ENHANCEMENT OF MÖSSBAUER LINES BY ACTIVE DATA ACQUISITION

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Submitted on 1 Jan 1979

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ENHANCEMENT OF MÖSSBAUER LINES BY ACTIVE DATA ACQUISITION

J. Sawicki, J. Stanek

To cite this version:

J. Sawicki, J. Stanek. ENHANCEMENT OF MÖSSBAUER LINES BY ACTIVE DATA ACQUISI- TION. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-58-C2-60. �10.1051/jphyscol:1979221�.

�jpa-00218589�

JOURNAL DE PHYSIQUE Colloque C2, suppldment au n ^O 3, Tome 40, mars 1979, page C2-58

ENHANCEMENT OF MOSSBAUER L I N E S B Y A C T I V E DATA A C 3 U I S I T I 9 N

J . A . Sawicki and J.J. Stanek

I n s t i t u t e of Physics, JagieZZonian University, Cracow 30059, PoZand

RBsum6.- On propose un proc6d6 d ' a c q u i s i t i o n de donn6es a c t i v e s . La m6thode combine un processus f o n c t i o n n e l i n s t a n t a n s e t un l i s s a g e des donnzes. Un c o n v e r t i s s e u r analogique d i g i t a l de type Gaussien e s t prSsent6. On o b t i e n t a i n s i un f o r t accroissement de l ' e f f e t r e l a t i f e t une diminution s i g n i f i c a t i v e de l a l a r g e u r de r a i e . La dur6e d e s temps d'accumulation e s t a u s s i largement r z d u i t e . A b s t r a c t . - A concept of a c t i v e d a t a a c q u i s i t i o n was proposed. The method combines i n s t a n t f u n c t i o - n a l p r o c e s s i n g and smoothing of d a t a . A Gaussian-type a n a l o g - t o - d i g i t a l c o n v e r t e r i s p r e s e n t e d . A

l a r g e i n c r e a s e i n t h e r e l a t i v e e f f e c t and a s i g n i f i c a n t l i n e narrowing were obtained. The d u r a t i o n s of some experiments were g r e a t l y reduced.

I n t r o d u c t i o n . - The s i g n a l - t o - n o i s e r a t i o has always been a c r u c i a l problem i n s p e c t r o s c o p i e s . S o p h i s t i - c a t e d techniques a r e a l r e a d y i n use which enhance t h e s i g n a l s of i n t e r e s t , b u t u s u a l l y i n t h e o p t i c a l and radiofrequency ranges. I n n u c l e a r s p e c t r o s c o p i e s and M'cissbauer spectroscopy i n p a r t i c u l a r , t h e s i n g l e p u l s e counting predominates. Experiments a r e u s u a l l y based on p r o p o r t i o n a l i t y i n o r d e r t o r e f l e c t Poisson o r Gaussian s t a t i s t i c a l d i s t r i b u t i o n of n u c l e a r e v e n t s . Conventional s c a l e r s o r multichannel analy- z e r s a r e s u f f i c i e n t f o r t h e p a s s i v e way of d a t a ac- q u i s i t i o n .

I n t h i s work a concept of a c t i v e d a t a acqui- s i t i o n i s d e s c r i b e d . An a c t i v e a c q u i s i t i o n makes t h e f i n a l count r a t e a well-defined f u n c t i o n of t h e i n - p u t r a t e . The concept w i l l b e i l l u s t r a t e d u s i n g MESS- bauer spectroscopy. Various o t h e r a p p l i c a t i o n s a r e p o s s i b l e .

A c t i v e d a t a a c q u i s i t i o n . - A c t i v e conversion of d i g i - t a l d a t a may be performed e a s i l y w i t h l o g i c c i r c u i t s o r computer p r o c e s s o r s . Here, an analog system cou- p l e d t o a m u l t i c h a n n e l a n a l y z e r i s d e s c r i b e d . The c o n v e r t e r e n a b l e s n o n l i n e a r p r o c e s s i n g of t h e analog i n p u t s i g n a l i n t o a number of counts i n s u c c e s s i v e channels o f time-mode MCA. The i n p u t s i g n a l , U ( t ) , i s added t o t h e p u l s e s from an a u x i l l i a r y g e n e r a t o r u s i n g t h e sum-invert a m p l i f i e r . The t o t a l s i g n a l i s t h e n d i s c r i m i n a t e d a t some l e v e l , Uo, w i t h t h e f a s t d i s c r i m i n a t o r . The o u t p u t p u l s e r a t e depends on t h e p u l s e amplitude d i s t r i b u t i o n of t h e g e n e r a t o r and on t h e i n p u t s i g n a l amplitude.

A l i n e a r f u n c t i o n c o n v e r t e r of t h i s type was used t o t r a n s f o r m a t y p i c a l m u l t i s c a l e r i n t o a s i g n a l a v e r a g e r . Recording of an average e r r o r s i g n a l of the PZijssbauer d r i v e i n a d i g i t a l from was f o r i n s t a n c e

p o s s i b l e . I n a n o t h e r a p p l i c a t i o n a d i g i t a l o f f s e t of count numbers was made i n o r d e r t o reduce much of t h e u s e l e s s counts, t h a t i s u s u a l l y r e g i s t e r e d by t h e m u l t i s c a l e r , e s p e c i a l l y a t h i g h count r a t e s .

We w i l l now c o n s i d e r a Gaussian-function con- v e r t e r i n more d e t a i l . I n t h i s case t h e thermally

s t a b i l i z e d p r e a m p l i f i e r was used a s a g e n e r a t o r . I t s n o i s e p u l s e s were found t o p o s s e s s approximately a normal d i s t r i b u t i o n of amplitudes. The r a t e of out- u u t u u l s e s was then :

where U 1 i s t h e mean amplitude of t h e g e n e r a t o r p u l s e s . A l l v o l t a g e s i g n a l s a r e expressed i n u n i t s of a, where a2 i s t h e v a r i a n c e of t h e g e n e r a t o r pul- s e amplitudes.

XEssbauer spectrum a m p l i f i e r . - The Gaussian conver- t e r was used i n t h e YEssbauer spectrometer. An espe- c i a l l y designed d i g i t a l - t o - a n a l o g c o n v e r t e r was used t o make t h e analog s i g n a l p r o p o r t i o n a l t o t h e count r a t e , n , i n s u c c e s s i v e channels, U(t) = p.n. Instead of an average count number ii i n a modified spectrum p o i n t , one w i l l o b t a i n a f t e r enough sweeps :

Q) U0+pn

. I

1

- ^Q) - ^m 121

where a normal p r o b a b i l i t y d i s t r i b u t i o n of i n i t i a l counts i s assumed.

c o n s i d e r a normal aiissbauer experiment w i t h t h e r e l a t i v e e f f e c t E = - no- iim/i&, and t h e signa.1- to-noise r a t i o p = ~ i iwhere ~iio and / Ii, ~a r e ~ average count r a t e s i n o r f a r o f f resonance. I n v i e w of t h e f u n c t i o n a l r e l a t i o n s h i p (Equation 2), a s p e c - trum o b t a i n e d w i t h a Gaussian c o n v e r t e r w i l l b e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979221

greatly modified.

In the example shown in figure 1, the single line was calculated using the parameters iio = 10,

& = 0.5, p = 3.3, and Uo = -10, and is compared with

an initial Lorentzian line.

-r o r

VELOCITY

Fig. 1 : Modification of a line-shape : solid line

- Lorentzian curve, dotted line - converted curve.

An increase of the effect by a factor of 20 and line narrowing are seen. It should be noted that the mo- dified spectrum may be noninverted or inverted com- pared to a normal one depending on the input pola- rity in sum-invert amplifier, p > 0 or p < ^0.

Computations of the modified effect,

- -

5 = (No-Nm)/Nm , and the signal-to-noise ratio,

=(Go- im)

/ I

i :

%

:_::il

a ¹

--

Zq

>

0-

3

9 ^UR

- 0 2 U-U,

Fig. 2 : Amplification of the relative effect com- puted with different sets of parameters.

It was assumed that the average amplitude of thein- put signal in the resonance, UR, equals U1 or U1-I.

Very large amplifications may be obtained. Verysmall effects may be amplified 2 or 3 times at most. The line intensity ratios are generally not conserved since small lines can be am~lified more than larger ones or vice versa, depending on the adjustable pa- rameters of the converter. The signal-to-noise ratio in modified spectrum is not significantly changed except for large p parameter values (p > 5) where the ratio R becomes smaller than the normal p value.

Instant smoothing.- A large increase in a relative effect and line narrowing enables an effective ap- plication of the procedure of smoothing. The smoo- thing of data points generally decreases the rela- tive effect and increases the linewidth. However,it markedly reduces the statistical variation of data points and thus markedly improves the signal-toaoise ratio.

Experimentally, the smoothing was performed simply by integration of the input signal U(t) with a time constant of approximately 2 - 3 dwell times, i.e.,

-

- one, that can be made in a computerized converter.

In such a case, the theoretical analysis of the li- neshape, the relative effect, and the signal-to- noise ratio in a smoothed spectrum may be made accu- rately.

The smoothing performed during spectrum aca- mulation is essentially equivalent to the smoothing on the final spectrum if the same filter is applied.

Experimental results.- The advantages of the method can be illustrated by some Mzssbauer spectra measu- red in the usual way and with the use of an active

converter. Spectra were measured under the same con- ditions at the detector and during the same time period.

In figure 3, the transmission spectra of so- dium nitroprusside are shown. The normal spectrum was purpously measured with a poor statistics. In a

spectrum measured with the active technique, the relative effect is about two times larger and the line positions determined with the Lorentzian line fit have more than three times the accuracy.

Conversion electron ~gssbauer spectra of 5%e implanted in silicon at a low dose are compared in figure 4. Again, a considerable improvement in the resolution has been obtained. Other experiments with

lo1'

C2-60 JOURNAL DE PHYSIQUE

a p o s s i b i l i t y f o r c o n s i d e r a b l e improvement of t h e s e n s i t i v i t y of t h e CEMS technique.

VELOCITY [mmlsl

Fig. 3 : ~ ' c i s s b a u e r t r a n s m i s s i o n s p e c t r a of sodium n i t r o p r u s s i d e : up - normal spectrum, down

-

trum measured w i t h t h e new method, i n v e r s i o n was a l s o a p p l i e d .

S u r p r i n s i n g l y quick measurements can be per- formed even w i t h t h e use of f a i r l y weak s o u r c e s . For i n s t a n c e , i n t h e case of CEMS s p e c t r a of 5 7 ~ e e n r i c h e d s t a i n l e s s s t e e l f o i l a resonance l i n e can be a l r e a d y d e t e c t e d i n a s i n g l e sweep, i . e . i n p a r t s of second. I n t e r e s t i n g a p p l i c a t i o n s can be imagined.

Fig. 4 : Conversion e l e c t r o n ~ G s s b a u e r s p e c t r a of 5 7 ~ e implanted i n S i a t t h e dose of 5 x 1 0 ~ + / c m ~ : up - normal spectrum, down

-

Remarks.- The a c t i v e p r o c e s s i n g of d i g i t a l d a t a du- r i n g measurements may be a p p l i e d i n v a r i o u s e x p e r i - ments, e s p e c i a l l y f o r t h e enhancement of t h e s i g n a l s of i n t e r e s t . The n o n l i n e a r conversion of d a t a des- c r i b e d above may g r e a t l y enhance t h e r e s o l u t i o n of Mgssbauer s p e c t r a , reduce t h e d u r a t i o n of experiments and e n a b l e t h e measurement of s m a l l e f f e c t s . Conver- t e r s of d i f f e r e n t types may be c o n s t r u c t e d , combined i n s e r i e s o r f e d back i n a m p l i f i e r s .

The f u r t h e r work on t h e a c t i v e ways of d a t a a c q u i s i t i o n i s i n p r o g r e s s . Other concepts f o r a more e f f i c i e n t use of t h e d e t e c t e d s i g n a l s a r e a l s o considered.