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A TIME-DEPENDENT METHOD FOR CHARACTERIZING THE DIFFUSION OF RADON-222 IN CONCRETE

Description: The porosity and diffusion length of concrete have been determined by measuring the time-dependent diffusion of radon through a thin slab of the material, One surface of the slab is exposed to a large, fixed radon concentration beginning at t=0. The radon that diffuses out of a portion of the opposite surface is collected during several contiguous time intervals. The total activity collected over a set of intervals beginning at t=0 and the steady-state flux of activity are used to calculate the porosity and diffusion length, As a test of these parameters, they are then used to predict the activity collected during other time intervals and for other sample thicknesses, Samples from two types of concrete were tested: one type yielded a porosity of 0.068 and a diffusion length of 12.6 cm; the respective values for the other were 0.32 and 16.9 cm. The predicted and experimental results agreed well, thereby verifying the assumption that concrete may be treated as a homogenous diffusion medium for radon.
Date: July 1, 1981
Creator: Zapalac, Geordie H.
Partner: UNT Libraries Government Documents Department

First working group meeting on the minority carrier diffusion length/lifetime measurement: Results of the round robin lifetime/diffusion length tests

Description: As was noted in the cover letter that accompanied the samples, the eleven bare silicon samples were from various manufacturers. Table I lists the codes for the samples and the manufacturer of each sample. It also notes if the sample was single or poly-crystalline. The samples had been polished on one side before being sent out for measurements, but no further processing was done. The participants of the study were asked to measure either the lifetime or diffusion length of each of the samples using their standard procedure. Table II shows the experimental conditions used by the groups who measured diffusion length. All the diffusion length measurements were performed using the Surface Photovoltage method (SPV). Table M shows the experimental conditions for the lifetime measurements. All the lifetime measurements were made using the Photoconductance Decay method (PCD) under low level injection. These tables show the diameter of the spot size used during the measurement (the effective sampling area), the locations where measurements were taken, and the number of measurements taken at each location. Table N shows the results of the measurements. The table is divided into diffusion length and lifetime measurements for each sample. The values listed are the average values reported by each group. One of the immediate artifacts seen in the data is the large variation in the lifetime measurements. The values from MIT and Mobil are generally close. However, the measurements from NCSU are typically an order of magnitude lower.
Date: November 1, 1995
Creator: Cudzinovic, M. & Sopori, B.
Partner: UNT Libraries Government Documents Department

Mutual Passivation in Dilulte GaNxAs1-x Alloys

Description: The dilute GaN{sub x}As{sub 1-x} alloys (with x up to 0.05) have exhibited many unusual properties as compared to the conventional binary and ternary semiconductor alloys. We report on a new effect in the GaN{sub x}As{sub 1-x} alloy system in which electrically active substitutional group IV donors and isoelectronic N atoms passivate each other's activity. This mutual passivation occurs in dilute GaN{sub x}As{sub 1-x} doped with group IV donors through the formation of nearest neighbor IV{sub Ga-}N{sub As} pairs when the samples are annealed under conditions such that the diffusion length of the donors is greater than or equal to the average distance between donor and N atoms. The passivation of the shallow donors and the N{sub As} atoms is manifested in a drastic reduction in the free electron concentration and, simultaneously, an increase in the fundamental band gap. This mutual passivation effect is demonstrated in both Si and Ge doped GaN{sub x}As{sub 1-x} alloys. Analytical calculations of the passivation process based on Ga vacancies mediated diffusion show good agreement with the experimental results.
Date: March 21, 2005
Creator: Yu, K.M.; Walukiewicz, W.; Wu, J.; Mars, D.E.; Scarpulla, M.A.; Dubon, O.D. et al.
Partner: UNT Libraries Government Documents Department

Surface photovoltage measurements and finite element modeling of SAW devices.

Description: Over the course of a Summer 2011 internship with the MEMS department of Sandia National Laboratories, work was completed on two major projects. The first and main project of the summer involved taking surface photovoltage measurements for silicon samples, and using these measurements to determine surface recombination velocities and minority carrier diffusion lengths of the materials. The SPV method was used to fill gaps in the knowledge of material parameters that had not been determined successfully by other characterization methods. The second project involved creating a 2D finite element model of a surface acoustic wave device. A basic form of the model with the expected impedance response curve was completed, and the model is ready to be further developed for analysis of MEMS photonic resonator devices.
Date: March 1, 2012
Creator: Donnelly, Christine
Partner: UNT Libraries Government Documents Department

Investigation of Deep Levels in GaInNas

Description: This paper presents and discusses the first Deep-Level transient spectroscopy (DLTS) data obtained from measurements carried out on both Schottky barriers and homojunction devices of GaInNAs. The effect of N and In doping on the electrical properties of the GaNInAs devices, which results in structural defects and interface states, has been investigated. Moreover, the location and densities of deep levels related to the presence of N, In, and N+In are identified and correlated with the device performance. The data confirmed that the presence of N alone creates a high density of shallow hole traps related to the N atom and structural defects in the device. Doping by In, if present alone, also creates low-density deep traps (related to the In atom and structural defects) and extremely deep interface states. On the other hand, the co-presence of In and N eliminates both the interface states and levels related to structural defects. However, the device still has a high density of the shallow and deep traps that are responsible for the photocurrent loss in the GaNInAs device, together with the possible short diffusion length.
Date: November 12, 1998
Creator: Abulfotuh, F.; Balcioglu, A.; Friedman, D.; Geisz, J. & Kurtz, S.
Partner: UNT Libraries Government Documents Department

Effect of Substrate Orientation on Phase Separation in Epitaxial GaInAsSb

Description: The effect of substrate misorientation on phase separation in Ga{sub 1-x}In{sub x}As{sub y}Sb{sub 1-y} nominally lattice-matched to GaSb is reported. The layers were grown at 575 C by organometallic vapor phase epitaxy on vicinal (001) GaSb substrates, miscut 2{sup o} {yields} (-111)A, (1-11)B, or (101). Ga{sub 1-x}In{sub x}As{sub y}Sb{sub 1-y} (x {approx} 0.1, y {approx} 0.09) layers, which have 300-K photoluminescence (PL) peak emission at {approx}2.1 {micro}m, grow step-bunched and exhibit minimal phase separation. The full width at half maximum of 4-K PL spectra is slightly smaller at 7 meV for layers grown on substrates miscut toward (1-11)B compared to 9 meV for layers grown on substrates miscut toward (-1-11)A and (101). Ga{sub 1-x}In{sub x}As{sub y}Sb{sub 1-y} layers with higher alloy composition (0.16 {le} x {le} 0.19, 0.14 {ge} y {le} 0.17), which have 300-K PL peak emission at {approx}2.4 {micro}m, have significant phase separation. These layers are characterized by increased lattice constant variations and epitaxial tilt, broad PL spectra with significant band tailing, and strong contrast modulation in transmission electron microscopy. The degree of decomposition depends on substrate miscut direction: Ga{sub 1-x}In{sub x}As{sub y}Sb{sub 1-y} layers grown on (001) 2{sup o} {yields} (1-11)B substrates are more homogeneous than those grown on (001) 2{sup o} {yields} (-1-11)A and (001) 2{sup o} {yields} (101) substrates. The results are attributed to the smaller adatom diffusion length on substrates miscut toward (1-11)B.
Date: April 20, 2001
Creator: Wang, C.A.; Calawa, D.R. & Vineis, C.J.
Partner: UNT Libraries Government Documents Department

TIME-DEPENDENT THERMAL-NEUTRON ENERGY SPECTRA IN A MONOATOMIC HEAVY GAS

Description: The time-dependent thermal neutron energy spectra, for times greater thsn the slowing-down time, were generated in a monoatomic heavy gas using multigroup formalism. These spectra were obtained for infinite as well as finite media of Be and graphite. The behavior of asymptotic energy spectra during the last stage of neutron thermalization and diffusion periods was studied. The thermalization time constant for the establishment of the final Maxwellian velocity distribution of neutrons, in a monoatomic heavy gas, was estimated to be equal to (l.l74 xi SIGMA /sub so/v/sub o/)/sup -1/. Total thermalization times for neutrons in Be and graphite were found to be equal to 114 and 238 mu sec, respectively. Using the energy-dependent transport mean free path, the diffusion cooling coefficient for Be was calculated to be equal to 0.890 cm/sup 2/. For graphite, under the constant diffusion coefficient assumption, the diffusion cooling coefficient was determined to be equal to 1.922 cm/sup 2/. (auth)
Date: July 11, 1960
Creator: Purohit, S N
Partner: UNT Libraries Government Documents Department

HIGH-FLUX REACTOR--MACHINE CALCULATION

Description: By means of 2-group, 3-region calculations the following features are investigated for a flux-trap reactor: finite fuel-layer thickness, moderation in the fuel layer, diffusion constant of the fuel layer, reflector thickness, and flux-trap radius. A 31-group calculation was used to study simultaneously finite fuel-layer thickness, moderation in the fuel layer, and epithermal absorptions as and fissions. The results obtained are very near those obtained for the idealized case in ORNL CF-58-1-4. (auth)
Date: April 15, 1958
Creator: Ergen, W.K.
Partner: UNT Libraries Government Documents Department

New Materials for Future Generations of III-V Solar Cells

Description: Three- and four-junction III-V devices are proposed for ultrahigh-efficiency solar cells using a new 1-eV material lattice-matched to GaAs, namely, GaInNAs. We demonstrate working prototypes of a GaInNAs-based solar cell lattice-matched to GaAs with photoresponse down to 1 eV. Under the AM1.5 direct spectrum with all the light higher in energy than the GaAs band gap filtered out, the prototypes grown with base doping of about 10{sup 17} cm-3 have open-circuit voltages ranging from 0.35 to 0.44 V, short-circuit current densities of 1.8 mA/cm2, and fill factors from 61% to 66%. To improve on the current record-efficiency tandem GaInP/GaAs solar cell by adding a GaInNAs junction, the short-circuit current density of this 1-eV cell must be significantly increased. Because these low short-circuit current densities are due to short diffusion lengths, we have demonstrated a depletion-width-enhanced variation of one of the prototype devices that trades off decreased voltage for increased photocurrent, with a short-circuit current density of 7.4 mA/cm2 and an open-circuit voltage of 0.28 V.
Date: October 6, 1998
Creator: Geisz, J. F.; Friedman, D. J.; Olson, J. M.; Kramer, C.; Kibbler, A. & Kurtz, S. R.
Partner: UNT Libraries Government Documents Department

Research on High-Bandgap Materials and Amorphous Silicon-Based Solar Cells, Final Technical Report, 15 May 1994-15 January 1998

Description: This report describes work performed by Syracuse University under this subcontract. Researchers developed a technique based on electroabsorption measurements for obtaining quantitative estimates of the built-in potential Vbi in a-Si:H-based heterostructure solar cells incorporating microcrystalline or a-SiC:H p layers. Using this new electroabsorption technique, researchers confirmed previous estimates of Vbi {yields} 1.0 V in a-Si:H solar cells with ''conventional'' intrinsic layers and either microcrystalline or a-SiC:H p layers. Researchers also explored the recent claim that light-soaking of a-Si:H substantially changes the polarized electroabsorption associated with interband optical transitions (and hence, not defect transitions). Researchers confirmed measurements of improved (5') hole drift mobilities in some specially prepared a-Si:H samples. Disturbingly, solar cells made with such materials did not show improved efficiencies. Researchers significantly clarified the relationship of ambipolar diffusion-length measurements to hole drift mobilities in a-Si:H, and have shown that the photocapacitance measurements can be interpreted in terms of hole drift mobilities in amorphous silicon. They also completed a survey of thin BP:H and BPC:H films prepared by plasma deposition using phosphine, diborane, trimethylboron, and hydrogen as precursor gases.
Date: December 28, 1998
Creator: Schiff, E. A.; Gu, Q.; Jiang, L.; Lyou, J.; Nurdjaja, I. & Rao, P. (Department of Physics, Syracuse University)
Partner: UNT Libraries Government Documents Department

Effusive-flow characterization of arbitrary size and geometry target/vapor transport systems: radioactive ion beam applications

Description: The principal factors that severely limit intensities of short-lived radioactive ion beams produced by the Isotope Separator On-Line (ISOL) technique are time delays due to diffusion of radioactive species from solid or liquid target materials and their effusive-flow transport to the ion source. Although diffusion times can be reduced by proper design of short diffusion length, highly refractory targets, effusive-flow times are more difficult to assess. After diffusion from the target material, the species must travel through the target material and vapor transport system to the ion source. The time required for effusive-flow transport to the ion source depends on the conduction path, chemical reactions between the species and target material and materials of construction as well as the physical size and geometry of the transport system. We have developed a fast-valve (1 ms closing time) for introducing gaseous or vapor-state species into the target/vapor transport/ion source/system th at permits measurement of effusive-flow times for any gaseous or vaporous species (chemically active or chemically inactive) through any vapor transport system, independent of size and geometry. Characteristic times are determined from the exponential decay of the momentum analyzed ion beam intensity for the species.during effusive-flow through the vapor transport system under evaluation. This report describes the effusive-flow apparatus and presents characteristic time spectra and characteristic effusive-flow time data for noble gases flowing through both serial and parallel-flow target reservoir systems.
Date: November 5, 2001
Creator: Bilheux, J.-C.
Partner: UNT Libraries Government Documents Department

A SUMMARY OF SHIELDING CONSTANTS FOR CONCRETE

Description: The present knowledge of the shielding constants of concrete is summarized. The densities, elemental compositions, and mixes, where available, are given for a wide range of concretes. From these data, various constants useful for shielding calculations were computed. These constants include the removal cross sections, total cross sections, average thermal neutron absorption cross sections, thermal neutron diffusion coefficients, reciprocal thermal neutron diffusion lengths, total gamma-ray linear attenuation coefficients, gamma-ray energy absorption linear attenuation coefficients, the effective atomic numbers for the determination of buildup factors, and the capture gamma-ray spectra. Experimental results are included where available. (auth)
Date: November 1, 1961
Creator: Walker, R.L. & Grotenhuis, M.
Partner: UNT Libraries Government Documents Department