Latvian SSR Academy of Sciences

Institute of Physics

LAPI - 027

PREPRIHT

April 1981

D.E.Aboltin and V.S.Zirap

RECOMBIHATION OP RADIATION DEFECTS IN AEKALI HALIDE

CRYSTALS AT LOW TEMPERATURES

Submitted to the International Conference

"Defects in Insulating Crystals"

Riga, May 18-23, 1981

Salaspils 1981

535.31АФИ - 027

Аболтинь Д.Э., Зирап Е.Э.

РЕКОМБИНАЦИЯ РАДИАЦИОННЫХ ДЕФЕКТОВ В ЩЕЛОЧНО-ГАЛОИДНЫХ КРИСТАЛЛАХПРИ НИЗКИХ ТЕМПЕРАТУРАХ

Представлено на Меядународную конференцию"Дефекты в диэлектрических кристаллах" ,Р и г а , 18-23 мая 1 9 8 1 .

Институт физики АН Латвийской ССРРижский район, СаласпилеЗ а к а з № 105. Подписано к печати 2 8 . 0 4 . 8 1Редактор И.О.ЕкабсонеПечать Л.В.Комарова и. Л.Р,Розенпяам

Латвийский государственный университет им.П.Стучки, 1981.

RECOMBIHATIOS OP HADIATIO» DEFECTS Ш ALKALI HALIDE CRYSTALS AT LOWTEMPEHATUEES

D.E.Abol t in and V^E.Zirap

Institute of Solid State Physics, Latvian State University,

8 Eengaraga Street, Riga 226063, USSR

I . I n t r o d u c t i o n

The accumulation of radiation defects (P centres and others) and their

thermal annealing in alkali halides usually takes place via two stage ionic-

electronic processes at room temperature and above [l-4] > as well as at

liquid helium temperature (LHeT) and above £>-I0]. The multistep thermal

annealing of F-type centres (P, F1, F,, e tc . ) is due to the thermally ac t i -

vated delocalization (and interaction with F-type centres) of cation vacan-

cies and probably other ionic defects at T>(200-300) К [1-4) , as well as I

centres and H centres Q?-I0, 15-18] at T>4.2 K.

At LHeT the processes of I and H centres delocalization and recombina-

tion withcCt P and F* centres are predominant, which leads to a release of

stored energy in the form of heat [9, 10] or luminescence [5-8, II-14] , at

that conduction electrons (e~) ars freed [5, 6, 8] . The recombination of

these electrons with V,, centres excites the luminescence of self-trapped

excitons (STE) during correlated annealing of triplet defect systems, such

as: {l...F...VK}-, {l...F'...Vg}-and { H . . . F ' . . . V K } - in crystals of KBr, KC1

and RbBr [6-8] and probably others.

In the case of KBr an attempt was made [13, 14] to explain the prin-

ciple peak of thermally stimulated luminescence (TSL) at T =29.5 К as в" and

ЗГ- emission of the STE [13] , or later [34] - only a part of this TSL peak

was interpreted as the 5T-e miss ion of the STE, which might have created as a

result of direct, H and F centre recombination. However, i t was shown [6, 8]

that a l l the peaks of TSL from 4.6 to 6.0 К in KBr are predominantly associa-

ted with the STE emission, at that the STE ars formed as a result of free

electron recombination with Vg centres. Just therefore the interpretation of

the TSL peak at. 29.5 К as the STE luminescence excited via direct F and H

centre recombination turns out incorrect [13] or at least doubtful [14] •

This work is aimed at complex investigation of the ionic-electronic

processes of the charge transport and the release of stored energy in the

form of luminescence when the annealing of colour centres (I,ОС, F, F 1 , V»

and H) in KBr, Bad and Sal X-irradiated at LBeT takes place. The mechanisms

of TSL and electron tunneling afterglow (ТА) in Had and Hal crystals X-

irradiated at LHeT are studied. In case of KBr data are discussed which lead

to the conclusion that the radiative electron tunneling "recharge" in the

defect pair { F . . . H } with a subsequent formation of a charged pair{0C...1}

is unlikely. I t is shown as well thst the formation of STE and their lumi-

nescence.as a result of the H and F centres direct recombination in NaCl,

Hal, KC1 and KBr could be noneffoctive. I t is observed the effect of the

Frenkel defect production efficiency increase if during the I-irradiation

2at 4.6 К an external DC voltage is applied to the sample of O r . L photo-

electric current ia the region of the ОС absorption band in K3r crystal X-

irradiated at 4,6 К is observed; the photocurrent spectrum correlates with

the cc absorption band: a photoionization of the oc-excitons occurs. The

DBchaniBms of the anion ffrenkel defect generation are discussed.

2. E x p e r i m e n t a l

A complex of thermally stimulated (IS) methods was used for the inves-

tigation of radiation defects and their interaction during TS annealing of

KBr crystal X-irradiated at 4.6 K: methods of thermally stimulated current

(TSC), depolarization current (TSDC), current of charge diffusion under the

gradient of radiation defect concentration (TSCD), desorption of gas from the

'sample as a result of halogen (H centres) release (TSDG), method of thermal

bleaching (ТВ) of induced absorption bands, as well as methods of recording

the optical absorption spectra, the spectra of TSL and the spectra and

kinetics of electron tunneling afterglow (ТА)._ p

Single-crystal specimens of Harshaw KBr having a 1.0 x ±,5 en cross

section and a thickness of O.I cm were coated with seaitransparent golden

electrodes on the largest faces. A guard ring on the smallest faces was made,

Guch в specimen was mounted in a liquid-helium cryostat "Liodel LT-3-1IO Heli-

Tran Liquid Helium Transfer Refrigerator" providing a vacuum about 1.3 x IC""-5

Pa. The stabilization or control (warm-up or cool-down) of temperature froc

4.6 К to 300 К was carried out by a change of helium flow through the cryos-

t a t .

The cryostat was equipped with a pair of saphire windows for the measur-

ing of optical absorption spectra and ТВ curves, by using a Cary 14R spectro-

photometer, and with an aluminium window-for X-irradiation, by using Ио-

target "OSG-50 X-ray tube" operating at 50 kV and 20 mA.

For simultaneous ( with ТВ curves ) electrical measurements of TSC (or

TSDC, or TSCD) a Teflon-insulated leads were made in the cryostat, A DC vol-

tage of U=300 V was applied to the electrodes of sample: (a) during X-irradia-

tion at 4.6 К when KBr crystal was e lectr ical ly polarized for a subsequent

measuring of ISDC (with U=0 V), or (b) during a linear rise of temperature

(after X-irradiation at 4,6 K) when the TSC curves were measured. Л vibrating

reed electrometer Model 31 was used for these current measurements. Such a

system provides a synchronous registration by recorders: (a) one of the ТВ

curves at the peak wavelengh of a selective induced absorption band (I, P,

"K-VK", H,, H1, V.), (b) the curve of TSC or TSDC, (c) the change of the

pressure in the sample chamber, i . e . the curve of TSDG, and (d) temperature

(T) of the sample - during linear warm-up with the rate about 0,03 K/s (in

the range 4,6 - 32 K) or 0.06 K/s (in the range 32 - 290 K).

The optical absorption spectra in the region of wavelengh 200 - 65 0 nm

were recorded at T = 4.6 К - before X-irradiation, after X-irradiation at

4.6 К and after subsequent linear heating of KBr to Тд- = 32 K, T2 = 60-62 К

and T̂ = 290 K. Such a procedure provided a high sensitivity and resolution

of a l l the used methods (TO, TSC, TSDC, TSCD, TSDG) for the same KBr sample.

3The measurements were carried out for the series of X-irradiation time - I h,

2 h, 4 h and 7 h 15 min.

The experimental technique for the luminescence measurements in eyngle-

crystal samples of IfeCl and Hal is given in the paper [35| .

3 . E x p e r i m e n t a l R e s u l t s f o r K B r C r y s t a l

X - i r r a d i a t e d a t L H e ' f a n d D i s c u s s i o n

3.1. Recombination of defects at T=4.6 - 32 К in KBr controlled by I-centre

delocalization

The X-irradiation of KBr at 4.6 К for 2 hours induces P t

nH-VK"t I and

«bands, as well as a small H! band (Pig. I ) . After a warm-up to 32.6 К

(curve £ ) a strong decrease of I and OCbands and a partial decrease offIE-VK" and P bands is observed while the small increase of H1 band appears.

The difference ber.veen the spectra "I" and "2" in Pig. 1 is shown as a curve

I in Pig. 2. I t can be seen that in the range of T = 4.6 -32.G К where the

I band annealing is practically completed (see Pig. 3) £. large decrease of

the complementary band oc takes place (in accordance with the data [lf;,Io]5,

but at the звте time an essential bleaching of other bands - P and "H-V.." -a

which traditionally are related to P and H centres, is recorded simultan-

eously.

I t must be noted that as well in the paper of h.Balaer [17] at 1 =

= 6-30 K, where about £0 % of I band anneals and a correlated annealinr of

the latt ice constant and volume of KBr crystal (by about 7*3-60 >i) takec

plb.ce, a partial (at least - two step) annealing of P band (by#v25 /o) arid

"H" band (byr-40 %) is observed (see Pig. 6 in [17] ) . However, i t is not

taken into account [17] that a lot of VK centres [5, S, 18, 19] are

produced by irradiation of KBr at LHeT. Because of the H band and V.r band

overlapping (Pig. I) and because of the height of the la t ter is about 25 -

30 % (from data in Pig. I and 2) or 30 % [19] relative to the total "H-V-."

band, some corrections in earlier works [10, 13-17, 20] probably are needed.

It is expected tha^ the presence of the V.. centres in KBr at Life I must be

taken into account in the analysis and calculations of the fractional volume

expansion шг Frenkel pair [15-17] and the stored energy per Prenkel pair

[Ю].

It is of utmost importance to take into account the accumulation of the

V̂ centres during irradiation of alkali halides at 4.2 К (as well as their

additional creation under the subsequent warm-up via localization of an К

centre in an oc centre [6-8] ) when the spectra of TSL in pure crystals ars

analysed according to STE luminescence mechanism, because the "У„ centers are

the most effective radiative recombination centers for free electrons (in

TSL) or for electrons of P and P1 centres (in tunneling afterglow) [b, 8 ] .

Otherwise misinterpretation of TSL mechanism at low temperatures may arise

[ 1 4 ] .

Attention must be paid at the annealing of the "H-VV," bands in the range

of T = 4.6 - 32.6 К (Pig. 2, curve I ) and T = 32.6 - 62.5 К (curve 2 ).

in the first range, where predominantly the V£ centres auneals because of

delocalization and interaction of the I centres with V̂ or F centers, the

maximum of the difference spectrum "I" is shifted relative to spectrum "2"

and corresponds with the VK band. In the second range where a delocalization

and recombination of the H centres with P centres predominates the diffe-

rence optical density spectrum "2" relates to the H band.

Pour substages in the annealing of the oc band were detected already

in the works by H.Itoh et a l . [15, 16] . Ebey evidently correspond to the

annealing substages of the I , P, and "H" bands and the la t t ice constant and

volume of the crystal (according to the research of RcBalzer [17]). In our

preliminary research [б] about the annealing of I, P and "I{-VT.fl bands and a

correlated TSC and TSDC peaks the main attention is paid to the discussion

of the data of the le.st substage of the I band annealing in O r at Т„ =

27.5 К, which associates with a pronounced ТВ step not only for the ОС band

but also for the P and VTr bands and results in the largest TSC and TCL

peaks.

The analysis of our experimental results of the whole set of the I, F

and "H-V-p." thermal bleaching curves, TSC and TSDC curves for the same speci-

ren of KBr measured after different X-irradiation times ( I , 2, A, 1 hr) as

•veil as upon the applied electric field or without th-эя allows UE TO conc-

lude [8] that the delocalization of the I centres end the correlated

racombinational bleaching of the P and Vy bands has a nultisxage character.

All these substages of T3 resuli-s in the electronic cnarpe Transport реакс

(TSC, TSDC, TSCD).

To sua up the detailed sxudy of the whole complex of thermally stimu-

lated relaxation curves (e. g. Pig. I — Pig. 6), the data of the oc band T3

[15, IS], the release of the stored energy [9, 10] and the volume relaxation

results [l7] , i t is concluded that a whole set of different relaxation

characteristics has a multistage nature during the warm-up of КЗг crystal

X-irradiated at LHeT [8]. In the range of T = 4.ь К - 32 К depending on the

kind and Temperature of irradiation i t can be resolved down то ten substages

of relaxation which are controlled by a delocalization and diffusion of the

I centres of different spatial distribution and configuration relative го

their complementary recombination partners either in defect pairs -|l...CC_f,

or in defect t r ip le ts -TI...F. ..VK%. These are substages with the values of

Tm = 8, I I , 13.5, 15, 17, 19, 21.5, 23.5,26 and 27.5+0.5 К [8].

Consequently, the annealing of the radiation defects in KBr at T =

4.6 - 32 К has an ionic - electronic character: the diffusion controlled

recombination of the I centres with the P and P' centres leads to the

electron release and subsequently their recombination with the V,, centres

results in the luminescence of STE [6—8] • Concurrent, with these processes

a recombination in the "pairs" {l...cc}- and £l...VKj- takes place at the

same time. Such a defect interaction reactions are very likely:

{l...P...VK}-»»{e~...VK } + Q -*. h*SIS , (I)

Т2 , ( 2 )

{ l . . . ot}->Beg. + Q , (3)

, (4a)

(4b)

(4c)Therefore the recombination in the defect t r i p l e t s according to the reac-

tion (I) gives r ise to the STE luminescence. According to the reactions (4e.-4c) in more complicated systems of defects, besides the two-stage ionic—elec-tronic processes an important role can be played by the tnree-stage ionic-electronic processes. Simultaneously with the heat (Q) release anc volumechange in such annealing substages of a lkal i halides ws expect: (a) a forma-tion of additional new К or other "H-type" centres by the reection (4a), aiv-i(b) a rise of the "thermally stimulated tunneling afterglow" (TSTA) if th=radiative electron tunneling takes place between the nev/ly icrne- сзпггс:; (.'or F ) and the centres already existing ( P or Vj. centres, respectively) -according to the reactions (4b, 4 c ) . The l a t t e r , i . e . TS2A processes wer\- re-served in [6, 8, 35] and are analysed in details in th is paper, ~A .

The reaction (4a) which leads to the formation of additional K1 centresin KEr (Pig. 2, curve 1) seems to be predominant at the last ainealiii^ sub-stage of the so-called "noncorre la ted" I centres at T^=27.5 К where annealrabout 10-15 % of tne P band, 15-20 ъ of the "li-Vj." band and 30-35 % of t:.-i :band. At that depending on the X-irradiation tine a creation of the H' fir.-iprobably other centres is observed by about 10-100 >. This is in accordancewith the data of R. Balzer [17] where an increase of the K' band is otiEerv=aduring the warm-up from ь К to 30 К.

At low doses of л-irradiation such a pairs, t r i p l e t s and more coinple^ de-fect systems may be relatively compact and isolated subsystems of aoi_ or lesscorrelated defects. This is caused by small displacements of the chargedPrenkel defects at LHeT [15-20] . If we take into account tnat the electron no-b i l i t y in KBr at LHeT is very high (about l(f ctnVvs) [2l] and that the elec-tron capture crosd—section of the CK and P centres is large, then in the casewhen electron-hole pair is generated by X-rays near a Frenxei pair ^ . . .oC^or { F . . . H } the t r i p l e t s of defects -[l.. ,F...VjJ and {н. . .F 1 ...Vk} can be pro-duced:

According to [22] i t is expected that the probability of an electron cap-ture by a dipole or a charged pair, e .g . | l . . .ocj- depends strongly on thepair separation and the electron location (angular and linear) relative "tothat pair . Therefore the pairs with a smaller separation between ОС and Icentre are more screened and look like neutral, with a smaller cross—sectionfor a distant e lectron. Such pairs are more protected with regard to an elec-tron capture and must be accumulated predominantly during irradiation a t Lllel.

б

Thus the accumulation of the defect triplets {l...F...V,^ should predominate

by the interactions of the electron-hole pairs with the more distant pairs

{ i . . . * } . Indeed, as stated above the annealing of the defect triplets and

other complexes according to the reactions (I), (2) and (4) takes place main-

ly in the last stage at Tm=27.5 К (Pig. 1-6) which corresponds to the more

displaced Frenkel defects, so-called "noncorrelated Prenkel pairs" [5-B,

15-18].

It is shown [5-7, 9, 14, 16] that depending on the nature of the ioniza-

tion radiation, the dose and the temperature of irradiation the last substage

of the I centre annealing at about 26-29 К in КВт may be approximated as a

kinetics of the first [5, 9, 14] or second[5-7, 16] order. The pre-exponen-

tial factors in various papers have different values in very wide range de-

pending on the experimental technique and the irradiation conditions of KBr:

2.8XIO5 s" 1 - from TSL data [14] , (4.6-25)xIO7 s"1 - fron: ISC data [5] and

JOI0-IO1-3 s" 1 - from ОС band ТВ data [15] . The low values of this factor indi-

cate that the recombination takes place after prolonged I centre diffusion.

In the range of the I centre multistage annealing (10-30 K) in K3r the

thermal activation energy of the relaxation processes was estimated: with the

rise of temperature the energy increases from 0.01 eV to 0.07 eV. These values

are in agreement with those in [5, 6, 9-II, 14-16]. It seems likely that the

energy values for the "last" substage of the I centre annealing Bj=0.06±0.0I

eV should be attributed to a highly displaced I centres in defect triplets

etc.

The decrease of the activation energy in the range С07-0.01 eV is ex-

plained [5-3, 15, 16] by the rise of the interactions for the more close

charged Prenkel pairs. At first, that is the Coulomb interaction energy which

is about 0.01 eV if the distance in the pair is ten lattice constants. There-

fore, such charged defects may be. considered as correlated. It is believed as

well [l5, 16] that the interstitial migration is nonisotropic and depends on

the relative I and Л centre configuration in the pair. Secondly, the strain

fields around the defect pair or tr iplet are of great importance. In case of

the pair "{l»..cCj the fractional volume expansion per Prenkel pair seems to

be larger for more separated pairs [l6] : at 5 К the expansion value is 3*3r-

0.3 but at 16 К - 6.6+1.0. Thus the strain fields should lower the thermal

stability for more distant defects relative to close defects.

It should be mentioned that a l l the TSC and TSDC peaks are caused predo-

minantly by the electron release as a result of the secondary reactions (I)

and (2). A direct thermal ionization of shallow electron traps seems to be

negligible because a l l the current peaks correlate just with al l the substa-

ges of the main defect annealing. In addition to that we failed to "re-excite"

the TSC peaks by a photostimulation in the P band at 4.6 K. Practically the

P centre photoionization quantum yield in KBr at LHeT is zero and electrons

should not be freed and TEC "re-excited".

It is expected that during the "P-stimulation" of KBr at 4.6 К the elec-

tron tunneling "recharge" in photoexcited close defect pairs | F S . . . HJ-and

{ P * . . . H A } - takes place:

{P...H (or Нд)} + hvjr»•§*...H (H A ^-> • £ . . . ! (or I A ) } . (5)

The decrease of the absorption in the region of the P and "H-H." bands end

associated increase of the absorption in the region of the ОС and' " I - I , "

bands is indeed detected (Pig. 2, curve 3 ) after the "P-stimulation". This

is in accordance with similar results in Kflr at LNT [23, 24] but in that

case i t should predominate the f r e e electron capture by the d. centres

with a subsequent formation of I» centres.

The electron tunneling transitions in nonexcited pairs are not observed

in the TSL spectra of KBr [6, 8] obviously because the stored energy per

charged Prenkel pair is higher than per neutral one [lO] . The main radiative

tunneling transitions take place in the pairs ^ . . . V ^ a n d { F ' . . . V , , / [6, 8,

35] • A spontaneous fa l l of the "P—j"lf absorption after X-irradiation of KBr

at 4.6 K- (Pig. 5, curve la at 4.6 K) in dark (during half an hour from the

value of the optical density 2.55 to 2,35) seera:? to oe caused by the electron

tunneling from the P or P1 centres to V.r centres. The conclusive proof in the

case of a high electron irradiation density of KBr at LHeT is given by tiie

absorption technique in [13] • It is confirmed that the tunneling transitions

take place just in the pairs £F. ,.VT,j- but not in the pairs ^F.o.ilJ-, з-э-

cauce the simultaneous decrease of the F and VT. bands leads to a rise of the

ОС band while there is no rise of the I band.

3.2. Recombination of defects controlled by n centre de localization at 'J =

= 32 - 60 К in KBr

The second range of temperature in KBr is from 32 К to 60 К (Pig. 5)

where a delocalization of the H centres and their re combination witn the F,

P' and ОС centres occurs [6, 8, 10, 14, 17] in a few substapes. The analysis

of the whole complex of thermally stimulated relaxation curves (Fig. 1-6) -

the P and H band thermal bleaching curves, the TSC, TSDC, TSCL and TGDG

curves - , BE well as the data of the stored energy release [9, ю ] , the

annealing of the la t t ice parameter and the crystal volume [15-17] and the

data of thermal bleaching [10, 14, 17, 20] allows us to distinguish at leest

six relaxation substages with the T = 31, 34, 38, 43.5, 46.5 and 53+ C.5 L

[8].The appearance of large TSC or TSDC peaks in a l l substages of the H and

"P-F t n band bleaching is evidently caused by the following secondary pro-

cesses [5, 6, a ] :

{H. . .F ' . , .V K } - • {e~...VK}+Q -» h9 S I E . (6)

As a result the correlated peaks of the STS luminescence arise [b-8].

The value of the thermal activation energy, £тт, for the H centre anne-

aling and the accompanied processes ri3e versus temperature and depending on

the experimental method range a wide interval of Е„ = 90 - 166 meV [5-9, 20,

25]. Prom the "H-V~" band decay time dependence on temperature after electron

pulse irradiation of KBr Ueta [25] determined E = 0.09 ± 0.01 eV and at t r ibut-

ed i t with the H centre migration and annihilation with the P centres.

However, our data and the data of TSL [6], TSCf5] , ТВ [20] and the heat

release [9] lead to the conclusion that the free migration of the H centres

requires a higher energy than 0.09 eV, i . e . ER = 0.11 - 0.13 eV [6] ,0.12 t o . 0 1 eV [20] or 0.09 - 0.17 eV [ 9 ] . V7e took these values because theya p p a r e n t l y correspond to the main substages of the H and P band annealing inthe range T = 37 - 55 К (see Pig. 3-6 and [Г7, 20]) and the correlated stageof the H1 band r i s e [I7 f 20] , the peaks of the heat re lease [9, 10], thestrongest current peaks (Pig. 3, 4, 6) and the most in tens ive gas desorptionpeaks (Pig.4, curve 2")«

As i t i s seen in F i£ . 4 there e x i s t s г close r e l a t i o n s h i p among the Hband ТВ (curve 2 ) , the gas emission (curve 2"), the TSDC (curve 2 ' ) and theP band ТВ (Pig. 5, curve 2 a ) . The most intensive processes correspond to Thelarges t ТВ substage of the H and P bands a t T = 43.5 К which should bere la ted to the beginning of the H c e n t r e q u a s i - f r e em o t i o n with the a c t i v a t i o n e n e r g y i\, = С ±2 t C. 01 eVTo, 2 0 ] . Besides the annealing goes on by the second order react ion [20] .A se t of the gas emission end TPDC peaks a r i s e s a t a l l the subsequent sub-stages of "the "K (375 nm)" and V. band IB in the th i rd re laxa t ion region а г7 = 60-260 К (Pig. 4 , curves 3, 3 1 , 3 " , 4 ) .

I t follows that the value В., = 0.09 eV [25] nay hardly ое- a t t r i b u t e d toa r i g h t migration energy of the f r e e H centre, as i t i s usually be l iev-ed. This value should be re la ted r a t h e r to the more or l e s s correlated К andF centres in p a i r s . I t i s possible t h a t in [25] because of a very high e l e c -tron i r r a d i a t i o n dens i ty the i n t e r a c t i o n between the p a i r s {l...cCj- and{e~...VK/ goes on and defect t r i p l e t s {_!.. .P...V.,_f are formed as wellduring the pulse e x c i t a t i o n . Indeed, we should s t r e s s the following p e c u l i -a r i t i e s in the data [25] : (a) a t T<290 К a V̂ band with a decay time ^ 1 , 3jfs i s resolved from the complex "H-Vy-V " band, (b) the Vv band decays bythe f i r s t order k i n e t i c s (obviously, a recombination of close defects in thep a i r s or t r i p l e t s takes p l a c e ) , (c) the determined in [25] value £..= 0,09 eV(Pig. I I , curve 'Ъ") for the absorption decay a t 3B5 nm ("H-V.," band) may bea t t r i b u t e d e i t h e r to the H centres or to the V,. c e n t r e s . Consequently, weare not sure that the value 0.09 eV i s the r i g h t one j u s t for the f r e emigration of the К c e n t r e s .

I t i s l i k e l y t h a t in the i n t e r p r e t a t i o n [25] of the "H-V,." and P banddecay k ine t ic s j u s t the I c e n t r e diffusion and subsequent recombi-nat ion in the defect t r i p l e t s •{_I...P...'V\rj should be taken into accountaccording to the r e a c t i o n s (I) and ( 4 ) . Because of the displacements of tneI and ОС centres a t higher temperatures are expected to be la rger than a tLKeT i t i s possible t h a t the defect t r i p l e t s with high, ef f iciency have beenformed in the ШТ-RT region in the experiments of [25] as wel l . So, af r a c t i o n of the I centres a t 290 К can behave l ike noncorrelated and indeedfree with the migration energy E-j->0.07 eV, possibly close to 0.09 eV. I t i sconsidered in the above mentioned conception that the r a d i a t i o n ( ionis ingr a d i a t i o n or l i g h t in the exci tonic band) in KBr primary can generatethrough the exc i tonic decay the I and (X centres as well [6, 7, 3 3 ] . Thisproblem i s discussed in the § 3«4.

3.3. The electric field influence upon trie colour centre accumulation at

LHsT

The X-irradiation of KBr at LHeT upon the application of DC field of

3 kV/cm on the sample electrodes causes a strong increase (by 10% or more )

of the I, cc , H, P and H1 centre concentrations (Pig. 7, curves I ' , 2 ' ;

Pig.8, curves I ' , 2 ' , 31) relative to thoee in the case of the absence of

the field during irradiation (Pig. 7, curves 1, 2', Pig. 8, curves I, 2, 3).

Only for the II centres this field induced effect lias a negative sign at one

hour irradiation (Pig.8).

Similar effects are observed at room temperature and higher in the

works [27, 28] as well. The increase of P and V band absorption and KJL

peak caused by the application either of A.C. field (5 0 Hz, 30-100 kV) onto

KBr crystal [27] or D.C. field (5.5 - 22 kV) onto- KC1 crystal [28] has boon

detected and studied. Because of high fields and RT region, the ionic -

electronic and dislocation processes can take place. As a result the field

can interact directly with the vacancy associations, dislocations and oxii'.T

defects xn the crystal and a release of the vacancy pairs and single vacan-

cies is possible [27, 28]. However, as distinct from the effect of A.C.

field [27] , in the case of D.C. field [28] a large shift of the P band

towards the short wavelengths and their broadening by several times, ac

well as the TSL реак broadening and low temperature shift in KC1 is obser-

ved. I t is believed [28] that the anion vacancies are formed fay the field

at the perturbed regions, e.g. dislocations of the crystal, therefore the

P centres turns out to be highly perturbed as well.

As distinct fron the field effect in the works [27, 2б] we do not

observe at LHeT any shift or broadening of the P, H, I and ОС bands induced

in K5r л-irradiated under D.C. field at 4.6 K. No detectable changer of thc-

T for the subsxages of annealing and the peaks of TSDC take place» Obvi-

ously, in our experiments at Life5? the field of 3 kV/cm is not high ei.ough

to form additional defects near the dislocations e tc . Consequently, i t fboula

be that secondary reactions go on at LBeT:(a) the stabilization of the

genetic pair \L...eCJ by way of the I and <r centre separation and dis-

placement by the field, (b) the rise of the colour centre accumulation

efficiency, i . e . the "bimolecularity" of the relaxation, processes by means

of enhanced electron drifts in the field [30] and (c) the warm-up of the

free electrons by the field up to the energies at which the ionisatiort by

electron impact takes place. The ionization by the electron impact mechanisr.

seems to be very important in the electroluminescence phenomena [29] of pure

alkali halides at low temperatures.

It is expected that the last mechanism should be predominant in the

field effects observed by us (Pig. 7, 8) as well. Indeed, the estimation for

KBr at LHeT shows that because of a very high electron mobility of ~I0

cm /Vs [2l] the electron drift may be large: in the case of the free electron

lifetime of I0~? s and the fields of 3 kV/cm the electron drift can reach the

values .^0.3 cm. This means that in our experiments when the distance

between the electrodes is 0,1 cm the electrons can gain energies up to 300

eV. Бс, the presence of the electric field during X-irradiation of i'.Br Et

LHeT can cause the generation of additional, secondary electron excitations

by electron impact ionization and thu3 the increaea of the crystal colou.rin,:

can arise.

3,4. Comments about the anion Fronkel defect generation in K3r

The well-known conclusion that the primary product of the nonradiati'-'.

excitoK decay is the P and H centres while the CC and I centres are forties

secondary as a result of the electron tunneling within the close defec-т

pair \l...eCJ-, is mainly based on the pulse electron beam irradiation ex-

periments in KBr [3l]and the subsequent works, However, such a conclusion

seems to be too categorical and not enougb valid, because some data and

discrepancies remain inexplicable in th3 work [3 l ] . At firax, the parallel

decay of the P and H bands does not result in the rise of the ОС banrl.

Secondly, the optical absorption spectrum at the end of the electron pulst;

shows a high induced absorption integral in the region cf trie i атасе r-V..I

absorption т-ail which gives evidence about very high disorder or, the crys-

ta l la t t ice . This effect is caused evidently by the charged anion Prer.!:*: j

defects at very high concentration of them. Thirdly, the measure пешс аг.г'

interpretation of the induced absorption spectra in the I, a and R band

region may be complicated because of the defect interaction at high derj.ci-

ties and the rise of the local electr ical, temperature and strain gradients

in the electron tracks during the electron beam irradiation. It s t i l l re-

mains inexplicable [31] why there was a failure relative to the fi oand

measurements because there was a lot of the P centres in КЗг. 'Ii'ns reason for

this nay be either tne nonperfectness of The aosorption measurement teciir.i-

que at photon energies above 5.5 eV or the specific behaviour cf the fi-t^r.C,

probably, cc-excitons at very high excitation densities.

Therefore, the categorical conclusion based on the date [31] , tl£-.

o n l y the P and II centres are generated primarily by tne nor.radiative- zz.-.

decay while the oc and 1 centres are the secondary product, seece : : i.c

insufficiently well-founded. Just after such considerations tae experi-jer.x

series of the simultaneously recording of the P band e tc . thermal bleacir.i"

and TSDC curves of КЗг either at T>290 К [зз] or at T>4,6 К [о, с] were

carried out. It is concluded [ЗЗ] that the p r i m a r y generation cf

the I and oc centres by the exciton decay in KBr is very likely er. well.

In addition to the nentioned above, i t is important to stress the

results of the picosecond laser spectroscopy obtained recently by ::.Schuue:-.

and R.Vogler [ 32] . They found that the laser excitation ±u the i band of t:ie

crystal KBr containing the P centres of above 10"' cm"; rerultea in tnc

decrease of the excited state lifetime for a fraction cf the P centres rr-

about five orders, down to 20 pe. This lifetime depends or. the local ?

centre concentration in the case of a nonhomogeneous distribution of then..

It is possible that a sinilar aituetion takes place in the pulse elec-

tron beam (e.g. [25, 3l] ) or laser irradiation experiments when the excite-

P centres of a h i g h l o c a l c o n c e n t r a t i o n car De pro-

duced. As a result, a fraction of the F centres can be formed in the pico-

second range by means of a very fast capture of the free electrons on tne ex

centres primarily generated by the excitonic mechanism. If i t is really so,

then the objections [25s 3l] raised against the F centre formation mecnanisT

by the electron capture on the anion vacancy stands no longer because ix

alkali halides KC1, KBr and probably others at very high pulse excitatioi.

densities the excited P centre lifetime can decrease by many orders, i . e .

fail down to T V<3Z О. Ь*в.

It should be noted that in the case of KI [34] the irradiation of the

crystal by the electron beam 10 ns pulses at RT results in a very fast gene-

ration of the ОС centres as well as the P centree with comparable efficien-

cies. The a and P.centre concentration at 30 ns after the irradiation cut

off is high enough (3 x 10 cm J and 2 x 10 cm -1) and comparable with

"hat of the P certres in [32]. Therefore, i t seems likely that the P centre

forniation by the "electron capture mechanism" can taiie place.

A . I v e s t i g a t i o n R e s u l t s o f t h e L e w - T e :: -

V a t u r e L u m i n e s c e n c e i n ii а С i a n d II a I

The invest igat ions of the spectra and k i n e t i c s of the e l e c t r o n tunne-

ling afterglow ('ТА) and thermally st imulated luminescence (TSL) during the

radia t ion deiect recombination in NaCl and Mai excited a t 4 .2 ii are carried

.rjt in [35] о The STE luminescence in HaCl (at 3,35 eV) and Hal (at 4.2 eV)

predominates in the spect ra of the ТА and TSL a t LHeT. I t i s shown [35] that

the ТА of the STE immediately a f te r the c r y s t a l exc i ta t ion a r i s e s as a

r e s u l t of the ?' centre e l e c t r o n tunneling to the V.. centre according to tne

react ion:

p. + vK

t u n ° > P + (VK + e~f => F - S - h O ^ . (7)

The question about the ПРuure of the STE emission in TSL a r i s e s .Recently i t was proposed by Tanimura e t a l . [14] that the STE lumi-

nescence in the Г •*— temperature peaks of TSL in iiaCl a r i s e s as a r e s u l t ofd i rec t ? and H с .̂cre recombination within t h e i r complementary p a i r s .

The r e s u l t s of the f r a c t i o n a l TSL measurements in KaCl X-irradiated forfive hours a t 4.2 К are represented in P i g , 9. I t turns out that the durableafterglow a t 4«,2 К a f t e r subsequent warm—up cycles "a", "b" , " c " and "d"(tnrough the next in turn TSL peak - see P i g . 9) res tores again with theasme spectrum and decay k i n e t i c s as the ТА immediately a f t e r the X-irradi-a t i o n . This the so-called thermally s t imulated tunneling afterglow (TSTA),as well as the ТА immediately a f t e r the X-irradiat ion, can be quenched bythe infrared l i g h t s t imulat ion in the P1 band region. On the other hand,such s t imulat ion e n t i r e l y does not influence the subsequent process of TSL.

The i n t e n s i t y of the TSTA during the f i r s t 100 s a f t e r a fast cool -down to 4 .2 К always ( i f the TSTA of the proceding cycle has been quenchedapontaneously or by means of the IR-i l lumination) i s p r o p o r t i o -n a l (Pig . 10.) to the "passed" a r e a of TSL ( i . e . the f rac t iona llightsum of TSL) but i t i s n o t p r o p o r t i o n a l to the TSLi n t e n s i t y a t the moment of the beginning of the cool-down. I t seems

elementary thermally stimulated nearing of some defects, for instance H and

P centres . Obviously, a t least one of the center in the tunneling recombi-

nation pair i s not generated primari ly but i s a p r o d u c t accumulated

during the thermally stimulated reaction among the primary generated

defects.

I t f a l l o w s from t h e аЬате m e n t i o n e d d a t a t h a t j u s t ttoe F 1 c e n -

t r e s a r e t h i s a c c u m u l a t e d p r o d u c t formed I n e a c h TSL p e a k a s a r e s u l t

of free electron capture on the ¥ centre с . A simple analys is of a l l pos-

sible interactions among the P, P ' , H, oc , I and Vj, centres during the

warm-up shows that the release of free e lectrons in the crys ta l , in which

a l l P1 centree are. destroyed previously by l i g h t , can take place only by the

reaction

I + P —> S + e~, (8)

as a r e s u l t of which a res torat ion of the regular l a t t i c e (R) and release

of free electron (e~) occures.The freed electrons a f t e r that e i t h e r

re combines d i r e c t l y with the V^ centres by the react ion

VK + e " " * ( VK + e ~ } * -*• Щ-ЕЕ*

which gives an intensive SIS luminescence ( ST and <5* 1) in the TSL peaks, or

are captured by the P centres r e s u l t i n g in a new P1 centre formation. Thus

the durable TSTA of the STE ar i ses according to the react ion (7) .

If the crys ta l с ; " i n s the. F 1 centres then the e lectron release i s

possible, on principlfc, by the react ion

Ы + P 1 — * R + e~

as wel l . However, vre could not find any influence of the "P1 - i l lumination"

on the TSL (at the same time TSL i s completely quenched a f ter "F-il lumi-

n a t i o n " ) . Рог that reason we draw a conclusion that the low-temperature

peaks of TSL in H a d a t T = 8.4, 12.6, 16.7 and 19.S К (with the a c t i -

vation energies 12, 30, 45 and 56,5 ± 0,5 meV, respect ively) and in Kal a t

T = 8,2, 10.1,11,5 and 1A-.8 К correspond to the di f ferent substages of the

I centre delocal izat ion and t h e i r recombination with the P centres accor-

ding to the react ion ( 8 ) .

The analogical STE luminescence in the low-temperature TSL peaks which

accompanies the, TSL has been investigated by us previously [ 6 ] . Therefore

we believe that the S T E l u m i n e s c e n c e i n f a c e -

c e n t e r e d a l k a l i h a l i d e s ar i ses o n l y b y t h e

e l e c t r o n r e c o m b i n a t i o n w i t h t h e V,. c e n t -

r e s and n o t b y a d i r e c t r e c o m b i n a t i o n o f

t h e c o m p l e m e n t a r y P r e n k e l d e f e c t s . Probably,

the JT -component of the STE luminescence (which, as i t i s suggested in

[14] , accompanies the recombination between the defects in the p a i r <TF-HJ-

in KBr) in tne paper [J4-] is misinterpreted because in the same spectra l

region the exciton-like tunneling luminescence in the pairs -JF-Vj,}-arises

[36].

It is of interest to note that the lightsum S^m^ emitted from the

ЫаС1 crystal during the first 1000 в in the TSTA curves reaches the values

0.07 relative to the lightsum S™, emitted in the TSL peaks. The relation

13

"TSTA^TSL B i l o u l < i ^ estimated probably O.I or higher i f «в take i n t oaccount the fraction of the TSTA which is already emitted during the TSLrecording. Such a high yield of the IS ТА in Had gives evidence (if we

consider that the electron capture cross-section is more higher for the Vvit.

centre than for the P centre) that in our samples Had, in spite of theirweaker colorability relative to KBr and KC1, the P centre concentration waslarge enough to excite and detect the luminescence (if i t indeed arises inthe spectral range 2-6 eV) caused by the {P-HJ- recombination.

On the other hand, the efficiency of the ТА. as well as the intensityof the "VK-peakn of TSL and the decay kinetics of ТА increase in the row ofthe KBr, NaCl and Hal crystals [37]. These data indicate that there are twocompetitive nonradiative exciton decay processes - the Frenkel defect orthe electron - hole pair generation - and that the last process is re la t i -vely more intensive in the crystals with a weaker exciton-phonon interacti-on because the self-trapping of the exciton is less effective. It seemslikely that the production of a large amount of the F centres in the alkaliiodides is accompanied by the creation of the Vv centres rather than the

X».

H centres. This conclusion evidently results from the facts that the STStunneling afterglow in Ual [35] and KI [ i l ] is observed, while the low -temperature TSL is absent, if the crystal is previously photoexcixed in laef i rs t excitonic absorption band. The X-irradiation of Hal leads to therelatively weak TSL peaks, which are caused, on our opinion, by the I centredelocalization, and very intense ТА of the self—trapped excitons. Does noti t mean that the Prenkel pair *{_I— ccj is generated by the radiationp r i m a r i l y ?

5. C o n c l u s i o n s

On the base o.f: the complex investigation of alkali halides by theoptical absorption, electrical, luminescent and gas desorption methoej atT ^ 4 . 2 К and taking into account the published data we can draw the follow-ing conclusive remarks about the mechanisms of the radiation defect accumu-lation and their radiative or nonradiative decay.

(1) The multis. „ge thermal annealing in the range of T = 4.6 - 32 Кtakes place in KBr X-irradiated at 4.6 K. It is controlled by the I centredelocalization, nigration and recombination within a more or less correlated"subsystems" of the main colour centres I, oc , P, F* and V~. centres. Accord-ing to the reactions (I) - (4) the complementary defect systems in the formof the defect pairs {l...cA, tr iplets {i...P...V^}, quartets £ l . . . F ' . . .Vp....VK} and so on should play an important role in the annealing of KBretc .

(2) About ten substages of the correlated defect annealing in KBr aredetected in the range of T = 4.6 - 32 К which are accompanied by the electrontransfer (the peaks of TSC, TSDC and TSCD), the recombination luminescence ofthe self-trapped excitons (the peaks of TSL), the main substages of the heatrelease and the annealing of the latt ice parametei" and the crystal volume.The thermal activation energy for these relaxation processes controlledby t̂ ie I centre delocalization and recombination increases depending on the

f ' .

suDstage temperature from G.QI to 0.07 eVo In the "1авг" substage of the 3band thermal bleaching a t Тщ » 27.5 ± 0.5 К the annealing of the defectt r i p l e t s e t c . predominantly taisee plaoe according to the react ions ( I ) , ( 2 ) ,(3) and (4) which r e s u l t in the l a r g e s t peake of the e l e c t r o n i c cunrent(TSC, TSDC, TSCD) and the excii;onic TSL and lead to a formation of a new H'c e n t r e s . The a c t i v a t i o n energy of t h i s substage i s E^ = 0.06 "£ 0.01 eV. I tis believed that t i e indeed f r e e (rendom) migration of the I centresrequires a higher than 0.06 eV energy, probably in the v i c i n i t y of 0,09 eV,

(3) И» thermal anneal ing of the H and P centres in KBr X-irradiated a t4.6 К occurrs in about s i x aubstages with the Tm «= 31, 34, 38, 43.5, 46.5and 53 1 0-5 К tfrh.ich are accompanied by the correlated peaks of the TSCand TSDC predominantly of e lectron n a t u r e , intense peaks of halogen emission•and gas desorption, as well as r e l a t i v e l y weak heat re lease peaks. I t seemsvery probable that the main annealing substage a t the T = 43.5 К i s a s soc ia-ted with the "beginning" of the H centre quasi-free migration and a subse-quent recombination with the F c e n t r e s . The ac t iva t ion energy for such pro-cess may be S^ = 0.12 t 0.01 eT.

(4) The effect of the tunneling "recharge" in the e x c i t e d defectpair •{_?* - H (or H^)J- during the photostimulation in the P band of KSr X-i r r a d i a t e d a t 4.6 К i s observed which r e s u l t s in the charged Prenkel defectpair-Toe - I Cor 1д)} formation. The tunneling "recharge" of the nonexcited?-H p a i r seems unl ikely because of the energy balance.

(5) I t was observed t h a t the Prenkel defect ( I , at , "H-VK"r F s H')a c c u m u l a t i o n e f f i c i e n c i e s i n c r e a s e by 10 Sor more if the X- i r radia t ion a t 4.6 К i s carr ied out u n d e r an appliedDC f i e l d of 3 kV/cm. I t i s proposed that t h i s f ield-induced ef fectcan ЪР predominantly caused by the warm-up of the free e l e c t r o n s by thef i e l d up to the energies a t which the ionizat ion by the e lec t ron impact

r:36chanism takes p l a c e .(6) On the basis of the analys is of the published data conclusion i s

iiade t h a t in a l k a l i h a l i d e s (KBr, KC1, KI and maybe o t h e r s ) there are pro-bably two competitive channels (mechanisms) of the p r i m a r y gene-r a t i o n of the anionic Prenkel defects — not only n e u t r a l (P, H) but a l s oc h a r g e d P r e n k e l d e f e c t s ( « ) 1) - as a r e s u l t of theexciton nonradiative decay. This i s in accordance with the data [6-8, I I ,32-34] .

(7) The se l f- t rapped exciton luminescence r i s e in the face-centereda l k a l i hal ides seems very unlikely by means of the complementary anionPrenkel defect d i r e c t recombination.

(8) In the a l k a l i hal ides with a low colorat ion e f f ic iency (e .g . i o d i -des) the anion e x c i t o n d i s s o c i a t i o n i n t o t h ee l e c t r o n - h o l e p a i r i s a competitive process r e l a t i v e to theprocess of the exci ton decay into the Prenkel defect p a i r . The p h o t o e l e c t r i -ca l d i s soc ia t ion of the ОС - excitons i s observed a t LHeT i n KBr X^irradi-ated a t 4.6 K.

15

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Crystals, Vol. 4, Iatvian State University, Riga 1975 (p.82-98).

[25] M.Ueta, J.Phys .Sac.Japan 23., 1265 (1967).

[26] M.Kristianpoller and M.Israeli, Phys.Rev. 32, 2175 (1970).

[27] S.Govinda, Physica JJB, 192 (1975).

[28] A.Subrahmanyam and K.Kao, phys.stat.sol. (a) 5_£, KI47 (1979).

[29] C.Paracchini, Phys.Rev. B8, 848 (1973).

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[32] M.Schubert and K.Vogler, phys.s tat .sol . (b) 101, 267 (ЗЭБС).

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16

6X1 _ _ h j l e V )3.0 2.0

Pig.I» The uiduoed opt ical absorption spectra in KBr X-irradiated for 2hours at 4.6 K: curves I , 2 , 3 and 4 show the spectra measuredimmediately after the irradiation ( I ) , after the subsequent f i r s t warm-upto 32.о К (2), after the second warm-up to 62.5 К (3) and after the sub-seauent cool-down and photostimulation in the F band ( s l i t with 2.0 aim) forI hr. under the DC f i e l d of 3 kV/cm (4) .

200\lnm) ——

300 400 500 600 700

60

Pig .2 . The spectra of the optical density changes (AD) measured at 4.6 £ui KBr X-irradiated for 2 hours at 4 .6 К after the subsequent influencesshown in Pig.I : a warm-up to 32.6 К ( I ) # a warm-up to 62.5 К (2), the "F-stimulation1 1 at 4.6 К under the DC f i e l d of 3 kV/cm (3) and a warm-upto 290 К (4) .

17

10

Pig. 3 . Thermal bleaching curves of the I band (curve I) and H band(curve 2 ) and synchronously with them measured curves of thermallystimulated current (curves I1 , 2' ) in KBr X-irradiated 2 hr. a t4-.6 K. ISC curves are measured under the DC field of 3 kV/cm.

£i Q2A :£

о

0.16

0.08

- x - ^ - i - j U ^ - • • • L40 180П 220 260

T O O — -

Pig. 4. Thermal bleaching curves of the I band (curve 1) , H band (2, 3) and V^ band (4) and synchronously with

them recorded curves of thermally stimulated depolarization current(l', 2',3') and thermally stimulated

deaorption of gaa (curves I", 2", 3") from the KBr crystal X-irradiated for 7 hr. 15 min. at 4.6 К under the

applied DC field of 3 kV/cm.

I centre delocalization H centre ^localization

ao"

ad

HA,V4, VF centre annealing

\

IJO-J

~Q8Q

d

. Q6

0.A

Q2

ОАО

Q32

024

0.16

0.08

0.16

Q

d008

60 100 140 180 220 260

P i g . 5 . T h e r m a l b l e a c h i n g c u r v e s o f t h e P b a i . j ( c u r v e s l a , ?.a, 3 a ) , T Ь ч л с ] ( l b ) , " H " b a n d ( 2 b , 3 b , I c , ? c ) , Ft .

b a n d ( 3 c ) a n d V . b a n d ( 4 b ) i n K B r X - i r r a r i i n t e r t n t 4 , 6 V. under t h e DC f i e L d o f ? V V / o m f o r ? h r . ( c u r v e s . T a - 3 a )

o r 7 h r . 1 5 m i n . ( c u r v o a Г Ь - ^ Ь ) , o r u n d e r t h e DC f i e U l o f ' ' . 6 7 k V / r > n f o r 1 h i - , ( n u r v e s I r - ? c ) .

20

10TOO

Pig. 6. Thermal bleaching curves of the n№-VK

n band (curve I) and the H

band (2) and recorded synchronously with them curves of the thermally

stimulated depolarization current ( I 1 , 2 1) in КВт Х-irradiated a t 4.6 К

for 4 hr . under the DC field of 0.67 kV/cm. Thermally stimulated current

of diffusion (curve 3) in the same KBr sample Z-irradiated at 4.6 К for I

hr. without the DC f ie ld. Uaermally stimulated heat release (curve 4) in

KBr after the reactor irradiation for I bx, (from [ 9 ] ) .

Г 5Еи

"о 4

2 Ь

1 h

О 2 Д 6

t (hours) >

Fig.7. The I centre (curves I, I') and ОС centre(curves 2, 2

1) concentrations accumulated in KBr at

4,6 К during the X-irradiation for I, 2, 4 lr . and7 hr. 15 min. - either upon the applied DC fieldof 3 kV/cm (curves I

1, 2') or without the field

(I, 2). The polnto "Д" relate to a lower field of0.67 kV/cm.

0 1 5 6 7t(hours) -

Pig. 8. The P-centre (curves I, I'), H centre (2, 21)

and H1 centre (3, 3') concentrations accumulated in

KBr at 4,6 К during the X-irradiation for I, 2, 4 hr.and 7 hr. 15 min. - either upon the applied DC fieldof 3 leV/cn (curves I

1, 2', 3') or without the field

(I, ?, 3). The pointa " Д " relate to a lower field of0,67 kV/r,m.

22

20

Т(К)

Pig. 9. The curves of the fractional thermally stimulated lumineecence ofNaCl X-irradiated a t 4 . 2 К for 5 hr. K» WBBB-TJP rate i s 0.5 K/min. Insach run (a, b, c, d) the cool-down tine i s about 10 в.

TSA intensity (arb.units)—»*•» а» ее о

О с

*•» -

(Л

•Г о»5-с

да -

L

О- 3