Showing papers by "Oak Ridge National Laboratory published in 1971"

Monte carlo channeling calculations.

Abstract: A Monte Carlo computer program for following the trajectories of high-energy ions in a lattice has been used to study depth dependence, half-angles, and minimum yields in channeling. The program uses the Thomas-Fermi interaction between ions and lattice atoms and gives the atoms' independent thermal displacements appropriate to the temperature of the lattice. The depth dependence of the nuclear interaction probability near the surface has been calculated and shown to be of importance in understanding other phenomena as well as having an intrinsic interest. Extensive calculations to explore the temperature and energy dependences of half-angles and minimum yields were done, and analytical formulas are given that summarize the computer results. The half-angle formulas give generally improved agreement with experiment and resolve certain discrepancies noted by Picraux, Davies, Eriksson, Johansson, and Mayer between their measurements and earlier theories. Minimum yields were calculated for both single- and double-alignment configurations. The results reveal an energy dependence as well as giving a better description of the temperature dependence and improved agreement with experiment. A relationship is given between the minimum yield and the yield from the surface.

Spermatogonial stem-cell renewal in the mouse†

Abstract: Type A spermatogonia in the mouse can be separated into five successive classes on the basis of nuclear morphology and stage of the cycle in which they occur. Enumeration of all types throughout the cycle of the seminiferous epithelium reveals that the As spermatogonia are the stem cells. They divide throughout the cycle and, especially at stages IX to I, form chains of cells which then give rise to spermatogonia A1 at stages II–VIII. The A1 cells divide in IX to form the A2, which divide in XI to form A3, and the A3 cells divide in I to form the class A4 spermatogonia. Spermatogonia A4 give rise only to the In type; there is no evidence for the formation of either As or A1 from A4 spermatogonia. Repeated injections of 3H-thymidine and tracing the history of labeled cells to 15 days after labeling supported the conclusion obtained from morphological and numerical data that As spermatogonia are the stem cells of the testis.

Lattice Dynamics of Rutile

Abstract: The phonon dispersion relation for rutile (Ti${\mathrm{O}}_{2}$) has been measured by the coherent inelastic scattering of thermal neutrons along principal symmetry directions of the Brillouin zone. Theoretical models based on rigid-ion and shell models, with either axially symmetric or tensor first- and second-neighbor forces, have been fitted to the measured dispersion relation. Only the shell model with tensor forces for all interactions except the second-neighbor oxygen-oxygen interaction was able to give acceptable qualitative agreement with the data, and that agreement is good for only some modes. A frequency distribution and Debye temperature spectrum are presented for that model. The temperature dependence of the frequency of the ${\ensuremath{\Gamma}}_{1}^{\ensuremath{-}}({A}_{2u})$ transverse optic mode is measured from 4 to 300\ifmmode^\circ\else\textdegree\fi{}K, and the behavior of the square of the frequency is in good agreement with that predicted by the static dielectric constant measurements of Parker.

Age-Associated Changes in the DNA of Mouse Tissue

Abstract: The template activity of DNA with calf thymus DNA polymerase has been studied in sections of fixed brain, liver, and heart tissues from young and senescent mice. The enzyme catalyzed greater incorporation of deoxyribonucleotide monophosphates into nuclei of old mouse neurons, astrocytes, Kupffer cells, and heart muscle fibers. The results are interpreted as the accumulation of DNA strand breaks with aging.

Definition of functional units in a small chromosomal segment of the mouse and its use in interpreting the nature of radiation-induced mutations.

Abstract: Complementation mapping of a small chromosomal region of the mouse has defined a linear series of functional units and has aided in the interpretation of the nature of radiation-induced genetic changes Mutations used for this purpose were derived from specific-locus mutation-rate experiments and were detected by their visible phenotype in combination with the test-stock's linkage-group-2 markers d and se (recombination frequency 016%) A total of 235 mutants of this type have been recovered, and 102 of these were used in complementation tests involving over 800 combinations and about 40000 progeny While the original screening for mutants employed only 2 markers, the subsequent analysis has, so far, revealed 16 complementation groups spanning 8 or 9 functional units Involvement in a mutation of 2 or more functional units is taken to indicate chromosomal aberration All mutations are consistent with a linear map, except for 2, which appear to “skip” units but have been shown not to result from recombinationally separable changes These 2 mutations, 1 of which gives “conversion”-like results in recombination tests, are interpreted as small rearrangements The remaining multi-unit mutations are, for the time being, taken to be deficiencies Although there is evidence that homozygous deficiency of either 1 of the marked loci gives a visible effect, this by itself is no proof against the single-unit mutations being “point” mutations (perhaps single base-pair changes) A strong effect of irradiated cell stage can be demonstrated, both on the locus-spectrum ( ie , relative frequencies of events involving d , se , or both) and on frequency of mutations that are interpreted as aberrations The latter ranges from 135% in most X-or γ-rayed spermatogonia, through 423% in postgonial stages, to 656% in oocytes Within spermatogonia, neutron irradiation and 24-h fractionation of X-rays shifts the distribution in the direction of postgonial results The rest of the irradiated spermatogonia closely resemble the control distribution

Lattice Dynamics of Grey Tin and Indium Antimonide

Abstract: We have measured the phonon-dispersion relations in symmetry directions for $\ensuremath{\alpha}\ensuremath{-}\mathrm{S}\mathrm{n}$ at 90 \ifmmode^\circ\else\textdegree\fi{}K and InSb at 300 \ifmmode^\circ\else\textdegree\fi{}K. A limited number of measurements were made at 240 \ifmmode^\circ\else\textdegree\fi{}K in $\ensuremath{\alpha}\ensuremath{-}\mathrm{S}\mathrm{n}$ to look for effects associated with the phase transition, but none were found. The two dispersion relations are qualitatively similar, the main differences arising from the lower symmetry and a slight general drop in frequency in going from $\ensuremath{\alpha}\ensuremath{-}\mathrm{S}\mathrm{n}$ to InSb. Systematic trends in the comparison with other group-IV and III-V semiconductors can be seen. We have found second-neighbor shell models which give a reasonable description of the dispersion relations but in which the parameters are poorly defined and apparently devoid of physical meaning. The microscopic implications of this fact are discussed.

Synthesis of Ribonucleic Acid and Protein in Plasmid-Containing Minicells of Escherichia coli K-12

Abstract: Unlike the deoxyribonucleic acid (DNA)-deficient minicells produced by F(-) parents, minicells produced by plasmid-containing strains contain significant amounts of plasmid DNA. We examined the ability of plasmid-containing minicells to synthesize ribonucleic acid (RNA) and protein. In vivo, minicells produced by F(-) parents are unable to incorporate radioactive precursors into acid-insoluble RNA or protein, whereas minicells produced by F', R(+), or Col(+) parents are capable of such synthesis. Using a variety of approaches, including polyacrylamide gel analysis of the RNA species produced and electron microscope autoradiography, we demonstrated that the synthesis observed in minicell preparations is a property of the plasmid-containing minicells and not a result of the few cells (approximately 1 per 10(6) minicells) contaminating the preparations. That the observed synthesis is of biological importance is suggested by the ability of plasmid-containing minicells to yield viable phage upon infection with T4.

Luminescence of Polypeptides and Proteins

Abstract: It is frequently advantageous to consider the evolution of ideas and results when surveying a particular area of reseach since the early results often contain the fundamental outlines of a problem which subsequently become obscured by the later accumulation of detailed knowledge The history of the development of protein fluorescence followed a logical pattern and so is quite suitable to discuss with a linear time scale Because there are many experimental complications presented to investigators of the luminescence of biological macromolecules, each advance in knowledge was and is intimately associated with technological breakthroughs in measurement rather than with advances in theory Also, because of considerable inherent difficulties in luminescence measurements, many of the initial observations announced were later shown to be in error So, with apologies to friends and former colleagues for remembering the errors, it is hoped that a brief account of the development of the field will prove to be of value to those who are tempted to enter this difficult area, which is one so susceptible to serious errors

Densities of States of Paramagnetic Cu-Ni Alloys

Abstract: The densities of states of the near ideally substitutional random-alloy system Cu-Ni are calculated using the coherent-potential approximation for a range of concentrations of the constituents across the complete alloy diagram. It is shown that previous model calculations for Ni-rich paramagnetic Cu-Ni alloys using the coherent-potential approximation can be extended to alloys of arbitrary concentration if appropriate Cu and Ni potential functions are used. The results obtained are consistent with experimental photoemission and soft-x-ray emission profiles in so far as comparisons can be effected. The results indicate the limits of this simple nonmagnetic random-alloy theory for discussing the conventional low-temperature specific-heat data of these alloys. For Cu-rich alloys the results correspond to a split-band regime while for Ni-rich alloys the $d$ bands associated with Cu and with Ni sites in the alloy overlap to a considerable extent. For all Cu-Ni alloys the rigid-band model and the virtual-crystal approximation are quite inappropriate.

Photon Interactions at a Rough Metal Surface

Abstract: We have analyzed the processes of diffuse scattering of photons and surface-plasmon creation by photons at a rough metal surface. We have approximated the metal by an electron gas of uniform density which is bounded by a nearly plane surface at which the density falls abruptly to zero. Quantum perturbation theory is used to evaluate the probability of occurrence of the various processes at the assumed "weakly" rough surface.

Diffuse Scattering from Defect Clusters near Bragg Reflections

Abstract: The diffuse scattering from crystals containing defect clusters with strong displacement fields has been calculated. The scattering is extremely large and concentrated in small regions near the Bragg reflections. For small concentrations the diffuse intensity is essentially determined by a "cluster form factor" depending explicitly on the displacement field. The scattering is studied in detail for small and large deviations from the Bragg reflection. Exact expressions are given for the moments of the scattering, depending sensitively on the displacement field in the core of the cluster. The diffuse scattering for strong displacement fields shows a pronounced asymmetry; its center does not coincide with the position of the Bragg peak, in agreement with recent experimental observations.

Total-Reaction-Cross-Section Measurements for 30-60-MeV Protons and the Imaginary Optical Potential

Abstract: We have measured total reaction cross sections for 30-, 40-, 49.5-, and 60.8-MeV protons incident on thin, separated isotopes covering the range from $^{12}\mathrm{C}$ to $^{208}\mathrm{Pb}$. Our results are consistent with previous data at 30 MeV, but disagree with earlier data at 60 MeV. We find a strong dependence of the reaction cross section on neutron excess for a series of Fe and Ni isotopes. Little, if any, such dependence is observed for the $N=28$ isotones. The data are well represented by the relation ${\ensuremath{\sigma}}_{\mathrm{R}}=\ensuremath{\pi}{({r}_{0}{A}^{\frac{1}{3}}+\overline{\ensuremath{\lambda}})}^{2}$ with ${r}_{0}=1.23\ifmmode\pm\else\textpm\fi{}0.01$ F. When analyzed with the conventional optical model, our data require the volume absorption to increase and the surface absorption to decrease with increasing proton energy ${E}_{p}$. The analysis reveals a striking $\frac{(N\ensuremath{-}Z)}{A}$ dependence for the product ${W}_{D}{a}^{\ensuremath{'}}$. Using the Oak Ridge parameters for the real and spin-orbit potentials, we arrive at the following parametrization for the imaginary potential: volume absorption potential, ${W}_{0}=(1.2\ifmmode\pm\else\textpm\fi{}0.09{E}_{p})$ MeV; surface absorption potential, ${W}_{D}=[4.2\ensuremath{-}0.05{E}_{p}+15.5\frac{(N\ensuremath{-}Z)}{A}]$ MeV; imaginary diffusivity, ${a}^{\ensuremath{'}}=[0.74\ensuremath{-}0.008{E}_{p}+1.0\ifmmode\times\else\texttimes\fi{}\frac{(N\ensuremath{-}Z)}{A}]$ F.

Measurement of Recessive Lethal Damage Over the Entire Genome and at Two Specific Loci in the ad-3 Region of a Two-Component Heterokaryon of Neurospora crassa

Abstract: Neurospora is a haploid organism, but with the use of a two-component heterokaryon, it can be used to study many of the same genetic effects that occur in diploid organisms (Atwood, 1949; Atwood and Mukai, 1953, 1954). Forced heterokaryons between two different haploid strains with different biochemical requirements produce three types of conidia; two different homokaryotic types that will grow only on supplemented media, and a heterokaryotic type that will grow on minimal medium. The heterokaryotic conidia of a two-component heterokaryon have at least one haploid set of chromosomes of each genotype together in the same cytoplasm rather than in the same nucleus, as in a diploid cell. The experiments of Atwood and Mukai (1953,1954) have shown that the heterokaryotic fraction of cells can be used to measure the dominant lethal damage that inactivates the cell as well as the recessive lethal damage that imposes new biochemical requirements upon the individual nuclei. The recessive lethal damage results from mutation of genes over the entire genome, de Serres and Osterbind (1962) have shown that this technique can be used to measure the recessive lethal damage occurring at specific loci and that this damage results both from point mutation and chromosome deletion. Thus, recessive lethal mutations at specific loci induced in a heterokaryotic cell result from the same spectrum of genetic alterations that is induced in a diploid cell.

Critical Binding of an Electron to a Rotationally Excited Dipolar System

Abstract: Calculations are made of the minimum dipole moments necessary to bind an electron to a nonstationary finite electric dipole in a number of rotationally excited states. The critical moment for a given dipolar system is found to depend on the dipole length, the moment of inertia, and the rotational quantum state of the finite dipole. The properties of the dipolar system are discussed as to their implications for electron scattering by polar molecules.

Australia antigen: large-scale purification from human serum and biochemical studies of its proteins.

Abstract: Biophysical techniques are described for the large-scale isolation of Australia antigen (Au) from unit quantities of human serum by using the batch-type zonal centrifuge rotors. A three-step procedure involving isopycnic banding of the particle in CsCl density gradients and rate-zonal centrifugation on sucrose gradients resulted in a highly purified Au preparation which was used for biochemical studies of Au proteins and as immunizing antigen for the production of reagent antiserum in animals. The spherical form of Au, which was devoid of detectable nucleic acid, was composed of two major proteins (AuP1 and AuP2) and a minor protein (AuP3) of 26,000, 32,000, and 40,000 molecular weight, respectively, as determined by acrylamide gel electrophoresis. The significance of these findings to the possibility of Au subtypes is discussed.

A new concept of spermatogonial stem-cell renewal in the mouse and its relationship to genetic effects ☆

Abstract: The stem cell of the seminiferous epithelium, designated spermatogonia As, is radiation resistant, has a long cell cycle time, and occurs as single, isolated cells. Renewal occurs by division to form more singles; formation of a “pair” of cells constitutes the initial step in differentiation. This model differs from previous ones in that spermatogonial types with high sensitivity to cell killing do not contribute to the stem-cell pool, and owing to their transient nature, are relatively unimportant in overall genetic effects in spermatogonia. Only As spermatogonia survived single radiation dose of 150, 500, 1000, and a fractionated 1000 R exposure. A combination of [3H]thymidine injection and irradiation showed equal spermatgonial labeling 8.5 and 17 days after 100 and 500 R, a decrease in percentage of labeled cells after 1000 R, and the highest value after 1000 R given as two 500 R fractions 24 h apart. These results suggest that differential cell survival may be a factor in observed mutation frequencies. The high percentage of labeled spermatogonia after 1000 R given in two 500 R fractions 24 hours apart suggests that selective action of the second exposure is an important factor in the enhanced mutation frequency observed after this treatment. The data also suggest that the spermatogonial population surviving 100 and 500 R is qualitatively similar, and that the kinetics of these cells is the same as that of As cells in controls. Thus extrapolation of the linear portion of the mutation rate curve would not be expected to underestimate the effect of low doses.