A general introduction on luminescence especially photoluminescence (PL) and

Transcription

1 A general introduction on luminescence especially photoluminescence (PL) and thermoluminescence ITL) has been presented A discussion on kinetics of luminescence and different decay mechanisms is included. The methods of preparation, properties, and applications of different types of luminescent materials are incorporated. The physical processes of doped phosphors and doped glasses are discussed in a more general way with special emphasis on glass formation from melt and sol-gel processing. The advantages and merits of glasses as hosts for various rare earth ions and luminescent species are detailed. The spectroscopic and optical features of rare earth ions in phosphors and glasses, which are instrumental in arriving at qualitative and quantitative results, are described. A brief account of enera transfer processes of rare earth doped glassy materials has been incorporated The properties of alkaline earth sulphide (AES) phosphors, their importance and applications in modern technology are also outlined.

2 Chapter Luminescence Luminescence is a science closely related to spectroscopy, which is the study of the general laws of absorption and emission of radiation by matter [I]. The existence of luminous organisms such as bacteria in the sea and in decaying organic matter, glow worms and fireflies have mystified and thrilled man since time immemorial. A systematic scientific study of the subject of luminescence is of recent origin, from the middle of nineteenth century. In 1852 English Physicist G.C.Stokes identified this phenomenon and formulated his law of luminescence now known as Stoke's law, which states that the wavelength of the emitted light is grater than that of the exciting radiation. German physicist E. Wiedemann introduced the term 'luminescence' (weak glow) into the literature in The phenomenon of certain kinds of substance emitting light on absorbing various energies without heat generation is called luminescence. Luminescence is obtained under variety of excitation sources [2]. The wavelength of emitted light is characteristic of the luminescent substance and not of the incident radiation [3]. The various luminescence phenomena are given names based on the type of radiation used to excite the emission (Table 1.I). Table 1.1. Luminescence phenomena and the methods of excitation

3 General introducrron -- 3 The last three phenomena are linked together for the time scale over which the light emission takes place. Fluorescence is a luminescent process, which persists only as long as the excitation is continued. The decay time of fluorescence is independent of temperature; it is determined by the probability of transition from the excited level to the ground state. Phosphorescence is luminescence observable after the removal of exciting source. The decay time is dependent on temperature. Substances emitting luminescence are called luminophors or phosphors. Luminescence study of materials in the form of polycrystalline phosphors, crystalline solids. noncrystalline solids and glasses are reported [3]. Most phosphors are basically semiconductors. describable in terms of energy band model with valence and conduction bands and with localized energy levels in the forbidden regions between the bands. The localized centers are associated with the impurities or imperfections in the host lattice. Impurities that provide levels, which permit radiative transitions, are called activators. Generally these levels close to valence band are when occupied by electrons can also act as traps for valence band holes. Certain other impurities can provide levels close to conduction bands, if empty may act as electron traps. Typical excitation mechanisms of luminescence involve raising an electron from the filled band or from a filled activator level to the conduction band, or from an activator ground level to some higher activator levels. Those electrons that reach the conduction bands can return to activator level or may be trapped. If trapped it can return to conduction band by absorbing sufficient thermal or other foms of energy. The return of an electron from conduction band to an empty activator level yields luminescence. Radiationless transition may also occur to trapped or free holes in which energy is transferred to lattice in the form of phonons [I]. The complex consisting of activator impurity and surrounding disturbed host lattice where these transitions take place is known as 'luminescence center'. Radiations from transitions between excited and ground state of activator produce fluorescence while the delayed return of electron from traps through the conduction bands yields phosphorescence. At low temperatures phosphorescence can be 'frozen in' such that thermal energy is not sufficient for the release of

4 Chapter I 4 trapped electrons. Thermoluminescence (TL) is the release of the ' frozen in' phosphorescence by raising the temperature. It is also known as thermally stimulated luminescence (TSL) or thermally stimulated relaxation Photoluminescence Luminescence in solids is the phenomenon in which electronic states of solids are excited by some energy from an external source and the excited energy is released as light. When the energy comes from short wavelength light, usually ultraviolet light the phenomenon is called photoluminescence (PL). PL in solids is classified in view of the nature of the electronic transitions producing the luminescence. In the case of PL a molecule absorbs light of wavelength h~, decays to lower energy excited electronic state and then emits light of wavelength h2 as it radiatively decays to its ground electronic state. Generally the wavelength of emission h2 is longer t w the excitation wavelength, but in resonance emission hl=h2.. Luminescence bands can be either fluorescence or phosphorescence, depending on the average lifetime of the excited state, which is much longer for phosphorescence than fluorescence. The relative broadness of the emission band is related to the relative difference in equilibrium distance between the excited emitting state and the ground electronic state. PL of a molecular species is different from emission from an atomic species. In the case Figure 1.1. Partial energy level diagram of a photoluminecent molecule. SI & S2 are singlet states and TI the triplet states

5 General introduction 5 of atomic emission both the excitation and emission are at the resonance wavelengths, in contrast excitation of a molecular species usually results in an emission that has a longer wavelength than the excitation wavelength. PL can occur in gas, liquid and solid phases. An energy level diagram as in Figure 1.1 can illustrate the radiative and non-radiative transitions that lead to the observation of molecular photoluminescence. The spin multiplicities of a given electronic state can either a electrons) or a triplet (unpaired electrons). The ground electronic state is normally a singlet state and is designated as SO. Excited electronic states are either singlet (SI. S2) or triplet (TI) states. When the molecule absorbs light an electron is promoted within '' seconds from the ground electronic state to an excited state that posses the same spin multiplicity as the ground state. This excludes a triplet-excited state, as the final state of electronic absorption because the selection rules for electronic transition dictates the spin state should be maintained upon excitation. A plethora of radiative and non-radiative processes usually occur following the absorption light en routc to the observation of molecular luminescence Non-radiative relaxation processes (a) Vibrational relaxation: -Excitation usually occurs to higher vibrational level of the target-excited state. The excited molecules normally relax rapidly to the lowest vibrational level of the excited electronic state. These non-radiative processes are called vibrational relaxation. It occurs within 10"~-10~~~ s, a time much shorter than the typical luminescence lifetime. So such processes occur prior to luminescence. (b) Internal conversion: - If the molecule is excited to a higher energy excited singlet state than SI (like S2 in Figure 1.1), a rapid non-radiative relaxation usually occurs to the lowest energy singlet excited state (SI). Relaxation processes between electronic states of like spin multiplicity such as SI and S2 are called internal conversion. It normally occurs on a time scale of s

6 Chapter 1 6 (c) Intersystem crossing: - Non- radiative relaxation processes can also occur between excited states of different spin multiplicity. Such relaxation process is known as intersystem crossing. The relaxation from SI to TI in Figure 1.1 is an example of intersystem crossing (d) Non-radiative de-excitation: - The above mentioned non-radiative processes occur very rapidly and release small amount of energy.the rest of the energy is dissipated either radiatively, by emission of photons, or non-radiatively by the release of thermal energy The non-radiative decay of excitation energy which leads to the decay of excited molecule to the ground electronic state is called non- radiative de-excitation. The amount of energy released in the form of heat is very small and cannot be measured experimentally. The evidence for non-radiative de- excitation process is the quenching of luminescence. In solid-state luminescent materials the crystal vibrations (phonons) provide the mechanism for non- radiative de-excitation Radiative processes -Fluorescence and Phosphorescence Fluorescence refers to the emission of light associated with a radiative transition from an excited electronic state that has the same spin multiplicity as the ground electronic state. The radiative transition SI+ So in Figure 1.1 represents fluorescence. Since fluorescence transitions are spin allowed they occur very rapidly and average lifetimes of the excited states responsible for are typically less than 1w6 s. Electronic transitions between states of different spin multiplicity are spin forbidden, however it becomes more probable when spin orbit coupling increases. The net result of spin orbit coupling is the mixing of excited singlet and triplet states. This mixing removes the spin forbidden nature of the transitions between pure singlet and pure triplet states. Therefore if intersystem crossing populates the triplet-excited state then luminescence might occur from the triplet state to the ground state. Phosphorescence refers to the emission of light associated with a radiative transition from an electronic state that has a different spin multiplicity from that of ground electronic state. The radiative transition TI -+ So in Figure 1.1 represents the phosphorescence. Since phosphorescence

7 General introduction -- 7 transitions are spin forbidden they occur slowly and so the average lifetime for such emission typically range from 10" to several seconds. Phosphorescence is also known as 'delayed fluorescence.' Configuration co-ordinate model of Luminescence Most of the luminescent solid materials exhibit broad bell-shaped absorption bands and emission bands corresponding to smaller energies. For explaining the luminescence spectra different models were suggested and configuration co- ordinate model is one among them. It is illustrated in Figure 1.2. The ordinate is the total energy of the system for ground and excited states of the center including both ionic and electronic terms. In the diagram the energy is shown to vary parabolically as some co-ordinate. The configuration co-ordinate represents changes of nuclear co-ordinates of all the lattice ions constituting the Configuration eo-ordinate * Figure 1.2 illustration of configuration co ordinate model luminescence center. There is a value for the co-ordinate for which the energy is minimum, but this value is different for ground and excited state because of different interaction of luminescent center with its neighbours [I]. Absorption of a quantum of light causes a transition from A to B. The lifetime of the excited state is of the order of 1w8s in the case of allowed transitions and is much longer than the lattice vibration period. So just after the absorption has terminated relaxation toward the minimum energy point in the excited state takes place

8 Chapter I 8 accompanied by the emission of phonons and reaches the position 'C'. From 'C' it comes to the ground state 'D' by emitting a quantum of luminescent light. Then the system goes back to A by means of energy dissipation through lattice vibrations. This model can also explain the case governed by Stocke's law. When the system is in equilibrium position 'C', it is not at rest, but migrates over a small region around "2' because of the thermal energy of the system. As a result the emission transition is not just to point 'D' on the ground state but covers a region around 'D'. In the vicinity of D the ground state curve shows a rapid change of energy so that even a small range of configuration co-ordinate leads to a large range of energies in the optical transition. This explains the broad emission and absorption bands observed Kinetics of Luminescence One generally comes across two kinds of luminescence processes. They are (a) kinetics of first order (monomolecular mechanism) (b) kinetics of second order (bi -molecular mechanism) (a) First order kinetics The number of excited electrons N decreases according to a constant probability law dn/n = -adt which gives the solution N = N,e-" The luminescence intensity I a dnidt Then I = 1,e'"' The major characteristic is lifetime, which is the average stay of ion in a given excited state. (b) Second order kinetics In this case the probability for recombination is proportional to the number of centers. Then

9 General introduction Therefore - 9 N = N,, /(l + Noat)... I. 6 Which shows that N decreases hyperbolically with time. Since I cc dnidt I = I, /(I + at)'... I. 7 where a = (1,a)' '... I. 8 Here the decay become more rapid as the excitation intensity is increased. The above kinetics can be applied only if the optical transition is associated with dipole radiation or the lifetime.r - second. In the case of phosphorescence the kinetics involved in the process depends on the spatial relation between luminescence centers and on the motion of conduction electrons. The after glow is longer at lower temperatures and shorter at higher temperatures showing the strong dependence of phosphorescence on temperature Different Decay mechanisms The process of phosphorescence arises due to a number of complicated terms such as transition between bands, activators with transfer of electrons and the hole involving temporary capture by traps, by the presence of traps with different depths, by possible repeated trapping etc. In order to explain the mechanism of decay, different methods are suggested. (a) Simple exponential decay The luminescence intensity from N atoms will be given by I a -dn/dt = un... I. 9 i.e. the number of photons emitted or the number of atoms de-excited at time t is equal to the rate constant u times the number of excited atoms existing at time t. Integration of this equation yields

10 Chapter I 10 where the rate constant a is equal to the reciprocal of the life time.r of the excited state -E/kT i.e. a=l/r=se I1 where S is the frequency factor, s is the time taken by the system to decay to e-'lo ie lo, where I0 is the luminescence emission intensity at t = 0 ( i.e. at the instant of cessation of excitation). Hence So a plot of logarithm intensity I versus time t is a straight line for an exponential decay. There are no external constraints on the free atoms assumed in this example so that r is about lo-' seconds. A decay curve is intensity versus time plot during phosphorescence, and the rate of decay at time t is the slope of the tangent of the decay curve at the same time. (b) Hyperbolic decay This type of decay takes place in a system where on irradiation shallow traps are also produced along with deeper traps. The decay of phosphorescence occurs generally in two or three groups depending on the nature and concentration of the traps. In such cases the observed intensity will be due to the superposition of all the exponentials col~esponding to different traps and represented generally as I, = l,cb where b is the decay constant. Also in this case logarithm of intensity I versus time t plots are nonlinear indicating the nonexponential form of decay. However the plots In I versus in t show linearity and the decay is said to be a hyperbolic one. Again I = ~,t-~ = CI~,,, m exp(-~,t)... I. 14 where 10, is the phosphorescence intensity due to electrons in traps of energy Em and

11 General introduction - --,,, 11 :*.,.. " ' is the probability of an electron escaping from'the trap. H~c~.&&halyses.,.- of the decay curve are usually done by adopting the peeling - off procedure, where each stage of decay will be associated with an exponential law [4]. The trap depth corresponding to each exponential is calculated from the slopes of straight lines of the semi-log plot using the relation E = kt ln(s I a)... I. 16 where a is the slope of each linear portion of the curve. The number of discontinuities in the in I versus In t plots correspond to the number of straight lines into which the semi log plots can be split. The value of b is indicative of the decay rate and also provides information about relative population of trapping states at various depths. The relative population of trapping levels N, at t=o can be obtained by the extrapolation of the In I versus time plot using the relation :, ~., < \. 'C ;.% where ~,=lip, is the life time of the electron trapped in the trap of depth En. So the ratio of N,(t)t=o/N.(t)t=l for the peeled off components of the graph indicate the idea about the distribution of trapping levels Techniques to study optical properties of condensed matter (a) Absorption spectra:- To study optical behavior of impurity atoms in condensed matter such as crystals, phosphors, and glasses very simple method is used. To characterize the material one can measure its absorption spectrum in the uv-visible region [5]. It helps to identify the active impurities and gives information on their environment. Now a days the measurement can be easily done by using computer-controlled spectrophotorneters. In crystalline hosts the polarization of absorption spectra can be used to conclude the site symmetries and crystal field parameters. Along with ground state absorption to investigate the excited state absorption characteristics, modified techniques are available by using two excitation sources, one powerful beam to excite the material and the other to measure the absorption after the excitation pulse.

12 Chapter I 12 (b) Excitation and emission spectra:- In certain materials ground state absorption is too weak to measure [1,6]. A method known as zero order excitation spectroscopy will provide equivalent information. Emission at all wavelengths is directed to a detector without dispersion. By varying the wavelength of exciting beam and recording the strength of emission at all wavelengths, a spectra equivalent to that of absorption of all luminescent centers can be recorded. By using modem spectrofluorimeters two different types, the emission and excitation spectra can be recorded. With special accessories the phosphorescence spectra also can be recorded. In excitation spectra the emission wavelength that corresponds to the emission peak of the sample is fixed whereas excitation monochromator is scanned. In emission spectra the excitation wavelength is fixed and emission monochromator is scanned. The excitation is usually fixed for a wavelength at which the sample has significant absorbance. The much higher sensitivity of luminescence technique compared to absorption technique is an obvious advantage for excitation spectra over absorption spectra. Also the relative ease of acquiring excitation spectra of materials like solids represents another advantage. Finally excitation spectra can provide valuable information about excited state processes such as energy transfer that cannot be obtained by absorption spectra. Thermoluminescence (TL) means not temperature radiation but enhancement of the light emission of materials already excited electronically by the application of heat. TL can be distinguished clearly from incandescence emission from a material on heating. In incandescence, which is classical in nature, radiation is emitted when the material is very hot. This radiation is in the invisible-far infrared but at higher temperature, shifts to the visible region [7]. The fundamental principles which govern the production of TL are essentially is the same as those which govern all luminescence processes and hence TL is one member of a large family of luminescence. The phenomenon is observed with

13 General introduction 1.4 some minerals and, above all, with crystal phosphors after they have been excited by light. The crystal phosphors or doped glasses, which respond to TL, contain certain traps. The traps are imperfections in the crystal lattice where electrons are captured after they have been ejected from a luminescent centre by excitation energy. The luminescent properties of phosphor centres are strongly dependent on the chemical nature of the host crystal, showing that the same activator ions, in different host crystals yield remarkably different-coloured emissions and decay times. Prolonging the emission time of phosphors up to days or even longer (production of phosphorescence of the phosphors) is possible by inserting traps into the host crystal. Trapped electrons cannot return directly to the centre. In order to be released from the traps they must first obtain additional thermal energy-in this case, thermal energy stimulates luminescence-after which they recombine with a centre and undergo radiative transition [8]. V K Mathur et a1 studied high dose measurements using thennoluminescence of CaS04:Dy and reported that the range of high dose measurements can be increased by an order of magnitude by increasing the concentration of dysprosium in CaS04:Dy. Recently many works were reported on CaSO., doped with rare earth ions ce3+ and sm3+ [ Earlier workers suggested that the glow peak and hence the trap depth depends on the property of host materials. But recent observations [12] on doped phosphors shows that the broad emission is quenched in favor of emissions from the rare earth (RE) impurity sites and the degree of quenching varies between the REs. The spectral measurements showed that the host material has minimal effect on the glow peak temperatures, T,. Above room temperature, the glow peaks are specific to the added RE ions and do not show common peaks Understanding of TL on the basis of energy band model. The phenomenon of TL can be understood by the energy band diagram of an insulator. in which the forbidden gap between the valence band and conduction band is of the order of few ev. According to this model the trace of impurities and defects responsible for the luminescence in solids introduced in the host

14 Chapter 1 14 lattice by heat treatment can be imagined to form discrete energy (donor1 acceptor) levels within the forbidden energy gap. Other impurities and lattice defects provide unoccupied energy levels called traps that have the capability of detaining the charge carriers before their recombination with the luminescent centers. These traps are denoted as hole traps or electron traps depending on whether the trapped carriers are holes or electrons [I]. A schematic representation of a simplified energy band model is shown in Figure 1.3 (a, b, c, d, e and f) [13]. When the substance is exposed to ionizing radiations, some of the electrons from the valence band are sufficiently excited to reach the conduction band. While most of them fall back immediately to valence band accompanied or un accompanied by light emission or internal heating, some of them get trapped in the forbidden gap at the donor level D or accepter level A (Figure 1.3 a). The temperature at which the light emission occurs depends on the depth at which the traps are located since it decides the amount of thermal energy required Irradiation (a) (b) (4 Figure 1.3. Band model diagrams for TL process in an insulating crystal (a) On Irradiation (b) - (9 Alternative process on heating. D -Donor level A- Acceptor level, Ee and Eh thermal activation energies.

15 General infroduction 15 to accomplish the detrapping and the consequent recombination of the electrons and the corresponding holes. In simple cases the liberated trap can move to its still trapped counterpart and recombine to give TL (Figure 1.3 b & c). Both acceptor and donor levels can simultaneously move to meet and recombine at an entirely new location known as luminescence or recombination centre L and give TL (Figure 1.3 d). A trapped charge always has a finite probability that it gets retrapped and then gets recombined. But it also has a finite probability that it get retrapped. (Figure 1.3 e). The recombination probability for the detrapped charge may change with time as the heating progresses. This is called as the second order process where the recombination probability is constant with time, are called the first order process. An isolated case where the detrapped charge gets recombined without having to be excited to the valence band or to the conduction band is also possible (Figure 1.3 f) which is also a first order process Thermoluminescence- configuration coordinate diagram When TL occurs in isolated luminescence centers the process can be described by the configuration coordinate diagram [ I]. The curves in Figure 1.4 represent the potential energy of the luminescence center as a function of the distance between h W 3 * r: T Excita 'on hv Emission hv' ---+ Configurational Displacement, X Figure 1.4. Configuration co-ordinate diagram for an impurity atom in an insulator an impurity atom and the first nearest neighbour called the configuration coordinate. The configuration coordinate diagram defines the potential energy Ug when the system is in the ground state and Ue when it is in an excited state [I].

16 Chapter I 16 During the irradiation the center will be raised to its excited state Ue (transition AA'), where the system will be in a higher vibrational state and so relaxes to the stable configuration B, dissipating the excess energy to the lattice in the form of heat. The centre will emit light through the transition BB' and will again be in the ground state Ug. The centre still being in the higher vibrational energy level in the ground state will come to its minimum energy position A with a further nonradiative loss of energy. The excited centre may also get trapped at T through CC', wherein after remaining for some time t it might escape to the excited state via CC' emitting light via BB'. Rejumping of the centre from T via C'C to the excited state depends upon the probability of escape p per unit time denoted by p = 1 1 =S ~ exp(- EIkT) where S is the frequency factor and E is the energy required for the release of the center from the trap to reach its excited state. The emission is now delayed (phosphorescence) by an amount of time that depends upon the time.r that the carrier spends in the trap. In the case of deep traps the heat energy at room temperature may not be sufficient to raise the centre to its excited state. Here heating of the luminescent material after the irradiation has been removed will raise the trapped center via C'C giving rise to emission via transition BB'. It is this emission during heating that is called TL, and a plot of intensity vs temperature is called glow curve Mathematical treatment The occurrence of TL during a thermal scan of a previously excited material is probably one of the most direct evidences that we have for the existence of electronic trapping levels in these materials. TL spectrum normally consists of a number of overlapped peaks, which are rarely isolated. Randall and Wilkins [14] proposed the first theory of TL for first order kinetics followed by Garlick and Gibson for second order kinetics [15]. During the irradiation of a sample by X-rays, Gamma rays or UV rays electrons are excited from the valence band to conduction band leaving holes in the valence band. Both electrons and holes move in the respective bands until either each finds a localized defect where they are trapped or might recombine directly with a

17 General introduction 17 charge carrier resulting the emission of light. The localized energy levels below the conduction band where the electrons are trapped are called electron traps. Similarly the energy levels just above the top of valence band is known as hole trap as it can trap holes present in the valence band. From thermodynamic considerations it can be shown that the mean time.r spent in trap by an electron or a hole can be written as r = S" exp (EkT)... I. 18 where S is known as the frequency factor, k is Boltzmann's constant and T is the temperature. During heating a trapped electron will gain energy to escape from the trap and might recombine with a hole trap resulting in the emission of light. In the first order kinetics (b=l) model of Randall and Wilkins, the possibility of re-trapping is neglected.in such a case the rate of recombination is equal to rate of release of electron from the traps. So the TL intensity I (t) as a function can be expressed as I(t) = -dn/dt = ns exp(-eikt)... I. 19 where n is the density of trapped electrons.for a linear heating profile T=T,,t pt... I. 20 where To is the initial temperature at time t and P is the rate of heating From eqs. (2) and (3) one can write I (T) = no (S) exp (-E/kT) exp [(-SIP) io exp (-E~T') d~i.1... I. 21 The TL intensity I(T) is maximum at a temperature T = T, given by PE/(~T,~) =S exp(e/kt,)... I. 22 From equation (5) it is clear that the peak temperature T, depends on E, S and P Factot-s affecting TL The factors on which the TL emission intensity and glow peak temperature depend on various factors. In general highly pure substances are poor TL exhibitors. For TL to be shown, the existence of defects or imperfections in the crystal lattice is essential. An impurity that causes TL is called an activator.

18 Chapter 1 18 Certain activators activate the TL in the host whereas some impurities quench or kill the luminescence present in the phosphor, are known as poisons. The activator that increases the luminescence already present in the TL material is called a sensitiser. Generally an activator is a luminescence emission center, a poison prevents the energy transfer to the emission center and sensitiser increases the energy absorption to useful luminescence emission [16,17]. Any thermal treatment after irradiation essentially erases the TL signal. A thermal treatment before exposure increases the number of defects in the substance resulting into enhanced sensitivity. The number of defects retained by the crystal also depends on the rate of cooling employed in the thermal treatment [18,19]. In the preparation of TL materialsflux is used to incorporate the dopants into the matrix and generally it does not introduce any new trapping sites. This indicates that flux serves to alter the relative importance of different group of traps and not their mean trap depth [20]. If very high dose ofirradiation is given to a sample it can enhance the TL [21]. In some cases high dose of radiation causes crystal damages like production of voids, aggregates etc. which reduces the TL output [22]. Some phosphors after irradiation with gamma rays are exposed to UV rays may show some bleaching effect in the gamma induced TL [23]. Preparatory parameters also affect the TL properties. The storability of the phosphor can be improved by varying the preparatory parameters like duration of firing rate of cooling ambient atmosphere etc [20]. Rao et a1 reported that the TL output decreases with decrease of grain size [24]. Size of the particle influences the excitation of a phosphor as well as the emission output by scattering and self absorption characteristics [25]. The fading behavior of TL with storage for different duration of time is an important parameter in TL dosimetry. Mathur et a1 [26] observed peak shift with decay time in case of CaS:Ce. This effect may be due to the broad distribution of trap depth, which is responsible for the release of electrons from the traps as they have two kinds of energies; one is the result of thermal vibration and the other due to the

19 General infroduction 19 interaction-taking place between electrons trapped at the same depth. It has been found that application of high electric fields on a TL phosphor during heating enhances the TL output and sometimes affects the nature of emission [27]. This effect was explained as due to either field ionization of electron traps or acceleration of electrons after thermal release from traps. In addition to the thermal and optical quenching discussed, quenching of luminescence is also achieved by concentration quenching. 'Nambi et a1 [I31 studied the effect of Dy and Tm doping on the TL output from CaS04:Dy and Tm are found to be important activators of the luminescence in the system such that the TL intensity increases as the dopant concentration increases. However at larger concentrations (= 0.1%) the TL output decreases gradually as the dopant levels are increased further. This effect is known as concentration quenching. It is stated that this behavior is characteristic of isolated activator centers. When one activator ion is located in a certain radius of another, the luminescence is quenched [3]. A separate effect known as impurify quenching is found to occur due to the action of 'killer' centres. Upon introducing certain elemental impurities especially heavy metals like Cu, Fe, Ni, and Cr into a TL material the intensity of luminescence emission is seen to reduce drastically [28] TL Applications The applications of TL are numerous. Besides the measurement of radiation exposure in nuclear plants and in medical fields, it is used in archaeology for the determination of the age of the ancient objects and for checking its genuineness. Also it is used for dating and deciding the origin of geological formation and possibly for ore prospecting and earth quake predictions. It can also be used in forensic science and in a wide range of quality control and analysis [29]. The TL investigations of certain metal oxides, ceramics, and various papers are reported which have been found to be suitable for accidental dosimetry to be used with TL technique [30]. The 'rl studies on different type of cements by Gartia et a1 suggested that the same could be used as a tool quality control and in forensic science [3 I]. Few of the recent trends are discussed below.

20 Chapter I 20 (a) Radiation Dosimetry A technique commonly applied in radiation dosimetry is the use of thermoluminescent dosimeters (TLDs). This technique is based on the use of crystalline materials in which ionizing radiation creates electron-hole pairs. During the period of exposure to the radiation, a growing population of trapped charges accumulates in the material. The trap depth is the minimum energy that is required to free a charge from the trap. It is chosen to be large enough so that the rate of detrapping is very low at room temperature. Thus, if the exposure is carried out at ordinary temperatures, the trapped charge is more or less permanently stored. After the exposure, the amount of trapped charge is quantified by measuring the amount of light that is emitted while the temperature of the crystal is raised. The applied thermal energy causes rapid release of the charges. A liberated electron can then recombine with a remaining trapped hole, emitting energy in the process. In TLD materials, this energy appears as a photon in the visible part of the electromagnetic spectrum. The total intensity of emitted light can be measured using a photo multiplier tube set up and is proportional to the original population of trapped charges. This is in turn proportional to the radiation dose accumulated over the exposure period. The readout process effectively empties all the traps, and the charges thus are erased from the material so that it can be recycled for repeated use. One of the commonly used TLD materials is lithium fluoride, in which the traps are sufficiently deep to prevent fading, or loss of the trapped charge over extended periods of time. The elemental composition of lithium fluoride is of similar atomic number to that of tissue, so that energy absorbed from gamma rays matches that of tissue over wide energy ranges [3]. Dosimeters are the instrument that measures exposure to ionizing radiation over a given period. UV and nuclear radiations incident on a TL sample produce mobile electrons and holes, which are caught in their respective trap states within the band gap of the sample. The populations of these occupied traps are proportional to the incident radiation dose. As TL intensity is a measure of the occupied trap density the sample can act as a TL dosimeter. There are three types of dosimeters

21 General introduction 21 worn by persons who work with or near sources of radiation. The film badge is the most popular and inexpensive. In it, photographic or dental X-ray film, wrapped in light-tight paper, is mounted in plastic. Badges are checked periodically, and the degree of exposure of the film indicates the cumulative amount of radiation to which the wearer has been exposed. Thennoluminescent dosimeters are nonmetallic crystalline solids that trap electrons when exposed to ionizing radiation and can be mounted and calibrated to give a reading of radiation level. The ion-chamber dosimeter, like the thennoluminescent one, is reusable, but it is self-reading for immediate determination of exposure [ Several of the TL materials e.g.cas04:dy, MgzSi04:Tb, CaFZ:Mn, LiZB407:Mn and LiF:Mg,Ti have been found especially valuable in this field. Glasses are also used as TLD materials Another area of widespread application of TLDs has been the intercomparison of radiation sources, particularly radiation therapy equipment on a national or international scale. Monitoring of radiation doses in body cavities of patients undergoing radiation therapy is one of the earliest applications of TLD. (b) Age determination This application of TL was first suggested by Daniels et al [36] who offered the premise that the natural TL from rocks is directly related to the radioactivity from uranium, thorium and potassium present within the material. This radioactivity results in the accumulation of so called 'geological dose'. If the rate of irradiation from the radioactive minerals is established, then the length of time over which the rock has been irradiated can be determined from the relation Geological age= absorbed dose1 dose rate. In addition to age determination utilized in geology TL is more sensitive for detecting traces of radioactivity than conventional methods. Thus the technique has found widespread application in radioactive mineral prospecting. (c) Defects in Solids. Townsend and Kelly 1371 estimated that the TL technique is capable of detecting defect levels in a specimen. When coupled with the ability to separate these

22 Chapter I 22 energies of these levels TL provides in principle a unique tool in the determination of the defect energies. Although it is fruitless to use TL alone to describe the defect structure of a solid, it is very useful technique when combined with other measurements. (d) Forensic Sciences In this application comparison of evidentiary materials with similar materials of known origin is made. The material can be glass, soil, insulation material etc that are encountered in criminal cases. This is exclusionary evidence, in which when the TL characteristics do not match it can be said certainty that the sample has not come from its known source [38]. The application of TL methods to detect art forgeries is also a part of forensic sciences. TL studies of dental enamel is also in progress, which is of great importance in criminal cases in relation with death of living things. (e) Biology and biochemistry Application of TL in the study of biological and biochemical systems is favored in recent times. Here the measurements are to be done at very low temperatures [39,40]. The attempt has been successful in the study of hydroxy and amino benzoic acids, urea, proteins, plant leaves and bacteria. The inter and intramolecular transfer of radiation damage in nucleic acids, proteins and their constituents could be correlated with their TL behavior. 1.4.Luminescent Materials Materials and substances that are capable of emitting light, particularly in the visible range are termed as 'luminescent materials'. There are innumerous luminescent materials but not all are efficient enough to be put to practical use. The more efficient materials are those, which are prepared in laboratories and have a specific composition. These materials exhibit a degree of repeatability and reliability in their performance, which qualifies them for practical application. Crystalline phosphors and non-crystalline glasses doped with certain rare earth ions and transition metals are examples for luminescent materials

23 General introduction l.Phosphors Solid materials in powder form that give luminescence when suitably excited are called phosphors. From 1950 onwards the extensive study of luminescence characteristic of sulphide phosphors has been started. The first phosphor synthesized was probably an impure barium sulphide prepared with very low luminescence efficiency and with the serious shortcoming that it was rather quickly decomposed in moist air, yielding hydrogen sulphide. A more stable sulphide type phosphor was produced in 1866 by heating zinc oxide in a stream of hydrogen sulfide. In 1887 it became known that these sulphides do not luminescence in a chemically pure state but only when they contain small quantities of a so-called activator metal. The sulphides of zinc and of cadmium are the most important basic materials of sulphide type phosphors. An important condition of getting highly efficient phosphors is that these sulphides must first be prepared to the highest possible chemical purity before the necessary amount of activator can be added precisely. Effect of impurities on the energy levels of sulphide phosphors has become an active field of investigation. Alkaline earth sulphide phosphors activated with specific metallic impurities and rare earth ions are of considerable practical importance. Many investigators have reported luminescence studies of alkaline earth sulphides activated with one or more rare earth ions and transition metal ions Most of the rare earth ions exhibit good fluorescence In many crystalline phosphors the luminescent emission originate in impurity systems called activators. In sulphide phosphors, however these properties seem to be associated more with lattice itself than activators. The impurities can be introduced in two ways (i) They may be impurity atoms occurring in relatively small concentration in the host material. (ii) They may be stochiometric excess of one of the constituents of the host material, which is called self-activation. The incorporation of an activator in crystalline solid gives rise to certain localized energy levels in the forbidden band. Depending upon the energy levels involved we can distinguish characteristic and non-characteristic luminescence. For characteristic luminescence the energy levels involved are those of the activator atoms or

24 modified perhaps by the host lattice. Here an activator atom absorbs the incident quantum of energy by the transition of one of these electrons from one quantum state to another. When the excited atom returns to the ground state, it loses a part of energy due to lattice interaction and hence emits a photon of less energy. In non-characteristic luminescence a charge transfer through the lattice takes place. This also involves the energy levels of the host lattice modified due to activator atoms [42,43]. The activator ions are surrounded by host-crystal ions and form luminescent centres where the excitation-emission process of the phosphor takes place. These centres must not be too close together within the host crystal. For high efficiency, only a trace of the activator may be inserted into the host crystal, and its distribution must be as regular as possible. In high concentration, activators act as "poisons" or "killers" and thus inhibit luminescence. The term killer is used especially for iron, cobalt, and nickel ions, whose presence, even in small quantities, can inhibit the emission of light from phosphors. The co-activator is an additional impurity, which is necessary for luminescence in sulphide phosphors. But it does not have the pronounced effect on emission spectrum that the activator has. Usually co-activators are identified as donors and the activators as acceptors. The lack of positive charges created due to the addition of monovalent or trivalent impurity ions gets compensated adding suitable flux materials such as sodium thiosulphate. The addition of flux only serves to alter relative importance of different groups of traps and not their mean depth or additional trapping levels. The flux facilitates the solution and distribution of the activators in the host crystal on firing. It probably acts to provide a charge compensating co activator, although the atoms of the flux do not always go into the lattice. If they do the flux may also furnish trapping centers [6]. Only small quantities of fluxes are integrated into the phosphor, but this small quantity is highly important for its luminescence efficiency. The fluxes act as coactivators by facilitating the incorporation of activator ions. Thus, many luminescent centres will be produced, and strong activation will result. In

25 General introduction 25 describing a luminescent phosphor, the following information is pertinent: crystal class and chemical composition of the host crystal, activator (type and percentage), coactivator (intensifier activator), temperature and time of crystallization process, emission spectrum and persistence. Copper-activated zinc and cadmium sulphides exhibit a rather long afterglow when their irradiation has ceased, and this is favorable for application in radar screens and self-luminous phosphors. Alkaline earth sulphides are very versatile phosphor materials. These sulphides produce different characteristic emission of different activators and coactivators and it is possible to prepare hundreds of different phosphors with different properties. Since their band gaps are large, the excited states of dopants are not densely distributed between valence and conduction bands. Also these host crystals provide environment around the impurities [20]. The study of phosphor chemistry has yielded a detailed picture of the role of the above-mentioned activators and fluxes. Philipp Anton Lenard,, was the first (1890) to describe activator ions as being distributed in zinc sulphide and other crystalline materials that serve as the host crystal. That luminescent properties of a centre are strongly dependent on the symmetry of neighboring ion groups with respect to the whole phosphor molecule is clearly proved by the spectral shifts of certain phosphors activated with lanthanide ions, which emit in narrow spectral regions Applications of Phosphors. Phosphors are transducers. whose output is light, in response to various forms of energy used as input. The present utilization of phosphors in a variety of devices is the result of scientific investigations on solid-state luminescence that began a century ago. Phosphors are widely used for lightning applications and in solidstate insulator (SSI) laser material research [50]. The classification of phosphors is based on the type of energy, to be converted. Although there is certain overlap in the type of materials used in lightning. CRT displays, and electra luminescence and in thermoluminescence, the mechanisms for light generation are quite different. In all cases the light generated may not be in the visible, but it could be

26 Chapter 1 26 in the IR or in the UV region. In either military or security applications, phosphors can be used for the detection of infrared or near-infrared, radiation by taking the advantage of outstanding detection sensitivity of photomultipliers. It should also be possible to resort to IR to visible conversion to improve the light output from incandescent sources rich in infrared output. Photostimulated luminescence (PSL) in Eu, Ce and Sm co-doped sulphides (CaS) has been studied by Kravets in order to develop a novel erasable and rewritable optical memory using the photoluminescence method. Some inorganic phosphors known as thermo luminescence (TL) phosphors, which satisfies the below mentioned features are used in solid-state dosimetry. (a). A high concentration of electron (or hole) traps and a high efficiency in the light emission associated with recombination. (b) Sufficient storage stability of the electrons (or holes) to cause no fading even during slight change in temperature. (c) Should have resistance against potentially disturbing environmental factors including light, humidity, organic solvents etc.(d) Low photon energy response and a linear response over a wide dose range.mgsi04 (Tb), A1203(Dy), CaF2(Dy), CaS04 (Dy), CaF2 (Mn), LiB407(Mn), LiF(Mg,Ti) etc are some commercially available TLD phosphors [32] Melt Glasses. Glass was first made in the ancient world, but its earliest origins are obscure. Egyptian glass beads are the earliest glass objects known, dating from about 2500 BC [6,51]. Glass is an inorganic solid material that is usually transparent or translucent as well as hard, brittle, and impervious to the natural elements. Glass has been made into practical and decorative objects since ancient times, and it is still very important in applications as diversified as building construction, house wares, and telecommunications. It is made by cooling molten ingredients such as silica sand with sufficient rapidity to prevent the formation of visible crystals. The varieties of glass differ widely in chemical composition and in physical qualities. Most varieties, however, have certain qualities in common. They pass through a viscous stage in cooling from a state of fluidity; they develop effects of colour when the glass