Capture Gamma-ray Spectroscopy Research Articles

Impact of the ENDF/B-VIII.0 library on modeling nuclear tools for oil exploration

In the oil field, exploration of the subsurface through well logging provides measurements of the characteristics of rock formations and fluids to help identify and evaluate potential reservoirs. Downhole nuclear measurements focus on formation properties such as natural radioactivity, formation density, and hydrogen content, as well as the identification of the elemental and mineralogical composition of the rock through spectroscopy. Accurate nuclear modeling is a fundamental part of nuclear well logging tool development, from concept through design to response characterization. Underlying the accuracy of nuclear modeling is a good knowledge of nuclear cross sections of the elements in the tool, borehole, and subsurface formations. The recent focus on replacing tools based on radio-isotopic sources with those based on D-T neutron generators opens many opportunities for new measurements but highlights the deficiencies of current cross sections. For example, in neutron-induced inelastic and capture gamma ray spectroscopy, major obstacles come from a lack of or inaccuracies in the cross sections of essential materials.

Designing tools for oil exploration using nuclear modeling

When designing nuclear tools for oil exploration, one of the first steps is typically nuclear modeling for concept evaluation and initial characterization. Having an accurate model, including the availability of accurate cross sections, is essential to reduce or avoid time consuming and costly design iterations. During tool response characterization, modeling is benchmarked with experimental data and then used to complement and to expand the database to make it more detailed and inclusive of more measurement environments which are difficult or impossible to reproduce in the laboratory. We present comparisons of our modeling results obtained using the ENDF/B-VI and ENDF/B-VII cross section data bases, focusing on the response to a few elements found in the tool, borehole and subsurface formation. For neutron-induced inelastic and capture gamma ray spectroscopy, major obstacles may be caused by missing or inaccurate cross sections for essential materials. We show examples of the benchmarking of modeling results against experimental data obtained during tool characterization and discuss observed discrepancies.

Exploring for oil with nuclear physics

Oil↓eld service companies help identify and assess reserves and future production for oil and gas reservoirs, by providing petrophysical information on rock formations. Some parameters of interest are the fraction of pore space in the rock, the quantity of oil or gas contained in the pores, the lithology or composition of the rock matrix, and the ease with which 'uids 'ow through the rock, i.e. its permeability. Downhole logging tools acquire various measurements based on electromagnetic, acoustic, magnetic resonance and nuclear physics to determine properties of the subsurface formation surrounding the wellbore. This introduction to nuclear measurements applied in the oil and gas industry reviews the most advanced nuclear measurements currently in use, including capture and inelastic gamma ray spectroscopy, neutron-gamma density, thermal neutron capture cross section, natural gamma ray, gamma-gamma density, and neutron porosity. A brief description of the technical challenges associated with deploying nuclear technology in the extreme environmental conditions of an oil well is also presented.

Inversion-based interpretation of logging-while-drilling gamma-ray spectroscopy measurements

Neutron-induced spectroscopy measurements are commonly used to quantify in situ elemental compositions of rocks from the processing of measured gamma-ray energy spectra. However, geometric effects on the measured spectroscopy logs, such as thin beds, dipping beds, and deviated well trajectories, can cause shoulder-bed averaging that compromises the assessment of the true layer elemental composition. We have developed an inversion-based interpretation method to evaluate layer elemental compositions from spectroscopy measurements acquired with a commercial 14-MeV pulsed-neutron logging-while-drilling spectroscopy tool. The algorithm is based on a new spectroscopy fast-forward simulation technique, and it estimates layer-by-layer elemental relative yields, weight concentrations, and their uncertainties. Calculations are performed with inelastic and capture gamma-ray spectroscopy measurements that arose from high- and low-energy neutron interactions, respectively. This strategy provides two sets of data that independently validate estimated elemental compositions and can ascertain chemical elements present in only one measurement mode. In laminated formations in which layer thicknesses are appreciably below the vertical resolution of the tool, it is impossible to quantify layer properties with inversion methods. We have therefore developed an additional interpretation method based on a spectroscopy mixing law to estimate elemental compositions within individual laminae. The new inversion-based interpretation methods were successfully verified with two challenging synthetic cases and implemented in two field cases with varying lithology and well trajectories. Our results found that the developed methods reduced shoulder-bed averaging effects on the measured spectroscopy logs by as much as a 0.4 yield fraction and a 0.17 weight fraction. Estimated elemental compositions with reduced shoulder-bed averaging effects improved the calculations in subsequent spectroscopy-based petrophysical interpretation.

Improvement of SNM Detection Performance by Fusion of Data From Multiple Inspection Systems

Data fusion has the potential to improve the performance of multiple systems over algorithms involving only Boolean combinations of decisions from the separate systems. To achieve this improvement, it is essential to utilize the complementary aspects of the systems involved. In this paper, we demonstrate that data fusion of x-ray radiography and neutron capture gamma ray spectroscopy provide complementary data to differential die away analysis, and thereby a substantial improvement in both detection performance and throughput.

A New Formation-Evaluation Technique for the Lower Tertiary in South Texas—Predicting Production in Low-Permeability, Fine-Grained Sandstones

Summary The lower Tertiary Wilcox, Yegua, and Vicksburg formations are prolific natural gas plays in south Texas that have been extensively drilled and produced from low-permeability, fine-grained sandstones. It is not unusual to encounter several potential pay zones in each well. What is lacking is a proven formation-evaluation method to determine the highest-productivity water-free producing zones from multiple intervals that can be casually identified on the logs as hydrocarbon-bearing. Connate-water resistivity (Rw) determination is not a major problem, given that many water-bearing sands are usually present. Accurately estimating true clay content, porosity, irreducible water volumes, and permeability present the greatest challenges. A new spectroscopy-based petrophysical interpretation methodology has been developed that makes use of several unique measurements, namely quantitative elemental concentrations and lithology logs obtained from capture gamma ray spectroscopy devices in open or cased wells. These measurements allow us to more accurately define the clay content, mineralogy, and matrix properties of each potential zone. One significant finding obtained from these measurements is the occurrence of calcite cements detected in many sands that cause a pessimistic density porosity to be computed if not accounted for. This calcite cementation appears to vary dramatically in a lateral sense, indicating that its presence should not be used to condemn an entire layer as being too tight for production, nor provide irrefutable evidence of the expected ability of the layer to contain a hydraulic fracture. The enhanced elemental and mineralogical analyses provided by the spectroscopy measurements also allow for more accurate bulk volume irreducible water calculations and a means to correct the neutron porosity for clay and matrix effects. By comparing irreducible water volume to bulk volume hydrocarbon, an accurate prediction of water production can be determined. Oil- and gas-bearing intervals can be identified easily from crossover of the matrix-corrected neutron porosity when used in conjunction with the matrix-corrected density porosity. Results are illustrated with several case studies from wells recently drilled and now producing from the Wilcox, Yegua, and Vicksburg formations.

Nuclear spectroscopy with slow neutrons

The aim of nuclear spectroscopy is to study nuclear excitations with a view to improving our knowledge of the forces play within the nucleus. Many complementary techniques have been developed to unravel the complex interplay of these forces and it is usually the combination of several experimental approaches such as the study of radioactive decay, transfer reactions, neutron capture, inelastic scattering, Coulomb excitation and other charged particle reactions which allows us to construct detailed level schemes and interpret them. We will concentrate, within the frame of this Workshop on the advanced instrumentation of the neutron capture gamma-ray spectroscopy, which plays an important and specific role in the comprehensive testing of nuclear models. Neutron capture gammaray spectroscopy now employs very sophisticated techniques to study nucear structure. The combination of precise measurements of primary gamma transitions with the precision obtained to data with curved and flat crystal and conversion electron spectrometers has considerably extended the possibilities in the study of nuclear structure. The nearly perfect resolution obtained with the most modern double flat crystal spectrometers allows the measurement of lifetimes of excited nuclear states in the range < 10−12 s via the observation of the Doppler broadening of gamma rays due to the recoil of the nucleus following emission of a primary gamma ray.

Capture gamma rays and related topics

Abstract The Eighth International Symposium on Capture Gamma-Ray Spectroscopy and RelatedTopics was held inFribourg, Switzerland, during 20–24 September 1993. The series dates back to 1969 when the first meeting was held in Studsvik, Sweden.

The dependence of elemental yield variance on detector type through mathematical modeling

The impact of detector choice on the elemental variance in the yields for capture gamma-ray spectrometry is studied assuming identical external environments and neutron source output. The tradeoff between lower sensitivity but moderate-resolution detectors, such as NaI and CsI, and the higher-sensitivity but lower-resolution detectors, such as GSO (gadolinium silicate) and BGO (bismuth germanate), is quantitatively assessed. Several common well logging environments are modeled with the Monte Carlo code MCNP and convolved with detector response maps for each detector type. The elemental standards for the environmental cases are computed, and the standard weighted-least-squares fitting procedure is applied. Five elements (H, Si, Ca, Cl, and Fe) are assumed present. The variance-covariance matrix from this procedure provides the uncertainty in the elemental yields. Laboratory results obtained using a BGO detector benchmarked the modeling. >

Analysis of some Elements in Fuel Oil by using Neutron Capture Gamma Ray Spectroscopy

The analysis of some elements in fuel oil was done by using neutron capture gamma ray spectroscopy. The gamma rays emitted after the reaction of (n, γ) from the fuel oil atoms bombarded with the thermal neutrons emitted from a 252Cf source were investigated in the energy interval 0 … 10 MeV. The impurity elements in the sample were determined.

53Cr(n, γ); Transition energies and levels excited in thermal neutron capture

The nuclear level structure of 54Cr was studied by thermal-neutron capture gamma-ray spectroscopy of an enriched sample of 53Cr and compared with a natural chromium sample by means of a Ge(Li) spectrometer, using an external beam at the SAFARI-1 reactor. Eighty of the ninety observed transitions allocated to 54Cr (as compared with the previously observed total of 42) could be placed in a level and decay scheme, representing more than 99% of the total observed intensity. The energy precision of several levels observed in other reactions was improved and limits were imposed on their spins. The sensitivity of the experiment allowed the observation of low-lying levels up to spin 4. A new level is proposed at 3925.5 keV, close to the known level of 3927.6 keV. Improved gamma branching ratios were obtained for a number of levels. The neutron binding energy was found to be in agreement with previous determinations.

Methods and new techniques in neutron capture gamma-ray spectroscopy

Neutron capture gamma -ray spectroscopy now employs the most sophisticated techniques to study nuclear structure, yet efforts are continuing to improve still further sensitivity and resolving power of the spectrometers used. The combination of precise measurements of primary gamma -transitions with the precision obtained to date with curved and flat crystal and conversion electron spectrometers has considerably extended the possibilities in the study of nuclear structure. Techniques are also available for the investigation of multiple neutron capture leading to nuclei which are up to three neutrons beyond stability. The nearly perfect resolution obtained with the most modern double flat crystal spectrometers allow the measurement of lifetimes of excited nuclear states in the range <10-12 s via the observation of the Doppler-broadening of gamma -rays due to the recoil of the nucleus following emission of a primary gamma -ray.

Some recent developments in interacting boson models

Neutron capture gamma ray spectroscopy has been a very powerful method for the study of nuclear structure. In particular the nucleus 168Er has been studied very precisely and almost complete information has been provided. The author takes 168Er to investigate how far the interacting boson model can go in explaining the structure of 168Er. The necessity of using hexadecapole (g) bosons is pointed out. The author discusses briefly the results of the spdf-boson model obtained by Otsuka and Sugita (1988). Their results are very promising for explaining the structure of actinide nuclei.

Boron determination by thermal neutron capture prompt gamma analysis

One of the long-standing applications of neutron capture gamma ray spectrometry is the determination of boron concentrations by in-beam prompt gamma analysis utilizing thermal neutrons. In this, the matrix-dependent shape variability of the Doppler-broadened gamma line has previously not been taken into account. The method described here recognizes the basic symmetry of the line and is of special significance in the frequent case of sodium interference, when it allows a better quantitative separation of the boron and sodium prompt gamma peaks.

Prompt gamma-ray neutron activation analysis facility testing by Sm, Gd and Mn determination in rock samples

The neutron capture gamma-ray spectroscopy facility assembled at the Institute of Radiochemistry, KfK (for analytical purposes) using a252Cf neutron source with a strength of 6·107 n/s, has been used to check its applicability and sensitivity for quantitative analyses of ores. The analysis of Sm, Cd and Mn in phosphate and monazite rock samples has been carried out. The results from this study show a variation of about 25% from the values determined by RNAA method. This discrepancy could be mainly due to the low signal-to-background ratio observed which is caused by (i) scattering of the source gammarays by the target, and (ii) interference from the 2223.1 keV neutron capture hydrogen gamma-rays produced by the moderated materials and from their compton scattering in the detector. To overcome these difficulties we suggest to introduce a 2.5 cm thick polyethylene sheet between the detector6Li-cap shielding and the target as well as to increase the detection solid angle. Also the strength of the252Cf neutron source should be increased by an order of magnitude and the neutron beam should be collimated to obtain the optimal thermal neutron flux with a low level of252Cf gamma-rays. This can be achieved by setting up between the neutron source and the target a conical polyethylene collimator with a thickness of 10 cm containing a 1 cm thick lead sheet.

In-Situ Elemental Analysis of Coal and Strategic Metals by Neutron Activation: ABSTRACT

Starting in 1969, the U.S. Geological Survey (USGS) developed neutron techniques for borehole measurement of the elemental composition of ores, and it successfully made a borehole ultimate analysis of coal in 1977. Borehole measurements permit real-time evaluation of ore quality without the expense of coring or the delays and expense associated with laboratory analyses. Two technological innovations have made such measurements possible: the availability of small californium-252 fission neutron sources from the Savannah River Operations Office of the Department of Energy, and the development, by USGS and Princeton Gamma-Tech, of the melting-cryogen-cooled high-purity germanium borehole gamma-ray detector. A technique of relating mass fractions to measured gamma-ray intensi ies, which eliminates the need for detailed knowledge of the geometry of the neutron distribution, was used to calculate elemental compositions without using test pits or computer borehole modeling. Most of the common elements in the earth's crust can be detected by neutron techniques. In coal all of the major constituents except oxygen (C, H, N, S, Si, Al, Fe, Ti) can be determined quantitatively by thermal neutron capture gamma-ray spectroscopy. The latest innovation in this field is the replacement of the 252Cf neutron source with a neutron generator, a type of ion accelerator. These generators, used for many years by the petroleum logging industry, produce neutrons having an energy of 14 MeV. The neutron generator is a safer tool than californium, because no radiation is e itted by the device until it is turned on in the borehole. Coupling a neutron generator with a high-resolution detector to form a borehole measuring system was pioneered by workers at Sandia National Laboratories. USGS has built and put into service one neutron generator based on the Sandia design, and now is building a second. This new device enables the experimenter to use higher energy (n,n^prime), (n,p), (n,2n), and (n,^agr) reactions as well as the (n,^ggr) thermal neutron capture reaction. Both the (n,n^prime) and the (n,p) reactions on 16O permit quantitative measurement of oxygen, and the inelastic scattering excitation of carbon in coal provides increased sensitivity over that of the (n,^ggr) reaction. Reactions caused by 14 MeV-neutron irradiation that are used in ex loration for strategic metals such as Cr, Ni, Mn, V, Co, Ti, and W are tabulated. End_of_Article - Last_Page 1442------------

The application of thermal neutron capture gamma rays to matrix identification in a transuranic waste assay system based on an electron accelerator

A Ge(Li) detector with a preamplifier modified to handle high count rates was used on-line with an electron linear accelerator for thermal neutron capture gamma-ray spectroscopy. The measurements identified seven potential matrix constituents in bulk containers of transuranic waste. This information is needed to make necessary corrections for matrix attenuation and absorption effects on neutron and photon assay results.

Analytical applications of neutron capture gamma-ray spectroscopy

Prompt gamma rays from thermally induced nuclear reactions have been used to estimate the boron, chlorine and phosphorus contents in industrial and reference materials. A neutron capture gamma-ray spectroscopy facility for analytical purposes using 252Cf sources has been designed and calibrated and is described in this paper. The facility is principally designed for measurement of the prompt gamma-ray spectra obtained due to thermal neutron capture using the internal target geometry. The capture spectra were recorded using a high resolution Ge(Li) system. Sensitivity curves using different standard concentration values of B, Cl and P are presented.

Application of a 14 MeV Neutron Source to the Detection of Special Nuclear Material Diversion

Some special nuclear material (SNM) is seriously vulnerable to diversion if currently used safeguards methods are applied to an unattended material/equipment pass-through. Without visual inspection, radiation shielding can be used to defeat direct detection of characteristic 235U radiation. Plutonium-239 is much less vulnerable to concealment by radiation shielding due to the prompt radiation from 240Pu which accompanies 239Pu in small amounts. An active neutron technique can be used to combine direct SNM detection and neutron shield detection to limit the amount of shielding to conceal SNM. While this technique is not presently adequate to meet the desired SNM detection threshold of 20g, an additional detection option of neutron capture gamma-ray spectroscopy is being studied.

Laboratory and Field Evaluation of an Inelastic Neutron Scattering and Capture Gamma Ray Spectrometry Tool

Abstract An experimental system for gamma ray spectroscopy logging has been developed which uses prompt and capture gamma radiation induced in formations by 14-MeV neutrons from a pulsed-neutron generator to determine relative concentrations of various elements in the formation. The logging system uses computer-processing techniques based on spectral modeling that has been developed to analyze the inelastic and capture gamma ray data obtained with a borehole spectrometer. The physics of the production of gamma rays from fast-neutron interactions with elemental nuclei in formations is discussed, leading to a simple but realistic interpretation model for the tool's response. This model is confirmed by laboratory and field tests. The relative spectroscopic contributions from carbon, oxygen, silicon, calcium, iron, chlorine, and hydrogen are used for various cased-hole and openhole logging applications. Particular emphasis is placed on the carbon/oxygen ratio used to obtain oil saturation independent of formation-water salinity. Carbon/oxygen ratio determinations made in the laboratory are compared with values predicted on the basis of known lithologies, porosities, and oil-saturation changes. In addition, the spectral contributions from iron, silicon, and calcium are used to interpret lithology; hydrogen, silicon, and calcium contributions are used to determine the effects of porosity; and chlorine and hydrogen contributions are used to investigate salinity changes. Field-test log examples of these elemental determinations are shown.