ON THE IMPACT OF THE PRE-IRRADIATION TLDs READOUT ON THEIR POSTIRRADIATION

International Journal of Physics

and Research (IJPR)

ISSN 2250-0030

Vol. 3, Issue 2, Jun 2013, 63-76

© TJPRC Pvt. Ltd.

ON THE IMPACT OF THE PRE-IRRADIATION TLDs READOUT ON THEIR POST-

IRRADIATION GLOW CURVES

ARAFA ABD EL-HAFEZ1, NABIL EL-FARAMAWY

2, JEEHAN FAHAD AL-SHAREEF

3 & SOAD EL-FIKI

4

1Ionizing Radiation Metrology Laboratory, National Institute for Standards (NIS), Giza, Egypt

2,4Department of Physics, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt

3Kingdom of Saudi Arabia, Riyadh

ABSTRACT

In this work the impact of reading out of the un-irradiated LiF TLDs based on the post irradiation readout was

investigated. Four types of LiF detectors were used. The first three TLDs were MTS-N, MTS-6 and MTS-7 which differ

only in the percentage abundance of Li6 and Li

7 isotopes. The present results were discussed and compared with similar

LiF (TLD-100, TLD-600 and TLD-700) fabricated by another company. MCP-N LiF dosimeters were also studied and

compared with similar detectors of different only in the doping materials. The current results indicated that all of the LiF

doped with Mg and Ti detectors were affected dramatically by the pre-irradiation readout process regardless of the

fabrication companies. Otherwise, the impact of different doping materials was contradictious. These dramatic changes

could be attributed to the occurrence of the nonradiative recombination through the irradiation process which yield high

temperature peaks in the glow curve of the investigated LiF: Mg,Ti in contrary to LiF: Mg,Cu,P detectors.

KEYWORDS: Zero Dose Readout, Pre-Irradiation Heat Treatment, Thermoluminescence, LiF:Mg,Ti, LiF:Mg,Cu,P

INTRODUCTION

There are several techniques for radiation dosimetry, among all these techniques, thermoluminescence dosimetry

(TLD) is now the most widely used technique in many fields like personal and environmental radiation exposure (Gilvin

and Perks, 2010, Sandouqa et al., 2011), geochronology and space dosimetry. In addition, TLDs are extensively used in

both of diagnostic (Shousha et. al., 2011) and therapeutic medical applications (Abd El-Hafez et. al., 1997a and b, Czopyk

et al., 2007, Sharma et al., 2011), and in other health related aspects as well.

Thermoluminescence phenomena results in forming the characteristic glow curves for the phosphor material. The

glow curves obtained for each material are different, and each glow peak is ascribed to the recombination centers and is

related to traps (Furetta, 2011). One of the major problems that afflict TLD in general is the complexity of the glow curve

obtained with many TLD materials. Hence, a glow curve analytical tool kit (GCAFIT) was used for assessing individual

behavior of each peak in terms of both intensity and position (Abd El-Hafez et al., 2011a).

Improper use of a thermoluminescent material in a dosimetric application may lead to numerous errors in

determined doses (Moscovitch and Horowitz, 2006). To assess the radiation doses precisely, the sources of dose errors

should be identified and minimized (Robertson1981). Reproducibility and overall accuracy of dose estimations with TLD

was assessed for standard procedures used in a variety of areas, such as radiation therapy, diagnostic radiology, mailing

dosimetry service for radiotherapy (ASTM, 1998; Izewska et al., 2007), and space missions (Jarvis et al., 2011). A

background signal of a TL dosimeter is due to several factors, such as tribo-and chemi-luminescence, stimulation of the TL

phosphor by UV and visible light, IR emission of the heating element and its surroundings, dark current fluctuations of the

PM tube, and residual signals of the TL phosphor from its previous irradiations. All those effects can be reduced or

64 Arafa Abd El-Hafez, Nabil El-Faramawy, Jehan Fahad Al-Shareef & Soad El-Fiki

eliminated if appropriate procedures are followed (Furetta, 2010). Typically, standard practice procedures for TL dosimetry

involve pre-irradiation background (also known as ‗‗zero dose‘‘) readings of TLDs especially at low dose levels, to be

subtracted from readings of irradiated dosimeters (ASTM, 1998; Izewska et al., 2007). However, some experimentalists do

skip this procedure at different circumstances: (1)if dosimeters are being irradiated to high doses compared to which

background values could be neglected, (2)when high level of dosimetry with minimized uncertainties is not within the

scope,(3) when dealing with large number of dosimeters, it was thought to be enough to select randomly few dosimeters

for pre-irradiation background measurements instead of reading all dosimeters for saving time, and (4) when the glow

curves are a subject of an analytical study regardless of doses delivered (Ixquiac-Cabrera, et al., 2011, Harvey, and

Kearfott, K. 2011).

The zero dose reading is very important when the dosimeters are used to measure low doses. As the measured

doses increase, the background doses and their variations become less important and can be entirely neglected at high

delivered doses. Quantitatively, a zero dose reading is a combination of two main components: a reading without a

dosimeter, Lo (dark current) and a reading of the unexposed dosimeter, Lu (Furetta, 2010). The mean value of the zero dose

reading LBG is given by

LBG= Lo +Lu (1)

In our previous work (Abd El-Hafez and Maghraby 2011, Abd El-Hafez et al., 2011b), we investigated

qualitatively: what extent pre-irradiation background measurements affect the final readout of the dosimeters after

irradiation. We selected commonly used lithium fluoride (LiF) from the numerous available thermoluminescent materials

(Horowitz, 1984; Horowitz and Yossian, 1995; Ranogajec-Komor, 2003). LiF as a TL dosimeter is known for its high

sensitivity, stability, and approximate tissue equivalency (Jung H. et al., 2003). Since the discovery of its useful TLD

properties at the University of Wisconsin (Daniel et al., 1953), LiF was extensively studied (e.g., Buenfil and Brandan,

1997; Yang et al., 2006; Fung, 2004; Kim et al., 2008; Harvey et al., 2010) and is now arguably the most extensively used

reference thermoluminescent material in a wide spectrum of applications. Selecting specifically LiF dosimeters to evaluate

the effect of pre-irradiation background measurements on the post-irradiation dosimeter readout values has made the

results interesting to a wide audience.

The readout of the un-irradiated thermoluminescence detectors (TLDs) plays important role when measuring low

doses in the range of personal and diagnostic dosimetry. Recently, Abd El-Hafez and Maghraby (2011), as a defence of the

pre-irradiation readouts (zero dose), noticed that readouts of LiF:Mg,Ti Harshow fabrication named TLD-100, TLD-600

and TLD-700 dosimeters background before irradiation (regardless of values of background) can enhance TL output and

may cause changes to the glow curves both qualitatively in terms of peaks positions and peaks relative intensities, and

quantitatively in terms of areas under the curve and the maxima of the peaks.

It may be concluded that pre-irradiation background measurements for the studied TLDs is not only important for

evaluation of residual doses when considering high level of accuracy, but its most importance (as a form of heat treatment)

in the enhancement of their sensitivities and adjustment of glow curves peaks distribution and relative ratios. Impacts on

glow curves enhancement were dramatically in cases of sensitized TLD-700 and sensitized TLD-600, and were minor in

case of CaF2:Tm TLD-300 dosimeters (Abd El-Hafez et al., 2011b). It is highly recommended to perform background

measurements before irradiation regardless how high the radiation dose is, and even the background value will be

neglected either the thermoluminescent dosimeters were sensitized or un-sensitized and or the Li isotopes are (enriched 6Li,

7Li or natural). The condition of better enhancement is that the TL material should have the critical mechanism is that of

On The Impact of the Pre-Irradiation TLDs Readout on Their Post-Irradiation Glow Curves 65

competition during the stage of readout. In contrary to CaF:Tm (TLD-300) which clearly shows that the critical mechanism

is that of competition during the stage of absorption of radiation.

Hence, it is of some importance to investigate if this enhancement in glow curves is observable in another

fabricated LiF such as MTS-6, MTS-7 and MTS-N; and to what extent the doping impurities such as Mg, Cu, P (MCP-N)

play a role, if it has the same enhancement associated with the pre-irradiation readouts.

INSTRUMENTS AND METHODS

Radiation Source and Dose Determination

The Cesium-137 gamma source model is GB-150, was manufactured by Atomic Energy of Canada Limited on

April 1970 with original activity of 1000 Ci. (3.7 x 1010

kBq) and was used for irradiation. Air kerma (Kair) was determined

according to the International Atomic Energy Agency (IAEA) code of practice TRS-(381) (IAEA, 1997). Kair

determination was performed using the secondary standard dosimetry system of the National Institute of Standards (NIS) –

Egypt, which is composed of the NPL-2560 electrometer (UK) and the NE2561 ionization chamber (UK), the secondary

standard calibration system was calibrated at the Bureau International des Poids et Mesures (BIPM).

The expanded uncertainty associated to Kair determination was about 0.9 % at 95% level of confidence (coverage

factor = 2).

TLD Reader

The used TLD reader is Harshaw/Bicron 4500 TLD Reader which is equipped with two photomultiplier tubes that

can read independently; the Reader operates on WinREMS Software, which runs under Windows on a separate computer.

Thermoluminescent Dosimeters

Four different LiF-based TLDs have been used in this study: Three types of LiF:Mg,Ti phosphors with

commercial names MTS-7 (enriched with 7Li), MTS-6 (enriched with

6Li), and MTS-N (with natural Li) were used beside

another type of LiF:Mg,Cu,P phosphors named as MCP-N manufactured by Institute of Nuclear Physics in Karkow,

Poland (IFJ). The dimensions of all the dosimeters were 3.2 x 3.2 x .9 mm3. The annealing were performed using an

electric furnace model (NEY VulcanTM

3-550) manufactured in (U.S.A).

Experimental Method

LiF dosimeters (all families) were annealed, and kept in dark. Two cases were investigated, in the first case:

background was readout, while the second case was irradiated without reading of the background. Each set is composed of

not less than four dosimeters.

All the TLDs were grouped together and exposed to a well-defined radiation dose from Cs-137 gamma source.

Dosimeters readings were acquired using linear heating rate 3°C/s over the range (100 – 400 C°). This low heating rate was

used in order to avoid possible shifts, and hence extreme overlapping in glow peaks (Abd El-Hafez et al., 1999; Yazici,

2004).

Thermoluminescent crystals are inherently re-usable. The crystals require special thermal treatments (annealing)

prior to re-use. The annealing was carried out in a separate annealing oven.

The MTS-N, MTS-6 and MTS-7 were annealed at 400 °C for one hour followed by fast cooling to 100°C and

maintained for two hour. Each dosimeter passes two cases; the first case (Case A) represents the readout after gamma

irradiation for dosimeters experienced pre-irradiation background measurements, while the second case (Case B)


represents the readout of dosimeters irradiated without experiencing pre-irradiation background measurements. After

annealing the TL detectors, the TL reader was switched on and left for about half an hour to reach the stability of the

electronic system. Moreover, the TLD readout was always after 24 hours post irradiation; in order to get rid of the short

lived TL peaks that occur at low temperatures.

RESULTS

A sensitivity correction factor is determined for each used detector by exposing all the dosimeters to the same

dose and repeating that four times. The final reading for TLDs is obtained by correcting each dosimeter‘s reading with its

sensitivity correction factor.

LiF:Mg,Ti (MTS-6)

Figure (1a) illustrates the glow curves of MTS-6 in the temperature range 100–400 oC with readout heating rate 3

oC/s and irradiated with 0.2 Gy from

137Cs gamma source, the dashed curve represents the case (A) without pre-irradiation

readout and the solid one represents the case (B) with pre-irradiation readout.

From the figure, it is so easy to notice that the change in sensitivity is not uniform over the heating range; there

are several obviously occurred changes. A remarkable enhancement of the glow curve as a whole is observed as the first

effect of pre-irradiation readout for the integral value of the glow curve and the maximum of the dosimetric peak 5 (P5).

The entire low temperature glow curve (P2:P5) is observed to be increased as a second effect of the pre-irradiation

readout. In contrary, the high temperature peaks (P7:P8) tends to be constant with a little decrease is noticed. Peak 3 (P3)

which is not apparent in the case without pre-irradiation readout is very clear and higher resolution is induced for P3 and

P5.The same behavior is noticed for MTS-6 irradiated with the other examined dose 0.4 Gy as in Figure (2a).

Figures (1b) and (2b) illustrate the ratio between the two glow curves point by point with and without pre-

irradiation readouts for the two examined doses 0.2 and 0.4 Gy respectively.

This ratio is not constant for all points, it has a value of one or less in the high temperature region from about

260:400 oC and has values more than 2 up to 6 at least at low temperatures from 100 : 260

oC. Several peaks in the ratio

curve with maxima correspond to the real positions of glow peaks which are not the apparent ones and represented by the

dashed vertical lines.

The reason of that behavior is that the temperature of the real peak stands few degrees of temperature less than the

apparent peak because of the contribution of the adjacent higher temperature one.

The lower values between the peaks of the ratio curve are represented by the dotted vertical lines. Differentiating

between the thermoluminescence intensity values of the pre-irradiation glow curve with respect to time of heating or

temperature, since the heating rate is constant and representing it as upper curve (c) in figures (1) and (2). From curves (b)

and (c), it can be predicted the existence of peaks 4 (not apparent in this case) and peak 6 (may appears only when analysis

the glow curve with computerized glow curve deconvolution).

The small peak at about 190 oC on the ratio curve (b) and on the differential curve (c) represents the position of

peak 4 of the thermoluminescent glow curve.

A small shift in the peak positions (the temperature at which the maxima of the peaks is occurred) is observed as a

result of pre-irradiation readout towards high temperatures for most peaks except for P3 shifted towards lower

temperatures.


Figure 1: (a) Illustrates the Glow Curves of MTS-6 in the Temperature Range 100–400 oC with Readout Heating

Rate 3 oC/s and Irradiated with 0.2 Gy from

137Cs Gamma Source, the Dashed Curve Represents the Case

(A) without Pre-Irradiation Readout and the Solid one Represents the Case (B) with Pre-Irradiation Readout.

(b) Illustrate the Ratio between the Two Glow Curves Point by Point with and without Pre-Irradiation Readouts.

(c) the Differential of the Thermoluminescence Intensity Values of the Pre-Irradiation Readout Glow Curve with

Respect to time of Heating or Temperature Since the Heating Rate is Constant

Figure 2: (a) Illustrates the Glow Curves of MTS-6 in the Temperature Range 100–400 O

c with Readout Heating

Rate 3 O

c/S and Irradiated with 0.4 Gy from 137

Cs Gamma Source, the Dashed Curve Represents the Case

(A) without Pre-Irradiation Readout and the Solid One Represents the Case (B) with Pre-Irradiation Readout.

(b) Illustrate the Ratio between the Two Glow Curves Point by Point with and Without Pre-Irradiation Readouts.


Respect to Time of Heating or Temperature Since the Heating Rate is Constant

LiF:Mg,Ti (MTS-7)

Figures (3a) and (4a) which represent the intensity of the glow curve of MTS-7 irradiated with the same dose 0.2

Gy and 0.4 Gy respectively without pre-irradiation (dashed curve) and with pre-irradiation (solid curve) readout. The same

behavior is recorded for MTS-7 as that for MTS-6 represented in figures (1a) and (2a). The ratio between the two

investigated case (with and without pre-irradiation readouts) for all points of the two glow peaks are plotted versus


temperature as shown in figures (3b:4b). A small shift in the peak positions is observed as a result of pre-irradiation

readout towards high temperatures for most peaks except for P3 shifted towards lower temperatures.

LiF:Mg,Ti (MTS-N)

The same behavior is recorded for MTS-N as shown in Fig. (5a) which represent the intensity of the glow curve of

MTS-N irradiated with the same dose 0.2 Gy without pre-irradiation (dashed curve) and with pre-irradiation (solid curve)

readout. The same behavior is noticed for MTS-N irradiated with the other examined dose 0.4 Gy as in Fig. (6a). Peaks 3

and 4 (P3 and P4) which are not apparent in the case without pre-irradiation readout are obvious and higher resolution is

induced for P3 and P5. Peak 4 appear as a hump in the case A (with pre-irradiation readout). The same behavior is noticed

for MTS-N irradiated with the other examined dose 0.4 Gy as in Fig. (6a). Noticeable decrease in peaks P7 and P8 of MTS-

N higher than that of MTS-6 and MTS-7 with corresponding smaller increase of peaks P2:P5 (low temperature peaks) than

that of MTS-6 and MTS-7. The ratio between the two investigated case (with and without pre-irradiation readouts) for all

points of the two glow peaks are plotted versus temperature as shown in figures (5b and 6b). The same small shift in the

peak positions is observed as a result of pre-irradiation readout towards high temperatures for most peaks except for P3

shifted towards lower temperatures.

LiF:Mg,Ti (MCP-N)

Representing the TL intensity glow curves of LiF:M,Cu,P (MCP-N) on the linear scale, it yields one peak only in

the two cases A and B (with and without pre-irradiation readouts). This single peak did not interpret the ratio curve (b) nor

the differential curve (c) as the discussed cases of LiF:Mg,Ti (MTS-6, MTS-7 and MTS-N). So, representing it on the log

scale as in Figures 7a and 8a yields two peaks at low temperature region which are responsible for observed high ratio in

that region on the ratio curve (b) beside the main peak at about 235oC. In contrary to the LiF:Mg,Ti dosimeters, little

effects of the pre-irradiation readout are noticed in the integral value of the glow curve (area under the glow curve) and in

the height of the main dosimetric peak.



Rate 3 O










Rate 3 O





(c) The Differential of the Thermoluminescence Intensity Values of the Pre-Irradiation Readout Glow Curve with


Figure 5: (a) Illustrates the Glow Curves of MTS-N in the Temperature Range 100–400 oC with Readout Heating








Figure 6: (a) Illustrates the Glow Curves of MTS-N in the Temperature Range 100–400 oC with Readout Heating






Respect To Time of Heating or Temperature Since the Heating Rate is Constant

DISCUSSIONS

For quantitative investigation of the impact of pre-irradiation readout on the post-irradiation readout, the values of

the ratio (R) which is defined as the intensity of the thermoluminescence signals with pre-irradiation readout to that

without pre-irradiation treatment. Table (1) represents this impact as the ratio between the total areas in the range 100-

400oC in both cases which increase with R between 1.83 and 2.69 for all LiF:Mg,Ti dosimeters and higher values of R for

the low temperature area ranged from 2.5 up to 4.13 with a lowest value for MTS-N.

The impact on the high temperature area is noticed as a small decrease with average value equals 0.785 for MTS-

6 and MTS-7. This decrease is enlarged for MTS-N to a value of 0.60. There is a little impact of 1.12 for MCP-N which

has different impurities than MTS dosimeters. As TLD models can be classified as models in which the critical processes

occur during the absorption of radiation, and models in which the critical processes occur during heating (i.e. during TL

readout).

The evidence, at least for LiF:Mg,Ti materials, whether the Lithium is 6Li or

7Li or natural Li, clearly shows that

the critical mechanism is that of competition during the stage of TL readout. For LiF:Mg,Cu,P (MCP-N), during heating,

electrons released from traps, may either recombine with trapped holes to produce TL or be retrapped in deeper traps,

which act as competing centers. The competition–during heating process was first suggested by Rodine and Land (1971).

This explains the different behavior between LiF:Mg,Ti and LiF:Mg,Cu,P.


Figure 7: (a) Illustrates the Glow Curves of MCP-N in the Temperature Range 100–400 oC with Readout Heating





(c) The Differential of the Thermolumine-Scence Intensity Values of the Pre-Irradiation Readout Glow Curve with

Respect To Time of Heating or Temperature Since the Heating Rate is Constant

Figure 8: (a) Illustrates the Glow Curves of MCP-N in the Temperature Range 100–400 oC with Readout Heating


137Cs Gamma Source, the Dashed Curve Represents the Case (A)

without Pre-Irradiation Readout and the Solid One Represents the Case (B) with Pre-Irradiation Readout. (b)

Illustrate the Ratio between the Two Glow Curves Point by Point with and without Pre-Irradiation Readouts. (c)

The Differential of the Thermoluminescence Intensity Values of the Pre-Irradiation Readout Glow Curve with


Table (1), also represents the ratio for the maxima of the apparent peaks for all the investigated dosimeters. The

lowest increase for the impact is observed for P2, the highest for P3 followed by P5 and the decrease for LiF:Mg,Ti is

observed for P7 and P8. It is well known that, especially, pre-irradiation heat treatments between 100 and 200 ºC cause a


series dramatic on the TL properties of TLD-100 as well as on the trapping parameters of the individual TL glow peaks

(Bos et al., 1992: Piter et al, 1996: Yazici, 1998).

Kitis and Otto (2000) for TLD-700 glow curves, observed a glow peak between P2 and P3 with a Tm very close to

the Tm of P2, and they observed a glow peak between P2 and P3 but with a Tm very close to the Tm of P3. It is first observed

by Johnson (Johnson, 1974) and labeled as P3a. A few other authors have reported this observation (Fairchild et al., 1978:

Perks and Marshall, 1991). It is not clear however, what experimentally conditions are necessary for the creation of this

peak (Horowitz and Yossian, 1995). It was reported (Yuan and McKeever, 1988 and Piters et al., 1996) that P2 and P3 have

been associated with with magnesium cation vacancy dipoles MgV and P4 and P5 with trimmers or higher order cluster of

(MgV)3. The basic processes in which MgV dipoles and clusters of dipoles (dimers, trimmers and so on) are involved can

be schematically presented by:

MgV (MgV)2 (MgV)3 precipitates (2)

Where, ‗precipitates‘ stand for higher order clusters. A high temperature has the effect of pushing the defect

equilibrium to the left and at a low temperature the defect equilibrium is forced to the right of equation (2).

Table 1: The Ratio between the Thermoluminescence Glow Curves (Peaks) with Pre-Irradiation

Readout to without Pre-Irradiation Readout for MTS-6, MTS-7, MTS-N and MCP-N

Irradiated with 0.2 and 0.4 Gy from 137

Cs γ-Source

Ratio (Case A/Case B)

MTS-6 MTS-7 MTS-N MCP-N MTS-6 MTS-7 MTS-N MCP-N

0.2 Gy 0.4 Gy

Total Area (100:400) 2.38 2.43 1.83 1.12 2.69 2.77 1.83 1.12

Area P2:P5 (100-250) 3.46 3.42 2.5 3.69 4.13 2.55

Area P6:P8 (250-400) 0.76 0.77 0.6 0.81 0.79 0.61

Max. of P2 3.1 2.7 2.3 3.60 3.4 3.2 2.3 3.05

Max. of P3 5.89 5.1 4.8 5.8 5.3 4.06

Max. of P4 2.8 3.02

Max. of P5 3.6 3.6 2.1 1.11 3.6 3.2 2.18 1.11

Max. of P7 0.78 .79 0.8 0.78 0. 9 0.59

Max. of P8 0.85 0.8 0.5 1.0 1.0 0.51

It was observed that the high temperature side in the case without applying the pre-irradiation read out process is

about 1.5 or 2 times greater than that in the case of applying it. The contribution of the low temperature side in the case of

applying the pre-irradiation read out process is greater than that in the case without applying it. Of course, one can ask;

when shall we apply the pre-irradiation read out process? In fact, this is depending on the natural type and the aim of the

experiment or the study. Many studies (Buenfil and Brandan, 1997: Berger and Hajek, 2008: Pardhan et al., 2009: Datz et

al., 2010) are based on using or studying the high temperature glow peaks.

Therefore, in such studies, it is not recommended to apply the pre-irradiation readout process especially with this

described heat treatment. We observe that the percentage contribution of P5 has increased by about three times in the case

of applying the pre-irradiation read out process. Therefore, in the studies which are based on using or studying P5 [e.g.

Chen and Hag-Yahya, 1996; Weizman et al, 2000; Weiss et al., 2008; Yazici, 2004) it is recommended to apply the pre-

irradiation read out process.

If we suggest that P4 is due to electron traps and P5 is due to hole traps, we can use Lawless et al. model (Lawless

et al., 2009) to explain the complexity structure of the glow P4 and P5. Their mode has one electron trap, one hole trap, and

one recombination center from which radiation is emitted when an electron recombines at the center. Under some


circumstances, this model shows two peaks. The first peak is due to thermal stimulation of electrons from the electron trap.

These recombine with holes in the center until those holes are depleted.

At this point, the TL emission due to electron recombination is stopped, and due to the lack of holes in center,

electrons emitted from the electron trap are re-trapped. This continues until free holes start to be created from thermal

stimulation of the hole trap. This replenishes holes in the center and allows electron recombination to continue. This results

in a second TL peak, in which the second peak has the initial rise and peak shape associated with the hole trap, not the

electron trap. If the time between the electron re-trapping in the hole centers and their recombination at the luminescence

center is too short, this will result in a complex overlapping of P4 with P5.

The shift in the peak quality, i.e., peak position towards higher temperature as an impact of pre-irradiation readout

was held responsible for the increase in the contribution of non-radiative transitions due to the presence of the competing

traps as previously concluded in our previous work (Abd El-Hafez et al., 2011b). It was inferred that the glow peaks

occurring at higher temperatures must exhibits higher thermal quenching. This is attributed to the energy dissipated in an

indirect transition, however, which is much less than the band-gap energy and may thus be dissipated either radiatively (via

photons) and non-radiatively (via phonons). Non-radiative capture of free carriers takes place because the lattice vibrations

cause the energy level to change its position in the forbidden gap.

For large enough vibrations, the energy level crosses into the conduction band and captures a free electron. The

lattice relaxation which follows the capture lowers the position of the level back into the energy gap, the excess energy

being propagated as lattice phonons (McKeever, 1985). However, the glow curve is controlled by the release of the charge

carriers from traps and not by the properties of the luminescence centers (Horowitz, 1984).

CONCLUSIONS

The pre-irradiation read out process causes dramatic effects on the behavior and shape, of the glow peaks in the

glow curves of the LiF:Mg,Ti (MTS-6, MTS-7 and MTS-N ) as well as (TLD-100, TLD-600, and TLD-700) dosimeters

manufactured by different companies with the same impurities.

The pre-irradiation readout process causes little impact only on the height of the glow peaks or area for the

LiF:Mg,Cu,P (MCP-N) as well as CaF2:Tm (TLD-300) which have no high temperature peaks and the critical mechanism

is that of competition-during the stage of absorption of radiation.Pre-irradiation readout causes an increasing of the main

dosimetric peak P5 in the glow curve by about three times double, but at the same time it causes a significant decreasing in

the area contribution of the high temperature glow peaks (P7, and P8) by about 0.6:0.8. Thus, according to the type and the

aim of the experiment, one can decide whatever to apply the pre-irradiation process or not.

In the studies which are based on using or studying the high temperature glow peaks, it is not recommended to

apply the pre-irradiation readout process on the dosimeters. While, in the studies which are based on using or studying the

main dosimetric glow P5, it is recommended to apply the pre-irradiation readout process on the dosimeters. Further studies

should be performed soon regarding behavior of each resolved peak as a function of radiation quality and dose level

effects.

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