Single Germanium Research Articles

Monte Carlo modelling for in vivo measurements of americium in a knee voxel phantom: general criteria for an international comparison

The general criteria and the scientific approach adopted for an 'International comparison on Monte Carlo modelling for in vivo measurement of Americium in a knee phantom' that is being organised within the EU Coordination Action CONRAD (Coordinated Network for Radiation Dosimetry) are described her. Detection system and a knee voxel phantom based on a computerised axial tomography of the Spitz anthropometric knee phantom with a homogeneous distribution of 241Am in bone have been considered for the simulation of three specific situations: (a) a single Low Energy Germanium detector for a point 241Am source in air; (b) the calculation of photon fluence spectra in air around the voxel phantom; and (c) the calculation of the energy distribution of pulses and peak detection efficiency in the real detection system geometry.

High-efficiency machining of single-crystal germanium using large-radius diamond tools

Round-nosed diamond tools with various nose radii are used for cutting single crystalline germanium and fundamental machining characteristics are examined. It is found that when cutting with small radius tools, the principal cutting force is dominant and crater wear on rake face is significant. However, when cutting with large radius tools, the thrust force becomes significant and tool failure is caused by formation of an angular flank wear land. The use of a large radius tool at a high tool feed improves ductile machining efficiency by a factor of over ten, thus is useful for reducing infrared lens manufacturing costs.

First-principles calculation of free Si(100) surface impurity enrichment

Low-temperature crystal growth techniques can deposit silicon films with impurity concentration orders of magnitude above their bulk solubility limits. First-principles calculations were performed of the energies (relative to the bulk) of single substitutional carbon, germanium, boron, and arsenic atoms at several positions within a thin (100) slab of silicon reconstructed as c(4×2). The energies of these impurities were found to be at least 0.2eV lower than in the bulk, corresponding to surface enrichments of 1000 or greater at a temperature of 500°C. General trends can be explained using the concepts of hybridization energy and lone pairs. The large surface reconstruction strain gives rise to this complex potential energy surface, and favors long-ranged order among impurities near the surface. As a result, we expect a complex dependence of trapped impurity concentrations on growth rate and temperature, with a high sensitivity to these parameters when the exchange rate of the impurity with neigboring sites is comparable to the monolayer deposition rate.

Gamma-ray imaging with a coaxial HPGe detector

We report on the first experimental demonstration of Compton imaging of gamma-rays with a single coaxial high-purity germanium (HPGe) detector. This imaging capability is realized by two-dimensional segmentation of the outside contact in combination with digital pulse-shape analysis, which enables to image gamma-rays in 4π without employing a collimator. We are able to demonstrate the ability to image the 662 keV gamma-ray from a 137Cs source with preliminary event selection, with an angular resolution of 5° and a relative efficiency of 0.3%. This efficiency expresses the fraction of gamma-rays that can be imaged, out of the total gamma-ray flux which is emitted into the solid angle of the detector. In addition to the 4π imaging capability, such a system is characterized by its excellent energy resolution and can be implemented in any size possible for Ge detectors to achieve high efficiency.

Green’s function-based multiscale modeling of defects in a semi-infinite silicon substrate

We have developed a Green’s function (GF) based multiscale modeling of defects in a semi-infinite silicon substrate. The problem—including lattice defects and substrate surface, i.e., an extended defect, at different length scales—is first formulated within the theory of lattice statics. It is then reduced and solved by using a scale-bridging technique based on the Dyson’s equation that relates a defect GF to a reference GF and on the asymptotic relationship of the reference lattice-statics GF (LSGF) to the continuum GF (CGF) of the semi-infinite substrate. The reference LSGF is obtained approximately by solving the boundary-value problem of a super-cell of lattice subject to a unit point force and under a boundary condition given by the reference CGF. The Tersoff potential of silicon, germanium and their compounds is used to derive the lattice-level force system and force constants and further to derive the continuum-level elastic constants (of the bulk silicon, needed in the reference CGF). We have applied the method to solve for the lattice distortion of a single vacancy and a single germanium substitution. We have further calculated the relaxation energy in these cases and used it to examine the interaction of the point defects with the (traction-free) substrate surface and the interaction of a single vacancy with a relatively large germanium cluster in the presence of the substrate surface. In the first case, the point defects are found to be attracted to the substrate surface. In the second case, the single vacancy is attracted to the germanium cluster as well as to the substrate surface.

Long-period grating Michelson refractometric sensor

This paper presents a single-probe Michelson interferometer that uses mode coupling in a long-period grating to establish the two optical paths in a single optical fibre. The interferometer phase shift depends on the refractive index of the material that surrounds the fibre probe, with the phase sensitivity directly proportional to the probe length. A simple phenomenological model explains the experimental results obtained with a liquid level sensor and a refractive index sensor. Its temperature sensitivity depends on the type of fibre that constitutes the probe. With a 45 mm long fibre probe, it is −2.5 and 12.8 degrees °C−1 for normal single mode fibre (SMF28) and germanium–boron co-doped fibre (PS1500), respectively.

An advanced Compton telescope based on thick, position-sensitive solid-state detectors

The development of high-resolution, position-sensitive solid-state detectors enables γ-ray instruments with improved sensitivity and imaging capabilities. For γ-ray astronomy, an improvement in sensitivity of 20–50 over previous missions is anticipated with the Advanced Compton Telescope (ACT) mission. The γ-ray astrophysics group at NRL has been developing germanium strip detectors for several years. We have shown that 3D locations for γ-ray interactions can be determined with mm accuracy, and have also demonstrated imaging capability within a single germanium strip detector and in detector arrays. Based on the realization that three Compton interactions would enable the energy and direction cone of the incident γ-ray to be determined, even when the total incident γ-ray energy is not deposited in the detector, we have also initiated work on thick silicon strip detectors. We are investigating the use of both thick Si(Li) and thick intrinsic silicon detectors. Progress on this work, including initial tests of the multiple-Compton imaging technique, is presented.

High-pressure ultrasonic setup using the Paris–Edinburgh press: Elastic properties of single crystalline germanium up to 6 GPa

A setup for the measurement of ultrasonic properties of single crystals at high pressure in the 10 GPa range is presented. In order to validate this new device, based on the “Paris–Edinburgh” press, the elastic properties of germanium have been measured and compared with the published data. The transit times of the ultrasonic waves are directly measured. The three elastic moduli and their pressure derivatives are obtained up to 6 GPa through three different methods of calculation, which are presented and compared. These results are in good agreement with previous results and hence validate the device. The Paris–Edinburgh press enables in situ measurement of ultrasonic and x-ray diffraction on the studied sample.

Hole trapping in self-assembled SiGe quantum nanostructures

The electronic behaviour of boron-doped single germanium layers grown in a silicon matrix by solid source MBE has been studied by deep level transient spectroscopy (DLTS) and photoluminescence (PL). The structures result from the Si-capping of nine monolayers of germanium on (100) silicon at different substrate temperatures (650, 550 and 450 °C). Due to the growth procedure, the buried germanium structures consist of a wetting layer (WL) (typical of the Stranski–Krastanov growth mode of germanium in silicon) plus the self-assembled structure under the silicon cap. The detailed morphology of the structure depends on the growth temperature. Emission and trapping behaviours of the layer have been studied by DLTS and PL measurements, while Transmission Electron Microscope (TEM) observations were used to characterise the actual film morphology. We first show that the major electrical signal arising from the structure comes from the WL. The corresponding ground state depth clearly follows the trend imposed by a typical quantum confinement: the thinner the well, the higher the level. We also show that the electrical signal is maintained in the low-temperature-grown sample even if light emission from the WL is non-existent. The growth procedure coupled with PL results also allowed us to confirm the high level of elastic relaxation inherent to dome islands nucleation. The hole trapping behaviour of the WLs have also been investigated. A surprising point-defect-like behaviour is observed, suggesting that the WL capture rate is charge state independent.

Development and applications of position-sensitive solid-state gamma ray detectors

The development of high-resolution position-sensitive, solid-state detectors will enable gamma ray detectors with improved sensitivity and imaging capabilities. The gamma ray astrophysics group at NRL has been developing germanium strip detectors for several years. We have shown that three-dimensional locations for gamma ray interactions can be determined with sub-millimeter accuracy, and have also demonstrated imaging capability within a single germanium strip detector. We have also initiated work on thick, silicon strip detectors. This was based on the fact that three sequential interactions can enable the energy and direction cone of the incident gamma ray to be determined, even without total energy deposition of the incident gamma ray. We are also working on low-power ASICs that are required to handle the large number of channels associated with arrays of strip detectors. Progress on this work will be presented, along with applications to high-energy astrophysics, medical imaging, nuclear physics, detection of fissile materials, and monitoring of environmental radioactivity.

High-Pressure Ultrasonic Measurements on Single Crystal

Elastic properties of single crystalline germanium have been determined up to 6 GPa. All three elastic constants and their pressure derivative have been obtained by transit time measurements in a large volume cell. Comparison with previously published data fully validate the proposed set-up. The design of this set-up allows simultaneous in situ ultrasonic and X-ray diffraction measurements.

Growth and electrical transport of germanium nanowires

Single crystalline germanium nanowires have been synthesized from gold nanoparticles based on a vapor–liquid–solid growth mechanism. Germanium powder was evaporated at 950 °C, and deposited onto gold nanoparticles at 500 °C using argon as a carrier gas. The diameter of the germanium nanowires ranged from 20 to 180 nm when gold thin films were utilized as the substrate, while the nanowires grown from 10 nm Au particles showed a narrower diameter distribution centered at 28 nm. The growth direction of germanium nanowires is along the [111] direction, determined by high resolution transmission electron microscopy. Transport measurements on individual Ge nanowires indicate that the wires are heavily doped during growth and that transport data can be explained by the thermal fluctuation tunneling conduction model.

Elemental investigation of some Egyptian vehicle motor alloys using neutron activation analysis

Six samples of different Egyptian motor vehicle alloys have been investigated by neutron activation analysis to estimate the concentrations of their elemental constituents, which affect their quality. A single high resolution hyper-pure germanium HPGe γ-ray detector and a multichannel analyzer are used to collect the γ-ray spectra. A HPGe–HPGe coincidence spectrometer was also used to confirm the identity of certain peaks. Thirteen trace elements (Sc, Cr, Fe, Co, Zn, Sn, Sb, La, Sm, Eu, Hf, Pt and Au) were observed in the spectra and their concentrations were determined. A comparative study on the element concentrations for the six samples is given.

Isotopically engineered semiconductors – new media for the investigation of nuclear spin related effects in solids

We report on the growth and characterization of isotopically controlled silicon bulk single crystals and germanium superlattice structures in which position and concentration of nuclear spins can be controlled intentionally. The isotopically controlled semiconductor structures enable a wide variety of nuclear-spin related studies involving nuclear-magnetic-resonance and electron-spin-resonance spectroscopy.

Gamma-ray Compton camera imaging with a segmented HPGe

In this paper, we investigate our concept to develop a γ-ray Compton camera out of a single coaxial High Purity Germanium (HPGe) detector. The imaging properties of the HPGe can be realized by way of a segmented outer contact and digital pulse-shape analysis. Limiting factors in performance will be related to the intrinsic electron momentum in Ge and the noise in the preamplifier JFETs. In addition to discussing these issues, we will present experimental and theoretical imaging studies that we have done using an existing segmented HPGe: the GRETA prototype detector at LBNL.

Efficiency Maps of the LLNL and JAERI Realistic Torso Phantoms Using a 70 mm Diameter Germanium Detector and Implications for the Minimum Detectable Activity

Counting efficiency surfaces of the LLNL and JAERI torso phantoms have been determined using a single large area germanium detector at various photon energies (17, 60, 122, 244, and 344 keV) by comparing count rates at many different positions over the torso phantoms' chest plate. The maps can be used to determine the optimum positions for placement of germanium detectors for lung counting and to compare the counting characteristics of the two torso phantoms. The 'efficiency' maps are quite different between the two phantoms at 17 keV; indicative perhaps of how real people will either differ from each other or differ from a calibration phantom. As the photon energy increases the 'efficiency' maps remain distinct, although they are more similar than at 17 keV, showing that the phantoms have different counting efficiency characteristics.

The High Purity Germanium Detector Whole Body Monitor at PSI

For the in vivo monitoring of radiation workers a new measurement system has been installed at PSI, Switzerland. A single p-type high purity germanium detector of 102% relative efficiency is mounted in a shielded room located in a subterranean shelter. The room has internal dimensions of 270 cm × 190cm × 220 cm and is constructed of iron from old railway rails. The detector is mounted on a mechanical support fixed on the ceiling, which permits movements in three directions and two axes. Measurements of radionuclides in the body are performed in a sitting position. The system is used to detect the presence of radionuclides that emit photons with energies between 50 keV and 2 MeV. A description of the system is presented. The results of calibration of the detector for total-body activity measurements are discussed. The performances of the new system and the old Nal(TI) body monitor used at PSI are compared.

The response of the Euroball Cluster detector to γ-radiation up to 10 MeV

The Euroball Cluster detector consists of seven closely packed hexagonal encapsulated germanium detectors which are mounted in a common cryostat and surrounded by a BGO suppression shield. By adding back the Compton scattered events between the seven germanium detectors the detector system behaves similar as a single germanium detector with a volume of 2000 cm 3. The response of this very large germanium detector to γ-rays up to 10 MeV has been measured using a short lived 24Al source produced at the Cologne FN Tandem accelerator. At 10 MeV the efficiency of the Cluster detector is nearly one order of magnitude larger than that of a 30% germanium detector.

Standard of Specular Reflectance at Glancing Incidence for the Infrared Region

The performance of high power carbon dioxide and free electron lasers depends upon the quality of mirrors used in them. A precise characterization of mirrors is required, and this is achieved by employing reference standards. Standards for the calibration of reflectance scale for the infrared region are not available. The present work describes the development of a specular reflectance standard at glancing angles of incidence from 10° to 70° for the 2,5 μm to 25 μm infrared region. Whereas aluminium and gold mirrors have been used as high reflectance value standards, single and double surface polished silicon and germanium have been employed as standards at low reflectance values.

赤外光学系における単結晶および多結晶ゲルマニウムの比較

赤外光学系における単結晶および多結晶ゲルマニウムの比較