Nagoya University
NagoyaUniversity

Effectof goldceating in quantitatiye
EPMA:A MC

study of monazite

KMO
Takenori
CenterforChronologieal
Research,
Nbgoya [lhiversity,Jbpan
We generally
use a conducting
ceatingin electron probe microanalysis (EPMA)of
geologicalsarnples to provide a path fbr the electron and to preventcharging. Acarbon(20- 30 nm in thickness) is the most common coating material in geologicalapplicationofEPMA
includingthe CHIME datingbecauseof low X-ray attenuation for
mostcharacteristic
X-ray lines, A gold coating has been also applied to the CHIME
dating(e.g.,Jercinovicand Williams,2005) to minimize
irradiation
effects by electron
bombardment.
EPMA ofrock-fbrrningminerals
One of the important
problemsin quantitative

isthe effect of a coating filmon matrix correction procedures. Correction
procedures
ZAF models(bothandBence-Albeemethod

using calculated

a-factor tables) have been

generallyestablished fbruncoated rnaterials. Therefbre,the effZct of a coating film
shouldbe carefu11y assessed to achieve accurate quantitative
EPMA of coated materials
as pointedout by Bastinand Heijligers(1991).We empirically know that the carboncoating
does not cause significant problem in quantitative
EPMA of most geologicalsamples.
However, itisanticipated that a gold coating has an effect on quantitative
analysis through scatteringof electrons
becauseofhigh atomic number and density.
The Monte Carlo simulations
werewith a 25 nm
perfbrmed fbrthe monazite
carbonand a 1O nm goldto understandeffects of a coating
filminquantitativeEPMA at
ED 15 keV and 25 keV This study used the conventional
single scattering modelslowing
down model) with the Mott (1929)
scattering using the Browning(continuousexpressionof the collision cross sections,
The kinetic
energy loss of an(1991)
electronwasobtainedby the Bethe (1930)
fbrmula modified
by Joy and Luo (1989)
Intensities
with the meanionization
potentialdescribedin Berger and Seltzer(1964).
1976).
of characteristic
X-rays were calculated by the Bethe-Powellfbrmula(Powell,==andThe randem
numbers
weregeneratedusing the Mersenne Twister (Matsumotowerecalculatedusingmassattenuation
Nishimura,1998). EmittedX-ray intensities
coefficientsby Henke et at. (1993)
at X-ray take-off angle 400.
the depth
At both Eo 15 keV and 25 keXL a 10 nrn gold coating distorted
distributionof generate
Th Ma intensities
in the monazite, while a 25 nm carbon coating
as the uncoatedmonazite.
A 10 nm gold coating isgave the same depthdistribution- 25 keV) timeshigherinthe k-ratio
ofthe
Th Ma line
1.06(Ee 15 keV) and 1.05(Eo==-29NII-Electronic Library ServiceNagoya University
NagoyaUniversity

between the monazite and the purethorium than a 25 nm carbon coating.
The anaiytical error ofthe Th Ma intensity
ofmonazite
isapproximately

1%

RSD at generaloperating conditions of the CHIME dating,and the uncertainty caused
by a goldcoating is not negligible,Hence, a 10 nm gold coating is a factorfbr
inaccuracyin quantitative
EPMAof monazite, while a 25 nm carbon
coating
doesnot
have significant effect.
References
Bastin G.F. andHeijligersH.J.M. In: Heinrich K.F.J.and

Newbury D.E. (eds),
Electron ProbeP]enum Press,New York, 163.
2uantitation,Bethe (1930)Annalen der PJtysik,5,325.
Berger M,J, andSeltzer S.M. (1964)
M4S4

SP-3012,National Aeronautics and Space Administration,

Washington D.C., 127pp.
Browning R. (1991)
Pig,sics
Letters,58,2845.
AppliedHenke B.L.,Gullikson
E.M. andDavis

J,C,(1993)Atomic Data andIVt{clearData

7lables,
54,181.

JercinovicM,J. and WilliamsM.L, (2005)American
Mlneralogists,9e,526
Joy D.C. and Luo S.(1989)Scanning,11, 176,
Matsumoto M. andNishimura

T. (1998)
ACM

7>'ansactionson Mbdelingand

ComputerSimulation,
8, 3.

Mott N.F.(1929)
Proceedingsqf'theRcyal Society (I/ondonj,
A124, 425,
Powell C.J.(1976)
Reviews ofMbdern
Physics,48, 33.

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