Monte Carlo simulation of Cu,Ni and Fe grade determination in borehole by PGNAA technique

A prompt gamma neutron activation analysis borehole logging method for copper, nickel and iron grade estimation is proposed. The performance of the method was simulated by MCNP5 code. Based on the theory of neutron–gamma distribution on the borehole condition, the BGO scintillator and ³He neutron tube are adopted to record gamma ray spectrum and thermal neutron simultaneously, and least square method is used for the characteristic gamma ray counts calculation in the high energy range. The results of detection limit of metal grade in borehole condition indicate that the effectiveness of this logging method.     

Monte Carlo source simulation technique for solution of interference reactions in INAA experiments: a preliminary report

A new method using Monte Carlo source simulation of interference reactions in neutron activation analysis experiments has been developed. The neutron spectrum at the sample location has been simulated using the Monte Carlo code MCNP and the contributions of different elements to produce a specified gamma line have been determined. The produced response matrix has been used to measure peak areas and the sample masses of the elements of interest. A number of benchmark experiments have been performed and the calculated results verified against known values. The good agreement obtained between the calculated and known values suggests that this technique may be useful for the elimination of interference reactions in neutron activation analysis.     

Neutron conversion efficiency and gamma interference with gadolinium

Gadolinium (Gd) neutron capture plays an important role in both Gd neutron capture therapy and neutron detection. Detailed information about the low-energy electron spectrum emitted after Gd neutron capture is essential for accurately determining the dose delivery to the target and healthy tissues, as well as the effectiveness of Gd against other neutron convertors such as boron (B) and lithium (Li). Owing to issues such as charge extraction associated with the low energy of internal conversion electrons (ICEs) and high gamma interaction of Gd, its competitiveness for certain applications remains debatable. We measured the ICE spectrum of Gd, compared the energy deposition rates of neutron capture products from Gd, B, and Li compounds, and discussed issues associated with gamma sensitivity of Gd.     

Monte Carlo simulation of PGNAA system for determining element content in the rock sample

The element content in rock sample can be determined by prompt gamma ray activation analysis technology. The neutron distributions under the conditions with different moderating materials, moderator size and distance from neutron source to lead-out hole were simulated using Monte Carlo method, and then the optimal structure parameters to get the highest thermal neutron flux was obtained. The PGNAA system with optimal parameters based on ²⁵²Cf neutron source was founded. In addition, the rock and element standard samples were irradiated by thermal neutron in this system. Moreover, the element content was calculated by processing gamma ray spectroscopy recorded, and it is in agreement with result of XRF method.     

Neutron and gamma ray doses from a 252Cf brachytherapy source in a water phantom

Axial and radial doses of Neutrons and gamma rays from an Isotron ²⁵²Cf Brachytherapy source were calculated in a Water phantom using Maienshein’s prompt fission gamma rays data and Maxwellian neutron energy spectrum. It was observed that neutron dose due to the source casing thickness does not contribute significantly to the total dose. Further the calculated secondary gamma ray dose rate is very small compared to the calculated primary gamma dose rate. Neutron and secondary gamma ray dose calculated in this study agree with the published data. Results of this study will be presented here.     

Characterization of a new external neutron beam facility at the Ohio State University

A thermal neutron beam facility has been designed and implemented at the Ohio State University Research Reactor. A project is underway to construct a large vacuum chamber such that the facility could have neutron depth profiling and neutron radiography capabilities as intended. The neutron beam is extracted from the reactor through a neutron collimator emplaced in Beam Port #2. The neutron spectrum entering the neutron collimator was unfolded from foil activation analysis results and also simulated with a full reactor core model in the MCNP Monte Carlo code. The neutron collimator uses polycrystalline bismuth as a gamma ray filter and single-crystal sapphire as a fast neutron filter. The beam is defined by multiple 3.0 cm diameter apertures made of borated aluminum. Characterization of the beam was performed using foil activation to find the flux and a low-budget neutron imaging apparatus to see the beam profile. The modulation transfer function was calculated to offer insight into the resolution of the imaging system and the collimation of the beam. The neutron collimator delivers the filtered thermal neutron beam with a ~4 cm diameter and a thermal equivalent flux of (1.27 ± 0.03) × 10⁷ n/(cm²s) at 450 kW power at the end of the collimator.     

Inversion of simulated positron annihilation lifetime spectrum using a neural network

Inversion of positron annihilation lifetime spectroscopy, based on a neural network Hopfield model, is presented in this paper. From a previous reported density function for lysozyme in water a simulated spectrum, without the superposition of statistical fluctuation and spectrometer resolution effects, was generated. These results were taken as the exact results from which the neural network was trained. The precision of the inverted density function was analyzed taking into account the number of neurons and the learning time of the neural network. A fair agreement was obtained when comparing the neural network results with the exact results. For example, the maximum of the density function, with a precision of 0.4% for the percentual relative error, was obtained for 64 neurons.     

A study on neutron energy spectrum estimation by LaBr3:Ce detector

A novel approach to estimation of neutron energy spectrum by LaBr₃:Ce detector which was usually used for gamma rays measurement was presented. In this approach, energy distribution of the neutron flux was devided into several bins, and simultaneous equations of these bins were setup based on the net counts of gamma peaks induced by neutron inelastic scattering with LaBr₃:Ce detector. With these equations, the neutron energy spectrum was derived by a deconvolution algorithm. This approach was tested using a 3 in.?×?3 in. LaBr₃:Ce detector exposured to an ²⁴¹Am-Be neutron source and proved practicable.

     

Evaluation of an alternative convenient irradiation system for determination of emission rate of radio-isotopic neutron sources

In present work, an alternative irradiation system based on a symmetric cylindrical tank filled with a moderator containing hydrogen, which was equipped with a NaI(Tl) scintillation detector, was proposed for using in determination of neutron flux. This irradiation system was designed by MCNP4C code, with considering a ²⁴¹Am–Be neutron source in several volumes and different materials. When the neutron is captured by hydrogen, a 2.22 MeV prompt gamma-ray is emitted. The gamma pulse-height spectrum shows a photo-peak around 2.22 MeV whose net area is proportional to the total emission rate of neutron. The simulation result showed that a cylindrical tank with 110 cm diameter and height filled with water can be a suitable system for neutron source strength calibration. Furthermore, a proper two-layer shielding must be placed between the source and detector for preventing neutrons and gamma rays to directly enter the detector.     

The McMaster University Nuclear Reactor (MNR) research facilities

The McMaster University Nuclear Reactor (MNR) is a unique Canadian facility in a university environment welcoming researchers from across Canada and abroad. The irradiation and analytical facilities available cover a broad spectrum of nuclear analytical techniques from standard INAA to fully automated counting systems for prompt gamma activation analysis and delayed neutron counting. In addition, neutron beams provide for a wide spectrum of applied and basic nuclear research applications. This paper describes the state-of-the-art facilities available at McMaster.     

Application of simulated standard spectra for the analysis of complex sample spectra from NaI(Tl) detectors

The most accurate method for the analysis of complex gamma ray spectra from scintillation detectors is least squares method. The major requirement of this method is individual standard spectra of all nuclides expected in the complex spectrum which is not possible and feasible for some nuclides. In the present work, an approach of using simulated standard spectrum of the radionuclides for the least squares analysis is studied. The paper describes the methodology used for the generation of simulated spectrum which is the main objective, and validation of results using standard sources in the Sodium Iodide (NaI(Tl)) based gamma ray spectrometer.     

Spectra and dose with ANN of 252Cf, 241Am–Be, and 239Pu–Be

Artificial neural networks have been applied to unfold the neutron spectra and to calculate the effective dose, the ambient equivalent dose, and the personal dose equivalent for ²⁵²Cf, ²⁴¹Am–Be, and ²³⁹Pu–Be neutron sources. The count rates that these neutron sources produce in a Bonner Sphere Spectrometer with a ⁶LiI(Eu) were utilized as input in both artificial neural networks. Spectra and the ambient dose equivalent were also obtained with BUNKIUT code and the UTA4 response matrix. With both procedures spectra and ambient dose equivalent agrees in less than 10%. The Artificial neural network technology is an alternative procedure to unfold neutron spectra and to perform neutron dosimetry.     

Utilizing Distinct Neutron Spectra and a System of Equations to Differentiate Competing Reactions in Activation Analysis

In activation analysis, the continuous neutron spectrum produced in a nuclear reactor can be both beneficial and detrimental. A continuous spectrum allows for activation of particular elements by using any number of neutron energies. However, upper threshold energies exist for most elements where reactions other than an (n, ) absorption will occur. In this situation, two different parent nuclides can be activated to the same product, which creates a problem in performing a quantitative analysis of one or the other nuclide. A methodology is presented where a system of equations is used to solve for the quantities of both nuclides by using different irradiation ports, and using specific neutron filters to "customize" the neutron flux.     

Design, testing and optimization of a neutron radiography system based on a Deuterium–Deuterium (D–D) neutron generator

Simulations show that significant improvement in imaging performance can be achieved through collimator design for thermal and fast neutron radiography with a laboratory neutron generator. The radiography facility used in the measurements and simulations employs a fully high-voltage-shielded, axial D–D neutron generator with a radio frequency driven ion source. The maximum yield of such generators is about 10¹⁰ fast neutrons per seconds (E = 2.45 MeV). Both fast and thermal neutron images were acquired with the generator and a Charge Coupled Devices camera. To shorten the imaging time and decrease the noise from gamma radiation, various collimator designs were proposed and simulated using Monte Carlo N-Particle Transport Code (MCNPX 2.7.0). Design considerations included the choice of material, thickness, position and aperture for the collimator. The simulation results and optimal configurations are presented.     

Study of Doppler broadening of gamma-ray spectra in 14-MeV neutron activation analysis

Neutron reactions producing characteristic photons of isotopes are important for nondestructive analysis of materials. Technique to determine the intensity of neutron induced gamma rays by fitting a spectrum with a Gaussian function using detector resolution curves derived from isotopic sources may fail if the peak is Doppler-broadened. This leads to the miscalculation of the area of the peak and, therefore, to misidentification of the material. This work shows that Doppler broadening occurs in the 14-MeV neutron analysis with photons emitted in inelastic scattering reactions on light nuclei with excited states whose lifetimes are much smaller than the time of flight of a recoiling nucleus in the material. It provides groundwork for analysis of gamma ray spectra utilizing detector response functions measured with a 14-MeV neutron source using actual geometry of an active interrogation system.     

Methods for preparing comparative standards and field samples for neutron activation analysis of soil

One of the more difficult problems associated with comparative neutron activation analysis (CNAA) is the preparation of standards which are tailor-made to the desired irradiation and counting conditions. Frequently, there simply is not a suitable standard available commercially, or the resulting gamma spectrum is convoluted with interferences. In a recent soil analysis project, the need arose for standards which contained about 35 elements. In response, a computer spreadsheet was developed to calculate the appropriate amount of each element so that the resulting gamma spectrum is relatively free of interferences. Incorporated in the program are options for calculating all of the irradiation and counting parameters including activity produced, necessary flux/bombardment time, counting time, and appropriate source-to-detector distance. The result is multi-element standards for CNAA which have optimal concentrations. The program retains ease of use without sacrificing capability. In addition to optimized standard production, a novel soil homogenization technique was developed which is a low cost, highly efficient alternative to commercially available homogenization systems. Comparative neutron activation analysis for large scale projects has been made easier through these advancements. This paper contains details of the design and function of the NAA spreadsheet and innovative sample handling techniques.     

Numerical simulation on scintillator detector response for determining element content in PGNAA system

For selecting gamma ray detector to determining element content using prompt gamma neutron activation analysis technique, the response of BGO and LaBr₃ scintillator detectors was compared with Monte Carlo simulation method. The simulation models under different formation composed of various elements were established by MCNP, and then gamma ray spectrum was processed by least squares method. It is concluded that the element calculation accuracy will be different when scintillator was changed. With higher energy resolution, LaBr₃ scintillator can be used to improve the accuracy of the element content calculation result, and is more suitable for plentiful element formation.

     

Utilization of a mixed-bed column for the removal of iodine from radioactive process waste solutions

A boron carbide capsule was previously designed and tested by Pacific Northwest National Laboratory (PNNL) and Washington State University (WSU) for spectral-tailoring in mixed spectrum reactors. The presented work used this B₄C capsule to create a fission product sample from the irradiation of highly enriched uranium (HEU) with a fast fission neutron spectrum. An HEU foil was irradiated inside of the capsule in WSU’s 1 MW TRIGA reactor at full power for 200 min to produce 5.8 × 10¹³ fissions. After 3 days of cooling, the sample was shipped to PNNL for radiochemical separations and analysis by gamma and beta spectroscopy. Fission yields for products were calculated from the radiometric measurements and compared to measurements from thermal neutron induced fission (analyzed in parallel with the non-thermal sample at PNNL) and published evaluated fast-pooled and thermal nuclear data. Reactor dosimetry measurements were also completed to fully characterize the neutron spectrum and total fluence of the irradiation.     

Instrumental multielement activation analysis of soil samples

Soil samples from a waste water cleaning facility in Berlin, Germany, have been analyzed using several activation analysis methods. 43 elements have been determined by instrumental high energy photon activation analysis (PAA), instrumental thermal neutron activation analysis (NAA) and 14 MeV neutron activation analysis (fNAA). Conventional gamma ray spectroscopy and low energy photon spectroscopy (LEPS) have been applied for product activity measurement. It has turned out that these methods in combination offer a wide spectrum of analytical information.     

Photoneutron spectra around an 18 MV LINAC

Photoneutron spectra around an 18 MV LINAC were calculated in order to observe the effect produced by media around the accelerator. Calculations were carried out with MCNP 4C code, three different cases were analyzed: Head model, Head and phantom model, and Head, air, phantom and wall model. The spectra were calculated in five detectors located at the irradiation room at different distances from the isocentre. A sixth detector, located near the entrance door was included to analyze how the maze change the neutron spectrum. Neutrons are mainly produced in the LINAC head change the shape of evaporation neutrons from the source term, some of these neutrons leak out the head with lesser energy, another neutrons goes with the treatment beam. At any site near the isocentre neutron spectrum has evaporation and thermal neutrons joined by a set of epithermal neutrons. As the distance from the isocentre increases evaporation neutrons tend to decrease while, epithermal and thermal neutrons tend to remain constant regardless de distance due to room return produced by the walls. The maze contributes to reduce the neutron fluence, reducing the evaporation neutrons; resulting spectrum is mainly the contribution of thermal and epithermal neutrons. Near the door these neutrons can produce activation and prompt gamma rays.