132 IEEE Transactions on Nuclear Science, Vol. 36, No. 1, February 1989

RECENT DJNELOPMENTS IN SCINTILLATOR RESEARCH

P. Schotanus. P. Dorenbos, C.W.E. van Eijk and R.W. Hollander

Department of Applied Physics, Radiation Technology Group, Delft University of Technology, Mekelweg 15, 2629 JB Delft,

The Netherlands

Abstract

Some results of recent scintillator research are presented. A description of the scintillation mechanisms of BaF, and of the influence of PbZ+ and La3+ doping on the scintillation characteristics is given.

Furthermore, the W scintillation properties of LaF, :Nd3+ are discussed. Scintillation light (A=173 nm) emitted by this crystal can be detected in a photosensitive MWPC. The decay time of the emission is 6 ns.

1. The Scintillation mechanisms in BaF,

Presently, barium fluoride (BaF,) is the fastest scintillator known. In addition to a slow emission component at 310 nm with a decay time of 600 ns, this high density scintillator has an UV emission band with two maxima at 220 and 195 nm [l]. These two components have an extremely fast decay time (600-800 ps) [1,2] and their intensity is independent of temperature [3]. These properties, together with a number of other favourable qualities of BaF,. have resulted in a widespread use of this relatively new scintillation material. E.g. the good time resolution achievable with BaF, together with its high density make it an attractive material for positron annihilation studies in materials science, high energy particle physics, and medical applications C4.51. Furthermore, BaF, seems particularly suitable for application in detectors at large future particle accelerators because of its fast scintillation components and outstanding radiation resistance.

Aleksandrov et al. [6] were the first who published a model to explain the fast luminescence of BaF,. They excited the material with synchrotron radiation and found that above photon energies of about 10 e V the slow component appeared. Above about 18 e V the fast components conld be detected.

created in the valence band and electrons in the conduction band. The recombination of these holes and electrons under the formation of so-called self- trapped excitons is associated with a radiative transition in a broad band around 300 nm (the slow component of BaF,). Self-trapped excitons are metastable states consisting of a hole, bound to an electron at a negative ion site, without the presence of an impurity (doping).

The fast luminescence components are related to ionisation of the cations, i.e. to formation of holes in the 5pBaz+ core band. Electrons are lifted into the conduction band, crossing the 2pF valence band. The resulting holes in the core ban-d are subsequently filled with electrons from the 2pF band (crossover transition) under the emission of W light.

Above an excitation energy of 10 eV, holes are

The important feature of the band structure is that the energy spacing between the 5pBaL+ band and the-2pF valence band is less than that between the 2pF band and the conduction band. This means that filling of the core-hole by an electron from the valence band cannot generate Auger electrons. The radiation resulting ffom the transition of an electron from the 2pF band to a hole in the 5pBa2+ band can therefore not-be re-absorbed.

The hole in the 2pF band, resulting from the transition to the 5pBaz+ band, may subsequently recombine with an electron from the conduction band under the formation of a self-trapped exciton. So the production of a hole in the core band results in two kinds of luminescent states: a valence electron-core hole pair and a self-trapped exciton.

X-rays, gamma-rays or charged particles results in both transitions from electron-core hole pairs and associated self-trapped excitons and transitions from directly created self-trapped excitons. The ratio between the two depends on the charge density in the track and the energy distribution of the &rays in the crystal. This explains the different emission spectra for e.g. alpha particles and gamma-rays [8].

The structure in the fast luminescence band with peaks at 220 and 195 nm is not caused by spin-orbit splitting of the Baz+ core levels since both emission bands have the same excitation threshold of 18.1 f 0.2 eV instead, it has been attributed to the density-of-states of the valence band [TI. It is therefore expected that the two fast components have the same decay time and temperature dependence. We have measured the decay times of the two bands and indeed we did not find any difference [l].

It must be noted that excitation of a crystal by

2. The influence of Pb2+ contamination on the BaF, scintillation characteristics

To determine the quality of BaF, scintillation crystals, often the optical transmission is measured since it is correlated with the scintillation light output. Incidentally, a strong absorption band around 200 nm has been reported [9,10]. Since this absorption affects the fast scintillation light output, it should be avoided. The mentioned absorption is found in crystals from different suppliers and can therefore be considered as a general problem.

scintillation emission spectrum of about ten different BaF, crystals, we demonstrated that the absorption at 205 nm is correlated with the presence of Pb2+ ions in the crystals [ll]. Also an extra scintillation emission at 257 nm was detected.

the crystals with X-rays from a 30 kV generator and by analyzing the scintillation light using a W monochromator (Jobin Yvon, Model UV-10) in combination with an XP2020Q photomultiplier. Fig.1

By measuring the optical transmission and the

The emission spectra were measured by irradiating

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I .o 6 0.8

2 0.6

0.4 P 0.2

.- m

ij F

% 2 I ,X' I , , I ,

200 300 Wavelength X [nm]

Fig.1. Optical transmission of 5 mm thick pure and Pb-contaminated BaF, crystals; the spectra are normalized at 300 nm.

shows the optical transmission spectrum for a sample containing 2 ppm Pb and that of a crystal without lead contamination. Fig.2 shows the emission spectrum of the contaminated crystal. Only a fraction ( + 30 % ) of the 257 nm emission has a decay time of the order of a microsecond (630 + 40 ns), the remaining part consists of slow scintillation components with decay times of the order of 0.1 ms. More details together with the experimental methods are presented in [ll].

by transitions between the 'P, and the 'S, states of the Pb2+ ion. The observed absorption and emission wavelengths are in good agreement with theory [12 ] .

The above absorption and emission bands are caused

The presence of Pb in BaF, can simply be explained by too low a temperature of the melt from which the crystals are grown and/or too large a concentration of the so-called "scavenger" which is added to the BaF, powder in order to avoid contamination of the crystal with oxygen and OH ions. Usually PbF, is taken for this which has a boiling point of 1563 K, which is only slightly lower than the melting point of BaF, (1630 K ) .

100 I

(0

c c .-

= 75 J i L 0 v

.z 50 (0 c a, c c

- 25

2 ppm Pb2* : BaF2

emiss ion n 200 300 400

Wavelength [nm]

Fig.2. Scintillation emission spectrum of BaF, contaminated with 2 ppm Pb; the spectrum is corrected for the quantum efficiency of the photomultiplier and the transmission of the monochromator.

3. La3+ doping of BaF, crystals

In an earlier paper, we reported the decrease of the intensity of the slow scintillation component of BaF, on doping the crystals with La3+ [I]. It was concluded that the intensity of the fast scintillation components (195, 220 nm) decreases much slower with increasing La3+ concentration than the slow component at 3 l O nm. Here we present some more detailed data about this phenomenon.

The crystals were obtained from Prof. G. Blasse, Physics Laboratory, University of Utrecht, and from Dr. H.W. den Hartog. Solid State Physics Laboratory, University of Groningen. both in The Netherlands. The crystals are cylindrical with a diameter of 8 mm; the thickness is beween 2 and 5 mm. Optical transmission measurements did not reveal any differences with pure BaF,; no absorption bands could be found between 180 and 600 nm.

by irradiating the crystals with X-rays. Fig.3 shows the intensity of the fast and the slow scintillation components as a function of the La3+ concentration. The error in the measurements is largely caused by differences in the surface conditions and the optical quality of the crystals. This affects the intensity

The scintillation emission spectra were measured

-4 c

~ a ~ + concentration [ m/o ~ a 3 ~ 1

Fig.3. Intensity of the fast and the slow scintillation components of La-doped BaF, as a function of the La3+ concentration, normalised at 0 m/o La3+ , measured under X-ray irradiation.

Fig.4. Relative light yield of BaF,:La3+ as a function of the La3+ concentration.

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at 195 and 220 nm much more than at 310 nm, as can be concluded from the much larger scatter in the corresponding datapoints. By examining only scintillation light emitted from a small spot (0.5 x 0.5 mm2) on the surface of the crystals, these effects are kept as small as possible.

The influence of La3+ doping on the fast scintillation components is clearly much smaller than that on the slow component.

We observed that after X-ray irradiation, the crystals had obtained a colour ranging from light pink to red. The intensity of the coloration, grows with the La’ concentration and is associated with an absorption band between roughly 400 and 600 nm, centered at 480 nm. This absorption band disappears slowly in time. After half a day to about three days at room temperature (300 K), the crystals appeared totally clear again. Although no detailed study of the effect has been performed yet, we found that the decolorization is strongly accelerated at an elevated temperature.

Next, the crystals were mounted on the window of an XP2020Q photomultiplier and the pulse height spectra for 662 keV gamma-rays (l’ICs) were measured using an amplifier with an integration time constant of 2 us. In this way, only scintillation components with a decay time of the order of one microsecond are considered. The position of the total absorption peak is a measure for the relative photoelectron yield of the crystals. The results are shown in Fig.4. Since the position depends on the the bulk properties of a crystal, only crystals with comparable dimensions and optical qualitities were considered.

In order to determine the decay times of the light emitted by a scintillator, light pulse shape studies were performed by means of the single photon counting technique described in detail elsewhere [l]. The crystals were irradiated with gamma-rays from a la7Cs source. Fig.5 shows the average decay time of the slow scintillation component as a function of the La3+ concentration. For the decay time of the fast scintillation components, 0.8 f 0.1 ns FWHM was found [l3]. Within the error of approx. 10 %, no significant change in decay time of the fast scintillation components of BaF, :La3+ could be found.

To explain the observed effects it is necessary to consider the formation of self-trapped excitons in BaF, in more detail. In BaF,, the absorption of radiation generates electrons and holes. Under adjustment of the positions of two fluoride ions. a

U a, > m

Fig.5.

- I ‘ 600‘ BaF, :La

3 400- a,

2 - 0

4 200 aJ W

2 4 6 8 10 12 14 ~ a ~ + concentration [ m/o 1

Average decay time of the slow component of BaF,:La3+ as a function of the La’+ concentration.

hole get easily trapped and forms a so-called V center [14]. When an electron encounters such a center, it can be bound to it and constitute the earlier mentioned self-trapped exciton. The electron can recombine with the hole via a triplet or a singlet state of the self-trapped exciton configuration under the emission of light (luminescence) [14]. However, the self-trapped exciton configuration can also decay by dissociation of the electron from the hole. In this dissociation, the electron exchanges position with a surrounding fluoride ion, in which case no luminescence occurs. This dissociation is temperature dependent because it is phonon assisted.

excitons is probably the explanation of the quenching of the luminescence intensity of the slow scintillation component of BaF, with increasing temperature. In other unactivated scintillators such as NaI(pure) and BGO. similar processes occur.

Let us now consider La-doped BaF,. To compensate for the higher positive charge of La’+ , interstitial F- ions are incorporated in the BaF, lattice. The decrease of the intensity of the slow component with increasing La3+ concentration can beattributed to the presence of these interstitial F ions since they both cause a larger dissociation rate of self-trapped excitons.

explained by a large concentration of La2+ ions, produced by electron capture in La3+ . The electron can be captured either from the conduction band or from a dissociated self-trapped exciton. The coloration disappears in time by recon,binatioii of the captured electrons with self-trapped holes. Obviously, more research is needed to provide a thorough proof of the above described model.

The decay time of the self-trapped exciton luminescence is determined by the thermal equilibrium between formation and dissociation of self-trapped excitons. When the dissociation rate increases, by increase of the temperature or by doping with La’ the decay time decreases.

The crossover transitions associated with the fast scintillation components are much less influenced by La3+ doping. ?he decrease of the fast scintillation intensity at high La3+ concentrations is possibly caused by the increased chance that a Ba2+ ion finds a La3+ ion or a interstitial F ion as its nearest neighbour. The hole in the core band will then be filled up e.g. by an electron from an interstitial ion. However, the exact quenching mechanism is still unknown.

The so-called dissociation of self-trapped

The coloration of irradiated La-doped BaF, can be

In scintillation detectors, the slow component of BaF, causes serious pileup at count rates higher than l o 6 , resulting dead time problems of the detector. For high count rate applications it can therefore be advantageous to use La-doped BaF,. A detailed study of the radiation damage of BaF,:La’+ crystals is desired.

4. UV luminescence from 5d-4f transitions in rare earth elements

For several applications, especially for fast timing purposes, high density fast scintillation materials are required. Examples of fast scintillators are e.g. CsF and BaF, in which crossover transitions are responsible for the luminescence [l5]. Another fast scintillator which recently has regained attention is undoped CsI. The luminescence is emitted in a band centered at 305 nm and is associated with a decay time of the order of 10 ns. The origin is not clear yet, possibly a

135

t r a n s i t i o n from an e x c i t e d s i n g l e t s t a t e of a s e l f - t rapped e x c i t o n . E s p e c i a l l y f o r h igh count r a t e a p p l i c a t i o n s , a s c i n t i l l a t o r with only f a s t decay components i s r e q u i r e d . I n t h e previous s e c t i o n i t was demonstrated t h a t by doping BaF, wi th L a 3 + , t h e slow component can be suppressed. Here w e d i s c u s s another c l a s s of f a s t s c i n t i l l a t i o n m a t e r i a l s .

of r a r e e a r t h e lements (Lanthanides) a r e c h a r a c t e r i s e d by s h o r t decay times (of t h e o r d e r of 10 n s ) and high quantum e f f i c i e n c i e s [16]. T h i s i s caused by t h e s h i e l d i n g of t h e 5d an6 t h e 4f s h e l l s by t h e 5s' and t h e 5p' o u t e r e l e c t r o n s h e l l s . The luminescence a s s o c i a t e d with t h e s e t r a n s i t i o n s provides t h e p o s s i b i l i t y t o c o n s t r u c t a f a s t s c i n t i l l a t o r .

Because of t h e above mentioned s h i e l d i n g of t h e 5d and 4f e l e c t r o n s h e l l s , t h e emission wavelength depends o n l y s l i g h t l y on t h e c r y s t a l mat r ix . The most f r e q u e n t l y used a p p l i c a t i o n of r a r e e a r t h luminescence i s t o dope a s u i t a b l e h o s t c r y s t a l wi th a r a r e e a r t h e lement . An example i s LaF,:Ce'+ , where luminesence o f t h e C e 3 + ion i s c h a r a c t e r i s e d by an emission band a t 286 nm and a decay time of approx. 20 n s [17].

d e t e c t i o n o f s c i n t i l l a t i o n l i g h t i n mul t iwi re chambers, w e i n v e s t i g a t e d t h e p o s s i b i l i t y t o d e t e c t UV s c i n t i l l a t i o n l i g h t from r a r e e a r t h doped f l u o r i d e s , The emission wavelength should p r e f e r a b l y l i e below 200 nm and t h e h o s t c r y s t a l should be o p t i c a l l y t r a n s p a r e n t i n t h a t reg ion . Neodymium-doped lanthanum f l u o r i d e meets t h e s e requirements C18.191.

LaF, is a non-hygroscopic s o l i d wi th a s p e c i f i c mass of 5.94 gfcm' and a r e f r a c t i v e index of 1 .70 a t 200 nm. The o p t i c a l t ransmiss ion extends from t h e i n f r a r e d i n t o t h e vacuum u l t r a v i o l e t wi th a c u t - o f f Wavelength a t 125 nm [ l9 ] .

c o n c e n t r a t i o n s between 0 .03 and 3 m/o were grown by D r . H . W . den Hartog, S o l i d S t a t e Physics Laboratory, U n i v e r s i t y of Groningen, The Nether lands. A l l c r y s t a l s , except t h e 0 . 0 3 m/o samples which a r e i r r e g u l a r l y shaped, a r e 8 mm diam. c y l i n d e r s wi th a t h i c k n e s s of 5 m m . Using t h e experimental methods descr ibed i n d e t a i l i n t h e prev ious s e c t i o n s and i n r e f . [ l ] , w e measured t h e s c i n t i l l a t i o n emission spectrum, t h e o p t i c a l t ransmiss ion and t h e decay time. F i g . 6 shows t h e emission spectrum of t h e 0 .03 m/o Nd sample under X-ray i r r a d i a t i o n wi th X-rays from a 30 keV g e n e r a t o r (copper anode) . The spectrum is c o r r e c t e d f o r t h e quantum e f f i c i e n c y of t h e p h o t o m u l t i p l i e r and t h e e f f i c i e n c y of t h e monochromator.

T r a n s i t i o n s between t h e 5d and t h e 4 f energy bands

S ince w e a r e e s p e c i a l l y i n t e r e s t e d i n t h e

LaF, s i n g l e c r y s t a l s doped with neodymium

I / / 1 , I I I

1 I I in

C 3

c -

?

in C W C c

-

160 200 220 240 260 280

wavelength X [nm]

F i g . 6 . S c i n t i l l a t i o n emission spectrum of LaF,:Nd'+ when i r r a d i a t e d with X-rays from a 30 kV g e n e r a t o r .

n L 0 -

10.' I IO

Nd3+ concentration [m /o]

Fig .7 . I n t e n s i t y of t h e 173 nm s c i n t i l l a t i o n component o f Nd-doped LaF, a s a f u n c t i o n of t h e dopant concent ra t ion . F u l l d o t s mark our measurements, c r o s s e s mark t h e d a t a from r e f . [ l 9 ] , normalised a t 0.5 m/o Nd.

The maximum l i g h t emission i s observed a t 173 ?r 2 nm whereas some s m a l l e r peaks can be d i s t i n g u i s h e d a t 217, 245 and 270 nm. The o r i g i n of t h e emission maxima i s i n d i c a t e d (see e . g . [20] ) . Th p o s i t i o n of t h e Nd3+ emission i s i n good agreement wi th r e f . 1181. The s m a l l e r peaks a r e caused by contaminat ion of t h e c r y s t a l wi th o t h e r r a r e e a r t h s . S ince P r and Ce have s t r o n g a b s o r p t i o n bands i n t h e VUV. t h e o p t i c a l t ransmiss ion of t h e c r y s t a l s i s poor below 200 nm 1201. A t y p i c a l va lue f o r t h e t ransmiss ion a t 180 nm is 10 % f o r a 5 mm t h i c k c r y s t a l [13].

Caused by some u n c l e a r quenching p r o c e s s , t h e i n t e n s i t y of te 173 nm emission decreases wi th i n c r e a s i n g Nd c o n c e n t r a t i o n , a s shown i n F ig .7 .

c r y s t a l s , on ly t h e decay time spectrum of t h e 0 .03 m/o doped c r y s t a l could be determined. The c r y s t a l was i r r a d i a t e d wi th 662 keV gamma-rays from a 1 3 ' C s source and t h e decay spectrum w a s measured u s i n g t h e method descr ibed i n r e f . [l]. For t h e decay component a t 173 nm, a decay t i m e of 6.3 C 0.5 n s was found

Because of t h e poor o p t i c a l t ranmiss ion o f t h e

N31.

LaF3:Nd3+ c r y s t a l s were t e s t e d i n combination wi th a TMAE-vapour f i l l e d mul t iwi re chamber. Two 0 .03 m/o c r y s t a l s were mounted 20 mm a p a r t on a g r i d mounted above a p o s i t i o n s e n s i t i v e MWPC. The c r y s t a l s have d i f f e r e n t dimensions: both bottom f a c e t s , f a c i n g t h e w i r e chamber, a r e r e c t a n g u l a r wi th a l e n g t h of 13 mm and a width of 7 .5 and 5 mm r e s p e c t i v e l y . The t h i c k n e s s o f t h e l a r g e s t c r y s t a l v a r i e s between 7 and 2 mm; t h e o t h e r one is 3 mm t h i c k .

The d e t e c t o r conta ined TMAE vapour a t 50°C; 173 nm UV photons a r e absorbed by t h e TMAE vapour and photoe lec t rons a r e c r e a t e d . which a r e subsequent ly d e t e c t e d i n t h e MWPC. The d e t e c t o r is descr ibed i n d e t a i l e lsewhere [6].

Fig .8 shows a two dimensional p o s i t i o n spectrum when t h e c r y s t a l s a r e i r r a d i a t e d wi th a l Z N a s o u r c e . The two c r y s t a l s can unambiguously be observed. The p o s i t i o n peaks have a width of 11 and 6 mm (FWHM) r e s p e c t i v e l y , i n t h e d i r e c t i o n of t h e s h o r t e s t a x i s of t h e c r y s t a l s . When s e l e c t i n g co inc ident 511 keV events by means of a co l l imated NaI(T1) d e t e c t o r , mounted above t h e SSPC, an i r r a d i a t e d s p o t on one of t h e c r y s t a l s r e s u l t s , and only one of t h e p o s i t i o n peaks shows up. I n t h i s way, t h e width of t h e p o s i t i o n peak of t h e l a r g e s t c r y s t a l was decreased t o 6 mm.

136

Fig.8. Image of two LaF3:Nd3+ crystals (20 mm distance, obtained with a photosensitive wire chamber filled with 50°C TMAE vapour. The crystals were irradiated with a ',"a source.

The pulse height is a direct measure for the number of photoelectrons created per absorbed gamma- ray. Under identical conditions, a BaF, crystal yielded approximately the same pulse height, which means that the photoelectron yield from LaF3:Nd3+ roughly the same as that from BaF,. Gamma radiation with energies of 122-136 keV from 5'C0 and of 59.5 keV from ,+lAm could also be detected, resulting in peak widths of 7 and 8 mm, respectively, for the smallest crystal.

is

We have demonstrated that scintillation light from a new UV scintillator can be detected in a photosensitive wire chamber. LaF,:Nd'+ is a relatively fast (6 ns decay time) VUV scintillator with a high specific mass. The observed photoelectron yield from our LaF,:Nd3+ crystals is of the same order of magnitude as that from BaF,. It is expected that a larger 173 Nd scintillation light output can be obtained from crystals with a smaller Nd doping. In addition, we expect that the optical transmission of the crystals can be improved by using material with a higher purity thus avoiding unwanted absorption bands. Taking into consideration that large crvstals can be grown, LaF,:Nd3+ seems to

good scintillator for a new generation of SSPCs improved characteristics.

References

P. Schotanus, C.W.E. van Eijk, R.W. Hollander and J. Pijpelink, "Development study of a new gamma camera". IEEE Trans. Nucl. Sci.; NS-34, NO. 1 (1987) pp. 272-276. M. Laval, M. Moszynski, R. Allemand, E. Cormoreche, P. Guinet, R. Odru, and J. Vacher, "Barium fluoride - inorganic scintillator for - subnanosecond timing", Nucl. Instr. and Meth., 206 (1983) pp. 169-176. P. Schotanus. C.W.E. van Eijk. R.W. Hollander and J. Pijpelink, "Temperature dependence of BaF, scintillation light yield", Nucl. Instr. and Meth., A238 (1985) pp. 564-565. R. Bouclier, G. Charpak, W. Gao, G. Million, P. Mink, S. Paul, J.C. Santiard, D. Scigocki, N. Solomey and M. Suffert. "Test of an electromagnetic calorimeter using BaF, scintillators and photosensitive wire chambers between 1 and 9 GeV", Nucl. Instr. and Meth.. A267 (1988) pp. 69-86.

E91

P. Schotanus, C.W.E. van Eijk and R.W. Hollander, "A BaF, -MWPC gamma camera for positron emission tomography", Nucl. Instr. and m, A269 (1988) pp. 377-384. Yu. M. Aleksandrov, V.N. Makhov. P.A. Rodnyi, T.I. Syreishchikova and M.N. Yakimenko, "Intrinsic luminescence of BaF, excited by synchrotron radiation pulses", Sov. Phys. Solid State. v01.26(9) (1984) pp. 1734-1735. M. Itoh, S. Hashimoto, S. Sakuragi and S. Kubota. "Auger-free luminescence due to interatomic transitions of valence electrons into core holes in BaF,", Sol . Stat. Comm., V01.65, NO. 6 (1988) pp. 523-525. T. Murakami, J. Kasagi, T. Tachibanaki, K. Yoshidea. Y. Shibata, T. Nakagawa, M. Ogihara,

"ProDerties of BaF, scintillators in charged S.M. Lee, T. Kubo and T. Motobayashi,

particle detection", Nucl. Instr. and Meth., A253 (1986). pp. 163-165. M. Murashita. H. Saitoh, K. Tobimatsu, M. Chiba and F. Takasaki, "Performance and radiation damage of BaF, calorimeter", Nucl. Instr. and Meth., A243 (1986) pp. 67-76.

[ l o ] T. Sung and C. Woody, " Light output and transmission properties of barium fluoride crystals of various geometries and manufacturers", Technical note No. ll9 Physics Department Brookhaven National Laboratory, associated Universities Inc. Upton, NY 11973, (1986).

[ll] P. Schotanus, C.W.E. van Eijk and R.W. Hollander, "The effect of PbZ+ contamination on BaF, scintillation characteristics", Nucl. Instr. and Meth. A272 (1988) pp.917-920.

[12] P. Schotanus, C.W.E. van Eijk. G. Blasse and H.W. Den Hartog. "luminescence of the divalent lead ion in barium fluoride crystals". Phys Stat., Sol. (b), Vol. 148. (1988), pp K77- K81.

emission tomography", PhD thesis, Delft University of Technology, Delft, The Netherlands, 1988.

"Spectral-kinetic study of the intrinsic- luminescence characteristics of a fluorite-type crystal". Opt. Spectrosc., Vol. 53(1), (1982).

[l5] Ya. A. Valbis, Z.A. Rachko and Ya. L. Yansons, "Luminescence due to electronic transitions between valence bands in cesium halides", Opt. Spectrosc. (USSR), Vol. 60(6), (1986) pp. 679- 680.

[16] K.H. Yang and J.A. Deluca, "Vacuum-ultraviolet studies of 5d34fn-' to 4fn and 4fnto 4fn transitions of Nd3+ -, E r 3 + -, and Tm'+ -doped

[l3] P. Schotanus, "A new gamma camera fo r positron

[14] N.N. Ershov, N.G. Zakharov and P.A. Rodnyi,

PP. 51-54.

trifluorides". Phys. Rev. B, Vol. 17(11), (1978) pp. 4246-4255.

[l7] D.J. Ehrich, P.F. Moulton and R.M. Osgood, Jr., "Optically pumped Ce:LaF, laser at 286 nm", Optics Lett., V01.5. No. 8, (1980), pp. 339-340.

emission from Nd3+:LaF,". Appl. Phys. B,

[l9] J.B. Mooney, " Some properties of single crystal lanthanum trifluoride", Infrared Physiscs, Vol. 6, (1966), pp. 153-157.

[20] Wm. S. Heaps, L.R. Elias and W.M. Yen, "Vacuum-ultraviolet absorption bands of trivalent lanthanides in LaF,", Phys. Rev. B, Vol. l3(l) (1976) pp. 94-104.

[18] P.W. Waynant, "Vacuum Ultraviolet laser

vol. 28, v01.2/3, (1982), pp. 205.