Thermally Stimulated Luminescence arising

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Radiological Physics and Advisory Division. Bhabha Atomic Research ...... Rivera T., Applied Radiation and Isotopes, Available online on 30 April 2012. 10. Chen R. and ... Technologists in Radiation Safety (Compiled and edited by P. K. Gaur),. Raiological ... 21. Gupta S. S. and Singh A. J., B. Electrochem., 545, 1989. 22.
PRINCIPLES OF THERMOLUMINESCENCE DOSIMETRY Dr. Munish Kumar Personnel Monitoring and Phosphor Development Section Radiological Physics and Advisory Division Bhabha Atomic Research Centre, Mumbai.

1. Luminescence - An overview There are two principal processes which cause the emission of light: 1) incandescence, which means heating an object to such a high temperature that the atoms become highly agitated leading to the glowing of the body. The emission of this light is explained by Planck's black body emission theory. It was for explaining the incandescence that the quantum nature of light was assumed for the first time by Planck. The light from the tungsten filament lamp or a burning piece of coal comes under this category. Only a small percent of the total energy of the hot body appears as visible light. Most of it is dissipated as heat. 2) A cooler and more efficient mechanism of light emission is luminescence. When radiation is incident on a material, some of its energy may be absorbed and re-emitted as light of longer wavelength (Stoke’s law). This is called luminescence. The wavelength of the emitted light is characteristic of the luminescent substance and not of the incident radiation. The emission of light takes place a characteristic time τc after the absorption of radiation and this parameter allows us to sub classify the process of luminescence. Based upon the characteristic time, the luminescence is further classified into fluorescence and phosphorescence. If τc < 10-8 s, then the luminescence process is called fluorescence whereas if τc > 10-8 s, the luminescence process is called phosphorescence [1]. In fact, seventy five percent of over 3000 minerals investigated show some kind of luminescence. Some luminescence under ultra-violet light and some need X-ray bombardment. The specific characteristic which provides an inorganic solid the luminescent property is the presence of atomic centres whose energy levels are protected from the thermal vibrations of the atoms of the bulk matter. For example the rare-earth ions dispersed in calcium salts and in many other crystals and glasses make very efficient luminescent centres. The transitions of inner 4f electrons in rare earths or 5f in actinides are well protected by the outer shell electrons. In zinc sulphide the copper ions, which substitute a fraction of zinc ions produce the well-known green luminescence. The emission is due to a complex interaction between the impurity and the lattice ions. It involves the coupling of the lattice vibrations with the electronic transition, with the result that, the emission is a broad structureless band even at low temperatures. Sometimes there are molecular complexes, which emit luminescence. A particularly striking example of protective shielding of molecular energy levels is the uranyl ion UO 2+ group responsible for luminescence behaves as a separate 2(NO3)2 nH2O. The UO2 molecule surrounded by a shield made up of the water molecules of hydration. Three major modes of vibration of O = U = O produce three sharp bands in the yellow green region under UV excitation. At times the centres consist of ionic vacancies. A very important example of this class is the negative ion vacancy in the alkali halides, in which an electron can be trapped. This is called F centre. The electron has different energy levels like the orbit of a hydrogen atom. Being much less tightly bound than the electrons of the lattice ions, its energy levels fall within the large band gap of the host lattice. This gives rise to absorption bands in the visible range and the corresponding luminescence.

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While the luminescence of inorganic solids is mostly due to impurity atoms or lattice defects, the centres in organic solids are molecular complexes [1].

Figure 1: The family tree of luminescence phenomena [2] There are a variety of luminescence phenomena (Figure 1) observed in the nature or in man -made substances and are given names which reflect the type of radiation used to excite the emission. Thus we have photoluminescence (excitation by optical or ultraviolet radiation), radio-luminescence (nuclear radiations i.e. β particles, γ rays and x-rays etc.) and cathodoluminescence (electron beam). In addition to excitation by radiation, luminescence can also be generated by chemical energy (chemiluminecsence), mechanical energy (triboluminescence), electrical energy (electroluminecsencer), biochemical energy (bioluminescence) and even sound waves (sonoluminescence) [1-2].

2. Thermoluminescence Thermoluminescence (TL) or thermally stimulated luminescence (TSL) is the emission of light from a wide band gap material called phosphor during heating, which has already been exposed to ionizing radiations (x, β and γ rays). During irradiation, the energy is stored in the phosphor, which is released during heating in the form of visible light and is detected using photo-multiplier tube (PMT). In thermoluminescence phenomena, temperature is the stimulating agent, so, if unexposed phosphor is heated, it will not emit any light (except black body radiation/infra-red). Hence the phosphor must be first exposed to ionizing radiation before getting TL signal. The existence of band gap is the essential criteria for the material to exhibit TL, so metals will not show TL. The wavelength emitted by the phosphor during heating lies in the visible region in general 2

but emission in infrared as well as ultra-violet region is also not ruled out. The plot of light output with time or temperature is called glow curve. There can be large number of glow curves having different glow peak temperatures, different heights/amplitudes in a given phosphor which is decided by the trap depth, heating rate, dose, population of different traps and their participation in the TL phenomenon. To enhance the TL sensitivity/output, dopants are added e. g. Dy in CaSO4. Details about TL models and associated kinetics can be had from literature [1-14].

Figure 2: The mechanism of thermoluminescence phenomena, VB - valence band, CB –conduction band and FB –forbidden band [9] During irradiation of phosphor, the electron-hole pairs are generated, which while moving in the phosphor can be trapped at the metastable energy levels which exists in the forbidden gap due to vacancies, defects or by the addition of impurties/dopants. It should be noted that the traps for electrons are usually deficient in negative charge, for example a negative ion vacancy also called F centre. The traps for holes are deficient in +ve charge. The traps can retain the captured electrons and holes for long periods at ambient temperatures. To free the carriers from the traps, the energy has to be supplied, which is done by heating the phosphor. During heating, the charge carriers are released from the traps and recombination of electron and hole tales place, which leads to emission of light in the form of TL. In Figure 2, kinetic processes happening during irradiation and readout stage are depicted. The theoretical expression for TL originally proposed by Randall and Wilkins is based on a model which assumes a free particle inside a potential well called the trap. The particles (electrons or holes) in the trap have an energy distribution that follows Boltzmann law. The particles which arrive at the top of the well are expected to escape from the well with a frequency approximately equal to the vibrational frequency of the particles, which obviously would depend on the temperature. This frequency is called the “attempt to escape frequency” designated by s in s-1. Thus the escape probability for a charged particle from a trap is given by p = s exp(-E/kT) where E is the well (trap) depth in eV, T the temperature in K and k the Boltzmann constant (J/K) [3-4]. When a pre-excited/irradiated sample is heated, the TL appears in the form of a transient glow. A trap of depth say 1 eV produces the glow peak at about 200 oC, with the

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trap emptying within a matter of seconds. It is important to explain at this stage how an electron trapped at a depth of 1 eV gets free at a temperature barely about 200 oC at which the average energy of the electrons (= 3/2 kT) is about 0.04 eV, which is too low in comparison to 1 eV. According to Maxwell-Boltzmann energy distribution, at 200oC the fraction of the total particle population which would have energy of 1 eV or more from their ground state (trap bottom) is expected to be only about 10 -13. It is only this incredibly small fraction which is capable of escaping from the trap. Then how does the trap empty so fast? The answer lies in the escape frequency s, which is of the order of 1013 s-1 (approximately equal to the vibrational frequency), the escape probability thus becomes 10-13x1013 = 1 s-1, which means that the fractional population which is 1 eV above the ground state of the trap escapes with nearly 100% probability. Once that has escaped, the thermal equilibrium gets quickly (in about 10 -12 s) restabilised with another equal fraction attaining the E >1 eV. With the process going on like this, the traps get emptied within a matter of seconds in the temperature range as cited above [3-4]. In Figure 3a, the energy distribution of particles (schematic) in thermal equilibrium at about 200oC is given whereas in Figure 3b, the schematic representation of variation of trap concentration, TL intensity and escape probability as a function of temperature is given.

Figure 3a: Energy distribution (N(E) versus E - schematic) of particles in thermal equilibrium at about 200oC. Particles (N) possessing energy above 1 eV are approximately 10-13 of the total particle population (N0) [3] TL Intensity

2.5

Trap concentration 2.0

1.5

1.0

Escape probability 0.5

0.0 300

325

350

375

400

425

450

475

500

525

550

575

Temperature (K)

Figure 3b: Schematic representation of variation of trap concentration, TL intensity and escape probability from trap as a function of temperature. 4

The phenomenon of TL was used for the dosimetry purpose around 50 years back and this application was suggested by an American scientist F. Daniel. Till then, various phosphors have been developed and are used in personnel, environmental and medical dosimetry applications. It should be noted that TL dosimeters are increasingly accepted for radiation dosimetry due to several reasons. The important ones are as follows: 1. Availability of tissue equivalent thermoluminescent materials. 2. Sufficiently high sensitivity and accuracy. 3. Availability of small size detectors adaptable for different applications. 4. Availability of materials with excellent long-term stability under varying environmental conditions. 5. Good sensitivity and reusability. 6. Linearity of dose response with dose and dose rate over a wide range.

2.1 Requirements for use of TL materials/phosphors for dosimetry applications For personnel monitoring/dosimetry purpose, the phosphor and the selected glow curve should satisfy following conditions [1-14]: 1. 2. 3. 4.

TL phosphor should have single glow curve (no interfering glow peaks) High TL sensitivity i. e. more light output per unit dose per unit mass Emission spectra in visible, preferably in the range 400 nm-500 nm Negligible thermal fading (loss of TL signal due to temperature - in fact the probability of release of electrons from the trap increases with the increase of temperature and shallow traps are more susceptible to fading). 5. Glow peak maximum should be preferably around 200oC (low temperature peaks are susceptible to thermal fading whereas for high temperature peaks, the black body radiation will be higher which is undesirable) 6. The glow curve selected for use in dosimetry should not exhibit thermal quenching 7. If possible, the phosphor should be tissue equivalent 8. Phosphor used should be cheap, easy to manufacture and should have simple annealing procedure 9. Linearity between dose and light output over wide range 10. Insensitive to light

Table -1 gives general characteristics of some commercially available TL dosimeters [5].

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Table-1 GENERAL CHARACTERISTICS OF SOME COMMERCIALLY AVAILABLE TL DOSIMETERS sensitivity

Fadingb (at 25 C)

Useful dose range

1

5%/year

20 μGy-10Gy

25

5%/year

0.2 μGy-10 Gy

605

0.20

4%/month

0.1mGy-3Gy

368

2

10%/2 months

10 μGy-10 3 Gy

190

490

10

4%/month

5 μGy-50 Gy

190

200-400

0.20

8%/2 months

0.1 mGy-0.5 Gy

11

200

380-400

40

very slight

10 μGy-1 Gy

15.3

220

480-570

30

1%/2 months

2 μGy-10 Gy

CaSO4:Tm

15.3

220

452

30

2 μGy-10 Gy

CaF2:Mn

16.3

260

500

5

1-2%/2 months 16%/2 weeks

CaF2 (natural) CaF2:Dy

16.3

260

380

23

very slight

10 μGy-50 Gy

16.3

215

480-570

15

8%/2 months

10 μGy-10 Gy

Al2O3 :C

10.2

190

420

60c

5%/year

0.1 μGy-10 Gy

TLD type

Effective At. No. Zeff 8.2

Main

Emission maximum (nm) 400

Relative

peak (oC) 200

LiF:Mg,Cu, P Li2B4O7:Mn

8.2

210

400

7.4

220

Li2B4O7:Cu

7.4

205

MgB4 O7:Dy

8.4

BeO

7.1

Mg2SiO4:Tb CaSO4:Dy

LiF:Mg,Ti

10 μGy-10 Gy

c Heating b Fading

rate 4 oC/s in the dark

2.1.1 Energy response The energy response to photons is the plot of TL output with the energy of the ionizing radiation when same dose (but of different energies) of ionizing radiation is delivered to the sample. This variation arises from the dependence of the material’s absorption coefficient on radiation energy and for photon irradiations; this is defined in terms of mass energy absorption coefficients of the material. The photon energy response is often defined with respect to the response of the TL material at gamma energy of 1.25 MeV (60Co). This leads to the term relative energy response (RER) which is defined as the ratio of the response of a given TL material at given energy to the response at 60Co gamma rays of energy 1.25 MeV. Ideally the response of the TL material is expected to be independent of energy and should mimics with the response of the tissue (Z Effective = 7.42), however many TL materials having ZEffective > 7.42 exhibit energy dependent response which can further be corrected using suitable filters. It should be noted that the suitability of dosimeters for personnel monitoring photon energy response should be studied in the energy range 15 to 1250 keV. Figure 4 shows the energy response of various TL materials. At lower photon energies, the response is higher due to higher probability of photoelectric interaction.

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Figure 4: Photon energy response curves for several thermoluminescent phosphors. A. CaF2; B. CaSO4; C. Al2O3; D. LiF:Mg,Ti; E. Li2B4O7, F. BeO; G. Mg2SiO4; H. MgB4O7; I. LiF:Mg,Cu,P [2, 5]. It should also be noted that for beta particles, the energy response is mainly decided by the thickness of the dosimeter. In fact the response becomes independent of energy if the range of beta particles is more than the thickness of the dosimeter. Hence for measurements of beta doses, dosimeters should be as thin as possible. It should also be noted that the dosimeters having typical thickness of 0.8 mm exhibit energy independent response for beta energy (Emax) > 1.7 MeV. The beta energy response of 0.4 mm and 0.8 mm thick CaSO4:Dy Teflon embedded discs is shown in Figure 5 [15].

Figure 5: Relative TL response as a function of beta energy for 0.4 mm and 0.8 mm thick CaSO4:Dy Teflon embedded discs [15]

2.1.2. Dose response As described earlier that the graph between TL output for a given glow curve in a TL phosphor versus the dose delivered should be linear. This helps in establishing a single calibration factor valid over a given range over which linearity exists. However it has been found that almost in all phosphors, linearity exists at lower doses i.e. TL 7

response per unit dose is constant in the range over which linearity exists. However at higher doses, the TL response per unit dose increases with increase of dose i.e. the glow curve exhibits over response (also called supralinearity). Further at very high doses, the TL response decreases with increase of dose and further at higher dose it saturates. Also beyond saturation, the response decreases with further increase in dose due to lattice damage and creation of new (absorption) centres. Several models have been proposed to explain supralinearity and most commonly used models are track interaction model and competeting traps model [7]. Schematic representation of dose response of a TL glow curve for a given TL material is shown in Figure 6. It should be noted that different TL glow curves of the same TL material may have different dose response. According to track interaction model, when the phosphor is irradiated, the electron - hole pairs produced will be initially distributed along the direction of the secondary charged particles. Also the luminescence centres are assumed to be produced along these tracks. At relatively low levels of absorbed dose only intra track TL emissive recombination is possible, as inter track distances are large compared with the electronhole migration distances. At relatively high levels of absorbed dose, the tracks are packed much closer together enabling inter track recombination along with intra track TL recombination which causes more TL output per unit dose as compared to that at lower doses and leads to supralinearity [7]. Further according to competing trap model, at lower doses, the distribution of electrons and holes is in all the traps in a given TL material. However with increase in dose, various competing traps may be completely filled and may not further act as competitors. Due to this, the electron-hole pairs produced have limited options for being trapped and these carriers are trapped in defects participating in the luminescence process.

Figure 6: Schematic representation of dose response of a TL glow curve [2] Hence more light per unit dose is emitted in the form of luminescence especially at higher doses. This explains the over-response or supralinearity of the given glow curve in a given TL material. It should be noted that the dose range of interest in personnel monitoring is 50 μGy to 1 Gy.

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2.1.3. Optimization of concentration of dopants and concentration quenching As described earlier that to enhance the number of defects (luminescence sensitivity) in insulating or in-organic materials, dopants are added. These defects may act as electron as well as hole traps and the dopants sometime may also act as luminescence/ recombination centre. It has been observed that for a given glow curve, the luminescence sensitivity/output increases with increase of concentration of dopants and reaches a maximum value. Beyond certain concentration, the luminescence efficiency starts decreasing with further increase of dopant concentration. This happens because the luminescence centres come very close to each other and their wave functions may also overlap at very high concentration of dopants (for 100 ppm doping, the dopants are separated by 20 lattice constants and are much closer at higher doping concentration). During luminescence emission by the emitting centre, the energy likely to be emitted in the form of thermoluminescence may be resonantly transferred to the nearby centre/dopant and this may continue along the chain of emitting centres. During this, there may be a killing or non-radiative centres in the chain of emitting centres (dopants) in the lattice which may absorb the energy which was likely to be emitted in the form of TL. These killing centres absorb this energy and dump that in the lattice in the form of phonons (non-radiative process). In view of this, the concentration of dopants in the host lattice needs to be optimized. In Figure 7a, the relative sensitivity of CaSO4:Dy (CaSO4:Tm) as a function of Dy or Tm concentration is shown [16]. Further in Figure 7b, the effect of variation in TL intensity of CaSO4:Dy as a function of Dy-concentration for various pre-irradiation annealing treatments is shown [17]. It appears from Figure 7b that high temperature annealing treatment influences the sensitivity of the dosimetry peak of CaSO4:Dy. In India, CaSO4:Dy is used as a dosimetry material for personnel as well as environmental applications [18]. It should be noted that at 2000 ppm (0.2 mole %) concentration of Dy, the dosimetry peak of CaSO 4:dy exhibits maximum sensitivity and same is used for environmental monitoring applications [19]. However, for personnel monitoring applications Dy concentration of 500 ppm (0.05 mole %) has been selected so that the dosimetry peak of CaSO4:Dy TL phosphor has optimum gamma response and negligible thermal neutron response (mainly due to Dy) [20]. In fact in environmental monitoring application the issue of thermal neutrons does not exist whereas in personnel monitoring applications, thermal neutrons are present in nuclear reactor environment. In addition to concentration quenching, trace impurities such as Fe, Cr, Ni, Co etc. in the host material may also introduce competitive/non-radiative paths and luminescence efficiency may further decrease due to the presence of these impurities [2]. The spectrographic analysis of trace elements in CaSO4:Dy phosphor shows the presence of Fe (10 ppm), Mn (