Slow positron beam design notes

1982 ◽  
Vol 60 (4) ◽  
pp. 551-557 ◽  
Author(s):  
K. F. Canter ◽  
A. P. Mills Jr.
Keyword(s):  
Figure Of Merit ◽  
Transverse Energy ◽  
High Vacuum ◽  
Positron Beam ◽  
Ultra High Vacuum ◽  
High Flux ◽  
Slow Positron ◽  
Beam Design ◽  
Positron Beams ◽  
Conversion Efficiencies

Recent methods of producing high flux. (105–106) s−1 slow positron beams are briefly reviewed. Currently, slow positron beams are produced most efficiently using a single crystal Cu(111) + S backscatter geometry moderator for ultra-high vacuum (UHV) conditions and an annealed W-vane converter for non-UHV conditions. The respective fast positron to slow positron conversion efficiencies for the Cu(111) + S and W-vane converters are (9 ± 3) × 10−4 and (1.2 ± 0.2) × 10−4. A new figure of merit, the "normalized brightness-per-volt", for converters is introduced which takes into account the transverse energy spread as well as the conversion efficiency. The importance of the normalized-brightness-per-volt in beam design and future methods to improve this figure of merit are discussed.

Shape-Controlled Growth of Indium and Aluminum Nanostructures on MoS2(0001)

2008 ◽  
Vol 8 (5) ◽  
pp. 2707-2712 ◽  
Author(s):  
S. S. Kushvaha ◽  
H. Xu ◽  
H. L. Zhang ◽  
A. T. S. Wee ◽  
X.-S. Wang
Keyword(s):  
Self Assembly ◽  
High Vacuum ◽  
Ultra High Vacuum ◽  
Scanning Tunneling ◽  
Deposition Flux ◽  
Average Height ◽  
High Flux ◽  
Molybdenum Disulphide ◽  
Tunneling Microscopy ◽  
Al Nanoparticles

The growth of indium and aluminum nanostructures on molybdenum disulphide (MoS2)(0001) substrate has been studied using scanning tunneling microscopy in ultra-high vacuum. At low coverage and room temperature (RT), mostly ultra-thin (∼1.2–2 nm) triangular In islands were observed on MoS2. With increasing coverage or high flux, large coalesced irregular islands along with triangular and round-shaped ones of increased average height were found. Triangular and round-shaped islands were obtained after annealing the RT-deposited In on MoS2 sample at 450 K. At ∼375 K, exclusively triangular In islands were observed. Al nanoparticles with diameter in 4–16 nm range were obtained after a low-flux deposited whereas ramified islands were observed in a high flux at RT. Ultra-thin (∼1.20–2 nm) Al islands and films were obtained on MoS2 after deposition at 500 K. These results demonstrate that the shape of In and Al nanostructures grown on MoS2 can be controlled in self-assembly by adjusting substrate temperature, deposition flux and amount.

Evaluation of Vacancy-Type Defects in Simox Substrates by a Slow Positron Beam and a Pulsed Positron Beam

1992 ◽  
Vol 262 ◽  
Author(s):  
H. Kametani ◽  
H. Akiyama ◽  
Y. Yamaguchi ◽  
M. Koumaru ◽  
L. Wei ◽  
...  
Keyword(s):  
Positron Lifetime ◽  
Silicon Layer ◽  
Positron Beam ◽  
High Dose ◽  
Positron Energy ◽  
Slow Positron ◽  
Depth Profiles ◽  
Positron Beams ◽  
Non Destructive ◽  
Incident Positron

ABSTRACTSlow/monoenergetic positron beams and pulsed positron beams have been used as a non-destructive probe to investigate vacancy-type defects in SIMOX substrates which were formed by high - dose oxygen implantation and high-temperature annealing. To obtain depth profiles of vacancy-type defects, a positron beam in the 0–30keV energy range was used. Doppler broadened annihilation spectrum and positron lifetime were measured as a function of incident positron energy. These measurements show the following results; vacancy -type defects exist near the surface of the top silicon layer even if the specimen was analyzed as defect -free Silicon by XTEM, and in the case of the as-implanted specimen, cavities in diameter of about 50–200A are created in the top silicon layer and they include high pressure gases.

scholarly journals High Concentration Erbium Implantation of Epitaxially Grown Caf2 /Si Structures.

1995 ◽  
Vol 392 ◽  
Author(s):  
S. Raoux ◽  
A. S. Barriere ◽  
H. J. Lozykowski ◽  
I. G. Brown
Keyword(s):  
Light Emission ◽  
High Vacuum ◽  
Charge Compensation ◽  
High Refractive Index ◽  
High Energy ◽  
Ultra High Vacuum ◽  
High Concentration ◽  
Integrated Devices ◽  
Compensation Mechanisms ◽  
Conversion Efficiencies

AbstractCalcium fluoride thin films grown on silicon substrates by sublimation under ultra high vacuum are well known to be highly efficient hosts for rare earth luminescence properties. For this reason we incorporate erbium by ion implantation in order to form optoelectronic integrated devices. Here we describe the incorporation conditions of erbium in CaF2/Si structures and their luminescence characteristics. The properties of the material have been investigated for implantation doses varying from 4×1014 to 1×1017 at.cm−2. The role of oxygen in the charge compensation mechanisms is investigated and it is shown that the maximum emission in erbium at 1.53μm occurs for an implanted dose of 2×1016 at.cm−2. This corresponds to an Er concentration three orders of magnitude greater than for the case of classical-erbium-doped semiconductors. At this high concentration (up to 15 at.%) the light emission mechanisms are of great theoretical interest. They involve strong Er-Er coupling effects: energy transfer, cross-relaxation phenomena and high conversion efficiencies.These properties make erbium-implanted CaF2/Si structures excellent candidates for the production of optically active waveguides. The guiding structure can be formed by high energy implantation to build a buried active region of high refractive index within the CaF2 thin film.

Thin Pyrolytic Graphite Films for Electron Microscope Substrates

1971 ◽  
Vol 29 ◽  
pp. 226-227 ◽  
Author(s):  
George H. N. Riddle ◽  
Benjamin M. Siegel
Keyword(s):  
Vacuum Chamber ◽  
Pyrolytic Graphite ◽  
High Vacuum ◽  
Dark Field ◽  
Ultra High Vacuum ◽  
Graphite Substrate ◽  
Nickel Substrate ◽  
Routine Procedure ◽  
Field Electron Microscopy ◽  
Dark Field Electron

A routine procedure for growing very thin graphite substrate films has been developed. The films are grown pyrolytically in an ultra-high vacuum chamber by exposing (111) epitaxial nickel films to carbon monoxide gas. The nickel serves as a catalyst for the disproportionation of CO through the reaction 2C0 → C + CO2. The nickel catalyst is prepared by evaporation onto artificial mica at 400°C and annealing for 1/2 hour at 600°C in vacuum. Exposure of the annealed nickel to 1 torr CO for 3 hours at 500°C results in the growth of very thin continuous graphite films. The graphite is stripped from its nickel substrate in acid and mounted on holey formvar support films for use as specimen substrates.The graphite films, self-supporting over formvar holes up to five microns in diameter, have been studied by bright and dark field electron microscopy, by electron diffraction, and have been shadowed to reveal their topography and thickness. The films consist of individual crystallites typically a micron across with their basal planes parallel to the surface but oriented in different, apparently random directions about the normal to the basal plane.

A High Voltage Electron Microscopy Study of Sub-Oxide Formation in Vanadium

1971 ◽  
Vol 29 ◽  
pp. 212-213
Author(s):  
R. H. Geiss ◽  
R. L. Ladd ◽  
K. R. Lawless
Keyword(s):  
High Vacuum ◽  
Microscopy Study ◽  
Electron Microscopy Study ◽  
Ultra High Vacuum ◽  
Pure Oxygen ◽  
Pressure Oxidation ◽  
High Voltage Electron Microscopy ◽  
Oxidation Study ◽  
Voltage Electron ◽  
Good Agreement

Detailed electron microscope and diffraction studies of the sub-oxides of vanadium have been reported by Cambini and co-workers, and an oxidation study, possibly complicated by carbon and/or nitrogen, has been published by Edington and Smallman. The results reported by these different authors are not in good agreement. For this study, high purity polycrystalline vanadium samples were electrochemically thinned in a dual jet polisher using a solution of 20% H2SO4, 80% CH3OH, and then oxidized in an ion-pumped ultra-high vacuum reactor system using spectroscopically pure oxygen. Samples were oxidized at 350°C and 100μ oxygen pressure for periods of 30,60,90 and 160 minutes. Since our primary interest is in the mechanism of the low pressure oxidation process, the oxidized samples were cooled rapidly and not homogenized. The specimens were then examined in the HVEM at voltages up to 500 kV, the higher voltages being necessary to examine thick sections for which the oxidation behavior was more characteristic of the bulk.

The High Resolution Scanning Transmission Electron Microscope

1974 ◽  
Vol 32 ◽  
pp. 316-317
Author(s):  
A. V. Crewe
Keyword(s):  
Electron Microscope ◽  
High Resolution ◽  
High Vacuum ◽  
Scanning Transmission Electron Microscope ◽  
Ultra High Vacuum ◽  
Resolving Power ◽  
Imaging Characteristics ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
The Moment

The high resolution STEM is now a fact of life. I think that we have, in the last few years, demonstrated that this instrument is capable of the same resolving power as a CEM but is sufficiently different in its imaging characteristics to offer some real advantages.It seems possible to prove in a quite general way that only a field emission source can give adequate intensity for the highest resolution^ and at the moment this means operating at ultra high vacuum levels. Our experience, however, is that neither the source nor the vacuum are difficult to manage and indeed are simpler than many other systems and substantially trouble-free.

Structure of Palladium Single-Crystal Films Prepared by Flash Evaporation onto (001) NaCl Substrates

1970 ◽  
Vol 28 ◽  
pp. 456-457
Author(s):  
L. E. Murr ◽  
G. Wong
Keyword(s):  
Single Crystal ◽  
High Vacuum ◽  
Ultra High Vacuum ◽  
High Rate ◽  
Flash Evaporation ◽  
Optimum Load ◽  
Single Crystal Films ◽  
Crystal Films ◽  
A Current

Palladium single-crystal films have been prepared by Matthews in ultra-high vacuum by evaporation onto (001) NaCl substrates cleaved in-situ, and maintained at ∼ 350° C. Murr has also produced large-grained and single-crystal Pd films by high-rate evaporation onto (001) NaCl air-cleaved substrates at 350°C. In the present work, very large (∼ 3cm2), continuous single-crystal films of Pd have been prepared by flash evaporation onto air-cleaved (001) NaCl substrates at temperatures at or below 250°C. Evaporation rates estimated to be ≧ 2000 Å/sec, were obtained by effectively short-circuiting 1 mil tungsten evaporation boats in a self-regulating system which maintained an optimum load current of approximately 90 amperes; corresponding to a current density through the boat of ∼ 4 × 104 amperes/cm2.

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