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Advanced biomolecular materials based on membrane-protein/polymer complexation

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The goal of this project was to apply neutron reflectometry and atomic force microscopy to the study of lipid membranes containing proteins. Standard sample preparation techniques were used to produce thin films of these materials appropriate for these techniques. However, these films were not stable, and a new sample preparation technique was required. Toward this goal, the authors have developed a new capability to produce large, freely suspended films of lipid multi-bilayers appropriate for these studies. This system includes a controlled temperature/humidity oven in which the films 5-cm x 5-cm are remotely drawn. The first neutron scattering experiments were then performed using this oven.
Date: December 1, 1998
Creator: Smith, G.S.; Nowak, A. & Safinya, C.
Partner: UNT Libraries Government Documents Department

Overview of the Target Systems for the Spallation Neutron Source

Description: The technologies that are being utilized to design and build the target systems for a state-of-the-art accelerator- based neutron source, the Spallation Neutron Source (SNS), are discussed. Emphasis is given to the technology issues that present the greatest challenges. The present facility configuration, ongoing analysis, and planned research and development program are also described.
Date: October 15, 1998
Creator: Gabriel, T.A.; Haines, J.R. & McManamy, T.J.
Partner: UNT Libraries Government Documents Department

{beta}{sub eff} measurements in reflected reactors using a Rossi-{alpha} technique

Description: In this report the authors calculate the effective delayed neutron fraction equation for reflected reactors using the Rossi-{alpha} technique. They determine that the equation for the delayed neutron fraction is just as valid for reflected systems as it is for unreflected systems.
Date: January 20, 1998
Creator: Doulin, V.A. & Spriggs, G.D.
Partner: UNT Libraries Government Documents Department

On the Rossi-{alpha} measurement of {beta}{sub eff} in reflected reactors

Description: In unreflected reactors, the probability of detecting chain-related counts is given by the well known Rossi-{alpha} expression. Using this the author derives an expression for the effective delayed neutron fraction, {beta}. He demonstrates that this expression for {beta} is equally applicable for a reflected systems in which two alphas are experimentally observed.
Date: March 13, 1997
Creator: Doulin, V.A. & Spriggs, G.D.
Partner: UNT Libraries Government Documents Department

SNS moderator design

Description: The pulsed-neutron source SNS facility will start operation at 1 MW. A later upgrade to 5 MW is planned. The facility consists of a linear accelerator, an accumulator ring, and a target station. The protons from the accumulator ring will be injected into the target station at 1 GeV. The subsequent spallation process will then produce low-energy thermal neutrons that may be used for a wide variety of experiments. In this paper the authors discuss neutronic calculations which address various aspects of the moderate design. The computer codes HETC and MCNP were used for these calculations with the former code performing the high-energy transport. Neutrons which fell in energy to 20 MeV or less were then passed to MCNP for further transport.
Date: December 1, 1997
Creator: Charlton, L.A.; Barnes, J.M.; Gabriel, T.A. & Johnson, J.O.
Partner: UNT Libraries Government Documents Department

The performance of reflectometers at continuous wave and pulsed-neutron sources

Description: To quantify gains from time-of-flight (TOF) methods, identical reflectometers viewing a continuous wave (CW) neutron source and a variety of pulsed-neutron sources were simulated using a Monte Carlo technique. Reflectivity profiles obtained for a simple thin-film, reflecting,sample were nearly identical in all simulations, and models fitted to the simulated data yielded parameters (film thickness, surface roughness, and scattering length density) that were equally accurate and precise in all cases. The simulations confirm the power of the TOF method and demonstrate that the performance of pulsed sources for reflectometry does not scale simply as the inverse duty factor of the source. In the case of long-pulse sources, the simulations suggest that pulse tails have little effect on results obtained from specular reflectometry and that maximum brightness of the neutron source should be the primary design criterion.
Date: December 1995
Creator: Fritzsimmons, M. R. & Pynn, R.
Partner: UNT Libraries Government Documents Department

K Reactor moderator distortion study

Description: Irradiation-induced moderator distortion has plagued the Hanford reactors from their earliest days until the present. The first-manifestation of graphite distortion was in the form of expansion of the B, D and F stacks. It was found that expansion was very temperature-dependent and could be reversed by raising the graphite operating temperature. The DR, H, C and K Reactors were designed with provisions to minimize, or accommodate expansion. It was not until after the K Reactors had been designed that graphite contraction was recognized as a threat to stack life. Under the present operating conditions, contraction is largely dependent upon neutron flux levels. To date, this study has been largely concerned with the K Reactors, since the vast bulk of the distortion data available is from the Ks. This study not only attempts to explain the mechanisms by which the K Reactors arrived at their present state of distortion but also provides a basis for forecasting future distortion. An understanding of the nature and magnitude of the forces at work within the stacks will enhance the possibility of devising means of conteracting the present trends.
Date: June 22, 1964
Creator: Alexander, W.K. & Russell, A.
Partner: UNT Libraries Government Documents Department

Assessment of aluminum structural materials for service within the ANS reflector vessel

Description: Most of the components in the Advanced Neutron Source (ANS) reactor, including the reflector vessel, will be built from the aluminum alloy 6061 (lMg,0.6Si) in its precipitation-hardened T6 and T651 conditions. The microstructural and mechanical characteristics of the alloy are described, and its operating boundaries of stress, temperature, and time in its unirradiated state are defined. The material`s responses to neutron radiation exposure in aqueous environments are reviewed in detail. The particular service conditions of stress, temperature, and radiation exposure expected for individual components in the ANS are listed, and the suitability of each component to meet the demands is assessed. Areas of uncertainties are outlined, and various suggestions and recommendations are made to give improved confidence in the predictions.
Date: August 1, 1995
Creator: Farrell, K.
Partner: UNT Libraries Government Documents Department

Coupled moderator neutronics

Description: Optimizing the neutronic performance of a coupled-moderator system for a Long-Pulse Spallation Source is a new and challenging area for the spallation target-system designer. For optimal performance of a neutron source, it is essential to have good communication with instrument scientists to obtain proper design criteria and continued interaction with mechanical, thermal-hydraulic, and materials engineers to attain a practical design. A good comprehension of the basics of coupled-moderator neutronics will aid in the proper design of a target system for a Long-Pulse Spallation Source.
Date: December 1, 1995
Creator: Russell, G.J.; Pitcher, E.J. & Ferguson, P.D.
Partner: UNT Libraries Government Documents Department

Coupling parameters for partially reflected reactors

Description: For situations in which the standard point kinetic model does not adequately characterize the kinetic behavior of a reflected system, the Avery-Cohn differential equations can be used. However, these equations require that one determine the coupling parameters between the core and the reflector, f{sub cr} and f{sub rc}. The coupling parameter, f{sub cr}, represents the probability that a neutron in the core will leak into the reflector, and the coupling parameter, f{sub rc}, represents the probability that a neutron in the reflector will scatter back into the core. As discussed in Reference 3, these two coupling parameters can be calculated from the multiplication factor of the bare core, k{sub c}, the effective multiplication factor of the integral system, k{sub eff}, and the fraction of system neutrons absorbed in the core region, P{sub ca}. The methodology presented in Ref. 3 was described for a fully reflected system, but it is also applicable to some types of partially reflected systems. In particular, it is applicable to those systems where neutrons leaving any core surface not contiguous to the reflector have a zero probability of entering the reflector. In other words, these surfaces have a view factor of 0 to all reflector surfaces in the system. However, if the view factor between an unreflected core surface and a reflector surface is not zero, then the aforementioned methodology has to be modified. To calculate f{sub cr}, one must include an estimate of the single-pass probability that a neutron escapes from the core to infinity, f{sub ci}. This is accomplished by including a view factor(s) in the calculations that accounts for the fraction of neutrons that are not traveling on a line intersecting some portion of the reflector. This paper illustrates this modification by assuming the partially reflected system shown in Fig. 1c.
Date: July 1, 1995
Creator: Busch, R.D. & Spriggs, G.D.
Partner: UNT Libraries Government Documents Department

Critical mass analysis for {sup 235}U and {sup 239}Pu systems moderated and reflected by D{sub 2}O

Description: Criticality dimensions for highly enriched {sup 235}U(93.5) and {sup 239}Pu(95.5) systems mixed with D{sub 2}O were studied. The objective of this work is to investigate the minimum critical mass and concentration of uranium and plutonium systems in a reflector-moderated arrangement. The present work demonstrates the critical instability of these systems that are moderated and reflected by D{sub 2}O and expands in previous unpublished work. These calculations were performed in a spherical geometry with the DANTS codes using the Hansen-Roach cross section library. Densities examined ranged from normal to very small, and are assumed to be uniform throughout the core. These spherical systems are reflected by 100 cm of D{sub 2}O.
Date: December 31, 1998
Creator: Loaiza, D. & Stratton, W.
Partner: UNT Libraries Government Documents Department