Thermal Neutron Absorption Cross Section Research Articles

Statistical uncertainties of elemental concentrations extracted from neutron-induced gamma-ray measurements

The uncertainties in elemental concentrations obtained from borehole thermal neutron capture measurements have been determined, when the normalization to absolute concentrations is by an associated element closure model that accounts for unmeasured elements in the rock. Typical uncertainties under normal logging conditions are between 2 and 3 wt.% for Ca and Si, an little over 1 wt.% for S, a few tenths of a weight percent for Fe and Ti, and about 0.2 p.p.m. for Gd. Adverse geophysical effects, large effective tool standoff, and large borehole thermal neutron absorption cross section increase the elemental concentration uncertainties because of an increase in the number of borehole gamma rays detected. Acceptable uncertainties can be obtained by slower logging speed or multiple passes. Regardless of geophysical parameter values, correct elemental concentrations are obtained within the measurement statistics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Texture Investigations by Neutron Time‐of‐Flight Diffraction

For the majority of isotopes the thermal neutron absorption cross section is two or more orders lower than that for X‐rays. This makes neutron diffraction well‐suited for bulk texture investigations. Some characteristics of neutron diffraction are discussed. The principles of neutron time‐of‐flight diffraction are described. The pole figure determination by means of TOF technique is considered. The main parameters of the present Dubna texture facility are given. Further developments of the experimental technique are considered. The application of the TOF technique for inverse pole figure measurement is discussed as an approach to direct observation of the texture forming process. The magnetic moments of neutrons can be used to study magnetic textures. Two different techniques are discussed.

Measurement of neutron absorption in geological material

The steady-state neutron-source method is applied to measure the thermal neutron absorption cross section of geological samples. On standard rocks the technique gives results consistent with previous studies. Measurements of North Sea sediments with high contents of clay minerals and large absorption cross sections are also performed, and demonstrate that such materials are within the range where the steady-state neutron-source method may be safely employed.

Determination of the thermal neutron absorption cross-section of small geological samples

A steady-state neutron source technique for determining the thermal neutron absorption cross section of geological samples is presented. It needs for analysis about 340 cm 3 of crushed material. No sample saturation is required. Variations of scattering properties of samples are monitored and taken into account in the interpretation procedure. Calibration of the system is achieved using a few calibration samples prepared from pure reactor graphite and boric acid. A total time of analysis is well below 2 h per sample. Uncertainty of the cross section determination with this technique has been estimated at less than 5%. Measured cross sections for several kinds of rocks have been compared with values obtained with other methods.

On the accuracy of a new method for measurements of thermal neutron absorption cross sections

The interpretation of thermal neutron absorption cross section measurements with Czubek's two-medium method has so far only been carried out with diffusion theory. In order to investigate the validity of this approximation, a two-medium plane case has been studied in transport theory for neutrons of one speed. The resulting coupled integral equations were solved by an iterative method. The deviation between the transport theory and diffusion theory results is less than one per cent for the thicker reflectors, increasing to about 1.5% for the thinner ones. The results suggest that the size of the inner region in the measurements is adequate. However, for the thinnest reflectors used, a correction may be necessary if high accuracy is striven for.

Simultaneous determination of hafnium and zirconium in low grade uranium ores using INAA

A non-destructive neutron activation analysis technique has been developed for the determination of hafnium and zirconium in low grade uranium ores. In order to calculate the fission contribution of235U, thermal neutron absorption cross-section /σa/ for94Zr has been determined. The study shows that 1 g of uranium produces the same activity as from 10.03 g of zirconium. Based on this fact, the degree of interferences have been calculated for each sample and the necessary corrections have been applied. The values have been compared with the reported IAEA and NBS values.

Neutron moisture meter : the dependence of their response on soil parameters

A multigroup diffusion theory calculation based on a nuclear reactor neutronics code is used to determine the response of a neutron moisture meter to changes in soil parameters such as dry soil density, soil water content, thermal neutron absorption cross-section and neutron scattering crosssection. Empirical equations which fit the results can be used to estimate the response at values of the soil parameters other than those used in the calculations. These equations can also be used to estimate the accuracy with which the parameters must be known to achieve a required accuracy in the derived soil water content.

The thermal-neutron absorption cross-sections of Mn and S

In a recent note a new value for the thermal-neutron capture cross-section of Mn was reported with significantly reduced uncertainty. The value was based on ratios of the cross-sections of H and Mn measured by Mn-bath dilution techniques. Recent measurements of the absorption cross-section of S, and of its ratio to that of H permit a simultaneous evaluation of the thermal-neutron cross-sections of H, S and Mn. The Mn cross-section remains unchanged at 13.41 barn, and a value of 535 ± 6 mbarn is recommended for S.

Determination of the thermal neutron absorption cross section of 1-liter rock samples by a pulsed source technique

A method is presented for measuring the thermal neutron absorption cross section of geological rock samples. Neutrons from a pulsed 150 kV ion accelerator are thermalized in a 1 1 sample surrounded by a water reflector. The water is in a cylinder concentric with the sample, and it aids in reducing neutron leakage from the system. A He-3 detector inserted in the sample monitors the thermal neutron die-away. The thermal neutron cross section is calculated from the die-away rate using diffusion theory in a fundamental mode analysis for the concentric cylinder geometry of the sample and water reflector.

Determining the thermal neutron absorption cross section of rocks

A new simple rapid and cost effective method of determining the thermal neutron absorption cross section of rocks is proposed. It uses samples of about 2 kg and a measuring set-up consisting of paraffin block, PuBe neutron source emitting roughly 5 × 10 5 n/s, BF 3 proportional counter and conventional counting equipment. The method has been tested with a set of 49 artificial samples and 95% confidence intervals of about 0.27 m −1 have been estimated for 2 × 300 s counting times. Measurements carried out for several basalt samples have been compared with results obtained by a pulsed neutron technique.

Time-Derived Sigma for Pulsed Neutron Capture Logging

Summary This paper reviews field applications of the Dresser Atlas Neutron Lifetime Log™ (NLL) service and examines recent technical advances in the instrumentation and data-analysis system. The major improvement discussed is a new method for computing S, the thermal neutron capture cross section of an earth formation. In the new method, the time after the neutron burst is measured for each gamma ray pulse detected by the instrumentation system within a gate of fixed width. This "average pulse time" is uniquely related to the thermal neutron decay rate observed in a borehole environment. The technique discussed is applicable for any condition where the neutron or gamma ray flux is time-dependent. The advantages of this signal-processing method, however, are most apparent for cases of rapid flux change with time, as in an exponential decay. A simple two-gate method of measurement suffers, statistically, from the lower count rate of the second gate. An average-pulse-time measurement over the same range covered by both gates of a two-gate system is less affected by statistical deviations than the ratio normally formed between the two gates. Some important advantages result from this type of signal processing when applied to the NLL service. First, the statistical accuracy of the log is improved; second, the instrumentation required for implementation of the method is not complicated. Also, the average-pulse-time measurement can be made with either digital or analog equipment, and since it involves only simple arithmetic computations, the method is much simpler to implement than either least-squares or correlation methods. Introduction The NLL service1 was introduced in the early 1960's; it was the first measurement through casing that could distinguish and quantitatively evaluate oil and gas in saline formation water environments. More recently, the Dual Detector Neutron Lifetime Log (DNLL) service2 has been used (1) to explore for hydrocarbon resources behind pipe, (2) to monitor the production behavior of wells, (3) to plan workovers properly, (4) to determine residual oil saturation for enhanced-recovery projects, etc. Although the carbon/oxygen3 (C/O) log provides a reliable cased-hole formation evaluation in fresh and brackish formation water environments, the DNLL service is applicable when logging through casing and/or tubing in saline environments. As discussed in detail in the literature, the basic measurement is the thermal neutron absorption (capture) cross section, S, of a subsurface formation. Advanced methods of processing the data recorded by such a pulsed neutron capture logging system are presently under development. These improvements include (1) the average-time technique4 and (2) the sigma-dependent positioning of a single gate exhibiting a fixed gate width of 600 µseconds. The basic theoretical considerations of these improvements are discussed, and supporting field data are presented.

Solution of the thermal neutron diffusion equation for a two-region system by perturbation calculation

The paper presents a method of solving the diffusion equation for the thermal neutron flux in a heterogeneous medium. Perturbation calculation is successfully applied for a cylindrical concentric system after testing this method for a spherical concentric geometry, analytically solved by Czubek (1981). The method makes possible the calculation of the thermal neutron decay constant and the space distribution of the thermal neutron flux in a two-region system. The condition of a constant value of the neutron flux in the inner part of the system must be met in this method. This method has an application in the measurement of the thermal neutron absorption cross-section (Czubek 1981).

Thermal neutron absorption cross-section for small samples: experiments in spherical geometry

A new method of determination of the thermal neutron macroscopic absorption cross-section Sigma a for small samples has been checked experimentally. The measurements have been performed with spherical geometry by the pulsed method. Two samples (aqueous solutions of boric acid) with known values of the absorption cross-section have been used. Good agreement of the measured values of Sigma a with the known ones has been observed.

A method for measurement of thermal neutron absorption cross-section in small samples

Theoretical principles of a method for measuring the absorption cross-section for thermal neutrons are presented in a one-velocity approach. In consecutive measurements the sample investigated is enveloped in shells of a known moderator of varying thickness and irradiated with a pulsed beam of fast neutrons. The die-away rate of thermal neutrons escaping from such a system is measured. The absorption cross-section of the unknown sample is found as the intersection of the experimental curve (die-away rate versus thickness of the moderator) with the theoretical one, calculated for the case when material buckling of the sample is zero.

Considerations for a cold moderator on a pulsed neutron source

The purpose of a cooled moderator on a pulsed neutron source may be to extend to longer wavelengths the neutron slowing down range with its narrow pulse widths, or to enhance the long wavelength intensity. The intensity and pulse length as a function of wavelength are determined by the rate of energy exchange, and by leakage and capture of neutrons. Energy deposition and radiation damage present serious problems, and restrict the choice of moderators. Radiation damage, in particular, may limit the lifetime and stability of the moderator, giving rise to problems of corrosion and, in the case of solid moderators, stored energy release. The advantages of solid ammonia are its high hydrogen atom density, and its thermal neutron absorption cross-section which allows the moderator to act as its own homogeneous poison. On the other hand this impairs its use for cold neutron production in those instances where neutron intensity rather than burst width is important. In such cases solid methane is favourable, though the radiation damage problem for methane at high intensity sources is severe, and liquid hydrogen is indicated.

A Study of the Effects of Diffusion on Pulsed Neutron Capture Logs(includes associated papers 7670 and 7901 )

The response of the Neutron Lifetime Log TM is studied in viewof the diffusion of thermal neutrons in a wellbore geometry. Qualitative theoretical arguments are given for the diffusioneffects of a log-inject-log (LIL) procedure. Experimentalverification that diffusion exactly cancels out the LILprocedure is presented. Introduction One application of pulsed neutron logging has been todistinguish qualitatively among gas-, oil- and saltwaterbearing formations. The parameter ordinarily measuredis the macroscopic, thermal neutron-absorptioncrosssection, which is sensitive to the chlorinecontent of the formation and can sense the saltwatersaturation of reservoir rock. A particularly usefuladvantage of pulsed neutron logs is that they can measurecased wells quantitatively. One presumed theoreticaldeficiency has been much discussed and contended. Neutron diffusion, a well known physical phenomenon, has been recognized or at least proposed - as a possiblesource of error when measuring cased wells. Certainlythis phenomenon sullies the otherwise simple relationship between the inherent nuclear properties of a formationand, measured by commercial instruments. This expectedinfluence of thermal neutron diffusion on the NeutronLifetime Log TM (NLL) for a particular case was investigated experimentally and is discussed in the first part of thispaper. Another application of pulsed neutron logs hasreceived considerable attention in recent years.Various people have tried to use pulsed neutron capture logs in alog-inject-log (LIL) procedure to ascertain residual oil saturation quantitatively in formations that have beendepleted by secondary recovery operations. It has beenargued that the effect of neutron diffusion on themeasured must be accounted for to attain the limitingaccuracy of the method. On the other hand, it has beenrecognized that the LIL procedure involves the comparisonof two separate measurements, each of which is influenced equally by diffusion effects (if any exist). This beingthe case, the net result of neutron diffusion is zero.Demonstration of this cancellation is verifiedexperimentally and discussed in the last pan of our paper. Theory of Measurement Thermal neutrons are not detected directly in the systemthat produces the NLL. Instead, gamma rays with energygreater than 2.2 million electron volts (MeV) aremeasured. This procedure effectively samples the thermal neutron population in the formations surrounding theborehole. The NLL instrument emits cyclical bursts of14-MeV neutrons, which suffer subsequent elastic andinelastic collisions and slow down to thermal velocities. Since protons are the best moderators of fast neutrons, any fluid-filled porosity results in a rapid slowdown.Typical sand formations saturated with water haveslowdown times of 10 to 20 seconds for 14-MeV neutrons. Thus, waiting an appropriate time after the fast neutronburst assures that observed gamma rays are caused bythermal neutron reactions. The detection of gamma rays generally is conceded tobe superior to neutron detection. This advantage isprimarily because most gamma rays can penetrate fartherto reach the detector than thermal neutrons. Furthermore, gamma rays travel with the speed of light, while neutronsdiffuse slowly, especially through the highly absorbingborehole fluids that come between the formation and thedetector. JPT P. 1788^

Thermal-Neutron Capture and Fission Cross Sections and Resonance Integrals for Curium-243

The thermal-neutron capture cross section and capture resonance integral for 243Cm have been measured in cobalt-monitored reactor irradiations of nearly isotopically pure 243Cm (99.94%) using cadmium filter techniques. The thermal-neutron absorption (capture plus fission) cross section and corresponding resonance integral for 243Cm have also been measured in a separate experiment relative to 248Cm. Values of σ2200(n, γ) = 130.7 ± 9.6, σ2200(n, f) = 609.6 ± 25.9, I(n, γ) = 214.7 ± 20.3, and I(n, f) = 1575 ± 136 b are reported.

ASTM Standardization Activities in the Fracture Testing of Beryllium

Abstract Interest in beryllium as a structural material stems from its low density (about one-fourth that of steel) and its high modulus (about 1.4 times that of steel). Beryllium also has a very low thermal neutron absorption cross section which makes it vattractive for use as a moderator or reflector in spacecraft propulsion systems employing nuclear reactors. However, structural applications have been limited because of beryllium's low “ductility.”

Diffusion and absorption parameters of heavy water

A pulsed-neutron experiment has been carried out on large systems of reactor grade heavy water. From the infinite medium decay constant the thermal neutron absorption cross-section of pure D 2O is estimated to be 1·274 ± 0·10 mb, and that for deuterium 0·485 ± 0·065 mb. The diffusion coefficient of pure D 2O was found to be 0·824 ± 0·0025 cm, in reasonable agreement with other measurements. The experiment was confined to large systems, so that it was not possible to estimate accurately the diffusion cooling coefficient: the value obtained was (12·9 ± 2·7) × 10 5 cm 4 sec −1, significantly greater than most previous values. An examination of all the experimental evidence suggests that the discrepancy may be due to the shape of the moderator.

Measuring the mean lifetime of thermal neutrons from a small specimen

The author describes a method of measuring the mean lifetime τ of thermal neutrons for specimens whose dimensions a r e commensurate with the thermal transport length. The method makes use of a pulsed neutron generator and is intended for determining τ for rocks by means of measurements on core samples. It can also be used to estimate the concentrations in rocks of elements with large thermal-neutron absorption cross sections.