Dual role of oxygen-related defects in the luminescence kinetics of AlN:Mn2.

This study presents the impact of temperature and pressure on AlN:Mn2+ luminescence kinetics. Unusual behavior of Mn2+ optical properties during UV excitation is observed, where a strong afterglow luminescence of Mn2+ occurs even at low temperatures. When the temperature increases, the contribution of the afterglow luminescence is further enhanced, causing a significant increase in the luminescence intensity. The observed phenomena may be explained by an energy diagram in which the ON-VAl complex in AlN:Mn2+ plays a key role. Hence the ON-VAl complex defect in AlN:Mn2+ plays a double role. When the ON-VAl defect is located close to Mn2+ ions, it is responsible for transferring excitation energy directly to Mn2+ ions. However, when the ON-VAl defect complex is located far from Mn2+ ions, its excited state level acts as an electron trap responsible for afterglow luminescence. Additionally, three models have been tested to explain the structure of the emission spectrum and the strong asymmetry between the excitation and emission spectra. From the most straightforward configuration coordinate diagram through the configuration coordinate diagram model assuming different elastic constants in the excited and ground-states ending by a model based on the Jahn-Teller effect. We proved that only the Jahn-Teller effect in the excited 4T1 electronic state with spin-orbit coupling could fully explain the observed phenomena. Finally, high-pressure spectroscopic results complemented by the calculations of Racah parameters and the Tanabe-Sugano diagram are presented.

Material jet printing of transparent ceramic Yb:YAG planar waveguides.

A new, to the best of our knowledge, 3D additive manufacturing technique utilizing particle-loaded ink jet printing to fabricate transparent ceramic Yb:YAG planar waveguides for laser gain media was demonstrated. Rheological optimization of YAG particle-loaded inks resulted in successful droplet formation and printing resolution. Planar waveguides composed of a Yb:YAG guide encased in undoped YAG cladding were printed with guide thicknesses ranging between 25 and 350 µm and consolidated to high optical quality via solid-state sintering. Sufficiently low optical (1-3%/cm) and intermodal scatter allowed single-mode propagation with a core/clad index difference of $\Delta {n}\sim{5.0} \times {{10}^{- 4}}$ (corresponding to 3 at.% Yb). The waveguides were cladding-pumped longitudinally with a 940 nm diode array resulting in 23.6% slope efficiency in 2 ms pulsed operation.

High-Z Sensitized Plastic Scintillators: A Review.

The need for affordable and reliable radiation detectors has prompted significant investment in new radiation detector materials, due to concerns about national security and nuclear nonproliferation. Plastic scintillators provide an affordable approach to large volume detectors, yet their performance for high-energy gamma radiation is severely limited by the small radiation stopping power inherent to their low atomic number. Although some sensitization attempts with organometallics were made in the 1950s to 1960s, the concomitant decrease in light yield has limited the usefulness of these sensitized detectors. Recently, with new knowledge gained during the rapid development of organic optoelectronics and nanotechnology, there has been a revived interest in the field of heavy element sensitized plastic scintillators. Here, the recent efforts on sensitized plastic scintillators are summarized. Basic scintillator physics is first reviewed. The discussion then focuses on two major thrusts in the field: sensitization with: (1) organometallics and (2) oxide and fluoride nanoparticles. The design rationales and major results are examined in detail, with existing limitations and possible future pathways discussed. Special attention is paid to the underlying energy deposition and transfer processes, as these determine the key performance metrics such as light yield and radioluminescence decay lifetime.

Experimental comparison of high-density scintillators for EMCCD-based gamma ray imaging.

Detection of x-rays and gamma rays with high spatial resolution can be achieved with scintillators that are optically coupled to electron-multiplying charge-coupled devices (EMCCDs). These can be operated at typical frame rates of 50 Hz with low noise. In such a set-up, scintillation light within each frame is integrated after which the frame is analyzed for the presence of scintillation events. This method allows for the use of scintillator materials with relatively long decay times of a few milliseconds, not previously considered for use in photon-counting gamma cameras, opening up an unexplored range of dense scintillators. In this paper, we test CdWO4 and transparent polycrystalline ceramics of Lu2O3:Eu and (Gd,Lu)2O3:Eu as alternatives to currently used CsI:Tl in order to improve the performance of EMCCD-based gamma cameras. The tested scintillators were selected for their significantly larger cross-sections at 140 keV ((99m)Tc) compared to CsI:Tl combined with moderate to good light yield. A performance comparison based on gamma camera spatial and energy resolution was done with all tested scintillators having equal (66%) interaction probability at 140 keV. CdWO4, Lu2O3:Eu and (Gd,Lu)2O3:Eu all result in a significantly improved spatial resolution over CsI:Tl, albeit at the cost of reduced energy resolution. Lu2O3:Eu transparent ceramic gives the best spatial resolution: 65 µm full-width-at-half-maximum (FWHM) compared to 147 µm FWHM for CsI:Tl. In conclusion, these 'slow' dense scintillators open up new possibilities for improving the spatial resolution of EMCCD-based scintillation cameras.

Characterization of an effective cleaning procedure for aluminum alloys: surface enhanced Raman spectroscopy and zeta potential analysis.

We have developed a cleaning procedure for aluminum alloys for effective minimization of surface-adsorbed sub-micrometer particles and nonvolatile residue. The procedure consists of a phosphoric acid etch followed by an alkaline detergent wash. To better understand the mechanism whereby this procedure reduces surface contaminants, we characterized the aluminum surface as a function of cleaning step using surface enhanced Raman spectroscopy (SERS). SERS indicates that phosphoric acid etching re-establishes a surface oxide of different characteristics, including deposition of phosphate and increased hydration, while the subsequent alkaline detergent wash appears to remove the phosphate and modify the new surface oxide, possibly leading to a more compact surface oxide. We also studied the zeta potential of <5 microm pure aluminum and aluminum alloy 6061-T6 particles to determine how surface electrostatics may be affected during the cleaning process. The particles show a decrease in the magnitude of their zeta potential in the presence of detergent, and this effect is most pronounced for particles that have been etched with phosphoric acid.

Electrolyte management for effective long-term electro-osmotic transport in low-permeability soils.

Electro-osmosis, a coupled-flow phenomenon in which an applied electrical potential gradient drives water flow, may be used to induce water flow through fine-grained sediments. Test cell measurements of electro-osmotic transport in clayey cores extracted from the 27-31 m depth range of the Lawrence Livermore National Laboratory site indicate the importance of pH control within the anode and cathode reservoirs. In our first experiment, pH was not controlled. As a result, carbonate precipitation and metals precipitation occurred near the cathode end of the core, with acidification near the anode. The combination of these acid and base reactions led to the decline of electro-osmotic flow by a factor of 2 in less than one pore volume. In a second experiment, long-term water transport (>21 pore volumes) at stable electro-osmotic conductivity (k(eo) approximately 1 x 10(-9) m2/s-V) was effected with anode reservoir pH > 8, and cathode reservoir pH < 6. Hydraulic conductivity (k(h)) of the same core was 4 x 10(-10) m/s under a 0.07 MPa hydraulic gradient without electro-osmosis. Stable electro-osmotic flow was measured at a velocity of 4 x 10(-7) m/s under a 4 V/cm voltage gradient, and no hydraulic gradient-3 orders of magnitude greater than the hydraulic flow. We also observed chloroform production in the anode reservoir, resulting from electrochemical production of chlorine gas reacting with trace organics. The chloroform was transported electro-osmotically to the cathode, without measurable loss to adsorption, volatilization, or degradation.