Investigation of Thin ThermoluminescentFilms of mixed CaSO4 and P(VDF-TrFE)

Copolymers and its Blends with PMMA for

Digital Radiography.

Maria R. M. Castro, Marcos C. Andrade and Luiz. 0. Faria

Centro de Desenvolvimento da Tecnologia Nuclear, R. Mario Wemeck s/n,, C. P. 941, 30123-970 Belo Horizonte- MG

Abstract: The thermoluminescence of [CaSO4:Mn]have been explored to produce radiographic imageswhen this material is mixed with pure ferroelectricpoly(vinylidene fluoride - trifluorethylene) [P(VDF-TrFE)] copolymer blended with poly(methylmethacrylate) [PMMA]. These polymeric materialshave been utilized mainly because of their goodoptical transparence, good resistance to ionizingradiation and heat diffusion. Thinthermoluminescent films (200 Sam) were prepared bycasting and irradiated with gamma and X rays in thepresence of human phantoms and metallicstandards. The radiographic images were generatedin a special device designed to heat the sample toCaSO4:Mn main TL peak temperature (1360C). Theemitted light was collected by a charged coupleddevice (CCD) camera. The good quality of theobtained radiographic images have demonstratedthe feasibility of using TL materials immersed inpolymeric matrix as an alternative way to obtain Xray digital images, which are commonly produced byphotolundnescence phosphors and X ray sensitiveCCD cameras. This investigation has shown thatcombining new high sensitive TL materials withCCD cameras which presents high response in theblue spectrum range could lead thermoluminescentfilms to show the same or even better performancesthan conventional digital radiographic devices.

INTRODUCTION

In spite of the great therapeutic utility and also theapplication in industrial activities, it was theapplication of X rays in the diagnostic field thatgenerated the first non-invasive image from inside ofthe human body. Since the X rays discovery in 1895, aseries of radiographic and measurement devices havebeen developed in order to explore this capacity,providing a series of powerful tools for medicalimaging professionals [1 ,2]. However, in our days, theinvestigation of new systems that could improve theimaging quality, together with the decrease in the

radiation doses delivered to workers and patients, isstill a very promising field of research.

In this context, the latest advances have beenconcerned to the digital radiography where fluorescentand phosphorescent materials, charged coupleddevices (CCD) and CT scanning have been currentlyexplored. Among the phosphorescent materials, mostof the progress has been done in the research ofoptically stimulated luminescence (OSL) baseddevices [3] where digital radiographic images must begenerated point by point because of spatial limitationsin the exciting laser. The main advantage of usingradiation storage phosphors relies on the fact that theoriginal X ray photons does not contribute to theimage on the moment of its generation as it does in Xray sensitive CCD cameras or fluoroscopic screens.These undesirable photons normally produce noise inthe radiographic image.

Taking into account that, in principle, the qualityof the instantaneous image emitted by fluorescent andphosphorescent screens should be similar, we haveinvestigated the possibility of collecting imagesemitted by a storage phosphor using a CCD device. Inthis work we report the results of a preliminaryinvestigation of another class of phosphorescentmaterials, i.e. the thermoluminescent (TL) phosphors,where the image can be generated just heating the TLscreen, without spatial limitations. TL materials havebeen widely used for radiation dosimetry since 1953[4,5] and their use in digital radiography was firstlyaddressed by Zangaro at al. [6]. After studying theproperties of several polymers, they used a TL filmmade of CaSO4 (TL) dispersed into PVDFhomopolymer to generate a point by point digitalimage using a fast laser heating [7], coupled to aphotomultiplier tube device. The generated imagespresented a very poor resolution. Now, making use ofcharged coupled devices and new polvmeric blends,we propose a device where a CCD camera collectsthe whole instantaneous image emitted by a TL filmmade of CaSO4 incorporated into a host P(VDF-TrFE)

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/ PMMA polymer blend. In this work, these blendshave been specially developed taking into accountheat diffusion, melting temperatures and opticaltransparence to visible light.

EXPERIMENTAL

The thermoluminescent films (about 150 [tm thick)used in this study is a compound of CaSO4:Mnobtained from IPEN, Sao Paulo, Brazil, incorporatedinto a polymeric blend by casting. The blends weremade of ferroelectric poly(fluoride trifluorethylene)[PVDF-TrFE] copolymer with 50 % wt. of TrFE,supplied by ATOCHEM, mixed with poly(methylmethacrylate) [PMMAJ from a commercial type(PEXIGLASSTM). The heating device was speciallyconstructed to simultaneously provide a low heatingrate (10 °C/s) to the total area of the TL film samples.Its TL glow curves were obtained in a HARSWAW4500 TLD reader. Thermal analysis was performed bydifferential scanning calorimetry (DSC) in a MettlerTAIO-DSC30 at 20 °C/min. The UV-VISmeasurements were taken in a Hitachi U-350 1Spectrophotometer. Samples were irradiated in aindustrial X ray device SEIFERT Pantak 320 and theimages were collected and digitized by a commercialstandard Sony DSC-V I CCD camera.

RESULTS AND DISCUSSION

One of the first parameters that has to be taken intoaccount in a TL film is concerned to the opticaltransmittance. It should be noted that this parameter isvery important because the TL visible light emittedfrom a granule localized inside of the film must reachthe CCD camera without any absorption. Zangaro atal. used PVDF as the host polymer, which has a goodoptical transmission coefficient (T = 0.9) at 50Onm [6].which is the peak of the CaSO4:Mn light emissionspectrum. However, PVDF presents a high infraredabsorption band at 10.6 gm which contributes to aundesirable low heat diffusion through the polymer. Itis expected that the heat supplied to the TL filmreaches the TL granules. also without any absorption.In order to improve this heat diffusion, instead ofPVDF we have used the P(VDF-TrFE) copolymer,which has CF2-CHF monomers randomly added to the(CH,-CF-,), chains of PVDF and a better heatdiffusion. Also, this material has proved to preserve itsmechanical properties when exposed to radiationgamma doses of 0.5 MGy [81. On the other hand, theoptical transmission coefficient of P(VDF-TrFE,)copolymers at 500nm is lower (T = 0.65) and then it isnecessary to blend it with another highly transparent

polymer in order to improve its optical transparence.Thus, we investigated the blending of P(VDF-TrFE)with amorphous PMMA, in view of its known hightransparence in the visible spectrum range.

In order to check the influence of blendingtransparent PMMA with semi-crystalline P(VDF-TrFE) [9], we show in Figure 1 the variation of theoptical transmittance efficiency for blends with 0, 5,10, 15, 25, 30 and 35 % wt. of PMMA. It is seen thatthe optical transmittance increases from 0.65 to 0.90 asthe content of PMMA in the blend increases from 0 to15 % wt., respectively. Above this content the opticaltransmittance is stabilized. Thus, taking into accountthat is preferable to have as high P(VDF-TrFE)content as possible in view of its thermal properties,we conclude that the 15 % wt. of PMMA is anoptimum content concerning to optical transparence.

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Figure 1. Optical transmittance at 500 nm forP(VDF-TrFE)/PMMA blends with 0, 5,10,15,25, 30and 35 % wt. of PMMA.

CaSO :Mn

15Time (sec)

Figure 2 - Thermoluminescent glow curve ofCaSO4:Mn incorporated into P(VDF-TrFE)/PMMAblend with 15 % wt. of PMMA. The temperatureprofile is on the right hand axis.

In this context, Figure 2 shows the TL output as afunction of time for samples of CaSO4:Mn

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incorporated into P(VDF-TrFE)/PMMA blends with15 % wt. of PMMA. The choice of CaSO4:Mn as thethermoluminescent material which converts Xradiation into visible light was mainly due to its stableand low temperature emission TL peak around 100 °C[10]. This choice allows the utilization of most of thepolymeric materials as TL host matrices because oftheir low degradation temperatures. It should be notedthat most of the TL materials have the main dosimetricpeak at temperatures above 200 °C [5]. In Fig. 2 wesee that after mixing the TL material with the polymerhost, the TL peak is shifted to 136 °C.

P(VDF-TrFE) is a semi-crystalline copolymer and,for 50 % wt. of TrFE contents, its crystallites melt attemperatures around 158 °C [9]. PMMA is anamorphous polymer and has no melting point. Mixingthese two materials in a 85/15 % wt. proportion willresult in a polymeric blend with a lower meltingtemperature (around 147 °C), as it can be seen in Fig.3,which shows the DSC thermograms for the purecopolymer and its blends with 15 % wt. of PMMA.

/PMMVA

lemperatumre ( C)

20 40 ;O Bo 100 120 140 160 180

Figure 3 - DSC thermograms for P(VDF-TrFE)/PMMA blends with 0 and 15 % wt. of PMMA.

The DSC peaks at 65 °C are related to the ferroelectricto paraelectric phase transitions in the crystalline partof the P(VDF-TrFE) copolymer and they will notaffect the heat diffusion at the temperature of the TLpeak emission. Note that the melting phase transitionshave a diffuse character and for the sample with 15 %wt. ofPMMA it starts between 130 and 140 °C.

Thus, combining the composition of the blendconstituents (15 % wt. of PMMA) that gives the betteroptical transmission efficiency, the temperature of themain TL peak when CaSO4:Mn is mixed (1360C) andthe diffuse character of the melting transitions of theP(VDF-TrFE)/PMMA blends, it is seen that theheating system, that will supply heat to the TL films,should have a maximum temperature limited to 140Oc.

The P(VDF-TrFE)/PMMA/CaSO4;Mn films wereirradiated with 0.5 Gy of X rays using a lead platebetween the beam and the film. The plate contains holeswith diameters ranging from 1 to 5 mm. After

irradiation the TL films were heated at a constantheating rate until 140 °C . The emitted light was thenphotographed with a CCD camera and the collecteddigital image was transferred to a computer (PC) whereit can be digitally refined by means of digital filters.

The correspondent thermoluminescentradiographic images can be seen in Figure 4, before theapplication of digital filters. On the right hand image itis possible to see all the pattern holes and also the lightemitted from the borders of the circular TL film.U_Fig. 4. Thermoluminescent digital radiographicimages from a 2 mm thick lead plate with holeswith diameters ranging from 1 to 5 mm. The TLfilm is composed by CaSO4:Mn incorporated intoa P(VDF-TrFE)/PMMA blend.

Two other images are shown in Figure 5 where wesee the TL digital radiographic images of a mandiblephantom and a radiographic pattern covered by asteel plate of 5mm.

Figure 5. Thermoluminescent digital radiographicimages of a mandible phantom (left) and aradiographic pattern with holes of 3, 1 and 4 mmcovered by a 5mm steel plate (right).

The above images deserve some comments. Thethermoluminescent digital radiographic imagesshowed in Figures 4 and 5 have a much better qualitythan the TL images generated by a laser scanningdevice [6]. They clearly demonstrate that a veryimportant technological step has been reached in thefield of TL radiographic images. However, theseimages also show that their quality needs to beimproved in order to reach the same level of thosegenerated by other devices such as X ray sensitiveCCD, OSL storage phosphors or even the conventionalradiographic emulsions. The same is true concerningto radiation doses delivered to the TL film to obtainthe images, which should be decreased by a factor of

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30. In this context, it should be noted that the TLimages were collected by a standard CCD camera witha very short collection efficiency around 500 nm. Infact, the images emitted from the TL films presentbetter resolution and quality when seen by humannaked eyes, which have higher sensitivity at thiswavelength. Note that, for instances, ultra highsensitive back-thinned CCD cameras have, in ourdays, about eight times more efficiency than thestandard ones. This fact lead us to think that in a nearfuture, both the dose level and the image quality willbe at the same level as those of conventional devices.Alternatively, these parameters could be alsoimproved using new TL materials with highersensitivity to X radiation than the CaSO4:Mn and withthe application of special digital filters to the collectedimages.

[7] J. Gasiot, P. Braunlich and J. P. Fillard, Laser Heating inThermoluminescence Dosimetry, Journal of Applyed Physics 53(7), 5209, 1982.

[8] C. Welter, L. 0. Faria and R. L. Moreira, Relaxor FerroelectricBehavior of y-irradiated poly(vinylidene fluoride -

Trifluorethylene) copolymers, Physical Review B 67, 144103(2003).

[9] A J. Lovinger, Developments in Crystalline Polymers, BassetDC editor, vol. 1, chapter 5, London: Applied Science, 1982.

[10] I. Motoji. and I. Noriaki, Properties of CaSO4Mn PowderforThermoluminescence Dosimeter, Journal of Nuclear Scienceand Technology 6 (3), 132 (1969).

CONCLUSION

In conclusion, instantaneous digital radiographicimages were successfully obtained from a heatedpolymeric thermoluminescent screen made ofCaSO4:Mn incorporated into a ferroelectric P(VDF-TrFE)/PMMA blend, establishing a new technologicalstep for TL radiographic imaging. UV-VISmeasurements have shown that 15 % wt, of PMMA inthe blend is the optimum content concerning to opticaltransparence. This investigation has shown that thecombination between new high sensitive TL materialswith more efficient CCD cameras, which presentsgood response in the blue spectrum range, could leadthe thermoluminescent films to showing the same oreven better performances than conventional digitalradiographic devices.

REFERENCES

[1] T. Fauber, Radiographic Imaging & Exposure, Elsevier, 2ndEdition, 2004.

[2] G. F. Knoll, Radiation Detection and Measurement,, John Wiley& Sons, New York, 1979.

[3] M. Sonoda, M. Takano, J. Miyahara and H. Kato, Computedradiography utilizing scanning laser stimulated luminescence,Radiology 148, 833 (1983)

[4] F. Daniels, C. A. Boyd and D.F. Saunders, Thermoluminescenceas a Research Tool, Science 117, 343 (1953).

[5] S. W. S. Mckeever, M. Moscovich and P. D. Townsend,Thermoluminescent Dosimetry Materials: Properties and Uses,Asford: Nuclear Technology Publishing, 1995.

[6] R. A Zangaro, Etude et Application du Sulfate de Calcium enCartographie de Radiation lonisante, These d'Etat, MontpellierUniversity, France, 1989.

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