method for correcting thermoluminescent signal fading - Ipen

2007 International Nuclear Atlantic Conference - INAC 2007
Santos, SP, Brazil, September 30 to October 5, 2007
ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN
ISBN: 978-85-99141-02-1
A METHOD FOR CORRECTING THERMOLUMINESCENT SIGNAL FADING OF
ENCAPSULATED LiF:Mg,Ti DETECTORS IN PTFE-Teflon®
Peterson L. Squair 1,2 , Paulo Henrique G. Rosado 1 , Paulo Márcio C. Oliveira 1,2 ,
Maria do S. Nogueira 1 , Teógenes A. da Silva 1,2
1 Centro de Desenvolvimento da Tecnologia Nuclear (CDTN / CNEN - MG)
R. Prof. Mário Werneck, snº, Cidade Universitária
Pampulha, 30123-970, Caixa Postal 941, Belo Horizonte, MG
pls@cdtn.br; phgr2005@yahoo.com.br; pmco@cdtn.br; mnogue@cdtn.br; silvata@cdtn.br.
2 Programa de Pós Graduação em Ciências e Técnicas Nucleares – PCTN
Universidade Federal de Minas Gerais – UFMG
Av. Antônio Carlos, 6627 – Campus UFMG
31270-090 Belo Horizonte/MG
ABSTRACT
Fading is the process when latent information of a detector is unintentionally lost mainly due to thermal
influence. Thermoluminescent (TL) detectors have different sensitivities as far as the fading effect.
Post-irradiation annealing of 10 minutes at 100ºC is the procedure adopted for the bare LiF:Mg,Ti TL detector
(TLD-100) aiming to eliminate low energy peaks and consequently to reduce the fading influence in the final
reading. However, encapsulated TL detectors are often used during in-field measurements. PTFE-Teflon ®
polymer is an example of encapsulation material that has a temperature resistance and it allows the
luminescence signal to pass through. Since encapsulated TL detectors cannot be submitted to annealing
treatment in an oven, another fading reduction method is needed. The TL evaluation method suggested in this
work is based on a specific glow-curve region. Two areas of the TL glow-curve were selected with the
WimRems software; they correspond to the high and low fading emission peaks, channels 070 to 110 and 110 to
190, respectively. The first peak area showed 50% of fading in about 22 hours; the other peak area showed only
5% in 60 days. For calibration purposes of the TL dosimetric system the second area is used without significant
reading loss.
1. INTRODUCTION
The thermoluminescence is the emission of light by an insulator or semiconductor that is
thermically stimulated after a prior absorption of the radiation energy given to this material.
When a thermoluminescent crystal is exposed to ionizing radiation, this radiation provides
energy to its electrons in their ground state, valence band, so that they can easily go to the
conduction band, leaving a hole in the valence band. The electron and the hole move freely
through the crystal until there are new bounds or until they are captured in meta state levels
of energy, which are usually named traps and placed in the band gap [1].
Fading is the process in which there is unintentional loss of the latent information, that is, its
response. Many are the causes of the process of fading, but the thermal is the main one. In the
thermal fading, the traps that present lower entrapment energy will fade faster than the more
energetic ones, due to their higher probability of transition. This can generate large errors in
the dose assessment [1].
Nowadays, hangers for the thermoluminescent detectors have been used. These hangers must
have some specific physical characteristics, such as little light attenuation. The hanger with a

PTFE-Teflon ® polymer, which contains thermoluminescent detectors encapsulated in its
interior, was efficacious to the use. However, after the identification of the detectors that was
carried out through codes, the use of pre-reading thermal treatments to reduce the fading
effect got prohibitive [2].
The development and implementation of a dosimetric system includes the realization of
characterization tests of the dosimeter that will be used. These tests assess the consistency of
the obtained outcomes by determining the characteristics of reference adopted. These patterns
of development are described by some standards and recommendations such as the ones from
CASMIE, abbreviation for the Brazilian Assessment Committee of Individual Monitoring
System [3], ISO 12794-1 [4] and IEC 1066 [5] that deals with extremity dosimeters.
This paper proposes a method for charge acquisition regarding a single region of the
thermoluminescent glow curve emission to the LiF:Mg,Ti (TLD-100) detector so as to reduce
the fading effect in the dosimetry.
INAC 2007, Santos, SP, Brazil.
2. MATERIALS
The equipment used for realizing the tests were: a 4500 model Thermo Electron
thermoluminescent reader; a 2210 model Thermo Electron 90 Sr/ 90 Y beta irradiator source;
dosimetric cards with two LiF:Mg,Ti detectors from Thermo Electron; a MTH 1380 model
Minipa thermohygrometer.
3. METHODOLOGY AND RESULTS
In order to reduce the fading effect without the need of a thermal reading, it was adopted a
method of charge acquisition which was related to a single region of the thermoluminescent
glow curve emission. Two regions of the TL glow curve were selected through the WimRems
software; they were related to the emission peak that presented high and low degree of fading
including two regions of interest (ROI); the channels 070 to 110 (ROI-1) and the 110 to 190
(ROI-2), respectively, from a total of 200 channels in which the other ones were discarded.
Region 1 (ROI-1) was identified as the “fading” region and region 2 (ROI-2) as the “dose”
region, due to its agreement with the format of the emission curve found after a period of two
months, as shown in figure 1. Region 2 represents the emission peaks 4 and 5 of the TLD
100, being used for the dosimetry.
So as to analyze the fading process it were used 4 TL cards, each one with two detectors,
resulting in a total of 8 measurements for each assessment. The detectors were given an
absorbed dose of 5 mGy in the 90 Sr/ 90 Y beta irradiator source. After the irradiation, a reading
of the detectors in the selected time for the analyzes of the fading effects was done, as shown
on Table 1, since the packaging temperature of the detectors was 20ºC with a maximum
variation of 2ºC and relative humidity between 40 and 65%. By using the data from Table 1 it
was possible to determine graphically the relative fading in the regions 1 and 2 (Figure 2).

INAC 2007, Santos, SP, Brazil.
Figure 1. Channels determination to dose evaluate
Table 1. Response variation in relation to time.

INAC 2007, Santos, SP, Brazil.
Figure 2. Fading results
4. CONCLUSIONS
Through this analyses it was verified that for the LiF:Mg,Ti (TLD-100) detector encapsulated
in PTFE-Teflon, the fading overestimates 7.3% for a period of three days and 4.4% for a
period of 2 months.
This reading procedure, which works specifically on some regions of the thermoluminescent
emission curve, can be used to reduce the effects that the fading causes in the final reading of
the dosimetric systems.
For further research works it is suggested a detailed analyses of the signal increase for
readings that lasts up to seven days and also the possibility of determining the irradiation
period.

INAC 2007, Santos, SP, Brazil.
REFERENCES
1. Maurício, C. L. P. Noções de Dosimetria Termoluminescente:Aplicação em Dosimetria
Individual, Apostila, Rio de Janeiro, IRD/CNEN, (1993).
2. P. L. Squair, M. S. Nogueira, P. M. C. Oliveira, “ Desenvolvimento e Fabricação de
Suporte para Detectores de Extremidades” International Nuclear Atlantic Conference -
INAC 2005, Santos, São Paulo, ISBN: 85-99141-01-5 (2005).
3. CASMIE Instituto de Radioproteção e Dosimetria / Comissão Nacional de Energia
Nuclear. Regulamentos Técnicos Referentes ao Processo de Certificação de Sistemas de
Monitoração Individual Externa, IRD/CNEN, (1995).
4. ISO International Organization for Standardization. Individual Thermoluminescence
Dosemeters for Extremities and Eyes; ISO 12794-1, Geneva, Switzerland, (2000).
5. IEC International Eletrotechnical Commission. Thermoluminescence Dosimetry Systems
for Personal and Evironmental Monitoring, IEC 1066, Geneva, Switzerland, (1991).