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Scintillating glass fiber neutron sensors: 3, Photon economy in scintillating fibers

Description: In an optical detector such as those constructed from scintillating glass fibers, the photons represent information. This study of the flow of information in a system of devices using PNL glass fibers was undertaken in order to resolve the conflict between expected and observed peak heights. This work concentrates on the number of photons produced and the fraction of photons trapped. It is found that the number of photons produced in bulk samples of the standard glass is about one-third that expected, based on published values; there is evidence that, in fiberized glass, this may be as small as one-fifth the expected value. Additionally, the fraction of trapped photons is found to be about three-fourths that expected because the glass has a smaller refractive index and the cladding a larger refractive index than published values in the spectral region of importance. These factors, taken together, are sufficient to resolve the conflict between the expected and observed peak heights. This analysis provides guidance for those who would use published materials properties to fabricate detectors in a new geometry where the materials properties may have been changed by the fabrication process.
Date: October 1, 1993
Creator: Abel, K. H.; Arthur, R. J. & Bliss, M.
Partner: UNT Libraries Government Documents Department

Streak tube photocathode development program. Phase 2, Final technical report

Description: This report details the progress made toward developing a streak tube with greater than 1% quantum efficiency at a wavelength of 1300 nm. The achieved performance is the result of approximately three years of effort. The goal of Phase 2 of this contract was to seal a working 1.3 {mu}m streak tube. This effort was focused in two areas. First there was a continuing effort to further develop and demonstrate the cathodes ability to meet the stated requirements. The second effort was aimed at solving the mechanical and process related problems related to sealing this cathode onto a EG&G streak tube.
Date: November 20, 1993
Partner: UNT Libraries Government Documents Department

Research on stable, high-efficiency amorphous silicon multijunction modules. Annual subcontract report, 1 January 1991--31 December 1991

Description: This report describes the progress made during Phase 1 of research and development program to obtain high-efficiency amorphous silicon alloy multijunction modules. Using a large-area deposition system, double-and triple-junction cells were made on stainless steel substrates of over 1 ft{sup 2} area with Ag and ZnO predeposited back reflector. Modules of over 1 ft{sup 2} were produced with between 9.2% and 9.9 initial aperture-area efficiencies as measured under a USSC Spire solar simulator. Efficiencies as measured under the NREL Spire solar simulator were found to be typically 15% to 18% lower. The causes for this discrepancy are now being investigated. The modules show about 15% degradation after 600 hours of one-sun illumination at 50{degrees}C. To optimize devices for higher stabilized efficiency, a new method was developed by which the performance of single-junction cells after long-term, one-sun exposure at 50{degrees}C can be predicted by exposing cells to short-term intense light at different temperatures. This method is being used to optimize the component cells of the multijunction structure to obtain the highest light-degraded efficiency.
Date: April 1, 1992
Creator: Banerjee, A.; Chen, E.; Clough, R.; Glatfelter, T.; Guha, S.; Hammond, G. et al.
Partner: UNT Libraries Government Documents Department

Research on polycrystalline thin film submodules based on CuInSe{sub 2} materials. Annual subcontract report, 11 November 1990--31 October 1991

Description: This report describes progress during the first year of a three-year research program to develop 12%-efficient CuInSe{sub 2} (CIS) submodules with area greater than 900 cm{sup 2}. To meet this objective, the program was divided into five tasks: (1) windows, contacts, substrates; (2) absorber material; (3) device structure; (4) submodule design and encapsulation; and (5) process optimization. In the first year of the program, work was concentrated on the first three tasks with an objective to demonstrate a 9%-efficient CIS solar cell. 7 refs.
Date: May 1, 1992
Creator: Catalano, A.; Arya, R.; Carr, L.; Fieselmann, B.; Lommasson, T.; Podlesny, R. et al.
Partner: UNT Libraries Government Documents Department

Thin film cadmium telluride, zinc telluride, and mercury zinc telluride solar cells. Final subcontract report, 1 July 1988--31 December 1991

Description: This report describes research to demonstrate (1) thin film cadmium telluride solar cells with a quantum efficiency of 75% or higher at 0. 44 {mu}m and a photovoltaic efficiency of 11.5% or greater, and (2) thin film zinc telluride and mercury zinc telluride solar cells with a transparency to sub-band-gap radiation of 65% and a photovoltaic conversion efficiency of 5% and 8%, respectively. Work was directed at (1) depositing transparent conducting semiconductor films by solution growth and metal-organic chemical vapor deposition (MOCVD) technique, (2) depositing CdTe films by close-spaced sublimation (CSS) and MOCVD techniques, (3) preparing and evaluating thin film CdTe solar cells, and (4) preparing and characterizing thin film ZnTe, CD{sub 1-x}Zn{sub 1-x}Te, and Hg{sub 1-x}Zn{sub x}Te solar cells. The deposition of CdS films from aqueous solutions was investigated in detail, and their crystallographic, optical, and electrical properties were characterized. CdTe films were deposited from DMCd and DIPTe at 400{degrees}C using TEGa and AsH{sub 3} as dopants. CdTe films deposited by CSS had significantly better microstructures than those deposited by MOCVD. Deep energy states in CdTe films deposited by CSS and MOCVD were investigated. Thin films of ZnTe, Cd{sub 1- x}Zn{sub x}Te, and Hg{sub 1-x}Zn{sub x}Te were deposited by MOCVD, and their crystallographic, optical, and electrical properties were characterized. 67 refs.
Date: April 1, 1992
Creator: Chu, T. L.
Partner: UNT Libraries Government Documents Department

Research on stable, high efficiency amorphous silicon multijunction modules. Semiannual technical progress report, 1 May 1991--31 October 1991

Description: Improvements towards a goal of a 12.5% initial triple-junction module efficiency require the use of a wide gap top-layer for improved open circuit voltage, higher transmission from the transparent front contact and more highly transmitting doped layers. To address the first issue, there has been continued development of a-SiC:H with the utilization of several novel feedstocks to control the atomic structure of the solid. These films have transport properties superior to the best results reported for a-SiC:H. Preliminary results with devices exhibits a stability comparable to a-Si:H, while previous results with a-SiC:H have generally shown for higher rates of degradation. Module fabrication has been refined to the extent that comparable module and small area device efficiencies are readily obtained. Despite the high initial efficiencies (9%--10%) obtained in 935 cm{sup 2} modules employing devices with 4000{Angstrom} thick middle junctions, higher than expected rates of degradation were found. The cause of the anomalous degradation was traced to shunts present in the device arising from defects in the tin oxide coating. NREL degradation results of triple-junction modules showed stabilized performance of the initial efficiency for modules prepared during the period in which shunts were a problem. 20 refs.
Date: February 1, 1992
Creator: Catalano, A.; Arya, R. R.; Bennett, M.; Chen, L.; D`Aiello, R.; Fieselmann, B. et al.
Partner: UNT Libraries Government Documents Department

High-efficiency, thin-film solar cells. Annual subcontractor report, 1 July 1991--30 June 1992

Description: This report describes work on a 3-year research program to investigate thin-film GaAs/GaInP cells using the cleavage of lateral epitaxial film for transfer (CLEFT) technique, and to determine the process to enable overgrowth of GaAs films using organometallic chemistry. Application of the CLEFT thin-film technique to GaInP/GaAs solar cells and organometallic overgrowth was investigated. A problem of alloy contamination was identified and controlled, leading to higher quality layers. Solar cell structures were grown and fabricated using previously determined growth parameters for GaAs and GaInP. With the improved materials developed significant improvements were made in solar cell performance. Conditions for in-situ overgrowth by organometallic chemical vapor deposition (OMCVD) were determined and continuous GaAs layers were grown over a separation mask layer. The layers were successfully separated from their substrate using the CLEFT process, demonstrating the application of overgrowth using OM chemistry with HCl.
Date: January 1, 1994
Creator: Gale, R. P.
Partner: UNT Libraries Government Documents Department

Role of polycrystallinity in CdTe and CuInSe{sub 2} photovoltaic cells. Annual subcontract report, 1 April 1990--31 March 1991

Description: The polycrystalline nature of thin-film CdTe and CuInSe{sub 2} solar cells continues to be a major factor in several individual losses that limit overall cell efficiency. This report describes progress in the quantitative separation of these losses, including both measurement and analysis procedures. It also applies these techniques to several individual cells to help document the overall progress with CdTe and CuInSe{sub 2} cells. Notably, CdTe cells from Photon Energy have reduced window photocurrent losses to 1 mA/Cm{sup 2}; those from the University of South Florida have achieved a maximum power voltage of 693 mV; and CuInSe{sub 2} cells from International Solar Electric Technology have shown a hole density as high as 7 {times} 10{sup 16} cm{sup {minus}3}, implying a significant reduction in compensation. 9 refs.
Date: December 31, 1991
Creator: Sites, J. R.
Partner: UNT Libraries Government Documents Department

Observation of a charge limit for photocathode electron guns

Description: The Photocathode Electron Gun (PEG) at SLAC is required to produce bunch intensities of up to 10{sup 11} electrons within 2 ns (8 Amps). Operation of PEG has demonstrated a `Charge Limit` phenomenon, whereby the charge that can be extracted from the gun with an intense laser beam saturates at significantly less than 10{sup 11} electrons (the expected `Space Charge Limited` charge) when the photocathode Quantum Efficiency is low. We report studies of this Charge Limit phenomenon observed with a GaAs photocathode.
Date: December 1, 1992
Creator: Woods, M.; Clendenin, J.; Frisch, J.; Kulikov, A.; Saez, P.; Schultz, D. et al.
Partner: UNT Libraries Government Documents Department

Photocathode transfer and storage techniques using alkali vapor feedback control

Description: Photocathodes of quantum efficiency above 1% at the doubled YAG frequency of 532 nM are very sensitive to the local vacuum environment. These cathodes must have a band gap of less than 2.3 eV, and a work function that is also on the order of {approximately}2 volts or less. As such, these surfaces are very reactive as they provide many surface states for the residual gases that have positive electron affinities such as oxygen and omnipotent water. Attendant to this problem is that the optimal operating point for some of these cesium based cathodes is unstable. Three of the cesium series were tried, the Cs-Ag-Bi-O, the Cs{sub 3}Sb and the K{sub 2}CsSb. The most stable material found is the K{sub 2}CsSb. The vacuum conditions can be met by a variety of pumping schemes. The vacuum is achieved by using sputter ion diode pumps, and baking at 250{degrees}C or less for whatever time is required to reduce the pump currents to below 1 uA at room temperature. To obtain the required partial pressure of cesium, a simple very sensitive diagnostic gauge has been developed that can discriminate between free alkali and other gases present. This Pressure Alkali Monitor (PAM) can be used cesium sources to provide a low partial pressure using standard feedback techniques. Photocathodes of arbitrary composition have been transferred to a separate vacuum system and preserved for over 10 days with less than a 25% loss to the QE at 543.5 nM.
Date: December 20, 1991
Creator: Springer, R. W. & Cameron, B. J.
Partner: UNT Libraries Government Documents Department

Rapid thermal processing of high-efficiency silicon solar cells with controlled in-situ annealing

Description: Silicon solar cell efficiencies of 17.1%, 16.4%, 14.8%, and 14.9% have been achieved on FZ, Cz, multicrystalline (mc-Si), and dendritic web (DW) silicon, respectively, using simplified, cost-effective rapid thermal processing (RTP). These represent the highest reported efficiencies for solar cells processed with simultaneous front and back diffusion with no conventional high-temperature furnace steps. Appropriate diffusion temperature coupled with the added in-situ anneal resulted in suitable minority-carrier lifetime and diffusion profiles for high-efficiency cells. The cooling rate associated with the in-situ anneal can improve the lifetime and lower the reverse saturation current density (J{sub 0}), however, this effect is material and base resistivity specific. PECVD antireflection (AR) coatings provided low reflectance and efficient front surface and bulk defect passivation. Conventional cells fabricated on FZ silicon by furnace diffusions and oxidations gave an efficiency of 18.8% due to greater short wavelength response and lower J{sub 0}.
Date: January 1, 1995
Creator: Doshi, P.; Rohatgi, A.; Ropp, M.; Chen, Z.; Ruby, D. & Meier, D. L.
Partner: UNT Libraries Government Documents Department

Optical generation of radio-frequency power

Description: An optical technique for high-power radio-frequency (RF) signal generation is described. The technique uses a unique photodetector based on a traveling-wave design driven by an appropriately modulated light source. The traveling-wave photodetector (TWPD) exhibits simultaneously a theoretical quantum efficiency approaching 100 % and a very large electrical bandwidth. Additionally, it is capable of dissipating the high-power levels required for the RF generation technique. The modulated light source is formed by either the beating together of two lasers or by the direct modulation of a light source. A system example is given which predicts RF power levels of 100`s of mW`s at millimeter wave frequencies with a theoretical ``wall-plug`` efficiency approaching 34%.
Date: November 1, 1994
Creator: Hietala, V. M.; Vawter, G. A.; Brennan, T. M.; Hammons, B. E. & Meyer, W. J.
Partner: UNT Libraries Government Documents Department

Study of non-linear photoemission effects in III-V semiconductors

Description: Our experience at SLAC with photoemission-based polarized electron sources has shown that charge limit is an important phenomenon that may significantly limit the performance of a photocathode for applications requiring high intensity electron beams. In the process of developing high performance photocathodes for the ongoing and future SLC high energy physics programs, we have studied the various aspects of the charge limit phenomenon. We find that the charge limit effect arises as a result of non-linear response of a photocathode to high intensity light illumination. The size of the charge limit not only depends on the quantum efficiency of the cathode but also depends critically on the extraction electric field. In addition, we report the observation of charge oversaturation when the intensity of the incident light becomes too large.
Date: October 1, 1993
Creator: Tang, H.; Alley, R. K. & Aoyagi, H.
Partner: UNT Libraries Government Documents Department

Polymers at liquid-liquid interfaces: Photophysics and photoredox chemistry. Progress report, April 1, 1991--March 31, 1992

Description: This report consists of three sections: study of triplet state electron transfer from polymer-bound anthracene and pyrene chromophores, adsorption of alternating block and comb polymers onto polymer surfaces, and discussion of transient absorption systems.
Date: December 1, 1991
Creator: Webber, S. E.
Partner: UNT Libraries Government Documents Department

Low-temperature emitter-passivation for silicon solar cells

Description: We have developed a process that provides much of the performance improvement possible from a high-quality emitter passivation while maintaining cell temperatures at or below 400{degrees}C. Using atmospheric-pressure chemical-vapor deposition (CVD) Of SiO{sub 2} at 400{degrees}C followed by a 400{degrees}C forming gas anneal, we have been able to produce a 3.1-fold increase in the blue response of cells compared to that of unpassivated cells. This has resulted in up to an 11% increase in AM1.5G short-circuit current due to the passivating qualities of the CVD oxide. This technique represents a low-cost approach to boost the performance of cells that do not presently employ passivating oxides. It would be of particular value to any silicon cell process in which material or other constraints preclude the use of a high-temperature thermal oxidation for the purpose of emitter passivation. This also applies to cells using material whose bulk lifetime is reduced by multiple high-temperature thermal excursions.
Date: November 1, 1992
Creator: Ruby, D. S. & Levine, J. D.
Partner: UNT Libraries Government Documents Department

A compact, low cost, 7 channel polychromator for Thomson scattering measurements

Description: A seven channel polychromator, utilizing high performance interference filters, has been tested for use in the multi-Nd:YAG laser Thomson scattering system for the DIII-D tokamak. Unique features of this polychromator are the combination of high throughput, easy alignment, flexibility, compact size, and low cost when compared with other alternatives. Light is introduced to the polychromator (f/1.75) via a fiber optic bundle which permits the use of small (3.0 cm dia) optics and leads to a compact design, an important design consideration for multiple polychromator systems. The light is cascaded through a series of different bandpass interference filters and relay lenses which are mounted on two precision parallel rails in such a way that alignment is trivial. The relay lenses are positioned directly in front of the filters so that light reflected from the filter passes through the lens twice. This leads to an efficient, compact design and reduces the angle of incidence (4{degree}) and the cone angle of light (4.5{degree}) seen by the filter, an important factor for narrowband (3.0 nm) filters. The transmission was optimized for 700--1100 nm by using broadband coatings throughout. The output images of each channel (2.3 mm dia) can be directly coupled to large format (3 nm dia) RCA silicon avalanche photodiode detectors, avoiding the losses caused by fiber optic coupling.
Date: October 1, 1990
Creator: Carlstrom, T.N.; DeBoo, J.C.; Evanko, R.; Greenfield, C.M.; Hsieh, C.-L.; Snider, R.T. et al.
Partner: UNT Libraries Government Documents Department

Rise time and recovery of GaAs photoconductive semiconductor switches

Description: Fast rise time applications have encouraged us to look at the rise time dependences of lock-on switching. Our tests have shown rise time and delay effects which decrease dramatically with increasing electric field across the switch and/or optical energy used in activating lock-on. Interest in high repetition rate photoconductive semiconductor switches (PCSS), which require very little trigger energy (our 1.5-cm long switches have been triggered with as little as 20 {mu}J), has also led us to investigate recovery from lock-on. Several circuits have been used to induce fast recovery, the fastest being 30 ns. The most reliable circuit produced a 4-pulse burst of +/{minus} 10-kV pulses at 7 MHz with 100-{mu}J trigger energy per pulse. 11 refs., 10 figs.
Date: January 1, 1990
Creator: Zutavern, F.J.; Loubriel, G.M.; O'Malley, M.W.; McLaughlin, D.L. & Helgeson, W.D.
Partner: UNT Libraries Government Documents Department

Improved amorphous Si solar cells. Quarterly progress report, August 1-October 31, 1980

Description: A model for explaining the growth, H incorporation and bandgap data in a-Si deposited from (SiF/sub 4/ + H/sub 2/) has been developed. It is proposed that the a-Si films are subjected to reactive ion etching and ion bombardment during growth, which may lead to lower defect densities (and hence lower H content) and microcrystallinity in doped films. A morphological study of B-doped a-Si:H has also been carried out. It is found that there are certain regions of pressure, temperature and relative B/sub 2/H/sub 6/ concentrations which lead to agglomerative growth. Study of drift mobility of electrons has continued. We have obtained very high drift mobilities, (approx. 0.8 cm/sup 2//V-sec) on thick films (approx. 3 ..mu..m). Thin films (approx. 1 ..mu..m) have significantly lower mobilities. It is proposed that this phenomenon is due to the existence of a transition layer between the substrate and high quality a-Si. We have continued nip device fabrication in a-Si:H. The following parameters have been obtained individually on cells of 0.24 cm/sup 2/ area. J/sub sc/ (internal) = 12 mA/cm/sup 2/ at 100 mW/cm/sup 2/, V/sub oc/ = 820 mV, and FF = 0.56 are obtained.
Date: January 1, 1980
Creator: Dalal, V.L.
Partner: UNT Libraries Government Documents Department

InP single-junction concentrator cell. Semi-annual report, October 1, 1980-March 31, 1981

Description: The purpose of this work is to develop a glass-sealed back-contacted InP or InGaAsP solar cell for use in concentrator systems. Work to be performed in developing this cell includes growing the InP epitaxial layers needed for cells and test structures, developing a model to be used in optimizing the cell structure, measurement of InP parameters necessary for use in the model, and developing the glass-bonding and back-contacting scheme. Accomplishments in these areas are discussed.
Date: January 1, 1981
Creator: Gregory, P.E.
Partner: UNT Libraries Government Documents Department

Cadmium sulfide/copper ternary heterojunction cell research. Final report, April 1, 1980-August 25, 1982

Description: The properties of polycrystalline, thin-film CuInSe/sub 2//CdS and CuInSe/sub 2//Zn/sub x/Cd/sub 1-x/S solar cells prepared by vacuum-evaporation techniques onto metallized-alumina substrates are described. An efficiency of 10.6% for a 1 cm/sup 2/ area cell and 8.3% for an 8 cm/sup 2/ cell when tested under simulated AM1 illumination is reported. The mixed-sulfide cells are described as exhibiting increased open-circuit voltages, slightly higher short-circuit currents, and improved efficiencies. Mixed-sulfide film preparation by evaporation of CdS and ZnS powders from a single source and from two sources is discussed with preference given to the later technique. Selenide-film preparation in a planetary or rotating substrate vacuum-deposition apparatus is described. A 1 cm/sup 2/ area cell without AR-coating produced by the planetary approach is reported to demonstrate a 7.5% efficiency. The results of cell heat-treatment studies showing a strong environmental dependence are presented and indicate the desirability of an oxygen-containing atmosphere. An automatic, computer-controlled, cell-measurement system for I-V, C-V, and spectral-response analysis is described. The results of the cell-analysis and cell-modeling studies on both the plain CdS and mixed Zn/sub x/Cd/sub 1-x/S thin-film devices are presented. Finally, data obtained from constant illumination and elevated temperature life-tests on the thin-film cells showing little degradation after 9300 hours is reported.
Date: August 1, 1982
Creator: Mickelsen, R. A. & Chen, W. S.
Partner: UNT Libraries Government Documents Department

Interdigitated back contact solar cell with high-current collection

Description: Internal current-collection efficiencies greater than 90 percent and energy-conversion efficiencies of 18 percent at 30 suns have been measured on a laboratory version of the interdigitated back contact (IBC) solar cell. The quantum efficiency at 600 nm was greater than 90 percent which implies a minority carrier lifetime of greater than 350 ..mu..sec and a front surface recombination velocity of less than 30 cm/sec on the better devices. To achieve these high-current collection efficiencies, a phosphorous gettering diffusion was performed on the front surface and then etched off. Also, thermal oxides were grown on the front and back of the cell to passivate the silicon surfaces. Although the internal collection efficiencies of the cell were high, series resistance caused the fill factor (FF) to decrease at concentrations above 30 suns. Dark current measurements on cells with a new grid spacing indicate that the series resistance is much lower than in the previous cell design. This should result in higher efficiencies at high concentration.
Date: January 1, 1981
Creator: Garner, C.M.; Nasby, R.D.; Sexton, F.W.; Rodriguez, J.L. & Norwood, D.P.
Partner: UNT Libraries Government Documents Department

Pulsed 4-MeV electron injector with an excimer laser driven photocathode

Description: The Relativistic Electron-Beam Experiment injector at the Los Alamos National Laboratory is used to generate a 4-MV pulse across an anode-cathode gap. A simple metal photocathode is illuminated by a pulsed excimer laser. Time-resolved measurements of current, voltage, and current density are made. The resulting quantum efficiencies are being used to obtain the required laser power for a multikiloampere, high-brightness electron gun to be used as an injector for a linear induction accelerator. 3 refs., 5 figs.
Date: January 1, 1989
Creator: Kauppila, T.J.; Builta, L.A.; Carlson, R.L.; Moir, D.C. & Ridlon, R.N.
Partner: UNT Libraries Government Documents Department

Short-pulse high-current-density photoemission in high electric fields

Description: We present the experimental results of photoemission studies on thin wires of gold-coated tungsten, held at surface fields in the range of 10/sup 6/ to 3 x 10/sup 8/ V/m, and illuminated by 10 ps long, 4.66 eV photon laser pulses. The wire cathodes arranged coaxially in an anode experienced a surface-field enhancement of 10/sup 2/ to 10/sup 3/ over the applied voltage. We obtained current densities exceeding 10 kA/cm/sup 2/ from a 50 ..mu..m diameter wire, from a (50 x 400) ..mu..m/sup 2/ area, under partially space-change-limited conditions. The quantum efficiency for emission-limited cases was in the range of 10/sup -5/. For these cases results using 50 ..mu..m and 4 ..mu..m diameter wires indicated linear dependence of charge density with optical energy density. The emission also scaled linearly with the emitting area. For surface fields above 3 x 10/sup 7/ V/m, a twofold enhancement of emission was observed for a tenfold increase in the field. 7 refs., 9 figs.
Date: January 1, 1987
Creator: Fischer, J. & Srinivasan-Rao, T.
Partner: UNT Libraries Government Documents Department

Nonphotochemical hole burning and dispersive kinetics in amorphous solids

Description: Results covering burn intensities in the nW to {mu}W/cm{sup 2} range, of dispersive hole growth kinetics are reported for Oxazine 720 in glycerol glasses and polyvinyl alcohol polymer films and their deuterated analogues. A theoretical model which employs a distribution function for the hole burning rate constant based upon a Gaussian distribution for the tunnel parameter is shown to accurately describe the kinetic data. This model incorporates the linear electron-phonon coupling. A method for calculating the nonphotochemical quantum yield is presented which utilizes the Gaussian distribution of tunnel parameters. The quantum yield calculation can be extended to determine a quantum yield as a function of hole depth. The effect of spontaneous hole filling is shown to be insignificant over the burn intensity range studied. Average relaxation rates for hole burning are {approximately}8 orders of magnitude greater than for hole filling. The dispersive kinetics of hole burning are observed to be independent over the temperature range of these experiments, 1.6 to 7.0 K. 6 refs., 20 figs., 1 tab.
Date: September 21, 1990
Creator: Kenney, M.J.
Partner: UNT Libraries Government Documents Department