A Cross Correlation Chopper for Molecular Beam Modulation

The measurement procedure based on the continuous thermal neutron beam modulation with a mechanical chopper was developed for delayed neutron yield measurement of the thermal neutron induced fission of [Formula: see text]Np. The idea of the procedure is similar to that widely used in modern computer communications to prevent unauthorized data access. The data is modulated with a predefined pattern before transmission to the public network, and only the recipient that has the modulation pattern is able to demodulate it upon receipt. For thermal neutron induced reaction applications, the thermal neutron beam modulation pattern was used to demodulate the measured delayed neutron intensity signals on the detector output, resulting in nonzero output only for the detector signals correlated with the beam modulation. The comparison of the method with the conventional measurement procedure was provided, and it was demonstrated that the cross-correlation procedure has special features making it superior to the conventional one, especially when the measured value is extremely small in comparison with the background. Due to the strong sensitivity of the measurement procedure on the modulation pattern of the neutron beam, one can implement the modulation pattern of a specific shape to separate the effect of the thermal part of the beam from the higher energy part in the most confident way in the particular experiment. The remarkable property of our method is related to the unique possibility of separation of the effects caused exclusively by thermal neutrons using the neutron time-of-flight measurement available on the IBR-2 pulsed reactor.

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Artefact in 20Å-Resolution Electron Images of Negatively Stained Twodimensional Membrane Protein Crystals

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100053231 ◽

1982 ◽

Vol 40 ◽

pp. 92-93

Author(s):

Douglas L. Dorset ◽

Barbara Moss

Keyword(s):

Electron Scattering ◽

Cross Correlation ◽

Transmission Function ◽

Group Symmetry ◽

Asymmetric Structure ◽

Object Model ◽

Phase Object ◽

Computing Systems ◽

Resolution Electron ◽

Plane Group

A number of computing systems devoted to the averaging of electron images of two-dimensional macromolecular crystalline arrays have facilitated the visualization of negatively-stained biological structures. Either by simulation of optical filtering techniques or, in more refined treatments, by cross-correlation averaging, an idealized representation of the repeating asymmetric structure unit is constructed, eliminating image distortions due to radiation damage, stain irregularities and, in the latter approach, imperfections and distortions in the unit cell repeat. In these analyses it is generally assumed that the electron scattering from the thin negativelystained object is well-approximated by a phase object model. Even when absorption effects are considered (i.e. “amplitude contrast“), the expansion of the transmission function, q(x,y)=exp (iσɸ (x,y)), does not exceed the first (kinematical) term. Furthermore, in reconstruction of electron images, kinematical phases are applied to diffraction amplitudes and obey the constraints of the plane group symmetry.

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Defects in Heavily-Doped MBE GaAs

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100054704 ◽

1982 ◽

Vol 40 ◽

pp. 442-445

Author(s):

C.B. Carter ◽

D.M. DeSimone ◽

T. Griem ◽

C.E.C. Wood

Keyword(s):

Molecular Beam Epitaxy ◽

Molecular Beam ◽

Heavily Doped

Molecular-beam epitaxy (MBE) is potentially an extremely valuable tool for growing III-V compounds. The value of the technique results partly from the ease with which controlled layers of precisely determined composition can be grown, and partly from the ability that it provides for growing accurately doped layers.

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STM studies of crystal growth

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100086179 ◽

1991 ◽

Vol 49 ◽

pp. 374-375

Author(s):

M. G. Lagally

Keyword(s):

Crystal Growth ◽

Molecular Beam ◽

Chemical Vapor ◽

Sputter Deposition ◽

Field Of View ◽

Scanning Tunneling ◽

Wide Field ◽

Tunneling Microscopy ◽

Wide Field Of View ◽

The One

It has been recognized since the earliest days of crystal growth that kinetic processes of all Kinds control the nature of the growth. As the technology of crystal growth has become ever more refined, with the advent of such atomistic processes as molecular beam epitaxy, chemical vapor deposition, sputter deposition, and plasma enhanced techniques for the creation of “crystals” as little as one or a few atomic layers thick, multilayer structures, and novel materials combinations, the need to understand the mechanisms controlling the growth process is becoming more critical. Unfortunately, available techniques have not lent themselves well to obtaining a truly microscopic picture of such processes. Because of its atomic resolution on the one hand, and the achievable wide field of view on the other (of the order of micrometers) scanning tunneling microscopy (STM) gives us this opportunity. In this talk, we briefly review the types of growth kinetics measurements that can be made using STM. The use of STM for studies of kinetics is one of the more recent applications of what is itself still a very young field.

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An in situ and ex situ TEM study of the formation of thin cobalt silicide films by molecular beam epitaxy on the silicon (100) surface

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s042482010010648x ◽

1988 ◽

Vol 46 ◽

pp. 884-885

Author(s):

D. Loretto ◽

J. M. Gibson ◽

S. M. Yalisove ◽

R. T. Tung

Keyword(s):

Molecular Beam Epitaxy ◽

Molecular Beam ◽

Defect Density ◽

High Vacuum ◽

Ultra High Vacuum ◽

Ex Situ ◽

Metal Base ◽

In Situ Experiments ◽

Potential Applications

The cobalt disilicide/silicon system has potential applications as a metal-base and as a permeable-base transistor. Although thin, low defect density, films of CoSi2 on Si(111) have been successfully grown, there are reasons to believe that Si(100)/CoSi2 may be better suited to the transmission of electrons at the silicon/silicide interface than Si(111)/CoSi2. A TEM study of the formation of CoSi2 on Si(100) is therefore being conducted. We have previously reported TEM observations on Si(111)/CoSi2 grown both in situ, in an ultra high vacuum (UHV) TEM and ex situ, in a conventional Molecular Beam Epitaxy system.The procedures used for the MBE growth have been described elsewhere. In situ experiments were performed in a JEOL 200CX electron microscope, extensively modified to give a vacuum of better than 10-9 T in the specimen region and the capacity to do in situ sample heating and deposition. Cobalt was deposited onto clean Si(100) samples by thermal evaporation from cobalt-coated Ta filaments.

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Near-Surface Aggregation of Sn In Heavily Sn-Doped GaAs Films Grown By Molecular Beam Epitaxy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100118710 ◽

1985 ◽

Vol 43 ◽

pp. 368-369

Author(s):

S. H. Chen

Keyword(s):

Molecular Beam Epitaxy ◽

Cross Section ◽

Electron Spectroscopy ◽

Molecular Beam ◽

Surface Segregation ◽

Secondary Ion Mass Spectrometry ◽

Specimen Preparation ◽

Near Surface ◽

Gaas Films ◽

Secondary Ion

Sn has been used extensively as an n-type dopant in GaAs grown by molecular-beam epitaxy (MBE). The surface accumulation of Sn during the growth of Sn-doped GaAs has been observed by several investigators. It is still not clear whether the accumulation of Sn is a kinetically hindered process, as proposed first by Wood and Joyce, or surface segregation due to thermodynamic factors. The proposed donor-incorporation mechanisms were based on experimental results from such techniques as secondary ion mass spectrometry, Auger electron spectroscopy, and C-V measurements. In the present study, electron microscopy was used in combination with cross-section specimen preparation. The information on the morphology and microstructure of the surface accumulation can be obtained in a fine scale and may confirm several suggestions from indirect experimental evidence in the previous studies.

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Application of cross correlation to HREM images of surfaces

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100128080 ◽

1987 ◽

Vol 45 ◽

pp. 754-755

Author(s):

D. E. Luzzi ◽

L. D. Marks ◽

M. I. Buckett

Keyword(s):

Cross Correlation ◽

A Priori ◽

Matrix Material ◽

Correlation Method ◽

Accurate Knowledge ◽

Complex Image ◽

Fourier Filtering ◽

The Matrix ◽

Low Pass ◽

Data Reduction Technique

As the HREM becomes increasingly used for the study of dynamic localized phenomena, the development of techniques to recover the desired information from a real image is important. Often, the important features are not strongly scattering in comparison to the matrix material in addition to being masked by statistical and amorphous noise. The desired information will usually involve the accurate knowledge of the position and intensity of the contrast. In order to decipher the desired information from a complex image, cross-correlation (xcf) techniques can be utilized. Unlike other image processing methods which rely on data massaging (e.g. high/low pass filtering or Fourier filtering), the cross-correlation method is a rigorous data reduction technique with no a priori assumptions.We have examined basic cross-correlation procedures using images of discrete gaussian peaks and have developed an iterative procedure to greatly enhance the capabilities of these techniques when the contrast from the peaks overlap.

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