Electron microanalysis of gallium and iron in plutonium-gallium and plutonium-iron alloys

JOURNAL

OF NUCLEAR

24 (1967)

MATERIALS

ELECTRON

104-106.

MICROANALYSIS

IN PLUTONIUM-GALLIUM M. R. HARVEY The Dow

Chemicd

Company,

Rocky

29 May

(1)

where KA is the measured relative intensity of element A, UAB is a conversion parameter for A in alloy AB and CA is the concentration of A in wt %. Their procedure consists of *

Work

performed

under USAEC

Contract

IRON ALLOYS

Golden, Colorado 80401,

*

USA

1967

determining

+ (1 -aAB)cA,

AND

CO., AMSTERDAM

RIEFENBERG

Flats Division,

Quantitative analysis with an electron probe microanalyzer has been discussed by many authors 1-s). Th ere is general agreement that the most accurate results are obtained by using known standards in the same composition range as the unknown. Frequently however, these standards are difficult and time consuming to prepare. As a result elaborate correction procedures have been developed to convert the observed X-ray intensities into concentrations. Depending on the particular system, fluorescence, adsorption and atomic number corrections have to be applied. These corrections require a knowledge of the mass absorption coefficients, the electron stopping power, backscatter coefficients, and fluorescence properties of the constituent elements. In systems such as Pu-Ga and Pu-Fe, some of these numbers are not well established. In addition, their particular application in systems with widely differing values of the atomic number is not as rigorous as in systems such as Fe-Ni or Cr-Fe. Ziebold and Ogilvie 7) have proposed a method which eliminates the necessity for preparing a large number of standards or applying corrections to observed intensities. They propose the following empirical equation

PUBLISHING

PLUTONIUM-IRON

and D. H.

Introduction

cApA=aAB

NORTH-HOLLAND

OF GALLIUM

AND

Received

1.

0

aAB from

an

alloy

of

known

composition and then applying it to the entire concentration range. Ziebold and Ogilvie’s justification for using this equation is based upon a very good correlation with a large number of systems. In addition they have shown that an equation of this form can be obtained by rearranging Castaing’s complete intensity-concentration expression 8). When this is done the conversion parameter has the following

form

aAB=

[o~BI~AI [f(~~)/f*(~*)l

[1 i ~l-l,

(2)

where &g and &A are the atomic number corrections, f(XA) and f*(x*) are the absorption corrections for pure A and for the alloy; and (1 +x)-l is the secondary fluorescence correction. All these terms are concentration-dependent, however they apparently counteract each other so as to yield a product which is essentially constant. In correlating this expression with twenty

systems Ziebold

aAB=o.g5[(a+X,A)/(o+X~)]

modified

eq. (2) to

[ZA/~B]~=‘~ x

X

[l + O.O71t]-1,

(3)

where rs is a constant dependent on accelerating potential, d and & are the absorption corrections for A in A and A in B ; and .??A and .& are the atomic numbers of A and B. A numerical solution for eq. (3) is readily which allows obtained for most systems comparison with the experimentally determined aAB.

AT(29-l)-1106. 104

ELECTRON

The present project to determine

MICROANALYSIS

was undertaken

if the relative

intensity

in order data for

GaKol and FeKa radiation obtained from various Pu-Ga and Pu-Fe alloys could be correlated with eq. (1). The only published

OF

GALLIUM

AND

105

IRON

plutonium lines. The beam current was held to less than & lo/’ deviation. Intensities were determined

by averaging

the counts on fifteen

separate areas and converted to relative intensities by normalizing with a pure gallium

work done

standard.

on the Pu-Ga system was by Hakkila et al. g), in which a Pu/l.O wt o/o Ga alloy was examined.

averaging

A conversion

Pu-Fe alloys were run in a similar manner.

reported

quantitative

parameter

which

on

a

@a, rU, of 0.33 was relative intensity-

concentration plot gave deviation from linearity.

a

strong

positive

Experimental

3.

was

determined

by shifting

off wavelength

+

and

by

the spectro-

-0.030

A. The

Results and discussion Fig.

Scott and Ranzetta lo), in studying the Pu-Fe system, observed a negative deviation from linearity which is indicative of over-riding absorption effects. This is strange because one would expect a similar behavior to that of the Pu-Ga system. 2.

meter

Background the count

1 shows the data plotted

according

to

eq. (1) for both systems. As can be seen from eq. (l), the ordinate intercept also yields the conversion parameter aAB. The values obtained (@a, pu = 0.806 and aFe, pU= 0.960) are compared with the individually calculated values, and with those calculated from eq. (3) in table 1. + The Pu-Ga data still show a positive deviation

procedure

Six alloys ranging from 0.56 to 2.12 wt y. gallium in the delta-stabilized region 11) and four intermetallic compounds (PusGas, PuGa, PuGaz, PuGas) were prepared by induction and arc melting, respectively. After homogenizing, the specimens were metallographically polished, with the final abrasive being lym diamond paste. Two iron compounds, PusFe and PuFez were arc melted, annealed and prepared for the microprobe in a similar manner. To avoid radioactive contamination of the microprobe, the samples were ultrasonically cleaned in Dow Chlorothene Nu* and rinsed with ethyl alcohol. The alloy surface was then flooded with alcohol and a film of Kodacel A-30** pressed onto it. The film was stripped off and the procedure repeated several times to remove any loose oxide. The surface was masked and the entire mount sprayed with clear lacquer. After removing the masking, the sample was placed in a Materials Analysis Corporation Microprobe, Model # 400. An accelerating potential of 19 kV was used to eliminate interference from the second order LB1 and L#?s * The Dow Chemical Company, Midland, Michigan. **

Eastman

York.

Kodak

Company,

Rochester,

New

Pu-Fe

n

0 Pu-Go

Go” - 0

”

”

S

0.5

”

’

’

1.0

Ga, Fe CONCENTRATION (wt.%)

Fig.

1.

Ziebold

plot of GaKa

and FeKn

radiation

for Pu-Ga and Pu-Fe alloys and compounds. left point

on Pu-Ga

alloys between t

line represents 0.56

Values of a~~(l.0

overriding indicative

Extreme

average

and 2.12 wt

are usually attributed

fluorescence

of six

o/o Ga. to an

effect while values > 1.0 are

of strong absorption

effects. This however

is not the entire picture because e simple calculation will

show

dominant

that

the

atomic

in the Pu-Ga

number

and Pu-Fe

correction

systems.

is

106

M. 'PABLE

Comparison Sample

HARVEY

wt O/oGa

parameters.

1

GAB

1

0.56

0.811

2

0.76

0.773

3

1.15

0.796

4

1.53

5

1.85

6

2.12

RIEFENBERG

3.90 wt o/o iron. This difference in concentration yields a variation in C/K of 0.02 whereas the

same

error

certainty

in PuFez

in

the

is negligible.

composition

The

of

the

un-

PuGFe

resulted in using the PuFez point for the extraempirical

0.807

relating

to concentration

0.795

followed

gallium and plutonium-iron alloys. Therefore, accurate quantitative analyses of gallium and

0.819

PuGa

22.6

0.813

PuGaz

36.8

0.827

PuGas

46.7

0.828

Intercept (determined by least-squares)

0.806

Calculated from eq. (3)

0.750

~

H.

0.806

14.9

wt %Fe

D.

polation in fig. 1. In summary, Ziebold’s

PusGas

Sample

ARD

1

of conversion

1

R.

aAB

PusFe

3.9

0.930

PuFe2

31.9

0.964

Intercept

0.960

Calculated from eq. (3)

0.814

iron

relative

intensity

for gallium

equation is

and iron in plutonium-

in unknown

alloys, through the entire can be obtained by composition range, determining the Ziebold conversion parameter from a single standard. If however, the standard is in the low concentration range, several other standards should be run and the parameter determined by extrapolation.

References on an intensity-concentration plot although the amount is much less than the data reported

1) R. Castaing, Thesis (University of Paris, 1951) 2) L. Marton (ed.), Advances in Electronics and

by Hakkila. The data reported here seem more reasonable since a calculation of the conversion

7

parameter using eq. (3) yields: UGa, pu= 0.750. Contrary to the data of Scott and RanzettalO), the Pu-Fe data also show a positive deviation from linearity although this is reduced even further from the Pu-Ga data because of the higher mass absorption coefficient of iron in plutonium. Again these results are predicted by use of eq. (3) which yields a&, ~,=0.814. The agreement of the data with eq. (1) in the Pu-Fe system is not as good as in the Pu-Ga system. This results from the extreme sensitivity of the function to concentration errors at low concentrations. For example, chemical analysis showed that the PusFe intermetallic (theoretical composition -3.79 wt o/o iron) was closer to

9

ElectronPhysics T.

0.

Ziebold

35 (1963)

13 (Academic Press, 1960) p. 317 and R.

E.

Ogilvie,

Anal.

Chem.

621

P. M. Thomas,

UKAEA

Report,

National

Lead

AERE-R-4593

(1964)

5,

J.

E.

Colby,

NLCO-929

9

W.

M.

Mueller,

Advances

(USA)

Report,

(1964) 0.

in X-ray

Mallet

and

Analysis

M.

Fay

7 (Plenum

(ed.), Press,

1964) p. 395

7)

T.

0.

Ziebold

36 (1964)

9

T. D. McKinley, Wittry

9)

(ed.),

)

K.

Chem.

Report,

(Wiley,

and C. F. Metz,

LA-3125

(1964)

J. Inst. Met.

160

F. H. Ellinger, Mat.

and D. B.

Microprobe

V. D. Scott and G. V. T. Ranzetta,

J. Nucl.

Anal.

F. J. Heinrich

G. R. Waterbury

(USA)

90 (1961-62)

11)

Ogilvie,

The Electron

1964) p. 378 E. A. Hakkila, Los Alamos

10

and R. E.

322

C. C. Land and V. 0. 12 (1964)

226

Struberg,