Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989)
8033.pdf
A MINIATURE ELECTRON PROBE FOR IN SITU ELEMENTAL MICROANALYSIS.
L. F. Lim1, A. E.
Southard3,1, S. A. Getty1, L.A. Hess1, J. G. Hagopian2,1, C. A. Kotecki1. 1NASA/GSFC, Greenbelt, MD, USA
([email protected]) 2Advanced Nanophotonics, Greenbelt, MD, USA. 3USRA, Greenbelt, MD, USA
Introduction: In situ probes will provide an important complement to the various sample return missions
envisioned for the next 35 years. The Mini-EPMA
under development will enable advanced, fine-scale in
situ mapping of the elemental composition of planetary
materials. Composition provides key evidence about
the processes by which rocks, soils, and ices were
formed and altered (e.g., accretion, differentiation,
hydrothermal alteration). This instrument will be a
valuable payload element for future landed missions to
airless bodies, including asteroids, comets, and various
planetary satellites. Operation in atmosphere would
require the addition of a vacuum housing.
Sub-mm spatial resolution: The focused electron
beam will permit sub-millimeter scale compositional
mapping in a flight instrument, a scale relevant to pet rographic structures. Modeling with SIMION [1] indicates that e-beam spot sizes under 100 µm are achiev able in a flight instrument with microscale field emitters in an array, with focusing achieved by a compact
electrostatic lens stack. Microfabrication techniques
are used to define the growth regions for the CNT
emitters as well as the grid electrode required to individually address each element in the array. The prototype cathode array will have 10 x 10 elements, leading
to a 10 x 10 compositional map of the target surface.
Spot pitch is tunable depending on science goals.
Flight instrument concept: In the mini-electron
probe (“EPMA”) flight concept (Fig. 1), electrons are
drawn out of an addressable-element carbon nanotube
field emitter array [2, 3] by the cathode/grid extraction
voltage, then accelerated by the lens stack into the
planetary/asteroidal/cometary surface at 15-20 kV,
exciting X-ray line emission characteristic of the elemental composition of the surface. The X-rays are then
measured by a silicon drift detector similar to those
used in laboratory energy-dispersive spectroscopy
(EDS) and analyzed using standard EPMA techniques
to give the surface composition of the region illuminated sequentially by each electron-beam spot (100
µm). In this way, a grid of e-beam spots activated in
sequence will non-destructively produce a fine-scale
map of elemental composition. Microfabrication techniques are used to define the growth regions for the
CNT emitters, as well as the grid electrode required to
individually address each element in the array.
Mass and power: A preliminary flight instrument
concept produced by the GSFC Instrument Design
Laboratory calculated a total instrument mass of 3.3–
3.6 kg. The model includes two electron guns and two
X-ray detectors for reliability. Peak power is estimated
at 12.7 W; average power at 5.7 W.
Figure 1. Preliminary concept for mini-EPMA flight
instrument
Figure 2. SEM micrograph of 10x10-element carbon
nanotube forest cathode prototype grown at GSFC.
References: [1] Dahl, D.A. (2000) International
Journal of Mass Spectrometry, 200(1-3):3–25. [2] S. A.
Getty, et al. (2007) Society of Photo-Optical Instrumentation Engineers (SPIE) Conference. [3] S. A.
Getty, et al. (2008) Society of Photo-Optical Instrumentation Engineers (SPIE) Conference.
Acknowledgments: This work is supported by NASA
ROSES 13-PICASSO13-0043.
J. Gaskin (NASA/
MSFC) contributed as co-author on the proposal. This
work also depends on the ongoing efforts of N. Costen,
A. Ewin, G. Hidrobo, C. Johnson, G. Manos, D.S.
Stewart, and E. Young at GSFC.
Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989)
8033.pdf