CHAPTER - IV

EFFECT OF PRE-IRRADIATION DEFORMATION ON

THERMOLCIMINESCENCE OF KCI :KBr MIXED CRYSTALS

4 -1 INTRODUCTION

The early investigations of colourability of deformedcrystals were motivated by the predictions of Seitz(1950,1954) that enhanced coloration should result from thepresence of vacancies introduced by deformation, or from theincrease in the density of dislocations whose jogs are sourcesof vacancies upon irradiation. These view points, particularlythe latter conditioned the experimental approach andinterpretation of results during most of the early work. Fromthe last three decades, however, ideas concerning the creationof lattice defects in alkali halides by ionizing radiationhave undergone drastic revision and the whole basis forinterpretation of results and designing experiments hasshifted to new ground. The traditional view of F-centreintroduction into alkali halides by X-rays and other ionizingradiation in the room temperature range, is that it occurs intwo stages. The first or early stage occurs more or lessrapidly with a small energy requirement per F-centre and

17approaching saturation for F-centre concentrations near 10-3 .cm . It has been regarded as arising from the population with

electrons of vacancies initially present or the radiolyticconversion of other defects (vacancy pairs, cation vacancies,larger c l u s t e r s ) i n t o F - c e n t r e s . It has been shown that theearly stage coloration can be suppressed by lowering thetemperature, indicating that the photochemical processresponsible may have an overall thermal activation. Also

iMi.t'lul examination (Alvarez-Rivas and Levy 1967, Levy et al 1 9 7 0 ) indicates that several separate processes may be participating. Results suggest that the so-called First stage is composed of a number of overlapping processes all of which contribute to easy F-centre formation.lt is generally agreed that prior deformation substantially enhances the early stage in most cases.

The second or late stage coloration is characterized bya much slower F-centre introduction rate which is initiallylinear and tends to saturate at F-centre concentrations in

18 - 3excess of 10 cm . The actual saturation value is sensitively dependent upon specimen origin, temperature of irradiation and radiation rate or intensity. For obvious reasons the second stage has been associated with the bulk or intrinsic defect creation proces and the effects of temperature, dose rate, etc. were considered to enter through their influence on the back-reaction or defect- recombination processes which eventually must determine the saturation behaviour. According to the Seitz model (1954), which requires dislocation jogs to be the source of radiation defects, deformaion would be expected to enhance the yield of colour centres in the second stage. In this model the energy for defect creation is supplied by the non-radiative recombination of electrons and holes at chargd dislocation jogs. The energy released is comparable to the band gap (6-8 eV) . Seitz also envisaged mobile excitons being captured and recombining at

dislocations which would deliver about the same energy to the site. However, it is now known that the exciton in these structures has a low mobility and is therefore not a candidate for transporting energy to these special sites (Kabler 1964). The release of such a large amount of energy at a particular site in the lattice would amount to a thermal spike or hot spot and the excess energy would be drained away by several hundred phonos. The half-jog is a natural source of vacancies since 'evaporation' of a cation vacancy from a negative jog leaves a positive charge of +1/2 e and the jog is 'cocked' for the creation of a negative ion vacancy. A full jog presumably would be capable of yielding vacancy pairs by a similar 'evaporation' process promoted by the energy supplied by recombination. The main difficulty of this model is that the radiation defects are created in the immediate vicinity of jogged dislocations and must depend upon diffusion to spread uniformly through the crystal. The yield of F-centres (number created per unit absorbed energy) is quite high, even near liquid helium temperature where diffusion of vacancies is severely restricted, and this was recognized as a serious flaw for the model (Seitz 1954). Nevertheless, the jogs were regarded as a well high inexhaustible source of vacancies and hence as the origin of late stage coloration.

Although the exact details of the displacement mechanism are still not clear (Sonder and Sibley 1972) .

Although interest in enhanced F-centre coloration in deformed alkali halide crystals goes back to Przibram (1927), the first systematic study was made by Gordon and Nowick in 1956 on NaCl. This work was motivated on the one hand by Seitz (1950) who predicted that plastic deformation creates Schottky pairs and on the other hand by Seitz (1954) proposal that radiolytic creation of defects occurs at dislocation jogs. Gordon and Nowick (1956) found that compressive strain of several percent enhanced the early stage of the F-centre growth curve upon X-irradiation at room temperature but that the late stage was scarcely affected. Since approximately the same amount of enhancement could be introduced by merely heating and slowly cooling the undeformed crystals, they attributed the early stage enhancement to the break up and dissolutin of small precipitates of impurity, already known to have a strong influence on the early stage coloration, rather than to vacancies created by dislocation interaction. Subsequently, Nowick (1958) extended the study by irradiating with ^ C o y-rays instead of X-rays which had the advantage of uniformly colouring thicker crystals. He found that in addition to early stage enhancement there was a decided increase in the rate of coloration in the later stages as well. He took this as confirmation of Seitz's model for defect creation at dislocation jogs.

In connection with a careful study of the kinetics of X-ray (140 kVp) creation of F-centres at room temperature, Mitchell et al (1961) irradiated KC1 crystals of nominal purity and subjected to a compressive strain up to 3%. They found substantial early stage enhancement but observed a definite decrease in the rate of colour centre generation at later stages.In attempting to rationalize their results with Nowick's, they pointed out that what he considered to be the late stage or intrinsic range could be treated equally well as a saturating early stage component with a logner time constant than normally observed for undeformed crystals. Clearly the decrease in F-centre yield in the late stage cannot fit into the framework of the Seitz model. On the other hand, they condcluded that the enhanced and extended early stage coloration was in good accord with the earlier proposal (Seitz 1950) that dislocation interactions introduced vacanceis upon plastic deformation. Bauer and Gordon (1962), however, pointed out that isolated negative ion vacanceis could hardly be involved since the deformation enhanced coloration for ~ 3% strain on both NaCl and KC1 was almost completely suppressed

the irradiation was performed at 200 K or below. This means that thermal activation is required for the enhancement whilst none is expected for electron capture by an anion vacancy. This strong temperature dependence of the major component of deformation enhanced coloration has been subsequently confirmed by Royce and Smoluchowski (1964) and Sibley and Russell (1967). Evidently the major part of the enhancement

occurs by early stage processes, i.e. thermally activated processes involving either interstitial capture or decomposition of debris components upon electron or holecapture.

Upon r r ' s u m p t i o n o l t J 1 < * i t i , i< 1 i , i L L o n t 11 r • r : h , j r ar . ’ L r j r . ' i n L i c

rapid rise and approach to saturation of early stage coloration is sharply visible (Agullo-Lopez and Levy 1963 ; Royce and Smoluchowski 1964, Sibley 1964, Sibley et al 1965). The presumption was that prior saturation of early stage processes due to imperfections initially present could be accomplished in this way so that they would be clearly separated from the deformation enhanced coloration. However, such a separation can never be complete since room temperature deformtion bleaches F- centres rather efficiently. The bleaching effect is clearly evident in the data of Royce and Smoluchowski (1964) since the F-centre density immediately after deformation is distinctly smaller than before deformation. However, the excess F-centres after deformation substantially exceeded any such bleaching effect. Royce and Smoluchowski (1964) examined the effect of deformation temperature upon deformation enhanced coloration near room temperature and found that although the rate of approach to saturation was much more rapid after ~ 3% strain at liquid nitrogen temperature than after the same deformation at room temperature, the saturation enhancement was essentially the same in each case. Now the flow stress is substantially higher

at liquid nitrogen temperature than at room temperature and this observation led them to the conclusion that although the details of debris configuration may depend upon deformation temperature, the amount of debris that can potentially contribute to early stage coloration is essentially independent of temperature and depends only upon strain.

Chang's (1965) experience is in sharp contrast to this behaviour. He has found that the extent of deformation enhanced coloration for room temperature 60Co gamma ray irradiation increases with decreasing deformation temperature. Moreover, the work of Sibley et al (1965) and Sibley and Russell (1967a,b) indicates that the deformation enhanced coloration is sharply amplified by the presence of impurity, particularly Ca^+ and Sr^+, which is known substantially to enhance the flow stress of the crystals. In both of these cases, the higher initial stress appears to result in a greater enhancement of coloration. The implication is that the rate of production of debris is proportional to th stress producing the plastic flow. Such a veiw is also supported by the studies of Souster and Pratt (1971) who have used specimens with different ratios of height to breadth as well as { m } cut crystals to cause alterations in the form of stress - strain curves. They have found that the enhanced F-centre production with compressive strain parallels the stress-strain curves, the effect being greater for the higher stress.

From this survey it is clear that, as the precision ofthe measurements improved and the techniques for separatingthe contribution to the coloration from deformation from otherradiolytic processes were devised, the evidence for an effectupon the late stage was refined. The one definite effect inthe late stage is a decrease rather than an increase inF-centre yeild (Mitchell et al 1961). This is presumablyassociated with electron trapping by debris which promotes theback reaction between vacancies and interstitials. Thechanging view of the origin of late stage F-centre coloration,which no longer favoured the Seitz dislocation mechanism, castdoubt on the possibility that the late stage could be enhancedby plastic deformation. Often the accepted model may render acertain blindness in th interpretation of experimental data;thus, it would be helpful to have a careful empirical fit tothe coloration curves of the deformed specimens which clearlyeither affirms or rules out a late stage contribution.Alvarez-Rivas and Levy (1967) were able to fit colorationcurves for NaCl obtained after successive exposures to X-rays.They found that prior to deformation the F-centre growthcurves could be described by an expression with one linearterm and three exponentially saturating terms in the following way ;

3Np = a <p + Y. /3. (1 - exp 3N0) ... (4.1)

i = lwhere a, ^ ancj y are constant coefficients and <p is the

accumulated X-ray exposure. Upon deformation they find the linear component unaffected, i.e. a is not changed, but that the slowest saturating component increases in amplitude proportionally to strain after a threshold compression of ~ 1 %. This information suggests that only the early stage coloration in NaCl is affected. However, recent studies on KC1 exposed to ^°Co y-rays, by Levy et al (1971), in which the absorption in the F-band was measured during irradiation and does not reflect the electronic redistribution which occurs after the crystals are removed from the radiation field, give quite different results. Again before deformation the F-centre growth curves could be fitted by a linear term and three saturating terms as in eq.(4.2.1). After deformation exceeds a threshold strain of 0.25%, the fitting could be accomplished by a linear term and only two saturating terms. The linear coefficient increased by as much as a factor of 8 after a 7.8% compressive strain. The two exponentially saturating terms were larger in amplitude but slower in approach to their maximum values than the corresponding pre-irradiation terms. After removal from the y-ray flux the F-centre density decays exponentially towards a stable value. The stable value is proportional to the square root of the strain.

In view of this marked difference in curve fitting in the case of NaCl and KC1, one hardly feels safe in excluding the possibility of enhancement throughout the late stage coloration. However, it should be remembered that the results

on KC1 were obtained during irradiation and, although the increase was linear with strain, the stable component increased with the square root of the strain. This together with the general increase in the time constants for achieving saturation in the various early stage components suggests that one of the saturating terms may have such a long time constant that it may not be separable from the linear term. The rather small radiation intensity would make the resolution of a slowly saturating component even more difficult. Indeed, the matter of dose rate itself seems to be an importnat experimental parameter (Sibley and Russell 1967) . In view of the weight of evidence it has been adopted that deformation influences the stable component of F-centre coloration only through a rapidly saturating component or set of components, which is to say only in the early stage coloration.

The questions then arise : (a) Why the great variety of responses ? and (b) What is the root cause of enhancement ? The work of Sibley et al (1965) and Sibley and Russell (1967a,b) first established that the amount of the enhancement !s sensitive to the presence of impurity. Indeed, Sibley et al (196b) found that in highly purified KC1 crystals there was scarcely any F-centre enhancement after 2% deformation followed by ^ C o gamma ray irradiation at ~ 35°C whilst there was a substantial enhancement in crystals containing Ca2+ and a smaller but pronounced enhancement in Pb2+ doped crystals given the same treatment. The absence of F-band enhancement in

the pure specimens should not be construed as indicating no effect since there was a very definite increase in M centre density relative to F centres as a result of deformation. Therefore, there was a modest increase in total anion vacancy content in nominally pure deformed crystals. Deformation had no detectable effect upon the M to F centre ratio in doped crystals. Subsequent experiments by Sibley and Russell (1967a,b) confirmed these observations and in addition showed that the magnitude of the effect in Ca2+ doped crystals was quite sensitive to the dose rate (rate of energy deposition) of the ionizing radiation, the increase in F-centre density for a given deformation being greater for lower irradiation intensities.

The present chapter reports the effect ofpre-irradiation deformation on the TL of pure KC1, pure KBr and KC1:KBr mixed crystals. Since TL intensity is directly related to the density of F-centres in crystals, it is anticipated that the effect of pre-irradiation deformation on the TL of crystals may throw light on the effect of pre-irradiation deformation on the coloration of crystals.

4 - 2 EXPERIMENTAL

For the measurements of TL and TL spectra, the techniques described in Chapter III were used. The crystals of pure KC1, pure KBr and KCl.-KBr mixed crystals were grown from slow cooling of their melt method described in Chapter II. For the

TL measurements, the crystals of small size 4 X 2 X 2 mm werecleaived from the central lower part of a large crystal . Beforeirradiation, the crystals were annealed at 400°C for 4 hours,then the crystals were deformed for different level of strain(e) along their (100) direction by using a tensile tester

_2(model 1.3 DKMI, Ahemdabad) where the strain rate was 3 X 10cm/sec. Finally, the y-irradiation was carried out for

6 0different, doses at room temperature using Co source.

In the present investigation, the TL glow curve of pre-deformed y-irradiated pure KC1, pure KBr and KCl:KBr mixed crystals were recorded. For measuring the effect of pre-irradiation deformation on the TL intensity of pure KC1, pure KBr and KCl:KBr mixed crystals, the crystals were deformed prior to irradiation by different levels of strain (e) and then exposed to y-irradiation for different y-dose and finally the TL glow curves were recorded. The TL spectra of pre-irradiation deformed pure KC1, pure KBr and KCl-.KBr [50:50] mixed crystals is also recorded.

For all the observations, a set of three or four crystals was uriod for oacli ntudy.

4 * 3 RESULTS

Fig.4.1(a,b,c) represents the TL glow curve of pure KC1, Pure KBr and KC1:KBr [50:50] mixed crystals having different value of pre-irradiation deformation of the crystals. It is

seen that no significant change occurs in the temperature corresponding to the peak of glow curves due to the pre-irradiation deformation of the crystals. However, the intensity of the first and second peaks of the TL glow curve increases with the pre-irradiation deformation of the crystals.

Fig. 4.2 (a, b, c) shows that the plot of log I versus strain (e) for pure KC1, pure KBr and KCl:KBr [50:50] mixed crystals for different values of 7 -dose. The plot of log I ̂

versus strain (e) are straight lines with positive slopes. This fact shows that the dependence of I on strain (c) should follow the relation,

I m l “ lml e x P ( “ ' c ) • • • ( 4 ' 2 )

where are the values of the intensity of first peak forundeformed crystals, and I represents the value of the TL intensity of first peak for the crystals deformed by strain (c) prior to irradiation

Fig . 4 . 3 (a, b, c) represents the plot of log 1 ^ versus strain (c) for pure KC1, pure KBr and KCl:KBr [50:50] mixed crystals for different values of ^r-dose. The plot of log I 2

versus strain (e) are straight lines with positive slopes. This fact shows that the dependence of I 2 on strain (c) should follow the relation,

TL

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T E M P E R A T U R E ( *C) -------->

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T E M P E R A T U R E ( * C ) — *

TL

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S T R A I N ( 6 p )

p ig **-2 ( a ) _ P L O T O F Log I m , V E R S U S S T R A I N ( £ ) FOR KC1 C R Y S T A L S , FOR D I F F E R E N T f - D O S E S .

TL

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S T R A I N (G ) — ►

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• — — • 135 Gy□ — — □ 3 1 5 Gyo — — o 6 2 5 Gy& — — A 1 3 50 Gy

1 8 0 0 Gy

101 L----------— ___________________i---------------------------------1--------------------------------------1--------------------------- ---------0-01+ 0 0 8 0 12 0 . 16

S T R A I N ( G )

Pig U- 2 ( c )_ P LOT OF L o g l m j V E R S U S S T R A I N ( G ) F O R KCl : K B r ( 5 0 : 5 0 )

M I X E D C R Y S T A L S , FOR D I F F E R E N T / - D O S E S .

S T R A I N ( E p ) ------ *

F i g - U .3 ( a ) - P L O T O F L o g I m 2 V E R S U S S T R A I N ( E p ) FOR K C l C R Y S T A L S , FOR D I F F E R E N T y ~ D O S E S .

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Fig. u . 3 ( b ) . PLOT OF L o g J - V E R S U S S T R A I N f 6 ) FOR K B r C R Y S T A L S

FOR D I F F E R E N T / - D O S E S .

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S T R A I N (G ) — ►

F > 9 - ^ 3 ( c L P L O T OF L o g I m V E R S U S S T R A I N ( € I FOR KCl : K B r ( 5 0 : 5 0 )

M I X E D C R Y S T A L S , FOR D I F P E R E N T / - D O S E S .

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W A V E L E N G T H ( n m ) -------►

5 ( a L T L S P E C T R A OF P U R E K C l C R Y S T A L S , D E F O R M E D P R I O R TO I R R A D I A T I O N .

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W A V E L E N G T H ( n m ) ------►

Fi g- U-5 ( b ) _ TL S P E C T R A OF PURE K B r C R Y S T A L S D E F O R M E D PRI OR TO I R R A D I A T I O N .

TL

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W A V E L E N G T H ( n m ) — ►

F i g ^ - 5 ( c ) _ T L S P E C T R A OF KCl ! K B r ( 5 0 : 5 0 ) M I X E D C R Y S T A L S , D E F O R M E D P R I O R TO I R R A D I A T I O N .

uncle1! 01 m< ni c ryst a 1 , «mcl I ropr<uitn the? va lut' ut the TL

;i.nt on::.i 1 y of second peak for the crystals deformed by strain

(c) prior to irradiation

The value of the slope of log I versus strain (c) and log I,, versus strain (e) can be calculated from the Figs.4.2-4.3 .

It is clear from the Fig.4.4 that as the strain rate of pre-deformation increases, the total TL intensity of pure KC1, pure KBr and KCl:KBr [50:50] mixed crystals also increases.

It is also clear from the Fig.4.5(a,b,c) that there is no significant change in the TL spectra of pure KC1, pure KBr and KC1:KBr mixed crystals due to the pre-irradiation deformation of the crystals.

4 - 4 DISCUSSION

4 -4 -1 EFFECT OF PRE-IRRADIATION DEFORMATION ON THE COLORATION OF CRYSTALS

Deformation results in the production of vacancies and other debris due to dislocation interactions. The defects created by dislocations may enhance the F-centre yield by trapping interstitials, i.e., by decreasing the extent of back reaction (Maratmendes and Coming 1977, Townsend 1979). In the crystals, where the extent of back reaction increases with deformation, the F-centre yield may decreases with the deformation.

Excluding flux-dependent effects one may now predict how an absorption band will develop as a function of irradiation dose. For intrinsic defects such as vacancies there may already be a number of sites in the crystal which were a consequence of cooling the crystal from the melt. This concentration can be calculated from a thermodynamic knowledge of the formation energy of the defect and the subsequent heat treatment of the sample. Irradiation of quite low energy which releases free charges allows this defect to produce an absorption band which saturates when the fixed number of sites comes to equilibrium with the other trapping processes (Agullo-Lopez 1967, Townsend and Kelly 1973). When a crystal is exposed to high energy radiation like X-rays or y-rays, the generation g of F-centres will depend on the absorptionCcoefficient of the crystal and the intensity of radiation. If gp is the rate of generation of F-centres and a is the recombination coefficient, then we have

where n^ is the density of F-centres at any time t of irradiation. Integrating the above equation and taking np = 0at t = o, we get

If the crystal is exposed for a shorter duration tc, then c*p t < 1 and we may write

dnF ... (4.4)

nF 1 - exp (- (*F t) . . . (4.5)

1 - 1 + a t:F c

or np = gp tc ... (4.6)

It has been found that g_ and a„ are affectedr rsignificantly by the deformation of crystals. In the first stage coloration gp increases linearly with the deformation of the crystals (Clark and Crawford 1973) and it may be expressed as

9F = 9F0 ( 1 + ^F c) ... (4.7)

where i3_, is a constant and q_~ is the value of g_ for r -̂ FO Fundeforiued crystal.

Trias, if a crystal is exposed to radiation for shorter duration of time t , then from equation (4.6) and (4.7), wehave

nF = 9F0 (1 + PF e ) tc ... (4.8)

The value of gpQ and /3p may be different for different types of electron trapping centres in a crystal.

4 - 4 - 2 EFFECT OF PRE-IRRADIATION DEFORMATION ON THE PROBABILITY v OF RADIATIVE RECOMBINATION OF THERMO-STIMULATED ELECTRONS

The thermally stimulated electrons from the filled traps moving in conduction band may either recombine with the hole centres, may fall into deep traps or may be captured by other compatible traps. Thus, the rate equation may be written as

= g - o'. N. V An - cr, N„ V An - <x-, N,, V An 3e 1 1 1 2 3 3dt... (4.9)

where g is the rate of generation of electrons in theconduction band due to the thermal stimulation. An is thechange in number of electrons in the conduction band at anytime t and v is the thermal velocity of electrons in the conduction band of the crystal.

The integration of equation (3.9) gives

geAn = ----------------y | 1 - expcr_ N„ +cr^N^+cr_,N < J 1 ^ 1 + ( T 2 N 2 + < J 3 ^ 3 V t

.. (4.10)

In equilibrium, we have

g~An°lNl+<r2N2+03N3 V

or g, ffiNl + <J2N2 + a3N3 V An 4.11)

The recombination of electrons with centres will give rise to the light emission, however, the recombination of electrons with deep traps N2 or other competing traps N3 will not give the light emission. Thus, the probability of electron-hole recombination may be given by

o* N1 V Aneh

P ... (4.12)o eh

Since the density of deep traps N2 is negligible as compare.I to N1, equation (4.12) may be written as

plastic deformation of alkali halide crystals whose density increases more or less linearly with the deformation of crystals (Ueta and Kamzig 1955), thus, the dependence of the number N£ of newly created electron traps on the strain may beexpressed as

N = M e e

where M is the multiplication factor.

Out of the deformation generated electron traps, afunction A of them may have electron capture-probability greater than that of dislocations and thus the number of deformation generated compatible electron traps N3 may be given by

\An ... (4.13)

is known that the electron-traps are created by the

N A M c ... (4.14)

From equations (4.13) and (4.14), Pgh may be written as

eh* 1 Ni

o^lN^ + A M e

or Peh1 +

cr A M £

eh ( 1 + ■y c) (4.15)

wher*cr A M

. . . (4.16)

As deformation bleaching is less (Hayashiuchi et al 199 0} N may be assumed to be a constant and 7 may be considered independent of the deformation.

For y c < 1, the above equation may be expressed asPeh = exp(- y £) . . . (4 .17

If 7j is the probability of electron hole radiative recombination, 77 of the thermally stimulated electrons in deformed crystal may be given by

V = 7} P ,o eh 7? exp { - r e 4.18

with electrons and only (1 - <5) A M e will be available for the competition. Thus, the following equation 7j = tj exp(-,yc)/ the deformation dependence v may be written as

where

n = VQ exp(- y 1 e) ...(4.19)

0"3 A M (1 - 8)ai Ni

4 - 4 - 3 EFFECT OF PRE-IRRADIATION DEFORMATION ON THE INTENSITY OF THERMOLUMINESCENCE

;:'rom equation (1.11) (described in Chapter I), (4.8) and (4.13; , the deformation dependence of TL intensity may beexpreeeed as

Ie = B1 Tlo exP (" jl e) gF0 (1 + ^F e) tc

or I = I (1 + (3 „ e) exp (- r'c) ... (4.20)c o F

where li is a constant and I = B_ t? g t 1 o 1 o FO c

For y 1 e << 1, the above equation shows that the TL intensity should increase linearly with the pre-irradiation deformation of the crystals.

For the crystals in which the extent of back reaction increases with deformation, (3 is negative and equation (4.20)rroay be expressed as

I = I (1 - e) exp(- c) ... (4.21)c o F

For y 1 c << l, the above equation shows that the TL

intensity should decrease linearly with the pre-irradiation deformation of the crystals.

4 . 4 - 4 EFFECT OF PRE-IRRADIATION DEFORMATION ON THE INTENSITY OF DEFORMATION-GENERATED TL PEAK

Due to the dislocation-dislocation interaction or dislocation-defect interaction, a new type of electron centres may be produced during the deformation of alkali halide crystals (Ausin and Alvarez-Rivas 1973, Bradbury and Lilly 1976). The number of deformation-generated compatible electron traps is given by = A M c. In the case of pre-irradiation deformation, a fraction 5 of may get filled with electrons. Thus, the number of electron traps filled during irradiation is 6 A M e and, thereby, the number of vacant electron traps is (1 - 8) A M e .

When an irradiated crystal will be heated, the electrons will start detrapping from the shallow traps. The thermally-stimulated detrapped electrons may either recombine with the holes and give rise to luminescence or they may be captured by compatible vacant electron traps, . The fraction of total thermally-stimulated electrons captured by compatible

1 - exp(- k 1 e 4.22)

From equation (1.2.6) (described in Chapter I), the total number of electrons released from different types of traps up to the temperature T , at which the TL due to deformation-generated new electron-traps begins, may be givenby

n exp °i

r rTr c -- ST exp 1

k T dTT -o

n exp °2

„T(3

exp r _E2 ] dT[ k T Jo

... (4.23)

where Tq is the room temperature, and are trap-depth of first and second types of traps, and nQ and nQ are the total

number of electrons trapped in the traps having depth E^ and , respectively.

Since upto the temperature T , all the electron traps in the E and E0 traps may get detrapped and we may write

n^T n + n +c °1 °2

or n„T = Y nF c . o_l 1(4.24)

where n is the initial number of electrons filled in i-type °i

trap.

From equation (4.21) and (A.7A), the total number of thermally stimulated electrons captured by the vacant compatible electron traps may be given by

n, = y 1 c T n t $ o± (4.25)

From equation (4.8), nQ and nQ may be expressed as

n°1 9F01 (1 + f3F1 C) tc

and n°2 gF02 (1 + l3F1 C) lc

... (4.26)

... (4.27)

where gFQ and gFQ are the values of gpQ and (3̂ and (3p are1 2 ‘ ~ "1 " 2

the values of (3F for the traps having trap-depth En and E0,respectively.

If we consider a crystal having only two types of shallow traps, then from equations (4.25), (4.26) and (4.27),we get

n. T 1 e ( )

1-----

H 1 + PF C tc + gF rl > 2 1 + PF2Ej tc + •♦ '

or n, K 1 ef r ( -

9f/ tc 1 + PF. + PF, c >1 1 2J 1 1 2 - j

... (4.28)

The reformation generated compatible electron traps have captured 8 A M c electrons during the irradiation and theyfurther capture n stimulated electrons from the shallow

traps. Thus, the total number of electrons captured by the deformation generated compatible electron traps is given by

+ gF2 l t c 1 +rl 2

... (4.29)In the temperature range where the TL from deformation

generated new electron traps starts, may mostly be captured by the hole centres, and, therefore, we have

V = Vo ... (4.30)

From equations (1.11) (In Chapter I), (4.29) and (4.30),we get

= B 2 gF +gF 1 21 + +^F 1 2

... (4.31!where B i s a constant

Equation (4.31) may be simplified for the following twocases.

Case I ; For Lower Values of c

In this case,

(4.31) may be expressed as^F + ^F 1 2

c < 1, and thus equation

B2 ^o

or I> I 1 E O ... (4.32)

where = B2 VQ 8 A M c + K 1 gF + gF 1 2J

Equation (4.32) shows that for lower values of e, the intensity of deformation generated peak should increase linearly with the strain of crystals.

Case II : For Higher Values of ein this case, we have

gF + gF, vF + pf 1 2J, and + ^ F 1 2J

Thus, equation (4.31) may be written as

tx c = b 2 ” 0 e 9f„ + g] + ^ F 1

or 1 1 c ... (4.33)

where 1 1 1 = t) k 1 e o 2 o gF + gF 1 2 rl 2The above equation shows that for higher values of e,

the intensity of deformation generated TL peak should increase linearly with square of the strain of crystals.

Thus, it seems that if the extent of back reaction increases with pre-deformation of the crystals, then the density of colour centres increases with the pre-irradiation deformation of the crystals and consequently the intensity of TL peak I and II increases with the pre-irradiation deformation of the crystals.

The intensity of deformation-generated TL peak should increase with the pre-irradiation deformation of the crystals because of two reasons. First, due to increase in the number of deformation generated compatible traps filled with electrons, and second, due to the increase in the number of thermally-stimulated electrons transferred from low energy traps to the deformation-generated high energy traps.

Ausin and Alvarez-Rivas (1973) have reported that for low dose of y-irradiation, the pre-irradiation deformation-generated new TL peak occurs around 275-2 85°C. We have found that for high dose of y - irradiation, the deformation-generated new TL peak does not appear in the pre-deformed crystals, however, it appears in post-deformed crystals. This fact shows that the energy released, i.e., the thermal vibration produced near the new F-centre formed as a result of non-radiative decay of exciton causes the diffusion of trapped interstitials in the highly deformed regions of KC1 crystals. Therefore, the deformation-generated new TL peak does not appear. It will be of worth to note that the post-deformation-generated TL peak also disappears after a certain value of the storage time of the crystals.

In case of post-irradiation deformation, the new traps generated are filled during the process of deformation. Therefore, strong lattice vibration occurring during the non-radiative decay of excitons during the y-irradiation plays

no part in causing the diffusion of interstitials produced in the highly deformed regions of the crystals. However, the fading of the intensity of the post-irradiation deformation-generated TL peak with the storage time indicates that the interstitials trapped in the high deformed region of the crystals diffuse towards the less deformed regions because of the thermal vibration of the lattice.

A good agreement is found between the experimental and theoretical results related to the effect of pre-irradiation deformation on the TL of KCliKBr mixed crystals.