IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
cuboidal in shape, quite
uniform and small (average size
about 25 micron), suitable for
manufacturing teflon discs in
as-prepared form whereas the
presently used conventional
phosphors are mostly (90%)
above 75 micron and has to be
ground for dosimetric
applications.
Long afterglow phosphors
Phosphorescence (or
afterglow) refers to the light
emission at room temperature
from a phosphor that persists
Fig.1 Comparison of TL glow curves
of CaSO 4 :Dy phosphor
after removal of the excitation
source. Presently oxide based
phosphors like aluminates and
Fig.2 Afterglow spectra of
(a) combustion (b) solid-state
synthesis
silicates are widely used as
afterglow materials. Rare earth
doped long glow aluminate
phosphors find application in
various fields due to their high
quantum efficiency and long
persistence time. This Centre
has developed one such longafterglow
phosphor,
Sr 4 Al 14 O 25 :Eu 2+ ,Dy 3+ .
This phosphor was
synthesized through both solid
state reaction and combustion
synthesis routes. In case of solid
state reaction, boric acid flux
was added to facilitate the
product synthesis at low
temperature. To improve the
afterglow (AG) characteristics
of the phosphor,
the stoichiometry was varied
by taking different
Sr/Al molar ratios.
The luminescence properties of
Eu 2+ in Sr 4 Al 14 O 25 :Eu 2+ ,Dy 3+
was also studied by substituting
Sr with other divalent cations
like Ca, Ba and Zn.
Photoluminescence (PL)
intensity of both the strontium
deficit and rich phosphors were
enhanced in non-stoichimoetric
(NS) host whereas no definite
correlation was observed
between the AG intensity and
non-stoichiometry. NS
compositions led to the
formation of different phases /
compounds of strontium
aluminates which resulted in
either blue or green shift of the
PL emission. However, the
afterglow emission was not
affected by the NS. Similarly the
divalent ion substitutions also
led to the formation of different
aluminate compounds which
could change the PL and
afterglow characteristics
significantly. Interestingly at
higher concentration of Ca in
the aluminate, the host exhibits
emission peaks at 440 and 530
nm. The blend of these lights
produces the white afterglow
emission. Silver doping in this
host enhanced the afterglow
intensity by nearly 10 times by
decreasing the trap depth and
increasing the trap density but
no significant change in the PL
or AG intensity was observed in
the presence of sodium. The
afterglow and PL emission of
the phosphor synthesized
through combustion route was
better than that
prepared through solid-state
method (Fig.2). The
greenish-blue AG from
Sr 4 Al 14 O 25 :Eu 2+ ,Dy 3+ as shown
in Fig. 3 persists for more than
7 h. The indigenous phosphor
developed by us gives an
equivalent intense emission and
its afterglow persistence time is
comparable to that of the
commercial ones.
Fig.3 Visually observed afterglow
from Sr 4 Al 14 O 25 :Eu 2+ ,Dy 3+
BASIC RESEARCH 163