Post on 16-Jun-2018
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.
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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.
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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)
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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)
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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)
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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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