Truman Wilson

Bright focused ion beam sources based on laser-cooled atoms

Nanoscale focused ion beams (FIBs) represent one of the most useful tools in nanotechnology, enabling nanofabrication via milling and gas-assisted deposition, microscopy and microanalysis, and selective, spatially resolved doping of materials. Recently, a new type of FIB source has emerged, which uses ionization of laser cooled neutral atoms to produce the ion beam. The extremely cold temperatures attainable with laser cooling (in the range of 100 μK or below) result in a beam of ions with a very small transverse velocity distribution. This corresponds to a source with extremely high brightness that rivals or may even exceed the brightness of the industry standard Ga+ liquid metal ion source. In this review we discuss the context of ion beam technology in which these new ion sources can play a role, their principles of operation, and some examples of recent demonstrations. The field is relatively new, so only a few applications have been demonstrated, most notably low energy ion microscopy with Li ions. Nevertheless, a number of promising new approaches have been proposed and/or demonstrated, suggesting that a rapid evolution of this type of source is likely in the near future.

Editors' Choice Communication—Comparison of Nanoscale Focused Ion Beam and Electrochemical Lithiation in β-Sn Microspheres

The development of Li focused ion beams (Li-FIB) enables controlled Li ion insertion into materials with nanoscale resolution. We take the first step toward establishing the relevance of the Li-FIB for studies of ion dynamics in electrochemically active materials by comparing FIB lithiation with conventional electrochemical lithiation of isolated β-Sn microspheres. Samples are characterized by cross-sectioning with Ga FIB and imaging via electron microscopy. The Li-FIB and electrochemical lithiated Sn exhibit similarities that suggest that the Li-FIB can be a powerful tool for exploring dynamical Li ion-material interactions at the nanoscale in a range of battery materials.

Updates of MODIS on-orbit calibration uncertainty assessments

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments have successfully operated for more than 17 and 15 years, respectively, on-board the NASA’s Earth Observing System (EOS) Terra and Aqua spacecraft. MODIS level 1B (L1B) data products include top of the atmosphere (TOA) reflectance factors for the reflective solar bands (RSB) and radiances for both the RSB and the thermal emissive bands (TEB), and their associated uncertainty indices (UI) at a pixel-by-pixel level. This paper provides a brief review of MODIS L1B calibration algorithms, including improvements made in recent years. It presents an update of sensor calibration uncertainty assessments with a focus on several new contributors resulting from changes in sensor characteristics and on-orbit calibration approaches and the impact due to these changes on the L1B data quality. Also discussed in this paper are potential changes that could be made to continue improving the quality of MODIS L1B uncertainty product.

Sensor performance assessment for Terra and Aqua MODIS using unscheduled lunar observations

The Moderate Resolution Imaging Spectroradiometer (MODIS) has been in operation for over 18 and 16 years on the Terra and Aqua spacecrafts, respectively. In order to maintain long-term calibration stability over the life of each mission, MODIS uses a set of on-board calibrators as well as observations of the Moon and selected Earth-view targets. The lunar observations nominally occur in a narrow phase angle range, 55°-56° , and use scheduled spacecraft maneuvers in order to bring the Moon into alignment with the MODIS space-view port. These observations are used to help characterize the MODIS scan-mirror response versus scan-angle. In addition to these scheduled lunar observations, MODIS also views the Moon through the space-view port without a spacecraft maneuver when the geometry is appropriately aligned. This occurs over a wider phase angle range, between 51°-82° degrees, than those of the scheduled moon observations. While the phase angle restriction of our scheduled observations provides consistency between the calibration events, the unscheduled Moon data can provide a valuable assessment of many calibration related investigations that use the Moon. In this paper, we compare the results of unscheduled versus scheduled lunar observations for several sensor calibration and performance assessments. These include the lunar calibration trending used to characterize the scan-mirror response versus scan angle and the electronic crosstalk correction of bands 27-30, which are currently used in the MODIS Level-1B data products, as well as sensor performance assessments such as band-to-band and detector-to-detector spatial registration.

Intercomparison of Terra and Aqua MODIS using unscheduled lunar observations

MODIS is the key instrument for the NASAs EOS Terra and Aqua missions, launched in December, 1999 and May, 2002 respectively. The reflective solar bands (RSB) on Terra and Aqua MODIS are calibrated independently using an on-board solar diffuser and a solar diffuser stability monitor. Near-monthly lunar observations are also a major component of the on-orbit calibration strategy facilitating a response-versus-scan angle (RVS) characterization on-orbit. With a few exceptions, the regularly scheduled lunar observations have been performed with the same phase angles ranging from -55◦ to -56◦ for Aqua MODIS and 55◦ to 56◦ for Terra MODIS. Previous efforts have demonstrated that the observations of the Moon serve as an effective mechanism to perform an on- orbit cross-calibration of the two MODIS instruments. In addition to the regularly scheduled lunar observations that require a roll maneuver, both MODIS instruments also view the Moon via its space-view (SV) port for three to four months in a year that covers a wider range of phase angles. In this paper, we expand on the previous effort to provide an assessment of the RSB calibration difference between Terra and Aqua MODIS based on unscheduled lunar observations made over a range of phase angles. Also, discussed in this paper are strategies that could benefit other EOS sensors such as SNPP and NOAA 20 VIIRS.

Evaluating calibration consistency of Terra and Aqua MODIS LWIR PV bands using Dome C

Over the years, data from Terra and Aqua MODIS (The Moderate Resolution Imaging Spectroradiometer) has provided invaluable information about Earths atmosphere, land and oceans. Both MODIS sensors have exceeded their designed lifetime of 6 years and are still in operation. Therefore, it is important to understand the on- orbit performance of both sensors. The thermal emissive bands (TEB) on MODIS are comprised of 16 spectral bands with wavelengths from 3.7 to 14.4 μm. The TEB are calibrated on orbit on a scan-by-scan basis using a blackbody calibration source. Since the mission beginning, a steady increase in electronic crosstalk has been observed in Terra MODIS bands 27-30. The MODIS Characterization Support Team (MCST) at NASA/GSFC has recently derived a set of correction factors that correct these bands for the entire mission. These corrections are being applied in Terra Collection 6.1. In this study, the effectiveness of this crosstalk correction is assessed. First, the observed brightness temperatures over Dome C for Terra with and without the crosstalk correction are compared. Then, the calibration consistency and stability between the TEB of Terra, with the crosstalk correction, and Aqua MODIS are also assessed. Finally, the relative bias between the two MODIS instruments is evaluated using the near-surface temperature measurements from an Automatic Weather Station (AWS).

Electronic crosstalk impact assessment in the Terra MODIS mid-wave infrared bands

The Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra spacecraft is one of the key instruments in NASAs Earth Observing System. Since 2000, MODIS has collected continuous data in 36 spectral bands ranging in wavelength between 0.4 μm and 14.2 μm. Since before launch, signal contamination in the form of electronic crosstalk has been observed in many of the MODIS thermal emissive bands, particularly for bands 27-30, a correction for which has been applied to the current Collection 6 algorithm. The mid-wave infrared bands in Terra MODIS, 20-25, also show signs of electronic crosstalk contamination, which can be seen clearly during observations of the Moon. In this paper, well present an impact assessment of electronic crosstalk on the mid-wave infrared bands in Terra MODIS. We will also derive correction coefficients from the lunar observations, which can be applied to correct the calibrated radiance in the MODIS Level-1B product. We will provide an analysis of these results and potential improvements to the MODIS Level-1B product.

Lunar calibration improvements for the short-wave infrared bands in Aqua and Terra MODIS

The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the key sensors among a suite of remote sensing instruments on board the Terra and Aqua spacecrafts. Since the beginning of each mission, regularly scheduled lunar observations have been used in order to track the on-orbit gain changes of the reflective solar bands. However, for the short-wave infrared bands, 5-7 and 26, the measured signal is contaminated by both electronic crosstalk and an out-of-band response due to transmission through the MODIS filters at undesired wavelengths. These contaminating signals cause significant oscillations in the derived gain from lunar observations for these bands, which limits their use in determining the scan mirror response versus scan angle at these wavelengths. In this paper, we show a strategy for correcting the electronic crosstalk contamination using lunar observations, where the magnitude and the source of the contaminating signal is clear. For Aqua MODIS, we find that the magnitude of the electronic crosstalk contamination is small, and the lunar calibration remains relatively unaffected. For Terra MODIS, the contamination is more significant, and the electronic crosstalk correction shows a significant reduction in the oscillations of the lunar calibration results.