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THE TEMPERATURE DEPENDANCE OF THE
EFFICIENCY OF RADIOLUMINESCENCE AND
COLOR CENTER GENERATION IN LiF CRYSTALS
K. Shvarts, A. Podin, E. Aluker, L. Intenberg, S. Tchernov
To cite this version:
JOURNAL DB PITYSIQUL Coiiuque C 4, SuppUment au no 8-9, rome 28, Aolit-Srptembre 1967, page C 4-178
THE
TEMPERATURE
DEPENDANCE OF
THE
EFFICTENCY
OF RADIOLUMINESCENCE AND COLQR CENTER GENERATION
IN
LiF
CRYSTALS
Tnstilutc of Physics, Academy of Scienccs of the Lalvian S. S.
R.,
RigaKkqume.
-
-. L'efiuacite du la creation Je cuntres colorCs et le rende~nent dc radiolurnincscencc dans Lit: d c diverses puretks ainsi quc de LiF dopi ['uranium, ont C t c ctudics entrc 100 et 400 OK.l. On lnontrc que l'cfficacitc de crkation dcs centres F c t X; depend de 1a purctk du cristal.
2. L'lnvestigation de la dependancc en tcmpiraiure dc I'eficacite de radioluminescence I / J , montre une diminution dc r/l.(T) dans lc do~naine oh Ies trous devicnnent auto-pikges.
3. Les spectres d e lurninesccnce ont et6 rnesurts pcndant l'cxcitat~un par dcs faisccaux d'klec- Irons de IO ]IS dc durtc et aprks cxcitation A difltrentes tempkratures.
Sur lid basc des rcnseigncments obtenus, le rBle deu procc5sus CIectron-trous dans kt creation dcs centres co1ori.s et dans la radiolumincscencc d u LiF est discutk.
Abstract. - The efficiency of color ccnters gcncration and thc radioluminescence yicld in lithiurn fluoride crystals of varying purity and also in uranium duped crystals (LiF-L) have been investigated in the region of tcmperature from 100 tn 400
1 . It is shown that the eccicncy of F-and X;--ccnterq generation dcpends on thc cry~lal purity.
2. Thc invcstigaticjn o f the tcniperature dependence of radiolurnincscencc yield qr, shows that there is abatement of V L ( T ) in all crystals in region whcrc holcs bcca~nc sclf-trapped.
3. Lurni~lesccnce spcctra havc bccn mcasurcd during excitation by l0 p sec electron pulses and arter cxcitalion at different temneratures.
On the basis of obtained data the role oC the electron-hole processes in color ccntcrs gcneration and in radioluminesccnce in LiF crystals is discussed.
Introduction. - The pr-oucsscs of color ucnler formation and of radioluminesccnce arc cl(~scly con- ncctcd. i t is shown in the works [ l , 2, 3,] that a causc of the decreasc of the aclivator luminesccncc yicld in alkali halide crystals is formation of activator color centcrs. Vilu and lilango [4] a n d Hcrsch [5] have shown that, in the region of low-tempcra- ture decrease of Pccnter generation cfficiency, thc intensity of intrinsic lurnincscencc (electron recom bi- naliur~ wit11 self-trapped holes) greatly jncreascs. 'l hus a definite compclition occurs between the processes of point dcrect generation and ctllor ccnter Corma- tion on thc onc hand, and the processes of radio- luminesccnce on thc other hand.
In thc prcscnt work the processes of color center gcneration and of radioluminescence in LiF and LiF-
U
crystals i n thc tcrnperature range 1000-400 OK haw been invcstigatcd.J. Experimental Methods and Results. - Color centers were investigated using thc registering spectro- photorncter on thc basc o f C@-4 with the clzarnber for
the operation a t 100°-400 O K . All absorption spectra measurements wcre made at 100 O K .
Thc conccntration of F and X;-centers was estimated by thc value of an absorption coefficient in the pcaks o r bands at 5,I e V and 3,6 eV assuming that the oscillatnr strengths are equal t o 0,s [6j and 2,6 [7] respectively. According to Klnzig
181
thc 3,6 &-band ctlrrcsponds to Xi--center up to 130- L40 O K . At a highcr temperature t h e absorption in the rcgion 3,6 CV is duc lo other color cefitexs.Radiolumincscencc was invcstigatcd i n t h e 100- 400 OKrange under cxcitation by pile radiation [9j, by X-rays [ I O J and bp clectrons.
Under cxcitation by pilc radiation thc tcmperature dependence of radiolumincsccncc intcnsity and the thermolumincsccncc curvcs wcrc invcsligatcd using thc Iight filtcrs for the scparating of various emission bands and using also thc cadnliurn filter for the thcrmal nculrons modcralion.
Under X-ray excitation (X-ray unit YPC-&CB-ZW, 45 kV, 20 m h ) intensity ol'~umincsccncc a n d therm<)-
THE TEMPERATURE DEPENDANCE OF THE EFFICIENCY C 4 - 179
TABLE
f-and Xj -center concentration after irradiation by X-rays at 100 °K Crystal ^ , ^ \
^ - - ^ " C o n c e n - L i F / L i F / 7 UP UJ LiF-U
^ ^ • ^ tration
nF, c m "3 2,4 x 1016 2,4 x 1016 1,6 x 1016 2,4 x 1016
nA- , c m -3 5,0 x 1016 4,7 x 1016 1,0 x 1016 3,0 x 1016
luminescence curves through light filters as well as li2n • . J ••' " — r ">-L " —H' f
emission spectra were measured with the registering t / \ J ^f ^\.
spectrophotometer on the base of OJ>-4. \m . — J — 1 — ~ / \ ~ ^—*fe
Emission spectra during the excitation by the elec- g / \ a<f^ %
tron pulses (10 kV, currents 0,5 and 5 mA) with pulse j-g _ V3* ^ I \ ^ ' / . _ ._$
duration of 10 psec as well as afteremission spectra \ \ f° LiFM with pulse delay of 10 u.sec and 20 usee were measured. 6 -——-t- \~^~ ~
The measurements were made in the temperature range \i (
from 100° to 400°K. 4 1 JL\- _ t The crystals grown in vacuum ( L i F / ) and in open / ^ / ° ^^—^ ^ " ^ v
(LiF II) as weil as specially activated by uranium 2 - ^ ^ J , ? ^ . ^ * * 1 1 ^ -*^—=~— (LiF-U) of the raw material « for mono crystals » were <
used for the investigations. Besides that the zone- a - — — •— purified crystals (Lif / / / ) were used. 7\ j
The results of investigation on temperature depen- / \ dence of the F-(fjF) and Xj -(^(xr) center generation and ~ l m
luminescence /L efficiency for Lif / / / crystal under j | \ \ ^ - ^ / " \
X-ray excitation are shown in figure 1. m w TOO 2S0 IW 3W MO The data on the concentration of .F-and JfJ-centers ^ ~" in various crystals are cited in Table (see also [11]). FIG, 1. — The temperature dependence of the luminescence
The results on temperature dependence of the radio- and color centers formation efficiency ;
luminescence intensity and spectra as well as thermo- tei,s — the coefficient of the induced absorption at 3,6 cV ; luminescence curves are given in figure 2. Emission Aas-the coefficient of the induced absorption at 5,1 eV ; spectra data under the electron pulse excitation (during ^ " x'™? i n d u o e d * ™ ; ™ « > intensity ;
K , . , • , , n , - . „ i /T-thermo luminescence (IT, and /T had been measured the excitation and with 10 (tsec. and 20u, sec. pulse without light filters),
delay) are represented in figure 3.
2. Discussion. — The point defect generation and purified crystal. Inequalities «F ^ nx- are explained by
radioluminescence processes in LiF crystals are studied the presence of the other traps of opposite sign (per-at a lesser extent than in other alkali halide crystals haps the impurity centers).
[II, 12, 13, 7], Also the difference in the position and halfwidth of At 100 °K in the main two kinds of defects F-and the thermoluminescence peak in the region of the X2 -centers [II] are formed in crystals at the first stage selftrapped hole thermal destruction and also the of excitation. The number of these centers varies in the different emission spectra of thermoluminescence for crystals of different purity (Table). the crystals LiF /, LIF //, LiF III and LiF-U [13]
In LiF /, LiF II and LiF-U crystals nx- > nF. For testify in favour of this.
LiF / / / «x,~ < %• *n LiF / / / crystal concentration of The !/F(T)-and tjXl (7)-dependences have a common
f-and Xz -centers (% and «Kj) is the smallest. This characteristic features. The character of rjF(T)
some-apparently is due to the higher purity of the zone- what differs only in the region of 200-300 °K, where the
K. SHVARTS, A. PODIN, E. ALUKER, L. INTENBERG, S. TCHERNOV
FIG. 2. - The radioIuminescence of LiF IIX and LiF-U crystaIs :
a : Tr, and IT-X-ray induccd lumincscence and thermoluminescence (without light filters) ;
6 : X ray induced lu~ninescencr: spectra at different temperatures.
Lip-U crystal :
a-1 : IT, under t h c excitation by cadmium (1 mm) fiItred pile radiation (light filter NC-3) ;
a-2 : TT. under the cxcitation by pile radiation (light filter XC-3) ;
a-3,4 : IL under X-ray excitation without light filter and with light Rlter YmC-I ;
Ip-tl~cr.moluminescen~~ after X-ray irradiation at 100 OK (light filter YmC-I).
small steps are observed for LiF 1 [ l 11 and for LiF 111
(Fig. l ) crystals. In all crystals the increase of q,(T)
begins at the te~nperaturc which considerably cxceeds the temperature of selftrapped hole destruction. It is interesting to note that the radioluminescence intensity in LiF III crystal decreases at low temperature too
(Fig. l). In the temperature range where selftrapped holes are stable, the efficiency of radioluminescence I, and of Pcenters formation q , is small.
By the increase the temperature the balance of energy between the color center formation and the radioluminescence changes [ l 51. It is shown especially distinctly in the 150-200 OK temperature range and in
the region of 250 O K . The investigations on tempera- ture dependence of the radioluminescence efficiency in different LiF crystals have shown that in all cases the decrease of the intensity is observed in temperature region of the hole selftrapping [10].
By lowering the ternperature the intensity of 2,4 eV
band in LiF-U decreases and 3,75 eV band, which is due for the more complex activator centers, increases.
FIG. 3. - Sp~ctra of LiF I11 crystal Iuminesccncc
under cxcitation by pulses of electrons (10 keV ; 10 pscc.) : This is in accordance with the results for other alkali
halidc crystals [3]. The general reguIarities in the
A) Current in the pulsc 0,5 ~ t t A ;
B) Current in the pulse 5 rliA ; l-during the excitation ; temperature dependence of the radioIuminescence of
2-10 psec after excitation ; 3-20 pscc aftcr excitation (the curves crystals coincide with that of other a! kaIi halide
THE TEMPERATURE DEPENDANCE OF THE EFFICIENCY C 4
-
181 The reason of the radioluminescence quenchingin LiF also are the competitive processes-the formation of color centers and the radiational recombination at the other centers.
The strong effect of hole selftrapping on the I(T) in LiF is explained apparently by two reasons. Firstly, the activator concentration (and thus impurities in the host crystal) in LiF is small and so the principal part there belongs to the structure defects. Secondly in radioluminescence an important part is played by a-band (4,5 eV [14, 131) whose intensity strongly decreases in the region of the hole selftrapping.
The decrease of all emission bands is observed in LiF crystals in the region of the holes selftrapping.
In other alkali halides the intensive emission which occurs by the recombination of electrons with self- trapped holes [l61 has been observed in this tempera- ture range.
The peak of the emission band in LiF is to be found in the 8 eV region (from the [l61 data, taking into account the correlation of this band with the peak of excitonic band). Apparently in LiF the intensive luminescence at the low temperature is to be observed in the vacuum ultraviolet region.
We were able to ascertain some peculiarities of the radioluminescence under excitation by neutrons. The dependences IL(T) for the X-ray and unfiltered pile radiation excitation coincide for LiF-U (though dE/dX in both cases differs by two orders of magni- tude) [10]. However IL(T) greatly differs under the excitation by fast neutrons and under the gamma- excitation (cadmium filtered pile radiation) in 200- 500 OK range (Fig. 2, curves 1 and 2). This difference is probably due to the action of the fast neutrons. In LiF crystals the displaced Li+ and F--ions (at initial energy of neutron E, = 2 MeV) have a recoil energy
(E,,,
= A E,, where A is atomic weight)( A
+
112220 and 85 keV respectively. These values are larger than the values of the critical ionisation energy for the moving Li' and F- in LiF crystals which according to the estimation of Seitz [l71 are equal to 68 keV and 25 keV respectively. Thus the fast neutrons in LiF crystals may cause ionisation and therefore radio- luminescence by the displacement of Li+ and F--ions. In such a way the displaced atoms create a high ionisa- tion density in the track (dE/dX).
Apparently the difference in IL(T) under the excita- tion by fast neutrons is connected with a high dE/dX.
However when the low effective capture cross-section of the fast neutrons is taken into consideration the peculiarities of their effect do not appear in the presence of thermal neutrons [10].
The influence of the excitation density has been observed also under the excitation by electrons (Fig. 3). The most essential result is that at high excitation density the fast processes which decay within 10-20 p-sec. are predominant in the radioluminescence. The increase of the electron excitation density (0,5 to 5 mA) (Fig. 3, A and B) essentially shortens the afte- remission time. The lowering of temperature changes the intensity and the spectral distribution of the radio- luminescence. These proceses strongly depend also on the excitation density.
The obtained results on the radiation defects genera- tion and on the radioluminescence indicate the close correllation between these processes and on their strong dependence on the excitation density and tem- perature.
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