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The Hexagon, Volume 92, Number 2, Summer 2001

The Hexagon, Volume 92, Number 2, Summer 2001

Date: Summer 2001
Creator: Alpha Chi Sigma
Description: Quarterly publication of the Alpha Chi Sigma chemistry fraternity containing articles related to chemistry research and the activities of the organization, including local chapters and groups.
Contributing Partner: UNT Libraries
Rediscovery of the Elements: Germanium

Rediscovery of the Elements: Germanium

Date: Summer 2001
Creator: Marshall, James L., 1940- & Marshall, Virginia R.
Description: Article recounting the history of the element Germanium, including background material on mining in Germany and the isolation of Germanium by Clemens A. Winkler. Tourist information is provided regarding Winkler's laboratory and mines open to the public as museums.
Contributing Partner: UNT College of Arts and Sciences
Final report

Final report

Date: August 7, 2002
Creator: Simmons, Joseph H. & Bukowski, Tracie J.
Description: This grant was a continuation of research conducted at the University of Florida under Grant No. DE-FG05-91ER45462 in which we investigated the energy bandgap shifts produced in semiconductor quantum dots of sizes between 1.5 and 40 nm. The investigated semiconductors consisted of a series of Column 2-6 compounds (CdS, CdSe, CdTe) and pure Column IV elements (Si and Ge). It is well-known of course that the 2-6 semiconductors possess a direct-gap electronic structure, while the Column IV elements possess an indirect-gap structure. The investigation showed a major difference in quantum confinement behavior between the two sets of semiconductors. This difference is essentially associated with the change in bandgap energy resulting from size confinement. In the direct-gap semiconductors, the change in energy (blue shift) saturates when the crystals approach 2-3 nm in diameter. This limits the observed shift in energy to less than 1 eV above the bulk value. In the indirect-gap semiconductors, the energy shift does not show any sign of saturation and in fact, we produced Si and Ge nanocrystals with absorption edges in the UV. The reason for this difference has not been determined and will require additional experimental and theoretical studies. In our work, we suggest, but ...
Contributing Partner: UNT Libraries Government Documents Department
Synthesis and Gas Phase Thermochemistry of Germanium-Containing Compounds

Synthesis and Gas Phase Thermochemistry of Germanium-Containing Compounds

Date: December 31, 2002
Creator: Classen, Nathan Robert
Description: The driving force behind much of the work in this dissertation was to gain further understanding of the unique olefin to carbene isomerization observed in the thermolysis of 1,1-dimethyl-2-methylenesilacyclobutane by finding new examples of it in other silicon and germanium compounds. This lead to the examination of a novel phenylmethylenesilacyclobut-2-ene, which did not undergo olefin to carbene rearrangement. A synthetic route to methylenegermacyclobutanes was developed, but the methylenegermacyclobutane system exhibited kinetic instability, making the study of the system difficult. In any case the germanium system decomposed through a complex mechanism which may not include olefin to carbene isomerization. However, this work lead to the study of the gas phase thermochemistry of a series of dialkylgermylene precursors in order to better understand the mechanism of the thermal decomposition of dialkylgermylenes. The resulting dialkylgermylenes were found to undergo a reversible intramolecular {beta} C-H insertion mechanism.
Contributing Partner: UNT Libraries Government Documents Department
High Growth Rate Deposition of Hydrogenated Amorphous Silicon-Germanium Films and Devices Using ECR-PECVD

High Growth Rate Deposition of Hydrogenated Amorphous Silicon-Germanium Films and Devices Using ECR-PECVD

Date: May 31, 2002
Creator: Liu, Yong
Description: Hydrogenated amorphous silicon germanium films (a-SiGe:H) and devices have been extensively studied because of the tunable band gap for matching the solar spectrum and mature the fabrication techniques. a-SiGe:H thin film solar cells have great potential for commercial manufacture because of very low cost and adaptability to large-scale manufacturing. Although it has been demonstrated that a-SiGe:H thin films and devices with good quality can be produced successfully, some issues regarding growth chemistry have remained yet unexplored, such as the hydrogen and inert-gas dilution, bombardment effect, and chemical annealing, to name a few. The alloying of the SiGe introduces above an order-of-magnitude higher defect density, which degrades the performance of the a-SiGe:H thin film solar cells. This degradation becomes worse when high growth-rate deposition is required. Preferential attachment of hydrogen to silicon, clustering of Ge and Si, and columnar structure and buried dihydride radicals make the film intolerably bad. The work presented here uses the Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition (ECR-PECVD) technique to fabricate a-SiGe:H films and devices with high growth rates. Helium gas, together with a small amount of H{sub 2}, was used as the plasma species. Thickness, optical band gap, conductivity, Urbach energy, mobility-lifetime product, I-V curve, and quantum ...
Contributing Partner: UNT Libraries Government Documents Department
Magnetic, Caloric and Crystallographic Properties of Dy5(SixGe1-x)4 Alloys

Magnetic, Caloric and Crystallographic Properties of Dy5(SixGe1-x)4 Alloys

Date: July 19, 2002
Creator: Ivchenko, Vitaliy Vladislavovich
Description: Polycrystals of the intermetallic compound of the Dy{sub 5}(Si{sub x}Ge{sub 1-x}){sub 4} system, where x = 0, 0.25, 0.5, 0.625, 0.675, 0.725, 0.75, 0.775, 0.825, 0.875, and 1, have been prepared by electric-arc-melting on water-cooled copper hearth in an argon atmosphere. A study of phase relationships and crystallography in the pseudobinary system Dy{sub 5}(Si{sub x}Ge{sub 1-x}){sub 4} using X-ray powder diffraction data and optical metallography was completed. It revealed that silicides in the composition range from 0.825 to 1 crystallize in the Gd{sub 5}Si{sub 4}-type crystal structure: germanides in the composition range from 0 to 0.625 crystallize in the Sm{sub 5}Ge{sub 4}-type structure, and alloys with intermediate composition range from 0.675 to 0.775 crystallize in the monoclinic Gd{sub 5}Si{sub 2}Ge{sub 2}-type structure. The -{Delta}S{sub m} values were determined from magnetization measurements for 7 alloys. The alloys with a monoclinic crystal structure which belong to an intermediate phase region have large MCE value, which exceeds those observed in the other two phase regions by 300 to 500%. The nature of the observed magnetic and structural transformations in the Dy{sub 5}(Si{sub x}Ge{sub 1-x}){sub 4} system seems to be similar with those reported for the Gd{sub 5}(Si{sub x}Ge{sub 1-x}){sub 4} system. However, the ...
Contributing Partner: UNT Libraries Government Documents Department
A high energy phase retarder for the simultaneous production of right- and left-handed circularly polarized x-rays

A high energy phase retarder for the simultaneous production of right- and left-handed circularly polarized x-rays

Date: September 1, 1997
Creator: Venkataraman, C.T.; Lang, J.C.; Nelson, C.S.; Srajer, G.; Haeffner, D.R. & Shastri, S.D.
Description: The authors have fabricated and characterized the performance of a monolithic Ge Bragg-Laue phase retarder capable of simultaneously producing both right- and left-handed circularly polarized x rays. The energy range of operation of the phase retarder is between 50 and 100 keV making it well suited to the measurement of spin-dependent Compton profiles within the impulse approximation, primarily because of the increased momentum resolution and larger Compton scattering cross section available at these higher incident energies. Although the phase retarder was optimized for operation at 86 keV, it can produce highly circularly polarized x rays over a substantial energy range. The performance of the phase retarder was tested via magnetic Compton scattering measurements on an Fe sample at the undulator A of the Advanced Photon Source. It was found to perform well in terms of flux and degree of circular polarization thereby greatly reducing the data collection times required for this inherently weak scattering process.
Contributing Partner: UNT Libraries Government Documents Department
Effect of uniaxial stress on gallium, beryllium, and copper-doped germanium hole population inversion lasers

Effect of uniaxial stress on gallium, beryllium, and copper-doped germanium hole population inversion lasers

Date: May 1998
Creator: Chamberlin, D. R.
Description: The effects of stress on germanium lasers doped with single, double, and triple acceptors have been investigated. The results can be explained quantitatively with theoretical calculations and can be attributed to specific changes in the energy levels of acceptors in germanium under stress. In contrast to previous measurements, gallium-doped Ge crystals show a decrease in lasing upon uniaxial stress. The decrease seen here is attributed to the decrease in heavy hole effective mass upon application of uniaxial stress, which results in a decreased population inversion. The discrepancy between this work and previous studies can be explained with the low compensation level of the material used here. Because the amount of ionized impurity scattering in low-compensated germanium lasers is small to begin with, the reduction in scattering with uniaxial stress does not play a significant role in changing the laser operation. Beryllium-doped germanium lasers operate based on a different mechanism of population inversion. In this material it is proposed that holes can transfer between bands by giving their energy to a neutral beryllium atom, raising the hole from the ground to a bound excited state. The free hole will then return to zero energy with some probability of entering the other ...
Contributing Partner: UNT Libraries Government Documents Department
Concentration increases in the isotopic germanium carrier solutions inferred from solution weights

Concentration increases in the isotopic germanium carrier solutions inferred from solution weights

Date: July 1, 1997
Creator: Rowley, J.K.
Description: Concentration increases in the isotopic germanium carrier solutions via evaporation would produce weight losses in the carrier solutions. Since the bottle containing each carrier is usually weighed before and after the removal of carrier for a given run, the possibility exists of finding evidence for increasing carrier concentration in this record. With few exceptions the weighings were performed on the same top-loading balance in the external chemistry lab. The primary purpose was to monitor carrier usage, not to look for evidence for increasing carrier concentration. However, in order to look for this evidence, it is necessary only to compare the weights of the closed bottles between the times of carrier removal. This comparison has been performed and is reported here. Bear in mind that there is some evidence that the bottle containing carrier was not always weighted in the same way (e.g. perhaps a plastic bag was not removed from the bottle or the bottle cap was removed before weighing). Another possible source of weighing errors is the occasional buildup of static charges, especially on dry winter days. Such problems of static electricity were easily recognized and overcome. For the most part, the resulting record agrees with the assumption that ...
Contributing Partner: UNT Libraries Government Documents Department
Research in chemical kinetics. Annual report, September 1, 1989--December 31, 1990

Research in chemical kinetics. Annual report, September 1, 1989--December 31, 1990

Date: December 31, 1990
Creator: Rowland, F.S.
Description: This report describes the progress in several projects: gas phase substitution reactions of thermal chlorine atoms with tetramethyl metallo-organic compounds; abstraction reactions by thermal chlorine atoms with tetramethyl metallo-organic compounds; gas phase thermal {sup 38}Cl reactions with M(CH{double_bond}CH{sub 2}){sub n}: non-RRKM decomposition of excited radicals (M = Sn, Si, n = 4; M = Sb, n = 3; M = Hg, n = 2); quantitative product identification for reactions of hydroxyl with {sup 14}CS{sub 2}; and statistical analysis of ground-based measurements of total ozone with Dobson spectrometers. The report also contains a research proposal for work from May, 1991 through April, 1992.
Contributing Partner: UNT Libraries Government Documents Department
Irradiation-induced phase transformations. Final report, July 1, 1995--June 30, 1997

Irradiation-induced phase transformations. Final report, July 1, 1995--June 30, 1997

Date: August 1, 1998
Creator: Atwater, H.A.
Description: During the course of this two year program, the authors attention focused largely on the synthesis, structure and properties of group IV semiconductor nanocrystals. They also drew to a close the investigations of defects in amorphous silicon. Work on control of nucleation in amorphous silicon and germanium is ongoing, and has taken important new directions at the interface between basic and applied research under DOE Office of Energy Efficiency support via a subcontract from the National Renewable Energy Laboratory and the BES Center for Synthesis and Processing of Advanced Materials` project on High Efficiency Thin Film Photovoltaics. During the course of this project, scientific and scholarly output included: (1) 10 invited talks related to work on Si and Ge nanocrystals; (2) 5 Applied Physics Letters published on Si and Ge nanocrystals; (3) 3 Caltech Ph.D. Theses on Si and Ge Nanocrystal work; and (4) New directions on control of crystallization in thin semiconductor films.
Contributing Partner: UNT Libraries Government Documents Department
Single-particle states in transcurium nuclei.

Single-particle states in transcurium nuclei.

Date: September 30, 1999
Creator: Ahmad, I.
Description: Identification of single-particle states in the heaviest known nuclei is important because their energies can be used to test the single-particle potential in these high-Z elements. These states can be identified by studying the decay schemes of very heavy odd-mass nuclides. For neutrons, the heaviest odd-mass nuclide available in milliCurie quantities is the 20-h {sup 255}Fm and for protons the heaviest nuclide available is the 20-d {sup 253}Es. These two isotopes were obtained from the Transplutonium Element Production Program at Oak Ridge and their spectra were measured with high-resolution germanium spectrometers. From the results of these measurements we have identified states in {sup 251}Cf and {sup 249}Bk up to 1 MeV excitation energy.
Contributing Partner: UNT Libraries Government Documents Department
Energy dependence of Ge amorphization by Ne, Ar or Kr ion irradiation

Energy dependence of Ge amorphization by Ne, Ar or Kr ion irradiation

Date: October 1, 1994
Creator: Birtcher, R.C.
Description: Ge has been irradiated at RT by Ne, Ar, or Kr ions of different energies, and the doses required for complete amorphization determined by in situ TEM and electron diffraction. Onset of amorphization was detected after the lowest ion doses reflecting amorphization by individual ions. The ion dose required for complete amorphization increased nearly linearly with ion energy over the range 0.5 to 3.5 MeV for all ions. Amorphization cross sections have been determined for all ions and energies used. The displacements per atom required for complete amorphization decreased with ion energy or mass, owing to decrease in radiation annealing of amorphous volumes as a result of a decrease in fraction of low energy transfers to Ge atoms. Increasing the relative fraction low energy transfers to Ge atoms by simultaneous 1 MeV electron irradiation increased the Kr ion dose required for complete amorphization by as much as a factor of 2.5. The effect is believed to be due to an increase in the fraction of freely migrating defects produced by low energy transfers to Ge atoms.
Contributing Partner: UNT Libraries Government Documents Department
Surface analysis of all elements with isotopic resolution at high ambient pressures using ion spectroscopic techniques

Surface analysis of all elements with isotopic resolution at high ambient pressures using ion spectroscopic techniques

Date: September 1, 1997
Creator: Smentkowski, V.S.; Krauss, A.R.; Gruen, D.M.; Holecek, J.C. & Schultz, J.A.
Description: The authors have developed a mass spectrometer capable of surface analysis using the techniques of secondary ion mass spectroscopy (SIMS) and mass spectroscopy of recoiled ions (MSRI). For SIMS, an energetic ion beam creates a collision cascade which results in the ejection of low kinetic energy secondary ions from the surface being analyzed. The low kinetic energy SIMS ions are very susceptible to charge neutralization with the surface, and as a result, the SIMS ion yield varies by orders of magnitude depending on the chemical state of the surface. SIM spectra contain elemental ions, and molecular ions. For MSRI, a pulsed ion beam induces a binary collision with the surface being analyzed and the surface species are recoiled into the forward scattering direction with a large kinetic energy. The violence of the binary collision results in complete molecular decomposition, and only elemental ions are detected. The high kinetic energy MSRI ions are much less susceptible to charge neutralization with the surface than the low kinetic energy SIMS ions. In MSRI, the ion yield typically varies by less than a factor of ten as the chemical state of the surface changes--simplifying quantitative analysis vs. SIMS. In this paper, they authors will ...
Contributing Partner: UNT Libraries Government Documents Department
The use of reactive ion sputtering to produce clean germanium surfaces in a carbon rich environment -- An ion scattering study

The use of reactive ion sputtering to produce clean germanium surfaces in a carbon rich environment -- An ion scattering study

Date: October 7, 1997
Creator: Smentkowski, V.S.; Krauss, A.R.; Gruen, D.M.; Holecek, J.C. & Schultz, J.A.
Description: The authors have used the ion spectroscopic techniques of direct recoil spectroscopy (DRS) and mass spectroscopy of recoiled ions (MSRI) to demonstrate that low energy reactive ion sputtering of Ge is capable of removing surface impurities such as carbon. The experiments were performed in a vacuum chamber maintained at 3.5 {times} 10{sup {minus}7} Torr. At these pressures, physical sputtering using noble gas is not effective for cleaning Ge surfaces as carbon re-deposits onto the surface. In this paper, the authors demonstrate that reactive sputtering of Ge using 4.0 keV nitrogen at a Ge surface temperature of {approximately} 740 K and above removes surface carbon and deposits nitrogen on the Ge surface. Heating the nitrogen exposed Ge surface to above {approximately} 880 K results in the desorption of nitrogen and generates an atomically clean Ge surface, under poor vacuum conditions.
Contributing Partner: UNT Libraries Government Documents Department
Epitaxial growth simulation employing a combined molecular dynamics and Monte Carlo approach

Epitaxial growth simulation employing a combined molecular dynamics and Monte Carlo approach

Date: July 1, 1995
Creator: Grein, C.H.; Benedek, R. & Rubia, T. de la
Description: The epitaxial growth of Ge on Si(OO1) is simulated by employing a hybrid approach based on molecular dynamics to describe the initial kinetic behavior of deposited adatoms and Monte Carlo displacements to account for subsequent equilibration. This method is well suited to describe initial nucleation and growth. Stillinger-Weber potentials are employed to describe interatomic interactions.
Contributing Partner: UNT Libraries Government Documents Department
Opacity measurements: extending the range and filling in the gaps

Opacity measurements: extending the range and filling in the gaps

Date: March 17, 1997
Creator: Back, C.A.; Perry, T.S.; Bach, D.R.; Wilson, G.; Iglesias, G.A.; Laden, O.L. et al.
Description: A series of experiments to explore Ge opacity at temperatures where the M-shell is almost filled will be discussed. Data are obtained at lower temperatures than previously explored and allow us to investigate the role of atomic structure calculations and their impact on opacity scalings. The experiment uses the Nova laser to irradiate a gold hohlraum within which a CH-tamped Ge sample is radiatively heated. A Nd backlight probes the sample 2 ns later to produce Ge spectral absorption features in the 1.2-1.5 keV energy range. Temperature is monitored by the use of an Al dopant and density is monitored by measuring the edge-on expansion of the sample. Temporal resolution of about 200 ps is obtained by using a short pulse backlight. Calculations in this photon energy region show significant changes in the spectral features.
Contributing Partner: UNT Libraries Government Documents Department
Magnetic properties of epitaxial and polycrystalline Fe/Si multilayers

Magnetic properties of epitaxial and polycrystalline Fe/Si multilayers

Date: August 1, 1995
Creator: Chaiken, A.; Michel, R.P. & Wang, C.T.
Description: Fe/Si multilayers with antiferromagnetic interlayer coupling have been grown via ion-beam sputtering on both glass and single-crystal substrates. X-ray diffraction measurements show that both sets of films have crystalline iron silicide spacer layers and a periodic composition modulation. Films grown on glass have smaller crystallite sizes than those grown on single-crystal substrates and have a significant remanent magnetization. Films grown on single-crystal substrates have a smaller remanence. The observation of magnetocrystalline anisotropy in hysteresis loops and (hkl) peaks in x-ray diffraction demonstrates that the films grown on MgO and Ge are epitaxial. The smaller remanent magnetization in Fe/Si multilayers with better crystallinity suggests that the remanence is not intrinsic.
Contributing Partner: UNT Libraries Government Documents Department
Measurement of the in-flight pusher density of an indirect drive capsule implosion core using x-ray backlighting

Measurement of the in-flight pusher density of an indirect drive capsule implosion core using x-ray backlighting

Date: May 30, 1996
Creator: Kalantar, D.H.; Haan, S.W.; Hammel, B.A.; Keane, C.J.; Landen, O.L. & Munro, D.H.
Description: Both the efficiency of an implosion and the growth rate of hydrodynamic instability increase with the aspect ratio of an implosion. In order to study the physics of implosions with high Rayleigh-Taylor growth factors, we use doped ablators which should minimize x-ray preheat and shell decompression, and hence increase in- flight aspect ratio. We use x-ray backlighting techniques to image the indirectly-driven capsules. We record backlit 4.7 KeV images of the full capsule throughout the implosion phase with 55 ps and 15{mu}m resolution. We use these images to measure the in-flight aspect ratios for doped ablators, and we infer the radial density profile as a function of time by Abel inverting the transmission profiles.
Contributing Partner: UNT Libraries Government Documents Department
Modification to the Klein-Nishina cross section for Ge electrons at high statistics limit

Modification to the Klein-Nishina cross section for Ge electrons at high statistics limit

Date: November 3, 1995
Creator: Wang, T.F.
Description: Modification factors for the Klein-Nishina cross-sections for gamma-ray with energies between 50 keV and 250 keV incident on Ge electrons have been obtained at the high statistics limit. In this limit, the Ge electrons can then be treated as they are obtained from the self-consistent augmented plane wave calculations, without considering the orientation of crystal lattice with respect to incident photons. The kinematics corrections (i.e. outgoing momenta), on the other hand, have to be taken into account on an event by event basis. Even so, the computing time has been reduced dramatically since the relativistic calculation of the modifications to the Klein-Nishina cross sections is the most tedious one. The modification factors are almost linear with respect to incident photon energy in the interesting energy range with respect to a given photon outgoing angle.
Contributing Partner: UNT Libraries Government Documents Department
Efficient production of 2--10 keV x-rays by laser heated ``underdense radiators``

Efficient production of 2--10 keV x-rays by laser heated ``underdense radiators``

Date: May 22, 1996
Creator: Suter, L.J.; Kauffman, R.L.; Maxon, M.S. & Davis, J.F.
Description: The next generation of high power lasers offers the prospect of creating multi-kilovolt x-rays with {gt}10% efficiency. Such efficiencies are achieved with ``underdense radiators``, a non- traditional source of laser generated x-rays. Applications of these sources with the proposed National Ignition Facility (NIF) include volume preheating of experiments; bright, multi-keV backlighting; pumps for fluorescent imaging of capsule dopants and Doppler velocimetry; uniform irradiation of large test objects. This paper presents two-dimensional numerical simulations for these high power lasers with unprecedented efficiency.
Contributing Partner: UNT Libraries Government Documents Department
Growth and characterization of GE nanocrystals

Growth and characterization of GE nanocrystals

Date: May 1, 1998
Creator: Guha, S., Naval Research Laboratory
Description: We have synthesized Ge nanocrystals of sizes 4, 8, and 12 nm by ion-implanting Ge+ ions into thermally grown Si0{sub 2} films and subsequent annealing of the films at 8300 C for 30 min in nitrogen. These films were characterized by x-ray, transmission electron microscopy (TEM), and Raman spectroscopy. A distribution of particle size was identified by TEM in a 1 00 nm band below the surface. Particle sizes were estimated by these 3 techniques.
Contributing Partner: UNT Libraries Government Documents Department
HPGe compton suppression using pulse shape analysis

HPGe compton suppression using pulse shape analysis

Date: April 15, 1998
Creator: Schmid, G.J.; Beckedahl, D.; Blair, J.J. & Kammeraad, J.E.
Description: We present a new technique for High Purity Germanium (HPGe) Compton suppression using pulse shape analysis (PSA). The novel aspect of our approach involves a complete unfolding of the charge pulse shape into a discrete sum of component y-ray interactions. Using the energy and position information obtained from such an unfolding, an algorithm is then applied which favorably rejects Compton escape events. The advantage of the current PSA approach, as compared with other recent approaches, is the potential to reject not only single-site escape events, but also multiple site escape events. Here we discuss the details of our algorithm, and present experimental results from a real-time implementation on a 5 cm X 5 cm HPGe. An experimental comparison with a standard BGO suppresser is shown. We also discuss the possible improvements to the current PSA approach that could be obtained if the HPGe could be highly segmented on the outer contact.
Contributing Partner: UNT Libraries Government Documents Department
Feasibility of organo-beryllium target mandrels using organo-germanium PECVD as a surrogate

Feasibility of organo-beryllium target mandrels using organo-germanium PECVD as a surrogate

Date: March 9, 1995
Creator: Brusasco, R.M.; Dittrich, T. & Cook, R.C.
Description: Inertial Confinement Fusion capsules incorporating beryllium are becoming attractive for use in implosion experiments designed for modest energy gain. This paper explores the feasibility of chemical vapor deposition of organo-beryllium precursors to form coating materials of interest as ablators and fuel containers. Experiments were performed in a surrogate chemical system utilizing tetramethylgermane as the organometallic precursor. Coatings with up to 60 mole percent germanium were obtained. These coatings compare favorably with those previously reported in the literature and provide increasing confidence that a similar deposition process with an organo-beryllium precursor would be successful.
Contributing Partner: UNT Libraries Government Documents Department
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