1.
Introduction

Thermoluminescence is a form of luminescence that is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon heating of the material. TL is one of the thermally stimulated phenomenon which provides a sensitive tool for investigation of trap energy levels in crystals. The main aim in this study has been to gather information about TL parameters like trap depth or activation energy (E) of electrons, frequency factor (S), capture cross-section (σ) for hole/electron etc. Actually, study of sulphide phosphors activated by specific impurities is of a considerable response in basic technology and science^{[1–7, 11]}. Another important application in particular of TL is a solid-state dosimetry of ionizing radiation. An interesting outgrowth of this is the dating of archaeological and geological specimens, which have been exposed to nuclear radiation for thousands of years^[12–18].



The luminescence intensity versus temperature curves, traditionally known as glow curves, came into extensive use after the research of Randall and Wilkins^[13]. Most of the work has been done on the alkali halides and on II–VI group materials. The wide band gap II–VI group semiconductors with energy gap more than 2 eV have been used in various optoelectronic devices. ZnSe, whose energy band gap is 2.67 eV, has seemly potential as an efficient orange–red light emitter. Vacancies and foreign impurities produce self-activated and luminescence centers which affect the optical and electrical properties in ZnSe. Due to the scientific and commercial interest ZnSe doped with Pr and Ag ions have been studied and investigated. The present paper focuses on the preparation, characterization and TL measurement of ZnSe:Tb and ZnSe:(Mn, Tb) phosphors. The TL glow curves of these phosphors were recorded from liquid nitrogen temperature to room temperature and above. TL parameters have been determined by employing Curie’s method, various heating rates method and initial-rise method.

2.
Experimental technique

ZnSe:Tb and ZnSe:(Mn, Tb) phosphors were prepared by firing an appropriate mixture of ZnSe, Tb₂O₃ and MnCl₂ with constant amount of NaCl for each sample in a silica crucible. The firing was done at 780 ± 20 °C for 1 h in nitrogen gas atmosphere of silica tubular furnace. The crucibles were taken out of the furnace when the firing was completed and then finally powdered so as to get uniform particle size. For the measurement of thermal glow curves at different temperatures, the authors fabricated the necessary apparatus as described elsewhere^{[2, 4, 7, 11]}. The apparatus consisted of two co-axial brass cylinders. The inner cylinder has an elliptical cross-section inclined at 45° to the horizontal and is placed inside the outer cylinder. The space between the two co-axial cylinders is evacuated with the help of an oil rotator pump. The temperature of the phosphors was measured by a chromel-alumel thermocouple with an accuracy of 1 K. The emitted light was detected by RCA IP21 photomultiplier tube operated at 1200 V DC supply. The output of PM tube was fed to multiflex galvanometer having current sensitivity of 10^?9 A/mm.



To study TL glow curves samples were excited for 3 min. by UV radiation (365 nm). When the phosphorescence intensity becomes feeble then the sample was heated at three different heating rates 0.4, 0.6, and 0.9 K/s to record the glow curves.

3.
Results and discussion

3.1
Thermal glow curves

3.1.1
At uniform single heating rate

Thermal glow curves for ZnSe:Tb and ZnSe:(Mn, Tb) phosphors from temperature 100 to 340 K were recorded at uniform heating rate 0.4 K/s and are shown in Figs. 1 and 2.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17050017-1.jpg'"
class="figure_img" id="Figure1"/>



Download



Larger image


PowerPoint slide






Figure1.
(Color online) Thermal glow curves at single uniform heating rate (0.4 K/s).






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17050017-2.jpg'"
class="figure_img" id="Figure2"/>



Download



Larger image


PowerPoint slide






Figure2.
(Color online) Thermal glow curves at single uniform heating rate (0.4 K/s).




It is clear from the figures that generally there is one prominent peak in all the samples between the temperature range 100 to 370 K. It is found that the intensity of peak maxima of TL glow curves decreases with increasing concentration of Mn²⁺ ions. This may be due to the concentration quenching^[19].

3.1.2
At three different heating rates

The glow curves obtained at three linear heating rates 0.4, 0.6, and 0.9 K/s for the ZnSe:Tb and ZnSe:(Mn, Tb) phosphors are shown in Figs. 3 and 4.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17050017-3.jpg'"
class="figure_img" id="Figure3"/>



Download



Larger image


PowerPoint slide






Figure3.
(Color online) Glow curves for ZnSe:Tb phosphors at three different heating rates.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17050017-4.jpg'"
class="figure_img" id="Figure4"/>



Download



Larger image


PowerPoint slide






Figure4.
(Color online) Glow curves for ZnSe:(Mn, Tb) phosphors at three different heating rates.




It is apparent from these glow curves (Figs. 3 and 4) that the position of peak maxima shifts towards higher temperature side, at higher rate of heating.

3.2
Evaluation of trap-depth

3.2.1
Curie’s method

The values of trap depth (E) corresponding to different peaks of Figs. 1 and 2 was calculated by employing the Curie’s relation^[18]

$${{E}} = frac{{{{{T}}_{ m{m}}} - {{{T}}_0}left( {{{beta }}/{{s}}} ight)}}{{{{k}}left( {{{beta }}/{{s}}} ight)}},$$

and are tabulated in Table 1. Lifetime (τ) of electrons in trap has also been calculated using relation

$${{tau }} = frac{1}{{{{s}};{ m {exp}}left( { - {{E}}/{{kT}}} ight)}},$$

and are tabulated in Table 1.

3.2.2
Various heating rates method

Glow peaks temperature corresponding to three different heating rates were determined from the glow curves of Figs. 3 and 4. The trap depth was calculated by using the relation^{[2, 6–8]}:

$${{E}} = {frac{{{{k}}{{{T}}_1}{{{T}}_2}}}{{{{{T}}_1} - {{{T}}_2}}}} { m {ln}} left[ { {frac{{{{{beta }}_1}}}{{{{{beta }}_2}}}} {{left( {frac{{{{{T}}_2}}}{{{{{T}}_1}}}} ight)}^2}} ight],$$

and values are tabulated in Table 2.

3.2.3
Initial rise method

Further, from these glow curves of Figs. 3 and 4 another graph was plotted between log of maximum brightness (ln I_m) and reciprocal of the maximum intensity temperature (1/T_m)^{[2, 7, 11]}. Such plots were found to be a straight line and are shown in Fig. 5.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17050017-5.jpg'"
class="figure_img" id="Figure5"/>



Download



Larger image


PowerPoint slide






Figure5.
(Color online) ln I_m versus 1/T_m plot.




The slopes of the straight lines give trap depth (E), which has been calculated and is tabulated in Table 2. The activation energy determined by these two methods is found to be in close agreement.

3.3
Evaluation of frequency factor

The frequency factor (S) or the attempt to escape frequency was calculated in these phosphors and tabulated in Table 2.



It is seen that the frequency factor is order of 10⁷ s^?1, which is also in close agreement with values reported for other ZnS/ ZnSe phosphors^{[2, 7, 8]}.

3.4
Evaluation of capture cross-section

The capture cross-section (σ) of electrons has been computed using the well-known Mott and Gurney relation^{[9, 12, 16, 17]} and is found to be of the order of 10^?22 m² (see Table 2). The capture cross-section calculated for the studied phosphors is found to decrease with increase of trap-depth. This is in conformity with the physical principle that the greater the depth the lesser the capture probability.

No	Phosphor	Peak temperatures T1 at β1, T2 at β2 and T3 at β3	At β1 & β2	At β2 & β3	At β1 & β3	Σ (10-22 m2)	From graph lnIm versus 1/Tm
1	ZnSe:Tb,
	Tb = 0.05%	181, 184, 187	0.36, 0.43	0.37, 0.96	0.36, 0.63	8.00	0.39
	Tb = 0.01%	160, 163, 166	0.28,0.03	0.29,0.08	0.28,0.03	0.71
2	ZnSe:(Mn,Tb),
	Mn = 1%, Tb = 0.05%	201, 204, 207	0.44, 6.34	0.45, 14.28	0.45, 9.59	95.85
	Mn = Tb = 0.05%	199, 202, 205	0.43, 4.83	0.44, 10.79	0.44, 7.30	74.49	0.51

Table2.
Values of peak temperature T_m (K), trap depth E (eV), frequency factor S (s^?1), and capture cross-section σ (m²) calculated by various heating rates method in these phosphors at three heating rates β₁ = 0.4 K/s, β₂ = 0.6 K/s & β₃ = 0.9 K/s:



Table options
-->


Download as CSV

No	Phosphor	Peak temperatures T1 at β1, T2 at β2 and T3 at β3	At β1 & β2	At β2 & β3	At β1 & β3	Σ (10-22 m2)	From graph lnIm versus 1/Tm
1	ZnSe:Tb,
	Tb = 0.05%	181, 184, 187	0.36, 0.43	0.37, 0.96	0.36, 0.63	8.00	0.39
	Tb = 0.01%	160, 163, 166	0.28,0.03	0.29,0.08	0.28,0.03	0.71
2	ZnSe:(Mn,Tb),
	Mn = 1%, Tb = 0.05%	201, 204, 207	0.44, 6.34	0.45, 14.28	0.45, 9.59	95.85
	Mn = Tb = 0.05%	199, 202, 205	0.43, 4.83	0.44, 10.79	0.44, 7.30	74.49	0.51

3.5
Evaluation of lifetime of electron

The lifetime of electrons in the trap has been calculated by using well-known relation $1/{{tau }} = {{s}}{
m {exp}}( - {{E}}/{{kT}})$ and is tabulated in Table 1. The lifetime of electron in traps is calculated for these phosphors and found to be in the range of 14 to 35 seconds between temperatures 100 to 350 °K^{[7, 11, 15, 17]}.

No	Phosphor	Tm (K)	E (eV)	τ (s)	σ (10?22 m2)
1	ZnSe:Tb,
	Tb = 0.05%	181	0.36	19.83	7.77
		250	0.50	26.95	4.33
	Tb = 0.01%	160	0.31	17.66	9.36
		265	0.53	28.50	3.85
2	ZnSe:(Mn,Tb),
	Mn = 1%,	130	0.25	14.58	12.14
	Tb = 0.05%	201	0.40	21.72	6.57
		313	0.63	33.46	2.70
	Mn = Tb =	142	0.28	15.81	10.97
	0.05%	199	0.39	21.69	6.63
		325	0.66	34.71	2.49

Table1.
Values of peak temperature T_m (k), trap depth E (eV), life time τ (s), and capture cross-section σ (m²) calculated by Curie’s method: β = 0.4 K/s, K = 480 K/eV and T₀ = 10 K.



Table options
-->


Download as CSV

No	Phosphor	Tm (K)	E (eV)	τ (s)	σ (10?22 m2)
1	ZnSe:Tb,
	Tb = 0.05%	181	0.36	19.83	7.77
		250	0.50	26.95	4.33
	Tb = 0.01%	160	0.31	17.66	9.36
		265	0.53	28.50	3.85
2	ZnSe:(Mn,Tb),
	Mn = 1%,	130	0.25	14.58	12.14
	Tb = 0.05%	201	0.40	21.72	6.57
		313	0.63	33.46	2.70
	Mn = Tb =	142	0.28	15.81	10.97
	0.05%	199	0.39	21.69	6.63
		325	0.66	34.71	2.49

4.
Conclusion

The glow curves obtained at three different heating rates (Figs. 3 and 4) do not appreciably differ in their shape except that the curves at higher heating rate are comparatively less diffuse. At higher rate of heating the curve as a whole is shifted towards the higher temperature side, because the traps are electron traps in these phosphors.



The activation energy determined by Curie’s method, various heating rates method and initial rise method are found in close agreement. It is found that the lifetime decreases with decrease of temperature; however the capture cross-section calculated for the studied phosphors is found to decrease with increase of trap depth. High-temperature TL glow peak shows the behavior displayed by phosphors may be useful for dosimetry application.