ON THE INFLUENCE OF DEFECTS ON POSITRON ANNIHILATION IN ALKALI-HALIDES

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ON THE INFLUENCE OF DEFECTS ON POSITRON ANNIHILATION IN ALKALI-HALIDES

L. Smedskjaer, S. Dannefaer, R. Cotterill, G. Trumpy

To cite this version:

L. Smedskjaer, S. Dannefaer, R. Cotterill, G. Trumpy. ON THE INFLUENCE OF DEFECTS ON

POSITRON ANNIHILATION IN ALKALI-HALIDES. Journal de Physique Colloques, 1973, 34 (C9),

pp.C9-97-C9-99. �10.1051/jphyscol:1973916�. �jpa-00215391�

JOURNAL DE PHYSIQUE

Col/oqzie C9, s u ~ ~ ~ p / ~ i i i e n t au no 1 1 - 1 2, To111e 34, Nouembre-Dicembre 1973, page (3-97

ON THE INFLUENCE OF DEFECTS ON POSITRON ANNIHILATION IN ALKALI-HALIDES

L. S M E D S K J E R , S. D A N N E F E R , R . M . J. C O T T E R I L L a n d G. TRUMPY D e p a r t m e n t o f Structural Properties o f Materials a n d L a b o r a t o r y o f Applied Physics I1

T h e Technical University o f D e n m a r k , Building 307, D K 2800 Lyngby, D e n m a r k

RCsumC. -

La technique d'annihilation positronique par l'utilisation de la durCe de vie et la correlation angulaire est presentee. Les resultats sont presentes pour

:

1 )

Monocristaux de NaCl et KC1 colorks additivement

; 2)

Conversion F B F-

;

3)

Trempe thermique

; 4)

DCfaut d'irradiation.

Abstract. - The technique of positron annihilation by use of lifetime and angular correlation will be presented. The main characteristic of this technique is that defects are important in the annihilation processes. Results will be presented for

:

1) Additively colored NaCl and KC1 single crystals

; 2)

F t o F- conversion

;

3)

Thermal quenching

; 4)

Radiation darnage.

These experiments show that F centers are effective as traps for positrons by the presence of a new

1.1

ns lifetime, and that trapping probability is proportional to the density of F centers, and further that F to F- conversion decreases this probability.

The introduction of defects by thermal treatment causes considerable changes in the annihila- tion spectra, and this defect influence on stage I coloration is also revealed in angular correlation measurements. From these experiments it is clear that stage I coloration takes place in a thin sur- face layer (in NaCI) within approximately four hours using a

1

~ n C i N a n source.

Before I present o u r results obtained with positron annihilation o n N a C l a n d KCI, I t h i n k its pertinent t o discuss shortly t h e positron annihilation technique (see Fig. 1).

DOPPLER WlOADENING I 1=:1$":

, < E L E C T R O N M O M E N T U M ) : ^{, -} r - , s : . ~ ::..<,,.

C . . . ~ . . . ....-.. .. ... , . ... ,: ..

DEATH

+=bL

^DEATH

FIG. I .

-Schematic drawing of the physical sitilation f o ~ three diKerent positron annihilation tcclilliqucs.

During t h e past decade positron annihilation has become a n increasingly popular technique for studying crystal defects. T h e principles o f the method a r e relatively simple, a n d these a r e illustrated schemati-

cally in figure 1. Fro111 a Na" source a positron is emitted simultaneously with a n 1.3 MeV gamma ray, and after t h e positron h a s become thermalized in the specimen t w o things m a y happen. It may either f o r m a positroniurn

((

uto~lz

^D,

which is a bound state between a n electron a n d a positron, a n d then anni- hilate f r o m this state, o r t h e positron may i ~ n n i h i l a t e freely. I n both cases t w o g a m m a rays a r e emitted, each with a n energy o f 0.5 MeV.

T w o properties c a n now be measured c x p e r i n ~ e n - t a l l y : T h e first o n e is t h e time elapsed f r o m the emission o f the birth g a m m a ray t o th: emission o f the annihilation garnrna rays. This constitutes the so called lifetime tneasuremcnt. T h e sccc>nd property is the ariglc between the two arlnihilation ganlrnah.

If the electron-positron pail. is at ~.cst this angle is 180" due t o monlentum c c ~ n ~ c r v a t i o n . but if

i t

is mo\,inp a small deviation is obscr\,cd. T h c mcasurc- ment of this deviation ccinstiti~tes the angular corrc-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973916

C9-98 L. SMEDSKJER, S. D A N N E F E R , R. M. J. COTTERILL A N D G. TRUMPY

lation curve, and as the positron is thermalized, it mainly expresses the electron monientum.

Now, if positronium is formed, this shows up in the lifetime measurement as a rather long lifetime of about 1 ns, while in angular correlation curve it is detected by a change in the shape when a magnetic field is applied to the specimen [I].

Figure 2 shows a lifetime spectrum for KC1 addi- tively colored t o the indicated F-center concentration.

K C I

Aadltlvelv colored

T I M E ( n s )

FIG. 2. - Lifetime spectrum for additively coloured KCI.

On the vertical axis we have the number of detected annihilations as function of the time elapsed from the emission of the birth gamma ray. If all the posi- trons annihilate with a single lifetime only one straight line should be present, but here we obviously are dealing with three lifetimes. These three lifetimes indicate that the positrons annihilate in three diffe- rent places with different electron densities and where the lowest density gives the longest lifetime. The probability for annihilation with one of the lifetimes is expressed by the intensity of the lifetime.

I n figure 3 we have shown an angular correlation curve for NaCl additively colored to the indicated F

N C C I P- a

NARRGvV AND B2OAD

C o M P o N E w T s S H O W N

P

? ^{' F '}

'.

^li)"

A N G L E IN M I L L I R A S l i ' . 5

FIG. 3.

-

Angular correlation curve for additively coloured NaCI. The circles show some of the measuring points.

center concentration. On the vertical axis we have the number of detected annihilations as function of the deviation from n radians. The curve can expe- rimentally be divided into two curves (dotted curves)

:

A broad one due to annihilation with high momen- tum electrons, and a narrow one due to annihilation of positroniun~. One can show that of the total amount of positronium formed only one quarter will contribute to the narrow component while the rest will contribute to the broad component [I]

The intensities of the components are defined as the area under each component divided by the total area.

I n figure 4 is shown the lifetime

t 3

and the intensity I, in additively coloured NaCl as a function of the

FIG. 4. - 53 and 1 3 versus initial concentration of F-centres.

F-centre concentration. The distinct lifetime of 1.1 ns with a n intensity proportional with the F-centre concentration is interpreted as positrons annihilating in F-centres, and from the value of the lifetime we assume that positroniu~n is formed. (In KC1 a lifetime of 1.04 ns was found.)

Exposing some of these additively colored crystals to a heavy irradiation dose by a 1 mCi positron sourcc.

this decreased the intensity from 8 :/, to about 4 y,,

in spite of an increase in the F-centre concentration by a factor of 5 or more. This observation suggests that defects other than just F-centres have been formed.

such as V-centres, but no positive identification call yet be given.

For the sake of complete~~ess we just mention that the two other lifetimes were 0.2 and 0.4 ns, and thal the 0.4 ns lifetime seems, at least partly, to be connected with annihilation in vacancies, while the 0.2 ns lifetime is belived to be due to free annihilation.

In order to investigate thc time development of the

ON T H E INFLUENCE O F DEFECTS ON,POSITRON ANNIHILATION IN ALKALI-HALIDES C9-99

irradiation damage in NaCl from the 1 mCi positron source used in angular correlation measurements, we measured the magnetic quenching effect at the top of the angular correlation curve (see Fig. 5).

I I I I I I I I I 1 TIME

0 1 2 3 4 5 6 7 8 9 . 1 0 ' s

FIG.

5. - Counting rate in the top of the angular correlation curve as function of time.

The upper points show the counts with a magnetic field of 20 kG, while the lower one corresponds to zero field, and the difference between these two curves is an expression for the formation of posi- tronium. The fast increase of the upper curve within the first 10000 s is thus taken as evidence for an increased positronium formation, and from optical measurements this increase seems to be connected with stage I coloration. Now, to determine the narrow and broad components, one has of course to measure the whole angular correlation curve and as this takes at least 20 h, the results actually correspond to past stage I irradiated crystals.

The curves shown in figure 6 represent the narrow components in differently treated crystals. The lowest one (squares) is for a cleaved crystal, where the

1 I 1 I I I I I l l I I

1 2 1 0 8 6 4 2 0 2 4 6 8 1 . 1 2

ANGLE IN MILLIRADIANS

FIG.

6. - Narrow component of the angular correlation curve for three differently treated NaCl crystals.

intensity is 6 % and the halfwidth is 3 mrad. The two other curves are for additively coloured crystals where the intensity is about 12 % and the halfwidth is 4.5 mrad, but as the same values were found for crystals which were thermally quenched in the same way as the coloured crystals, this suggests that it is the quenching and not the colouring, which causes the change

;

and as the total positronium formation is four times the intensity of the narrow component, the quenching results in quite a large change from 24 to 48 %.

Further, one can calculate from the halfwidths by using the Heisenberg uncertainty principle, that in cleaved crystals the mean space in which the posi- tronium atom is confined is 3.5 A, while in the quenched ones it only is 2.5 A.

Reference

[ I ] Positrotr A t ~ t ~ i l ~ i l r t i o t ~ , edited by A. T. Stewart and L. 0. Roel- lig (Academic Press) 1967.

DISCUSSION

A. DUPASQUIER.

-

In the abstract of your contri- light, and as function of temperature the intensity

bution it is mentioned that you observed an effect of the long-lived component showed a distinct decrease

due to the F to F' conversion. Could you now say a in the temperature area where the maximum conver-

few words about this subject

?

sion to F- centers takes place. However, it is remar-

ed that also rather strong changes in the other inten-

S. DANNEFAER.

-

We made this measurement on sities take place. The intensity of the

s l ~ o r f i . ~ t

lifetime

KC] additively colored to about 1.5 x 10'' F/cn13. increased by approximately 40 x. This we interprete

The crystals were continuously irradiated with F- as positron annihilation in F - centers.