CHAPTER-IV

LYOLUMINESCENCE IN γ-IRRADIATED Ce, Dy AND Eu DOPED BORATE BASED

PHOSPHORS

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4.1 INTRODUCTION

The luminescence produced due to recombination process during

dissolution of irradiated materials to certain suitable solvent is known as

lyoluminescence (LL). The quantum of light yield produced is

proportional to absorbed dose in the solid. The lyoluminescence

phenomenon, owes its origin to the transfer of energy from the radiation-

induced free radicals to the solvent molecules, a part of which gets

transposed to visible light. The lyoluminescence is a quite general

phenomenon occurring with very different materials but most of the

systematic studies turned to the dosimetric application have been

concentrated on alkali halides and on two classes of organic compounds

regarded as tissue equivalents, saccharides and amino acids. Recent work

in this field appears to deal more with improvement of techniques, both

of detection and preparation of materials, aimed at achieving more

reliable dosimetry (Ettinger and Puite, 1982). However there are number

of various factors that influence the light yield during the dissolution of γ-

irradiated phosphors in solvent which have not been investigated in

detail. Mass of the sample, pH of the solvent, temperature of the solvent,

irradiation dose etc. are some example of parameters that influence the

LL intensity. For the purpose of developing LL dosimetry material, these

factors require detail investigation.

The word lyoluminescence was coined by Wiedemann and

Schamidt (1895) who first observed the phenomenon on dissolving NaCl

irradiated with cathode rays in water. In the same year they reported a

further observation with MgSO4:Cd. There were no further reports until

1959 when Ahnstron and Ehrenstein (1959) observed persistent

luminescence on dissolving glucose irradiated with X-rays, γ-rays and

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neutrons to absorbed doses of several MGy. Westermark et al (1960),

investigated the lyoluminescence of both inorganic and organic materials,

and also observed light emission from irradiated polystyrene dissolved in

liquid scintillator. Subsequent studies included those of Ahnstrom (1961),

Eriksson (1962) and Ahnstrom (1965). Observations of inorganic

lyoluminescence were reported by Lelievre and Adloff (1964); Avotinsh

et al (1981) and by Arnikar et al (1970; 1971). Arnikar et al (1977),

termed the phenomenon aquoluminescence for dissolution in water and

sololuminescence as the more general term to include solvents other than

water (Arnikar et al, 1971).

The application of lyoluminescence to radiation dosimetry was

discussed by Atari et al (1973). Investigations of the lyoluminescence of

inorganic materials both for the possible application to dosimetry and to

understand the mechanism of light production were continued by Atari,

Ettinger and co-workers (1974, and 1977) and by Arnikar and colleagues

(1975, 1977a, and 1977b). However, greater emphasis has been put on

the lyoluminescence of organic materials, particularly saccharides, in

view of their similarity of elementary composition to tissue (Atrari and

Ettinger, 1973). The possibility of the development of a fairly true tissue

equivalent dosimeter was presented by Atari and Ettinger (1974).

Organic materials other than saccharides have also been studied,

namely amino acids, amino sugars, proteins and nucleic acid (Ettinger

and Atari, 1974; Thwaites et al, 1976; Ettinger et al, 1977and Puite et al,

1980); but in general, the lyoluminescence phosphors of choice have been

saccharides (Ettinger et al, 1977; Chazhoor and Mishra, 1979; Bartlett,

1979; Bartlett, 1982; Busuoli and Monteventi, 1982 and Puite et al,

1977). Puite and co-workers (1977) have reported the application of

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lyoluimescence of mannose to the dosimetry of low energy X-rays and

fast neutrons for application to radio-biology and radiotherapy, and of

glutamine (1980) for high dose intercomparisons. Ettinger et al (1977)

and Puite (1977) have also considered the application of lyoluminescence

dosimetry in commercial irradiation of foodstuffs. Ziemer et al (1978),

reviewed lyoluminescnce dosimetry and investigated the lower limit of

detection using trehalose dehydrate. The effect of storage on the

lyoluminescence response of mannose was investigated and the

dependence of the change in response on observed dose has been

determined by Bartlett (1979). Kundu and his co-workers (1984 and

1992) have studied LL of certain organic solids. The mechanism of

radical generation, peroxide formation and light emission has been

summarized by Chatterjee et al (1993), giving a comprehensive

calculation of kinetics and the light yield.

The study of LL investigation of inorganic phosphors yielded

useful results. Inorganic LL phosphors other than alkali halides were

investigated in the beginning of nineties. Copty Wergles et al (1990)

investigated LL during dissolution of calcite and aragoniticsnail irradiated

with 60Co gamma ray in HCl. LL induced by the dissolution of UV

irradiated Tb doped potassium peroxodisulphate in aqueous solution was

reported by Kulama and Haapakka (1994). Matachescu et al (1997) have

reported the mechanism of analytical application of luminol-specific

extrinsic LL of UV irradiated potassium peroxidisulphate. LL

phenomenon in the super molecular aggregation of antinuclear gold

complex was studied by Vickery et al (1997). Chattopadhyay et al (2000)

developed a nonlinear differential equation and analyzed it by a dynamic

system analysis as well as exact numerical integration. Raman et al

(2001) reported the LL of trehalose dihydrate. They observed that LL

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shows linear gamma sensitivity in the dose range 0.1x102 to 5x102 Gy.

LL property of gamma irradiated Eu activated KNaSO4 and K3Na(SO4)2

phosphors were reported by S.J. Dhoble (2002). He observed linear

response of absorbed gamma dose from 0.06 to 10 Ckg-1. He also studied

the fading of LL intensity and found that this phosphor is suitable for LL

dosimetry. Dependence of the lyoluminescence of divalent impurity

doped NaCl on temperature was studied by Gour et al (2004). The effect

of pH of solution on the lyoluminescence of divalent impurity doped

sodium chloride was studied by Chourasia et al (2004). Particle size

dependence of lyoluminescence of gamma irradiated sodium bromide

was reported by Banerjee et al (2005). Hakansson et al (2006) studied the

lyoluminescence of aluminium induced by lanthanides chelates in

alkaline aqueous solution.

Lyoluminescence (LL) of MgSO4:Dy phosphors has been studied

by Kher et al.(2008). The maximum LL intensity was observed for 0.05

mole% of Dy in MgSO4. Thermoluminescence (TL) glow curves of

MgSO4: Dy phosphors are complex and peaks around 1100C, 1500C and

2200C were observed. The effect of post-irradiation annealing on the LL

shows that with removal of 2200C TL peak, the LL intensity decreases

markedly.

KNaSO4:Tb3+ phosphors were synthesized by melt technique with

different concentration of Tb3+ ions and MgSO4:Dy3+ phosphor were

prepared by solid state diffusion method by Upadhyay et al (2008).

Lyoluminescence and photoluminescence (PL) characterization of

KNaSO4:Tb and MgSO4:Dy3+ phosphors were reported in this paper.

116

They showed that KNaSO4:Tb3+ and MgSO4:Dy3+ phosphors are suitable

as a lyoluminescence dosimetry phosphor for ionizing radiations.

The lyoluminescence (LL) intensity of gamma irradiated KCl:Sr

microcrystalline powder of different particle sizes have been investigated

by Sahu et al (2009). They found that in the mesh range of 150-355 µm,

the peak LL intensity initially increases with increasing particle size,

attains an optimum value for particular size and then it decreases with

further increase in particle size of the sample.

The temperature and mass dependence of lyoluminescence

intensity of γ-irradiated colored potassium chloride powder were studied

by Nayar et al (2010). They found that peak lyoluminescence intensity

increases with increasing amount of solute added up to 50 mg and then

tends to saturate. Further they reported the dependence of decay time on

mass and dependence of LL intensity on temperature. They found that

initially the peak LL intensity increases with temperature up to 60°C,

then decreases with further increase in temperature and decay time tends

to decrease with increasing temperature.

Rare earth Ce and Dy doped polycrystalline Li2BPO5 phosphors

were prepared by high temperature solid state reaction by Puppalwar et al

(2011). They reported that LL emission of Li2BPO5:Ce,Dy phosphor

occurs in blue – yellow region of the spectrum and response curve of LL

with gamma rays exposure upto 3.5 kGy is linear.

Lyoluminescence and mechanoluminescence of Cu activated

LiKSO4 phosphors for radiation dosimetry is studied by Dhoble et al

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(2011). Synthesis of Cu2+ activated LiKSO4 phosphor and its possible use

as lyoluminescence (LL) and mechanoluminescence (ML) dosimeter

were discussed.Cu activated LiKSO4 phosphor showed less fading in LL

and ML over a long time, linear gamma-ray dose response up to 1 kGy,

emission around 455 nm, and simple glow curve. These properties make

this phosphor a useful candidate for LL and ML dosimetry.

Survey of literature show that no systematic investigation has been

made on the lyoluminescence of borate based phosphors. Borates are

good candidate for thermoluminescence dosimetry and many

thermoluminescence materials also show LL when dissolved in suitable

solvent. The present chapter reports the systematic investigation of LL of

borate based phosphors. The objective is to find suitability of these

phosphors for dosimetry and to use this for understanding of other

luminescence phenomena.

4.2 EXPERIMENTAL

Ce, Dy and Eu doped MB4O7 (M= Ba, Ca, Sr) and LnCa4O(BO3)3

(Ln= Gd, La, Y) phosphors were prepared by the methods described in

chapter-II. Gamma irradiation was carried out using 60Co source. The LL

read out system fabricated for lyoluminescence measurements consists of

a lyoluminescence cell, a light detector, signal processor and signal

recorder. Fig. 4.1 shows the schematic diagram of the LL reader system.

The lyoluminescence cell and the photomultiplier tube (PMT) are kept in

the light-tight box. A circular hole was cut on the top surface of the box

into which a glass tube (8 mm diameter and 4 cm length) was fitted. This

glass tube is acted as the dissolution vessel. It rests just in front of the

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window of the PMT. The solvent is injected into the vessel with the help

of a conventional hypodermic syringe.

The light emitted during the dissolution was detected by RCA-931

photomultiplier tube. For biasing the various dynodes of the PMT, a high

voltage power supply was used. The output of PMT was fed to storage

CRO. All the experiments have been conducted in a dark room at room

temperature. The mass of the samples used for recording LL data is kept

approximately (~1mg) in each case and dilute HCl has been used as the

solvent to dissolve the sample. The volume of the solvent was 2.5 ml in

each case. For reading LL spectra filters were placed between LL cell and

the PMT. For each point at least three observations were recorded in

identical experimental conditions to ensure the reproducibility.

Fig. 4.1 Experimental setup used for LL measurement

RCA 931A

PMT

Storage CRO

Shutter

H. T. Power Supply

Powder Sample RCA 931A

PMT

Storage CRO

Shutter

H. T. Power Supply

Powder Sample RCA 931A

PMT

Storage CRO

Shutter

H. T. Power Supply

Powder Sample

Solvent

RCA 931A

PMT

Storage CRO

Shutter

H. T. Power Supply

Powder Sample

Light Tight Wooden Box

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4.3 RESULTS

It is observed that when Ce, Dy and Eu doped MB4O7 (M = Ba, Ca,

Sr,) and LnCa4O(BO3)3 (Ln = Gd , La ,Y) phosphors exposed to gamma

dose were dissolved in solvent , MB4O7 did not show any measurable

intensity in solvent while LnCa4O(BO3)3 (Ln= Gd , La ,Y) phosphors

showed prominent peaks when it was dissolved in dilute HCl .

Figure 4.2a, 4.2b and 4.2c shows the time dependence of LL

intensity of γ-irradiated Ce, Dy and Eu doped GdCa4O(BO3)3,

LaCa4O(BO3)3 and YCa4O(BO3)3 phosphors respectively. It is seen that

on dissolving gamma irradiated samples in to dilute HCl, initially the LL

intensity increases with time, attains maximum value Im at a particular

time tm (i.e. time corresponding to LL peak) then it decreases and finally

disappears. Similar pattern was observed for all the samples, however

relative intensity is different.

Figure 4.3a, 4.3b and 4.3c show the dependence of total LL

intensity on concentration of dopant for Ce, Dy and Eu doped

GdCa4O(BO3)3, LaCa4O(BO3)3and YCa4O(BO3)3 phosphors respectively.

It is observed that undoped phosphors show very weak LL intensity. LL

intensity initially increases with increasing dopant concentration attains

maximum value for a particular concentration, and then decreases with

further increase in concentration. It is also observed that tm i.e. time

corresponding to LL peak does not change significantly with change in

concentration of dopant. It is also observed that Ce is more effective

dopant for the enhancement of LL intensity in borate based phosphors.

120

Figure 4.4a, 4.4b and 4.4c show the total LL intensity as a function

of gamma ray dose given to the samples for Ce, Dy and Eu doped

GdCa4O(BO3)3, LaCa4O(BO3)3and YCa4O(BO3)3 phosphors respectively.

It is observed that the LL intensity increases with gamma ray dose given

to the samples and attains saturation for higher values of gamma doses.

Figure 4.5a shows LL emission spectrum of Ce doped γ-irradiated

LnCa4O(BO3)3 (Ln = Gd , La ,Y) phosphors. It is observed that all the

studied phosphors emit LL mainly at 455 nm (in blue region).

Figure 4.5b shows LL emission spectrum of Dy doped γ-irradiated

LnCa4O(BO3)3 (Ln = Gd , La ,Y) phosphors. Two peaks were observed in

the range 470-480 nm and 570-580 nm for all the samples.

Figure 4.5c shows LL emission spectrum of Eu doped γ-irradiated

LnCa4O (BO3)3 (Ln = Gd , La ,Y) phosphors. A single peak around 606

nm was observed for LnCa4O (BO3)3 phosphors.

Maximum LL intensity observed in GdCa4O(BO3)3:Ce(0.5mol%)

as compared to other borate based phosphors. Therefore, the LL glow

curves of GdCa4O(BO3)3:Ce(0.5mol%) sample have been shown in

further results.

Fig. 4.6 shows the dependence of total LL intensity on mass of

dissolved γ-irradiated GdCa4O(BO3)3:Ce(0.5mol%) sample. It is seen

that initially the LL intensity increases with the mass of the crystallites

added in a fixed volume of dilute HCl and then it seems to attain a

saturation value for higher mass of the crystallites dissolved. It is also

seen that time corresponding to LL peak shifted towards higher value side

with increasing the mass of solute.

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0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2 2.5 3 3.5

Time (ms)

LL in

tens

ity (

arb

unit)

Ce(0.5 mole %)

Dy (0.5 mol%)

Eu(0.5 mol%)

Figure 4.2 (a): Time dependence of LL intensity of γ-ray-irradiated GdCa4O(BO3)3:RE phosphors dissolved in dilute HCl. ( γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

0

5

10

15

20

25

30

35

40

45

50

0 0.5 1 1.5 2 2.5 3 3.5

Time (ms)

LL in

tens

ity (a

rb u

nit)

Ce(2.0 mole %)

Dy (0.5 mol%)

Eu(0.5 mol%)

Figure 4.2 (b): Time dependence of LL intensity of γ-ray-irradiated LaCa4O(BO3)3:RE phosphors dissolved in dilute HCl. ( γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

122

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3 3.5

Time (ms)

LL in

tens

ity (a

rb u

nit)

Ce(2.0 mole %)

Dy (0.05 mol%)

Eu(0.2 mol%)

Figure 4.2 (c): Time dependence of LL intensity of γ-ray-irradiated YCa4O(BO3)3:RE phosphors dissolved in dilute HCl. ( γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5

Dopant concentration (mol%)

Rel

ativ

e to

tal

LL in

tens

ity (a

. u.)

Ce

Dy

Eu

Figure 4.3 (a): Relative total LL intensity of γ-ray-irradiated GdCa4O(BO3)3:RE phosphors dissolved in dilute HCl as a function of dopant concentration ( γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

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0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5

Dopant concentration (mol%)

Rel

ativ

e to

tal L

L in

tens

ity (a

rb u

nit)

Ce

Dy

Eu

Figure 4.3 (b): Relative total LL intensity of γ-ray-irradiated LaCa4O(BO3)3:RE phosphors dissolved in dilute HCl as a function of dopant concentration ( γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5

Dopant concentration (mol%)

Rel

ativ

e to

tal L

L in

tens

ity (a

rb u

nit)

Ce

Dy

Eu

Figure 4.3 (c): Relative total LL intensity of γ-ray-irradiated YCa4O(BO3)3:RE phosphors dissolved in dilute HCl as a function of dopant concentration ( γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

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0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5

Gamma-ray dose(kGy)

Rel

ativ

e to

tal

LL

in

ten

sity

(ar

b u

nit

)

GdCa4O(BO3)3:Ce(0.5 mol%)

GdCa4O(BO3)3:Dy(0.5 mol%)

GdCa4O(BO3)3:Eu(0.5 mol%)

Figure 4.4 (a): Relative total LL intensity of γ-ray-irradiated GdCa4O(BO3)3:RE phosphors dissolved in dilute HCl as a function of gamma ray doses given to the sample (amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5

gamma-ray dose (kGy)

Rel

ativ

e to

tal

LL

in

ten

sity

in

arb

un

it

LaCa4O(BO3)3:Ce( 2.0 mol%)

LaCa4O(BO3)3Dy(0.5 mol%)

LaCa4O(BO3)3:Eu(0.5 mol%)

Figure 4.4 (b): Relative total LL intensity of γ-ray-irradiated LaCa4O(BO3)3:RE phosphors dissolved in dilute HCl as a function of gamma ray doses given to the sample (amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

125

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2

Gamma-ray dose (kGy)

Rel

ativ

e to

tal

LL

in

ten

sity

(ar

b u

nit

)

YCa4O(BO3)3:Ce(2.0 mol%)

YCa4O(BO3)3:Dy(0.05 mol%)

YCa4O(BO3)3:Eu(0.2 mol%)

Figure 4.4 (c): Relative total LL intensity of γ-ray-irradiated YCa4O(BO3)3:RE phosphors dissolved in dilute HCl as a function of gamma ray doses given to the sample (amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

0

2

4

6

8

10

12

400 450 500 550 600 650 700

Wavelength (nm)

Rel

ativ

e to

tal

LL

in

ten

sity

(ar

b u

nit

) GdCa4O(BO3)3:Ce(0.5 mol%)

LaCa4O(BO3)3:Ce( 2.0 mol%)

YCa4O(BO3)3:Ce(2.0 mol%)

Figure 4.5 (a): LL emission spectra of γ-ray-irradiated Ce doped oxyborate phosphors dissolved in dilute HCl (γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

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0

2

4

6

8

10

12

400 450 500 550 600 650 700

Wavelength (nm)

Rel

ativ

e to

tal

LL

in

ten

sity

(ar

b u

nit

) GdCa4O(BO3)3:Dy(0.5 mol%)

LaCa4O(BO3)3Dy(0.5 mol%)

YCa4O(BO3)3:Dy(0.05 mol%)

Figure 4.5 (b): LL emission spectra of γ-ray-irradiated Dy doped oxyborate phosphors dissolved in dilute HCl (γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

0

2

4

6

8

10

12

400 450 500 550 600 650 700

Wavelength (nm)

Rel

ativ

e to

tal

LL

in

ten

sity

(ar

b u

nit

) GdCa4O(BO3)3:Eu(0.5 mol%)

LaCa4O(BO3)3:Eu(0.5 mol%)

YCa4O(BO3)3:Eu(0.2 mol%)

Figure 4.5 (c): LL emission spectra of γ-ray-irradiated Eu doped oxyborate phosphors dissolved in dilute HCl (γ-ray dose 1.1 kGy, amount of solute 1 mg, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

127

0

10

20

30

40

50

60

70

80

90

0 0.5 1 1.5 2 2.5 3 3.5

Time (ms)

LL in

tens

ity (a

. u.)

0.6 mg

1 mg

1.6 mg

2 mg

2.6 mg

Figure 4.6: Time dependence of LL intensity of γ-ray-irradiated GdCa4O(BO3)3:Ce(0.5 mol%) phosphors dissolved in dilute HCl for different mass of solute. ( γ-ray dose 1.1 kGy, amount of solvent 2.5 ml, pH 4, particle size 90-120 µm)

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8

pH value

Rel

ativ

e to

tal L

L in

tens

ity (a

rb u

nit)

Figure 4.7: Relative total LL intensity of γ-ray-irradiated GdCa4O(BO3)3:Ce(0.5 mol%) phosphors dissolved in dilute HCl as a function of pH value of the solvent (amount of solute 1 mg, amount of solvent 2.5 ml, particle size 90-120 µm)

128

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3

Time (ms)

LL

inte

nsi

ty (

arb

un

it) 30 °C

45 °C

60 °C

75 °C

90 °C

Figure 4.8: Time dependence of LL intensity of γ-ray-irradiated GdCa4O(BO3)3:Ce(0.5 mol%) phosphors dissolved in dilute HCl for different temperature of solvent (γ-ray dose 1.1 kGy, amount of solvent 2.5 ml, mass of solute 1 mg, pH 4, particle size 90-120 µm)

0

0.5

1

1.5

2

0 0.5 1 1.5 2 2.5 3

Volume of solvent (ml)

Rel

ativ

e to

tal L

L in

tens

ity (a

rb u

nit)

Figure 4.9: Relative total LL intensity of γ-ray-irradiated GdCa4O(BO3)3:Ce(0.5 mol%) phosphors dissolved in dilute HCl as a function volume of solvent (γ-ray dose 1.1 kGy, mass of solute 1 mg, pH 4, particle size 90-120 µm)

129

0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2 2.5 3

Time (ms)

LL in

tens

ity (a

rb u

nit)

0-75 µm

75-90 µm

90-120 µm

120-150 µm

150-180 µm

Figure 4.10: Time dependence of LL intensity of γ-ray-irradiated GdCa4O(BO3)3:Ce(0.5 mol%) phosphors dissolved in dilute HCl for different particle size (γ-ray dose 1.1 kGy, amount of solvent 2.5 ml, mass of solute 1 mg, pH 4)

0

2

4

6

8

10

12

0 5 10 15 20

Storage time (day)

Rel

ativ

e to

tal L

L in

tens

ity

(arb

un

it)

GdCa4O(BO3)3:Ce(0.5 mol%)

LaCa4O(BO3)3:Ce( 2.0 mol%)

YCa4O(BO3)3:Ce(2.0 mol%)

Figure 4.11: Relative total LL intensity of calcium oxy borate phosphors dissolved in dilute HCl . (γ-ray dose 1.1 kGy, amount of solvent 2.5 ml, mass of solute 1 mg, pH 4)

130

Fig. 4.7 shows the dependence of total LL intensity on the pH of

dilute HCl for γ-irradiated GdCa4O(BO3)3:Ce(0.5mol%) sample. It is seen

that the LL intensity is optimum for a particular pH value of the solvent.

Results show that the LL peak intensity increases from a nearly zero

value at a pH of 1 to a maximum value of 4 and then it decreases rapidly

to nearly zero again at a pH of about 7.

Fig. 4.8 shows the LL glow curve of γ-irradiated

GdCa4O(BO3)3:Ce(0.5mol%) sample for different temperature of the

solvent (dilute HCl). It is observed that LL intensity increases with time,

attains an optimum value and then decreases and finally disappears. It is

also seen that peak intensity Im increases with increasing temperature of

the solvent, attains an optimum value at 75°C and then decreases with

increasing temperature of the solvent. It is also observed that tm shifted

towards shorter time value with temperature of the solvent.

Fig. 4.9 shows the dependence of the total LL intensity of γ-

irradiated GdCa4O(BO3)3:Ce(0.5mol%) sample on volume of the solvent

(dilute HCl). It is seen that LL intensity increases with increase in volume

of the solvent and attain an optimum value for a particular volume of

solvent without any appreciable change in tm.

Fig.4.10 shows the time dependence of the LL intensity for

different grain size of γ-irradiated GdCa4O(BO3)3:Ce(0.5mol%) sample.

It is seen that on dissolving the γ-irradiated sample of a particular grain

size to a fixed volume (2.5ml) of dilute HCl, initially the LL intensity

increases with time, attains a maximum value Im at a particular time tm,

then it decreases and finally disappears. It is also observed that tm shifts

towards higher value with the increase of grain size.

131

Fig. 4.11 shows the effect of storage on LL (at room temperature in

dark) of γ-irradiated GdCa4O(BO3)3:Ce(0.5mol%), LaCa4O(BO3)3:Ce

(2mol%), and YCa4O(BO3)3:Ce(2mol%). It is clear that fading in LL

intensity of the entire sample is very low.

4.4 DISCUSSION

The phenomenon of the emission of light on dissolution of

irradiated samples to the solvent is known as lyoluminescence, and has

been investigated by a number of workers for dosimetric applications.

Several inorganic and organic samples possess LL properties. In

inorganic materials such as alkali halides the release of trapped electrons

from F centers is responsible for the light emission. In case of some

organic materials such as saccharide, trapped free radicals are involved in

process of LL.

It is observed that the LL intensity of LnCa4O(BO3)3 phosphors

may depends on different parameters in the following way:

i. Radiation dose: The intensity of LL emission initially rises with

the increase in radiation dose given to the sample and then attains a

saturation value for higher values of the radiation dose.

ii. Density of defect centres (nF): The intensity of LL emission

depends linearly on the density of defect centres.

iii. Time of dissolution (t): The LL intensity initially increases

attains an optimum value and then decreases exponentially with the

time of dissolution of the sample.

iv. Temperature of the solvent (T): LL intensity initially increases

with temperature, attain optimum value then decrease with further

increase in temperature.

132

v. Mass of the dissolved sample: The LL intensity initially increases

and then tends to attain a saturation value for higher mass of the

solute added to the solvent.

vi. Type of the sample: The LL intensity is different for different

sample.

vii. pH of the solvent: The LL intensity attains an optimum value for

as particular value of pH of the solvent.

viii. Volume of the solvent: The LL intensity is optimum for a

particular volume of the solvent.

ix. Type of impurity present: This causes increase in LL intensity.

x. Particle size of the sample: The LL intensity depends on the

particle size of the sample.

4.4.1 Mechanism of the LL in borate based phosphors

Borates under γ-rays irradiation are known to produce various

defect centers such as B2+, BO32-, O-

, (B3O6)3-, O3

-, O2- etc (Porwal et al,

2005; Lochab et al, 2007). On dissolving such borates in suitable solvent

the LL is observed due to release of trapped energy during dissolution as

per mechanism suggested by Arinkar et al (1971). Schematically, the

mechanism of LL in LnCa4O(BO3)3 may be described by the following

equations:

e- (trapped) + Aqueous Solvent → e-aq, (a)

e-aq + X → Xaq

*, (b)

Xaq* → Xaq + hυ, (c)

where, X =Radicals generated due to γ-rays irradiation.

It is possible that steps (b) and (c) occur together without the

intermediate state Xaq*. A similar mechanism was also suggested by

Ahnstrom (1965).

133

Generally, in the lyoluminescence experiment, X- or γ-irradiated

crystallites of a given mass are dissolved in a given volume of a solvent.

If the solute contains N molecules at any time t and α is the rate of

dissolution in the solvent, then the rate of decrease in the number of

solute atoms may be written as

αN.

dt

dN =− (4.1)

For N=N0 at t = 0, the integration of Eq. (4.1) gives

N = N0 exp (-αt). (4.2)

Assuming the linear dependence of the rate of generation g of the

hydrated electrons on nF and (-dN/dt), we get

g = γ nF (-dN/dt)

or

g = γ nFN0α exp (-αt), (4.3)

where, γ is a factor correlating the number of hydrated electrons with the

number of dissolved defect-centres of the crystal.

The number of hydrated electrons will increase due to the

dissolution of the crystallites and it will decrease due to the

recombination of hydrated electrons with the holes present on the surface

of the crystallites. In this way, the rate equation may be written as

,n vNσg

dt

dnrr−= (4.4)

where, n is the number of hydrated electrons at any time t, σr is the

capture cross-section of holes, Nr is the density of recombination centres

i.e. holes and v is the average velocity of the hydrated electrons.

134

Now, substituting the value of g from Eq. (4.3) into Eq. (4.4), we

have

n vNσαt)exp( αn Nγdt

dnrrF0 −−=

or

n,βαt)αexp(nNγdt

dnF0 −−= (4.5)

where, β = σr Nr v, is the rate constant for the recombination of

hydrated electrons with holes.

Now, the lifetime of hydrated electrons is given by

.vNσ

1

β

1τ

rr

== (4.6)

For n = 0 at t = 0 the integration of Eq. (4.5) gives

.βt)}exp(αt){exp(α)(β

Nnαγn 0F −−−

−= (4.7)

If η is the probability of radiative recombination, then the LL intensity

may be given by

βt)}.exp(αt){exp(α)(β

NnαβγηI 0F −−−

−= (4.8)

It is evident from the above equation that the LL intensity I is zero,

at t = 0, and t = ∞. Thus, the LL intensity should be maximum at a

particular value of time, tm. The value of tm can be determined by

differentiating I with respect to t and then equating it to zero. This gives

αexp(-αtm) = β exp (-βtm) (4.9)

135

or

( ).β/αlnα)(β

1t m −

= (4.10)

The value of intensity Im corresponding to the peak of LL intensity

versus time curve, may be given by

)βtexp(1α

β

α)(β

NnαγβηI m

0Fm −

−

−=

or

[ ] ( ){ }.β/αlogα)β/(βexpα

α)(β

α)(β

NnαγβηI 0F

m −−×

−

−= (4.11)

As the dissolution of a solute in a solvent is a slow process as

compared to the process of recombination of hydrated electrons with

holes, we may assume β>>α, and thus Eq. (4.11) may be written as

Im = ηγ nF N0α. (4.12)

The total LL intensity IT i.e. the total area below the LL intensity

versus time curve may be given by

∫∞

=0T IdtI

or

( ) [ ]dt t)exp(βαt)exp(αβ

Nn α γβ ηI

0

0FT −−×

−= ∫

∞

or

136

IT = η γ nFN0. (4.13)

The above equation shows that the value of IT will be directly

proportional to nF and N0.

4.4.1.1 Rise and decay of LL intensity

For low value of t, Eq. (4.8) may be written as

I = η β γ α nF N0 t . (4.14)

The above equation shows that when the solute will be added to the

solvent then initially the LL intensity will increase linearly with time.

Since β>>α, for higher values of t, Eq. (4.8) may be expressed as

I = η γ α nF N0 exp(-αt). (4.15)

The above equation shows that the LL intensity should decrease

exponentially with time. Thus, by plotting a curve between log I and t,

one will be able to determine the value of α from the slope of the curve.

As α increase with temperature, an increase in the values of I with

temperature is expected. However, at higher temperature the thermal

bleaching of defect-centres takes place and the LL intensity may decrease

with temperature. Thus, the LL intensity should be optimum for a

particular temperature of the solvent.

137

4.4.1.2 LL glow curve

It is found that when γ-irradiated LnCa4O(BO3)3 sample of a given

mass is dissolved in a fixed volume of dilute HCl, then the LL intensity

initially increases linearly with time, attains an optimum value, then it

decreases and finally disappears. These results are in accordance with

Eqs. (4.8), (4.14) and (4.15).

4.4.1.3 Dependence of LL on dopant concentration

It is observed that the LL intensity depends on the concentration of

the activators (Impurity) in the crystallites. Initially the LL intensity

increases with the concentration of activator, attains an optimum value

for a particular concentration and then it decreases with further increase

in concentration of the dopant. When the concentration of the dopant is

increased, initially the number of luminescent centre and the formation of

defect centre increase, thereby increasing the LL intensity. Later on when

the concentration exceeds a particular level, the concentration quenching

starts and the efficiency of radiative transition decreases. The critical

concentrations of different activators for optimum LL intensity are

different in the present investigation. It may be speculated that the factors

leading to an increase in the probability of non-radiative transitions

results as a decrease in the optimum luminescence efficiency.

4.4.1.4 Dependence of LL on γ-ray dose

In the observation of the dependence of LL intensity on the γ-

irradiation dose given to the LnCa4O(BO3)3 crystal, It is seen that

138

initially the LL intensity increases with the radiation dose and then it

attains a saturation value for higher radiation doses given to the

crystallites. This can be understood as follows:

When a LnCa4O(BO3)3 crystal is exposed to high energy radiation

like γ-rays or X-rays, excitation of electrons of borate atoms from

valence band to conduction band takes place. Some of the excited

electrons return immediately from the conduction band to the valence

band, however, some of the electrons in the conduction band get trapped

in the negative ion vacancies during their movement and consequently the

formation of defect centres takes place. Initially the number of defect

centres increases with the radiation dose given to the crystals and thereby,

the LL intensity increases with the radiation dose. However, for long

duration of the irradiation of the crystals the recombination between

electrons and holes takes place and consequently the density of defect

centres in the crystals attains a saturation value.

As a matter of the fact, the LL intensity also attains a saturation

value for high radiation dose given to the crystallites.

4.4.1.5 LL Spectroscopy

Spectroscopic studies of the LL emission indicate that rare earth

impurity doped in the borate based phosphors act as luminescence

centers. It can be assumed that energy released shown in equation (c) is

transferred non-radiatively to the rare earth ions present in host lattice

139

which excites that rare earth ions and the de-excitation of the rare earth

ions give rise the luminescence.

4.4.1.6 Dependence of LL on mass of solute

It is observed that initially the LL intensity increases with the mass

of the crystallites added in a fixed volume of dilute HCl and then it attains

a saturation value for higher mass of the crystallites added. Such results

are predicted from Eq. (4.13), where N0 is related to mass of the sample

added into the solvent. When higher mass of the crystallites are added to

the solution, a saturation will occur and only a fixed amount of the solute

will be dissolved, hence, the LL intensity will attain a saturation value

beyond a particular mass of the solute.

4.4.1.7 Dependence of LL on pH of the solvent

In the observation of the dependence of LL intensity on the pH of

the solvent it is seen that the LL intensity is optimum for a particular pH

value of the solvent. The effect of pH on the intensity of

lyoluminescence of irradiated LnCa4O(BO3)3 has been studied by varying

the pH of the medium in which the sample were dissolved by adding to it

a calculated amount of HCl . Results show that the peak height increases

rapidly from a nearly zero value at a pH of 1 to a maximum value of 4

and then it decreases rapidly to nearly zero again at a pH of about 7. It

has been shown that the maximum intensity in lyoluminescence is

observed over the pH range in which the stability of the hydrated

electrons is optimum.

140

4.4.1.8 Dependence of LL on temperature

As the probability of radiative recombination increases with

increasing temperature, the LL intensity should increase with increasing

temperature. When the temperature of the solvent was increased LL

intensity increased with temperature attained an optimum value at 75°C

then decreased with further increase in temperature. The decrement of LL

at higher temperature is because of thermal bleaching (eq.4.15) .

4.4.1.9 Dependence of LL intensity on volume of the solvent and

particle size of the sample

It is observed that the LL intensity initially increases with increase

in the volume of the solvent, attains an optimum value for a particular

volume and then decreases with further increase in the volume. This can

be interpreted as the rate of dissolution increases with the increase in the

volume of the solvent and consequently LL intensity increases initially.

For a higher volume of the solvent in which a particular mass of the

solute is dissolved, the average distance between the hydrated electron

and holes on the surface of solute will increase. It seem that this quenches

the occurrence of radiative recombination between the hydrated electrons

and holes near the surface of the solute, and therefore resulting in a

decrease in LL intensity with increasing volume of the solvent.

It is found that LL intensity initially increases with increasing

particle size of the sample. This may be due to greater stability of defect

centres in crystallites of large dimensions whereas in the case of smaller

dimensions there is a rapid decay of defect centre resulting in poor

radiolysis products. Above a certain particle size, the LL intensity

141

decreases with increase in particle size. This is in accordance with

equation (4.12) which clearly indicates that Im should decrease firstly due

to the formation of less density of defect centres in larger grain, and

secondly due to decrease in the rate of dissolution with increasing grain

size of crystallites. It is also observed that the time tm corresponding to

the peak of the LL intensity increases with increasing particle size. This

may be due to longer time duration needed for dissolving the samples of

larger particle size. Since the dissolution rate decreases with increasing

particle size, Equation (4.10) indicates that tm should increase linearly

with particle size of the sample.

Conclusively, it may be said that there is a good qualitative

correlation between the experimental and theoretical results.

******

142

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