[en] Quantification of radioactive contamination in the environment is often desirable so that health implications can be determined and appropriate remedial measures taken. In-situ gamma spectrometry is a potentially powerful technique that has the advantage of speed and spatial averaging over a large area. Calibration, however, is complex not least because the distribution of the activity in the field is generally unknown. To overcome this problem, methods have previously been proposed to derive information about the vertical activity distribution. All of these methods assume a fixed simple activity distribution model, the parameters of which are derived from varying in-situ spectral region ratios. In this work, these methods have been extensively assessed and compared both theoretically and experimentally for the quantification of ¹³⁷Cs contamination. In all cases, the best method was that which used a lead plate to alter the contributing angular interval. Experimentally, this Lead Plate Method predicted the activity concentration to within a factor of between 1.50 and 1.66 on average. A new method, using collimated detector measurements within a shallow well was developed and theoretically optimised in terms of number and depths of detector positions and type of collimation. This optimised Submerged Detector Method was assessed and compared with the Lead Plate Method both theoretically and experimentally. The latter comparison involved 19 sites across Wales, UK where there is a wide range of ¹³⁷Cs activity levels and vertical distributions. The Submerged Detector Method was found to be more accurate both in the theoretical modelling assessment and in the experimental comparison, where the method was found to predict the activity concentration to within a factor of 1.35 on average. Distribution profiles predicted by the Submerged Detector Method were also found to be generally closer to the true profiles than those obtained using the Lead Plate Method. (author)

[en] Problems with scaling of conductive-system experimental M_dat^#double_prime#(ω) and σ_datprime(ω) data are considered and resolved by dispersive-relaxation-model fitting and comparison. Scaling is attempted for both synthetic and experimental M^#double_prime#(ω) data sets. A crucial element in all experimental frequency-response data is the influence of the high-frequency-limiting dipolar-and-vibronic dielectric constant var-epsilon_D∞, often designated var-epsilon_∞, and not related to ionic transport. It is shown that var-epsilon_D∞ precludes scaling of M_dat^#double_prime#(ω) for ionic materials when the mobile-charge concentration varies. When the effects of var-epsilon_D∞ are properly removed from the data, however, such scaling is viable. Only the σprime(ω) and var-epsilon^#double_prime#(ω) parts of immittance response are uninfluenced by var-epsilon_D∞. Thus, scaling is possible for experimental σprime(ω) data sets under concentration variation if the shape parameter of a well-fitting model remains constant and if any parts of the response not associated with bulk ionic transport are eliminated. Comparison between the predictions of the original-modulus-formalism (OMF) response model of 1972 - 1973 and a corrected version of it that takes proper account of var-epsilon_D∞, the corrected modulus formalism (CMF), demonstrates that the role played by var-epsilon_D∞ (or var-epsilon_∞) in the OMF is incorrect. Detailed fitting of data for three different ionic glasses using a Kohlrausch - Williams - Watts response model, the KWW₁, for OMF and CMF analysis clearly demonstrates that the OMF leads to inconsistent shape-parameter (β₁) estimates and the CMF does not. The CMF KWW₁ model is shown to subsume, correct, and generalize the recent disparate scaling/fitting approaches of Sidebottom, Leon, Roling, and Ngai. [copyright] 2001 American Institute of Physics

[en] Some aspects of a future experiment using stopped K⁺s which could confirm the single-event observation of Brookhaven E787 and obtain a statistically interesting determination of Standard Model parameters are discussed. (author)

[en] The results of accurate calculations of collision integrals for the attractive static screened Coulomb potential are presented. To obtain high accuracy with minimal computational cost, the integrals are evaluated by a quadrature method based on the Whittaker cardinal function. The collision integrals for the attractive potential are needed for calculation of the electrical conductivity of a dense fully or partially ionized plasma, and the results presented here are appropriate for the conditions in the nondegenerate envelopes of white dwarf stars. 25 refs

[en] Two different analyses of impedance data obtained from ultra-pure water allowed to equilibrate with the atmosphere have recently appeared. They both thus show much smaller low-frequency resistances than does ultra-pure water. Different fitting models were used in these analyses and led to appreciably different parameter estimates from their data fits. Their two “pure” water experimental data sets are here analyzed with a Poisson-Nernst-Planck model that incorporates the possibility of dissociation of a neutral species to positive and negative charges of arbitrary mobilities, anomalous diffusion in the interface region, and reaction of mobile ions at the electrodes. Complex-nonlinear-least-squares fitting of these data sets with either charges of a single sign mobile or with those of both signs mobile showed that the one-mobile choice was far superior to the two-mobile one. These results were compared both with newly calculated theoretical ultra-pure water immittance ones and with the results obtained in the earlier two papers, where different Poisson-Nernst-Planck-related fitting models were employed. Both involved the restrictive assumptions of full dissociation and two-mobile behavior with equal mobilities of the positive and negative charges. The dominant mobile charge species present in the equilibrated “pure” water data sets (protons for the ultra-pure water), involved mobile impurity ions, possibly oxygen ones. The Poisson-Nernst-Planck model used here is simpler than the other models, and it led to better fits of the data sets and to more physically significant parameter estimates than did the earlier fits

[en] Observations of soft X-rays emitted by the hot stellar remnant of classical nova outbursts provide a means of probing the late stage of the evolution of the remnant. Determination of the rate of evolution of the remnant may discriminate between theoretical ideas of how mass is lost from the nova envelope during optical decline and provide an estimate of the white-dwarf mass that is independent of the distance. As a step in this direction, model atmosphere calculations are presented of the X-ray emission from the photospheres of hot, high-gravity stars, which can be used to interpret forthcoming Rosat data. 36 refs

[en] The rapid and accurate quantification of radioactivity in the ground is important in many instances to enable appropriate decisions to be made about limiting its use. In-situ gamma spectrometry has been shown to be a potentially powerful technique for providing this information but its accuracy is dependent on knowledge of the activity variation with depth which is generally unavailable. Methods that allow this distribution to be rapidly determined have previously been published but all rely on a fixed depth-distribution model which is unrealistic. A novel technique using submerged collimated detector measurements termed the Submerged Detector Method (SDM) has been proposed, and theoretically assessed in earlier work and has given promising results. In this work, the SDM is experimentally assessed at 19 sites in Wales, U.K., of variable ¹³⁷Cs activity levels and profile shapes. Practicality and accuracy both in the derived activity per unit area and the profile shapes were assessed against the best of the other methods. The results confirm that the SDM gives more accurate results with the derived activity per unit area on average within ∼40% of the true value and the depth distributions being more accurately determined

[en] Stratified hydrogen/helium envelope models in diffusive equilibrium are calculated for a 0.6-solar-mass white dwarf for effective temperatures between 10,000 and 80,000 K in order to investigate the observational constraints placed on the total hydrogen mass. Convective mixing is included ab initio in the calculations, and synthetic spectra are used for comparing these models with observational materials. It is shown that evolutionary changes in the surface composition of white dwarfs cannot be explained by a model in which a small amount of hydrogen floats to the surface from initially being mixed in the outer parts of a helium envelope. It is pointed out that the shape of the hydrogen lines can be used for constraining theories of convective overshoot. 91 refs

[en] The stability of the electrode materials used in hot phosphoric acid fuel cells, in particular the conductive carbon electrocatalyst support, is a continuing concern. The Tafel slope of the carbon corrosion reaction appears to be indicative of the degree of disorder on the carbon surface--the larger the Tafel slope, the greater the degree of disorder. The effect of heat-treatment was found to depend on the structure of the parent carbon. Differences in the electrochemical behavior of samples of heattreated Vulcan XC-72R were correlated with the structural properties of the carbon produced, as reflected by lattice parameters. Changes in BET surface areas and lattice parameters as well as in electrochemical behavior showed evidence that the addition of boron resulted in ''graphitization'' that was not achieved by heat-treatment alone

[en] Simple theories of hydrogen shell flashes on accreting white dwarfs are used to find the requisite conditions for classical novae. It is shown that enhancement of CNO abundances, relative to the Sun, is necessary for fast novae. Yet slow novae, such as DQ Her, can still occur with large CNO abundances. An analysis of observational data indicates that the white dwarfs in nova binaries have masses typically 1.1 solar masses. For some old novae, the accretion rate, deduced from the accretion disk luminosity, is too high to permit a strong enough hydrogen flash to give the observed nova strength. Some possible resolutions to this paradox are suggested. (Auth.)