312 E.C. Silva et al. / Radiation Measurements 42 (2007) 311 – 315
Table 1
Integrated thermoluminescent outputs from
K
2
YF
5
:Dy
3+
and K
2
YF
5
:Tb
3+
crystals after gamma irradiation with a 1 mGy dose
Tb
3+
concentration Dy
3+
concentration TL output from TL output from
in K
2
Y
1−x
Tb
x
F
5
(at%) in K
2
Y
1−x
Dy
x
F
5
(at%) K
2
Y
1−x
Tb
x
F
5
(a.u.) K
2
Y
1−x
Dy
x
F
5
(a.u.)
0.0 0.0 1.9 1.9
0.2 0.2 23.7 44.5
– 1.0 – 207.9
– 2.0 – 121.8
– 5.0 – 97.8
10 10 723.4 37.1
20 – 174.7 –
50 – 57.4 –
100 100 23.4 23.4
Within the present work, the main dosimetric and TL char-
acteristics of K
2
YF
5
doped with different Dy
3+
concentrations
have been studied and the effect of the Dy
3+
concentration on
the properties of K
2
YF
5
has been investigated.
2. Experimental
K
2
YF
5
doped with 0.2, 1.0, 2.0, 5.0 and 10.0 at% Dy
3+
as
well as K
2
DyF
5
and undoped K
2
YF
5
were synthesized with
hydrothermal technique. Crystals of these fluoride compounds
up to 1 cm
3
in size were grown by a direct temperature-gradient
method as a result of the reaction of potassium fluoride aque-
ous solutions with appropriate mixtures of 99.99% pure RE ox-
ides under hydrothermal conditions. Polished crystal platelets
with thickness of about 1 mm were utilized for the TL measure-
ments. In addition, unmounted commercial LiF:Mg,Ti (TLD-
100) chips manufactured by the Harshaw–Bicron Chemical
Company and CaSO
4
:Mn chips manufactured by Instituto de
Pesquisas Energeticas e Nucleares (IPEN–CNEN) were used
in order to check the delivered doses used to obtain the relative
TL sensitivities of synthesized K
2
YF
5
crystals.
The samples were exposed at room temperature (RT) to pho-
ton fields with gamma ray energies of 662 and 1250 keV from
137
Cs and
60
Co gamma sources, respectively, and with effec-
tive X-ray energies of 33.3, 41.1 and 52.5 keV, of which the
last two were the W60 and W80 spectra as defined by ISO
4037-1 series. Some samples were presensitized with delivered
3 kGy dose from a
60
Co gamma source. The measurements of
TL glow curves were performed with a Harshaw–Bicron 4500
TLD reader operating with a linear temperature profile over a
range from 50 to 300
◦
C in the resistive mode by using a heating
rate of 10
◦
C/s and reading cycles of 35 s. Samples were an-
nealed during secondary readings and the residual signal (read-
ing 2/reading 1) was 0.01%. The samples were weighted and
all data were normalized to the mass.
3. Results and discussion
First, the dependence of the TL efficiency on the Dy
3+
con-
centration in K
2
Y
1−x
Dy
x
F
5
crystals following
60
Co gamma
radiation to a 1 mGy dose has been investigated. The maximal
TL outputs integrated within 50–300
◦
C temperature range are
observed for K
2
YF
5
containing 1.0–5.0 at% of Dy
3+
. For the
Dy
3+
concentrations below and above this range, the TL out-
put is nearly similar to that of undoped K
2
YF
5
as it can be seen
from the data presented in Table 1. The data concerning the TL
sensitivity of K
2
YF
5
doped with Tb
3+
are also listed in Table 1
and by taking into account that these data have been obtained
under the same conditions as in the case of K
2
YF
5
doped with
Dy
3+
(Faria et al., 2004), they can be compared. In particular,
the K
2
Y
0.99
Dy
0.01
F
5
composition exhibits the highest TL sen-
sitivity to gamma rays among other K
2
YF
5
crystals doped with
Dy
3+
, whereas the maximal TL sensitivity has been found for
K
2
Y
0.9
Tb
0.1
F
5
in the terbium series. It should be also noted
that the TL sensitivity of K
2
Y
0.9
Tb
0.1
F
5
is higher by a factor
of 3 than that of K
2
Y
0.99
Dy
0.01
F
5
. In this context, it is assumed
that trivalent RE ions incorporated into a host can be traps for
electrons or holes created during irradiation depending on type
of RE ions (Sidorenko et al., 2006; Kui et al., 2006) and thus
one can expect that the higher the concentration of RE ions in
a host, the higher the TL sensitivity of this composition. Obvi-
ously, the optimal concentration of RE ions is caused by con-
centration and temperature quenching of luminescence for an
appropriate RE ion in a host.
By taking into account that K
2
Y
0.99
Dy
0.01
F
5
has the max-
imal TL sensitivity, the shape and temperature positions of
the TL glow peaks for this composition have been studied
more comprehensively than those for other samples synthesized
within this research. The TL glow curve from K
2
Y
0.99
Dy
0.01
F
5
following gamma irradiation to a 7 mGy dose without any pre-
sensitization is presented in Fig. 1(a), whereas Fig. 1(b) shows
the glow curve from the same composition presensitized to a
3 kGy dose and irradiated after that at the same conditions. As
one can see, the TL glow curve shifts maximum from 104 to
131
◦
C after the sensitization process with the appearance of a
lower temperature peak around 95
◦
C although the relative peak
heights almost do not change depending on pretreatment. The
same effect is also observed for the K
2
Y
0.99
Dy
0.01
F
5
samples
irradiated with gamma doses ranging from 0.01 to 7.0 mGy. On
the other hand, sometimes during repeated irradiation-reading
processes, some sensitized samples alternatively show the same
TL glow curve structure as before the sensitization.
In order to elucidate this bivariance of TL glow curve
structure, the curve fittings for unsensitized and sensitized