Browsing by Author "Bertram, WK"
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- ItemAnalysis of (n-2n) cross sections(Australian Atomc Energy Commission, 1969-11) Bertram, WKThe statistical model is used to derive an expression for the (n-2n) excitation function, taking into account (n-3n) competition. The (n-2n) cross sections thus obtained are found to agree with experiment over a wide range of energies extending well beyond the (n-3n) threshold.
- ItemCompound nucleus formulation of reaction matrix theory(Australian Atomic Energy Commission, 1972-02) Cook, JL; Bertram, WKIt is shown that multilevel resonance parameters for each element of the reaction matrix cannot be determined from available data. However, additional constraints may be introduced without affecting agreement with experiment. The Bohr compound nucleus hypothesis is applied to the T-matrix and it is found, as in Newton's model, that the channel matrix can be inverted analytically to provide simple formulae for cross sections, for both the real Wigner-Eisenbud reaction matrix and Moldauer's complex reaction matrix. Wigner-Eisenbud theory leads directly to Newton's strong correlation model and its unacceptable consequences. Moldauer's theory does not and can explain cross section behaviour adequately while being consistent with Bohr's hypothesis. Cross sections can be written as a sum of single level contributions, as in the Adler-Adler formulation. Finally, Moldauer's statistical theory is shown to be applicable, and expressions are derived fr the averaged cross sections as functions of the complex Moldauer resonance parameters.
- ItemCracks and pores - their roles in the transmission of water confined in cementitious materials.(Springer, 2010-10-01) Bordallo, HN; Aldridge, LP; Wuttke, J; Fernando, K; Bertram, WK; Pardo, LCCement paste is formed through a process called hydration by combining water with a cementitious material. Concrete, the worlds most versatile and most widely used material, can then be obtained when aggregates (sand, gravel, crushed stone) are added to the paste. The quality of hardened concrete is greatly influenced by the water confined in the cementitious materials and how it is transmitted through cracks and pores. Here we demonstrate that the water transport in cracks and capillary pores of hardened cement pastes can be approximately modeled by simple equations. Our findings highlight the significance of arresting the development of cracks in cementitious materials used in repository barriers. We also show that neutron scattering is an advantageous technique for understanding how water transmission is effected by gel pore structures. Defining measurable differences in gel pores may hold a key to prediction of the reduction of water transport through cement barriers. © 2010, Springer.
- ItemDelineating the first few seconds of supramolecular self-assembly of mesostructured titanium oxide thin films through time-resolved small angle x-ray scattering(American Chemical Society, 2008-10-07) Luca, V; Bertram, WK; Sizgek, GD; Yang, B; Cookson, DJThe early stages of evaporation induced self-assembly of titanium oxide mesophases from a precursor solution containing TiCl4 and the Pluronic triblock copolymer F-127 in HCl-water-ethanol solution have been studied using time-resolved SAXS techniques. Two experimental protocols were used to conduct these experiments. In one of these, the precursor solution was pumped around a closed loop as solvent was allowed to evaporate at a constant humidity-controlled rate. In the second protocol, a film of precursor solution was measured periodically as it dried completely to a residue under a stream of dry air. This permitted the detailed monitoring of changes in solution chemistry as a function of the elimination of volatile components followed by the actual drying process itself. The SAXS data were modeled in terms of two Guinier radii for soft nanoparticles while a broad Gaussian feature in the scatter profiles was accounted for by particle-article scattering interference due to close packing. For the initial precursor solution, one Guinier radius was found to be about 17 (A) over circle while the other ranged from 4 to 11 (A) over circle. Changing the rate of evaporation affected the two radii differently with a more pronounced effect on the smaller particle size range. Analysis gave an interparticle distance in the range 55-80 (A) over circle for the initial precursor solution which decreased steadily at both of the humidities investigated as evaporation proceeded and the particle packing increased. These results represent the first attempts to monitor in a precise fashion the growth of nano building blocks during the initial stages of the self-assembly process of a titanium oxide mesophase. © 2008, American Chemical Society
- ItemThe effects of multiple scattering on the analysis of USANS data(Australian Institute of Physics, 2004-02-04) Bertram, WKSmall-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) are widely used to investigate the micro- and nano-structure of condensed matter. However, measurements on materials such as cements, clays etc. are often affected by multiple scattering. If a substantial number of neutrons experience more than one scattering event within a sample, the result is broadening of the angular distribution profile. Often samples cannot be made thin enough to guarantee single scattering and even in cases where it is possible to make thin samples, surface effects can sometimes lead to results that are not representative of the bulk material. In addition, for many SANS and USANS measurements there is no reliable method for determining whether or not a sample is thin enough to produce predominantly single scattering. Here we present a new method for extracting singly-scattered data from data affected by multiple scattering. One of the main advantages over our previously published method  is that this method is much easier to apply as it requires no initial modeling and fitting of the data.
- ItemA method for inverting the channel matrix in nuclear resonance theory.(Australian Atomic Energy Commission, 1971-11) Bertram, WKA technique is presented for inverting the channel matrix which occurs in R-matrix theory. The collision matrix can be expressed in a form which does not involve matrix inversions; it can be evaluated by the application of a recurrence relation. The connection between this method and the level matrix formalism is examined.
- ItemThe micro pore structure of concrete determined by small angle neutron scattering(Australian and New Zealand Institutes of Physics, 1994-02-10) Sabine, TM; Bertram, WK; Aldridge, LPSmall angle neutron scattering (SANS) is a complementary technique to electron microscopy for the elucidation of the shape and size of inhomogenities in the nanometric size range. SANS has the advantage that a large volume ( - 0.5 cm3 ) of the specimen is illuminated by the beam, and experiments can be earned out at atmospheric pressure on water saturated samples. It has the disadvantage mat, except for very simple systems, interpretation of the experimental data is not unambiguous. We have collected SANS data on a range of samples of concrete containing different water/cement ratios. The data was collected on the LOQ instrument at the ISIS spallation neutron source. We discuss interpretation of the data in terms of microscopic inhomogenities in the structure of concrete.
- ItemModelling the microstructure of cement(Australian and New Zealand Institutes of Physics, 1994-02-10) Bertram, WK; Sabine, TM; Aldridge, LPTo gain a better understanding of the factors that influence the strength and durability of concrete the microporosity of hydrated cement paste has been investigated, by small angle neutron scattering (SANS). The great advantage that SANS has over the more conventional methods of microporosity analysis such as mercury or gas intrusion, is that for SANS measurements it is not necessary to dehydrate the samples and the process is non-intrusive. However the results from SANS are often open to several different interpretations and it is necessary to obtain additional information where possible .One method of obtaining such additional information is through computer modelling of the process of cement hydration. In this poster we present the results of a 3- dimensional computer model that simulates the growth of a layer of calcium-silicate-hydrate (CSH) on a surface. The model used is similar to that for random aggregates (TA Witten and LM. Sanders, Phys. Rev. Lett. 19,1400 (1981)). The model calculates the porosity and the pore size distributions, which can men be compared with those obtained from SANS measurements.
- ItemOn the non-invariance of distributions of reaction matrix parameters under changes in boundary conditions(Australian Atomic Energy Commission, 1972-03) Bertram, WK; Cook, JLContrary to current opinion, the statistical distributions of level spacings and reduced widths when applied to the reaction matrix, are not invariant under changes in the boundary condition matrix or the matching radius. General arguments are given, together with specific examples which violate the invariance requirements. We conclude that it is the parameters of the collision matrix which should be analysed and considered as the invariant parameters.
- ItemSolution of the inverse reaction problem for complex potentials(Australian Atomic Energy Commission, 1971-02) Bertram, WK; Cook, JLA method is given for solving the inverse reaction problem to obtain complex potentials as in the optical model of the nucleus. The method will reproduce reaction data to the accuracy with which the reaction matrix can be least squares fitted to a sum of simple poles.
- ItemStatistical distribution functions for products of variables with a gaussian distribution with zero mean(Australian Atomic Energy Commission, 1974-09) Bertram, WK; Clancy, BE; Cook, JL; Rose, EKThe statistical distribution of a product of variables which have a Gaussian distribution is investigated. These distributions are found to be given, in general, by special functions. Expansions for these functions for small values of the variable and their asymptotic behaviour are derived. The functions are tabulated for products of up to seven variates. Some simple integrals related to the functions are given.