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Radiation Measurements 43 (2008) 315 – 318 Characteristics of LiF:Mg,Cu,P thermoluminescence at ultra-high dose range P. Bilskia,∗, B. Obryka, P. Olkoa, E. Mandowskab, A. Mandowskib, J.L. Kimc a Institute of Nuclear Physics (IFJ), Krakow, Poland bInstitute of Physcis, Jan Dlugosz University (AJD), Czestochowa, Poland cKorean Atomic Energy Research Institute (KAERI), Dejoan, Republic of Korea The behaviour of LiF:Mg,Cu,P (MCP-N) detectors at ultra-high doses up to 500 kGy has been investigated. Some very signiﬁcant changes of the glow-curve shape have been revealed. The most important ﬁnding is the appearance of a new peak at exposures above 50 kGy, the positionof which shifts toward higher temperatures with increasing dose (from 405 to 470 ◦C at 500 kGy), in contradiction to standard TL models.
The E and s trapping parameters also increase with increasing dose. This new peak appears to have potential application for ultra-high-dosemeasurements. The TL emission spectrum also changes at such high doses with the appearance of several new bands of wavelengths up to800 nm.
2007 Elsevier Ltd. All rights reserved.
Keywords: Thermoluminescence; High dose; Lithium ﬂuoride IFJ, Krakow. The standard 240 ◦C/10 min annealing cycle wasapplied. Gamma-ray exposures were performed using the high- LiF:Mg,Cu,P thermoluminescent detectors are now becom- activity Co-60 source at KAERI and the therapeutic Co-60 ing standard in modern TL dosimetry. Their main virtue, which source in Krakow. Doses ranged between 0.5 and 500 kGy. At gained them their present popularity, is their very high sensi- each dose four to six detectors were used. Thermolumines- tivity. LiF:Mg,Cu,P detectors are able to measure doses at mi- cence was measured up to 600 ◦C using a RA'94 reader at the crogray levels or even below. Another interesting feature is their IFJ at a linear heating rate of 2 ◦C/s. The reader was equipped dose response, which does not show supralinearity up to satura- with a BG-12 ﬁlter. Glow-curves were deconvoluted into sin- tion at about 1 kGy. Very remarkable is also that the glow-curve gle ﬁrst-order peaks using GlowFit, a new program recently shape remains nearly unchanged over several decades: between developed in Krakow Spectrally micrograys and a kilogray These properties are resolved measurements were performed at AJD Czestochowa quite unlike those of LiF:Mg,Ti.
using a spectrometer connected with a liquid nitrogen-cooled Most, if not all, earlier studies of this material at high doses CCD 1024E camera. TL spectra were recorded from room tem- perature up to 350 ◦C in a vacuum.
were limited to the above-mentioned dose range.
The present paper reports on investigations of properties of 3. Results and discussion
LiF:Mg,Cu,P at doses much higher than 1 kGy, i.e. above thenominal saturation level of this material.
presents the measured glow-curves with peak heights normalized to unity over the dose range 0.5–5 kGy. It is ap- 2. Material and methods
parent that increasing the dose results in growth of the high-temperature peaks. At 1 and 2 kGy this is observed only as a All measurements were performed using LiF:Mg,Cu,P more pronounced tail of the main peak 4, but, at 3 and 5 kGy, (MCP-N) thermoluminescent detectors manufactured at the other peaks arise. The glow-curve over this dose range can be easily and consistently deconvoluted into six ﬁrst-order peaks illustrates changes of the glow-curve shape E-mail address: (P. Bilski).
1350-4487/$ - see front matter 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.radmeas.2007.10.015 P. Bilski et al. / Radiation Measurements 43 (2008) 315 – 318 Fig. 1. LiF:Mg,Cu,P glow-curves over the dose range 0.5–5 kGy (peak heights Fig. 3. LiF:Mg,Cu,P glow-curves over the dose range 5–50 kGy.
normalized to unity). The arrow indicates increasing doses.
Fig. 4. LiF:Mg,Cu,P glow-curves over the dose range 50–500 kGy.
Fig. 2. The deconvoluted LiF:Mg,Cu,P glow-curve after 5 kGy exposure.
observed at the doses ranging from 5 to 50 kGy. Over this dose peak should not depend on dose. For second-order kinetics range the previous main peak 4 becomes strongly suppressed and even in the more general framework of the simple trap and at 50 kGy it is practically absent. The high-temperature model, the peak should shift with increasing dose, but towards peaks still increase with dose and dominate the whole glow- lower temperatures The new peak may be well curve. At 50 Gy a new peak, located at temperatures above ﬁtted with a ﬁrst-order kinetic function, this deconvolution 400 ◦C, appears. The glow-curves for this dose region cannot leading to very high values of energy and of the frequency be successfully deconvoluted even with 10 peaks.
factor (E = 4.47 eV and s = 5.6 × 1028 s−1 at 500 kGy).
Some very interesting features are observed at doses ex- The values of these kinetic parameters also increase with in- ceeding 50 kGy The new peak, which appeared at ca.
creasing dose The obtained results, i.e. unphysical 400 ◦C, still grows with increasing dose but, surprisingly, its values of frequency factors, as well as the anomalous shift position (temperature) also becomes dose dependent—shifting of the high-temperature TL peak with dose, clearly indicate towards higher temperatures with dose. The difference in the that the classical ﬁrst-order kinetic theory does not apply in peak position between 500 and 50 kGy is as much as 60 ◦C. It this case. Therefore, the obtained values presumably have should be emphasized that such an effect is not predicted by no physical meaning. Thus, a question arises concerning a the standard TL models. The position of a ﬁrst-order kinetic possible theoretical account for this phenomenon. No one P. Bilski et al. / Radiation Measurements 43 (2008) 315 – 318 Fig. 5. Position (open circles, left-hand scale) and kinetic energy (full circles,right-hand scale) of the new high-temperature peak in LiF:Mg,Cu,P vs. dose.
Fig. 6. Emission spectra of LiF:Mg,Cu,P measured at doses 0.0065, 20 and500 kGy, integrated over the temperature range (60–350) ◦C.
justiﬁcation, based on standard kinetic models, is known to theauthors.
However, an explanation of this unusual behaviour of the new peak is proposed by in an accompa-nying paper, whereby traps and recombination centres may beconsidered as a system of interacting clusters. With increasingdose the size of the clusters grows, simultaneously increasingthe effective activation energy and shifting the TL peak towardshigher temperatures. The very-high-frequency factors are pro-duced by a cascade detrapping mechanism due to simultaneousoccurrence of localized and delocalized transitions. One shouldnote that the nature of these ‘defect clusters' is unknown at thismoment, and the problem should be studied in the future.
Examination of emission spectra also revealed interesting features. Within the spectrum, which for lower doses consists ofa narrow peak at about 350–380 nm, several longer-wavelengthbands gradually appear with increasing dose. At 20 kGythey are observed as a new emission peak at about 550 nmAt the highest exposure, the emission spectrum ex-tends beyond 800 nm, while the former main emission below Fig. 7. Dose response for the integrated TL signal of LiF:Mg,Cu,P within 400 nm is nearly absent. Generally, it seems that the higher ranges 250–350 and 350-600 ◦C. Solid lines indicate linear trends.
the dose, the more of the longer-wavelength bands appear.
Unfortunately, for technical reasons, emission spectra weremeasured only up to 350 ◦C, therefore the main emission at 4. Summary
the highest doses was not recorded. Nevertheless, the obtainedresults clearly indicate that at the highest doses different re- The behaviour of MCP-N (LiF:Mg,Cu,P) detectors at ultra- combination centres become activated.
high doses has been investigated. While the shape of the In order to quantify the dose response of the high-temperature glow-curve at doses ranging from a microgray to a kilogray is part of the glow-curve, the TL signal was integrated over ﬁxed practically identical, signiﬁcant changes are observed at higher temperature regions: 250–350 and 350–600 ◦C. The depen- doses. According to these changes in the shape of the glow- dence of these integrals on dose is presented in The curve, three sub-ranges of dose can be distinguished: 1–5, simple (however, not necessarily linear) dose response of the 5–50 and 50–500 kGy. Within the ﬁrst sub-range, the nominal new high-temperature peak suggests the possibility of its ap- main peak 4 at ca. 230 ◦C remains the most prominent one in plication for dosimetry of ultra-high doses. This would make the glow-curve, changes being observed only as a growth of MCP-N a unique detector capable of measuring doses from be- the high-temperature tail of this main peak. Within the second low 1 Gy to at least 0.5 MGy, i.e. over a dynamic range of 12 sub-range the glow-curve becomes very complex, consisting orders of magnitude.
of at least 10 peaks, with a reduced (already saturated) peak 4.
P. Bilski et al. / Radiation Measurements 43 (2008) 315 – 318 Also a new emission spectrum band at 550 nm appears. Within the third sub-range a completely new peak appears at very hightemperatures. Its position was found to shift towards higher Bilski, P., 2002. Lithium ﬂuoride: from LiF:Mg,Ti to LiF:Mg,Cu,P. Radiat.
temperatures with dose, in contradiction to standard TL mod- Prot. Dosim. 100, 199–206.
Bos, A.J.J., 2007. Theory of thermoluminescence. Radiat. Meas. 41, els. The kinetic energy and frequency factors of this peak also demonstrate unusual features. These observations make the Horowitz, Y.S., 1993. LiF:Mg,Ti versus LiF:Mg,Cu,P: the competition heats new peak very interesting from the theoretical point of view. A up. Radiat. Prot. Dosim. 47, 135–141.
possible explanation of this phenomenon, based on the model Mandowski, A., 2007. How to detect trap cluster systems? Radiat. Meas., of interacting clusters, was given recently by Olko, P., Bilski, P., Michalik, V.M., 1994. Microdosimetric analysis of the At doses around 500 kGy, an emission band beyond response of LiF thermoluminescent detectors for radiations of different 800 nm emerges in the emission spectrum. This new TL peak qualities. Radiat. Prot. Dosim. 52, 405–408.
in MCP-N is of considerable theoretical interest and could Puchalska, M., Bilski, P., 2006. GlowFit: a new tool for thermoluminescence potentially be exploited in ultra-high-level dosimetry, both glow-curve deconvolution. Radiat. Meas. 41, 659–664.
subjects being currently pursued.
Velbeck, K.J., Luo, L.Z., Ramlo, M.J., Rotunda, J.E., 2006. The dose–response of Harshaw TLD-700H. Radiat. Prot. Dosim. 119, 255–258.
This work was partly supported by a research project from the Polish Ministry of Science over the years 2006–2008(no. N20703931/1541).
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