3
I OFL.
ORNL P
1701

EEEFE EFE
125 L4 LG
*
MICROCOPY RESOLUTION TEST CHART
NATIONAL QUREAU OF STANDARDS - 1963
ORNL
120,
Cont-6509517
NOV 1 5 1965
65-94
NOTE:
This is a draft of a paper to be presented at
the 2nd International Luminescence Symposium
on the Physics and Chemistry of Scintillators,
Munich, Germany, Sept. 4-10, 1965.
THE SCINTILLATION PROCESS IN NONACTIVATED ALKALI HALIDES
R. B. Murray

RELEASED FOR ANNOUNCEMENT
IN NUCLEAR SCIENCE ABSTRACTS
SOLID STATE DIVISION
Oak Ridge National Laboratory
Operated by
UNION CARBIDE CORPORATION
for the
U. S. Atomic Energy Commission
Oak Ridge, Tennessee
September, 1965
to
m
*
2.
THE SCINTILLATION PROCESS IN NONACTIVATED ALKALI HALIDES
**
R. B. Murray
Solid State Division, Oak Ridge National Laboratory
Oak Ridge, Tennessee, U.S.A.
ABSTRACT
· Various nonactivated alkali halide crystals exhibit scintillation
luminescence of high efficiency at low temperatures, a process previously
ascribed to the radiative decay of exciton states. This paper reviews
the properties of this intrinsic luminescence and describes recent ex-
periments with alkali iodide crystals which indicate that the scintilla-
tion luminescence in the "pure" crystal arises from the recombination of
an electron with a self-trapped hole. This recombination process results
in an exciton state (electron bound to self-trapped hole) in which the
lattice preserves the axial distortion along a <210> axis characteristic
of the trapped-hole center. This effect is demonstrated experimentally
through the polarization of luminescence from oriented self-trapped
hol.es.
Research sponsored by the U. S. Atomic Energy Commission under contract
with Union Carbide Corporation.
- 2.
INTRODUCTION
Activated alkali halide scintillation crystals at room tempera-
ture are widely used as scintillation detectors and spectrometers. In
such crystals it is generally agreed that the scintillation lumines-
cence arises from the radiative decay of an excited state associated
with the impurity ion, e.g., the Ti* ion in NaI(TI) or KI(TI). On the
other hand, nonactivated alkali halide crystals at room temperature
generally exhibit only a very weak scintillation pulse, an effect so
small that it cannot be utilized. At low temperatures, however, sev-
eral nonactivated alkali halides are known to be extremely good scin-
tillators, in the sense of a high scintillation efficiency (i.e.,
energy emitted in a scintillation pulse/energy deposited). For
example, nonactivated NaI exhibits a y-ray scintillation efficiency
at liquid nitrogen temperature (LAT) of 25% Nonactivated CsI 18 re-
ported to have a scintillation efficiency at LNT which is even higher
than that of Nal. Further, the luminescence observed during excitation
by x rays or charged particles is also stimulated by ultraviolet light
whose wavelength corresponds to the first exciton band. The emission
bands characteristic of nonactivated alkali iodides at low temperature
(nominally LNT) are summarized in Table I. Low temperature luminescence
is not restricted to the iodides, but occurs in other alkali halides as
well. However, the present paper is principally concerned with alkali
iodides so that we have included only these crystals in Table I.
big...
....
.
..
-
3 -
TABLE I. Luminescence Emission Bands of
Nonactivated Alkali Iodides
Crystals
Peak a (mu)
References
LII
366, 403
3
NaI
295
This work, also 4
302, 371
KI
RDI
318
This work
CSI
325, 356
Various experimental results indicate that the low temperature
luminescence in the nonactivated alkali iodides arises from transitions
characteristic of the pure crystal, and is not associated with impuri-
ties. This (tentative) interpretation is based on the repeated obser-
vation of the bands of Table I in a variety of crystals, and the cor-
relation of these emission bands with the photoluminescence emission
stimulated in the excitɔn region. Thus, several workers' have attrib-
uted this luminescence to the radiative decay of excitons.
In this paper we present experimental results on the low tempera-
ture scintillation luminescence of nonactivated KI, NaI, and RbI. The
purpose of this work has been to examine the origin of the luminescence
in each of these crystals, and especially to compare the results with a
model which was proposed previously" for the way in which exciton states
might be formed during irradiation. In particular, this model suggests
that the recombination of an electron with a self-trapped hole is respon-
sible for the formation of a localized exciton and hence the scintilla-
tion emission.
-
4
.
SELF-TRAPPED HOLE IN AIKALI HALIDES
We will first review briefly some of the known properties of
the self-trapped hole (v center) in alkali halides. The Ve center
has been identified in several alkali halides of the NaCl structure,
by optical absorption measurements and electron paramagnetic resonance
studies, as a stable molecular ion formed from two adjacent halides,
of the form X. This center has an axis of symmetry along any one
of six equivalent <110> directions, as sho:n in Fig. 1. In order to
form stable self-trapped holes it is necessary to trap the electron
which has been removed from one of the halides; otherwise electron-
hole annihilation occurs promptly and Vv centers are not observed.
For this reason investigations of V centers are performed in crys-
tals which are doped with en electron-trapping impurity such as Ti,
NO, Ag, or Eu. Electron spin resonance studies have shown that the
Vy center is a property of the pure crystal lattice, so that the im-
purity presumably is not directly associated with the center but
serves only as a remote electron trap.
Preferential orientation of Vk centers along one of the <110>
directions can be achieved by. bleaching with linearly polarized light
whose wavelength corresponds to one of the Vx optical absorption bands.
This effect is 1llustrated in Fig. 2, showing the prominent V absorp-
tion bands in KI at 404 and 800 mu measured with polarized light, both
before and after bleaching with 404-mu light polarized along the (0īl)
direction.* If the lower curve of Fig. 2 is subtracted from the upper
W
The polarization direction refers to the direction of the electric
field.
IT.
- 5-
curve (OD - OD), the resulting spectrum (called the anl.sotropic
absorption) exhibits bands characteristic of the V center and is
zero elsewhere.
Finally, we note that V centers are stable only at low tempera-
tures. As the temperature is raised the self-trapped hole begins to
migrate through the lattice from one site to another, until it en-
counters an electron-excess center and is annihilated., or until it
is trapped at an impurity site. The temperature at which self-trapped
holes become unstable (i.e., lifetime of order seconds) is a character-
istic of the particular alkali halide. In KI this occurs at about 105°K.
LUMINESCENCE IN NONACTIVATED KI
We will first discuss" studies of the low-temperature luminescence
in KI. This work has been reported previously and is only summarized
here. The emission spectrum of nonactivated KI, upon x-ray excitation
at LNT is shown in Fig. 3 in the lower curve. (All spectra presented
in this paper have been corrected for the monochromator-detector re-
sponse function.) There are two emission bands, located at 302 and
371 mfl. These bands are also observed at liquid relium temperature
(LHT) and at higher temperatures, although the intensity decreases
rapidly above ~ 100°K (see ref. 5 for more details). In order to pro-
duce stable V centers we have used, in some cases, crystals of KI(T1)
(nominal 0.1 mole %), so that the emission spectrum from KI(TI) is also
of interest. This is shown in the upper curve of Fig. 3. Known TI
emission bands-º are indicated by arrows; a weak Tl emission band at
304 mu overlaps the 302-mu emission of the nonactivated &rystal.
lo
.6.
We now ask: What are the products of irradiating a crystal of
KI(Tl) at low temperature with x rays or energetic electrons? On the
basis of previous experiments by Hersh and coworkers and, on the
basis of the observed absorption bands in the present work, we con-
clude that there are four principal products which anter into the
present considerations: F centers, Vx centers, Tlº ions (formed by
capture of an electron at Ti*), and ti** ions (formed by capture of
a hole at 11). Now if a crystal of KI(T1) is irradiated at LIT and
then illuminated with monochromatic light in the F band or in the Tiº
band (w).lu in KI(T1)), an intense luminescence is observed, and the
emission spectrum is shown in Fig. 4 in the upper curve. This spec-
trum is the same upon excitation in the F band and at 1.14, and we
note that it is the same spectrum as that of KI(T1) in Fig. 3. From
more detailed studies of the absorption bands and their bleaching prop-
erties we conclude that illumination in the F band and at lolj releases
electrons which annihilate Vy centers and mit centers, and at the same
time produces the luminescence spectrum of Fig. 4 (top). If the irrad-
iated crystal of KI(T1) is heated to a temperature just above 105°K,
the self-trapped holes are destroyed; if the crystal is then cooled
to LNT and 1lluminated with F light, or 1.14 light, the resulting
emission spectrum is that shown in Fig. 4 as the lower curve. It is
.
observed that the 302- and 371-mu bands are eliminated as a consequence
of the thermal destruction of Vr centers, whereas the Tl emission is
actually stronger. This result is taken as evidence supporting the
origin of the 302- and 371-mu bands as radiation from the recombine.
tion of electrons with self-trapped holes.
-
.7.
More definitive evidence is obtained by preferential orienta-
tion of V, centers prior to the release of electrons in the crystal
by optical stimulation.
If the
enters are first oriented, it is
observed that both the 302- and 371-me emission bands are partially
plane polarized. This polarization effect is illustrated in Fig. 5
for the case of KI(Eu), and shows the intensity of the two emission
bands transmitted through a Glan-Thompson polarizer prism as a func-
tion of the rotation of the polarizer axis relative to the fixed
crystal. In this experiment preferential orientation of Vx centers
was established by bleaching at 800 mu with the electric vector paral-
lel to an axis at 135° (see Fig. 5). It is assumed that the 800-mu
band in KI 18 predominantly o-polarized (1.e., the transition occurs
with electric vector parallel to the molecular axis), so that bleach-
ing has the effect of populating the direction perpendicular to that
of the electric vector.* On this basis the electric vector of 11ght
emitted at 371 mu was perpendicular to the V axis, and the 302-mu
band was polarized parallel to the Vx axis. A similar effect was
observed in KI(TI) (same polarization properties). Further, the
polarization has been observed at both LHT and LNT.”
The foregoing experimental results lead to the conclusion that
the two emission bands characteristic of "pure" KI do arise from the
radiative decay of excited states resulting from the recombination
It is known from bleaching experiments in KI that both the 404- and
bands have the same polarization properties, i.e., both are
O polarized or both are a polarized, but optical experiments alone
cannot determine which is the correct assignment. The work of
Delbecq, Hayes, and Yuster, utilizing spin resonance, shows that
the Vk bands are definitely o polarized in LiF, Kci, and KBr. We
therefore assume the Vx bands of KI to be predominantly o polarized.
WE
T.
1
..
1
- 8 -
of electrons with self-trapped holes. In the "pure" crystal this
recombination of course occurs promptly during irradiation, as there
are no other significant defects competing for electron capture. It
is for this reason that V centers are not observed after the irradi-
ation of a "pure" crystal.
NaI
Luminescence from nonactivated NaI at low temperatures has been
studied by Van Sciver," who observed a prominent emission band at
295 mu upon excitation by ultraviolet light of energy equal to or
greater than that of the fundamental absorption band. This same
luminescence was also observed in NaI upon excitation by gamma rays
or alpha particles. The luminescence from nonactivated NaI at LNT
and LHT has been measured, in this work, upon x-ray excitation and
is shown in Fig. 6. From these measurements the peak wavelength 18
found to be 295 mil at LNT, anu 293 mů at LHT.
Self-trapped holes have not been identified previously in alkali
iodides other than KI. In order to determine whether a recombination
process similar to that in KI occurs in NaI, it is first necessary to
establish whiether Vy centers exist and at what temperature they become
unstable. Knowing that self-trapped holes become unstable in KI at
~105°K, we can attempt to predict roughly the temperature range in
which self-trapped holes would be stable in NaI and RBI, by very sim-
ple considerations of the extent of overlap of neighboring halide ions
along a <110> axis. Figure 7 presents a scale drawing of the lattices
of NaI, KI, and RbI, using tabulated ionic radii and lattice constants.
-9.
We characterize the overlap of adjacent Iº ions by the parameter
D/a as shown; for halide ions which are just touching this paran-
eter is 1.0, and as the halides are further separated it becomes
smaller.* We assume that in a crystal with closely spaced halide
ions (large D/av2), it will be relatively easy for the self-trapped
hole to move from one site to another, hence a low "critical" temper-
ature. On this basis we expect the I, to become unstable in NaI at
a temperature below 105°K, and in RbI at a temperature above 105°K.
In searching for stable V centers in NaI we have irradiated
crystals of NaI(TI) and NaI(Eu) at LHT. In both crystals the
radiation-induced absorption spectrun is complex, but we attempt to
identify bands due to Vk centers by their optical anisotropy. Ir
both NaI(Tl) a
ME
radiation-induced absorption bands occur
in the vicinity of 400 mu and 800 m, similar to the prominent VK
bands of KI. If a crystal of NaI(TI) or NaI(Eu) is irradiated at
LHT, and then bleached with 430-mų light which is linearly polarized.
with electric vector along a <110> axis, it is found that an optical
anisotropy is induced, indicating the presence of anisotropic defects.
The absorption spectrum measured with light polarized parallel" and
perpendicular to the electric vector of the bleaching light is simi-
lar to that of Fig. 2. By subtracting the lower curve from the upper
(OD, - OD,, ) we obtain the anisotropic absorption spectra of NaI(Ti)
and NaI(Eu), shown in Fig. 8. For comparison the anisotropic absorp-
tion due to Vy centers in KI is shown as the bottom curve. It is seen
.
Strictly speaking we should consider the overlap between a neutral I
atom and one of its neighbors; the conclusions are the same either
way.
- 10.
that these spectra are quite similar. Since the absorption spectrum
of self-trapped holes should be determined largely by the identity of
the halide ion, one would expect the anisotropic absorption of IM
molecular ion to be approximately the same for all alkali iodides
having the same crystallographic structure. On the basis of Fig. 8
we are led to the tentative conclusion that Ve centers are formed in
NaI at LHT.
We next ask: At what temperature do these centers become un-
stable in NaI? There are two features of the absorption spectrum
which can be investigated in order to answer this question. First,
it is known that an anisotropic distribution of V, centers is stable
only at low temperatures; as the crystal is warmed, Vx centers re-
orient by thermal excitation, and the distribution becomes random
over 6 equivalent <110> directions with the concomitant loss of opti-
cal anisotropy. As the temperature of the crystal is raised the ani-
sotropy disappears rather abruptly, and this occurs at a temperature
somewhat below the "critical" temperature at which self-trapped holes
are annihilated.
In the present case of NaI we have measured the
anisotropic absorption at 875 miu as the temperature is raised slowly
and find a sharp loss of anisotropy at a temperature of about 53°K,
see Fig. 9. A second feature of the absorption spectrum which is
correlated with the instability of self-trapped holes is the exis-
tence of rather sharp bands in the near ultraviolet region which have
been interpreted as arising from the center formed by capture of a
hole at a ti* site (which we denote for simplicity as Ti++). In the
case of KI(T1) two of these sharp "thallium perturbation bands" increase
11 .
abruptly as the crystal is warned through ~105°K, at which temperature
self-trapped holes are freed. In NaI(Tl), a narrow absorption band at
312 mu is observed upon irradiation at LJIT, and is very weak. Warming
the crystal results in an abrupt increase in this band at ~60°K, see
set
..
.
.
Fig. 9.
Taking into account the results illustrated in Figs. 8 and 9, it
seems very reasonable to conclude that Vy centers are formed in NaI
crystals which are doped with an electron trapping impurity, and that
these centers are stable for temperatures less than about 58°K. In
fact, it would be a remarkable coincidence if we were dealing with
another type of defect in NaI, whose optical properties are so similar
to those of the V center in KI. In the following discussion we will
therefore assume that this is the correct assignment. We note that
the "critical" temperature in NaI is considerably lower than that for
self-trapped holes in KI, in qualitative agreement with the ideas pre-
sented previously.
If a crystal of NaI(Tl) or NaI (Eu) is irradiated with 1.7-MeV
electrons at LHT (forming Vy centers), and is then illuminated with
light in the wavelength region around 600 mu (corresponding to the
F band"), a luminescence is observed whose spectrum contains the
The F band in irradiated NaI has not been observed previously to the
author's knowledge. We have found that the only band resulting from
the irradiation of "pure" NaI at room temperature (with 1.7-MeV elec-
trons) is centered at 604 mu. This peak wavelength correlates very
closely with that of the known F band in numerous other alkali halides
on a Mollwo-Ivey plot. We conclude that this 604-me band is the F
band of Nal. This band is extremely weak, showing that NaI is very
difficult to color by irradiation at room temperature.
- 12 -
prominent 295-mi emission. In NaI(TI) there are two additional emission
bands at longer wavelength (analogous to the known Tl bands in KI(Tl) at
LHT”), and in NaI(Eu) there is an additional weak band near 425 ml which
is presumed to be associated with the Eu. If an anisotropic distribu-
tion of Vk centers is achieved by bleaching with polarized light at 430
mu in either NaI(Tl) or NaI(Eu), it is found that the 295-mu luminescence
stimulated by ~600-inų light is partially plane polarized. The polariza-
tion results for NaI(Eu) are shown in Fig. 10. In this particular ex-
periment the orientation of V centers was not carried to its maximum
value, so that the polarization effect of Fig. 10 is smaller than that
which could be achieved by a longer bleaching. The qualitative effect
is clear, however, and establishes the connection between the 295-mu
emission and the recombination process at Vy centers. If we assume that
the orienting transition is o polarized, then the luminescence is polar-
ized perpendicular to the Vk axis.
RbI
The X-ray excited emission spectrun of nonactivated RbI at LNT
is shown in Fig. ll. The prominent emission band is centered at 318
mu at LNT and 315 mu at LHT. The peak intensity of this band occurs
in the region of 40°K. It decreakes rapidly from 50° to 100°K, and is
very weak at temperatures above 130°K.
Irradiation of RbI(T1) at LNT with 1.7-MeV electrons produces a
very distinct F band at 732 me, and additional absorption bands at
shorter wavelength, the most prominent occurring at 308 mu and ~400 mil.
Irradiation of RbI(Tl) at LHT produces no measurable F band at all, but
- 13 -
the bands at 308 mu and 405 mu are clearly resolved, and an additional
band at 782 mu appears. (The latter band also occurs upon irradiation
at LNT but is obscured by the long-wavelength tail of the very strong
F band.)
If the crystal is irradiated at LHT and then exposed to 405-mu
light polarized along a <110> axis, an anisotropic absorption results
and is shown in Fig. 12. This spectrum is clearly very similar to
that of NaI and KI, and is thus attributed to the anisotropic absorp-
tion of Vy centers in RbI. The optical anisotropy at 405 mu is stable
at temperatures somewhat above LNT, and disappears rapidly at about
100°K. This can be compared with V, centers in KI where the anisotropy
drops abruptly at 88°-90°K. This comparison of KI and RbI is thus con-
sistent with the qualitative ideas of Fig. 7. We have not measured the
temperature at which V centers disappear in RbI, but it is clearly well
above the "critical" temperature of KI.
If V centers are. produced in RbI(11) by irradiation at LNT, and
the crystal is then illuminated with F-band light, a luminescence results
whose spectrum is very similar to that of Fig. 11. If the V, centers are
oriented by bleaching with polarized light in the usual way, a distinct
polarization is observed in the 318-my emission. The polarization re-
sults are shown in Fig. 13. Illustration of the Vx axis at 135° in Fig.
13 assumes that the orienting absorption transition at 405 mp is o polar-
ized. The polarization results of Fig. 13 are in accord with the concept
of the origin of the 318-mu band as radiation from the recombination of
electrons with self-trapped holes.
- 14.
OTHER ALKALI HALIDES
The origin of low-temperature luminescence in other alkali
halides has been studied by Kabler, 18 who concluded that the x-ray
excited luminescence in KCl, KBr, Naci, and KI is due to the recon-
bination of electrons with self-trapped holes in the perfect lattice.
The techniques used in his study were basically the same as those de-
scribed here, i.e., release of electrons by optical stimulation of
electron-excess centers, and their recombination with oriented V
centers. Taking Kabler's work together with that reported here gives
a total of six alkali halides in which the luminescence characteristic
of the "pure" crystal is attributed to the recombination of electrons
with self-trapped holes. The characteristics of the luminescence from
these crystals is summarized in Table II.
DISCUSSION
Spin resonance experiments have been shown that the self-trapped
hole is a property of the pure crystal lattice, i.e., it is not adjacent
to another defect such as an impurity ion or a vacancy. If an electron
recombines with a Vy center to produce an excited state, then this state
is one which is a property of the pure crystal lattice and which has, in
fact, the normal charge. It follows that such an excited state must be
one of the exciton states of the crystal. This is, of course, consis-
tent with the observation of the "pure" crystal luminescence by UV
excitation in the fundamental band. However, we must note that the
excited state formed by the recombination of an electron with a ver
center is a state in which the lattice retains the axis of symmetry
- 15 -
Table II. Emission Bands Attributed to the
Recombination of Electrons with Self-Trapped Holes
Crystal
Peak 1 (mu)
Polarization Relative to
References
VK Axisa
NaI
295
This work
КІ
302
(1)
(11)
(1)
(11)
371
RbI
318
This work
KC1
537
F
=
KBT
280
546
t
NaCl
230
Unknown
369
Unknown
"Polarization assignments in parentheses are based on the assump-
tion that the orienting transition is o polarized. In KCl and KBT
this is known to be the case from electron spin resonance studies.
'
i
,
T
i
..
.'- . .
.
1.
- 16 -
characteristic of the 1%, as polarized luminescence is observed from
preferentially oriented self-trapped holes. Thus, the excited state
must be one in which the captured electron is confined to an orbital
about the Vy center and this configuration is retained during the life-
time of the radiative state; i.e., the nuclear coordinates do not relax
to restore cubic symmetry about an excited I° ion. It should be noted
that polarized luminescence is observed in both KI and RbI at LNT, and
this is a relatively high temperature as it is approaching the point
at which oriented I, ions lose their preferential orientation in a
matter of seconds. Thus, even at such a "high" temperature, the ex-
cited state (e + Vx) is stable against reorientation during its life-
time (of order 10-6 sec for ki24,15), and this is a time interval which
is orders of magnitude longer than that required for a nuclear relaxa-
tion.
In view of the apparent close relationship between (e + Vy) lumi-
nescence and the photoluminescence stimulated in the fundamental band,
we are led to ask if optically excited excitons can assume an (e + V)
configuration prior to radiative decay. Unfortunately there is no direct
way to answer this question unambiguously at the present time, although
it appears to be a very reasonable hypothesis on the basis of recent
studies of relaxation effects which occur in electronic excited states
of defects in alkali halides. For example, theoretical studies of the
excited states of the F center16,17 have shown that very considerable
lattice distortions occur, and that the shift in electronic energy
levels as a result of this distortion is responsible for the large
. 17 -
observed Stokes shift. In the case of optical excitation of an exciton
in an alkali halide, the excited electron leaves behind a hole which is
at least partially unshielded, and hence capable of forming a molecular
bond with one of its 12 nearest neighbor halide ions. If such a bond
is formed, a nuclear relaxation would very likely occur to give a dis-
tortion along a <110> axis, resulting in an (e + V) configuration.
This process has recently been discussed by Wood.
-
.
-
.
i..
;
.
.
re
ce
M
i
.
.
.
.
REFERENCES
1. W. J. Van Sciver and L. Bogart, IRE Trans. Nucl. Sci. NS-5,
90 (1958).
2. B. Hahn and J. Rossel, Helv. Phys. Acta 26, 803 (1953).
3. H. G. Hanson, J. J. Manning, and R. B. Murray, Oak Ridge National
Laboratory Report, ORNL-2609, p. 138 (1958), unpublished.
W. J. Van Sciver, Phys. Rev. 120, 1193 (1960). Also, IRE Trans.
Nucl. Sci. NS-3, 39 (1956).
5. R. B. Murray and F. J. Keller, Phys. Rev. 137, A942 (1965).
6. H. Enz and J. Rossel, Helv. Phys. Acta 31, 25 (1958).
7. For a review see J. B. Birks, The Theory and Practice of Scintilla-
tion Counting, Pergamon Press (New York) 1964, p. 81 ff.
8. R. Gwin and R. B. Murray, Phys. Rev. 131, 508 (1963).
9. See C. J. Delbecq, W. Hayes, and P. H. Yuster, Phys. Rev. 121,
1043 (1961).
10. R. Edgerton and K. Teegarden, Phys. Rev. 129, 169 (1963). Also,
Phys. Rev. 136, A1091 (1964).
11. H. N. Hersh, J. Chem. Phys. 31, 909 (1959). Also, R. G. Kaufman
and W. B. Hadley, J. Chem. Phys. (submitted 1965).
F. J. Keller and R. B. Murray, Phys. Rev. Letters 15, 198 (1965).
13. M. N. Kabler, Phys. Rev. 136, A1296 (1964).
14. J. Bonanomi and J. Rossel, Helv. Phys. Acta 25, 725 (1952).
15. N. N. Vasil'eva and 2. L. Morgenshtern, Opt. Spectry. (USSR) 9,
357 (1960).
16. R. F. Wood and H. W. Joy, Phys. Rev. 136, A451 (1964).
17. W. B. Fowler and D. L. Dexter, Phys. Rev. 128, 2154 (1962).
18. R. F. Wood, Solid State Communications 4, 39 (1965). Also, Phys.
Rev. Letters (submitted 1965).
*nomine
RE
::.
ine
L
FIGURE CAPTIONS
Fig. 1
Configuration of self-trapped hole (Vx center) in an alkali
halide. Shaded circles represent halide ions, open circles
represent alkali ions.
Fig. 2
Absorption spectrum due to Vk centers in KI (NO2) measured
with linearly polarized light.
Fig. 3
Emission spectra from nonactivated KI (lower curve) and
KI(11), 0.1 mole % (upper curve) during x-ray excitation at
78°K. Curves have been corrected for monochromator-detector
response function. Band pass was 1.3 to 2 mie Peak wave- '",
lengths of known Tl bands are indicated.
Fig. 4
Upper curve: Luminescence emission upon stimulation at 1.lu
from KI(T1) which was irradiated at LNT. Emission spectrum
was measured at LNT. Lower curve: Emission spectrum at LNT
after crystal had been warmed above 105°K, at which tempera-
ture self-trapped holes are destroyed.
-,-.1
Fig. 5
Relative intensity of light transmitted through polarizing
.
I
. -
-
prism as a function of angle of rotation of the prism.
The
N
,LY
crystal is fixed and the prism is rotated; the angle @ is
.
measured from a fixed axis (horizontal) to the polarization
axis of the prism. Experiment performed with KI(Eu) irradia-
ted at LNT, measured at LHT, excitation by unpolarized F light.
The V axis is at 45° if the orienting transition is o polarized.
Emission spectrum from nonactivated NaI at LNT and LHT, X-ray
Fig. 6
excitation.
pi
ti
-
Figure Captions (con't)
Fig. 7
Lattice configuration of NaI, KI, and RbI, using tabulated
Fig. 8
ionic radii and lattice constants.
Anisotropic absorption spectrum in NaI( Eu) and NaI(Tl), ir-
radiated with 1.7-MeV electrons at LHT. Anisotropy induced
by bleaching with 430-74 light, electric vector along [0īl)
direction. Bleaching experiment and measurement of spectra
were performed at LHI'. For comparison, the anisotropic ab-
sorption spectrum from Vk centers in KI (at LNT) is shown
12
as the lower curve.
Fig. 9 Showing the optical anisotropy at 875 mt, and the absorption
coefficient of a band at 312 mu in NaI(TI), as the crystal is
slowly warmed from LHT.
Fig. 10 Relative intensity of 295-mul emission in NaI(Eu) excited by
light at 620 mil, as a function of the angle of rotation of
a polarizing prism. Crystal was irradiated at LHT and experi-
ment was performed at LHT. V centers had been previously
oriented by bleaching with linearly polarized 430-mu light.
Fig. 11 Emission spectrum of nonactivated RbI at LNT, X-ray excitation.
Fig. 12 Anisotropic absorption spectrun from RbI(Tl), irradiated with
1.7-MeV electrons at LHT and illuminated with 405-mu light
polarized along a <110> axis. Spectrum measured at LHT.
Fig. 13 Relative intensity of 318-mu emission in RbI(Tl) excited by
F-band light, as a function of the angle of rotation of a
polarizing prism. Crystal was irradiated at LNT and experiment
was performed at LNT. V centers had been previously oriented
Figure Captions (con't)
Fig. 13 (con't)
by bleaching with polarized 405-mu light. The V-axis
is oriented at 135° if the orienting transition is o
polarized.

ORNL-DWG 65-8727
S
ROK.
.
ti
E
more
:
.
Fig. I
11
ORNL-DWG 65-1232R
KI (NO3)
ALOPTICAL DENSITY [014] AFTER [011] BLEACH
I LOPTICAL DENSITY [011] AND OPTICAL DENSITY (071]
BEFORE [011] BLEACH
RADIATION INDUCED OPTICAL DENSITY
LOPTICAL DENSITY [011] AFTER
[011] BLEACH
4
300
400
500
600 700
i (mu)
800
900
1000

Fig. 2
UNCLASSIFIED
ORNL-DWG 64-7403R
75
BOTH SPECTRA AT 78°K
X-RAY EXCITATION
τι
KI (TI)
RELATIVE INTENSITY
- KI (NON-ACTIVATED)
250
300
350
400
1 (mu)
450
500
550


Fig. 3
..
It..
.
STA
UNCLASSIFIED
ORNL-DWG 64-7678R
KI (TI) AT 78°K
EXCITATION BY
1.14 LIGHT
AFTER ELECTRON
IRRADIATION
RELATIVE INTENSITY
-AFTER THERMAL
DESTRUCTION OF
W CENTERS
250
300
350
400
1 (my)
450
500
550
Fig. 4
UNCLASSIFIED
ORNL-DWG 64-7738R
1.6
W=300 m
1=375
m
l
RELATIVE INTENSITY
TRANSMISSION
DIRECTION FOR Ē
1 POLARIZERWte
VR-AXIS –
- HORIZONTAL
VR-AXIS
CRYSTAL

KI (Eu), LIQUID HELIUM
TEMPERATURE
o
45
90
0 (deg)
135
180
Fig. 5

ORNL-DWG 65-8379
150
Nol "PURE"

LIQUID HELIUM
TEMPERATURE
RELATIVE INTENSITY
-.
.
.
..
......
.
.
...
SLIT WIDTH
-
...
-
-
-
-
-
...
- -
.
-
- ...
-
-
-...
..
..
.
.
.
.
LIQUID NITROGEN
TEMPERATURE
RELATIVE INTENSITY
-
-
-
SLIT WIDTH
--
.-.--.
--
---
-
200
300
400
500
À (mue)
Fig. 6
.
:
:
+ ما
+ ما
ORNL-DWG 65-8380

+ ما
+ ما
+ و
- 0.840
R6*
+ ما
= = 0.952
+ ما
+ 9

ل
+ ما
+ با
+ ما
) (
Fig. 7
5+
]
+
[
+ ما
و
).
+
))
+
* = 0.872
7 = 105°K
+

ORNL-OWG 65-7577
A(mk)
500
300
400
750
1500
0.09

0.08
0.07
0.06
0.05
+
Nal(Eu)
0.04
0.03
OPTICAL ANISOTROPY, OD, -OD,
Nal(TI)
OPTICAL ANISOTROPY, OD, -OD...
0.24
0.20
0.16
0.12
0.08
KI(TI)
2
ENERGY (ov)
Fig. 8
ORNL-DWG 65-7576
-
OPTICAL ANISOTROPY
AT 875 my NaI ( Eu)
INTENSITY (RELATIVE UNITS )
THALLIUM PERTURBATION
BAND FROM CAPTURE OF
HOLE, NA I(TI)
.
10 ·
20
60
30 40 50
TEMPERATURE (°K)
70
80

Fig. 9
ORNL-DWG 65-8377
1.12

1.08
VX AXIS,
IF ORIENTING
TRANSITION IS
O-POLARIZED
1.04
RELATIVE INTENSITY
0.92
0.88
0.84
0
180
30 60 90 920 950
POLARIZER ROTATION ANGLE (deg)
Nol (Eu) Liquid Helium Temperature.
Fig. 10
....
.
ORNL-DWG 65-8733

T = 78°K
RELATIVE INTENSITY
LL
O
300
400
600
700
500
i (mu)
Emission Spectrum of RbI, X-Ray Excitation.
Fig. 11
ORNL-DWG 65-8732
400
ilmu)
500
750
.
1000
1500
..
..
.*"
RbI(TI)
OPTICAL ANISOTROPY, OD, -OD,1
1
VOLNUD
-
4.0
3.5
3.0
9.5
9.0
0.5

2.5 2.0
E (ev)
Fie: 12
ORNL-DWG 65-8842
1.4
1=318 mye
RELATIVE INTENSITY
VR AXIS
o
30
60 90 120
POLARIZER ANGLE (deg)
150
180
RbI (TI) 78 OK
Fig. 13
went

"min
"-
"
-
"
- -
-
- ,
' i. - --
.
m i
Zri
.-
1 / 13 / 66
DATE FILMED
END