R. H. Howell

Characterization of Free Volume in a Bulk Metallic Glass Using Positron Annihilation Spectroscopy

The free volume of metallic glasses has a significant effect on atomic relaxation processes, although a detailed understanding of the nature and distribution of free volume sites is currently lacking. Positron annihilation spectroscopy was employed to study free volume in a Zr-Ti-Ni-Cu-Be bulk metallic glass following plastic straining and cathodic charging with atomic hydrogen. Multiple techniques were used to show that strained samples had more open volume, while moderate hydrogen charging resulted in a free volume decrease. It was also shown that the free volume is associated with zirconium and titanium at the expense of nickel, copper, and beryllium. Plastic straining led to a slight chemical reordering.

Detailed study of nuclear fusion from femtosecond laser-driven explosions of deuterium clusters

Recent experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with sufficient kinetic energy to drive nuclear fusion. Irradiating deuterium clusters with a 35 fs laser pulse, it is found that the fusion neutron yield is strongly dependent on such factors as cluster size, laser focal geometry, and deuterium gas jet parameters. Neutron yield is shown to be limited by laser propagation effects as the pulse traverses the gas plume. From the experiments it is possible to get a detailed understanding of how the laser deposits its energy and heats the deuterium cluster plasma. The experiments are compared with simulations.

Production of slow positrons with a 100‐MeV electron linac

Intense, pulsed beams of low‐energy positrons have been produced by a high‐energy beam from an electron linac. The production mechanism has been studied near 100‐MeV incident energy and several characteristics that affect slow‐positron production have been identified. These characteristics include the geometry of the electron‐positron converter and positron moderator, the thickness of the converter, and the energy of the electron beam. The production efficiency for producing low‐energy positrons has been determined experimentally. The results imply that low‐energy positron beams from a linac can be of much higher intensity than those beams currently derived from radioactive sources.

Atomic-layer engineering of cuprate superconductors

A technique for atomic layer-by-layer synthesis of cuprate superconductors and other complex oxides has been developed. Thin films with excellent transport properties and atomically flat surfaces and interfaces are obtained. The samples are engineered by stacking molecular layers of different compounds to assemble multilayers and superlattices, by adding or omitting atomic monolayers to create novel compounds, and by doping within specified atomic monolayers to fabricate, for the first time, intra-cell barriers. Apart from manufacturing trilayer Josephson junctions withIcRn>5 mV, this technique enables one to customize both the materials and the devices according to the needs of a specific experiment. A number of fundamental issues, such as the dimensionality of the HTSC state, existence of long-range proximity effects, occurrence of resonant tunneling with a specified number of hops, etc., have been addressed in this way. Synthesis of the first “artificial” metastable HTSC compounds is also reported.

Chemical ordering around open-volume regions in bulk metallic glass Zr52.5Ti5Al10Cu17.9Ni14.6

We provide direct experimental evidence for a nonrandom distribution of atomic constituents in Zr52.5Ti5Al10Cu17.9Ni14.6 bulk metallic glass using positron annihilation spectroscopy. The Ti content around the open-volume regions is significantly enhanced at the expense of Ni and Cu. Our results indicate that Ni and Cu atoms closely occupy the volume bounded by their neighboring atoms while Al, Ti, and Zr are less closely packed, and more likely to be associated with the open-volume regions. The overall distribution of elements seen by the positron is not significantly altered by annealing or by crystallization. Theoretical calculations indicate that the observed elemental distribution is not consistent with the known crystalline phases Zr2Cu and NiZr2, while Al3Zr4 shows some of the characteristics seen in the experiment.

Production and use of low-energy, monoenergetic positron beams from electron LINACS

Low-energy intense positron beams derived from pair production can be made at high-energy electron linacs and such beams are in operation or under installation at several linac facilities. Using a pulsed position beam made at a 100 MeV electron linac, we have measured the intensity and velocity distribution of positronium emitted from materials by measuring the time-of-flight of annihilating positronium. The time-of-flight data are augmented by positron lifetime and angular correlation measurements performed with the beam. Positronium spectra have been measured for a number of metallic samples. Several new observations have been made including details of the energy distribution of positronium emission formed by a thermalized positron and a conduction electron and the production of positronium from energetic positrons scattered out of the sample.

Positron annihilation spectroscopy and small-angle neutron scattering characterization of the effect of Mn on the nanostructural features formed in irradiated Fe–Cu–Mn alloys

The size, number density, and composition of the nanometre-sized defects responsible for the hardening and embrittlement in irradiated Fe–0.9 wt% Cu and Fe–0.9 wt% Cu–1.0 wt% Mn model reactor pressure vessel alloys were measured using small-angle neutron scattering and positron annihilation spectroscopy. These alloys were irradiated at 290°C to relatively low neutron fluences (E > 1 MeV, 6.0 × 1020 to 4.0 × 1021 n m−2) in order to study the effect of manganese on the nucleation and growth of copper-rich precipitates and secondary defect features. Copper-rich precipitates were present in both alloys following irradiation. The effect of Mn was to reduce the size and increase the number density of precipitates in the Fe–Cu–Mn alloy relative to the Fe–Cu alloy. Vacancy clusters were observed in the Fe–Cu alloy, but not in the Fe–Cu–Mn alloy. These results suggest a strong effect of Mn on vacancy diffusion and clustering.

Temperature dependence of positron annihilation in a Zr–Ti–Ni–Cu–Be bulk metallic glass

A strong temperature dependence of positron annihilation with low-momentum electrons is reported for a Zr-based bulk metallic glass in the temperature range 50-300 K. The observed behavior was rationalized in terms of shallow versus deep positron traps. An interpretation of the data was presented based on the idea that there were two different types of open-volume regions: Bernal interstitial sites and thermally unstable larger holes. Bernal interstitial sites, intrinsic to the glass structure, were found to be insensitive to annealing. Alternatively, the larger holes were removed by annealing. The strong correlation between these larger holes and diffusion and viscous flow processes suggests that they may act as diffusion and flow defects.

Direct observation of carbon-decorated defects in fatigued type 304 stainless steel using positron annihilation spectroscopy

We have directly observed carbon decoration of defects in fatigued 304 stainless steel using positron annihilation spectroscopy. The formation and evolution of defects during fatigue was determined by positron annihilation lifetimes and electron momentum distributions in a series of samples. We find an initial rapid change in the defect concentrations that saturates around 10% of the cycles to failure into two distinct open-volume defect populations that both trap the positrons. Analysis of the momentum distributions of atomically bound electrons demonstrates that one of the defects has high levels of carbon decoration. Electron momentum distributions also show evolution in the carbon decoration of the defects with increasing fatigue all the way to failure.

The positron microprobe at LLNL

The electron linac based positron source at Lawrence Livermore National Laboratory (LLNL) provides the worlds highest current beam of keV positrons. We are building a positron microprobe that will produce a pulsed, focused positron beam for 3-dimensional scans of defect size and concentration with sub-micron resolution. The widely spaced and intense positron packets from the tungsten moderator at the end of the 100 MeV LLNL linac are captured and trapped in a magnetic bottle. The positrons are then released in 1 ns bunches at a 20 MHz repetition rate. With a three-stage re-moderation we will compress the cm-sized original beam to a 1 micro-meter diameter final spot on the target. The buncher will compress the arrival time of positrons on the target to less than 100 ps. A detector array with up to 60 BaF2 crystals in paired coincidence will measure the annihilation radiation with high efficiency and low background. The energy of the positrons can be varied from less than 1 keV up to 50 keV.