS scintillators loaded with neutron absorbers (6Li, 10B) is prone to be limited to values well below 100%: the non-transparency of the scintillator results in a wide dynamic range of the detected signals with some fraction always falling under the detection threshold set to satisfy the requirements on the gamma sensitivity, multi-count ratio, and the background count rate of the detector. The question of the limited trigger efficiency of such detectors is very seldom discussed in publications, yet it is very important as the product of the trigger efficiency and the probability of neutron absorption in the detector defines its overall efficiency. Available data indicate that the trigger efficiency of PMT-based detectors used in neutron diffraction experiments can be as low as $\sim 50$ %. In this work we evaluate the trigger efficiency of a SiPM-based ZnS:6LiF scintillation neutron detector developed to replace a 3He-detector of the POLDI time-of-flight diffractometer at the Paul Scherrer Institut in Switzerland. Requiring the gamma sensitivity with a 60Co source $\le 10^{-7}$ , the multi-count ratio $\le 10^{-3}$ with the detector dead time set to $10~\mu \text {s}$ , and the background count rate $\le 10^{-3}$ Hz we obtain the maximum possible trigger efficiency of $\sim 85$ % with the main restriction coming from the gamma sensitivity condition. Considering the neutron absorption probability of $\sim 80$ % at 1°A, the detector efficiency at this wavelength amounts to $\sim 70$ %, which is comparable to that of the current 3He-detector of the POLDI instrument.

A study on artificial rare earth (RE2O3) based neutron absorber

A new concept of a neutron absorption material (i.e., an artificial rare earth compound) was introduced for criticality control in a spent fuel storage system. In particular, spent nuclear fuels were considered as a potential source of rare earth elements because the nuclear fission of uranium produces a full range of nuclides. It was also found that an artificial rare earth compound (RE2O3) as a High-Level Waste (HLW) was naturally extracted from pyroprocessing technology developed for recovering uranium and transuranic elements (TRU) from spent fuels. In this study, various characteristics (e.g., activity, neutron absorption cross-section) were analyzed for validating the application possibility of this waste compound as a neutron absorption material. As a result, the artificial rare earth compound had a higher neutron absorption probability in the entire energy range, and it can be used for maintaining sub-criticality for more than 40 years on the basis of the neutron absorption capability of Boral™. Therefore, this approach is expected to vastly improve the efficiency of radioactive waste management by simultaneously keeping HLW and spent nuclear fuel in a restricted space.

Evaluation of two thermal neutron detection units consisting of ZnS/6LiF scintillating layers with embedded WLS fibers read out with a SiPM

Two single channel detection units for thermal neutron detection are investigated in a neutron beam. They consist of two ZnS/6LiF scintillating layers sandwiching an array of WLS fibers. The pattern of these units can be repeated laterally and vertically in order to build up a one-dimensional position sensitive multi-channel detector with the needed sensitive surface and with the required neutron absorption probability. The originality of this work arises from the fact that the WLS fibers are read out with SiPMs instead of the traditionally used PMTs or MaPMTs. The signal processing system is based on a photon counting approach. For SiPMs with a dark count rate as high as 0.7MHz, a trigger efficiency of 80% is achieved together with a system background rate lower than 10−3Hz and a dead time of 30μs. No change of performance is observed for neutron count rates of up to 3.6kHz.

Multi-group formulation of the temperature-dependent resonance scattering model and its impact on reactor core parameters

A multi-group formulation for the exact neutron elastic scattering kernel is developed. It incorporates the neutron up-scattering effects stemming from lattice atoms thermal motion and it accounts for them within the resulting effective nuclear cross-section data. The effects pertain essentially to resonant scattering off of heavy nuclei. The formulation, implemented into a standalone code, produces effective nuclear scattering data that are then supplied directly into the DRAGON lattice physics code where the effects on Doppler reactivity and neutron flux are demonstrated. The correct accounting for the crystal lattice effects influences the estimated values for the probability of neutron absorption and scattering, which in turn affect the estimation of core reactivity and burnup characteristics. The results show an increase in values of Doppler temperature feedback coefficients up to −10% for UOX and MOX LWR fuels compared to the corresponding values derived using the traditional asymptotic elastic scattering kernel. This paper also summarizes research performed to date on this topic.

Deterministic modeling of higher angular moments of resonant neutron scattering

An exact scattering kernel formulation for anisotropic scattering up to angular order 10 has been developed and implemented into a deterministic code. The effects of accounting for lattice dynamics on the modeling of neutron scattering in 235U, 238U, 238Pu, and other nuclides have been demonstrated. The new formulation essentially reproduces other investigators previous results for isotropic scattering and quantifies the departures from the isotropic values when higher angular orders are accounted for. The correct accounting for the lattice effects influences the estimated values for the probability of neutron absorption and scattering, which in turn affect the estimation of core reactivity and burnup characteristics. It is shown that, when using the exact scattering kernel formulation, the probability for upscattering significantly increases with increasing temperatures. For example, upscattering for 238U from below the 20.67 eV resonance increases from 5.57% at 300 K to 30.41% at 1000 K, respectively. Thus, it is shown that the exact scattering kernel is strongly sensitive to temperature, a fact of major importance for High Temperature Reactor fuels. The slowing down process is important in thermal reactors because it results in the neutrons entering the thermal energy range in which the majority of fission events occur. Correctly modeling the slowing down and hence slowing down source into the thermal energy range and consequently allowing the correct modeling of the thermal energy neutron fluxes (or the correct thermal range portion of the spectrum) is paramount to the correct prediction of criticality and safety features such as the Doppler effect. These artifacts are important for all thermal spectrum reactors. In High Temperature Reactors such as the NGNP or the Deep Burn HTR these effects are even more important.

Influence of the effective mass on the relative neutron density distribution inside a large sample in prompt-gamma neutron activation analysis

A Monte Carlo study was carried out to determine the influence of the effective scattering mass (M e) of the atoms on the neutron density profile inside and outside the sample illuminated by a thermal neutron beam as in large-sample prompt-gamma neutron activation analysis (LS-PGNAA). From theory it is known that the spatial neutron density distribution (n(r)) inside a large sample is not the same for atoms with the same macroscopic scattering and absorption cross-section (Σ s and Σ a) but different M e, due to anisotropic scattering at low M e. The probability of neutron absorption in the sample was found to be the same for materials with equal Σ s and Σ a but different M e, even though the neutron density distribution in the sample was found to change slightly. In view of typical sample, collimator and detector dimensions, it is concluded that M e does not need to be taken into account in a correction method for neutron self-shielding in LS-PGNAA.

Thermal neutron detection with the lithium borate scintillator

Single crystals of /sup 6/Li/sup 158/Gd(/sup 11/B/sub 3/O)/sub 3/ were grown, then powdered and mixed with powdered Schott glass and epoxy. The dough was solidified in a Teflon mold to a scintillator foil of 3 mm thickness and size 50 mm/spl times/50 mm. By attaching the /sup 6/Li borate foil to a conventional RCA photo multiplier (PM) and to a position sensitive Hamamatsu PM the properties as a scintillator for neutron detection were investigated with neutrons, and rays of high energy betas and gammas. The neutron absorption probability is sufficient large (36% for 0.1 nm wave length neutrons). Practically all neutron events yield sufficiently large pulses. The averaged neutron pulse is 4.8 times larger than that of a /sup 6/Li glass and 0.55 times smaller than that of a /sup 6/Li iodide. The photon energy equivalent to an average neutron pulse is 2.5 MeV (for /sup 6/Li glass and /sup 6/Li iodide the energies are 1.5 and 2.4 MeV, respectively). The spatial resolution obtained with the 45 mm/spl times/55 mm Hamamatsu PM is 0.54 mm FWHM.

Efficiency of neutron-absorbing materials on the basis of cadmium

Measurements for the absorption efficiency of the following cadmium-containing neutron absorber compositions are reported: CdO-ZrO/sub 2/, CdO-CaO, CdS-SiC, and CdS-B/sub 4/C. The efficiency of the materials in tablet form was measured with the uranium-graphite F-1 reactor and in the critical water-water Mayak assembly in the I.V. Kurchatov Institute of Atomic Energy. The efficiency of the cadmium-containing compositions in the F-1 reactor was measured by reactivity compensation. Neutron absorption probability in dependence on the optical thickness of the absorber was calculated using a Wigner approximation.

On the probability of neutron absorption in plane and spherical blocks

On the probability of neutron absorption in plane and spherical blocks

Probability of neutron absorption in a block in the range of unresolved levels

Probability of neutron absorption in a block in the range of unresolved levels

Calculating the efficiency of multi-layer control rods

It is shown that in order to find the effective boundary conditions in the case of multi-layer cylindrical control rods, the probability of neutron absorption for such a rod must be calculated. Two assumptions are used in obtaining the probability of absorption: 1) the angular distribution of the neutrons does not change when they pass repeatedly through any of the internal layers of the rod, and 2) the effect of scattering in each layer of the rod may be evaluated by an approximate method described in [4].

On the Diffusion of Decaying Particles

The probability of neutron absorption on a spherical surface for a radial source in a medium for which the particle has prescribed lifetime and rate of diffusion is calculated. This helps illuminate the more intricate, related problem for a charged particle diffusing in the presence of a radial inverse-square electric field to the extent that an explicit relation for the relative absorption probabilities that accompany changes in the direction of the drift field is secured. An asymptotic solution for the absorption probability is also provided which in first-order approximation may be applied in the range of interest for transistors.