Publications

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Neutron sensing is critical in civilian and military applications. Conventional neutron sensors are limited by size, weight, cost, portability and helium supply. Here the microfabrication of gadolinium (Gd) conversion material-based heterojunction diodes for detecting thermal neutrons using electrical signals produced by internal conversion electrons (ICEs) is described. Films with negligible stress were produced at the tensile-compressive crossover point, enabling Gd coatings of any desired thickness by controlling the radiofrequency sputtering power and using the zero-point near p(Ar) of 50 mTorr at 100 W. Post-deposition Gd oxidation-induced spallation was eliminated by growing a residual stress-free 50 nm neodymium-doped aluminum cap layer atop Gd. The resultant coatings were stable for at least 6 years, demonstrating excellent stability and product shelf-life. Depositing Gd directly on the diode surface eliminated the air gap, leading to a 200-fold increase in electron capture efficiency and facilitating monolithic microfabrication. The conversion electron spectrum was dominated by ICEs with energies of 72, 132 and 174 keV. Results are reported for neutron reflection and moderation by polyethylene for enhanced sensitivity, and γ- and X-ray elimination for improved specificity. The optimal Gd thickness was 10.4 μm for a 300 μm-thick partially depleted diode of 300 mm 2 active surface area. Fast detection (within 10 min) at a neutron source-to-diode distance of 11.7 cm was achieved with this configuration. All ICE energies along with γ-ray and K α,β X-rays were modeled to emphasize correlations between experiment and theory. Semi-conductor thermal neutron detectors offer advantages for field-sensing of radioactive neutron sources.

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Portable applications of microdischarges, such as the remediation of gaseous wastes or the destruction of volatile organic compounds, will mandate operation in the presence of contaminant species. This paper examines the temporal evolution of microdischarge optical and ultraviolet emissions during pulsed operation by experimental methods. By varying the pulse length of a microdischarge initiated in a 4-hole silicon microcavity array operating in a 655 Torr ambient primarily composed of Ne, we were able to measure the emission growth rates for different contaminant species native to the discharge environment as a function of pulse length. It was found that emission from hydrogen and oxygen impurities demonstrated similar rates of change, while emissions from molecular and atomic nitrogen, measured at 337.1 and 120 nm, respectively, exhibited the lowest rate of change. We conclude that it is likely that O2 undergoes the same resonant energy transfer process between rare gas excimers that has been shown for H2. Further, efficient resonant processes were found to be favored during ignition and extinction phases of the pulse, while emission at the 337.1 nm line from N2 was favored during the intermediate stage of the plasma. In addition to the experimental results, a zero-dimensional analysis is also presented to further understand the nature of the microdischarge.

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Si-MOS based QD qubits are attractive due to their similarity to the current semiconductor industry. We introduce a highly tunable MOS foundry compatible qubit design that couples an electrostatic quantum dot (QD) with an implanted donor. We show for the first time coherent two-axis control of a two-electron spin logical qubit that evolves under the QD-donor exchange interaction and the hyperfine interaction with the donor nucleus. The two interactions are tuned electrically with surface gate voltages to provide control of both qubit axes. Qubit decoherence is influenced by charge noise, which is of similar strength as epitaxial systems like GaAs and Si/SiGe.

Si-MOS based QD qubits are attractive due to their similarity to the current semiconductor industry. We introduce a highly tunable MOS foundry compatible qubit design that couples an electrostatic quantum dot (QD) with an implanted donor. We show for the first time coherent two-axis control of a two-electron spin logical qubit that evolves under the QD-donor exchange interaction and the hyperfine interaction with the donor nucleus. The two interactions are tuned electrically with surface gate voltages to provide control of both qubit axes. Qubit decoherence is influenced by charge noise, which is of similar strength as epitaxial systems like GaAs and Si/SiGe.

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The temporal evolution of spectral lines from microplasma devices (MD) was studied, including impurity transitions. Long-wavelength emission diminishes more rapidly than deep UV with decreasing pulse width and RF operation. Thus, switching from DC to short pulsed or RF operation, UV emissions can be suppressed, allowing for real-time tuning of the ionization energy of a microplasma photo-ionization source, which is useful for chemical and atomic physics. Scaling allows MD to operate near atmospheric pressure where excimer states are efficiently created and emit down to 65 nm; laser emissions fall off below 200 nm, making MD light sources attractive for deep UV use. A first fully-kinetic three-dimensional model was developed that explicitly calculates electron-energy distribution function. This, and non-continuum effects, were studied with the model and how they are impacted by geometry and transient or DC operation. Finally, a global non-dimensional model was developed to help explain general trends MD physics.

Neutron sensing is critical in civilian, military, industrial, biological, medical, basic research, and environmental applications. Conventional neutron sensors are limited by size, weight, cost, portability, and helium supply. Here the microfabrication of Gd conversion material-based heterojunction diodes is described for detecting thermal neutrons using electrical signals produced by internal conversion electrons (ICE). Films with negligible stress were produced at the tensile-compressive crossover point, enabling Gd coatings of any desired thickness by controlling the radiofrequency sputtering power and using the zero-point near p(Ar) of 50 mTorr at 100 W. Post-deposition Gd oxidation-induced spallation was eliminated by growing a residual stress-free 50 nm neodymium-doped aluminum cap layer atop Gd. Resultant coatings were stable for at least six years demonstrating excellent product shelf life. Depositing Gd on the diode surface eliminated air gap, leading to improved efficiency and facilitating monolithic microfabrication. The conversion electron spectrum was dominated by ICE with energies of 72, 132, and 174 keV. Results are reported on neutron reflection and moderation by polyethylene for enhanced sensitivity and g- and X-ray elimination for improved specificity. Optimal Gd thickness was 10.4 um with 300 um thick partially depleted diode of 300 mm2 active surface area. Fast detection within 10 minutes at a neutron source-to-diode distance of 11.7 cm was achieved using this configuration. All ICE energies along with g-ray and Ka X-ray were modeled to emphasize correlations between experiment and theory and to calculate efficiencies. Semiconductor thermal neutron detectors offer advantages for field-sensing of radioactive neutron sources. ACKNOWLEDGEMENTS Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. We thank Edward Cole, David Wheeler, Robert Koudelka, and Lyle Brunke for productive interactions and materials support.

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We report on the development of a highly miniaturized vacuum package for use in an atomic clock utilizing trapped ytterbium-171 ions. The vacuum package is approximately 1 cm3 in size and contains a linear quadrupole RF Paul ion trap, miniature neutral Yb sources, and a non-evaporable getter pump. We describe the fabrication process for making the Yb sources and assembling the vacuum package. To prepare the vacuum package for ion trapping, it was evacuated, baked at a high temperature, and then back filled with a helium buffer gas. Once appropriate vacuum conditions were achieved in the package, it was sealed with a copper pinch-off and was subsequently pumped only by the non-evaporable getter. We demonstrated ion trapping in this vacuum package and the operation of an atomic clock, stabilizing a local oscillator to the 12.6 GHz hyperfine transition of 171Y b+. The fractional frequency stability of the clock was measured to be 2 × 10-11/τ1/2.

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