MÃ¶ssbauer Spectroscopy and Its Chemical Applications - American

5 Conversion Electron Mössbauer Spectroscopy of Europium-151 and Thulium-169 G. K. SHENOY, D. NIARCHOS, P. J. VICCARO, and B. D. DUNLAP

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch005

Argonne National Laboratory, Argonne, IL 60439 We demonstrate the feasibility of conversion electron Mössbauer spectroscopy of rare-earth systems using the 21.53-keV transition in Eu and the 8.4-keV transition in Tm. The resonance spectra of Eu are measured using L conversion electrons with kinetic energy of about 13.5 keV. For Tm the conversion process involves M electrons that have kinetic energy of approximately 6.1 keV. The com parison of the conversion electron spectra to transmission data for a number of europium-based compounds indicates that an enhancement of the resonant effect occurs in most cases using the conversion electron technique. For Tm, a dilution of the effect occurs from photoelectrons of thulium. 151

169

151

169

169

T n recent years, the use of spectroscopy

5 7

F e and

1 1 9

S n conversion electron Mossbauer

( C E M S ) has g r o w n s t e a d i l y ( I ) . I n this t e c h n i q u e , t h e

i n t e r n a l c o n v e r s i o n a n d associated A u g e r electrons w h i c h result f r o m t h e d e - e x c i t a t i o n of M o s s b a u e r n u c l e i f o l l o w i n g resonant a b s o r p t i o n of i n c i d e n t g a m m a rays are d e t e c t e d . T h e p r i n c i p a l difference b e t w e e n this t y p e of M o s s b a u e r spectroscopy a n d the m o r e u s u a l o n e i n v o l v i n g t h e d e t e c t i o n of r e s o n a n t l y a b s o r b e d g a m m a rays i n a t r a n s m i s s i o n o r s c a t t e r i n g g e o m etry is t h e m u c h s m a l l e r r a n g e o f t h e c o n v e r s i o n e l e c t r o n i n a g i v e n a b s o r b e r c o m p a r e d t o t h e g a m m a r a y . A s a result, C E M S is sensitive to those resonant n u c l e i c o n t a i n e d i n a t h i n l a y e r ( 5 0 - 3 0 0 0 A ) a t t h e surface of t h e absorber.

O n t h e other h a n d , m e t h o d s b a s e d o n g a m m a

r a y d e t e c t i o n p r o b e d e p t h s o n the o r d e r of tens of m i c r o n s . T h i s u n i q u e p r o p e r t y o f C E M S m a k e s i t s u i t a b l e f o r s t u d y i n g surfaces. I n p r i n c i p l e , C E M S c o u l d b e u s e d f o r a n y of the M o s s b a u e r isotopes f o r w h i c h t h e n u c l e a r d e - e x c i t a t i o n has a l a r g e e n o u g h p r o b a b i l i t y o f o c c u r r i n g t h r o u g h t h e i n t e r n a l c o n v e r s i o n process.

H o w e v e r , except f o r

0065-2393/81/0194-0117$05.00/0 © 1981 American Chemical Society

Stevens and Shenoy; Mössbauer Spectroscopy and Its Chemical Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

118 5 7

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS

F e and

1 1 9

S n , l i t t l e is k n o w n c o n c e r n i n g t h e a p p l i c a t i o n o f t h e C E M S

t e c h n i q u e to other isotopes. O t h e r c a n d i d a t e s f o r w h i c h C E M S

could

p r o v e u s e f u l a r e some o f t h e M o s s b a u e r isotopes f r o m t h e r a r e - e a r t h series. W i t h this i n m i n d , w e h a v e i n v e s t i g a t e d C E M S a p p l i e d to t w o o f the r a r e - e a r t h isotopes,

1 5 1

21.53-keV t r a n s i t i o n i n 8.4-keV t r a n s i t i o n i n

1 5 1

1 6 9

E u and

1 6 9

Tm.

O u r results i n d i c a t e t h a t t h e

E u is i n f a c t v e r y c o n v e n i e n t f o r C E M S .

The

T m , o n t h e other h a n d , does n o t a p p e a r t o b e

as f a v o r a b l e . F o r b o t h isotopes, t h e c o n v e r s i o n e l e c t r o n spectra a r e c o m p a r e d t o s i m u l t a n e o u s l y m e a s u r e d g a m m a - r a y t r a n s m i s s i o n spectra. A n e v a l u a t i o n of the s e n s i t i v i t y of C E M S is m a d e i n e a c h case, a n d i n a d d i t i o n , t h e Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch005

o r i g i n of n o n r e s o n a n t a n d resonant electrons is discussed.

Experimental F o r t h e 8.4-keV 3 / 2 - 1 / 2 * t r a n s i t i o n i n T m , a n E r : A l source at a m b i e n t t e m p e r a t u r e w a s used. T h e T m 0 o x i d e a b s o r b e r w a s a p p r o x i m a t e l y 2 m g / c m of c o m p o u n d . T h e 21.53-keV 7 / 2 - 5 / 2 resonance i n E u was investigated using a S m 0 source at a m b i e n t t e m p e r a t u r e . T h e absorbers c o n s i s t e d of a p p r o x i m a t e l y 5 m g / c m of c o m p o u n d . T h e t r a n s m i s s i o n spectra a n d c o n v e r s i o n e l e c t r o n spectra w e r e c o l l e c t e d s i m u l t a n e o u s l y u s i n g t h e same absorber. T h e e l e c t r o n detector w a s of t h e b a c k s c a t t e r t y p e s i m i l a r i n d e s i g n to that g i v e n i n R e f . 2 i n w h i c h the flow gas w a s H e - 1 0 % C H . T h e t r a n s m i s s i o n spectra w e r e c o l l e c t e d b y p l a c i n g t h e a p p r o p r i a t e p r o p o r t i o n a l c o u n t e r at t h e exit w i n d o w of t h e c o n v e r s i o n e l e c t r o n detector. T h e spectra w e r e a c c u m u l a t e d i n a multichannel analyzer operated i n the time mode a n d synchronized to t h e s i n u s o i d a l m o t i o n o f t h e v e l o c i t y t r a n s d u c e r o n w h i c h t h e source was mounted. 1 6 9

2

1 6 9

3

2

+

1 5 1

1 5 1

2

+

3

2

4

Results 1 5 1

and

Discussion

E u 21.53-keV Resonance.

keV gamma ray by

1 5 1

T h e resonant a b s o r p t i o n of t h e 21.53-

E u results i n t h e e x c i t a t i o n f r o m t h e 5 / 2

g r o u n d state t o t h e 7/2

+

+

nuclear

e x c i t e d state. T h e s u b s e q u e n t d e - e x c i t a t i o n c a n

o c c u r t h r o u g h either t h e e m i s s i o n of a g a m m a r a y o r t h r o u g h t h e ejection of a n e l e c t r o n f r o m one o f t h e i n n e r shells. T h e p r o b a b i l i t y f o r e l e c t r o n e m i s s i o n ( i n t e r n a l c o n v e r s i o n ) is a p p r o x i m a t e l y 28.6 (3) t i m e s t h a t f o r g a m m a - r a y e m i s s i o n ( i n t e r n a l c o n v e r s i o n coefficient a =

28.6).

T h e p a r t i a l i n t e r n a l c o n v e r s i o n coefficients f o r e a c h o f t h e e l e c t r o n i c shells h a v e b e e n d e t e r m i n e d p r e v i o u s l y (3) f o r t h e 21.53-keV t r a n s i t i o n . A s c a n b e seen f r o m T a b l e I , L i c o n v e r s i o n is most p r o b a b l e

(71%),

p r o d u c i n g electrons w i t h a k i n e t i c e n e r g y o f a p p r o x i m a t e l y 13.5 k e V . H i g h e r - e n e r g y electrons f r o m t h e M shells a r e also present. I n a d d i t i o n , t h e filling of t h e v a c a n c i e s p r i m a r i l y i n t h e L shells results i n e i t h e r l o w e n e r g y x-rays o r s e c o n d a r y l o w - e n e r g y electrons.


5.

Europium-151

SHENOY E T A L . Table I.

and

Partial Internal Conversion Coefficients (3) 21.53-keV 7/2 -> 5/2 Transition in Eu +

Shell

+

Binding Energy (keV)

K

1 5 1

Conversion

48.5 8.05 7.62 6.98 ~ 1.40

I* La Lm Mi-Mx

for the a

Electron Energy (keV) 13.5 13.9 14.6 ~ 20

71 9 4 16

The total conversion coefficient is «T =

a

119

Thulium-169

28.6.


A l l electrons p r o d u c e d i n this m a n n e r t h a t r e s u l t f r o m t h e resonant a b s o r p t i o n process constitute t h e resonant effect i n C E M S .

A l l other

sources of electrons s u c h as p h o t o e l e c t r i c a b s o r p t i o n of t h e source g a m m a rays o r those g a m m a rays r e s u l t i n g f r o m n o n r e s o n a n t r e - e m i s s i o n esses i n the absorber w i l l c o n t r i b u t e to t h e b a c k g r o u n d .

The

proc

efficiency

or s e n s i t i v i t y of the t e c h n i q u e is t h e n d e t e r m i n e d b y the n u m b e r

of

resonant electrons t h a t escape t h e surface of the a b s o r b e r c o m p a r e d

to

those

arising from

nonresonant

processes b o t h

i n the

absorber

and

detector. A n estimate of t h e thickness of the l a y e r p r o b e d b y t h e C E M S

tech

n i q u e c a n be m a d e b y c o n s i d e r i n g the r a n g e of t h e c o n v e r s i o n

electrons

in a given compound.

that for

I t has b e e n s h o w n e x p e r i m e n t a l l y ( 4 )

electrons w i t h a k i n e t i c e n e r g y b e t w e e n a p p r o x i m a t e l y 5 a n d 15 k e V , the r a n g e f o l l o w s a n a t t e n u a t i o n l a w of t h e f o r m

I = h where 7

0

e'f

is t h e i n i t i a l flux a n d I is the

final

flux after t r a n s v e r s i n g a

d i s t a n c e z i n t h e m a t e r i a l . T h e a t t e n u a t i o n coefficient /*, i n A " is g i v e n b y 1

H — 4.43 X 10"

3

/E

P

z/2

w h e r e p is the d e n s i t y i n g / c c of the m a t e r i a l a n d E is the i n i t i a l k i n e t i c e n e r g y of the e l e c t r o n i n k e V . F o r L i electrons w i t h energy 13.5 k e V i n e u r o p i u m m e t a l (p =

5.26

g / c c ) , t h e h a l f - t h i c k n e s s is a p p r o x i m a t e l y 1500 A . T h e a t t e n u a t i o n l e n g t h f o r the 2 1 . 5 3 - k e V g a m m a r a y of

1 5 1

E u , o n the other h a n d , is the o r d e r of

60 p. W i t h o u t c o n s i d e r i n g s e c o n d a r y p r o d u c t i o n o f photoelectrons

pro

d u c e d b y r e s o n a n t l y scattered g a m m a rays d e e p i n s i d e t h e m a t e r i a l ( 5 ) , t h e r e l a t i v e r a n g e of g a m m a rays a n d c o n v e r s i o n electrons i n d i c a t e s t h a t C E M S is sensitive to a r e l a t i v e l y t h i n surface l a y e r of t h e m a t e r i a l . I n F i g u r e 1 w e s h o w t h e c o n v e r s i o n e l e c t r o n spectra f o r three c o m p o u n d s of e u r o p i u m c o m p a r e d to c o r r e s p o n d i n g s p e c t r a t a k e n s i m u l t a n e o u s l y i n the t r a n s m i s s i o n geometry.

F o r the two trivalent

compounds


120

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS


I 1 1 1 1 I I I | I I I 1 1 I I I II I I I 1 1 I I 1 I I 1 I I I 1 I

I

I

I

I

-16

I

I

I

I

I

"8

I

I

0

i +

*8

1

I6-I6

i l

I

I

I

I

-8

I

I

I

0

I

I

I

I

1 I

I

*8

I

*I6

VELOCITY (mm/sec)

Figure 1. Transmission Eu Mossbauer effect spectra at 300 K for (a) Eu O , (c) EuF , and (e) Eu SiO . The conversion electron spectra also are shown for (b) Eu 0 , (d) EuF , and (f) Eu SiO^ A Sm O source was used. 151

2

s

s

2

2

Eu 0 2

3

2

u

s

(Figures l a and b) and E u F

151

2

3

2

s

( F i g u r e s l c a n d d ) the most striking

difference b e t w e e n the c o n v e r s i o n e l e c t r o n a n d g a m m a - r a y t r a n s m i s s i o n s p e c t r a i n e a c h c a s e is t h e 6- to 1 0 - f o l d e n h a n c e m e n t of t h e resonant effect a c h i e v e d w i t h the C E M S t e c h n i q u e . F o r e a c h c o m p o u n d t h e resonance w i d t h s f o r t h e c o n v e r s i o n e l e c t r o n a n d t r a n s m i s s i o n s p e c t r a are a p p r o x i mately equal.

C o n s e q u e n t l y , t h e e n h a n c e m e n t of t h e effect represents

a n increase i n s e n s i t i v i t y w i t h o u t loss of r e s o l u t i o n . F o r the s p e c t r a of d i v a l e n t c o m p o u n d E u S i 0 2

4

shown i n Figures l e

a n d f, a n e n h a n c e m e n t of the resonant effect s i m i l a r to t h a t f o u n d f o r t h e t r i v a l e n t c o m p o u n d s is f o u n d . I n a d d i t i o n , t w o other differences

between

t h e c o n v e r s i o n e l e c t r o n a n d g a m m a - r a y t r a n s m i s s i o n s p e c t r a are e v i d e n t . F i r s t of a l l , a w e a k resonance at —0.33 ± 0.1 m m / s c o r r e s p o n d i n g to a


5.

Europium-151

SHENOY E T A L .

and

Thulium-169

121

i I I I I l l I i | i I l i I i i i i

i i i i i i i i i I I i i i i i -16

-8

0

*8

1 1 1

*I6


VELOCITY (mm/sec)

Eu

3 +

Figure 2. Conversion electron Mossbauer spectrum for 170 europium in magnesium

Eu ppm

151

species is present i n t h e c o n v e r s i o n e l e c t r o n s p e c t r u m ( F i g u r e

a n d absent i n the t r a n s m i s s i o n one ( F i g u r e l e ) .

a p p r o x i m a t e l y the s a m e signal-to-noise r a t i o f o r the m a i n E u it a p p e a r s t h a t t h e E u

component

3 +

If)

S i n c e b o t h spectra h a v e resonance,

2 +

arises p r i m a r i l y i n t h e surface l a y e r

2000 A ) p r o b e d b y C E M S .

(~

T h e s e c o n d aspect of the C E M S d a t a for E u S i 0 2

s o m e s t r u c t u r e i n the E u

4

is the p r e s e n c e of

resonance w h i c h is n o t a p p a r e n t i n the t r a n s

2 +

m i s s i o n d a t a . T h i s s t r u c t u r e appears to b e associated w i t h the p r e s e n c e of a s m a l l q u a d r u p o l e i n t e r a c t i o n at t h e E u field

g r a d i e n t is a s s u m e d , the v a l u e of

2 +

e qQ 2

site. I f a n a x i a l e l e c t r i c =

1 mm/s

— 12 ±

is

o b t a i n e d f r o m a fit to the d a t a . T h i s v a l u e is the same as t h a t d e t e r m i n e d (6)

b e l o w the m a g n e t i c t r a n s i t i o n of 9 K for this c o m p o u n d i n a t r a n s

mission geometry.

T h e presence of a q u a d r u p o l e i n t e r a c t i o n of t h i s o r d e r

w o u l d a c c o u n t for the a s y m m e t r i c b r o a d e n i n g o b s e r v e d i n o u r t r a n s m i s s i o n d a t a for E u S i 0 2

4

at 300 K . T h e i n c r e a s e d r e s o l u t i o n t h e C E M S d a t a

i n d i c a t e d b y these results f o l l o w s f r o m the f a c t that t h i c k n e s s - b r o a d e n i n g effects are m i n i m a l for this t e c h n i q u e b e c a u s e of t h e t h i n l a y e r p r o b e d . A f o u r t h e x a m p l e of

1 5 1

E u C E M S t h a t illustrates the s e n s i t i v i t y of

t h e t e c h n i q u e is e u r o p i u m - d o p e d m a g n e s i u m m e t a l . F i g u r e 2 shows t h e C E M S d a t a at 300 K f o r 170 p p m e u r o p i u m i n m a g n e s i u m . seen, b o t h a E u ±

0.1 m m / s

2 +

species at —13.0 ±

are d e t e c t e d .

0.3 m m / s a n d a E u

3 +

As can

T h e t o t a l resonant a m p l i t u d e i n t h e

resonance is a p p r o x i m a t e l y 3 % .

be

one at —0.27

T h e r a n g e of t h e L i 1 3 . 5 - k e V

Eu

3 +

conver

s i o n electrons i n m a g n e s i u m is c h a r a c t e r i z e d b y a h a l f thickness of a b o u t 4500 A . O n the o t h e r h a n d , the h a l f t h i c k n e s s for p h o t o e l e c t r i c a b s o r p t i o n of the 2 1 . 5 3 - k e V g a m m a r a y is a p p r o x i m a t e l y 1.6 m m , i n d i c a t i n g a m i n i m a l c o n t r i b u t i o n of p h o t o e l e c t r o n s r e s u l t i n g f r o m t h e m a g n e s i u m m a t r i x . T h i s , i n t u r n , f a c i l i t a t e s t h e d e t e c t i o n of s m a l l q u a n t i t i e s of e u r o p i u m i n t h e m a t e r i a l . T h i s r e s u l t demonstrates t h a t u n d e r f a v o r a b l e c o n d i t i o n s C E M S is c a p a b l e of d e t e c t i n g s m a l l a m o u n t s of surface, s u c h as those

europium impurities i n the

realized i n i m p l a n t a t i o n experiments

(^

atoms/cm ). 3


10

1 8

122

M O S S B A U E R S P E C T R O S C O P Y A N D ITS C H E M I C A L A P P L I C A T I O N S

1 6 9

3/2

1 6 9

T m results i n t h e t r a n s i t i o n f r o m t h e l / 2

n u c l e a r e x c i t e d state.

+

T h e resonant a b s o r p t i o n of t h e 8 . 4 - k e V

T m 8.4-keV Resonance.

gamma ray b y

d e c a y is l a r g e w i t h a

T

=

+

g r o u n d to t h e

T h e i n t e r n a l c o n v e r s i o n coefficient

291 ( 7 ) .

for

the

T a b l e II shows the b i n d i n g energies

f o r the K, L, a n d M shells of t h u l i u m , a n d as c a n b e seen, o n l y M s i o n is possible. T h e t h e o r e t i c a l estimate ( 8 )

of t h e p a r t i a l

coefficients shows t h a t M i s h e l l c o n v e r s i o n is most p r o b a b l e .

conver

conversion T h e kinetic

e n e r g y of t h e r e s u l t a n t c o n v e r s i o n electrons is a p p r o x i m a t e l y 6.1 k e V . A n estimate of the h a l f thickness for electrons w i t h this energy i n t h u l i u m metal based z Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch005

1/2

~

on

t h e same

e m p i r i c a l r e l a t i o n g i v e n earlier results i n

250 A . (8)

Table II. Estimated Partial Internal Conversion Coefficients for the 8.4-keV 3 / 2 - > l / 2 Transition in Tm° +

Binding Energy (keV)

Shell

1 6 9

% Conversion

69.4 8.6-10.1 2.3 1.5-2.1

K Li-L

in

Mi

M -M U

+

Y

Electron

— — 95 5

° The total conversion coefficient is a = 291 T

Energy (keV)

— — 6.1 - 6

(7).

T h i s v a l u e is n e a r l y a f a c t o r of t e n s m a l l e r t h a n t h a t f o r

Eu

1 5 1

c o n v e r s i o n electrons i n e u r o p i u m m e t a l a n d i n d i c a t e s t h a t a m u c h t h i n n e r surface l a y e r is p r o b e d w i t h

1 6 9

Tm CEMS.

O n e m i g h t expect that

1 6 9

T m w o u l d be a more favorable

for C E M S than

1 5 1

Eu.

B o t h the resonance f r a c t i o n a n d the i n t e r n a l c o n

v e r s i o n coefficient are l a r g e r f o r the 8.4-keV t r a n s i t i o n i n 1 5 1

Eu.

candidate

1 6 9

T m than i n

H o w e v e r , F i g u r e 3 shows t h a t t h e resonant effect f o r T m 0 2

3

is

s m a l l e r for the C E M S d a t a t h a n for the t r a n s m i s s i o n d a t a . T h i s is just the opposite of w h a t w a s o b s e r v e d f o r resonant effect for

1 6 9

1 5 1

Eu.

T h e a p p a r e n t decrease i n

T m C E M S is a t t r i b u t a b l e to a n increase i n the b a c k

g r o u n d c o n t r i b u t i o n , associated i n p a r t w i t h a l a r g e increase i n p h o t o electron production.

F o r t h u l i u m m e t a l , f o r e x a m p l e , t h e r a t i o of

the

resonant a b s o r p t i o n cross section of the 8.4-keV g a m m a r a y to that f o r p h o t o e l e c t r i c p r o d u c t i o n is a p p r o x i m a t e l y six. W h i l e this is a b o u t times s m a l l e r t h a n t h a t f o u n d for

1 5 1

severe r e d u c t i o n i n t h e o b s e r v e d i n t e n s i t y i n to t h e t r a n s m i s s i o n d a t a .

four

E u , i t c a n n o t f u l l y a c c o u n t for t h e 1 6 9

T m C E M S data compared

I n addition, the relation used for the half-

thickness c a l c u l a t i o n is p e r h a p s not v a l i d i n the 6 - k e V r a n g e , a n d as a c o n s e q u e n c e , w e m a y b e s a m p l i n g c o n s i d e r a b l y s m a l l e r n u m b e r s of reso n a n t atoms.

T h e s e results o n

1 6 9

T m C E M S suggest t h a t the

technique

m a y be useful only under special circumstances.


5.

SHENOY E T A L .

Europium-151

and

123

Thulium-169

i IIi i Ii i i |i i i Ii i i I

Figure 3. Transmission Tm Mossbauer effect spectrum (a) at 300 K compared to the conversion electron spectrum (b) for Tm O . An Er:Al source was used.


169

2

VELOCITY (mm/sec)

s

169

Acknowledgment T h i s w o r k w a s s u p p o r t e d b y t h e U . S . D e p a r t m e n t of E n e r g y . Literature Cited 1. Tricker, M . J., Chapter 3 in this book. 2. Spijkerman, J. J. "Mössbauer Effect Methodology"; Gruverman, I. J., E d . ; Plenum, 1971; Vol. 7, p. 85. 3. Antman, S.; Petterson, H.; Zehlev, V.; Adam, I., Z . Physik 1970, 237, 285. 4. Cosslett, V . G.; Thomas, R . N . Brit. J. Appl. Phys. 1964, 15, 883. 5. Tricker, M . J.; Ash, L. A.; Cranshaw, T. E. Nucl. Inst. Meth. 1977, 143, 307. 6. Kalvius, G . M.; Shenoy, G . K. Z. Naturforsch. 1971, 26a, 353. 7. Aratamonova, K. P.; Voronkov, A . A . ; Grigor'ev, E. P.; Zolotavin, A. V . Izv. Akad. Nauk SSSR, Ser. Fiz. 1976, 40, 32. 8. Hager, R . S.; Seltzer, E. C . Nucl. Data 1968, A4, 1. R E C E I V E D June 27,

1980.


MÃ¶ssbauer Spectroscopy and Its Chemical Applications - American

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