-.*

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FINAL REPORT

SCINTILLATION MATERIALS FOR MEDICAL APPLICATIONS

Grant No: DE-FG02-90ER6 1033

December 1, 1997 to November 30, 1999

A. Lempicki, PIC. Brecher

A. J. WojtowiczP. Szupryczynski

Boston UniversityDepartment of Chemistry590 Commonwealth Ave.

Boston, MA 02115

We have no object~on ftm a patentstandpoint to the publication wdissetinat$on of thXs material.

w-Office of lnt~l~~

Prop&r~ CounseMB Field of f~ce,

DISCLAIMER

This repofi was.prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.

DISCLAIMER

Portions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.

Table of Contents

1. Introduction2. The Scintillation Phenomenon3. Material Development

3.1. PET3.2. Broadening the Horizon

3.2.1. Tantalates3.2.2. CT Materials3.2.3. Search for Faster Decay3.2.4. Ceramics

4. Conclusion5. References6. Papers published under the grant7. Appendix

1. Introduction

The past decade has witnessed an intensive search for new scintillators, driven mostly by thestringent and conflicting needs of medical and high energy physics applications. The search hasbeen very competitive and carried out worldwide, with principal efforts in the US, Netherlands,Russi~ Japan and, on the manufacturing front, China. Has it been successful? In terms of un-covering materials capable of better performance in either application, the answer is certainly yes.But it is also true that no material has yet been found to be fully satisfactory in either case, forcing theultimate users into difficult and frustrating compromises. At the present time, high-energyphysics finds lead tungstate to be best for its needs, while PET imaging must cope with thesomewhat unpredictable LSO. So, as is usually the case, success is in the eye of the beholder.

Our own effort of the last three years was outlined in the renewal proposal of 1996 [1], whichemphasized two major areas of research:

(a) Improvement of the understanding of the scintillation phenomenon and the factors thatlimit the scintillatorperformance;

and

(b) Development of new scintillator materials and/orimprovement of those already in use.

While we have made substantial progress in both categories, out greatest advances have come inthe fmt area, where we obtained the first comprehensive understanding of the critical role of

2

b.

i

trapping centers associated with defects in the crystalline lattice [2,3]. We found that these trapsare not simply passive sources of loss, merely reducing the light output, but can have a major im-pact on the kinetics of the scintillation process. These deleterious effects are not necessarily anintrinsic property of the particular scintillator material but, in at least some cases, are a directconsequence of the methods by which the crystals are grown. This offers hope that, at least inprinciple, improvement in growth techniques could improve both the yield of good quality crystalsand the performance of which they are ultimately capable.

Except for this, we find the outlook for improving currently known materials to be rather bleak.Our efforts in this area yielded no significant progress, and we feel that any hope for successresides in a sustaked, well-funded and specialized research effort. With regard to new materials,we feel that a similar judgment holds. Here success depends very much on the definition andultimate use of the new scintillators. While we can identi~ a number of candidates with highstopping power, high scintillation efficiency, chemical and mechanical stability and low price, theultimate stumbling block remains speed. It as in PET applications, speed is of the essence, thenno dopant other than Ce will provide suitable performance, and the roster of potential materialsystems shrinks almost to the vanishing point, and we are not particularly optimistic that any newand substantially better rapid scintillator can be achieved. On the other hand, if speed could berelaxed (as it might be for CT and digital radiology), then we see much greater opportunities. Thematerial needs in those fields are no less pressing, and the outlook for improvement is fkr brighter,than in PET.

2. The Scintillation Phenomenon

During the course of this DOE program, we have been singularly successfid in elucidating thevarious factors that govern the complex scintillation phenomenon. As an extension of this effort,our most recent focus has been in defining the major barriers to material improvements. Possiblythe most important of these is the presence of defects and traps in the lattice. It has been knownfor some time that when the activator (in our case Ce) is optically excited into its own absorptionbands below the band gap of the host, the rise of the emission is essentially instantaneous and thedecay faithfi.dly follows an exponential shape. When excitation is accomplished by ionizing radi-ation, however, the decay almost always departs significantly from such simple behavior, oftenexhibiting ftite rise times, retarded decays, and a generally nonexponential shape of the scintilla-tion pulse. This is a direct-consequence of the formation of free carriers by the ionizing radiation,requiring spatial transport of their stored energy to the activator ions, a process that is particularlyvulnerable to the trapping and detrapping of carriers. Depending on the trap depths and the ambienttemperature, this can substantially reduce the light yield and lengthen the decay of the emissionwell beyond the radiative limit, to the serious detriment of detector sensitivity and speed.

Carrier trapping processes are of course well known in luminescence and are usually addressedby the techniques of thermoluminescence, wherein a sample excited at low temperature is thenslowly heated, causing release of trapped carriers and subsequent light emission. The dynamicsof this emission is the principal tool for studying the properties of the traps. Thermoluminescenceof scintillators excited at room temperature and then heated has been the subject of several papers,emanating mostly from Prof. Van Eijk and his group at the Technical University in Delft [4].Such high-temperature thermoluminescence originates from deep traps that are stable at or nearroom temperature; while these are of no consequence for the scintillation kinetics under ambientconditions, their very presence will make some carriers unavailable to excite the emission centers,reducing the total light. Perhaps more importantly, the presence of deep traps is often indicativeof the presence of other shallower ones which, counterintuitively, can be far more deleterious tomaterial performance.

3

In the past year, we have investigated the relationship between scintillation and thermolumines-cence over a considerably broader range of conditions, finding dramatic connections heretoforeunknown [5,6]. The first new development was revealed by greatly expanding the temperaturerange. With our specially designed cryostat, we were able to excite samples at temperatures aslow as 4 K and then warmed them up to 400 K. This enabled us to study traps much shallowerthan those found by room temperature excitation, so shallow that most investigators had previ-ously dismissed them as inconsequential. As it turns out, however, such shallow traps are in factprincipally responsible for the nonexponential decay of scintillation. Our low temperature thermo-luminescence work is a major advance, and has turned conventional wisdom on its head.

We have also demonstrated a systematic connection between thermoluminescence and scintilla-tion light yield. Scintillators are commonly characterized by a light output (LO), expressed interms of the number of visible photons generated by a gamma photon of unit energy (usuallytaken as 1 MeV). The temperature dependence of LO had never before been studied, except quitenear room temperature where thermal stability of scintillators is important for practicalapplications such as high energy physics or CT scanning. As we have shown, the temperaturedependence of LO in the range of 50 K to 400 K can be quite complicated, with well-defined localminima and maxima. While these observations were initially baffling, we have now developed acomprehensive model that explains all”the relevant features [3].

At the same time, we were also successful in characterizing the influence of the deeper trapsresponsible for the high-temperature thermoluminescence. With the advice and participation ofour consultant, Prof. R. H. Bartram, an ingenious experiment was set up at the University ofConnecticut in Storrs [5]. In this experiment a steady-state gamma beam derived from a van deGraaff generator was directed at a scintillator sample, whose emission intensity was measured.After a predetermined time T the beam is turned off and the time integrated light output S isrecorded. The sample is then immediately warmed up and the total thermoluminescent output Gis recorded. The plot of G/S vs. T is analyzed to yield information how much LO is lost due tothe presence of deep traps [7]. These results revealed a surprising difference between YAP:Ceand its isostructural LuAP:Ce analog, the latter showing significantly greater loss of scintillationlight at room temperature.

The nature of these deep traps was explored by Prof. A. J. Wojtowicz (Co-PI in this project) and hisgroup in Torun, Poland. High temperature thermoluminescence and complementary isothermaldecay (ITD) experiments dn YAP:Ce and LuAP:Ce scintillator materials [7] demonstrated that thegreater scintillation light loss of LuAP:Ce is due almost exclusively to a single dominant trap ofvery unusual characteristics, which is absent from YAP:Ce. All the other traps in both materials fit acommon pattern, with LuAP traps always deeper than their YAP counterparts. These observa-tions explain all the puzzling differences in performance of the two materials, including higher LOof YAP at room temperature, slower than radiative scintillation decay in YAP (25 ns instead of17) and slower rise time in LuAP (600 instead of 350 ps in YAP) [8]. These results have been used topredict the performance of a hypothetical trap-free aluminate material, indicating the potential forsignificant improvement, especially in high speed applications. A more detailed survey of this workwith a review of relevant publications is presented in the Appendix. Finally, tying all these resultstogether, we developed theoretical formulations of the kinetics of a scintillator in the presence oftraps. These took the form of a complete set of coupled differential equations (both linear andnonlinear), solved both analytically and numerically [3], along with a simpler approach utilizingreasonable approximations, well suited to the case of many different traps. These theoreticalapproaches reproduced quite well the results obtained fi-om the low-temperature measurements.

Our findings can be summarized as follows:

.<

1.

2.

3.

4.

.

Both low and high temperature traps contribute to the loss of LO. In general, there is a sig-nificant inverse correlation between the light output and the number and intensity of TL glowpeaks. LO of samples of a given material, nominally of same composition, vary more than anorder of magnitude.

The role of the traps can be quite complex. We fmd in the particular case of the aluminatesthat rather small changes in trap depths translate into large differences in scintillator perform-ance. Moreover, correspondence in lattice structure should not be taken to imply similarcorrespondence in trap patterns.

Although the concentrations of traps of various depths are not known with any precision, itcan still be shown that they cannot by themselves account for the entire shortfall in LO fromthat predicted by theory. In other words, even in the absence of traps nonradiative recombinationprocesses retain an important role. Indeed, the best scintillator specimen that we have everseen (a particular crystal of LSO), with the least thermoluminescence and the highest LO, stillhas a total efficiency of only 36% of theoretical. It would appear that minimization of trapdensity is a necessary but by no means sufficient condition.

All of the presently known inorganic Cc-activated scintillators are limited to a significantdegree by ~pefiections, which generate traps and/or recombination centers. We feel-tiat trapmanagement is the key to improvement of scintillation light output: either in preventing thetraps from forming, or circumventing their deleterious effects. Philosophically the former ispreferable, but would require a lengthy and expensive program to improve material quality. Justsuch an effort has been under way on LSO for many years, with yet no end in sight. Analternative approach would involve intentional codoping designed to introduce “ultra-shallow”traps. This approach relies on the fact that capture cross-sections of very shallow traps areusually much larger than those of deep traps, and, in adequate concentrations, can reasonablybe expected to dominate the kinetics of the material and reduce the role of detrimental inad-vertent traps. Although somewhat speculative, related techniques are well known in bothphosphor and semiconductor technology, and this approach is far better suited to our experi-mental capabilities. Practicality wins; philosophy loses.

We have to remember that the title of our grant is “Scintillation Materials for Medical Applications”.Although applications to PET have been most challenging, there are certainly other medicalmodalities requiring scintillators, which are somewhat less demanding but equally important.Sufl-lce it to say that a certain relaxation of speed opens many new material possibilities with appli-cations to area tomography and digital imaging, to just mention just two. For these reasons wehave decided to end the program by an excursion into the unknown, while freeing ourselves fromthe rigid constraints of PET. First, however, let us summarize the current status of the latter.

3. Material Development

3.1. PET

At the present time there are two scintillator materials whose properties make them clearlysuperior to all other candidates for PET applications: lutetium orthosilicate (LSO) and lutetiumaluminum perovskite (LuAP), both of them doped with cerium. The latter, fwst developed by ourgroup at Boston University, demonstrates a substantial advantage in both speed and density, butto date its light output (efficiency) remains significantly lower than the best exhibited by itscompetitor. The superior performance of LSO, however, has been achieved only through a massiveoptimization effort involving hundreds of boules, most of which were well short of optimal, andtheir growth even now is far from under control. LuAP, however, has been grown not more thanabout a dozen times, and we can only speculate on its ultimately achievable petiormance.

5

At the beginning of the current reporting period we attempted to arrange a significant expansionof the materials effort on LuAP. To this end, the PI of this project obtained some independentfinding for the continuation of crystal growth by Litton-Airtron and DoE issued a small grant toDr. Tom Lograsso of Ames Laboratory. Although the fi.mds for Airtron were quickly exhausted,they did reveal that an increase of Ce concentration is counterproductive, resulting in crystalfracture, presumably from the strain generated by the size disparity between the Ce and Lu ions.

With Litton-Airtron utilizing the Czochralski technique for crystal growth, the Ames work ex-plored the possibility of alternative processes. The most suitable new approach was judged to bethe optical floating zone. Preliminary experiments were quite promising in that the liquid zonewas easily established and maintained, and polycrystalline rods were readily produced. Themajor difficulty proved to be nucleation of the appropriate perovskite phase, since the kinetics ofnucleation favor the garnet phase. We know, however, that this difficulty is not insurmountable,since the same problem was met and overcome by Airtron in their experiments. Unfortunately,the work at Ames did not proceed fi.u-therbecause of a lack of funding.

Further work on the development of LuAP will require substantial tiding. At a Workshop onFlash Radiography at Los Alarnos the participants exhibited strong interest in LuAP, whose proper-ties favorable for PET are equally applicable to FR. Since the effort necessary for the materialoptimization of LuAP and its development into a practical detector is outside the mission of thepresent DoE program, we must leave its future to other governmental programs or the marketplace.

Paradoxically, the very work that extends our level of understanding of the scintillationphenomenon(next section) also demonstrates the formidable limitations of the materials. It is becomingincreasingly evident how unlikely it is that we will find significantly better scintillator materialsfor PET. The primary reason is the large number of limiting and often contradictory requirements,including high density, rapid decay, high light output, and emission wavelength in or near thevisible. These in turn require that the emission must arise from an allowed optical transition,dictating that the dopant must be the Ce3+ ion [9].

In our quest for improved materials, we have suspected for some time that an important factor isthe perfection of a crystal, in terms of the absence of point defects that can serve as carrier traps.Such traps are responsible for the long components of scintillation, which can represent a majorloss from the prompt component. The thermoluminescence measurements described in theprevious section have rqow provided experimental confirmation of this belief, and haveestablished an approximate measure of the degree of improvement that control of such traps couldreasonably provide. The work described in Ref. 6 indicates that elimination of some traps couldimprove the light output of LuAP by about 150A.

Utilizing this relationship, we setup a detailed comparison between LSO (both “poor” and “good”specimens) and LuAP [10]. The analysis of these experiments demonstrates that “good” LSO isremarkably free of the shallow traps that give rise to low temperature (=180 K) glow peaks, andthat its decay is remarkably free of long components. On the other hand, both LuAP and “poor”LSO have a prominent glow peak in this region and certainly show long components of the decay[4]. A quantitative interpretation of these results has proved to be exceedingly difficult andshould be the subject of further work. Hopefidly, out of this could emerge a clearer picture of thefactors that reduce the scintillation output and a means for counteracting them.

3.2. Broadening the horizon

For PET scintillators the activator of choice is obviously Ce because of its speed. The allowed5d-$transitions of other rare earth ions, so far, have not produced any competitor. Given thisstate of affairs, for Ce to be an efilcient emitter, the band gap of the host must be around 7 eV or

6

more. This mea.hs excellent transparency in the UV, which very rarely couples with high density(vide fluorides). The unique requirements of PET are likely to be met in only a few crystals, most ofwhich have already been discovered and discarded [11].

Consequently, we feel that the perovskites YAP and LuAP deserve a continuation of work. LuAP isa direct result of this particular DoE-sponsored effort, while YAP, despite its inferior density, hasthe advantage of being easier to grow. With carrier trapping now established as the majormechanism limiting scintillator performance, the primary goal should be to identifj the trapsinvolved and to fmd ways either to prevent them from forming during crystal growth or, morelikely and practically, to neutralize their detrimental effects by intentional codoping. Specificexamples, such as the positive effect of Mo-codoping on cathodoluminescent efficiency ofYAP:Ce, are already known. We believe that a number of ions bearing a higher positive charge,such as Zr, Hf, Ta, Mo, W, Pb, and Sn, can act as donors in the YAP or LuAP lattice, introducingthe required shallow levels. Even shallower levels could be generated by substitution of isoelec-tronic trivalent ions such as Sc, La, Lu (in the case of YAP), and Y (in the case of LuAP), dependingon differences in ionization potential, among others, to generate the requisite effect. While suchcodoping could introduce yet other problems, such as competitive emissions (Pb, Sri), or undesiredcharge states (Pb, Sn, Ta, Mo, W), we believe we have a reasonable chance of success, especiallyin the case of Zr and Hf, well known for their stable 4+ charge state. We must emphasize, however,that the success of such an effort will depend critically the ability to obtain the necessary crystalsand hence on the willingness to commit adequate finding for this purpose.

An obvious question is whether there maybe yet other materials less subject to limitations statedabove. To answer this, we must once again be reminded of the basic requirements for a PETscintillator. Although some compromises can be made, these include:

Exponential decay on the order of 50 ns or less, over at least two orders of magnitude.

Light yield greater than 25,000 photons/MeV

Density greater than 8 g/cm3

High transparency in the near UV (for fast emission) and in the case of the prefened Ceactivator, a band gap on the order of 8 eV and a crystalline field capable of accommodatingthe excited states of Ce within this band gap.

Manufacturing by simple and reasonably economic techniques.

No new material satis~ing all these criteria has been found and is unlikely ever to be found. Anyexcursion in this area must therefore be considered as entailing very high risk and expense, withlittle likelihood of success. Given this rather pessimistic conclusion we must ask whether thereare any more promising alternatives where we can use to advantage the enormous amount ofinformation that we have been able to gather on the phenomenon of scintillation. A partialanswer to this question is contained in the following.

3.2.1. Tantalates

The motivation for this work was to explore the possibility of using tantalates of trivalent lantha-nides (Ln) as scintillator materials for y-ray detection. A number of these compounds are goodluminescent hosts that have played. a role as X-ray phosphors [12,13]. In view of their generallyhigh densities, their use as scintillators was fwst suggested (in the open literature) by Weber et al.[1 1]. An additional attraction is the prospect of replacing Lu, which is both expensive andcontains a radioactive isotope that contributes to a background count, with the nonradiative Ta asthe density-determining ion. On the other hand, where density is of paramount importance, as forPET applications, it can be brought to an unusually high value by using Lu as the rare earth

7

t

Table 1. Tantalate compounds and structures

Ln203/Ta205 ~ Formula : Structure ‘ Space group ~ DensityRatio \ (# cm3)

1:1 , LnTa04 ~ Fergusonite; ~! M: 12/m; M’:P2/a ~ 9.8monoclinic :. ...... . . . . .. ... . .. . . ... . . ..... ...... .... ... ... . ... . ... .. . ... .... ... ............. ... . . .................. ...... .. ...

1:3 ~ LnTa30g ~ Perovskite; ~orthorhombic !..--..———.-.—.-.—...—*-... —. —.-—-.-...&-—..—.—________.-,._._+__-.-_-._-... __._..._-...J...-----------------------

1:5 ; LnTa5014 \ Pna21 ~ .... ....11:7 ! LnTaTOlg ~ hexagonal ~ P63/mcm !-...-—--——-...——-—-.-... -..._.—_.-—-—-...-.—&—--_._-_._.._, .... .. .... ... . ... ... . .. .... . .... ........ . .. ...... ......... ... .. ... ...3:1 j Ln3TaOT ~ cubic ~ Fm3m ~ 9.7

component. For very well known reasons, the presence of Ce3+ is regarded as essential for modemscintillators. Hence tantalates containing Ce are regarded as an important goal of scintillatorresearch. Unfortunately no reports on the luminescence of Cc-doped tantalates have appeared sofar, and our work was the f~st attempt to fill this gap.

Rare earth tantalates assume a broad range of compositions and structures, some of which arelisted in Table 1. Consequently the chemistry and crystallography of these materials are quitecomplex, and have been the subjects of extensively studies by Russian scientists [14] as well as ina series of papers originating from the National Bureau of Standards [15]. The 1:1 compound hastwo structural modifications M and M’ [14]. With a density of 9.75 g/cm3 it is one of the densesttransparent and nonradioactive materials known. It is also the one to which most of the tantalateliterature is devoted.

In general the luminescent properties depend upon the nature of the Ln ion. If Ln is opticallyinert (L% Y, Lu), the luminescence is strictly a host property. Alternatively, Ln can be one of thelanthanides with an incomplete~shell (either stoichiometrically or as a dilute dopant with an inertion), in which case we will observe the characteristic emissions that arise in the d-for f-ftransitions of the activator. Depending on the concentration of the active ion, its emission maybeaccompanied by some admixture of emission from the host lattice. In general the knowledge ofthe luminescence of the tantalates is rather incomplete. In the review of the Russian work (Ref.14) the authors state that luminescence concentration quenching differs significantly among thevarious tantalate hosts, with the higher ratios of tantalum to RE oxides (corresponding to greaterisolation of the rare earth) generally leading to lower concentration quenching. For the 1:7compound, LaTaTOlg doped with Eu, quenching was not observed at all; i.e. the most efficientluminescence was observed fi-om stoichiometric EuTaTO1g.

Even less is known about luminescence of 1:3 compounds which crystallize in the perovskite type ofstructure, whose unit cells are either tetragonal (RE = L% Ce, Eu, Gd, Dy, Ho, Y) or rhombic(RE = Pr, Nd, Tb). Aside from Ref 14, scant information can be found aside from apassingremarkabout LaTa30yCe in Ref. 12. Only one-third of the RE sites of the typical perovskite structure isfilled with RE ions, with the rest remaining empty. Finally, the corresponding ytterbium tantalatecrystallizes in the monoclinic system.

For single crystal scintillators, the 1:3 compound LaTa30g (density 8.1 g/cm3) is of particularinterest, since it melts congruently at 1850 C and does not undergo a phase transition below itsmelting point, making it a good candidate for crystal growth by the Czochralslci method. Anotheradvantage that LaTa30g brings as a potential host for Ce3+ ion is its crystallographic similarity toCeTa~Og, both of which crystallize in the same type of structure (perovskite, tetragonal system)with almost identical unit cell dimensions. Remembering that La3+ and Ce3+ ions have verysimilar ionic radii (1.03 and 1.01 respectively) we expect that cerium would be readily incor-

8

porated into the LaTasOg lattice even in relatively high concentrations. The possibility of a rela-tively high Ce admixture maybe quite important, depending upon the mechanism of scintillation.Further the 1/3 occupancy of the Ln sites should reduce the likelihood of concentrationquenching, even at high concentrations.

This approach differs from that presented by Weber et ‘al. [1 1]. These workers examined aLaTa30g sample activated with only 0.5?40of cerium, obtaining a scintillation output (excited withX-rays) below that of the BGO standard and a decay that could be fitted to two exponential withtime constants of 1.7 ns (29?40)and 35.5 ns (71 Yo). This low light output, observed from a singlespecimen with low cerium concentration, cannot justifi excluding this material from con-sideration, particularly in the absence of any details on its preparation, which we know can be acritical factor in determining performance.

Basically two preparation methods have been reported for producing tantalates. One, often calledthe “ceramic method”, is based on solid state reaction between the two oxides, Taz05 and Ln203.It is accomplished by a number of sequential beatings of cold pressed starting materials atprogressively higher temperatures. In the other method the starting composition is mixed with acompound that acts as a flux, and the mixture is heated for 10-20 hours. The flux enables thereaction to proceed at a lower temperature and yields a product in fme powder form with smallparticle size. The last factor is quite important for fabrication into practical devices.

Using the ceramic method, we prepared samples of LaTaqOg with cerium content: O, 1, 5, 10,20and 50 atomic ‘%0 with respect to lanthanum. A stoichiometric mixture of Ta205 (Fluka, 99.90A)and La203 (Alfa Aesar, 99.9°/0) with or without the addition of an appropriate amount of Ce02(Alfa Aesar, 99.99%) was ground in a mortar with acetone to get a powder mixture. After drying,the powder was pressed into a pellet and heated on an alumina plate at 1200 C for a few hours.Then the sintered sample was ground again and the procedure was repeated. Subsequent heatingwas performed at 1400 C for 10-15 hrs. One series of samples was prepared in air and anotherone in an argon-hydrogen (O.1‘Yo)mixture. A few specimens prepared in air were heated for 25hrs. The specimens were in the form of discs about 1 cm in diameter and 3-4 mm thick, in whichform they were used for all measurements.

All the specimens showed a prominent emissionband centered at around 380 run, as illustrated inFigure 1. Although these, emissions were excitedoptically, by 166 nm radiation, the energy carriedby such photons is sufficient to span the band gapand generate electrons and holes, in a rough simula-tion of the scintillation mechanism. For those wil-ling to bet on a long shot, we feel that, given enougheffort, these tantalates (particularly the Lu analog)have a reasonable chance of being developed into asuitable scintillator for PET applications.

3.2.2. Materials for Computerized Tomography

Computed Tomography (CT) has been known foryears but is lately undergoing very important ex-pansion into spiral and area CT. Although thenecessary material characteristics (Table 2) are lessdemanding than for PET, there are relatively fewmaterials in current use. These are summarized inTable 3.

80

0300 400 500

Wavelength (rim)

Figure 1. Emission spectrum of LaTa30gsintered powder doped with 8 mole percentCe, under 166 nm optical excitation.

9

L

.

Table 2. Necessary Characteristics of CT Scintillators(after Blasse and Grabmaier [131)-.

Property Desired Value

l,~~tial resolution j 0.5-lm ~—-.. --- ——...--——.-.——..——..-..—.”.”.-..—.. -.— —

The key feature of CT is the contrastresolution, which should be accurateto a few parts per thousand, requirin

Fa high dynamic range in excess of 10.The requirement on afterglow origi-nates from the rotation of the CT de-tector ring, which should not accumu-late counts originating from a previousrevolution. The value given in the tablecan be met with materials havingexponential decay constants of lessthan 300 ps, provided that no compo-nents with longer decays are present.Conversely materials whose radiativedecays are much shorter can toleratelong;omponents up to 300 ~s.

Cd tungstate is a commonly used material in CT (Analogic Systems) and represents a particulartype of luminescent system. Its broad band emission is characteristic of undoped (pure) materialand is usually referred to as host emission. Its origin is the formation of an exciton (bound electron-hole pair) resulting from the photoelectric absorption of the X-ray photon. The radiative decay ofthe exciton is almost certainly caused by an encounter with a crystalline defect, whose perturbingeffect causes the radiative collapse of the exciton. Such events depend upon the migration ofexcitons in the lattice and are therefor strongly temperature dependent. Although this is not anabsolute rule and exceptions are possible, one would on general grounds expect a doped lumines-cent system to have better temperature stability. This follows from the fact that radiative recom-bination processes within an ion, especially if it is weakly coupled to the lattice, should be lesstemperature-dependent than the delocalized emissions of the lattice. Hence other things beingequal, one would tend to look for improved CT materials among doped, rather than pure hosts.

3.2.3. Search for Faster Decay

Here we seek a dopant, preferably trivalent, to insert into the cation sites of a LU203 host. Thedecay should be on the order of microseconds (or faster), some three orders more rapid than thatof EU3+. This is not an eksy task since the all the very efficient 4~4~emissions of the trivalentkmthanides are excluded, being strongly forbidden and therefore slow. Moreover, in those rareearths (e. g., Ce, Pr and Nd) that exhibit 5d–4~allowed transitions, the emissions are situated at toohigh an energy to be accommodated in the oxide. Our search was therefore directed at trivalentions such as Cr3+ and Ti3+ among the transition metals, and Bi3~ and Sb3+ among the so-called S2ions [13]. In principle any one of these could be of some promise for the present-application.

rPhosphor

CSI:TI

(Y,Gd)203:Eu,P],.—— ___Gd202S:Pr,Ce,F

Table 3. Important properties of CT detectors

State ~Emission ~ Relative \ Attenuation ~ Decay \ Afterglow~ Wave- \ Light ~Coefficient for ~ Time ~ 3 ms afier

/ length * Output ~ 150 keV \ ~X-ray turn-off[ (rim) , (%) [ (cm-l) \ (us) ~ (0/00)

crystal i 550 I 100 ~ 3.21 { 1 ~ = 100. ..——__._.&___ .. ..&__ -—-.&__.____&._.._.. .._.._”.__.&----------------------crystal ! 480 ! 30 j 7.93 ‘ 5 ! <1-.—..._...——_.____.. . ..__...._.+__——_——_-..._.&.,____________________: -- ———__....____._.. .._.-.

ceramic I 610 ; 65 \ 3.40 ! = 1000 ! =30-——_.—_—. —-._._ __”.”__&_...-- . .....-.--_ .+----- &.—..-....—-.-.-. -..... ..----ceramic ! 511 [ 75 [ 6.86 ‘ =3 ~ <1

10

.

Transition Metal Activators

Materials doped with Cr and Ti are known fortheir very good photoluminescence efficiency,which accounts for their important role as tunablelasers. The luminescent transitions are opticallyallowed (in the case of Cr in so-called “lowfield” materials [16]) and their decay times, in themicrosecond range, are quite applicable to CT.However these materials have not been in-vestigated in the context of scintillators and weknow of no data on their spectra and decay timesunder ionizing (X-ray) irradiation. Consequentlywe considered it worthwhile to make a quickexploration, in case a potential gem might havebeen overlooked. We excited two standard Crdoped materials, alexandrine and GSGG:Cr, with20 keV X-rays and compared their emissionswith CSI (Figure 2). We found the respectiveintemated intensities of alexandrine and GSGG to

‘ ---- &J,.sc@o.&o140- .--... Alemdrite

- — w-l-l123-

z“~ Ico -

-Qs.~ -.@

g~ ;

40 -

20 -

n40) m ml 70) m

Wavelength (rim)

Figure 2. Emission spectra of Cr-doped Alex-andrineand GSGG under 20 keVX-ray excita-tion, in comparison with CSI:T1.

be 3> and 23% of CSI. In neither case did we find a competitor for CSI in terms of total intensity,although for a narrow band detector at around 700 nm, alexandrine would be preferable. These re-sults, while not exactly breakthroughs, should certainly be followed up, particularly since the twomaterials have been optimized for lasers and not X-ray detectors. In addition the GSGG afterglowwas quite large, amounting to approximately 30/0after 2500 ms. The densities of these materials areof course outside the range of interest for the CXT application, so that the main point of theseexperiments was to confmn that Cr3+can be excited by ionizing radiation, a fact neither obvious nornecessarily expected on general grounds. With the crystal growth technology of these materials sowell developed, these results also suggest that examination of the denser analogs, such as lutetium-containing garne~ might be well worthwhile.

Titanium-doped materials typically show decays in the microsecond range and thus are also compat-ible with CT. We found that the laser rnateriaI YAP:Ti can also be excited by X-rays but the efficien-cy appears to be an order of magnitude lower than in, say, alexandrine. This material is difficult togrow at Cr doping levels above O.1’%and hence the perforrmmce of this specimen may not be charac-teristic of the material in general. The decay trace is a good exponential with a time constant of 70 vs.

Of course the really interesting case would be LuzOs:Cr but whether the oxide would quali~ as alow field material is an open question. Our attempts to prepare Lu203:0.5’%Cr encounteredserious difficulties and were abandoned.

In sum, in the case of transition metal scintillators the jury is still out. Data like those of Figure 2are by no means discouraging but again any hopes for success in this area would require a sub-stantial commitment of time and support.

S*Activators

An alternative to transition metal activators would be trivalent S2ions such as Bi or Sb, where theexcited d~”spstate provides the allowed emission [13]. To explore this approach we synthesizedpowder samples of LuQ03:0.5%Bi. The color of these samples and their emission spectra underX-ray excitation depended on the details of the preparation and thermal treatment, but we foundthat under optical excitation the intensity could be quite respectable, with an initial decay time of0.3 VS. The spectrum of this emission is shown in Figure 3. In view of the high density of the ma-

17

,

terial and the intrinsically fast decay, we considerthese results to be quite encouraging and believethat this system clearly merits fimther study.

3.2.4. Ceramics

Another category of materials that have recentlyattracted interest for scintillator applications is theceramics. Interestingly some scintillators, mostnotably Gd oxysulfide (GOS) [17], have not evenbeen prepared in single crystal form, but only aspowders or ceramics. In other cases ceramics mayjust be a less expensive form than a single crystal.

Ceramics can be further subdivided into those thatare perfectly transparent (the so-called transparentoptical ceramics or TOCS), whose constituents arecubic phases [18, 19]) or the large majority thatare merely translucent. ‘The two ceramics listedin Table 2 belong to the second category, with thegadolinia-yttria system doped with Eu featured inGE tomographs, while the ceramic based on Gdoxysulfide doped with Pr is favored by Siemens.

5Q0 1 I I 1

460nm

400

100

0 I 1 1 1 1

300 400 500 600

Wavelength (rim)

Figure 3. Emissionspectrum ofLu203 dopedwith O.5% Bi, excited by .20keVX-rays.

The gadolinia-yttria system is also a precursor of TOCs, which were developed only quite recently inresponse to the need for transparent but passive applications such as high-pressure arc lampenvelopes and heat-resistant windows for heat seeking missiles. As host materials for luminescentions, however, TOCS have received relatively little attention. Some work at GE is reported inpatents and review articles, with the main subject being Eu-doped Y203-Gdz03. In general, theproperties of TOCS as luminescence hosts remain largely unknown, with the exception of researchcarried out at Boston University by Brecher and coworkers. This work, carried under an NIHgrant, made it possible for the first time to compare the luminescent properties of a single crystaland a TOC of the same composition. Having attained considerable expertise in the properties of Cein crystals, we chose the material system of Y3A1501z:Ce (YAG) as the main vehicle for this study.Its cubic structure allows, the fabrication of completely transparent ceramic specimens, while itselectronic structure is such that efticient Ce luminescence can be excited by both optical (less thanband gap) and ionizing (greater than band gap) irradiation. The study was principally concernedwith the mechanism of scintillation in a ceramic and the role played by grain boundaries. Thiswork resulted in the development of the fust ceramic scintillator to have a light output in the samerange of magnitude as that of the corresponding single crystal, and, through comparison withsingle crystal material of the same composition and containing the same activators, demonstratedthat a ceramic is much more complicated than merely a closely-packed assemblage of randomlyoriented crystallite. YAG is not a particularly dense material (4.6 g/cm3) and has already beensupplanted, in related work, by its lutetium analog, resulting in a density increase to 6.4 g/cm3.

LU203 Transparent Ceramics

It is rather unfortunate that the binary oxides of the lanthanides (Y203, LU203, etc.), whose densityand cubic structure (see Table 4) would make them excellent candidates for TOC scintillators, do nothave band gaps large enough to accommodate the excited states of Ce, resulting in an intense chargetransfer absorption and the total absence of any significant Ce emission from these oxides. How-ever, since CT does not involve the extremely high counting rates that PET-type gamma-ray de-tectors must deal with, it does not require ultrafast (nanosecond range) luminescence decays. Re-

12

..

.

laxation of the speed requirement means that we are no longer restricted to allowed optical transi-tions, which in the context of lanthanides means interconfigurational d-to-~ transitions, and emis-sions in the near UV because of the dependence of the lifetime on the square of the wavelength.No longer is cerium (with its uniquely rapid decay) the only rare earth acceptable as the activator,allowing a much broader range of ions and greater flexibility for the vagaries of energy transfer.

In the rapidly expanding area of digital radiography, speed is of much lesser importance than forboth PET and CTF. Hence scintillators involving the strong but slow ~-~emissions of the rareearths can be used, provided they offer high stopping power, efficiency and good match of theiremission to the spectral response of CCD detectors. Additionally transparency is very importantbecause light scattering is a serious limiting factor in resolution.

A host of particular interest is LU203, whose 9.42 g/cm3 density is one of the highest ever observed ina transparent colorless material, and whose cubic crystal structure makes it an ideal candidate for aTOC. Moreover, the isostructural yttrium analog has long been known to be a highly efficientcathodoluminescent host for europium, whose emission just happens to fall at wavelengths near thepeak sensitivity of CCDS. Bearing these considerations in mind, we prepared (by a hot pressingtechnique) TOC specimens of Eu-doped LU203, which we excited with 20 keV X-ray irradiation.

The resultant intense red emission of the Eu doped ceramic is shown in Figure 4. The integratedintensity amounts to 60°/0 of that of CSI:T1, which immediately indicates that we are alreadyamong the most efficient scintillators known. Even more impressive is the fact that when thewavelength distributions of the respective emissions are weighted according to the response of theCCD, the effective intensities are virtually identical.

It is fi.nther instructive to compare the X-ray absorption characteristics of Lu203 with those of stan-dard phosphors Gdz02S and CSI. From the densities and effective Z numbers of these materials, wecan readily calculate their X-ray absorption coefficients as a function of photon energy; these re-sults are given in Figure 5. From these, in turn, we can estimate the stopping ability of a thin layerof each phosphor. Using the energy distribution of a common dental X-ray machine, we calculatethat a 200 ~m layer will absorb the following fractions of the total emitted energy: CSI, 75.6Yo;

1 L~O; 5°hEu+0.50/700

“w 400 500 60) 700 600 900

Wavelength (rim)

Figure4. SpectraofLu20j:EuceramiccomparedwithCSI:Tl,underexcitationby 20 keVX-rays.

l“ooooo~),l]. !!l ! I , I 1::

;J

_,

5

j

E

I.O.d 1000 I

8=

8

.= +

ELq 10: qz ir 1Ti-

~

1:11’, !,4,’ 1 [, ,1,! ,

0 10 20 30 40 50 60 70 80 90 100Photon Energy (keV)

Figure 5. Calculated absorption coefficient of X-i-ayscintillators as afunction ofphoton energy.

73

..*

Table 4. Density and structure of binary oxides Table 5. Ionic radii for some trivalent ions

Oxide Density (g/cm3) ~ Structure Ion ~ Radius (~)

Yz03 5.01 ! Cubic Lu ~ 0.85..-..-—-—...--..--..——....... ......... .. . . .. . .... . .. ..... .. .......... .. ... .......... . .. ........... ............ .... .. . .. .... ... .. ........La203 6.5 . Hexagonal Cr ! 0.63-..-..__-_ —...—_,.-—.——-.—.,—_—_—--.._..._..-_..._—._.._—....-—. ..--..._-..____....._-..._.-.-__-.--._- ._.,,. .....Gd203 7.41 - Cubic Ti ~ 0.76._..__. —--. —-.—.. .. . .. ... .... . . .... ..... ... ... .. . . . . . . .Ybz03

..______._... -—.-..-__..___.._..._._..-.-___-_.917 i Cubic Bi !———.—. 0.96_—L._.—__________________________ .“-..—._..-._ —_._.._-._—.——.—-. . .... . .. ......

LUZ03 9.42 j Cubic Sb ~ 0.76

Gd202S, 76.OYO;Lu@s, 84. l?Ao.Indeed, except for the narrow region between 30 and 63 keV dueto the particular position of the X-ray edges, LU203 is clearly superior across the board.

As we can see from Table 4 there are two forms of the binary lanthanide oxides, hexagonal andcubic. The cubic structure provides two different sites of symmetry C2 and &j for the Ianthanide andtherefore also the trivalent dopant. The hexagonal structure has only one site of symmetry, C3V.For fiture reference Table 5 provides radii for several ions which were considered as dopants.

The results shown in Figure 5 certainly understate the relative superiority of LuzOs with respect toGOS, because the absorption were calculated on the basis of the full crystalline densities. Inactuality, unless special densification techniques (incidentally, rather similar to those in TOCfabrication) are used, the phosphor densities are no more than about 50’% of the single crystal. Inany case such compaction of a hexagonal material as the oxysulfide, despite the increase in den-sity, cannot result in transparency, which is obtainable only with cubic materials. GOS plates, nowwidely used in DR, are notorious for light scattering, which diminishes their resolution capability.

The LU203:EU TOC, with its rather long decay time (= 2 ins), may or may not be applicable to CT,depending on the scan rate. On the other hand its narrow emission spectrum, consistingessentially of one line a 610nm will be much better matched to the spectral response of CCD typedetectors. This is the subject under current investigation under a current NIH grant.

In the context of the present DOE grant we consider the LU203 transparent ceramic a most im-portant host. The challenge is to fmd dopants whose efilciency would be as high as EU3+but witha substantially shorter decay time to satisfi the scan rates of conventional CT.

Tantalate Transparent Ceramics

This is the most speculative item we have to offer, but with potentially the greatest payoff. It hadits genesis in an unobtrusive entry in Table 1, where the presence of a cubic phase (the 1:3 com-pound Ln3Ta07) suggests the possibility of a tantalate TOC. This particular tantalate is even lessknown than the others; the only reference to it that we were able to uncover was is in the book byRoazhdestvenskii et al. [14], who report the cubic structure and cell dimensions but nothing more.We produced good powder samples of this compound using a generic flux method described byBrixner, and confirmed the cubic structure by X-ray analysis petiormed by M. Downey of U. Mass. -Lowell. We subjected samples doped with Tb or EU to X-ray excitation, and clearly observed therespective characteristic emissions, as shown in Figure 6. While we have not yet succeeded inobserving emission from Ce in this material, the absence of a significant body color suggests that,unlike the cubic sesquioxide, the tantalate may have a sufficiently large band gap to accommodatethe relevant Ce energy levels. Since Ce emission has indeed been observed in other tantalates, wecannot yet rule out this one.

One of the major attractions of this tantalate is its high X-ray absorption coefficient, practically iden-tical to that of LU203, which has already been shown in Figure 5. Both of them are superior to thepopular material, CSI:T1. Moreover, neither the tantalate nor lutetium oxide have any absorption

14

,

edges in the region of X-ray tube emission, be- 100

tween 10 and 64 keV, and therefore will notsuffer any resolution loss due to X-ray escape.This is particularly important for dental radi- 80ography, which utilizes X-ray energies in thatrange. Since lutetium has a naturally occurring sradioactive isotope, whose decay could con- S 60

ceivably interfere with the detectability of low $intensity signals, it could be important to have a 3

“g 40nonradioactive alternative. The tantalate gives ~us this possibility without any significant sac- =rifice in stopping power. Indeed, Ln3TaOTdoped with a suitable activator (Eu, Tb) could 20even become an important X-ray powder phos-phor in its own right.

o

4. Conclusion

r , , I , ,

t!

I I I500 550 600 650

Wavelength (rim)

From the very beginning of our program we re- Figure 6. Emission spectra of doped Ln3Ta07garded the understanding of the scintillation p.nwkr un&-20 keVX-myewittkm Heavy trace,mechanism as our primary mission. If in addi- 10 mole % Eu; thin trace, 2 mole % Tb. Notetion this understanding could lead to the dis- the strong blue component of the Tb emission,covery of a new material, so much the better. similar to that of Gd202S, but relatively rareWhen we began this work some nine years ago, among Tbphosphors.the theoretical basis for the scintillation phe-nomenon was in disarray. The initial and final steps were reasonably well characterized, but therewas no consensus on the crucial intermediate, the transfer of energy from the lattice to the emittingcenter. In the over 40 publications that resulted from this program, we demonstrated that despite thehighly insulating nature of the hosts and the great magnitude of the band gap, the primary meansof transport is through mobile charge carriers and their sequential capture by the emitting center.Although radical at the time, this picture is now generally accepted throughout the field. Subse-quently, we also recognized the critical role that trapping centers localized at lattice defects canplay in the process, not merely as passive sources of loss but as active participants in the kinetics.In this sense shallow traps &m wreak more havoc than deep ones, impeding the rate by which carrierscan reach the emitting centers and seriously slowing the resulting decay. And we establishedlow-temperature thermoluminescence as a comprehensive tool for quantizing these effects.

As for new and better materials, our work also had an impact. We were among the f~st to recognizethe potential of LuAIOs (lutetium aluminum perovskite, or LuAP) as a detector for PET applications.Although this material has not supplanted LuSiOs (lutetium oxysilicate, or LSO) in terms of lightoutput or absence of afterglow, LuAP still exhibits by far the highest figure of merit (light outputdivided by decay time) of any scintillator material currently known. Our work has also boughtinto stark view the dismaying realization of just how improbable it is that a material will ever befound that will be capable of any more than an incremental improvement in performance.

Inevitably, many nagging questions remain. Our greatest regret is that we have been unable toprovide any formula for enhancing the transfer process and suppressing the long decay componentof the scintillator material. We know that that the basis for the superiority of LSO’S “good”crystals in terms of light output and unimpeded decay is intimately connected with thek low con-centration of traps, but we have not been able to unearth a means for controlling them. Indeed,even the tremendous materials development effort on LSO has not eliminated the wide variability

75

..

between individual specimens, with the majority still plagued by low light output and long after-glow. A comprehensive model relating growth parameters with crystal perfection is desperatelyneeded, but nowhere in sight.

We wish to finish on a high note. On the basis of our work on ceramics, we feel that we have barelyscratched the surface in tapping the potential of this material form. Ceramics, and TOCS in particu-lar, represent an emerging technology with broad applicability to radiation detection, and it is ourstrong belief that they will greatly advance the technology and usefulness of digital radiography.

5. References

1. A. Lempicki et al., Proposal: “Scintillation Materials for Medical Applications”, Boston Uni-versity, Chemistry Department, 1996.

2. A. J. Wojtowicz, J. Glodo, W. Drozdowski, K. R. Przegietka, “Electron Traps and ScintillationMechanism in YAIOg:Ce and LuA103:Ce Scintillators”, 1 Lumin. 79,275-291 (1998).

3. A. Lempicki, R. H. Bartrarn, J. Lumin.81,13 (1998).4. ~. W. E. van Eijk, Nucl. Instr, & Meth. A 392,285 (1997).

5. R. H. Bartram, D. S. Hamilton, L. A. Kappers, A. Lempicki, “Electron Traps and Transfer Effi-ciency of Cerium-Doped Aluminate Scintillators”, J. Lumin. 75, 188-191 (1997).

6. A. J. Wojtowicz, J. Glodo, A. Lempicki, C. Brecher, “Thermoluminescence and ScintillationMechanism in YA103:Ce”, J. Physics: Cond Matter 10,8401-8415 (1998).

7. W. Drozdowski, D. Wisniewski, A. J. Wojtowicz, A. Lempicki, P. Dorenbos, J. T. M. de Haas,C. W. E. van Eijk, A. J. J. Bos, “Thermoluminescence of LuA103:Ce”, Proc. 1996 Int’1. Conf onLuminescence and Optical Spectroscopy of Condensed Matter, ICL96, eds. J. Hhl% P. Reineker,and R. S. Meltzer, J Lumin. 72/74,756-758 (1997)8. S. E. Derenzo, W. W. Moses - private communication.

9. A. Lempicki, A. J. Wojtowicz, C. Brecher, “Inorganic Scintillators”, in Wide-Gap Materials:Theory andApplications, ed. S. R. Rotman, Kluwer Academic Press, 1997.

10. A. Lempicki, J. Glodo, “Ce doped Scintillators: LSO and LuAP”, Nucl. Instr. & Meth. A416,333-344 (1998).

11. M. J. Weber, S. E. Derenzo, C. Dujardin, W. W. Moses, “Dense Ce3+-Activated ScintillatorMaterials”, Proc. M. Con$ on Inorganic Scintillators and their Applications, eds. P. Dorenbosand C. W. E. van Eijk, Delft University Press 1996, p. 325.

12. L. H. Brixner, Mat. Chern.and Phys. 16,253 (1987).13. G.Blasse, B. C,Grabmaier,Lu~inescentMaterials, Springer-Verlag, Berlin, Heidelberg, 1994.

14. F. A. Roazhdestvenskii, M. G. Zuev, A. A. Fotiev; “Tantalates of Trivalent Metals”, Nauka,Moscow 1986- in Russian.

15. R. J. Cava, R. S. Roth, “Structure of LaTaOz at 300 C by Neutron Powder Profile Analysis”; JSolid State Chem. 36, (1981], 139-147; R. S. Roth, T. Negas, H. S. Parker, D. B. Minor, C. Jones,“Crystal Chemistry of Cerium Titanates, Tantalates and Niobates”, Proceedings of the 12*~RareEarth Research Conference, 2, (1976), 737-46 and references listed therein.

16. P. T. Kenyon, L. Andrews, B. McCollum, A. Lempicki, IEEEJ.@ant. Electr., QE-18, 1189(1982)

17. Y. Ito, H. Yamada, M. Yoshida, H. Fujii, G. Todzq H. Takeuchi, Y. Tsukuda, Jpn.J Appl. Phys.27, L1371 (1988).

18. W. H: Rhodes, J Am. Ceram. Sot. 64, 13 (1981).

19. G. C. Wei, M. R. Pascucci, E. A. Trickett~ C. Brecher, W. H. Rhodes, SPIEProc. 968,5(1988); C. Brecher, G. C. Wei, W. H. Rhodes, J Am. Ceram. Sot. 73,1473 (1990).

16

6. Papers Published under the Grant

1999

A. J. Wojtowicz, P. Szupryczynski, J. Glodo, W. Drozdowski and D. Wisniewski, “Radiohunines-cence and Recombination Processes in BaFz:Ce”, submitted to J Ply-x: Corzdens. Matter (1999)

J. Glodo, P. Szupryczynski and A. J. Wojtowicz, “Thermoluminescence and Scintillation TimeProfiles of BaFz:Ce”, Acts Phys. Polon. A95, 259-68 (1999)

A. J. Wojtowicz, P. Szupryczynski and W. Drozdowski, “Radiative Recombination in Cc-, Pr-,and Tb-doped Barium Fluoride”, accepted for publication in J. ofAlloys and Compounds

J. Glodo and A. J. Wojtowicz, “Thermoluminescence and Scintillation Properties of LuAP andYAP”, accepted for publication in L ofAlloys and Compoun&

A. J. Wojtowicz, “Some Aspects of Solid State Radioluminescence”, Acts Plzy.s.Pol. 95, 165-178 (1999).

R. H. Bartram, D. S. Hamilton, L. A. Kappers, A. Lempicki, J. Glodo, J. S. Schweitzer and C. L.Melcher, “Electron Traps and Transfer Efficiency in Ceriurn-Doped Lutetium Orthosilicate Scin-tillator”; RadiationEflects & Defects in Solids 150, 11-14 (1999).

1998

A. Lempicki andR. H. Bartrarn, “Effect of Shallow Traps on Scintillation”,J Lumin.81,13-20 (1998)

A. J. Wojtowicz, J. Glodo, W. Drozdowski, K. R Przegietka, “Electron Traps and ScintillationMechanism in YA103:Ce and LuAIOs:Ce Scintillators”, J Lumin. 79,275-291 (1998)

A. Lempicki and J. Glodo, “Ce doped scintillators: LSO and LuAP”, iVucl.Inst. & Meth. A416,333-344 (1998).

A. J. Wojtowicz, L. Glodo, A. Lempicki and C. Brecher, “Thermoluminescence and ScintillationMechanism in YAIOs:Ce”; J. Physics: Cond. Matter 10,8401-8415 (1998)

A. J. Wojtowicz, W. Drozdowski, D. Wisniewski, K. Wisniewski, K. R Przegietk~ H. L. Oczkowskiand T. M. Piters; “Thermoluminescence and Scintillation of LuA103:Ce”, Radiation Measure-ments 29,323-326 (1998)

1997

R. H. Bartram, D. S. Haniilton, L. A. Kappers and A. Lempicki, “Electron Traps and TransferEfficiency of Cerium-Doped Aluminate Scintillators”, J. Lumin. 75, 188-191 (1997).

A. J. Wojtowicz, J. Glodo, D. Wisniewski and A. Lempicki, “Scintillation Mechanism in Re-activated Fluorides”, Proc. 1996 Int’1. Conf. on Luminescence and Optical Spectroscopy of Con-densedMatter, ICL96, eds. J. Hala, P. Reineker, and R. S. Meltzer,J Lumin.72/74,731-733 (1997).

R. H. Bartram and A. Lempicki, “Electron Multiplication in Scintillators and Phosphors”, Proc.1996 Int ’1 Conj on Luminescence and Optical Spectroscopy of Condensed Matter, ICL96, eds.J. HAl~ P. Reineker, and R. S. Meltzer, J Lumin.72/74,734-736 (1997).

W. Drozdowski, D. Wisniewski, A. J. Wojtowicz, A. Lempicki, P. Dorenbos, J. T. M. de Haas,C. W. E. van Eijk and A. J. J. Bos, “Thermoluminescence of LuA103:Ce”, Proc. 1996 Int ‘1.Conj on Luminescence and Optical Spectroscopy of Condensed Matter, ICL96, eds. J. H&la, P.Reineker, and R. S. Meltzer, Z Lumin.72/74,756-758 (1997).

D. Wisniewski, A. J. Wojtowicz and A. Lempicki, “Spectroscopy and Scintillation Mechanismin LuA103:Ce”, Proc. 1996 Int ‘1.Con~ on Luminescence and Optical Spectroscopy of CondensedMatter, ICL96, eds. J. H41a, P. Reineker, and R. S. Meltzer, J Lumin. 72/74,789-791 (1997).

17

*.

A. Lempicki, A. J. Wojtowicz and C. Brecher, “Inorganic Scintillators”, book chapter in lVide-Gap Materials: Theory and Applications, ed. S. R. Rotman (Kluwer Academic Press, Norwell,MA, 1997), p. 235-301.

R. H. Bartrarn and A. Lempicki, “Electron-Hole Pair Production in Scintillators”, R-oc 13’hM ‘1.Conj on Defects in InsulatingMaterials, eds. G. E. Matthews and R. T. Williams (Mat ‘1s.Sci. Forum Vols. 239-241, Trans Tech Publications, Switzerland, 1997), p. 241-244.

A. Watterich, L. A. Kappers, O. R. Gilliam, R. H. Bartrarn, A. Lempicki and M. H. Randles,“Electron Spin Resonance of Dopant and Impurity Centers in LuA1OS Single Crystals”, Proc.13thInt ‘1. Conj on Defects in Insulating Materials, eds. G. E. Matthews and R. T. Williams(Mat ‘1s.Sci. Forum Vols. 239-241, Trans Tech Publications, Switzerland, 1997), p. 253-256.

M. Moszynski, D. Wolski, T. Ludziejewski, M. Kapus@ A. Lempicki, C. Brecher, D. WisniewskivmdA. J. Wojtowicz, “Properties of the New LuAP:Ce Scintillator”, Nucl. hstr. &Meth. A385, 123(1997)

1996

R. H.” Bartram and A. Lempicki, “Efficiency of Electron-Hole Production in Scintillators”,J. Lumin. 68,225-240 (1996).

A. J. Wojtowicz, A. Lempicki, D. Wisniewski, M. Balcerzyk and C. Brecher, “Carrier Captureand Recombination Processes in Ln3+-Activated Scintillators”, l.EEE Trans. Nucl. Sci. 43, 2168-2173 (1996).

A. Lempicki, C. Brecher, D. Wisniewski, E. Zych and A. J. Wojtowicz, “Lutetium Aluminate:Spectroscopic and Scintillation Properties”, IEEE Trans.Nucl. Sci. 43, 1316-1320(1996); Con-ference Record, 1995 IEEE Nuclear Science Symp. & Medical Imaging Conj, ed. P. A. Moonier(IEEE Inc., Piscataway, NJ, 1996).

D. Wisniewski, W. Drozdowski, A. J. Wojtowicz, A. Lempicki, P. Dorenbos, J. T. M. de Haas,C. W. E. van Eijk and A. J. J. Bos, “Spectroscopy and Thermoluminescence of LuAIO~:Ce”,Acts Phys. Pol. A90, 377 (1996).

A. J. Wojtowicz, “New High Performance Scintillators Based on RE-Activated Insulator Materi-als”, Acts Phys. Polon.A90,215 (1996).

A. Lempicki, C. Brecher, D. Wisniewski and E. Zych, “Cerium-Doped Aluminate Scintillators”,Proc. Int ‘1. Con$ on Scintillators and their Applications, SCINT95, eds. P. Dorenbos and C. W.E. van Eijk (Delft University Press, the Netherlands, 1996), p. 340-343.

A. J. Wojtowicz, “Scintillation Mechanism: The Significance of Variable Valence and Electron-Lattice Coupling in RE-Activated Scintillators”, Proc. Int ‘1. Conf on Scintillators and theirApplications, SCINT95, eds. P. Dorenbos and C. W. E. van Eijk (Delft University Press, theNetherlands, 1996), p. 95-102.

M. Moszynski, D. Wolski, T. Ludziejewski, A. Lempicki, C. Brecher, D. Wisniewski and A. J.Wojtowicz, “LuAP, A New Fast Scintillator”, Proc. Int ‘1. Con$ on Scintillators and theirApplications, SCINT95, eds. P. Dorenbos and C. W. E. van Eijk (Delft University Press, theNetherlands, 1996), p. 348-351.

1995

A. Lempicki, M. H. Randles, D. Wisniewski, M. Balcerzyk, C. Brecher and A. J. Wojtowicz,“LuA103:Ce and Other Aluminate Scintillators”, IEEE Trans. Nucl. Sci. 42, 280 (1995); Con-ference Record, 1994 IEEE Nuclear Science Symp. & Medical Imaging Con#., ed. R. C. Trendier(IEEE Inc., Piscataway, NJ, 1995), p. 307-311.

A. Lempicki, “The Physics of Inorganic Scintillators”, J Appl. Spect. 62,209-231 (1995); Recordof the Conference on Excited States of TransitionElements (Kudowa, Poland, Sept. 1994).

1- 18

.&

,

1994

A. Lempicki, M. H. Randles, D. Wisniewski, M. Balcerzy~ C. Brecher and A. J. Wojtowicz,“LuAIOs:Ce and Other Aluminate Scintillators”, IEEE Trans. Nucl. Sci. 42, 280 (1995); Con-ji+ence Record, 1994 IEEE Nuclear Science Symp. & Medical Imaging Con-f, ed. R. C. Trendier(IEEE Inc., Piscataway, NJ, 1995), p. 307-311.

A. Lempicki and A. J. Wojtowicz, “Fundamental Limitations of Scintillators”, J Lumin. 60/61,942-947 (1994).

A. J. Wojtowicz, M. Balcerzyk and A. Lempicki, “The Scintillation Properties of CeXLal.XF3”,J Lumin. 60/61,987 (1994).

A. J. Wojtowicz, M. Balcerzyk, E. Berman and A. Lempicki, “Ce.Lal.XF3: Optical Spectroscopyand Scintillation Mechanisms”, Phys. Rev. B 49, 14880-14894 (1994).

A. Lempicki and A. J. Wojtowicz, “Fundamental Limitations of Scintillators”, J Lumin. 60/61,942 (1994)

A. J. Wojtowicz, M. Balcerzyk, D. Wisniewski, A. Lempicki, C. L. Woody, P. W. Levy, J. A.Kierstead and S. Stoll, “Scintillation Light Trapping and Radiation Darnage in CeF3”, IEEETrans.Nucl. Sci. 41,713-718 (1994).

A. J. Wojtowicz, D. Wisniewski, A. Lempicki, and L. A. Boatner, “Scintillation Mechanisms inRare Earth Orthophosphates”, Conference Record, EURODIM ’94, (Lyon, France, 1994).

A. J. Wojtowicz, A. Lempicki, D. Wisniewski, and L. A. Boatner, “Cerium-Doped Orthophos-phate Scintillators”, Proc. MRS Symposium on Scintillator and Phosphor Materials, eds. M. J.Weber, P. Lecoq, R. C. Ruchti, C. Woody, W. M. Yen, and R. Y. Zhu (Materials ResearchSociety, Pittsburgh PA, 1994), Vol. 348, p. 123-129.

A. J. Wojtowicz, A. Lempicki, D. Wisniewski, C. Brecher, R. H. Bartram, C. Woody, P. Levy,S. Stoll, J. Kierstead, C. Pedrini, D. Bouttet, and Cz. Koepke, “Scintillation Mechanism andRadiation Damage in CeXLal-XF3Crystals”, Proc. MRS Symposiumon Scintillator and PhosphorMaterials, eds. M. J. Weber, P. Lecoq, R. C. Ruchti, C. Woody, W. M. Yen, and R. Y. Zhu(Materials Reseai-ch Society, Pittsburgh PA, 1994), Vol. 348, p. 455-461.

1993

A. Lempicki, A. J. Wojtowicz, and E. Berman, “Fundamental Limits of Scintillator Performance”,Nucl. Instr. &Meth.A333,304-311 (1993).

A. J. Wojtowicz, M. Balc&zyk and A. Lempicki, “Luminescence and Scintillation Properties ofCeXLal.XF3 Monocrystals”, Acts Phys. Polon. 84,963 (1993).

A. J. Wojtowicz, A. Lempicki and E. Berman, “Scintillation Mechanisms in Stoichiometric CeriumCompounds”, Heavy Scintillatorsfor Scient@c and IndustrialApplications (Proceedings of the“C~stal 2000” Int ‘1.Workshop), eds. F. DeNotaristefani, P. Le Coq, and M. Schneegans (EditionsFrontiers, Gif-sur-Yvette Cedex, France, 1993), p. 179-183.

1992

A. J. Wojtowicz, E. Berman and A. Lempicki, “Stoichiometric Cerium Compounds as Scintil-lators - Part II: CeP5014”, IEEE Trans.Nucl. Sci. 39, 1542-1548 (1992).

A. J. Wojtowicz, E. Berman, Czl Koepke, and A. Lempicki, “Stoichiometric Cerium Compounds asScintillators - Part 1: CeF3”, IEEE Trans.Nucl. Sci. 39,494-501 (1992).

A. J. Wojtowicz, E. Berman, Cz. Koepke and A. Lempicki, “Cerium Compounds as Scintillators”,Conference Record, 1991 IEEE Nuclear Science Symp. & Medical Imaging Conj, eds. G. T.Baldwin and F. Kirsten (IEEE Inc., Piscataway, NJ, 1992), vol. 1, p. 153-157.

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*

APPENDIX

Prepared by

A. J. Wojtowicz, P. Szupryczynski

Boston University, Chemistry Department

This summarizes our activities under the grant: Scintillation materials for medical applications,DE-FGO2-9OER61O33, in the period Aug. 1, 1998 – Nov. 30, 1999

TABLE OF CONTENTS

1. Samples

2. Experiments

3. Discussion

4. Papers and presentations; summaries

1. Samples

The experiments presented in this report were performed on samples of oxide (LuAP:Ce, YAP:Ce,YAG:Ce, YAG:Ce, Mg) and fluoride materials (BaFz:Ce, BaF2:Pr, BaF2:Nd, BaF2:Tb). TheLuAP samples were grown by Airtron, YAP by Union Carbide and YAG by ITME, Poland. Allfluoride samples were grown by Optovac under the SBIR project in 1997.

2. Experiments

The experiments outlined in this report were performed at Chemistry Department, Boston Uni-versity. These experiments include:

2.1. X-ray excited luminescence spectra

The luminescence spectra under X-ray excitation provide a convenient and accurate method ofquickly assessing the dominant radiative decay mode of electronic excitations of the materialunder ionizing excitation. These spectra were routinely measured for each of the samples studied.They provide information about the contributions of competing “host” emissions and the branch-ing ratio at the activating rare earth ion (d-~vs. j~transitions).

2.2. Low temperature thermoluminescence (ltTL)

Low temperature thermoluminescence experiments (starting at about 20 K up to about 340 K)provide a tool to estimate energy depths and frequency factors of traps that are shallow enough toinfluence the kinetics of the scintillation process itself. These experiments were performed onoxide materials to obtain the parameters of electron traps and on fluorides in which case theyprovide information on hole trapping characteristic of the fluoride host. Glow curves weremeasured and interpreted in the context of a simple f~st order kinetic model. Electron trap pa-rameters in LuAP, YAP and YAG have been extracted horn fits to those curves. In the case offluorides (up to now for BaF2:Ce) a more complex mixed order kinetic model was employed and

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parameters characterizing thermal release of holes from Vk, H and Vka, Vka’ centers have beenextracted. Further experiments on other fluorides are in progress.

2.3. Isothermal thermoluminescence decays (ITD)

This technique is complementary to ltTL. Afler a certain predefmed temperature (at or near theTL glow peak) is reached the sample is kept at a constant temperature and the decay pattern ofthe phosphorescence is recorded. The technique can be used to study cases in which glow curvesfrom different traps overlap and also cases in which a single trap releases trapped charges undera higher order kinetic when the decay is not exponential. Measurements on LuAP:Ce, YAP:Ce,BaF2:Ce and BaF2:Pr have been performed so far. Until now only in the case of BaFz:Ce havetheoretical interpretation and modeling been carried out. Both experimental and theoretical workwill be continued.

2.4. Scintillation time proffles (STP)

The measurements of rise and decay time under gamma excitation were performed using thesynchronous photon counting method devised and described long ago by Thomas and Bollinger.Since our set-up does not enable us to measure short rise times we have used it to measuretemperature dependent scintillation decay times on many materials including LuAP, YAP, YAGand the fluorides. These results provide information about traps that take part in the scintillationtraps and are responsible for longer than usual decay times in the scintillation time profiles. Thedecay times that can be practically measured using this technique range fi-om about 1 ns to 10 ps.

2.5. Slow decay measurements under pulsed X-ray excitation (SSTP - slow scintillationtime profdes)

This method was developed using a chopper and a photon counting Stanford boxcar to measureslow decays under the modulated X-ray excitation. The technique enables measurement ofdecay times (0.5 ms to about 10 ms) that are not accessible using a standard gamma excitationand synchronous photon counting technique (1 ns to about 10 p.). This experiment has so fmbeen performed on only one material (BaFL:Ce) delivering information on trap lifetimes thatcould not be obtained by any other means.

2.6. Scintillation light yield as function of temperature (LY vs. T)

Since at high enough temperatures the lifetime of any trap becomes short enough to contribute toa measured scintillation light yield (determined by a predefmed fixed time window or the so-called shaping time of the experimental set-up) there are large variations in the measured scintil-lation light yield with the temperature. A simple fust order kinetic model enables us to obtaintrap parameters from fits to experimental points and to obtain trap parameters independently ofthe TL a.mlor ITD methods. These experiments have been performed and then interpreted in thecase of LuAP, YAP and also on pure and Cc-doped BaF2. Measurements have also beenperformed on YAG. More experimental and theoretical work on YAG and other fluorides isrequired to conclude this line of investigation.

3. Discus&ion

For a number of years the group at Boston University has presented strong evidence that thedominant mechanism of radioluminescence in efficient doped materials is due to the consecutivecapture of mobile charge carriers by an activator ion. In particukw the group demonstrated thatthe dopant-induced recombination of electron-hole pairs generated by ionizing radiation pro-vides most of the scintillation light in materials such as LuP04:Ce, LuAP:Ce, LSO:Ce, YAP :Ceand YAG:Ce, In these materials a valence band hole is captured first, creating a Ce4+ ion. In thenext step an electron is captured into one of the Ce4+-localized states with extended Bohr radius

m

and large capture cross section, from which it relaxes to the lowest excited electronic state (d-state)of the Ce3+ ion. Finally the radiative d-~transition completes the cycle producing a visible scin-tillation photon and the unexcited Ce3+ ion. In this scenario, in the absence of electron traps, thebranching of holes between Cc-ions and any other hole-traps (loss centers) determines the scin-tillation efficiency. Unfortunately almost all real materials contain electron traps that take part inthe scintillation process. Deep electron traps are responsible for the further loss of scintillationlight while shallower traps divert part of the scintillation into slower components, decreasing thezero-time amplitude of scintillation, which is important for fast timing applications. Very shallowshort-lived electron traps increase the rise time of the scintillation pulse causing further loss inthe zero-time amplitude.

In some materials other processes, such as reversed recombination ardor exciton-mediated ener-gy transfer, may play a more prominent role but, as a rule, such materials are characterized bylower scintillation efficiencies. In the particular case of alkaline earth fluorides a self-trappingprocess provides a strong competition for direct trapping of valence band holes by activator ions.Consequently a fast direct component is absent or relatively weak and the scintillation time pro-file, even in the absence of the STE emission, displays significant contributions of slow compo-nents that are due to both the reversed recombination and the excitonic energy transfer mechanisms.The energy is then apparently lost not only via a hole-trapping type of nonradiative recombina-tion centers and competing slow STE emission centers, but also because of H-, Vka- and Vka’-center formation. All of these centers, including vk centers responsible for STE emission, inaddition to the already well known effect of introducing additional deleterious absorption bymeans of radiation damage, also play a role. These are directly responsible for much of the lossin the zero-time amplitude in the scintillation time profile or, in other words, for slower thanusual rise and decay times in the scintillation pulse.

The results of our earlier investigations that included LY vs. T, ltTL and STP on oxides (LuAP,YAP) and fluorides (BaF2, BaF2:Ce) and which only recently have been published, filly supportall the earlier models. Upon analyzing these results we concluded that traps were identified in bothclasses of materials, traps that are responsible for the slower than radiative decay as well as forthe occasional longer than usual rise times of scintillation pulses. In particular results for LuAP andYAP and their trap-based interpretation explain some largely misunderstood differences betweenthe two materials, such .as the slower scintillation decay time in YAP, its larger room temperaturescintillation light yield and a slower rise time in LuAP. Although some previously unobservedpatterns emerge regarding traps in both materials (traps in LuAP are shallower than correspondingtraps in YAP), more studies are required to identifi the traps, to determine their ofigin, and tofmd ways to reduce their deleterious influence on the performance of both materials.

Since hole trapping in fluorides is an intrinsic process characteristic of the fluoride host, it wouldappear that large scintillation losses and slow scintillation time profiles should be m“avoidable.However, one case, E%.F2:Ce, displays a measurable direct component in scintillation that is dueto extremely fast hole trapping by Ce3+ ions. Since electrons do not self-trap in the fluoride hostthe process of consecutive hole and then electron capture by the Ce3+ ion renders a self-trappingprocess irrelevant for this component. Interestingly such a possibility was earlier dismissed onthe grounds that self-trapping must be an extremely efficient process while the hole capture bythe Ce ion, on the contrary, must be inefficient because of the repulsive 3+ charge associated withthe Ce3~ ion. Nevertheless, we fmd that direct recombination via the Ce3+ ion contributes to thescintillation. We believe that the process of hole capture is facilitated by a nearby compensatinginterstitial F1- ion that tends to reduce the effect of the 3+ charge on the Ce ion. Any modifica-tion of the host that would enhance the hole capture process by the Ce ion is likely to improvethe performance of the BaF2:Ce scintillator material.

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We expect that the new techniques we have recently developed, including ITD and SSTP, willprovide more new, accurate and detailed information about performance of both oxide andfluoride scintillator materials and in particular about traps in those materials. Any progress inimproving oxide and fluoride materials is not possible without more detailed understanding oftraps and defects.

4. Papers and presentations (summaries)

1) Andrzej J. Wojtowicz, Piotr Szupryczynski, Jaroslaw Glodo, Winicjusz Drozdowski andDariusz Wisniewski, “Radioluminescence and Recombination Processes in BaFL:Ce”, submittedto L Phys.: Condens. Matter

In this paper we report measurements of X-ray- and vacuum W- excited luminescence, lumines-cence excitation spectra and time profiles, low temperature thermoluminescence and isothermaldecays as well as X’ray and gamma-excited scintillation time profiles and scintillation light yieldsat various temperatures on BaF2:Ce an~ for reference, on undoped BaF2, two well known scintil-lator materials. For the fust time we fmd that all these results can be consistently interpreted inthe frame of a model that includes Cc-related recombination centers and several charge “traps”.The charge trapping at most of these “traps” has its origin in self-trapping and trapping of holes atregular (Vk) and interstitial (H) fluorine sites. We have identified and characterized two differentmodes of thermally activated vk release that precede radiative recombination at Ce sites. Thesetwo modes are responsible for a 7 ns rise time and a slower 114 ns components in the scintillationtime profile at room temperature (297 K) that produce about 67!40of the scintillation light detectedwithin a 0.5 ps time window. The remaining 33°A is due to a prompt component decaying withthe Ce3+-radiative lifetime of about 30 ns that originates from the direct recombination of chargecarriers at Ce3+ions. We also estimate that scintillation light loss due to even slower components, at23.1 ps (H-centers), 1.1 ms and 7 ms (VkA and VkA’ centers), exceeds at least three times theamount of light emitted in the 0.5ps time window. Therefore in addition to their well known roleas defect centers actively participating in the formation of stable radiation damage centers thesespecies are also involved in the radiative recombination process itself. The prospects of improve-ments in performance of the BaF2:Ce scintillator are also briefly discussed.

2) A.J. Wojtowicz, P. Szupryczynski, W. Drozdowski, Radiative recombinationin Cc-, Pr-, and Tb-doped barium fluoride, J. of Alloys and Comp. 300-301(2000) 199-206. Acknowledgment: KBN (Poland), TMR (EC), USDoE DE-FG02-96-ER82 117 i DE-FG02-90-ER61 O33, ALEM Assoc. Boston.

Open }shell rare-earth ions doped into the solid state matrix oftentimes easily change theircharge state via interactions with charge carriers generated by ionizing radiation. This featurepromotes a desirable efficient radiative recombination of separated charge carriers at rare-earthion sites but it may also help to stabilize various radiation defects that destructively interfere withthe scintillation process itself. The well known effect responsible for the scintillation light lossdue to absorption introduced by these so-called “radiation damage” centers in alkali halides hasbeen identified and studied for a long time.

In this communication we concentrate on a different and much less known and studied effect inwhich radiation induced centers directly and actively participate in the scintillation process itself.We present and discuss some selected recent results that illustrate the importance of competitionbetween the prompt radiative recombination via rare-earth ions and generation of the radiationdamage centers in barium fluoride crystals activated with Ce, Pr and Tb. We demonstrate thatresults of measurements such as radioluminescence spectra, VUV spectroscopy, low temperature

23

thermoluminescence glow curves, isothermal decays, and scintillation time profiles can beconsistently explained hi the context of a simple model that includes one recombination center(RE-ion) and a number of charge traps.

We find that the trap model of radiation damage centers such as vk describes reasonably well theirparticipation in the scintillation process that includes creation (equivalent to charge carrier trap-ping) and thermally activated decomposition (charge carrier release). These effects are shown toquantitatively account for important characteristics of the scintillation process such as “large varia-tions in the scintillation light yield with temperature and longer decay times in the scintillationtime profiles that effectively lower the scintillation light yield at ambient temperatures.

3). . J. Glodo, A.J. Wojtowicz, Thermoluminescence and scintillation propertiesof LuAP and YAP, J. of Alloys and Comp. 300-301 (2000) 289-294.Acknowledgment: KBN (Poland), USDoE (DE-FG-02-90-ER61 O33), TMR (EC), ALEMAssoc. Boston.

In this communication we report on the application of low temperature thermoluminescence (ltTL)associated with shallow traps in the study of scintillation properties of cerium doped LuAP andYAP crystals. We show that existence of shallow traps and their interference with scintillationprocess readily explain changes in light yield and time profiles with temperature. The analysis oftwo major glow peaks at 183 and 270 K of LuAP:Ce ields trap parameters: The activation ener-

,1 ,gy E = 0.507 eV; the frequency factor s = 3.65xIO s- and E = 0.786 eV, s = 1.77x1013 S-l,respectively. A glow curve of YAP:Ce also shows two major glow peaks at 108 K and 154 K, al-though this case is more complex and involves a distribution in the energies. Assuming a Gaussiandistribution with the standard deviation c ~ 0.018 eV we find trap parameters to be: E = 0.30 eV(the mean); s = 5X1012S-1 and E = 0.5 eV; s = 7X1014 s-l, respectively. Then using the obtainedtrap parameters we calculate examples of time profiles and light yield characteristics to comparethem to the experimental results.

4) A. J. Wojtowicz, Some aspects of solid state radioluminescence, invited plenary lecture,presented at the Jablonski Centennial Conference on Luminescence and Photophysics, 23-27July 1998, Torun, Poland, Acts Phys. Polon. A95 (1999) 165-178.

In this paper we review results of radioluminescence studies on two scintillator materials, LuA103and YA103, activated with Ce. The experiments include measurements of thermoluminescence,isothermal phosphorescence decays, scintillation light yield as function of temperature, and scin-tillation time profiles under ga.mrna excitation. Experimental results are interpreted in the contextof a simple kinetic model that includes a number of electron traps. We have identified andcharacterized a number of deep and shallow traps and demonstrated that traps in LuA103 :Ce aredeeper than corresponding traps in YA103:Ce. Unlike deep traps which are responsible for somescintillation light loss but otherwise do not have any impact on generation of scintillation light,shallow traps are shown to actively interfere with the process of radiative recombination via Ceions. We demonstrate that shallow traps are responsible for some as yet unexplained observa-tions including a higher room temperature light yield of YA103:Ce and its longer scintillationdecay time, as well as a longer scintillation rise time in LuA103:Ce.

5) J. Glodo, P. Szupryczynski, A. J. Wojtowicz, Thermoluminescence and scintillation timeprofiles of BaF2:Ce, presented at the Jablonski Centennial Conference on Luminescence andPhotophysics, 23-27 July 1998, Torun, Poland, Acts Phys. Polon. A95 (1999) 259-68.

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In this communication we present results of measurements of low temperature thermoluminescence,isothermal decays, steady state radioluminescence yield, and scintillation time profiles at varioustemperatures on two scintillator materials, BaFiCe and undoped BaFq. We find that all these resultscan be consistently interpreted in the context of a model that includes several relatively shallowcharge traps. We have identified and characterized one particular shallow trap that causes the decayof the dominant scintillation component of BaFl:Ce to be slower than radiative, as well as a setofothers that are responsible for even slower components in the scintillation time profile of this material.

I 6) A. J. Wojtowicz, J. Glodo, A. Lempicki, and C. Brecher, “Recombination and ScintillationProcesses in YAIOq:Ce”, J l%ys.: Condens. Matter 10 (1998) 8401-15.

This paper reports spectroscopic and scintillation studies of the well-established scintillator ma-terial YA103:Ce. Standard measurements of luminescence emission and excitation spectra havebeen accompanied by investigations of thermoluminescence and scintillation light yield over a widetemperature range, and by decay measurements under pulsed gamma and WV excitations at vari-ous temperatures. These measurements are interpreted in the framework of a model that includes a

recombination center (Ce3+) and a number of electron traps. We demonstrate that some unusual andlargely unexplained features of the YA103:Ce scintillator, such as a substantial disparity betweenscintillation and radiative decay,times, the presence of slow components in scintillation decays, anda strong temperature variation of scintillation light yield between 150 and 300 K, have their origin inthe processes of charge carrier capture and emission by electron traps. Although the nature of thesetraps remains elusive, most of the trap parameters, such as frequency factors, energy depths, andrelative populations have been estimated. This makes it possible to predict the characteristics oftrap-free material and the potential improvements that could be achieved thereby.

I 7) A. J. Wojtowicz, J. Glodo, W. Drozdowski, K. R. Przegietk~ “Electron Traps and Scintilla-tion Mechanism in YA103:Ce and Lti103:Ce Scintillators”, J Lumin. 79 (1998) 275-291.

In this paper we present the results of thermoluminescence, isothermal decay and scintillationlight yield measurements on two isostructural scintillator materials, YA103:Ce and LuA103:Ce.In addition to the variety of deep traps identified by thermoluminescence and isothermal decays,scintillation light yield experiments demonstrate the presence in both materials of a number ofrelatively shallow traps. While the deep traps may reduce the scintillation light yield, they do notinfluence the kinetics of the process. The shallow traps, on the other hand, by interfering withthe process of radiative recombination of charge carriers via Ce3+ ions, can strongly affect notonly the yield of the scintillation process but its kinetics as well. The presence of shallow trapsprovides a consistent explanation for a number of poorly understood relationships between thetwo scintillator materials, including a higher room temperature scintillation light yield and longerscintillation decay time in YA103:Ce, and a longer scintillation rise time in LuA103 :Ce. Theoreticalanalysis indicates that elimination of these traps would make the two materials nearly identical inscintillator performance. Although the specific identity of all traps remains elusive, the performa-nce of both scintillator materials is now, in practical terms, fidly understood.

I8) A. J. Wojtowicz, W. Drozdowski, D. Wisniewski, K. Wisniewski, K. R. Przegietk~ H. L.Oczkowski, T. M. Piters, “Thermoluminescence and Scintillation of LuA103:Ce”, presented atthe LUMDETR conference, Ustron ’97, Rad. Mess. 29 (1998) 323-326.

Thermoluminescence (TL) has proved to be a usefid tool for investigating the kinetics and effi-ciency of light generation in scintillator materials. One particularly important issue has been therole of lattice defects and carrier traps in the mechanism by which energy reaches the emittingcenter. In this paper we address the problem of TL traps and scintillation light output of the fast

I 25

1.

..

LuAP:Ce scintillator material. We report the results of two complementary measurements, TLand scintillation light output (LO) as a fimction of temperature. The latter represents the fustmeasurement of this quantity over such a wide temperature range (250-600 K) and its correlationwith TL indicates that the traps play major role in the loss of scintillation light output.

We fmd that the two different experiments - the well established TL technique and the relativelynew measurement of the scintillation light vs. temperature - both provide important informationabout electron traps in the scintillator material. However, we must conclude that the apparentcorrelations between the two experiments are misleading, since they probe different sets of traps.The TL experiment, at least in the 270-770 K range, provides information about relatively deepertraps (E> 0.7 eV), while the LO vs. temperature experiment probes shallower traps (E ~ 0.7 eV).TL experiments at lower temperatures, say 50-300 K, may reveal other traps, whose values dotruly correlate with the light output at room temperature. Preliminary and yet unpublished TLdata obtained at Boston University do show glow peaks at 170 and 260 K. These experimentswill be continued and will be filly reported elsewhere.

The effect of traps on the scintillation light output of LuAP:Ce cannot be overestimated. Toargue that deep traps can be removed from consideration as loss factors simply by filling them isfallacious, since the radiation dose necessary to accomplish this in a finite time is many orders ofmagnitude higher than is normally encountered. Moreover, even shallow traps, which retaintheir electrons for only milliseconds at room temperature, can cause relatively large apparentlosses in light output (estimated here at 30-35 Yo),making them unavailable for prompt radiativerecombination at Ce3+ ions on the scintillation time scale of nanoseconds. It is clear that traps ofvirtually any depth can contribute to a loss in the fast response of a scintillator material. Moreexperiments, exploring the fill range of trap depths, are clearly needed in order to fully under-stand and assess the impact of electronic traps on performance of ionizing radiation detectors.

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