Showing papers by "Oak Ridge National Laboratory published in 1979"
TL;DR: The generalized cross-validation (GCV) method as discussed by the authors is a generalized version of Allen's PRESS, which can be used in subset selection and singular value truncation, and even to choose from among mixtures of these methods.
Abstract: Consider the ridge estimate (λ) for β in the model unknown, (λ) = (X T X + nλI)−1 X T y. We study the method of generalized cross-validation (GCV) for choosing a good value for λ from the data. The estimate is the minimizer of V(λ) given by where A(λ) = X(X T X + nλI)−1 X T . This estimate is a rotation-invariant version of Allen's PRESS, or ordinary cross-validation. This estimate behaves like a risk improvement estimator, but does not require an estimate of σ2, so can be used when n − p is small, or even if p ≥ 2 n in certain cases. The GCV method can also be used in subset selection and singular value truncation methods for regression, and even to choose from among mixtures of these methods.
3,697 citations
TL;DR: The available body of information on fluorescence, Auger, and Coster-Kronig yields, radiative and radiationless transition rates, level widths, and x-ray and Auger line widths is presented in this article.
Abstract: The available body of information on (a) fluorescence, Auger, and Coster‐Kronig yields, (b) radiative and radiationless transition rates, (c) level widths, (d) x‐ray and Auger line widths, (e) x‐ray and Auger spectra, and (f) Coster‐Kronig energies has been used to generate an internally consistent set of values of atomic radiative and radiationless yields for the K shell (5 ?Z?110) and the L subshells (12 ?Z?110). Values of fluorescence yields ωk, ω1, ω2, ω3, Coster‐Kronig yields F1, F1.2, F1.3, F1.3, F2.3. Auger yields ak, a1, a2, a3, and effective fluorescence yields ν1 and ν2 are presented in tables and graphs. Estimates of uncertainties are given. Updated and expanded graphs of partial and total widths of K, L1, L2, and L3 levels are presented as well as a reference list of papers published since about 1972.
1,703 citations
TL;DR: In this article, the authors presented semi-empirical values of the natural widths of K, L1, L2, and L3 levels, Kα1 and Kα2 x-ray lines, and KL1L1, KL 1L2 and KL2L3 Auger lines for the elements 10?Z?110.
Abstract: Semi‐empirical values of the natural widths of K, L1, L2, and L3 levels, Kα1 and Kα2 x‐ray lines, and KL1L1, KL1L2 and KL2L3 Auger lines for the elements 10?Z?110 are presented in tables and grapahs. Level width Γi (i=K, L1,L2, L3) is obtained from the relation Γi=ΓR,i/ωi, using the theoretical radiative rate ΓR,i from Scofield’s relativistic, relaxed Hartree‐Fock calculation and the fluorescence yield ωi from Krause’s evaluation. X‐ray and Auger lines widths are calculated as the sums of pertinent level widths. This tabulation of natural level and line widths is internally consistent, and is compatible with all relevant experimental and theoretical information. Present semi‐empirical widths, especially those of Kα1 and Kα2 x‐rays, are compared with measured widths. Uncertainties of semi‐empirical values are estimated.
1,251 citations
TL;DR: In this article, the doublefolding model was used to calculate the real part of the optical potential for heavy-ion scattering, and the resulting potentials were shown to reproduce the observed elastic scattering for a large number of systems with bombarding energies from 5 to 20 MeV per nucleon.
1,078 citations
TL;DR: In this paper, the RIS process can be saturated with available pulsed lasers, so that one electron can be removed from each atom of the selected type, and a compact theory of RIS process is presented which delineates the conditions under which rate equations apply.
Abstract: Resonance ionization spectroscopy, RIS, is a multistep photon absorption process in which the final state is the ionization continuum of an atom. The RIS process can be saturated with available pulsed lasers, so that one electron can be removed from each atom of the selected type. This method was first applied to the determination of the absolute number of Hes(2/sup 1/S) excited states produced when pulsed beams of protons interacted with helium gas. Laser schemes for RIS are classified into five basic types; with these, nearly all of the elements can be detected with commercially available lasers. A periodic table is included showing schemes applicable to all of the elements except He, Ne, F, and Ar. A compact theory of the RIS process is presented which delineates the conditions under which rate equations apply. Questions on the effects of collisional line broadening, laser coherence time, and nonresonant multiphoton ionization processes are discussed. The initial demonstration of one-atom detection of Cs is described. By using laser beams to saturate the RIS process and proportional counters as single-electron detectors, one-atom detection is made possible. With RIS, one-atom detection is highly selective, has the ultimate in sensitivity, and has excellent space and timemore » resolution. Furthermore, a modification of the technique in which single electrons (or single ions) are detected with a channel electron multiplier permits single-atom detection in a vacuum. The authors describe how RIS can be used for photophysics measurements such as far wing collisional line broadening, measurements of photoionization cross sections for excited states, and collisional redistribution among excited states.« less
452 citations
TL;DR: In this article, a new fundamental radiation-induced defect in high-purity synthetic silica has been identified by electron-spin-resonance studies of $ √ 17 √ O$-enriched Si as a peroxy radical bonded to one Si in the glass matrix.
Abstract: A new fundamental radiation-induced defect in high-purity synthetic silica has been identified by electron-spin-resonance studies of $^{17}\mathrm{O}$-enriched Si${\mathrm{O}}_{2}$ as a peroxy radical ${\mathrm{O}}_{2}^{\ensuremath{-}}$ bonded to one Si in the glass matrix. The precursors of these defects are envisioned to be \ensuremath{\equiv} Si-O-O-Si \ensuremath{\equiv} structures, some of which preexist in the silica, are formed in greater numbers during neutron bombardment, and which may release an electron either during irradiation or subsequent annealing.
305 citations
TL;DR: Experimental and theoretical studies of runaway electrons in toroidal devices are reviewed in this article, with particular reference to tokamaks, and the complex phenomenology of runaway effects, which have been the subject of research for the past twenty years, is organized within the framework of a number of physical models.
Abstract: Experimental and theoretical studies of runaway electrons in toroidal devices are reviewed here, with particular reference to tokamaks. The complex phenomenology of runaway effects, which have been the subject of research for the past twenty years, is organized within the framework of a number of physical models. The mechanisms and rates for runaway production are discussed first, followed by sections on runaway-driven kinetic relaxation processes and runaway orbit confinement. Next, the equilibrium and stability of runaway-dominated discharges are reviewed. Models for runaway production at early times in the discharge and the scaling of runaway phenomena to larger devices are also discussed. Finally, detection techniques and possible applications of runaways are mentioned.
305 citations
TL;DR: In this article, a statistical model for the calculation of inelastic mean free paths and energy losses of electrons in solids is described, and results for the application of the model to the six solid media Al, Au, Ag, Cu, Ni, and Si are presented.
222 citations
TL;DR: The combined effects of BHT and oxygen, when given in the correct temporal sequence, were synergistic and not simply additive and concluded that pulmonary fibrosis can result from interaction between an agent causing acute lung damage and a second toxic agent which compromises reepithelialization.
202 citations
TL;DR: Inelastic mean free paths and energy losses in aluminum metal are calculated for electrons with energies from a few eV to 10 4 eV above the Fermi level as mentioned in this paper.
164 citations
TL;DR: In this paper, it was shown that a charged particle moving with velocity in a medium of resonance frequency may set up two types of electron-density fluctuations, which constitute a mode of energy transport from the particle track leading eventually to particle-hole excitations.
Abstract: It is shown that a charged particle moving with velocity $\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}$ in a medium of resonance frequency ${\ensuremath{\Omega}}_{0}$ may set up two types of electron-density fluctuations. Collective fluctuations trail the particle, composing a conical pattern in a relatively extended periodic wake of wavelength $\ensuremath{\sim}\frac{2\ensuremath{\pi}v}{{\ensuremath{\Omega}}_{0}}$. They constitute a mode of energy transport from the particle track leading eventually to particle-hole excitations. Single-particle interactions give rise to bow waves ahead of the particles of wavelength $\frac{2\ensuremath{\pi}\ensuremath{\hbar}}{\mathrm{mv}}$. The gradient along $\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}$, at the site of the ion, of the wake potential set up by density fluctuations, multiplied by the ionic charge, yields an expression for the retarding force of the medium on the projectile in exact agreement with the Bethe stopping-power formula appropriate to the medium. The wake of a dicluster causes forces between the constituent ions which account quantitatively for measurements of the breakup behavior of swift molecular ions in thin foils. The increase in the energy straggling of a test charge, when moving in the wake of a leading ion, is shown to be small compared with the straggling induced by its own wake under normal conditions.
TL;DR: In this paper, a combination of binary and tertiary gas mixtures has been discovered for which the electron energies and the overall scattering cross sections that correspond to these are reduced, resulting in an increase in the drift velocity, w, of the electrons in the gas mixture for a range of E/P (pressurereduced electric field) values.
TL;DR: In this article, it is proposed that diffusion of hydrogen parallel to the $c$ axis proceeds by a proton jump from one proton ion to another along the channel as represented by O${\mathrm{H}}^{\ensuremath{--}}$\ifmmode\cdot\else\textperiodcentered\fi{}${H${O}}^{2}+}$
Abstract: The predominant hydrogen-containing species in Ti${\mathrm{O}}_{2}$ (rutile) are O${\mathrm{H}}^{\ensuremath{-}}$ (hydroxyl) ions, in which the oxygen occupies a regular oxygen ion site, and the O-H bond is perpendicular to the $c$ axis. It is proposed that diffusion of hydrogen parallel to the $c$ axis proceeds by a proton jump from one ${\mathrm{O}}^{2\ensuremath{-}}$ ion to another along the channel as represented by O${\mathrm{H}}^{\ensuremath{-}}$\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{O}}^{2\ensuremath{-}}$ \ensuremath{\rightarrow} ${\mathrm{O}}^{2\ensuremath{-}}$\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{H}}^{+}$\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{O}}^{2\ensuremath{-}}$ \ensuremath{\rightarrow} ${\mathrm{O}}^{2\ensuremath{-}}$\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}H${\mathrm{O}}^{\ensuremath{-}}$. It is also proposed that diffusion perpendicular to the $c$ axis proceeds by a rotation of the O${\mathrm{H}}^{\ensuremath{-}}$ bond to move the proton from one channel to an adjacent channel, followed by a proton jump to another ${\mathrm{O}}^{2\ensuremath{-}}$ ion in the same channel. From a potential-energy model, which includes a Morse function to represent the O${\mathrm{H}}^{\ensuremath{-}}$ bond, as well as electrostatic and repulsive terms, the activation energies for hydrogen and tritium diffusion parallel to the $c$ axis were calculated to be (including a zero-point energy correction) 0.60 and 0.69 eV, respectively, in good agreement with the respective experimental values of 0.59 and 0.75 eV. The calculated activation energy for diffusion perpendicular to the $c$ axis was 1.23 eV (no zero-point energy correction), as compared to the experimental values of 1.28 and 1.11 eV, respectively, for hydrogen and tritium. The calculated equilibrium orientation of the O${\mathrm{H}}^{\ensuremath{-}}$ ion in Ti${\mathrm{O}}_{2}$ and the calculated stretching frequency of this species were also in good agreement with the respective experimental results.
Journal Article•
TL;DR: The overall results suggest the possibility that ara-C is a weak competitive inhibitor of DNA polymerases associated with ultraviolet-induced excision repair.
Abstract: The antineoplastic agent arabinofuranosyl cytosine (ara-C) produces an inhibition of the pyrimidine dimer excision system of human DNA repair. Alkaline sucrose gradient analysis of DNA from normal human skin fibroblasts exposed to 20 J/m/sup 2/ of ultraviolet radiation (254 nm) shows an accumulation of DNA single-strand breaks when DNA repair is attempted in the presence of 10 ..mu..M ara-C. Cells from complementaion groups of xeroderma pigmentosum that are defective in early steps of excision repair show reduced numbers of DNA single strand breaks/10/sup 8/ daltons when compared with normal cells. Cesium chloride gradient analysis of radioactive precurser uptake during repair replication indicates that ara-C causes a 6 to 56% reduction in the number of nucleotide bases inserted into the DNA at concentrations of 1 and 10 ..mu..M, respectively. These concentrations result in the substitution for deoxycytidine (dCyd) by ara-C of 40 and 100%, respectively, in repaired regions. Repair inhibition is reversed by 50% upon removal of ara-C and by > 95% with the addition of 100 ..mu..M dCyd. Chromatography of digested DNA shows that incorporated ara-C is not removed during dCyd reversal, suggesting that ara-C incorporation per se does not play a significant role in the inhibition of repair synthesis.more » The repair inhibition observed here is dependent on 2 mM hydroxyurea, presumably due to reduction in the intracellular pool of dCyd. The overall results suggest the possibility that ara-C is a weak competitive inhibitor of DNA polymerases associated with ultraviolet-induced excision repair.« less
Journal Article•
TL;DR: The tumor promoter 12-O-tetradecanoylphorbol-13-acetate and the antileukemic agent mezerein were equipotent on a molar basis in inducing hyperplasia, inflammation, and ornithine decarboxylase activity, as well as in reducing cyclic adenosine 3':5'-monophosphate accumulation in response to beta-adrenergic stimulation.
Abstract: The tumor promoter 12- O -tetradecanoylphorbol-13-acetate and the antileukemic agent mezerein are diterpene esters of plant origin with certain structural similarities. Both compounds, when applied topically to mouse skin, were equipotent on a molar basis in inducing hyperplasia, inflammation, and ornithine decarboxylase activity, as well as in reducing cyclic adenosine 3′:5′-monophosphate accumulation in response to β-adrenergic stimulation. In contrast, mezerein was much less effective as a tumor promoter; the phorbol ester at 8.5 nmol/application yielded 78-fold more tumors than did 8.5 nmol mezerein per application to similarly initiated SENCAR mice. The superiority of the phorbol ester was nearly as great in CD-1 mice. These results suggest that although the induction of hyperplasia and ornithine decarboxylase activity may be necessary components of the carcinogenic process, they are not sufficient; 12- O -tetradecanoylphorbol-13-acetate must accomplish an essential event not accomplished by mezerein.
TL;DR: In this article, the physical and electrical properties of ion-implanted silicon annealed with high-powered laser radiation are described, with a focus on the comparison of materials properties that can be achieved with laser annealing to those which can be obtained by conventional thermal anealing.
Abstract: The physical and electrical properties of ion-implanted silicon annealed with high-powered laser radiation are described. Particular emphasis is placed on the comparison of materials properties that can be achieved with laser annealing to those which can be achieved by conventional thermal annealing. Applications of these techniques to the fabrication of high-efficiency solar cells, and potential applications of this new technique to other materials areas are discussed.
TL;DR: The objective of this paper is to determine the relative volatility of two to five ring PAH structures in water, and estimate the importance of the volatilization process in removing PAH from aquatic environments.
Abstract: The movement of organic contaminants from water to the atmosphere can be important in reducing concentrations of foreign substances in aquatic ecosystems (HILL et al. 1976, DILLING 1977, MACKAY and LEINONEN 1975). Polycyclic aromatic hydrocarbons (PAH), a class of chemical produced in the combustion and pyrolysis of organic fuels, possess properties which suggest that their volatility in solution may be significant (MACKAY and WOLKOFF 1973). Since many PAH are potent carcinogens, a knowledge of their persistence, transport, and fate in aquatic environments is critical. The objective of this paper is to determine the relative volatility of two to five ring PAH structures in water, and estimate the importance of the volatilization process in removing PAH from aquatic environments. The theoretical basis for describing the transfer of vola: tile substances across an air-water interface has been succinctc}y detailed (LISS 1973, LISS and SLATER 1974). The rate of the process under a given set of conditions is described as a simple first order exponential decay with rate constant KL/depth. K L, the overall mass transfer coefficient, has units of distance time (such as cm/hr) and three components: kg, k I and H, related by the expression
TL;DR: Junctions between Xenopus laevis oocytes and follicle cells have been identified as gap junctions by the passage of microinjected fluorescent dye from oocytes to follicles by regulating the opening or assembly of these junctions.
Abstract: Junctions between Xenopus laevis oocytes and follicle cells have been identified as gap junctions by the passage of microinjected fluorescent dye from oocytes to follicle cells. The opening or assembly of these junctions, or both, appears to be regulated by gonadotropins.
TL;DR: In this paper, models with k variables were taken, for the sake of argument, to exactly represent a system and models with n variables were then developed for the same system and differences in output examined.
01 Jan 1979
TL;DR: This chapter will first deal with optimizing the instrumental factors which influence microanalysis, namely: the specimen/detector geometry; the choice of accelerating voltage, electron gun, imaging mode, and most importantly the elimination of instrumental artifacts.
Abstract: X-ray microanalysis using a modern transmission or scanning transmission electron microscope (TEM/STEM) is an extremely powerful analytical tool. The ability to observe and characterize the morphology, crystallography, and elemental composition of regions of a specimen as small as 20 nm in diameter is a major breakthrough for materials science. In this chapter the practical aspects of the application of x-ray microanalysis to nonbiological systems will be considered (the use of this technique in biological research is the topic of Chapter 5). This chapter will first deal with optimizing the instrumental factors which influence microanalysis, namely: the specimen/detector geometry; the choice of accelerating voltage, electron gun, imaging mode, and most importantly the elimination of instrumental artifacts. The application of the principles of thin film analysis to simple single phase samples will then be considered, followed by a discussion of multiphase systems where analysis and interpretation becomes more complex. These examples will be drawn primarily from the author’s own experience, because of the difficulties inherent to reconstructing complete experimental details from the literature. The published applications of several other authors are cited in the bibliography.
TL;DR: Measured cross sections for electron capture by a variety of multiply charged ions in collisions with atomic and molecular hydrogen are presented in this article, where the data are primarily for He-like and Li-like ions of B, C, N, and O in the velocity range 0.4-1.8 cm/sec.
Abstract: Measured cross sections for electron capture by a variety of multiply charged ions in collisions with atomic and molecular hydrogen are presented. The data are primarily for He-like and Li-like ions of B, C, N, and O in the velocity range 0.4-1.0 \ifmmode\times\else\texttimes\fi{} ${10}^{8}$ cm/sec. The electron-capture cross sections for these ions are typically 3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ in the velocity range tested. Comparison is made between these data and a number of recent theoretical predictions. While no simple scaling rules apply to these low-velocity data, for initial charge $q\ensuremath{\ge}+4$ the cross sections are essentially characterized by initial ionic charge rather than by projectile mass or electronic structure.
TL;DR: Current risk assessments, which consider technetium and neptunium as potentially capable of migrating from high-level radioactive waste repositories, may be overestimating their potential hazard to the public since the Fe(II) content of many subsurface waters may maintain these elements in less soluble oxidation states.
Abstract: Relatively mobile TcO +– and NpO 2+ can be chemically reduced to less soluble oxidation states in the presence of igneous rocks, as predicted by oxidation-reduction measurements Current risk assessments, which consider technetium and neptunium as potentially capable of migrating from high-level radioactive waste repositories, may be overestimating their potential hazard to the public since the Fe(II) content of many subsurface waters may maintain these elements in less soluble oxidation states
TL;DR: In this article, the authors observed unpredicted spectral peaks in the range from 10 to 130 keV for planar-channeling electrons, whereas only a large low-energy enhancement was seen for axial channeling electrons.
Abstract: Radiation from 56-MeV and 28-MeV electrons channeled along major planes or along a major axis of an 18-..mu..m thick silicon crystal has been observed. Unpredicted spectral peaks in the range from 10 to 130 keV were observed and resolved for planar-channeling electrons, whereas only a large low-energy enhancement was seen for axial-channeling electrons.
TL;DR: Vitellogenin can be selectively precipitated from the plasma of estrogen-treated animals by Mg2+ in the presence of EDTA and can also be isolated by DEAE-cellulose chromatography, resulting in preparations that contain <2% albumin.
TL;DR: A comparison of hydroxyurea effects on the different DNA repair assays indicates inhibition of one step in DNA repair also results in varying degrees of inhibition of other steps as well.
TL;DR: In this article, a perturbation formulation for the space-energy dependent burnup equations describing depletion and transmutation of nuclide densities in a coupled neutron-nuclide field, such as a reactor core, is developed in a form consistent with the computational methods used for depletion analysis.
Abstract: A perturbation formulation is developed for the space-energy dependent burnup equations describing depletion and transmutation of nuclide densities in a coupled neutron-nuclide field, such as a reactor core. The formulation is developed in a form consistent with the computational methods used for depletion analysis. The analysis technique currently employed in most burnup calculations is first reviewed as a method for describing the nonlinear coupling between the flux and nuclide fields. It is shown that based on the present formulation three adjoint equations (for flux shape, flux normalization, and nuclide density) are required to account for the coupled variations arising from variations in initial conditions and nuclear data. The adjoint equations are derived in detail using a variational principle, and an algorithm is suggested for solving the coupled equations backward through time. Perturbation expressions are used to define sensitivity coefficients for responses that depend on the coupled interaction between the neutron and nuclide fields. The relation between coupled and noncoupled sensitivity theory is illustrated. Finally, two analytic example problems are solved that determine the sensitivity of some final nuclide concentration to changes in initial conditions. Results obtained from direct calculation and from the coupled perturbation theory are compared.
TL;DR: The potential existing vegetation cover of the United States was determined, using a computer algorithm developed to subtract Conservation Needs Inventory land-use data from Kuechler's Potential Natural Vegetation of United States.
Abstract: The potential existing vegetation cover of the United States was determined, using a computer algorithm developed to subtract Conservation Needs Inventory land-use data from Kuechler's Potential Natural Vegetation of the United States. This was done on a county-by-county basis for the 48 conterminous states. The results indicate that 23 of the 106 vegetation types have each been reduced by over 50% by man-induced land uses. In contrast, only 26 types have had 5% or less of their potential area converted to urban, agricultural, pasture, or other miscellaneous uses. Tule marshes showed the greatest destruction, losing 88% of their original area, primarily to agriculture. Bluestem prairie and its transition zone with oak-hickory forest declined by 85% and 77%, respectively, primarily due to agriculture. Over 41 x 106 ha of these two vegetation types have been converted to other uses. The agricultural states of Iowa, Illinois, and Indiana have the least amount of land covered by natural vegetation - 8, 11, and 18%, respectively. Despite the drawbacks of using Kuechler's Potential Natural vegetation of the United States and CNI land-use data, the results give a fairly accurate and startling portrayal of the area occupied by natural vegetation.
TL;DR: Guiding-centre trajectories in non-circular axisymmetric tokamak plasmas are studied in this article, where it is shown that the existence of an absolute minimum B in the higher- (7.7%) equilibrium results in an orbit topology dramatically different from that of the lower case.
Abstract: Guiding-centre orbits in non-circular axisymmetric tokamak plasmas are studied in the constants of motion (COM) space of (v, ζ, ψm. Here, v is the particle speed, ζ is the pitch angle with respect to the parallel equilibrium current, J||, at the point in the orbit where ψ = ψm, and ψm is the maximum value of the poloidal flux function (increasing from the magnetic axis) along the guiding-centre orbit. Two D-shaped equilibria in a flux-conserving tokamak having values of 1.3% and 7.7% are used as examples. In this space, each confined orbit corresponds to one and only one point, and different types of orbit (e.g. circulating, trapped, stagnation and pinch orbits) are represented by separate regions or surfaces in the space. It is also shown that the existence of an absolute minimum B in the higher- (7.7%) equilibrium results in an orbit topology dramatically different from that of the lower- case. The differences indicate the confinement of additional high-energy (v → c, within the guiding-centre approximation), trapped, co- and counter-circulating particles, with an orbit ψm falling within the absolute B-well.
TL;DR: Nitrogen mineralization and net nitrification rates were 3–7 times greater in soil incubations from a girdled Liriodendron tulipifera (L.) stand than in a control stand, suggesting that nitrification is regulated by the availability of NH4+ in these stands.
Abstract: Nitrogen mineralization and net nitrification rates were 3–7 times greater in soil incubations from a girdled Liriodendron tulipifera (L.) stand than in a control stand. Neither litter nor root extracts had an inhibitory effect on nitrogen mineralization or nitrification rate. A lack of nitrification inhibitors also was demonstrated by the fact that ammonium added to the control stand was completely converted to nitrate upon incubation. Additions of sucrose increased CO2 evolution and decreased nitrogen mineralization and nitrification rates in the girdled plot soil, suggesting that nitrification could be effectively controlled by competition for NH
4
+
supplies by heterotrophic soil organisms. CO2 evolution rates during incubation showed that heterotrophic as well as nitrifier activities were greater in the girdled plot soil than in the ungirdled plot soil, but the ratio of C to N mineralized was lower in the girdled plot soil. These results collectively indicate that nitrification is regulated by the availability of NH
4
+
in these stands, and that the latter is strongly regulated by heterotrophic demand for N.
TL;DR: The phenotypic expression time, that is, the time interval after mutagen treatment which is necessary befor all mutant cells are able to express the TG-resistant phenotype, has been found to be 7–9 days in this CHO/HGPRT system when the cells are subcultured every 48 h.
Abstract: Mutation induction at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus in Chinese hamster ovary (CHO) cells (referred to as the CHO/HGPRT system) can be quantitated by selection for the phenotype of resistance to 6-thioguanine (TG) under stringently defined conditions. The phenotypic expression time, that is, the time interval after mutagen treatment which is necessary befor all mutant cells are able to express the TG-resistant phenotype, has been found to be 7–9 days in this CHO/HGPRT system when the cells are subcultured every 48 h. Subculture in medium with or without hypoxanthine (HX) utilizing trypsin, ethylenediaminetetraacetic acid (EDTA), or ethylene glycol bis( β -aminoethyl ether)- N , N , N ′, N ′-tetraacetic acid (EGTA) for cell removal yields identical results. When subculture at intervals greater than 48 h is employed, a slight lengthening of the expression time is observed. An alternative method to regular subculture has also been achieved by maintaining the cells in a viable, non-dividing state in serum-free medium. This procedure yields a similar time course of phenotypic expression and thus shows that continued cell division is not essential to this expression process. In addition, this observation offers methodology which can significantly reduce the investment of time and money for mutation induction determinations in this mammalian cell gene mutation assay.