Electronegativity

The NRL SIMS-SSAMS:

A Unique Instrument for Cosmo- and Geochemistry

o The Naval Research Laboratory (NRL) acquired and built a

unique Secondary Ion Mass Spectrometer (SIMS) coupled to a

Single Stage Accelerator Mass Spectrometer (SSAMS).

o Addresses topics in nuclear forensics, geochemistry,

cosmochemistry, and materials science [1].

o SIMS analyses of high-mass (> 50 u) ions complicated by

presence of molecular isobars, which require high mass

resolution or energy filtering to resolve [2,3].

o AMS uses gas strippers or foils to remove molecular isobars.

o SIMS-SSAMS utilizes the benefits of both techniques, while

eliminating or reducing the drawbacks of each.

o SSAMS can accept (+) or (-) ions, unlike tandem AMS.

o SIMS provides spatial and compositional information.

o The SIMS-SSAMS can analyze nearly the entire periodic

table.

References: [1] E.E. Groopman (this conference). [2] Zinner, E. & Crozaz, G. (1986) Int. J. Mass Spectrom. Ion Processes, 69, 17-38 [3] Fahey, A.J. (1998) Int. J. Mass Spectrom. 176, 63-76 [4] Kennedy, A.K. et al. (2011) Can. Mineral., 48, 1423-1443 [5] Suchanska, M. (1997) Prog. Surf. Sci., 54, 265-209 [6] Pillatsch, L.

et al. (2010) Surf. Interface Anal. 42, 645-648 Acknowledgements: We would like the thank the National Museum of Natural History for providing a sample of N. Korean monazite NMNH 147239.

HV Deck

N. Korean Monazite NMNH 147239

155Gd+ 156Gd+

+ LaO+ + CeO+

SIMS-SSAMS: Instrument Description

Initial Results:

Future Upgrades:

Initial Results:

Introduction:

±20 kV

CRM U500

U Isotope Abundances

Isotope CRM

U500

Natural

U

234U 0.52% 0.006%

235U 49.70% 0.72%

236U 0.08% 0.00%

238U 49.71% 99.27%

239Pu 0.00% 0.00%

NIST Certified Reference Material (CRM)

U500 (U3O8) Particles & Clusters

No Molecular

Interferences!

Ar gas flow

0.2 SCCM

SE Images

SIMS Mass Spectrum

SSAMS Mass Spectrum

Readily Make: • POSITIVE ions

• (+) or (-) ions

• NEGATIVE ions

• Noble gases make

POSITIVE ions

Faraday Cups

o Modified Cameca ims-4f SIMS and National

Electrostatics Corp. (NEC) SSAMS (∓300 keV)

with custom coupling section.

o Molecular ions are dissociated a gas stripper tube

filled with Ar.

o Removes molecular isobars.

o Can measure fragments, e.g., U+ from

breakup of UO2+.

o Can use any gas, e.g., Ar, N2, O2, He.

o Electrostatic peak switching in SSAMS for

isotope ratios.

o Beam imagers after gas stripper and at end for

beam tuning.

o Integrated 6-way cross in coupling section for

future ion sources, e.g., ICP, gas sources.

SIMS Modifications:

o LabVIEW software controls hardware via 3 fiber optic interfaces to: universal serial bus (USB), Ethernet, or National Instruments Inc.

“Peripheral Component Interconnect Extensions for Instrumentation” (PXI) chasses.

o Fully extensible. Can customize measurements.

o SIMS on 1-m-high platform to match High-Voltage (HV) Deck of SSAMS.

o SIMS output detectors electrostatically selected (in purple cube on diagram): an electron multiplier (EM) run in pulse-counting mode, a

pass-through position to the SSAMS, or a Faraday cup (FC).

o SIMS optical elements and output section modified to prevent beam clipping.

o U mass spectra filled with

molecular isobars.

o 236U+ and 235UH+ require

mass resolving power

(m/Δm) ~ 38,000 to resolve.

o With the SSAMS, 236U+

measured even with low

mass resolution due to

molecule destruction!

o Scattering losses in gas

stripper are low compared to

molecule destruction: ~1 ion

lost for every 60,000

molecules destroyed!

o Molecules efficiently destroyed over wide range of gas flow rates.

o Atomic ion signal reduced primarily by scattering from stripper gas.

o For every atomic ion scattering loss, ~60,000 molecules destroyed!

o Can also measure atomic ion fragments from dissociation of molecular

ions injected by SIMS, e.g., U+ from UO2+.

o Atomic ion fragments energy partitioned from parent molecules, so

fragments of two initial molecular isobars do not interfere.

Example:

o 248[ThO]+ (232Th + 16O) and 248[CaPb]+ (40Ca + 208Pb) from Grenville skarn titanite (CaTiSiO5; see other

poster), which contains up to 100 ppm Pb and 1000 ppm Th [4].

o Each injected ion has energy of 304.5 keV (4.5 keV from SIMS + 300 keV from SSAMS).

o Energy partitioning ~ mass partitioning, mass(fragment)/mass(molecule): mass(232Th+/ 248[ThO]+) =

232/248 ≈ 0.935; mass(208Pb+/ 248[CaPb]+ ) = 208/248 ≈ 0.839.

o 232Th+ and 208Pb+ fragments have energies (effective masses) of ~ 284.7 keV (216.9 u) and 255.4 keV

(174.5 u), respectively, so they are easily mass filtered by the SSAMS magnet. o Certificate ratios from 3

CRMs (U030a, U500,

U850) span 6 orders of

magnitude.

o No observable mass bias.

o Scatter in measurements

(130 meas. on 52 particles)

from iterative improvements

in measurement parameters

and a faulty power supply.

o SIMS contains electrically isolated flight tube through magnet: use for electrostatic peak switching (±600 V).

o Increase SIMS extraction voltage to 10 kV.

o Replace ims 4f sample chamber with 6f sample chamber: can do O2 flooding, laser post-ionization [5].

o Use F- or SF5- primary beams, which have increased brightness relative to O- and/or are useful for gentle sputtering [6].

o Convert LabVIEW-based software to Python + Qt.

o Measurement of Heavy Rare

Earth elements notoriously

difficult due to molecular

isobars.

o Removal of LaO+ and CeO+

reveals underlying Gd

isotopic composition.

Evan E. Groopman1,2, Albert J. Fahey2, Kenneth S. Grabowski2, Kamron C. Fazel2

1NRC Research Associateship Programs, The National Academies of Sciences, Engineering, and Medicine, 500 Fifth Street NW, Washington, DC 20001 2U.S. Naval Research Laboratory, Accelerator Mass Spectrometry Section, 4555 Overlook Avenue SW, Washington, DC 20375