Optical constants of plutonium at 5461 Å

JOURNAL

OF NUCLEAR

24 (1967) 232-233. @

MATERIALS

OPTICAL

CONSTANTS

OF PLUTONIUM

D. T. LARSON

The Dow Chemical Company,

is fast becoming

18 July

an important

contractor

for the U.S.

Atomic

Energy 232

80402,

USA

1967

components

extinction settings of the ellipsometer were found using the method described by Archer 4). Readings were taken in the four zones, given by McCrackin et al. 2), and the averages were used to calculate the ellipsometer parameters, A and Y. A is defined as the phase change of the parallel and normal components to the plane of incidence and Y is the arctangent of the amplitude ratio of the parallel and normal A prime

CO., AMSTERDAM

AT 5461 B

Rocky Plats Divition *, Golden, Colorado

tool to study corrosion and passivity of metals. In order to pursue such studies, it is necessary to know the optical constants of the substrate. The optical constants for delta-plutonium at 5461 a were determined using an ellipsometer l-3). Delta-plutonium (98.90 wt “,& Pu, 1.01 wt o/o Ga, and 0.09 wt o/o impurities) was used in these studies. The surface finish was prepared with successive grinding stages and a final polishing stage using 1 ,um diamond grit. After polishing, the plutonium coupon was placed in a vacuum system, and the system was evacuated to 5 x 1O-g Torr. Argon ion bombardment at 1500 V, 0.6 mA/cm2, and 30 to 40 micron argon was used to prepare the “clean” surface and was continued until there was no further change in ellipsometer readings. A Rudolph 43303-200 E, photoelectric ellipsometer was used, and the order of the optical components in relationship to the sample was polarizer, sample, compensator, and analyzer. Ellipsometer measurements were made with the plutonium sample in the vacuum chamber The equipped with 7056 Pyrex windows.

*

PUBLISHING

end D. L. CASH

Received

Ellipsometry

NORTH-HOLLAND

4). These parameters

measured for

the “clean” surface were used to calculate 5) the refractive index, n, and the absorption coefficient, 12. The complex index of refraction is related to the optical constants by the relationship fi = n - ki. Table 1 shows the optical TABLE Optical

constants

1

of delta-plutonium

5461 A and an angle

determined

of incidence

at

of 70”.

a=n-ki Sample

I

n

I

k

1

/

2.08

3.64

2 3

;

2.00

3.69

2.14

3.63

4

2.04

3.52

5

2.10

3.57

--i-ln=2.07

& 0.03

k=3.61

f

0.04

?i=2.07-3.61i

constants calculated for five different deltaThe probable errors 6) plutonium samples. calculated from these data are 0.03 and 0.04 for n and k, respectively. The average value for the complex index of refraction at 5461 A constants were is fi=2.07-3.6li. These measured using an angle of incidence of 70”. The accuracy of the optical constants is dependent. on inherent errors in the ellipsometer and the experimental problem of producing a “clean”, smooth surface. Averages of readings in four zones are reported to eliminate systematic errors resulting from polarizer, Commission

Contract

AT(29-l)-1106.

OPTICAL

compensator, Also,

this

CONSTANTS

and analyzer zero misalignment technique

eliminates

errors

7).

from

interference in the compensator 8). The accuracy for the average readings in four zones is d = f 0.04" and !P= f 0.02" while the scales analyzer, and of the polarizer, compensator, are measured in 0.01” angle of incidence increments.

One difficulty in sample preparation

is that argon bombardment tends to etch the plutonium surface 9). The etching of the surface

OB

233

PLUTONIUM

sufficient

to use with ellipsometry

when studying

plutonium

techniques

oxidation.

References 1) J. Kruger, Corrosion 22 (1966) 88 2) F. L. McCrackin,

E. Passaglia,

R. R.

Stromberg

and H. L. Steinberg, J. Res. Natl. Bur. Std. (U.S.) 678

(1963)

363

3)

A. C. Hall,

4)

R. J. Archer,

J. Opt.

5)

A. Vasicek

Sot.

J. Opt.

Am.

in Ellipsometry

will have some bearing on the optical constants

of

obtained. To determine the thickness and index of refraction of a thin film with an ellipsometer, it is necessary to know the optical constants of a “clean” metal surface; however, Winterbottom states 7) that errors in the optical constants of the substrate have only a second order effect on the calculated thickness or index of refraction of a thin film. The accuracy of the optical constants at 5461 d for deltaplutonium reported in this note should be

R. R. Stromberg

surfaces

and

55 (1965)

Sot. Am.

thin

A.

G.

films

(ed.

and J. Kruger;

Worthing

experimental 7) A.

B.

and

J.

Winterbottom, no.

1,

F.

Passaglia, Bur. Std.

Treatment

New York,

Optical

surfaces (The Royal Norwegian Report

E. Natl.

D. C., 1964) p. 25

Geffner,

data (Wiley,

970

in the measurement

Miscl. Publ., no. 256, Washington, 6)

911

52 (1962)

Bruns,

of

1959) p. 158

studies

of

metal

Scientific Society,

Trondheim,

Norway,

1955) p. 67 8)

II.

T.

J. Opt. 9)

K.

Yolken, Sot.

Imlah

R.

Am.

M.

Waxler

57 (1967)

in Extractive

and

J.

Kruger,

283

and physical

metallurgy

of plutonium

and its alloys (ed. W. D. Wilkinson;

Interscience,

New York,

1960) p. 189