Transcript Evaluation of the characteristics of TLD LiF:Mg.Ti

Evaluation of the characteristics of TLD LiF:Mg.Ti-100 Powder: A Measure of
Consistency Between Multiple Batches of Powder
Paola Alvarez,Jose Francisco Aguirre, Susan Smith, David Followill
Dept of Radiation Physics, UT MD Anderson Cancer Center, Houston, TX
The RPC and RDS have used TLD LiF-100
encapsulated powder to verify the output for
60Co to 23 MV photon beams and 6 to 23 MeV
electron beams, for the past 14 years. The
capsules are 1.5 cm in length and 0.4 cm
diameter and filled with 20 to 22 mg of powder.
The reading of the dosimeter is defined as the
ratio between the signal from the reading
process and the mass of powder. The mass is
measured with highly sensitive digital balances.
The make and model of TLD readers used are
Harshaw 3500 for RPC and Rexon UL-320 for
RDS.
Commissioning of each batch of powder
encompassed determining the linearity response
vs. dose level, energy and fading characteristics
of each batch of powder to determine the
correction factors for the calculation of dose.
These characteristics are reader independent
and batch dependent. During each reading
session the system sensitivity, dose response, is
defined. This parameter works like a calibration
factor for the reader in use.
Distribution of Signal
Average signal: 0.225 V/mg +/- 0.75%
30
25
20
The system sensitivity is determined every
reading session. This parameter is the
signal/mg per unit known dose of 60Co. The
sensitivity is based on 60Co irradiations to an
accurate dose of 300 cGy. This correction is
used as a calibration factor for the TLD reading
during a reading session.
Results
More than 15 batches of TLD powder were
commissioned to determine correction factors
for the calculation of dose. The reproducibility
of the TLD has always been the first
characteristic determined for each new batch
of TLD. This test is also performed during
commissioning of a new reader. Results show
a uniformity in the TLD response with a
standard deviation of 1.2%. Example of data
for one batch is shown on Figure 1.
6 MeV
9MeV
12MeV
20MeV
6MV
10MV
15MV
18MV
Average
Correction
1.038
1.034
1.028
1.049
1.013
1.040
1.046
1.061
%Standard
deviation
0.7%
1.4%
1.1%
1.1%
0.8%
1.7%
0.9%
1.1%
15
Table 1: Energy correction factors.
10
5
0
0.220
0.221
0.222
0.223
0.224
0.225
0.226
0.227
0.228
0.229
Signal (V/mg)
Fig 1: Reproducibility study for one TLD batch
The correction for fading, defined between 1 to
180 days, is a known characteristic of the LiF
crystal. This correction factor varied by 1%
between the multiple batches. See Figure 2
Fading correction
1.04
1.03
1.02
The system sensitivity is highly dependent on
each TLD batch, specific TLD reader and reading
session conditions. The sensitivity shows
variability that depends on the specific powder
batch and changes in the reflecting characteristics
of the heating planchet. The system sensitivity,
while accounting for the changes in readers and
reading sessions, varied by as much as 20%
between batches. See Figure 4. The changes in
sensitivity from session to session follow a pattern
that can be used as an indicator of the time to
either clean or change the planchet.
Correction
1.01
1.4
1.00
0.99
0.98
The energy correction factor is defined as the
change in TLD response because of the energy
of the beam. The RPC/RDS TLD systems are
based on the use of acrylic miniphantoms
under specific irradiation geometry and 60Co is
the reference energy.
Energy
0.97
0.96
0.95
0
20
40
60
80
100
120
140
160
180
200
Days from irradiation to reading
B97
B92
Fig 2: Fading correction for two TLD batches
Relative System Sensitivity
Materials
Reproducibility of the system is defined as the
analysis of the response from reading many
capsules irradiated to the same dose under
same geometry. On the order of 50 capsule
randomly selected from the batch are used for
this test. Irradiations are done in a 60Co beam
at a dose level of 300 cGy.
The correction for fading takes into account
the loss of signal between the irradiation and
read date. TLD capsules are irradiated at
same dose in a 60Co beam. Irradiations are
done between 1 to 180 days before the read
date. All readings are done within a single
session.
The linearity correction is based on irradiations
in a 60Co beam. The dose range is between 25
and 600cGy. The normalization point is
300cGy. The correction is defined as a way to
characterize the lack of linearity based on dose
level delivered to the TLD.
Results (continued)
Results (continued)
1.2
Change Planchette
1.0
Change Batch
Change Batch
Change Batch
0.8
The linearity correction, showed a maximum
variation of ±3% between batches. See Figure 3
0.6
2/13/02
9/1/02
Linearity correction
3/20/03
10/6/03
4/23/04
11/9/04
5/28/05
12/14/05
7/2/06
Read date
1.15
Fig 4: System sensitivity for one specific reader
1.10
Conclusion
1.05
Correction
The Radiological Physics Center (RPC) and the
Radiations Dosimetry Services (RDS) have used
LiF:Mg.Ti-100 encapsulated powder for many
years to measure the dose delivered from
photon or electron beams as a part of their audit
services. The determination of dose based on
this thermoluminescent dosimeter (TLD) has
characteristics that influence the calculation of
dose through the signal emitted by the powder
when it is thermally stimulated. The
reproducibility of the signal, fading of signal after
irradiation, lack of linear response and
energy/block dependence was analyzed for each
batch of powder used going back to 1988.
The gathered data on these parameters from
batch to batch has shown a remarkable
reproducibility that has been determined though
the analysis of the commissioning data for the
past 14 years. The variations in these factors
have been studied and are presented in this
poster.
Methods
Relative Frequency (%)
Introduction
1.00
0.95
0.90
0.85
0.80
0
100
200
300
400
500
600
700
Dose (cGy)
B99
B01
B03
B04
B05
B07
B09
B11
Fig 3: Linearity correction for eight different TLD batches
The energy correction factors, as defined for
the RPC/RDS TLD audit systems varied within
±1.7% (one std dev.) between batches. The
correction factors are defined for various
electron and photon energies. Values for these
corrections are shown on Table 1.
LiF:Mg.Ti-100 powder shows a predictable
behavior in terms of the characteristics of
fading, linearity and energy/block correction.
The system sensitivity or calibration factor for
each TLD batch, although variable, follows a
pattern that allows the determination of
maintenance actions when changes are
observed.
Each batch of LiF-100 TLD powder showed
minimal
variability
in
their
powder
characteristics, except for system sensitivity.
However, an accurate calculation of dose using
TLD with a minimal uncertainty, requires a new
commissioning for each new batch.
Work supported by PHS CA010953 awarded by NCI, DHHS