STIMULATED PHOTON- AND ELECTRON EMISSION FROM LATTICE DEFECTS IN BARIUM- AND STRONTIUM SULPHATE

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STIMULATED PHOTON- AND ELECTRON

EMISSION FROM LATTICE DEFECTS IN BARIUM- AND STRONTIUM SULPHATE

G. Holzapfel, M. Krystek

To cite this version:

G. Holzapfel, M. Krystek. STIMULATED PHOTON- AND ELECTRON EMISSION FROM LAT- TICE DEFECTS IN BARIUM- AND STRONTIUM SULPHATE. Journal de Physique Colloques, 1976, 37 (C7), pp.C7-238-C7-240. �10.1051/jphyscol:1976758�. �jpa-00216917�

C7-238 ^JOURNAL DE PHYSIQUE Colloque C7, suppKment aau ^no 12, Tome 37, Ddcembre 1976

STIMULATED PHOTON- AND ELECTRON EMISSION

FROM LATTICE DEFECTS IN BARIUM- AND STRONTIUM SULPHATE

G. HOLZAPFEL and M. KRYSTEK

Physikalisch-Technische Bundesanstalt, Institut Berlin, 1 Berlin 10, Abbestr. 2-12, FRG

Rksumk. - L'Btude des pieges d'electrons dans les sulfates alcalino-terreux actives par des terres rares a kt6 axke principalement sur le BaS04 et le SrS04. Ces derniers, cristallisent dans le m6me rkseau de barite et sont dopQ en E u ~ + . L'kmission simultanee de photons et d'electrons (lumines- cence et exokmission thermostimul6es) ont pour origine des pieges d'klectrons equivalents, mais il n'y a pas d'interfkrence entre ces phknomenes. La dependance des niveaux d'energie des pieges par rapport aux parametres de reseau est aussi discutke ; elle signale des defauts intrinseques.

Abstract. - Investigation of electron trapping sites in rare earth activated alkaline earth sul- phates is focussed on Euz+ doped Bas04 and SrS04, both crystallizing in the barite lattice. Simul- taneous photon- and electron emission (thermally stimulated luminescence and exoemission) originate in equivalent electron traps, but the phenomena do not interfere. The dependence of the trap energy levels on the lattice parameters, which points to intrinsic defects, is discussed.

Sulphate phosphors became interesting on account of their high response to ionizing radiation 11, 21.

Optimum efficiencies are achieved with earth alkali sulphates, especially CaSO,, BaSO, and SrSO,.

Activators for thermoluminescence (TL) are conve- niently provided by rare earth (RE) admixtures.

The T L emission characteristics are largely deter- mined by the valency of the incorporated RE ions.

RE3+ ions produce individual line-structured spectra which can be identified with transitions between known RE3+ levels shielded against the host crystal lattice field. In contrast the RE2+ ion, which is repre- sented only by Eu2+, exhibits a non-structured, broad TL emission band, suggesting strong coupling with the lattice. The RE3+ line emission spectra are enti- rely independent of the host lattice, whereas the Eu2+

emission band shifts on the wavelength scale when the lattice structure is varied.

Another feature, which is controlled by the trapping sites, is the glow peak location on the temperature scale. Here the TL glow peaks are independent of the RE3+ species but dependent on the individual sul- phate lattice. This glow peak shift is even more pro- nounced for Eu2+ doping.

These facts suggest trapping sites which are relat- ed to the interatomic distances of the sulphate lattices.

Excluding CaSO,, BaSO, and SrSO, crystallize in the same lattice and would appear to be suitable for inves- tigation of the nature of trapping sites in more detail.

Consideration of the TL emission spectra indicates that only Eu2+ doping can produce classical recom- bination phosphors with the trapping sites clearly separated from the activator sites.

General information on electron traps in ionic lattices is often available from measurements of exoe- lectron emission 131. This may take place simultaneou- sly with luminescence from recombination phosphors.

Common to both is the detrapping of electrons by thermal or optical stimulation ^(I). The mobile elec- trons may then recombine with holes trapped in acti- vators, so causing photon emission (TL). Alternati- vely, the detrapped electrons may escape from the surface (TSEE). Outside the crystal the exoelectrons can be sensitively detected by particle counters. As a result of their small energy the electrons are emitted only from a very thin surface layer, whereas photon emission occurs in the bulk of the emitter. If pene- tration of the surface is not seriously affected by the charge of the escaping particle, TL and TSEE will yield comparable results [4].

Since activators are not required for exoemission, the initial trap spectrum of phosphors can be inves- tigated before and parallel with TL activation. The results for BaSO, and SrSO, increasingly doped with Eu2+ are shown in figure 1. The undoped materials already show strong TSEE which remains unaffected by the Eu2+ doping up to concentrations of loz0 cmd3.

This evidently means that electron traps are not intro- duced into the sulphate lattice b y the activator ; rather they appear to be formed by natural defects (or impurities) in the original material. This result is supported by closer inspection of the correspon- ding luminescence and exoemission glow curves

(1) Code : TSEE Thermally Stimulated Exoelectron Emission OSEE Optically Stimulated Exoelectron Emission.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976758

STIMULATED PHOTON- AND ELECTRON EMISSION FROM Bas04 AND SrS04 C7-239

Analysis of the glow curves by a high resolving, computer-assisted method [4, 51 revealed f i ~ s t order kinetics governing both the TL- and the TSEE pro- cess. This indicates that the effects do not interfere as already shown by the independence of the TSEE efficiency with regard to the growing TL activation (Fig. 1). The term data (activation energy, frequency factor) could therefore be uniquely evaluated and defi- nitely attributed to single trapping levels, after sepa- ration of minor satellite levels [4] (Table I). The fre- quency factors are of the order of the DEBYE fre- quency. Identical activation energies prove that the same type of trapping site is effective for TL and TSEE and that these traps are not distorted by the vicinity of the surface.

The energy shift of the trapping levels when the sul- phate is altered may be correlated to lattice parame- ters [2]. A decrease in the average nearest neighbour

'//-

1

U Sr a1 TL

c P

-

-

0

400 500 K 600

Temperature

-

-u

6 1

FIG. 2. - TI- and TSEE glow curves of Bas04 and SrS04 ; heating rate 0.14 Kis. Dashed curves computed using the term

data of table I.

'

distance (BaSO, 0.286 nm, SrSO, 0.275 nm) results in a shift of the activation energy to higher values (BaSO, 1.08 eV ; SrSO, 1.27 eV). Hence it should be discussed whether a relation similar to the well known MOLLWO rule can be found.

An immediate application to our results does not appear feasible since the MOLLWO-rule relies on optical absorption measurements. The thermal acti- vation energies obtained from TL- and TSEE- mea- surements differ from the optical activation energies by the FRANCK-CONDON ratio [6]. Due to varying polarisation properties this ratio may change even in

5 ^{- -}

lYg

b) SrS04 ^{I I} I I

I I

to

-

O - , n # , -

01 03 mol% 0.5 Eu,O, concentratton

-

b) TSEE FIG. 1. - Relative TL- and TSEE efficiencies of Bas04 and ;;;

SrS04 dependent on the dopant concentration.

-

_z.

& Q L -

-

(Fig. 2) which exhibit characteristic single peaks. The

2

TL-and TSEE glow curves are almost identical without _0,2

j

any change in TSEE between zero and high Eu2+ ^ernlss~on

doping. The specific peak shift from BaSO, to SrSO,

occurs for TSEE as well as for TL.

o I

Term data of prin~i~val electron trap levels in BaSO, and SrSO, Material

term data

BaSO, SrSOa

TSEE

(*) Normalized to q = 1 Kjs.

C7-240 G. HOLZAPFEL and M. KRYSTEK the same lattice structure, i. e. from BaSO, to SrSO,.

Leaving SrSO, for later investigation, only the optical activation energy of BaSO, is known from optical measurements (absorptance and respectively reflec- tance) combined with photothermal exoelectron emis- sion (selective OSEE) [7].

Another aspect refers to the lower symmetry of the barite lattice, since the MOLLWO-rule.

EOP' = A . d - @ (1)

(Eop, optical activation energy, d lattice constant) was originally formulated for NaC1-type lattices [8].

The constants A, a, however, change with the lattice type. In principle, if the lattice structure is non-cubic, one cannot expect the simple relation (1) to be valid a priori. Hence the intrinsic nature of the defects producing the electron traps in the highly sensitive sulphate phosphors must be substantiated by conti- nued study.

References

[1] YAMASHITA, T., NADA, N., ONISHI, H., KITAMURA, S., [4] HOLZAPFEL, G., KRYSTEK, M., Phys. Status Solidi (a) 37 Proc. 2nd Int. Conf. Luminescence Dosimetry, Gatlin- (1976) 303.

burg, C0nf-680920 and Physics 21 [5] HOLZAPFEL, G., KRYSTEK, M., WOLBER, L., Vide 30 (1975)

(1971) 295. I n7

A",.

[2] DIXON, R. L., EKSTRAND, K. E., J. Lumin. 8 (1974) 383.

[31 BOHUN, A+, SCHARMANN, A., K ; ~ x ~ ~ ~ ~ , H., proc., 4 th 161 NINK, R.2 HOLZAPFEL, G . 5 . 9 Physique Collo4.34 (1973) C 9.

Int. Symp. Exoelectron emission and dosimetry, [71 ^NINKy R. Optik 41 515.

Liblice 1973, Czechosl. Acad. Sc. Ed. A. Bohun [8] MARKHAM, J. J., F-Centers in Alkali Halides (Academic

Prague (1 974). Press, New York and London) 1966.