A detailed study of' thermoluminescence (TL) of luminescent materials such as

doped phosphors and glasses is reported The TL properties of well known CaS

phosphors singly and multi activated by rare-earth cerium and samarium ions

are investigated in detail. The effect uf using UV and gamma radiation as

exciting source on TL parameters qf doped phosphors is discussed .The TL

emission characteristics qf' gamma irradiated phosphate glasses doped with

transition metal ion ~ n ' and rare earth ion ~ e j - are discussed and the results

are incorporcrted Sol-gel derived and heat-treated cu2'doped silicate glasses

are also subjected to TL studies. From the results obtained different types of new

TL materials are proposed

Chapter 5 108

Thermoluminescence (TL) is the phenomenon of light emission upon heating a

material, which has been previously excited. All types of radiations such as

gamma rays, X-rays, alpha rays, beta rays and light rays can 'excite' a material

but to widely different extents. Out of the excitation energy imparted a very large

portion is almost instantaneously dissipated by various processes such as heat and

light and only the balance is absorbed and stored in it. On subsequent heating the

energy may be released and some of it may be in the form of light, which we call

thermoluminescence. The underlying mechanism involves the role of (i) crystal

defects which allows the storing of energy derived from exposure to radiation

through the trapping of carriers at these defects and (ii) subsequent release of

stored energy as visible light when these trapped carriers, after having been freed

by thermal stimulation recombine at the luminescent centers provided by

impurity atoms in the solids. In recent times, the phenomenon has been correctly

termed as thermally stimulated luminescence (TSL) 11- 41.

Thermoluminescence 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 [ 5 ] . 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. To get TL emission from a material

three essential conditions are necessary. Firstly the material must be an insulator

or a semiconductor. Secondly the material should have some time-absorbed

energy during exposure to radiation. Thirdly heating the material triggers the

luminescent emission. Once TL emission has been observed the material will not

show it again after simply cooling the specimen and reheating it but has to be

exposed to radiation to obtain TL again. The plot of intensity of the emitted light

Thermoluminescence studies of doped phosphors and glasses 109

against temperature is known as glow curve. Randall and Wilkins first developed

the theory for the calculation of TL parameters such as the activation energy E,

escape frequency factor S and the order of kinetics involved .The glow curve can

yield valuable information about the role of various impurities present in the

sample. Different workers have reported large number of works in the field of TL

on different materials. Large number of dielectric materials exhibit TL emission

including minerals, rocks, inorganic single crystals and polycrystalline

semiconductors and insulators, phosphors, glasses and ceramics, organic

compounds, biological and biochemical materials [6]. TL properties doped

phosphors of SrS, CaS, ZnS MgS etc are widely studied by UV excitation and

reported by many workers. [7-121

Mathur et a1 [13] studied high dose measurements using thermolurninescence 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. A further increase in high dose measurements is

possible by considering the ratio of two high temperature peaks. The ratio of two

peaks observed in glow curve is an intrinsic property of the material. Nair et a1

[14] carried out TL studies on CaS04:Ce phosphors with the aim of studying TL

process as well as the energy transfer and the effect of charge compensation.

Following gamma-irradiation, a drastic reduction in ce3+ fluorescence was

observed in all these samples. To explain the peculiar nature of the TSL glow

curve of CaS04:Ce, the gamma-ray-induced oxidation of ce3' ions

(i.e.,ce3++Ce"+e-) is proposed as a viable mechanism of hole trap production.

Thermoluminescence (TL) spectral analysis techniques have been applied to

doped calcium sulphate samples designed for radiation measurements at elevated

temperatures. CaS04:Dy. when co-doped with Ag, provides a TL dosimetric peak

near 350°C. which is useful for radiation measurements at high temperatures.

Dopants of Ce, Mn and Dy variously move the peak temperature from 400

degrees C to 200 degrees C. It is proposed that the dopants form part of large,

complex defects, instead of isolated trapping and recombination centres [15]. The

Chapter 5 110

effect of co-dopant when present in thermoluminescent phosphors induces higher

luminescence efficiency either due to better incorporation of activator ions or due

to improvement in energy transfer processes. Co-doping with Sm and Ce, the

CaS phosphors resulted in increasing the TL sensitivity. The high TL output of

these TL phosphors could be used in dosimetric practice for special short-term

measurements [16]

TL studies of rare earth ion and transition metal ion doped in different types of

glasses were reported recently [17-211. Irradiation of glasses leads to the

formation of structural defects [18]. Studies on radiation induced glass centers

provide a unique probe for elucidating not only the nature of the defect but also

the glass structure as well. So far no reports are seen on TL of gamma ray

irradiated phosphate glasses. In the present work we investigated TL properties of

all the prepared samples under UV and gamma ray excitation. Different methods

are adopted for the analysis of the data and the results are incorporated.

5.2.Theoretical methods of analysis

The models and factors affecting TL are discussed in detail in chapterl. The TL

emission characteristics are to be analyzed to get the mechanisms involved in the

process. Also TL applications of studied samples were determined only with

knowledge of TL parameters. So discussion of the important and usually applied

methods is included in the methods of analysis.

Singh et a1 [22] developed a method for the evaluation of activation energy of

thermoluminescence peak recorded with hyperbolic heating scheme by taking

into account temperature dependent frequency factor. They have arrived at a

number of expressions of activation energy involving the peak temperature

andlor temperatures corresponding to the two points of inflection of the peak. It

has been observed that the temperature dependence of frequency factor might

lead to an error of the order of 10% in the determination of activation energy.

Research activities in the area of TL over the years led the scientists in three

different directions. The first approach is concentrated on the determination of

Thermoluminrscence sludies of doped phosphors and glasses 1 1 1

trapping parameters such as activation energy (E), frequency factor (S) and order

of kinetics (b) relevant to the TL glow curves [2]. The second approach is related

to studies on defect centers [23]. The third approach makes use of the capability

of deep traps in insulating materials to store charge caniers above room

temperature for along tlme that leads to two main applications in the field of

dosimetry and dating [24]. There are many methods generally used in the

determination of TL parameters from an experimental glow curve. The methods

of analysis employed in our studies are discussed in the fallowing.

5.2.1.Heuristic methods

These are the first approximations for the evaluation of activation energies. Even

though the accuracy expected in these methods is poor in general, in recent

publications many workers have reported results based on these empirical

relations. Urbach [25) deduced an empirical relation from TL experiments in KC1

for the activation energy (E) or trap depth as

where T, is the glow peak temperature in Kelvin.

Randall and Wilkins [26-271 obtained an expression for trap depth at a

temperature T, of the glow peak intensity as

Table 5.1. Values of To and K for various (PIS) (D. Curie) -

0 = (PIS) (OK)

1 o - ~ 1 o-?

10" 1 0" - -

1 o - ~

lo-" 10'" 1 O - ~ lo-l4 10."

K ("Wev)

833 725 642 577 524 480 44 1 408 379 353 33 I 312

To

35 28

10 7 6 6 5 5 4

Chapter 5 112

......... E =kT,ln S 5. t

where S is known as the frequency factor and k is Boltunann's constant

Randall and Wilkins assumed ~ = l ~ ~ s e c ' l for alkaline earth sulphides .A similar

method was introduced by Curie [28] namely

E=[Tm-To (P /S)]/K (PIS ......... 5. 3

where To and K are functions of the parameter (PIS) given in the Table 5.1

5.2.2.Peakshape method

By using the glow shape method Grosswiener [29] obtained a relation using the

ascending part of glow curve (Figure 5.1) for the case of first order kinetics as

E, = 1.5 1 kT,Tl/r ......... 5. 4

where T, is the glow peak temperature and TI (rising end) and T2 (falling end)

are the temperature at the half widths of the glow peaks, k is Boltzmann constant

and r = T,-TI

By using the descending part of glow curve Lauchtehik [30]

Temperature

Figure 5.1. A schematic TL glow curve showing the maximum temperature

Tm,the low and high temperatures of half intensityTl and T2 and

the derived half widths W = T ~ - T ~ , & T ~ - T , , ~ ~ ~ 7=Tm-TI

Thermoluminescence studies of doped phosphors andglasses 113

obtained the relation as

E& = k 'rmZi8 ......... 5. 5

for first order kinetics and

~ = 2 k T ' d i 6 ......... 5. 6

for second order kinetics where 6 = (T2 -T,,,).

Analyzing the glow curve. Chen [3 11 put forward the relation for the trap depth

and frequency factor as

E, = 2.29 kTm2/ w ......... 5. 7 and

S = 2.67 ( P I o) 10 ......... 5. 8

respectively where is the rate of heating

5.2.3.Symmett-y factor and order of kinetics

The order of kinetics of a TL peak is manifested in its position, shape and

intensity [ 3 2 ] . The symmetry factor p, which is connected with the shape of the

TL peak, is used to determine the order of kinetics. The symmetry factor pg, =6/w,

where 6 is the half width towards the falling side of the TL peak and o is the total

width at half intensity such that o = T +6, T being the half width towards the

rising side of the TL peak. Haperin and Braner [33] suggested that the first order

kinetics is characterized by i*g< e" (e is the base of natural logarithm) where as

pg< e-' corresponds to the second order kinetics. It is to be noted that in general,

experimental TL peaks are not isolated. Usually there might be satellite peaks

either on the rising side or on the falling sides or on both the sides. In such a

situation it becomes difficult to determine the temperature at half intensity. Gartia

et a1 [34] introduced a method of determination of symmetry factor at any

fractional intensity points like x = 115, 112, 213 and 4fS.Thus the symmetry factor

at any fractional intensity can be defined as

p g ( ~ ) = (Txi -Tm) /(T: -T;) = ox ......... 5. 9

where T i and T,' are the temperatures corresponding to the fractional intensity x

in the rising and falling sides of a TL peak . 6 , and ox represents half width at the

Chapter 5 114

falling side and full width for Urn= x. It is found that the effect of satellite peaks

is less for higher values of X.

5.2.4.Various heating rate method.

In the above-mentioned methods the determination of the activation energy (trap

depth) require the prior knowledge of the order of kinetics. Various heating rate

(VHR) and initial rise (IR) methods can be used to determine the activation

energy even if the order of kinetics is not known

These methods are based on repeated measurements of a certain peak at different

heating rates keeping all other parameters constant. One advantage of this method

is that only properties related to the maximum point itself viz T,, the peak

temperature or the corresponding intensity I, are to be measured. Booth [35]

suggested the use of two heating rates for the evaluation of the trap depth as

where EB is the trap depth by Booth method, T,I and Tm2 are the peak

temperatures corresponding to the linear heating rates Pland p2, and k the

Boltzmann's constant. Hoogenstraaten [36] suggested the use of several heating

rates and according to this method, a plot of ln(~,*/ 0) verses IIT, should yield a

straight line of slope Elk, from which the trap depth can be calculated. Chen and

Winer [37] suggested that the plot of I, versusl/T, gives a straight line with slop

Elk. Gartia et a1 [34] also suggested another version of two heating rates method

according to which the activation energy EG is given by

EG = kTml Tm2 /(Trnl-Tm2). In[Im~/Imzl ......... 5. 11

where I,, and Im2 are peak intensities

5.2.5. Initial rise method

The initial rise method suggested by Garlick and Gibson [38] is based on the

assumption that for T 5 T, and I _< I, , the TL intensity can be written as

I = Const .exp (-EIkT) so that the plot of in (I) against llT results in a straight line

Thermolurninescence srudies of doped phosphors and glasses 115

with slope Elk. Since only a small portion of TL curve is used this method is very

popular. The trap depth by IR method is given by

5.3.Experimental Procedure

The methods of' preparation of doped CaS phosphors are discussed in detail in

Chapter I1 .The glass samples used for TL study were prepared as discussed in

Chapter 111 and Chapter IV. Experimental set up assembled in the laboratory

(Figure 5.2.) consists of a wooden box with a window, a PMT and a sliding

heater system. The PMT is fixed above the window facing the inside of the

wooden box. The sliding heater system consists of two similar stainless steel

plates sandwiching a heater coil mounted on a wooden board capable of freely

sliding into the outer box through one side.

The supply voltage to the heating coil can be varied so that the rate of heating

can be set to any desired value. A chromel-alumel thermocouple is spot welded at

the bottom centre of the lower metal plate so that the thermocouple leads can be

connected to a digital multimeter to measure the thermo-emf and hence the

surface temperature of the heater plate. The sample phosphor or glass can be

placed on the top center of the upper plate. The sliding heater system can be

pushed into the wooden box so that on pushing fully the sample comes vertically

under the PMT window and the set up becomes light tight. The required high

voltage for PMT is provided from a high voltage supply. The out put from the

PMT is amplified and given to Tektronix Digital Oscilloscope (THS 730A) for

recording the plot of TL intensity against time (Glow curve). The glow curve can

be saved in the instnunent and can be obtained as print out with the help of a

computer and the printer. The observations required for calculations of TL

parameters can be taken accurately from the oscilloscope data to minimize the

errors. Figure 5.3 shows the photograph of the digital oscilloscope with gamma

irradiated CaS:Sm,l TL glow curve on the screen.

Chapter 5 116

To study the TL of doped phosphors due to the excitation of UV irradiation,

about 25mg of sample is uniformly spread over lcm2 area at the center of the

upper heater plate and excited for 5 minutes from a UV source arranged nearby.

- 4 High voltage

Light detector (PMT)

Heater System Oscilloscope

I Figure 5.2. Schematic diagram of the experimental set up for the measurement

of Thermoluminescence (TL)

After excitation the sample is allowed to emit phosphorescence decay for 5

minutes. Then it is heated at a fixed rate of heating and the glow curve is plotted

as described above. The experiment is repeated for different rates of heating and

for different samples. The initial rise (IR) method of TL analysis was conducted

for samples containing sm3+ ions. For this the samples are exposed to UV

radiations for 5 minutes and after 5 minutes it is heated in the sample holder. The

output is observed till the emission intensity reaches the first glow peak. By

plotting 1/T vs Ln(1) the trap depth can be determined from the slope of the plot.

The glow curves of the same samples were also plotted after thermal cleaning of

the first glow peak. ie by heating the samples to a known temperature so that the

shallow traps can be emptied. The TL intensities are different for different cases

and hence the amplification is to be varied to get well-defined curves.

Chapter 5 118

estimated as AT = PL'C~IK, where P is the rate of heating, L the sample

thickness. p the density, C the specific heat capacity and K the thermal

conductivity of the material. So the temperature observed by thermocouple Tc is

to be corrected as T = Tc - AT12 for the glass samples.

5.4.Results and discussion

(a) TL of Phosphors due to UV irradiation

The TL glow curve of pure CaS, irradiated with UV radiation are recorded in the

oscilloscope and shown in Figure 5.4. It showed two peaks, one with maximum

intensity at 80°C and the other as a shoulder at 1 70°C, for a low rate of heating of

2"CIs. The intensity ratio is less than 112 and the peaks are resolved. The trap

depths of the two peaks are determined by different methods and are tabulated in

Table 5.2. The presence of the peak at 1 70°C is rarely reported in literature. For a

very low rate of heating, the appearances of two peaks in the temperature region

60 to 1 10°C are widely reported [39]. The peaks were explained as due to host

lattice defects and sulphur vacancies. The deep electron traps, which appear to be

related to calcium vacancies, are likely due to calcium interstitials. Two glow

peaks implies the existence of two traps, which invariably have different cross-

sections leading to a more complex intensity-dose relationship [40]. The results

obtained for pure CaS at different rate of heating showed a gradual change in the

peak temperature. The different rate of heating, influence of flux and the variation

of firing temperature may be the reasons for the shifting of reported TL peak

temperature (411 The UV excited TL glow curves of calcium sulphide (CaS)

doped with cerium (CaS:Cey), samarium (CaS:Sm,) and doubly doped with

samarium and cerium (CaS:Sm,,Ce,) are recorded . The glow curves of CaS and

CaS:Ce, are found to be gaussian with two glow peaks with considerable

separation so that each glow peaks can be very well distinguished. The

representative glow curve of CaS:Ce is shown in Figure 5.5. The glow curves of

different heating rates are also recorded and found that the TL intensity and peak

temperature increases with increase of rate of heating.

Thermoluminescence studies ofdopedphosphors and glasses 119

Representative glow c w e of C a S : S q X = ~ ~ o l w t ~ ~ showing the variation of

intensity and peak temperature with rate of heating is given in Figure 5.6 . Peak

temperatures T,I and 'rm2 of the two peaks of doped CaS phosphors at a rate of

heating 2'Cls are tabulated in Table 5.2. The activation energy in each case is

calculated by different methods of analysis assuming monomolecular kinetics, by

using equations j.(1).5.(2) and 5.(3/ and is also included in Table.5.2. In

(CaS:Ce), Ce acts as resonance centers for the TL transition in CaS , thereby

enhancing the TL glow [42]. In doped samples, the broad emission is quenched

in favor of emissions from the rare earth (RE) impurity sites. 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-max. Above

room temperature, the glow peaks are specific to the added RE ions and do not

show common peaks [43]. The isolated glow c w e s of CaS and CaS:Ce, are also

analyzed by peak shape method and tabulated in Table 5 3.

Temperature "C

Figure 5.4 TL glow c w e of UV irradiated CaS for P = 2 ' ~ / s

Figure 5.5 TL glow curve of UV irradiated CaS:CeYl for P = 2O~/s

-~ 0 .

Figure 5.6 TL glow curves of U V irradiated CaSSm,l for different

rate of heating

Thermoluminrscrnce srudies of dopedphosphors and glasses 121

Table 5.2.Trap depth values (eV) of doped CaS phosphors irradiated with UV

Table5.3. TL parameters of pure and ce3+ doped CaS phosphors irradiated with

UV rays. by glow peak shape method

glow shape method after deconvolution of glow curves.

As concentration of dopant increases it is found that the intensity of first trap

which is representing the shallow trap increases and for x = 0.009 wt% both

peaks shows equal intensity. Also variations in peak temperatures are noted. The

122 Chapter 5

variations of intensity of glow peaks are due to the creation of new traps of the

same type. The broadening of the peaks is due to the creation of large number of

traps of depths close to that of the main trap. The glow curves obtained for

undoped and doped CaS are also analyzed by deconvolution of the multi peaks

with the help of computer programme [ORIGIN 501 and the corresponding trap

depth values are tabulated in Table 5.4. A representative glow curve

(deconvoluted) of CaSSm,d is shown in Figure 5.8. Deconvolution can be applied

to all samarium doped samples.

arm inmk arm li_

Figure 5.7. TL glow curves for various UV irradiated CaS: Smx samples

Thermoluminescence studies ofdopedphosphors and glasses 123

0.006

Temperature ("c) Figure 5.8. TL glow curve (deconvoluted) of UV irradiated CaS:Smx4 sample

Figure 5.9. 1IT vs Ln Intensity plot of UV irradiated CaS:Sm, samples by

initial rise method

Chapter 5 124

Pagonis et a1 [44] presented an improved experimental procedure of separating a

composite thermoluminescence glow curve into its components. Careful

monitoring of the isothermal cleaning process using the initial rise method

ensures the complete thermal removal of TL peaks. Several standard methods

(initial rise and whole glow curve) are used to obtain the energy values and

frequency factors of the traps. The method has been used successfully to analyze

the well-known composite TL glow curve of the dosimetric material LiF

(TLD-100). Although the method works best for TL glow curves described by

first order kinetics, it should also be applicable to more general kinetics.

The presence of double peak in samples doped with Sm and Ce, with out clear

separation leads us to investigate the trap depth of first shoulder or peak by initial

rise method and that of the second peak by plotting TL glow curve after thermal

cleaning of first peak. The 1/T versus Ln (I) plots for IR method are given in

Figure 5.9.The trap depth values are calculated as per equation (5.12) and are

tabulated in Table 5.5. Glow curves of samarium doped samples after thermal

Figure 5.10. Representative TL glow curve of UV irradiated CaS:SmXl for p = 2 ' ~ / s after thermal cleaning

Thermoluminescence siudies of doped phosphors and glasses 125

cleaning of first peak are given in Figures 5.10. The trap depth parameters

calculated by equation (5. (7) } and {5.(8)} and are tabulated in Table 5.6.

Table 5 .5.Trap depth values of samples of doped CaS by initial rise method

Changes are observed in the glow c w e s of doubly doped phosphors under UV

Sample I Slope (Elk) =d (Ln ( x and y in wt%) I)/d( 1 IT)

CaSSmo,, -7128 5

irradiation. As cerium concentration in the sample increases the double peak

Trap depth Ela = -k, Slope eV

0.5 178

observed for low concentration disappears and for a concentration of 0.002 wt%

the curve appears to be a single guassian

Table 5.6 TL Parameters of doped CaS phosphors irradiated with UV rays after

thermal cleaning

Figure 5.11 represents TL glow curves of CaS (Sm,:Ce,) for different

concentrations. The TL glow curves of CaSSm,Ce, samples are also analysed by

deconvolution method. In Sm-Ce co-doped samples deconvolution was possible

only to CaS Sm,,=ooll: Ce,, o.oooz).A representative of the deconvoluted glow

curve is shown in Figure 5.12. The initial rise method and thermal cleaning of

Chapter 5 126

T ~ " C ~smperwrr "c

Figure 5.11. TL glow curves of UV irradiated CaS:Sm,Ce, samples for

p = 2O~/s (x and y in wt%)

shallow trap method are also employed for all CaS(Sm,:Ce,) samples and 1/T vs

LI(1) plots are shown in Figure 5.13.The glow curves of samples CaS (Sm,:Ce,)

obtained after thermal cleaning of shallow traps are found to be single gaussian.

Figure 5.14. shows the representative glow curve obtained after thermal cleaning.

The corresponding data are tabulated in Tables 5.2, 5.4, 5.5 and 5.6. The TL

parameters are calculated by peak shape method. The results obtained from the

initial rise method for shallow traps agree with the results of other methods. The

Trap depth values obtained from thermal cleaning of shallow traps are found to

match with the values of trap depth of the second peak in the glow curves of sm3+

doped samples by other methods.

It is found that the UV irradiated CaS (Sm,o.ool:Ce,o.ooz) phosphor showed a

single guassian at 150°C in the glow curve, which suggests that the same can be

used as a TL material in radiation dosimetry.

Thermoluminescence studies of doped phosphors and glasses 127

Temperature OC

Figure 5.12. TL glow curve (deconvoluted) of UV irradiated CaS:Sm,CeYl sample

I hi. ) I ITCK'I

Figure 5.13. I/T vs Ln Intensit) plot of UV irradiated CaS:Sm,Ce,

samples by initial rise method

Chapter 5 128

The low values of trap depth and monomolecular process of low temperature peak

indicate that the involved hole and electron traps are located near each other [41].

Temperature OC

Figure 5.14. Representative TL glow curve of UV irradiated CaS:Sm,CeY2 for

p = 2 ' ~ / s after thermal cleaning

~ ~ r e ' ' c rampas~lrs ('c)

Figure 5.15 TL glow curve of y irradiated (a) CaSSm,, and (b) CasSm,CeYz

samples for B= 2 O ~ l s

Thermolumine.scence sludies ofdopedphosphors and glasses 129

Table 5.7 TL Parameters of doped CaS phosphors irradiated with y rays

(b) TL of Phosphors due to gamma excitation

Gamma irradiated pure CaS showed very small TL output and the glow peak was

obtained at 16SUC. But all samarium doped and cerium co doped CaS phosphors

showed TL glow curves with single peak around 170°C with slightly varying

intensity. Representative glow peaks are shown in Figure 5.15 and the

corresponding TL parameters in Table 5.7. Also it is found that for doped

samples irradiated with gamma rays. TL fading is not at all affected. The

involvement of deep traps may be the reason for this. The TL traps related to high

temperature glow peaks. exhibited by doped CaS, excited with gamma radiation

might be attributed to the distribution of trapped hole and electron centres

involving the defect complexes in some manner or other [41].

(c) TL of Phosphate glasses due to gamma excitation

The TL glow curves of undoped, manganese doped, cerium doped and Mn-Ce

co-doped phosphate glasses were recorded after irradiation with gamma radiation

of 15 Gy from a "CO source. A pure phosphate glass sample does not show any

TL emission. Figure 5.16 (a), (b) and (c) represents the glow curves of singly

doped ~ n ~ ' . ce3* and a representative of doubly doped ~ n ~ + : ~ e ~ + ions in

phosphate glass. ~ n ' ' doped sample showed a broad and intense TL glow curve

with peak temperature at around 220°C. As cerium concentration increases the

broadness of the glow curve decreases giving a clear shape for the glow curve.

Chapter 5

Temperature "C 0.12 I

Temperature "C 0.7 I

Temperature "C

Figure 5.16 TL glow curves of y irradiated Mn, Ce, doped sodium

for b= 1 .S~C/S

130

phosphate glasses

Thermoluminescence studies of doped phosphors and glasses 13 1

Keeping ce3' content as constant ~ n ' + concentration is varied for other set of

samples and the glow curves are ploned. The glow curve of glass sample G

(Mnr:Cea3) with peak at 180°C, was found to be narrowed compared to other

glow curves. The summary of the data on TL peak temperatures and

corresponding activation energy and trap depth parameters are tabulated in

Table 5. 8. The trap depth calculations were carried out by peak shape method for

first order kinetics. The trap depths of the glass samples were calculated by

equarions (5. 4) and l5.51, the frequency factor by equation (5.8) and symmetry

factor by equalion (5.9) The high dose of gamma radiation on glass produces

secondary electrons from the sites where they are in a stable state. These

electrons with excess energy traverse in the glass lattice depending on the

composition of the glass and are finally trapped forming color centers in the glass

[45]. The trapping sites may be the metal cations, which constitute the glass

structure and the structural defects due to impurities. The Na20 is principally

responsible for non-bridging oxygen on which holes are trapped when the glass is

gamma irradiated

The thermoluminescence is the consequences of radiative recombination of

electrons and holes thermally released from the corresponding trapping sites. The

observed thermoluminescence in Mn2' doped and ce3' can be attributed to such

radiations. Also it is noticed that Mn2' in phosphate glass network was found to

be in more tetrahedral configuration. It is well known that a tetrahedral

configuration of transition metal ion is more absorptive than octahedral

configuration to ionizing radiation, generating high concentration of colour

centers and hence giving out more TL output [45] This is the reason for high TL

output for Mn2' doped glasses than ce3' doped and ce3+ co-doped samples. The

proportion of dopant for Mn as 1 wt% and Ce as 0.3% gave reasonable TL glow

output, at a peak temperature of 180°C for a dose of 15 Gy with a clear shape. So

this wt % combination of Mn and Ce can be selected as a good TL material for

high temperature dosimetry. The TL observed for Mn and Ce doped glasses for

UV irradiated with a less intensity compared to gamma irradiated samples also

can be explained by the same processes. The symmetry factor is found to be =0.5

Chapter 5 132

which is < 1,shows that the kinetics involved is first order .It implies that the

process of retrapping is negligible and the traps should be situated very close to

the luminescence centers.

Ram Reddy et al [17] carried out TL studies on X-ray irradiated Pb0-A1203 -

BzO, glasses doped with different rare earth ions. They observed that pure glasses

exhibit glow peak at 370K. When the same glasses are doped with different rare

earth ions no additional peaks are observed but the glow peak temperature T,

shifted gradually towards the higher temperatures with gain in TL intensity . Also

they reported that the above trend continued up to samarium, (Z=62) doped glass

and for further increase in the value of Z, peak temperature T, shifts towards

lower temperature with reduced intensity. The action of X-ray irradiation on glass

is to produce secondary electrons from the sites where they are in a stable state

and have an excess energy. Such electrons traverses the glass lattice depending

up on their energy and the composition of the glass and are finally be trapped

thus forming colour centers in the glass.

Rabie et a1 [IS] reported the TL studies of rare earth doped cabal glasses

irradiated with gamma radiations and found glow peaks at 125'C and 260°Cfor

pure and ~ 0 ~ ' doped glasses while a single peak at 300°C for sm3' doped

glasses. TL studies of transition metal ions doped borate glasses were carried out

by Krishnamurthy et a1 [45] under X-ray irradiation and reported the possible use

of these glasses in radiation dosimetry. Gel derived copper doped silica glasses

were also irradiated with same dose of gamma radiation and TL studies were

performed at a rate of heating 2OCIs in air. It exhibited a single broad glow peak

at around 150°C. Compared to ~ n ~ + doped phosphate glasses the TL emission

intensity is small. The parameter values are enclosed in Table 5.9. High

temperature TL of gamma irradiated copper activated silica glass was reported by

Debnath et al [19]. In the copper activated silica glass prepared by melt quench

method and reported by them showed two peaks, one at 145'C and other at 257'C

for a high rate of heating 13.15Ws.

Table 5.8. TL parameters of gamma irradiated phosphate glasses (Na2O-P2Os-MnxCe,) doped with ~ n ~ + and ce3+ ions

Sample T, K TI T2 o = T2. TI T= Tm-T, S= T2.Tm ~lp= 61 w Ez Es E o 5=2.67(~/o)10 (x and y in K K K K K x l o 3 s-'

Table 5.9. TL parameters of gamma irradiated sol gel derived silicate glasses doped with cu2+ ion

Therrnolurninescence studies of dopedphosphors and glasses 134

The samples derived by sol gel method and heated to 1000°C showed only one

peak at 150°C due to ~d ions. The absorption and emission studies reported in

Chapter IV proved that the samples heated to 1000°C contain CU' ions. It was

reported that TL in the ~d activated glass comes from the Cu+ centers, which are

in fact associated with the recombination sites of the released electrons. No

significant fading of the trap centers with time was noticed. Since gel derived

silica can be prepared at low temperatures and it satisfies the conditions for a

good TL dosimetric material this can be recommended as a good gamma ray

dosimeter for measuring gamma radiation of moderate levels.

5.5. Conclusions

The TL properties of well known CaS phosphors singly and multi activated by

rare-earth cerium and samarium ions are investigated in detail. The effects of

using UV and gamma radiation as exciting source on TL parameters of doped

phosphors are discussed. Pure CaS, which showed two peaks in the glow curve,

one at temperature around 80°C with high intensity and the other at 170°C with

low i~tensity for UV excitation. Addition of activator cerium impurity increases

the intensity of low temperature peak and not influenced in the peak

temperatures. When samarium ions are added as impurity its influence was

observed in both intensity and peak temperatures. As concentration of dopant

increased the low temperature peak gradually shifted to high temperature region

and high temperature peak showed a shift to low temperature region. The

intensity of high temperature peak increases as concentration increases. At a

dopant concentration of 0.009 wt % of the host, both peaks of glow curve showed

equal intensity and temperature difference between the peaks were reduced from

90K to 40K. The cerium and samarium co-activated CaS phosphor showed a

peculiar type of glow curve. As cerium concentration increased the first peak

observed as a shoulder gradually disappeared and for a dopant concentration of

CaS(Sm,=o ool:Cefl.ooz) it showed a single guassian at 1 SO°C in the glow curve.

This TL study of CaS phosphor doped with samarium and cerium rare earth ions

and irradiated with UV radiation concludes that by selecting proper percentage of

Chapter 5 135

dopant concentration. CaS phosphors satisfies the conditions for a good TL

dosimetric material and can be used as a TL material in radiation dosimetry.

The results obtained from TL studies of doped CaS phosphors by initial rise

method and thermal cleaning method. suggest that these methods can be applied

satisfactorily for the analysis of TL materials which gives multi peaked TL glow

curves that are difficult to isolate or deconvolute. The gamma ray irradiated TL

studies of doped CaS phosphors confirms the involvement of deep traps in the TL

process. The glow curve shows the single peak around 160°C, due to the deeper

traps produced by high energy of gamma radiation. The presence of single peak

at high temperature leads this material for gamma ray dosimetry. The results of

TL emission of' gamma irradiated phosphate glasses doped with transition metal

ion Mn2+ and rare earth ion ce3+ are also discussed. ~ n ~ + and ce3+ doped

phosphate glasses showed single and well-shaped glow curves at high

temperature (=I 80°C). The broad shape observed in our study may be due to the

large size of the samples. Making of glass sample of any size by melt quench

method is possible and so by selecting small size melt glass, single guassian at

high temperature are obtained under gamma ray excitation. Also proper annealing

and insulation by thin polythene will minimize the hygroscopic effect, which is a

disadvantage for sodium phosphate glasses. By taking these precautions sodium

phosphate glasses doped with Mn2+ and ce3+ can be used as one of the best

materials in radiation dosimetry.

cu2' ion doped silicate glasses, prepared by sol-gel method and heat treated to

1000°C are also used for TL studies. The gamma irradiated silicate glass samples

doped with copper showed TL due to the presence of Cu' ions in the high

temperature heat treated samples, which are in fact associated with the

recombination sites of the released electron. Gel derived silicate glass doped with

copper can be prepared at low temperatures and it satisfies the conditions for a

good TL dosimetric material. So this can be recommended as a good gamma ray

dosimeter for measuring gamma radiation of moderate levels. , I , , .

Thermoluminescence studies of doped phosphors and glasses 136

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