Showing papers by "Chalk River Laboratories published in 1987"

Genetics of the polycross : 1. Experimental results from Norway spruce.

Abstract: Rates and patterns of male gamete incorporation for a polycross mating design were studied for two independent years of pollination in Norway spruce, Picea abies (L) Karst. Segregation distortion in a subset of maternal clones was documented for one locus. We have proposed a model, involving the existence of a linked lethal allele, which accounts for these observations. Significant temporal and maternal clonal differences were observed in the rates at which single locus and multilocus gametes were incorporated. Striking differences in apparent fertility existed among four clones which produced unique multilocus gametes. One clone, in particular, was shown to be contributing three times as many gametes to the next generation as predicted by the hypothesis of equal clonal male contribution. These deviations from expectation were also detected in the genotypic distributions of the resultant filial generation. Ramifications of these results on family structures in the filial generation, effective size of the male population, and possible bias in inferences of genetic differences and parameter estimation are discussed.

Molecular Evidence for Cleavage of Intradimer Phosphodiester Linkage as a Novel Step in Excision Repair of Cyclobutyl Pyrimidine Photodimers in Cultured Human Cells

Abstract: SUMMARY A re-analysis of the metabolic fate of ultraviolet light (u.v.)-induced cyclobutyl pyrimidine dimers in the DNA of dermal fibroblasts from patients with different genetic forms of xeroderma pigmentosum (XP), a rare cancer-prone skin disorder, has provided new insight into the mode of dimer repair in normal human cells. When DNA isolated from post-u.v. incubated cultures was subjected to enzymic photoreactivation (PR) to probe dimer authenticity, single-strand scissions were produced in the damaged DNA of incubated XP group A and D cells, but not in DNA from XP group C cells or normal controls. Since enzymic PR treatment ruptures only the cyclobutane ring, these results suggested that in dimer excision-defective XP group A and D strains, the intradimer phosphodiester bond may have been cleaved without site restoration. Such a cleavage event had not previously been detected; the possibility that this reaction may be an early step in the normal excision-repair process is supported by the observed release of free thymidine (dThd) and its monophosphate (TMP), but not of thymine, upon photochemical reversal of the dimer-containing excision fragments isolated from post-u.v. incubated normal cells. The combined number of dThd and TMP molecules released was equal to ≈80% of the number of dimers photoreversed; for such release to occur, the dimer must both be at one end of an excised fragment and contain an internal phosphodiester break. Taken together, these data lead us to propose a novel model for dimer repair in human cells in which hydrolysis of the intradimer phosphodiester linkage precedes the concerted action of a generalized ‘bulky lesion-repair complex’ involving conventional strand incision/lesion excision/repair resynthesis/strand ligation reactions.

Genetic diversity within and among 11 juvenile populations of green alder (Alnus crispa) in Canada

Abstract: Germinated seeds from 11 populations of green alder [Alnus crispa (Ait.) Pursh] sampled in four Canadian provinces were analysed for electrophoretically demonstrable diversity of 10 enzymes encoded by 15 structural loci. Of these, nine were polymorphic, and on average, 52% of the loci per population were polymorphic. Assuming a diploid model of expression, average level of expected heterozygosity was 0.11 with nearly all populations in Hardy-Weinberg equilibrium for the set of polymorphic loci analysed. No significant inbreeding and associated subpopulation structuring were noted. Rates of gene flow appeared high within and among populations. Although little divergence was observed among populations, genetic and geographical distances between populations were related. Discriminant and cluster analyses revealed a pattern of genetic variation associated with geography. Populations from northern Quebec were poorly differentiated, whereas western populations from Alberta exhibited a larger degree of genetic differentiation. Introgresive hybridization with the sympatric species Alnus sinuata (Regel) Rydberg and partial isolation in the West are suggested as an explanation for this larger differentiation. The occurrence and significance of rare alleles is discussed in relation to the importance of geographical distance in the process of population differentiation in this species.

A further calibration of the harwell series II Bi-implanted RBS standards

Abstract: An extensive investigation of the Harwell series II RBS standards, using selected samples and edge pieces from each of the six implantation runs, shows that the previously reported [1] implantation problem is much more complex than had been assumed. The following conclusions can now be drawn: o (i) Only two of the implantation runs produced Bi contents within ±1% of the series I value; most of the other runs exhibit significantly higher Bi levels. (ii) Even within a single implantation run, gradual variations as large as ±2.5% occur across the set of four wafers. However, over the dimensions of a single sample (1 × 1 cm), the uniformity was always better than ±1%. In view of the above complexities, each series 11 standard requires cross-calibration whenever an absolute accuracy of better than ± 2.5% is desired.

Tritium implantation and depth profiling in fusion related materials

Abstract: Tritium behaviour in solids and particularly its permeation and inventory in the first wall, limiters, breeding blanket materials and in other structural elements of fusion reactors is a subject of great concern in all projects aiming at D + T fusion. In the present work elastic recoil detection (ERD) under 4 He bombardment and the T(d, α)n nuclear reaction analysis (NRA) in the forward detection geometry were applied to the depth profiling of tritium at submicron distances below the surface of selected fusion related materials. Experimental results obtained for tritium implanted in titanium, graphite and lithium aluminate LiAlO 2 are presented as the examples.

Corrosiveness of Decontamination Solutions to Sensitized AISI 304 Stainless Steel

Abstract: An experimental study of the corrosiveness of citric acid, oxalic acid, and EDTA mixtures to sensitized AISI 304 stainless steel (SS) was conducted. In initial tests, the corrosivity of various solutions at 125 C was assessed by measuring anodic polarization curves and estimating corrosion current densities (CDs), critical CDs for passivity, and passivation CDs. Based on the electrochemical results, solutions were ranked from highest to lowest according to their corrosiveness when applied to sensitized AISI 304 SS as follows: citric acid/oxalic acid, citric acid/oxalic acid/EDTA, and citric acid/ EDTA. Scanning electron microscopy (SEM) and metallographic examinations of specimens exposed at rest potential to solutions of varying corrosiveness show that intergranular attack (IGA), IGA with pitting at grain boundaries, and general pitting occurred only in samples immersed in solutions containing oxalic acid. No surface attack was detected in specimens immersed in citric acid/EDTA mixtures. These r...

Neutralization effect for Ne + scattering from a Cu(110) surface

Abstract: Intensity and structure of the energy spectra of Na + and Ne + ions scattered from a Cu(110) surface are governed by multiple scattering and neutralization effects. These were studied for ion energies between 600 eV and 1 keV and in the temperature range from 100 to 600 K in the experiment and by computer simulation. Na + scattering directly reflects the crystallographic structure of the (110) surface. The temperature effects can be used to analyze thermal motions of surface atoms in terms of a surface Debye temperature for specific vibrational directions. The contributions of single and multiple scattering events to the energy spectra are analyzed and for Ne + a strong trajectory-dependent neutralization is found. The comparison of the neutralization of Ne + and Na + leads to a Ne + ion survival probability of a few percent for single scattering, less than 1% for double scattering, and a value of less than 10 −3 for scattering from atoms below the top atomic layer. A simple neutralization model is developed to explain the observed survival probabilities.

Direct mass measurements at the TASCC facility: The Manitoba-Chalk River on-line mass spectrometer☆

Abstract: The Chalk River On-line Isotope Separator has been used by itself to determine the masses of unstable nuclei. The addition of a high-resolution mass spectrometer for the further analysis of beams from the separator, currently on-line at the TASCC facility at Chalk River, offers an improvement in the accuracy and sensitivity of the mass measurement system. The characteristics and projected performance of the proposed apparatus are described.

Axial Strains at a Girth Weld in a 914 mm Linepipe

Abstract: High resolution neutron diffraction measurements have been made of axial strains in and adjacent to a multipass girth weld in a complete section of 914 mm (36″) linepipe. The experiments were carried out at the NRU reactor, Chalk River, with 1 mm wide apertures before and after the sample to define the location of the strain measurement. The diffraction angle was measured to a precision of ±0.003° leading to an accuracy of 1 in 104 in interplanar spacing. The strains were measured in the subset of grains whose [110] direction of the body centred cubic structure was aligned along the axis of the pipe. The strains were measured at ten positions through the 16 mm thickness of the pipe on the weld centre and offset by 4, 8, 20 and 50 mm from the weld centre. In addition, the spatial variation of axial strain as a function of axial distance from the weld centre was mapped out along the mid-thickness, near the root of the girth weld, and at the three-quarter-thickness position. The mid-thickness is in a state of tensile strain with respect to the outside and inside surfaces of the pipe. The effect is strongest at the weld centre and is less, though still present, well away from the weld. The strain peak at mid-thickness extends 5 mm from the weld centre, followed by a minimum at about 9 mm from the weld centre quite close to the edge of the weld. A further slight but extended maximum in strain is centred at 20 mm from the weld centre, well outside the weld. At a position 3 mm from the inside surface of the pipe, a broad strain minimum on the weld centre and a broad maximum centred at 20 mm from the weld centre are observed. The results are compared with strain gauge measurements on the weld surface and through-wall residual stress distributions measured using a layering technique.

Level structure of 68 149 Er81 and high-spin isomerism in proton-richN=81, 82, 83 nuclei

Abstract: The level structure of theN=81 nucleus149Er has been studied by γ-ray spectroscopy following the reaction92Mo+255 MeV60Ni. Yrast levels in149Er are established up to ~3.3 MeV, including 0.61 and 4.8 μs isomeric states. Most of the observed levels are interpreted as seniority-three states arising from the coupling of s1/2, d3/2 and h11/2 neutron holes with πh112/n. Isomers identified in the reaction96Ru+255 MeV58Ni are tentatively assigned to151Yb. TheB (E2) values of high-spin isomers inZ=66−70,N=81−83 nuclei are surveyed.

Mössbauer and Exafs Study of 57Fe Labelled Pt/SiO2 Catalysts

Abstract: Specimens of highly dispersed Pt/Silicalite catalyst, decorated with 57Fe by impregnation in aqueous ferric nitrate solution, have been studied after decoration, and various pretreatments including exposure to carbon monoxide, by in situ Mossbauer spectroscopy. The form of Pt was followed by EXAFS analysis near the Pt LIII edge. The microstructural studies were combined with CO chemisorption and infrared absorption measurements. It is observed that iron in Pt clusters is very reactive even at ambient temperatures and that the interaction of Fe with CO is strongly accelerated by platinum.

Experimental Considerations in the Practice of Metal Ion Separations by Dynamic Exchange Techniques

Abstract: Dynamic equilibria involving ion exchange and/or hydrophobic interactions combined with postcolumn detection can be used for the sensitive and precise determination of a wide variety of metal ions. Conventional high-performance chromatographic instrumentation can be used for such analyses, and recently we have examined some of the practical aspects of this approach to metal-ion determinations. This paper summarizes these results along with some earlier data in an effort to provide a balanced perspective of the associated advantages and pitfalls. Factors discussed include: eluent purity; effect of eluents on selectivity; limitations of construction materials and corrosion of metal system; column stability; and performance of dynamic and static (permanently coated) system.