Thermal Neutron Cross Section Research Articles

Isotopic Enrichment and Thermal Neutron Cross Section Measurement of Erbium-167

The thermal neutron cross section of Er167 has been obtained by measuring the relative number of Erl68 atoms formed by radiative neutron capture, when isotopically pure Er167 samples were exposed to a highly-thermalized neutron flux. A four-stage mass spectrometer was used to provide a 106/1 isotopic enrichment of Er167. It was also employed it measure the postirradiation Er168/Er167 isotopic ratios of the samples, which were placed in the D2O moderated Argonne CP-5 reactor. The integrated neutron flux, and effective neutron temperature in this reactor were also monitored mass spectrometrically, by observing isotopic ratio changes in nuclides whose cross section as a function of temperature are known. Two rare earth isotopes Sm149 and Gd157, were selected as the "non-l/v" absorbers that yielded this data. The effective neutron cross section of Er167 was found to be 740 ± 21 barns, at an effective neutron temperature of 42.8 + 2.5°C, with an integrated neutron flux of 3.37 ± 0.08 × 1019 neutron/cm2. The thermal (2200 m/sec) neutron cross section of Er167 was then calculated to be 699 ± 20 barns.

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.

Effective Thermal-Neutron Cross Sections for Materials with Grain Structure (Addendum)

"Effective Thermal-Neutron Cross Sections for Materials with Grain Structure (Addendum)." Nuclear Science and Engineering, 26(2), p. 288

Effective Thermal-Neutron Cross Sections for Materials with Grain Structure

Effective Thermal-Neutron Cross Sections for Materials with Grain Structure

Studies of Producing Reservoirs With the Neutron Lifetime Log

MARQUIS, G.L., WICHMANN, P.A., JUNIOR MEMBER AIME, MILLIS, C.W., LANE. WELLS CO., DIV. OF DRESSER INDUSTRIES, INC., HOUSTON, TEX. Abstract Reservoir evaluations are made on information obtained at the time wells are drilled and completed and on the basis of reservoir performance. Important data normally available from open hole measurements include gas-oil contacts, water saturations and water table levels. These parameters change as a reservoir is produced and the results manifest themselves as changes or a combination of changes in oil produced, water produced, gas-oil ratio or bottom-hole pressure. The Neutron Lifetime Log gives a cased hole measurement of a formation parameter sensitive to the amount and type of fluid in the formations. This parameter-thermal neutron capture cross-section - qualitatively distinguishes potentially productive intervals from non-productive intervals under a wide range of wellbore conditions. Well studies are presented indicating results observed from applying cased hole measurements of capture cross-section to reservoir evaluation. Introduction The Neutron Lifetime Log uses a pulsed accelerator neutron source to measure a specific formation characteristic, the thermal neutron capture cross-section. Every element has its unique capture cross-section for thermal neutrons. As a result, each combination of formation and contained fluids penetrated by the borehole has its own particular composite thermal neutron capture cross-section. Where the contrast in capture cross-section properties of the contained formation fluids is significant, and where fluids are found in sufficient quantities, this phenomenon can be exploited to determine water saturation. The log can be run in either cased or uncased holes. Obviously, because of the lack of such a tool in the past, the greatest application has been in cased holes, primarily to determine the feasibility or need of workover operations due to occurrences such as bypassing of oil, normal depletion or offset drainage. Because of its success in these areas it is logical that the measurements can and should be applied to full scale reservoir studies, either as a complement to openhole logging data or as a method of continually updating various reservoir parameters such as fluid contacts, much as periodic measurements are made to determine changes in reservoir pressure. Thermal Neutron Capture Cross-Section Capture cross-section is a function of neutron energy and can be defined only for a specific energy. Neutron capture cross-section of a medium can be defined as a statistical probability of capture of that energy neutron in the medium. It is a convenient and useful analogy to liken this statistical probability to an area which target nuclides present to the moving neutrons. The term thermal neutron refers to any neutron in thermal equilibrium with its surroundings and identifies the neutron energy level or velocity. At 20C, the most probable thermal neturon velocity is 2,200 m/sec. The macroscopic thermal neutron capture cross-section for a cubic centimeter of material is the arithmetic sum of the capture cross-sections of each individual nuclide. This is expressed either as cm2/cm3 or barns/cm3 with a barn the equivalent of 10(-24) cm2. Table 1 shows the thermal neutron capture cross-section of several elements commonly found in the strata of sedimentary basins. The magnitude of the cross-section of chlorine with respect to the other most common elements present in formation waters and hydrocarbon fluids forms the basis of a quantitative determination of water saturation. Since the composite thermal neutron capture cross-section of any medium is based on the absolute finite number of nuclides of all its constituents, this quantity as applied to a fluid reservoir can be expressed as: TABLE 1-THERMAL NEUTRON CAPTURE CROSS-SECTIONS. Element a barns/atom Boron 750.0000 Lithium 070.0000 Chlorine 032.0000 Sodium 000.4900 Sulfur 000.4900 Calcium 000.4200 Hydrogen 000.3300 Silicon 000.1300 Magnesium 000.0590 Carbon 000.0045 Oxygen 000.0002 JPT P. 412ˆ

Iodine-129 as a "Nonradioactive" Tracer

The nuclide I129 has a great potential for utilization as a chemical and biological tracer (1). Its physical half-life of 1.6 X 107 years provides a prolonged shelf life for iodinated derivatives, and its low radiation energy (0.038-Mev y and 0.150-Mev /-) facilitates safe handling and minimizes internal molecular radiation damage to organic compounds. Large doses could be employed as a tracer in man without significant radiation hazard, and other stable or radioactive isotopes of iodine could be used concomitantly with I129 and still permit quantitative radioisotopic discrimination as well as simultaneous chemical analysis. The low specific activity resulting from its long lifetime and its weak radiation energy limit the value of this nuclide as a radioactive tracer, since it cannot be efficiently counted by scintillation detectors with pulse-height discriminators. However, I129 has a high capture cross section for thermal neutrons (31 barns) (2), a property favorable to neutron activation analysis. Through neutron capture I129 produces 1130 (half-life 12.5 hours), thus enhancing its specific activity by a factor of nearly one million. The product I130 is readily counted by instruments available in most laboratories, and interference from short-lived nuclides such as I128 (halflife 25 minutes) can be resolved by disparity in half-lives. This essentially nonradioactive tracer has potential biological and medical usefulness if technical problems associated with its measurement by activation analysis prove amenable to control.

The spin of the 2 sec isomeric state of In 116

The cross section ratios for the formation of the 2 sec and the 54 min isomeric states of In 116 due to neutron capture in the three lowest resonances of the In 116 compound nucleus were measured. A comparison of the results with the isomeric cross section ratios calculated by the γ-ray cascade statistics yields for the spin of the 2 sec state, a value J m2 = 5. The activation cross section for thermal neutrons was calculated from the isomeric cross section ratio in the 1.46 eV resonance to be σ m2(0.0253 eV) = 81±8 b.

A stainless steel with high capture cross section for thermal neutrons

A stainless steel with high capture cross section for thermal neutrons

Neutron-Capture Cross Section of 35-h Rh105

The cross section for the reaction 35-h Rh105 (n,γ)30-sec Rh106 has been measured using a TRIGA reactor in the pulsed mode of operation as a neutron source. The effective thermal-neutron (0 to 0.5 eV) cross section was found to be 13 300 ± 1 500 b, relative to a 36-b cross section for Co59. The cadmium ratio was observed to be ≥ 20 in a neutron flux for which a cadmium ratio of 7 was measured for cobalt.

The effect of temperature on the diffusion length and the scattering cross section of thermal neutrons in graphite

The effect of heating graphite on the diffusion length and effective scattering cross section of thermal neutrons was investigated. It was established that in the 15–350 °C range the diffusion length changes mainly in accordance with the law 1/v for the absorption cross section. The slight deviation from this law is due to the increase of 0.5 mb/deg in the scattering cross section with an increase in temperature.

THE COBALT RESONANCE CAPTURE INTEGRAL

The resonance integral of cobalt has been measured by comparison of the cadmium ratios for gold and cobalt in 1/E slowing-down spectra in two thermal reactors. The value obtained was 69.9 barns, for a cadmium cutoff of 0.5 ev, with the 1/v part included. The error, including uncertainties in the thermal neutron cross sections for cobalt and gold and the resonance integral for gold as well as the experimental error, is estimated to be ±3.5 barns. The reduced resonance integral, i.e. after subtraction of the 1/v part, is 53.1 ± 3.5 barns, and the s√(T0/T) value in the Westcott nomenclature is 1.60±0.08.

The effect of temperature and microstructure of sintered beryllium oxide on the scattering cross section of thermal neutrons

The total effective neutron cross section σt for sintered beryllium oxide in the 0.0025–10 ev neutron energy range was measured by means of a mechanical selector at four temperatures of the scattering sample: 290, 800, 1300 and 1500 °K. Measurements of σt for beryllium oxide samples of different microstructure were also carried out at room temperature. Slight anisotropy of scattering, due to texture, and a marked influence of the size of the crystal grain on σt, due to the extinction effect, were noted.

Determination of Carbon-Hydrogen Ratios by Neutron Scattering.

A series of organic compounds, including hydrocarbons, alcohols, ketones, and esters were analyzed for hydrogen by a thermal neutron scattering technique. The neutron transmission was found to vary smoothly with the hydrogen content over the range investigated (8 to 10% hydrogen) indicating that the thermal neutron cross section per hydrogen atom is constant for all these compounds. The lower limit of sensitivity for determination of hydrogen content was found to be approximately 0.1% hydrogen by weight. The neutron source consisted of a Pu-Be mixture emitting 8.2 x 10/sup 6/ n/sec which were thermalized by D/sub 2/O. A collimated beam was directed at an A/sub 1/ cell containing the sample, and the transmitted neutrons measured with a BF/sub 3/ counter. (auth)

Cross Sections for theB11(n, γ)B12Reaction

Cross sections for the reaction ${\mathrm{B}}^{11}(n, \ensuremath{\gamma}){\mathrm{B}}^{12}$ were measured by an activation technique for neutron energies from 139 to 2325 kev and at thermal energy. Resonances in the reaction were observed at neutron energies of 430, 1030, 1280, and 1780 kev. Since the resonance at 1030 kev did not correspond to a known level in ${\mathrm{B}}^{12}$, the total neutron cross section for boron was measured with 4-kev resolution in the neutron energy region between 950 and 1350 kev. An $S$-wave resonance was observed in the total cross section at 1027\ifmmode\pm\else\textpm\fi{}10 kev. This resonance corresponds to a level in ${\mathrm{B}}^{12}$ at an excitation energy of 4.31 Mev; it most probably has spin and parity of ${1}^{\ensuremath{-}}$, and a width of 10\ifmmode\pm\else\textpm\fi{}4 kev. From an analysis of the capture cross section resonances, the radiation widths of the levels were found to be 0.3 ev at 430 kev, 0.3 ev at 1027 kev, 0.2 ev at 1280 kev, and 0.9 ev at 1780 kev, with estimated uncertainties of 50%. The measured value of the ${\mathrm{B}}^{11}(n, \ensuremath{\gamma}){\mathrm{B}}^{12}$ cross section for thermal neutrons was 5\ifmmode\pm\else\textpm\fi{}3 mb.

Application of stable boron isotopes

This article presents a brief survey of the basic applications of stable boron isotopes as materials with an altered isotope composition in cases where the differences between the nuclear properties of boron isotopes are used directly. The neutron flux transformation into heavy ionizing particles by means of the B10 (n, α)Li7 reaction and the large value of the effective thermal neutron cross section of this reaction make it possible to use B10 in medicine, nuclear studies, and radiation chemistry. The differences between the neutron cross sections of B10 and B11 make It possible to use these isotopes in reactor construction and, in particular, to use B10 in materials for control rods and reactor shields.

THERMAL NEUTRON CROSS SECTIONS AND RESONANCE INTEGRALS OF THE REACTIONS O17(n,α)C14, Ar36(n,α)S33, AND N14(n,p)C14

The thermal neutron cross sections of the exothermic reactions O17(n,α)C14, Ar36(n,α)S33, and N14(n,p)C14 have been measured, relative to the activation cross section of gold, by using a pulse ionization chamber technique. The results obtained are 0.235 ± 0.005 barn, 5.52 ± 0.11 millibarns, and 1.83 ± 0.03 barns respectively. All the reactions were shown to have the same cadmium ratio as the B10(n,α)Li7 reaction with accuracies of ±10%, ±40%, and ±3% respectively. The O17 result shows that capture in this nuclide contributes significantly to neutron absorption in heavy water.

Half-life of 134 m Cs and the cross-section for its formation by neutron activation

(In English) The thermal neutron cross section of Cs/sup 133/ to produce Cs/sup 134//sup m/ is redetermined to have a value of 2.44 plus or minus 0.15 barns. The half life of Cs/sup 134//sup m/ is 2.895 plus or minus 0.005 hr as determined from decay data obtained from a gamma spectrometer and beta scintillation and proportional counters. (N.W.R.)

Thermal neutron cross sections for producing isomers (II)

Thermal neutron cross sections for the production of isomers have been measured for the following cases, by making irradiations with the swimming pool reactor at Trombay, Bombay: Se 78 (n, γ)Se 79m, Sr 84 (n, γ)Sr 85m, Sr 84 (n, γ)Sr 85, Rb 85 (n, γ)Rb 86m, Ba 132(n, γ)Ba 133m, Ba 134(n, γ)Ba 135m Ba 136(n, γ)Ba 137m, Os 190(n, γ)Os 191, 191m and some others. Activation method was employed using scintillation technique. Isomeric ratios seem to follow the rule of Mateosian and Goldhaber.

Scattering of Thermal Neutrons in Graphite

Previous calculations of the total cross section of thermal neutrons in graphite were modified, taking into account contributions to neutron scattering only from the planes which gave x-ray reflections. Only 18 reflections of the expected 40 were obtained. Calculated results, using this method, coincided with the experimental points within 2%. (M.C.G.)

The thermal neutron cross-section of the reaction 137Cs( n, γ) 138Cs

The thermal neutron cross-section for the reaction 137Cs( n, γ) 188Cs has been found to be 0·110 ± 0·033 barns in a flux having a resonance flux, φ r φ th , equal to 0·00277, where φ r φ th is the resonance flux per unit interval of ln E, per unit thermal flux, and the resonance neutron energy ranges from 0·4 eV up to the energy of fission neutrons.