Dhoble, S J

Dubey, R N

Tiwaki, Manoj

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/ndian J. Phys. 79 (1). 65-68 (2005)U PThermoluminescence studies of CaS phosphors activated by Ce-^ ,^ Na  ^ and Ag^M anoj Tiwari*. Vijay Singh, S JjDhoble' and R N Diibey’Department o f  Physics. Regional Insrmile ol Education, Bhopal-462 013 Madhya Pradesh. India •Department o f  Physics, Kamla Nehru C'ollegc, .Sakkard|ra Square. Nagpur-^440 009, Maharashtra. India -Department o f  Physics, Maulana A /ad  C ollege o f  T ecl^ ology, Bhopal-462 007 Madhya Pradesh. IndiaE-mail. s|dhobleCa^re|iilfmail comR ece ived  2S N nvem her 2002, ci^'epted 12 O ctober 2004A bstract : Thermoluminescence (TL) o f  C'c' .^ Na^ and Ag^ activated C'aS phosphors has been studied The TL intensity decreases by the addition of monovalent activator (Na" )^ in C'aS C'e^  ^ phosphor This may be due to the foimation o f C'e'*"'^  ions during Y-irradialion. which do not participate in the TL process. Only one type o f  defect is formed in C'aS due to its exposure to y-radiation. Doping with C'e^ "'^  or Ag^ ions in the C'aS forms new defects, which are responsible for the additional TL peaks observed in these samplesK eyw ord s : Therm olum incsccnce, trapping param etei, C'aSPACS N os. 7 8 .6 0 .Kn. 6 1 .7 2 .W wAlkaline earth sulphide phosphors have aroused much interest after (he discovery  o f  infrared  stim ulation  and sensitized luminescence (1,21. The knowledge o f defect structure and its connection with distribution o f energy levels in the band gap of solids is an important prerequisite for thorough understanding of luminescence phenomenon. Thermc^luminescence (TL) and decay characteristics o f phosphors have been widely used as a means of investigating the m echanism  o f luminescence. The decay characteristics offer significant inform ation about the kinetics involved in the lum inescence process and the energy distribution o f trapping centers. B lokinsev (3 1 and De Groot [4J were the first among the early workers, who derived theoretical tim e-dependence o f  the decay  o f  p h o sphorescence  from  consideration o f electron transitions between various energy levels.CaS phosphors were obtained using solid state reaction with control bearing betw een activators and CaSO^ as starting material and fire at higher tem perature (80CTC) for 2 hours. The reducing agent was AR grade carbon  pow der used in the synthesis. This phosphor has been denoted as CaS: Ce^" .^ CaS phosphors were prepared by solid state diffusion method at higher temperature (800®C) for 2 hours using Na2SO^ as a flux* Corresponding AuthorWith Starting material CaSO^; the prepared phosphor denoted as CciS: (Na***). The concentration t)f Ce and Ag werechanged to optim ize the lumen output and is denoted as CaS: Ag'^  ^ phosphor. A fter heat treatment, the resultant powder pulverized in a dry atmosphere and stored, keeping in view the extreme purity as the main consideration in preparation. Phosphor was confirm ed taking into consideration o f X RD  patterns.Therm olum inescence glow curves were recorded with the usual set- up consisting o f a small metal plate heated directly using a temperature programmer, a photomultiplier tube (PM T), a DC amplifier and a millivolt recorder. 5 mg o f phosphor was heated each time at the rate o f 5®C per second. Exposure to y - rays was performed using a ^*^ Co source.Thermoluminescence has been used by a number o f workers for obtaining the trapping parameters. Randall and W ilkins 151 were the first to analyse the phenom enon. They proposed the following equation for the TL process./  = c dndtI = esn  exp f —V k T ( 1)© 20051ACS66 M anoj Tiwari, Vijay Singh, S  J  D hohle a n d  R  N  D ubeyIn this equation, E is the trap depth, .s the frequency factor, c an arbitrary constant and n the number o f populated traps. It IS assumed that the thermally released electron recombines at the luminescence center and there is no retrapping, Garlick and Gibson 16| modified the equation by considering the retrapping and proposed the following equatit>n for ,second order :/  -  c d ^dt/ — —r.sN “ "(if) (2)Here, N  is the total number of traps. According to Garlick and Gibson, the second order approximation applied when retrapping and recombination probabilities are equal. It was recognized that the order of kinetics need not be 1 or 2, but can be assumed to be o f any intermediate value. A general order equation was proposed by May and Partridge 17].dtcs n exp(if) (3)s \  the pre-exponential factor, is a constant having dimensions of cm^. s*“‘ which is not considered as a frequency factor anddefined as s ' = sn^ n is the concentration o f trap electronin cm~^ and b is the order o f kinetics. It can be shown that in general, order kinetics is similar to frequency factor forRandall and Wilkins; where is the initial number of populated traps. A number o f methods have been suggested to obtain trapping parameters with the help o f  these equations [8]. Chen f8] has connected the order o f kinetics with an experimentally obtainable quantity, the form factor . All these formulations assume the order o f kinetics, which represents the trapping probability to be constant.In eq. (3), b is between 1 and 2 and gives the fraction of electrons going to recombination centers. Solving eq. (3) and applying the condition o f maximum, we obtain a reasonable approximation for most o f the cases.- ^  = S c x p f ^  * r i  I kT2kT„-]• (4)where S -  s '  ^ heating rate (K /scc). Using eq. (4), frequency factor S  can be calculated for 6 = 1 .kT^(5)Chen [8) has related eq. 3 with experim entally obtainablequantity, from factor denoted by . Chen [8] also emf>irica!ly im proved  som e o f  the e x p re ss io n s  fo r trap  dep th  and coordinated the methods, which used different parts o f the glow curves for obtaining the trapping parameters.A reasonable method which does not require prior knowledge of b is based on the accurate measurements of the temperature of TL peak maximum ), low temperature half width ( r  ) and high temperature half width ( c')). M ethods using <5, r  alongwiihwere developed by Luschik (9| and I lalperin and Braner 110] Chen [8] later summed up these three methods in the form ot following equations:E ^ C a Ba{2kT„ ,), ( 6)where a, Ca, Ba are constants, E  is the activation energy. Depending on whether r ,  i5 or co is used. This equation takes the forms := [l.51 + 3(M .^ - 0 .4 2 ) ] ^ - [ i .58 + 4 .2(^,, -0.42)]2*7;„.(7)= [0.976 + 7.3(^^ -  0.42)'1^ - (8)£■,„ = [2 .5 2 + 10 .2 (/i^ - 0.42) -2A7-„, .•1 CO(9)where is the tem perature at which maximum intensity i.s observed, and are the tem peratures on low and high temperature sides, respectively at which the intensity falls to half of the maximum value.^  = t;  - t;„, tu = ~ t = -  r,5and Bv ~  — • ^ COExcept for extrem e values of (< 0.42, > 0.52), the trap depths obtained using eq. (7-9) are consistent.Typical TL glow curves are shown in Figure 1 (exposed toy- ray = 1.16 C/kg). Tl.- in pure CaS is very weak (Figure 1, curve-a), only one peak is seen at 362 K. The CaS: Ce phosphor shows two peaks around 362 K and 426 K(Figure I, curve-b). The most prominent TL peak appears around 426 K and 362 K. The 426 K TL peak intensity increases with increasing Ce^ "*^  shows that concentration in CaS. TL in CaS: (Na“^) (Figure 1, curve-c).the 426 K peak intensity decreases due to Na’*' ions in CaS: Ce-^ '* phosphor. During y-ray irradiation, holes are trapped at Ce^ **" as well as at other site. Formation of Ce"*  ^is a wellknown mechanism of Ce^ **' ions during y- or X-ray irradiation [11]. On heating, therm ally released holes from Ce'^^ site recombine with the electrons at unspecified sites, y-ray induced oxidation of Ce^^T herm o lum inescence  stud ies o f  C aS  phosp h o rs a c tiva ted  by Na* a n d  A 67ions, viz., + e“ shows the hole trap production.Among the ions, Ce exhibits 3+ as well as 4+ valence states. The higher TL peak tem perature o f CaS: Ce indicates that the thermal activation energy o f  holes from sites is higher than that of the host lattice-related sulfide radicals. On the contrary, 2-k valence state can also be stable in the Ca^ "*^  sub-lattice, lownsend and W hite [12] have recently com m ented that the » RE “^  redox model in CaSO^ : RE has a conceptual problem since the hole is not only captured by RE-"*" ion but also Stimulates RE*'"^  luminescence.F igure 1. Typical TL glow  curve o f  various phosphors exposed to y-ray M 16( Vkg) (a)C aSpure, (b)C aS  .r e '* .  (c)C a S  (Na^). (d) CaS Ag^(lower concentration) (0.1 mol% ), (e) C'aS : Ag*  (higher concentration)( I 0 mol%). (0  CaS . Ce-V. Ag^, (g) C'aS C c'M N a^) Ag-"Ag"^  ion activated in CaS host shows two peaks at 487K, 503K and shoulder observed at higher tem perature at 573K for low concentration o f Ag^ ions (0.1 mole% ) (Figure 1, curve-d). With increasing the concentration  o f  A g“*" ions, the higher temperature shoulder is m anifested as a prom inent glow peak (Figure 1, curve-e). TL peaks o f CaS: C e ^ \ Ag"  ^(Figure 1 curve- f) was observed at 426 K, 487 K, 503 K and shoulder at 583K; the I'L intensity of 426 K peak increased by 1.6 tim es as compared to that in CaS:Ce. This may be cither due to som e competitive process or energy transfer from Ag"*^  to Ce '^*'. However, this is only a speculation as we do not have any experimental evidence lor this. CaSiCe* '^*’ (Na”*") Ag"  ^ phosphor shows the prom inent peak at 429K and two shoulders at 362K and 573K (Figure 1, curve-g). The prom inent peak observed around 426 to 429K for CaS: Ce"^, CaS: Ce^-" (Na^) and CaS: Cc^^ (Na*^) Ag^ phosphors may be due to Ce"^ ions in CaS host lattice. In addition, shoulders are seen at 573K due to A g ions in  CaS host. TL intensity as per the results m entioned above, decreases due to presence of Na'*^  ion.The decrease in TL intensity in CaS.Ce, Na and CaS:Cc, Ag, Na may be due to destruction o f  high tem perature TL traps due to addition o f Na in these phosphors.For isolating the TL peak, various cleaning procedures were attempted and the shape o f the cleaned peak was examined. The procedure which gave pure peak was selected. This consi.sts heating o f the phosphor at less than 10 K tem perature o f TL peak for 3 min and then cooling to room tem perature. Heating beyqfid this tem perature, did not change the shape of the peak but ^ c re a s e d  TL intensity. The isolated peak was used for calculating trapping parameters.tShe value o f E was obtained from the shape o f  the isolated peal<| The highest temperature peak can be m ost easily isolated theripially. A peak around 362 K was observed (Figure 2, curve- a) in|pure CaS and E value is calculated using the Chen formulae[8] a|id presented in Table 1. A I'L  peak around 426K was isolated for c|aS: Ce-^ '*’ phosphor, shown in Figure 2 (curve-b). To test the single nature o f the trap depth, the sam ple was heated till the peak tem pera tu re  w as ach ieved , qu ick ly  coo led  to  room  temperature and then again heated. It was found that the peak temperature rem ains the same, so in the present sam ple, no spread o f E is indicated. The E values are shown in Table 1. Similarly, isolated peaks are shown in Figure 2 o f CaS: Cc'^ '*' (Na"^) (curve-c) and CaS: A g“^ (curve-d). The 429 K isolated peak cannot be obtained in CaS: Ce^^, Ag** and CaS: Ce^ *** (Na"*") Ag"*' phosphors due to presence o f high tem perature (573K) peak in the TL glow curve. Therefore, trap depth calculation o f CaS: Ce^ "*", Ag"*^  and CaS: Ce^ "*^  (Na"*") Ag"*^  phosphor is not possible by Chen [8] formula.F ig u re 2 . Therm ally cleaned glow  peak for use in trapping param etercalculations ; (a) CaS pure, (b) CaS : Cc^”**, (c) CaS : Cc^* (Na*), (d) CaS :Ag* (higher concentration) (1 .0  mol% ).6 8 Manoj Ttwari, Vijay Singh, S J Dhohle and R N DubeyTable 1. Trapping parameters.Phosphors PeakTemp( K )T,(K)T m( K ) ( K )Order of kineticsEx(eV)Es(eV)E.(eV)CaS puic 362 330 362 370 0.42 1 0.647 0.671 0.662CaSCc^  ^ 426 403 426 451 0.52 2 1.083 1.060 1.080CaSCc’MNaM 426 401 426 453 0.52 2 0.982 0.083 0.987Thcrmoluminescence glow curve shows the increase in TL intensity with introduction of Ce in CaS lattice. In CaSrCe^ '^  phosphor, TL intensity increased by Ag  ^ion as a co-activator probably due to either some competitive process(s) or to energy transfer from Ag"*^ to Ce^  ^ ions. Only one type of defect is formed in CaS due to its exposure to y-ray radiation. Ce^  ^and ions in CaS lattice result in increased formation of new defects, which are responsible for the additional TL peak observed in CaS:Ag and CaSrCe^ "^ , Ag^ '** phosphors.AcknowledgmentOne of us, SJD, is thankful to the University Grants Commission, New Delhi for financial assistance.References[IJ R Pandey. A B Kunz and J M Vail J. Mater. Res 3 1362 (1088)[2] Y Kancko and T Kocla ./. Cryst Growth 86 72 (1988)[3] D Blokhinsev ./. f:xpt. Thor Phys. 7 1242 (1037)[4] W DeCiroot PhysU a 6 275 (1939)[5] J J Randall and M H F Wilkins Pmc. Roy Soc. A148 366 (104.S)[6] G F J Garlick and A F Gibson Pror. Roy. Soc. 60 574 (1948) f7] C E May and J Patridge J. Chem. Phys. 40 401 (1964)[8] R Chen J. AppL PIm. 40 570 (1969)(OJ Ch D Luschik Acad Nauk. (USSR) 101 651 (1955)[10] A Halperin and A Braner Phys. Rev. 117 408 (1960)[11] P Rodnyi, E Melchakov, N Zakhurov, I Munro and A Hopkirk / Lum. 65 85 (1995)[12] P D Townsend and D R White Radial P ro t D osun  65 281 (1996)

Thermoluminescence Studies of CaS Phosphors Activated by Ce3+, Na+ and Ag

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Thermoluminescence Studies of CaS Phosphors Activated by Ce3+, Na+ and Ag

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