Using the theory of the nature of lattice vibrations around an impurity of different mass in a cubic crystal, detailed calculations are made of the optical absorption expected from compensated group III and group V impurities in Si due to creation of single phonons. Provided good compensation can be achieved it appears that this absorption should be clearly visible in concentrations of about 1019 cm-3. The calculated frequency spectrum of Johnson is used and it is shown that localized and band modes will give comparable effects, which change considerably with the mass of the impurity.

https://iopscience.iop.org/article/10.1088/0370-1328/81/3/309/pdf

Theory of Optical Absorption by Vibrations of Defects in Silicon

Determination of Resonance Parameters and their Covariances from Neutron Induced Reaction Cross Section Data

The resonance structure in neutron induced reaction cross sections can be used to determine the elemental compositions of materials or objects. The occurrence of resonances is the basis of neutron resonance capture analysis (NRCA) and neutron resonance transmission analysis (NRTA). NRCA and NRTA are fully non-destructive methods to determine the bulk elemental composition without the need of any sample preparation and resulting in a negligible residual activity. They have been applied to determine the elemental composition of archaeological objects and to characterize reference materials used for cross section measurements. For imaging applications a position sensitive neutron detector has been developed within the ANCIENT CHARM project. The detector is based on a 10 × 10 array of 6Li-glass scintillators mounted on a pitch of 2.5 mm, resulting in a 25 × 25 mm2 active area. The detector has been tested at the time-of-flight facility GELINA and used at the ISIS spallation source to study cultural heritage objects.

https://iopscience.iop.org/article/10.1088/1748-0221/7/03/C03009/pdf

Neutron resonance spectroscopy for the characterization of materials and objects

Neutrons can be used as probes to investigate and study properties of materials and objects. The deep penetration of neutrons into matter allows the study of bulk properties. One evolving activity in this field concerns the existence of resonances in neutron-induced reaction cross sections in the neutron energy range from thermal up to a few megaelectronvolts for certain nuclides. Since these resonances appear at neutron energies, which are specific for each nuclide, they are very suitable for the determination of the elemental composition of objects and materials. Neutron resonance capture analysis (NRCA) and neutron resonance transmission analysis (NRTA) are two methods used for exploiting the resonance structure in the neutron-induced reaction cross sections. Both methods are nondestructive, determine the bulk elemental composition, do not require any sample preparation, and result in a negligible residual activity. NRCA is applicable to almost all stable elements, while NRTA is preferred for light elements or nuclei near a closed shell. Quantitative NRCA and NRTA have been used for the analysis of archeological objects and reference materials used for cross-section measurements. Other applications of neutron resonance spectroscopy range from radiography of nuclear fuel (including spent fuel), detection of explosives and drugs, to thermometry and diamond mining.


Keywords:

neutron;
resonances;
epithermal;
capture;
transmission;
Doppler broadening;
archeology;
reference material;
nuclear fuel;
thermometry;
radiography;
nondestructive assay

Neutron Resonance Capture and Transmission Analysis

A new R-matrix analysis of the {sup 235}U cross-section data in the 0- to 2250-eV energy region is presented. The analysis was performed with the SAMMY computer code that has recently been updated to permit, for the first time, inclusion of both differential and integral data within the analysis process. Fourteen differential data sets and six integral quantities were used in this evaluation: two measurements of fission plus capture, one of fission plus absorption, six of fission alone, two of transmission, and one of eta, plus standard values of thermal cross sections for fission and capture, and of K1 and the Westcott g factors for both fission and absorption. An excellent representation was obtained for the high-resolution transmission, fission, and capture cross-section data as well for the integral quantities. The result is a single set of resonance parameters spanning the entire range up to 2250 eV, a decided improvement over the present ENDF/B-VI evaluation, in which 11 discrete resonance parameter sets are required to cover that same energy range. This new evaluation is expected to greatly improve predictability of the criticality safety margins for nuclear systems in which {sup 235}U is present.

R-Matrix Analysis of 235U Neutron Transmission and Cross-Section Measurements in the 0- to 2.25-keV Energy Range

A study has been made of the Doppler broadening of resonances in the total cross section for interaction of slow neutrons with nuclei bound in crystals. The resonance at 6.71 ev in Os metal and the resonance at 6.65 ev in U metal were chosen as examples of moderate crystal binding, while the 6.65-ev resonance in U/sub 3/O/sub 6/, for which k THETA /sub o/ = 0.043 ev, was chosen as an example of strong binding. The shapes of the resonance lines were determined as a function of temperature by measuring the neutron transmission of thin and thick samples by means of the Argonne fast chopper. These shapes were compared with theoretical line shapes calculated by means of Lamb's theory of Doppler broadening as applied to the spectra of lattice frequencies implied by simple models of the crystal lattices. For moderate binding, a simple Einstein model which reproduces the observed specific-heat behavior of the crystal above 40 EK gives accurate resonance-line shapes at all temperatures. However, for U/sub 3/O/sub 6/ the line shapc implies a generalized Nernst-Lindemann model of the lattice with a frequency spectrum g( nu ) = 0.9 delta (h nu /sub 1/0.013 ev) + 0.1 delta (hmore » nu /sub 2/ -- 0.052 ev). The proportions of the high- and low- frequency components are quite different fiom the values resulting from specific- heat data. Possible interpretations of the U/sub 3/O/sub 8/ results in terms of simple lattice models are presented. (auth)« less

J. E. Lynn

Papers

Resonant absorption of neutrons by crystals