K. Verghese

Measured molecular coherent scattering form factors of animal tissues, plastics and human breast tissue.

Photon scattering angular distributions from various animal tissues were measured at two energies of a monochromatic synchrotron x-ray beam. Two plastics and human breast tissue were also measured. From these two measurements, the molecular coherent scattering form factor of each material was extracted. A new data analysis technique that uses Monte Carlo based corrections for air scattering, incoherent scattering and multiple scattering was used. The form factors of the 16 materials are presented in tabular form, suitable for use in computer calculations.

On the solid angle subtended by a circular disc

Abstract The solid angle subtended by a circular disc from a point is approximated by the analytical expression for the solid angle for n-sided, regular polygon of area equal to that of the disc. When the point is farther from the plane of the disc than 0.1 the disc radius, polygons of 20 and 36 sides are required for approximating the solid angle subtended by the disc to within 1 and 0.1% respectively. In addition, the average solid angles subtended by circular discs from circular discs are calculated and given in tabular form for a range of disc sizes.

Hydrogen diffusion and solubility in pyrolytic carbon

Abstract Tritium diffusion coefficients and deuterium solubilities have been measured for laminar pyrolytic carbon in the temperature range 900–1500°C. The tritium diffusion coefficients were much lower than those for metals at equivalent temperatures, but the activation energy for the diffusion was much higher (∼-100 kcal/mole). Tritium diffusion coefficients measured for silicon-doped pyrolytic carbon were over an order of magnitude higher than the values for the undoped laminar pyrolytic carbon. The solubility of deuterium in laminar pyrolytic carbon was found to decrease with increasing temperature and exhibited a pressure dependence of p. 1 2

An investigation of the possible interaction mechanisms for Si(Li) and Ge detector response functions by Monte Carlo simulation

Abstract Simple physical mechanisms implemented by either analog Monte Carlo simulations or analytical models have been used to investigate the Si(Li) and Ge detector response functions for X- and gamma rays, respectively. The mechanisms investigated include the various possible combinations of partial losses of photoelectric and Auger electrons from the detector surfaces and complete losses of the various photons involved such as the Si K X-ray, the 0.511 MeV annihilation photons, and single or multiple Compton scattered photons. The most probable interaction mechanisms for each detector response function feature are identified and the simple analytical functions that have been used in the past are justified.

Surface effects on tritium diffusion in niobium, zirconium and stainless steel

Abstract Tritium diffusion in niobium, Zircaloy-2 and stainless steel has been studied by measurement of both tritium concentration profiles and surface tritium release rates. Concentration profiles show buildup of tritium in the surface layers of a specimen with classical diffusion behavior at depths greater than about 5 μm from the surface. Application of a two-region diffusion model to the experimental data gives tritium diffusion coefficients in the surface films which are lower than the bulk diffusion coefficients by two orders of magnitude in stainless steel and eight to ten orders of magnitude in niobium and Zircaloy over a temperature range of interest for fission and fusion reactor systems. The surface effect appears to be a consequence of oxide film formation and is not due to the helium injected into specimens along with the tritium.

An improved Si(Li) detector response function

Abstract An improved Si(Li) detector response function model over the range from 5 to 60 keV has been developed. The values of the model parameters are obtained by least-squares fitting of the pulse-height spectra from a number of pure-element samples excited by 109 Cd or 241 Am sources. Simple functional forms of the separable parts of the detected X-ray pulse-height spectrum as functions of incident energy are summed to produce the generalized response function. Compared to previous work [Nucl. Instr. and Meth. A243 (1986) 121, ref. [1]] the following changes and improvements have been made: (1) the entire useful energy range of the Si(Li) detector from 5 to 60 keV is treated rather than the previous upper limit of about 20 keV, (2) a Compton-scattering continuum function from both the detector and the surrounding material has been added to account for the additional features observed with high-energy X-rays, (3) consideration of radiative Auger satellite X-ray lines has been found to account for the previously observed discrepancy in the model parameters for K α and K β X-rays, (4) the resulting generalized detector response function has a much simpler and more consistent form than the previous one, ref. [1] and (5) the new model parameters expressed as functions of energy are more consistent with physical phenomena than before and are not just fitted polynomials. Applications of the new Si(Li) detector response function are discussed.

A semi-empirical model for the gamma-ray response function of germanium detectors based on fundamental interaction mechanisms

Abstract A model for describing the response function of large volume gamma-ray detectors over the range of incident gamma ray energies between 1.0 and 6.5 MeV has been developed and applied to a 39% high purity germanium detector. Functional forms for describing the various features of the response function are based either on empirical functions or on the shape of exact analytical expressions for the interaction mechanisms as in the case of multiple Compton scattering of the primary and the annihilation photons. These functions are combined and fit by an appropriate linear-nonlinear least squares method to measured single energy photon spectra. The model parameters so obtained are then fit to simple functions of the incident gamma-ray energies to form the complete response function of the detector. Validity of the model is demonstrated by synthesizing a 49Ca spectrum using the response function and comparing it to the equivalent measured spectrum.

Solid angle subtended by a circular cylinder

Abstract The solid angle subtended at a point by the lateral surface of a right circular cylinder is approximated by replacing the cylinder surface area seen from the point by a regular polygonal cylindrical surface. This approximation combined with a similar approximation for the solid angle subtended by a circular disc, enables the calculation of the total solid angle from a right circular cylinder to any desired degree of accuracy.

Monte Carlo calculation of efficiencies of right-circular cylindrical NaI detectors for arbitrarily located point sources

Abstract An efficient Monte Carlo program for calculation of total intrinsic efficiency, peak-to-total ratio and source intrinsic efficiency of bare right-circular cylindrical NaI(Tl) detectors from an arbitrarily located isotropic point source emitting photons of energy 1 MeV or below is outlined. Total variance reduction based on physical principles is used throughout to insure that each proton history is successful in order to minimize computer time. Results show close agreement with other Monte Carlo calculations for sources located along the detector axis and with experiments for sources located off-axis.

Tritium diffusion in zircaloy-2 in the temperature range −78 to 204° C

Tritium diffusion measurements in Zircaloy-2 were carried out over the temperature range −78 to 204 °C by direct measurement of tritium diffusion gradients. The 6Li (n, α)3H reaction was used to inject tritium into the specimens and to produce initial tritium concentration in the range 0.0065 ppm to 0.013 ppm 3H by weight. Two diffusion components were identified from the concentration profiles: a surface trapping region approximately 5 μm thick and a normal diffusion profile characteristics of bulk diffusion. Surface release measurements of tritium verified the existence of a surface trapping layer. The bulk diffusion component was consistent with classical diffusion solutions and was given by: D = 0.00021−0.00018+0.005 exp−(8500 ± 200 cal/RT) cm2 · sec−1. The surface trapping was attributed to oxide films formed on the Zircaloy-2 at room temperature. The apparent diffusion coefficients for the surface region were consistent with: D = 4.0−3.3+19.7 × 10−14 exp−(7200 ± 1500 cal/RT) cm2 · sec−1 over the temperature range 25 to 411°C.