M E C H A N I S M O F P H O T O S T I M U L A T E D L U M I N E S C E N C E : B a F C I : E u 2 + *)

ZHAO HUI, WANG YONGSHENG, XU ZHENG, TANG HUIJUN, Xu XURONG

Institute o[ Optoelectronic Technology, Northern Jiaotong University, Beijing, 100044, P.R. China

Received 15 September 1998; final version 18 December 1998

We propose a new parallel model based on the analysis of photostimulated lumines- cence (PSL) process and existing theoretical models. While solving this model, the general expression for PSL is gained. BaFCl:Eu 2+, one of the PSL materials, is prepared and a series of spectra are measured. We compared the theoretical and experimental results in terms of the decay process, the difference between two kinds of color centers and the relationship between irradiation dose and luminescent intensity.

1 I n t r o d u c t i o n

As an excellent mater ial for photost imulated luminescence (PSL), BaFCI:Eu 2+ has been widely used in many fields Ill. Storage screen based on PSL has been used in Compute r Radiography system for medical X-ray detection [2]. The PSL process can be s imply visualized as follows. Exposure of BaFCI:Eu2+(X = CI,Br) to X-rays or ultraviolet (UV) radiation creates electrons and holes in the BaFX(X = CI,Br) lattice. Some fraction of the electrons is t rapped at anion vacancies (yielding F centers), and equal number of holes is t rapped at other unspecified sites. The elec- trons captured in F centers can be released from their t raps by exposure to 633 nm radiation f rom a He-Ne laser and will then recombine with the t rapped holes. Par t of the energy released by this recombinat ion process appears as Eu ~+ luminescence at 386 nm, completing the PSL cycle.

Until now, the transfer process of electron between F centers and luminescence centers is still uncertain and two different models have been proposed in this field: conduction band model [3,4] and tunnel model [5-7]. In this paper, the electron transfer process in PSL cycle is analyzed in detail and a new parallel model is proposed.

2 T h e o r e t i c a l a n a l y s i s

2.1 C o n d u c t i o n b a n d m o d e l a n d t unne l m o d e l

As mentioned above, two different models of PSL have been proposed: conduc- tion band model and tunnel model. The former one can be described as follows [3]: par t of Eu 2+ in BaFX:Eu2+(X = CI,Br) could release electrons during the X-ray irradiation to the conduction band and become Eu 3+. These electrons are easily

*) Supported by the National Nature Science Foundation of China.

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Zhao Hui e t al.

captured by the vacancies of haloid ions to form the F centers. When the sample is stimulated by visible light, the captured electrons could absorb the st imulated energy and be released to conduction band. Then they could recombine with Eu 3+ and the emission occurs. Experiments with photoconduction are the basis of con- duction band model. But these experiments have not been well repeated by others and the conditions are uncertain. From this model quadratic relationship between PSL intensity and X-ray irradiation dose is obtained. This does not coincide with the experimental results which give linear relationship. Furthermore, this model suggests that the valency of Eu ion was changed during the PSL process. This disagrees also with the experimental result that the storage effect is created by UV-irradiation with energy smaller than the ionization energy [8].

Tunnel model [6] suggests that some complexes of F center and adjacent Eu 2+ are formed during the irradiation. In the readout process, electrons of F centers relax to "tunnel level" and then recombine with Eu ion by tunnelling. In this model, the forming of the complex is still in dispute and the relaxation from excited state of F center to "tunnel level" is a upward relaxation, which is converse with common relaxation. Furthermore, this model could not yield convincing evidence to deny the conduction band process.

2.2 Paral le l model

The main difference between these two models is the channel of electron transfer in readout process: conduction band or tunneling. From the view of physics, when the electrons in F centers absorb energy and go over to excited energy level, both the entrance to conduction band by the thermal excitation and the direct recombination with Eu ion by tunneling are possible. We suggest that these two processes both exist in the readout process and each has a certain probability. Obviously, these probabilities should vary in different materials or under different irradiation and stimulation conditions. In some cases, one of the processes may have much higher probability than the other one, then one of the models could be appropriate for this case. But in order to explain all cases correctly, both processes have to be considered.

The energy level scheme of PSL materials, as well as the transfer and transition of electrons in readout process are shown in Fig. 1. F stands for F center and E for luminescent center, nfo , nf, nc, he, ne0 stand for the electron populations in the ground and excited states of F center, conduction band, excited and ground states of luminescent center, respectively. Under the X-ray irradiation, the excited electrons are captured by traps, and after the transient process the populat ions of electrons in the sample could be described as follows:

ne ---- ne ---- nf ~-- O, nf o = 3'D, (1)

where D stands for irradiation dose and 3' is a constant. This is the initial condition for the readout process.

1206 Czech. J. Phys. 49 (1999)

Mechataism of photostimulated luminescence: BaFCI:Eu 2+

C.B.

wc wt

We W W f V1

h v 2

~o F

E

/7 c

Fig. 1.

V . S .

Energy levels in the PSL material and the channels of the electron transfer in the readout process.

In the readout process, electrons in the ground of F centers are st imulated by readout light to the excited state. Then they have three chances: to relax to the ground state of F centers, to transit to the conduction band by the thermal exci- tation and to tunnel to the luminescent centers. In the PSL materials, the number of released electrons in the readout process is much smaller then that of captured electrons, so the processes described by Wee, Wcf could be approximately regarded as one molecule process. Tha t is, Wee and Wcr remain constant in the PSL process. Based on the above analysis, onege t s the equations describing the readout process:

dnfo = 7 t f W f - n f o W l o ,

dt d n f dt = t t f° W f ° + n c W c f - - i t t fWfc - - ; t f W f - - l t f W t ,

dnc --" 71fWfc - - I l c W c f - y t c W c e ,

dt dne

= ncWce + nfWt - nCWe, dt

d n e o = n e W e " dt

(2)

From the initial conditions: nf = 0, nf o = 7D, at least in the initial stage of the readout process, we have Wfnf << Wfo nf o. From this approximation, and by the use of I(t) oc Wene(t), we solve the equations and obtain the PSL intensity

W~71D (E]e,.,t + E2e,2t + E3e_Wtot + E4e_W,t) ' fo

(3)

Czech. J. Phys. 49 (1999) 1207

Zhao Hui et al.

where

+~)(~l-Wf°+r2 r2) [ Wee+ Wt_ ] E~ = (we ~7)-,~ (wc~ + w ~ + ,.~) ,

~:~--w'°+~' ~,) [w~ + wt._ ] E2 = (We ~-t~ (WCf + Wee + r2) ,

- W, [ wt w ] E 3 - - We W c e ~ - ~ f c ( c f - ~ - W c e - W f ° ) '

E4 = - ( E l + E2 + Ea) ,

rl = l (--al+~l/a~l--4a2), r2= l ( - -a l - -~/a21--4a2) ,

al = Wcf + Wce-F Wfc'k" Wf + Wt , a~ = (Wf¢ + W~, + Wt)(Wcf + Wc~) - W f c W ¢ f ,

and 71 is a constant.

3 E x p e r i m e n t a l

BaFCI:Eu 2+, a kind of PSL material, was prepared. The excitation and emission spectra were measured. The excitation spectrum is a band spectrum peaking at 286 nm and the emission spectrum is composed of two components: the line compo- nent at 362.2 nm and the band component around 386 nm. Based on these results, the stimulation spectrum of BaFCI:Eu 2+ was gained by changing the st imulated

40, /

-I o=

v

30, " o /m e-

/ e- 20, o E -..1 ._1 J / " =

I / % 10'

/

/ O,

0 10 20 30 40 50 60

Time(sec)

Fig. 2. Decay curve of PSL stimulated at 450nm (a) and 550nm (b); line - - theory, points - - experiment. (continued)

1 2 0 8 Czech. J. Phys. 4g (19g9)

Mechanism o f photostimulated luminescence: BaFCI:E,u 2+

40

~ 3 o , 8 ¢-

2 0 , ¢-

-.I

1 0 - / /

/ 0

0

/

/

i / /

/

\ \

b)

• • • I

'o 2'0 ' ' ' 1 30 40 ,50 O0

Time(sac)

Fig. 2. (continued)

wavelength from 400 nm to 650 nm. It has two maxima, one is around 450 nm and the other at 550 nm. After these, the decay curves of the PSL were measured under the stimulaltion at 450 nm and 550 nm, respectively, as shown in Fig. 2. The sample was irradiated by X-ray (10 mA, 25 kV) for 5 minutes before these measurements to complete the storage process. The relationship between the PSL intensity and the X-ray irradiation dose was also measured by changing the irradiation time, as shown in Fig. 3.

Fig. 3.

A I

O - J

4 .

3 .

2 . ~ B

I I I I • " '

-9 -8 -7 -6 -5 -4

Log(D)

The relationship between the X-ray irradiation dose and the PSL intensity•

Czech. J. Phys. 49 (1999) 1209

Zhao Hu i et al.

4 R e s u l t s a n d d i s c u s s i o n s

4.1 The decay of PSL

We fit the experimental curves of the PSL decay with the theoretical result (3), as shown in Fig. 2 and Table 1. | t is obvious that the theoretical results of parallel model coincide with the experimental results.

Table 1. Fitting parameters of the decay curves.

Wavelength We Wf o Wt Hit Wfc Wee Wcf of stimulation

450nm 0.1007 0.3983 0.0609 0.2276 0.0076 0.4013 0.2801 550nm 0.108310.4547 0.0855 0.0963 0.1279 0.4182 0.3432

4.2 The difference between two kinds o f F centers in BaFCI:Eu 2+

There are two kinds of F centers in BaFCI:Eu 2+, F ( F - ) centers and F (CI - ) cen- ters. They correspond to the two peaks in the stimulation spectrum. Investigating the relationship between the relative intensity of these peaks and F/CI ratio [9] we found that F ( F - ) centers correspond to the peak at 450nm and F (CI - ) cen- ters to that one at 550 nm. Furthermore, we studied the tempera ture effect of two kinds of F centers and found that the PSL of F ( F - ) centers (st imulated at 450 nm) is almost independent of temperature but that of F (CI - ) centers (stimulated at 550 nm) increases with temperature [10]. From these results, we can conclude that the electron transfer of F ( F - ) center is mainly realized by tunneling process and that of F (CI - ) center involves conduction band. Our fitting results of Wt = 0.2276, Wfc = 0.0076 while stimulated at 450nm and Wt = 0.0963, Wfc = 0.1279 while st imulated at 550 nm coincide with the above results and analyses.

4.3 T h e re la t ionship between the X-ray irradiation dose and PSL intens i ty

The linear relationship between the X-ray irradiation dose and the PSL intensity was theoretically obtained in the parallel model. This result coincides with our experiments, as shown in Fig. 3. As mentioned above, quadratic relationship was obtained for the conduction band model. This is also an evidence for the correctness of the parallel model.

5 C o n c l u s i o n s

Basing on the analyses of the PSL process and existing theoretical models, we propose a new model of PSL - - the parallel model. Solving this model, the general expression for PSL is gained. We compared the theoretical and experimental results in terms of the decay process, the difference between two kinds of color centers and

1210 Czech. J. Phys. 49 (1999)

Mechanism of photos t imula ted luminescence: BaFCI:Eu 2+

the relationship between irradiation dose and luminescent intensity. We found that parallel model agrees with experiments in these terms. For the support of this model, however, further experimental works are still needed, especially in other PSL materials.

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[5] P. Avouris and T.N. Morgan: J. Chem. Phys. 74 (1981) 4347.

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