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The development of laser wakefield accelerators (LWFA) over the past several years has led to an interest in very compact sources of X-ray radiation – such as “table-top” free electron lasers. However, the use of conventional undulators using permanent magnets also implies system sizes which are large. In this work, we assess the possibilities for the use of novel mini-undulators in conjunction with a LWFA so that the dimensions of the undulator become comparable with the acceleration distances for LWFA experiments (i.e., centimeters). The use of a prototype undulator using laser machining of permanent magnets for this application is described and the emission characteristics and limitations of such a system are determined. Preliminary electron propagation and X-ray emission measurements are taken with a LWFA electron beam at the University of Michigan.
Identifying youth who may engage in future substance use could facilitate early identification of substance use disorder vulnerability. We aimed to identify biomarkers that predicted future substance use in psychiatrically un-well youth.
LASSO regression for variable selection was used to predict substance use 24.3 months after neuroimaging assessment in 73 behaviorally and emotionally dysregulated youth aged 13.9 (s.d. = 2.0) years, 30 female, from three clinical sites in the Longitudinal Assessment of Manic Symptoms (LAMS) study. Predictor variables included neural activity during a reward task, cortical thickness, and clinical and demographic variables.
Future substance use was associated with higher left middle prefrontal cortex activity, lower left ventral anterior insula activity, thicker caudal anterior cingulate cortex, higher depression and lower mania scores, not using antipsychotic medication, more parental stress, older age. This combination of variables explained 60.4% of the variance in future substance use, and accurately classified 83.6%.
These variables explained a large proportion of the variance, were useful classifiers of future substance use, and showed the value of combining multiple domains to provide a comprehensive understanding of substance use development. This may be a step toward identifying neural measures that can identify future substance use disorder risk, and act as targets for therapeutic interventions.
We have determined the extinction efficiency factor Q for the objects in the Table by means of maps, scans, or photometry at 1 mm using a composite bolometer at the prime focus of the ESO 3.6m telescope (Arnold et al., 1978; Arnold, 1979). The beam is 2′ in diameter.
The extended ranges (2–3 times theoretical) for hydration from an ambient atmosphere or water immersion and other anomalous ranges for property changes in ion-implanted fused silica are explained on the basis of a stress corrosion model (Michalske-Bunker). The results for the hydration of implanted soda-lime glass are similar to fused silica with the added feature of compositional modification due to the near-surface removal of alkali.
Sorption and desorption measurements were made of strontium and cesium onto clinoptilolite and Calico Hills tuff. The object was to see whether there was a correlation between sorption of strontium and cesium onto Calico Hills Tuff and the clinoptilolite based on the content of clinoptilolite in the Calico Hills Tuff. If sorption onto Calico Hills Tuff is solely due to the presence of clinoptilolite, then the ratios of the sorption ratios on tuff to those on clinoptilolite at similar conditions should be the weight fraction of the clinoptilolite in the tuff. Since the tuff contained about 50% clinoptilolite, the ratios would be expected to be about 0.5 if sorption was due solely to clinoptilolite. The experimental evidence showed that the ratios were generally near 0.5 for both cesium and strontium sorption and that ion-exchange processes were operative for both the clinoptilolite and the tuff. However, the ratios differed to a small extent for different conditions, and there were indications that other sorption processes were also involved.
Many properties of implanted fused silica (e.g., surface stress, hardness) exhibit maximum implantation-induced changes for collisional energy deposition values of ∼1020 keV/cm3. We have observed a second critical energy deposition threshold value of about 1022 keV/cm3 in stress and hardness measurements as well as in many other experiments on silicate glasses (leaching, alkali depletion, etching rate, gaseous implant redistribution). The latter show evidence for damage depths exceeding TRIM ranges by about a factor of 2. For crystalline quartz, a similar threshold value value has been found for extended damage depths (greater than TRIM) for 250 kev ions (H-Au) as measured by RBS and interference fringes. This phenomenon at high damage deposition energy may involve the large stress gradients between damaged and undamaged regions and the much increased diffusion coefficient for defect transport.
Cantilever-beam measurements of ion-implantation induced stress in (InGa)As/GaAs, Ga(AsP)/GaP, and Ga(AsP)/GaAs strained layer superlattices (SLSs), grown either by molecular-beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD), have shown that a mechanism for precipitous stress-relief can be operative, f or room-temperature damage -energy deposition values above - 2 × 10 keV/cm. This phenomenon is correlated with the initial residual compressive stress on the composite structure and is determined by the differences in lattice parameter between the substrate and the buffer alloy-layer.
Damage-profile tailoring using multi-energy ion implants can produce smooth side walls and deeper etched features with ion-bombardment-enhanced wet etching of LiNb03 than is possible with a single-energy implant. High ion fluences can produce buried microcracks which may contribute to propagation losses commonly observed in ion-implanted waveguides.
Single crystals of the silicate neptunite were irradiated with 600 keV Ar2+ and 1.5 MeV Kr+ and analysed by transmission electron microscopy. Amorphization was observed in a surface layer several hundred angstroms thick following Ar2+ irradiations up to 5.0×l013 Ar/cm2, yet the Ar2+ ions travelled an average of 1/2 μm in depth. The microstructure of the amorphous surface layer depends on the ion fluence, but the amorphous layer thickness remained constant. At the highest fluence, a narrow region below the amorphous layer shows a brittle-to-ductile strain transition, due to tensional volume-expansion of the adjacent ductile amorphous layer. With 1.5 MeV Kr1+, amorphization of the electron transparent region was completed after a fluence of 1.7×l014 Kr+/cm2, and no further damage was observed up to 5.1×1015 Kr+/cm2. However, following a low fluence of 2.0×1011 Kr+/cm2, a single crystal of neptunite became a polycrystalline aggregate (grain size 10 nm) within 7 days of room temperature aging.
The H and Li profiles in proton-exchanged (PE) LiNbO3, have been measured using elastic recoil detection (ERD). Profiles were determined as a function of crystal orientation, time after PE, annealing temperature, MgO-doping, and added Li-benzoate to the benzoic acid proton source. The proton-exchange process produces Hx Li1−x NbO3 structures where x stabilizes near values of x=0.5 or x=0.7-0.75 depending on specific conditions. The ERD measurements represent the first direct and simultaneous measurements of H- and Li-concentrations in PE LiNbO3 and their variation with process parameters.
Cantilever-beam bending and RBS channeling measurements have been used to examine implantation-induced disorder and stress buildup in In0.2Ga0.8As/GaAs SLS structures. Implantation fluences from 1011 to 1015/cm2 were used for 150 keV Si, 320 keV Kr, and 250 keV Zn in SLS and GaAs bulk materials. The critical fluence for saturation of compressive stress occurs prior to amorphous layer formation and is followed by stress relief. For all the ions the maximum ion induced stress scales with energy density into atomic processes and stress relief occurs above ∼1 × 1020: keV/cm3. Stress relief is more pronounced for the SLSs than for bulk GaAs. We suggest that stress-relief may lead to slip or other forms of inelastic material flow in SLSs, which would be undesirable for active regions in device applications. Such material flow may be avoided by limiting maximum fluences or by multiplestep implantation and annealing cycles (or hot implants) at high fluences.
We have examined the optical and transport properties of In.2Ga.8As/GaAs straled-kayer superlZotices (SLS's), which have been implanted either with 5 × 1015/cm2, 250keV Zn+ or with 5 × 1014/cm2, 70keV Be+ and annealed under an arsenic overpressure at 600 °C. For both cases, electrical activation in the implantation-doped regions equalled that of similar implants and anneals in bulk GaAs, even though the Be implant retained the SLS structure, while the Zn implant intermixed the SLS layers to produce an alloy semiconductor of the average SLS composition. Photoluminescence intensities in the annealed implanted regions were significantly reduced from that of virgin material, apparently due to residual implant damage. Diodes formed from both the Be- and the Zn-implanted SLS's produced electroluminescence intensity comparable to that of grown-junction SLS diodes in the same chemical system, despite the implantation processing and the potential for vertical lattice mismatch in the Zn-disordered SLS device. These results indicate that Zn-disordering can be as useful in strained-layer superlattices as in lattice-matched systems.
Cantilever beam measurements of induced lateral stress have been made for He, Xe and Pb ion-implantation into the complex borosilicate radioactive waste glasses (PNL 76–68, SRP), a commercial borosilicate (Pyrex), and into fused Si02 (Suprasil 1). For the borosilicate glasses, implantation caused a buildup of stress to a saturation value (∼ 1 × 105 dynes/cm). The maximum occurred at ion fluences greater than those found for maximum stress in fused silica and without a stress relief mechanism. The separate family of stress curves for Pyrex and PNL 76–68 coincide when stress is plotted as a function of energy into electronic processes. Pyrex has 13 wgt.% B2O3 and 81 wgt.% SiO2, while PNL 76–68 and SRP have 10% B2O3, 40% SiO2 and N5 B2O3 and 58% Si0 2, respectively. Pyrex is known to be phase-separated, and Ghe waste glasses are susceptible to phase separation. It is suggested that the B2O3 phase may be more easily compacted than the SiO2 phase. The evidently greater effectiveness of the ionization component of the ion energy in creating damage in borosilicate glasses should be of considerable interest in waste glass studies because of the large amounts of ionization from α-decay (∼ 5 MeV He).
Sorption isotherms and apparent concentration limits for Tc(VII) and Np(V) for a variety of groundwater/basalt systems were determined using Grande Ronde basalt samples representative of the Hanford Site candidate high-level waste repository. Under oxic redox conditions (air present), little or no sorption of technetium was observed; neptunium exhibited low to moderate sorption ratios. Under anoxic redox conditions (oxygen-free), low to moderate sorption of technetium was often observed, but the extent of sorption was highly dependent upon the groundwater composition and the method of pretreatment (if any) of the basalt. Sorption isotherms for technetium under reducing redox conditions (hydrazine added) indicate an apparent concentration limit of approximately 10−6 mol/L Tc. No apparent concentration limit was found for neptunium for concentrations in groundwater up to.∼10−6 mol/L and 8 × 10−7 mol/L under oxic and reducing (hydrazine added) redox conditions, respectively.
Valence control and valence analysis experiments suggest that the sorption or precipitation of Tc and Np from groundwater in the presence of basalt may result from a heterogeneous reaction occurring on the surface of the basalt. One of the critical factors of this reduction reaction appears to be the accessibility of the reactive ferrous iron component of the basalt. The laboratory simulation of groundwater redox conditions representative of the repository environment through the use of solution phase redox reagents is of questionable validity, and information obtained by such experimental methods may not be defensible for site performance assessment calculations. Anoxic experiments conducted in an argon-filled glove box appear better suited for the laboratory simulation of in situ redox conditions.
We have examined the microstructure and the transport properties of nitrogen-implanted silicon-on-insulator wafers, as well as the performance of integrated-circuit transistors fabricated in this material. The insulating regions were fabricated in silicon by the unpatterned implantation of 4×1017 /cm2, 300 keV nitrogen dimers followed by annealing at 1473 K for 5 hours. For these parameters, the buried nitrogen-implanted layer crystallized into α-silicon nitride, and contains ≈20% excess silicon in the form of silicon inclusions of 5–15 nm diameter. The surface silicon layers are characterized by low-mobility, p-type conduction. The buried dielectric has a resistivity of approximately 108 Ωcm. Functional p-channel, integrated circuit transistors have been fabricated in n-type epitaxial silicon grown over the buried-nitride wafers. These transistors devices are similar in performance to those fabricated in bulk silicon,(hole mobilities in inversion layers of 140 cm2/V-s), and demonstrate the suitability of the buried nitride process for integrated circuit applications.
Implantations of He and Ti were made into LiNbO3 and the H and Li profiles determined by elastic recoil detection (ERD) techniques. The loss of Li and gain of H depends upon the supply of surface H (surface contaminants or ambient atmosphere). For 50 keV He implants into LiNbO3 through a 200 Å Al film, the small Li loss is governed by the interface H. This is also the case for He implants into uncoated LiNbO3 in a beam line with low hydrocarbon surface contamination; similar implants under conditions of greater hydrocarbon deposition result in proportionally larger Li loss and H gain in the implant damage region. The exchange is possible only for those He energies, i.e., 50 keV, where the damage profile intersects the surface. For Ti implants Li is lost with little H gain. For this case the Li loss is believed to result from radiation-enhanced diffusion. Where He implantation is used to establish waveguiding in LiNbO3, the presence or absence of H in the implanted region is crucial with regard to refractive index stability, due to the replacement of H by Li from the bulk.
Metal colloids in glasses can yield an enhanced (χ((3)) susceptibility which leads to an intensity dependent refractive index. Ion implantation is a convenient means of introducing the metal species. The host glass plays an important role in colloid formation. We have characterized Ag-colloid formation in various silicate glasses and, in addition, have studied the formation of colloids in Ag-doped phosphate glass as a function of N and H implantation. Some preliminary results for Cu-implanted glasses are presented.