Fluoropolymer studies for radiation dosymetry

Abstract

The polymers Tetrafluoroethylene- hexa- fluoropropylene (FEP) and Tetrafluoroethylene- per- fluoromethoxyethelene (PFA) are normally used as anti-adherent coatings and can also be applied for several applications in research. For example, they can be used as radiation dosimeters for X-ray and gamma photons, electrons, protons and other ionazing particles. In order to determine radiation induced damage, that can compromise applications in dosimetry, FEP and PFA films were bombarded with protons of 1 MeV at fluences from 1 × 10(11) protons/cm² to 1 × 10(16) protons/cm². During the bombardment, the chemical species emission was monitored with a Residual Gas Analyzer (RGA), and results show that the CF3 radical is the specie preferentially emitted. The bombarded films were also analyzed with Optical Absorption Photospectrometry (OAP) which shows quantitative chemically specific evidence of the damage caused by the proton bombardment. Our results show that damage to polymers is detectable for all fluences used in this work, but damage that can compromise applications in dosimetry occurs only for fluences greater than 1 × 10(14) protons/cm².

Fluoropolymer studies for radiation dosymetry

Marco Aurélio Parada^I; Renato Amaral Minamisawa^I; Adelaide de Almeida^I; Claudiu Muntele^II; Robert Lee Zimmerman^II; Iulia Muntele^II; Daryush Ila^II

^IDepartamento de Física e Matemática, FFCLRP, Universidade de São Paulo,Av. Bandeirantes 3900, CEP 14040-901, Ribeirão Preto, SP, Brazil

^IICenter for Irradiation of Materials, Alabama A&M University, Normal, Alabama 35762-1447, USA

ABSTRACT

The polymers Tetrafluoroethylene- hexa- fluoropropylene (FEP) and Tetrafluoroethylene- per- fluoromethoxyethelene (PFA) are normally used as anti-adherent coatings and can also be applied for several applications in research. For example, they can be used as radiation dosimeters for X-ray and gamma photons, electrons, protons and other ionazing particles. In order to determine radiation induced damage, that can compromise applications in dosimetry, FEP and PFA films were bombarded with protons of 1 MeV at fluences from 1 × 10¹¹ protons/cm² to 1 × 10¹⁶ protons/cm². During the bombardment, the chemical species emission was monitored with a Residual Gas Analyzer (RGA), and results show that the CF₃ radical is the specie preferentially emitted. The bombarded films were also analyzed with Optical Absorption Photospectrometry (OAP) which shows quantitative chemically specific evidence of the damage caused by the proton bombardment. Our results show that damage to polymers is detectable for all fluences used in this work, but damage that can compromise applications in dosimetry occurs only for fluences greater than 1 × 10¹⁴ protons/cm².

1 Introduction

Polymers are compounds formed by macro-molecules obtained with thearrangement of small molecules (monomers). They have chemical and physical properties that are different than those of the original monomers. Usually, they do not react with acids or alkalis, and present high mechanical and electrical resistance and have several applications[1, 2] which include clothes manufacturing, food and pharmaceutical packing, and medical supplies. Fluoropolymers can be used as radiation dosimeters [3, 4, 5] and can be applied as a spacecraft surfacecoating to avoid damage due to the high level of ionizing radiation met in its trajectory [6, 7]. These polymers can support high temperatures without damage.

During bombardment by ionizing radiation, the absorbed energy can break the polymeric chains with the releasing of fluorine, carbon and oxygen in amounts proportionally to the absorbed energy [8].

In this work we present experimental evidence of damage to Tetrafluoroethylene- hexa-fluoropropylene (FEP) and Tetrafluoroethylene-per-fluoromethoxyethylene (PFA) films after 1 MeV proton bombardment. The damage was determined through Residual Gas Analysis (RGA), and Optical Photospectrometry (OAP) techniques [9]. The OAP gave spectra which were shown to be dependent on the radiation dose absorbed in the polymeric films.

2 Material and Methods

PFA is a fluorinated polymer with oxygen cross links between chains expressed by the molecular formula –[(CF₂CF₂) _nCF₂C(OR_F)F]_n–. It melts at a temperature around 304ºC, has a density of 2.15 g/cm³ and a molecular weight from 2 × 10⁵ up to 4.5 × 10⁵ dalton [10]. FEP is expressed by –[(CF₂CF₂) _nCF₂C(CF₃)F]_n–, has a density of 2.15 g/cm³, a molecular weight from 2.5 × 10⁵ up to 6 × 10⁵ dalton [10], and melts at a temperature around 265ºC.

PFA and FEP films with 20 × 20 mm² and 25 mm thick were bombarded with 1 MeV protons from the Alabama A&M University Pelletron at fluences of 1 × 10¹¹, 1 × 10¹², 1 × 10¹³, 1 × 10¹⁴, 1 × 10¹⁵ and 1 × 10¹⁶ protons/cm² (one film for each fluence). The current was kept below 1 mA, to avoid sample heating. Residual Gas Analyses (Stanford Research Systems, Model 200) were conductedin real time during the bombardment.

In order to determine the polymer damage mechanisms, virgin and bombarded samples were analyzed with Optical Absorption Photospectrometry (OAP), in the UV-VIS region. The optical absorbance measured at a wavelength of 250 nm was determined for the absorbed radiation dose for each proton fluence value.

3 Results and Discussion

Fig. 1 and Fig. 2 present the RGA measurements that determine the gas emitted during bombardment of the films. There is clear evidence that proton bombardment breaks chemical bonds in the polymeric chains with attendant modifications of the polymer. The CF₃ radical accounts for a larger part of partial pressure detected during the bombardments, but ^.CF, ^.CF₂, ^.COF and CF₂CCF₃ and other species were also emitted from both films. PFA has the CF₃ as an original radical. A possible explanation that could justify the relative abundance of CF₃ radical emission from the FEP films, which has no CF₃ radical, is that a single fast proton breaks almost simultaneously neighboring C-F and C-C bonds and the active F would recombine with other liberated products, specially with CF₂ radical to form the observed CF₃ radical.

The results measured with Optical Absorption Photospectrometry are shown in the Fig. 3 and Fig. 4. In these figures we compare the spectra obtained from films bombarded at fluences of 1 × 10¹¹, 1 × 10¹², 1 × 10¹³, 1 × 10¹⁴, 1 × 10¹⁵ and 1 × 10¹⁶ protons/cm². With increasing fluence, the spectra reveal increasing absorbance at longer wavelengths to the increased formation of carbon double bounds.

In the Fig. 5 and Fig. 6 are shown the optical absorbance versus estimated absorbed dose for the PFA films bombarded at fluences 1 × 10¹¹, 1 × 10¹², 1 × 10¹³, 1 × 10¹⁴, 1 × 10¹⁵ and 1 × 10¹⁶ protons/cm² and for FEP bombarded at fluences 1 × 10¹¹, 1 × 10¹², 1 × 10¹³, 1 × 10¹⁴ and 1 × 10¹⁵ protons/cm². The OAP spectrum for FEP could not be obtained at the 1 × 10¹⁶ protons/cm² owing to excessive damage of the bombarded film. OAP results show that the absorbance is directly proportional to dose in the range of 10³ - 10⁵ Gy, for both polymers. For 10⁶ - 10⁷ Gy for FEP and 10⁶ - 10⁸ for PFA, one can notice that the absorbance is not directly proportional to the dose, indicating that the damage mechanism is more efficient for the fluences lower than 1 × 10¹⁵ and 1 × 10¹⁶ protons/cm².

These films become fragile after bombardment at fluences greather than 10¹⁴ protons/cm² such that their use as proton dosimeters, and for many other applications, may be compromised.

4 Conclusions

Residual Gas Analysis and Optical Photospectrometry are effective to evaluate charged particle bombardment damage in thin polymer films. Optical absorption spectra can be used to determine the absorbed dose in fluoropolymer films after exposure to ionizing radiation. The optical absorbance is dose dependent in the region from 10³ up to 10⁵ Gy.

RGA measurements during proton bombardment show that ^.CF₃ is the preferentially emitted specie, probably because ^.CF₃ is a radical in the PFA polymeric chain. ^.CF₃ observed to be emitted also from bombarded FEP, which has no CF₃ radical, is produced by another mechanism.

Acknowledgments

The present work was supported by grants from CAPES/CNPq (Brazil) and AAMURI (USA).

Received on 05 October, 2003

• [1] J. Chubb, Journal of Electrostatics, 54, 233 (2002).
• [2] L. S. Pinchuk, V. A. Goldade, G. M. Sessler, A. G. Kravstov, E. A. Tsvetkova, Medical Engineering & Atoms, 24, 351 (2002).
• [3] M. A. Parada, A. De Almeida, Nuclear Instruments and Methods in Physics Research, Beam Interactions with Material and Atoms, 191, 820 (2002).
• [4] G. M. Sessler, Electrets, Spring-Verlag, New York, 1980.
• [5] G. M. Sessler, Journal of Electrostatics, 51,137 (2001).
• [6] K. K. Groh, J. R. Hall, M. P. Espe, D. R. Cato, J. K. Sutter, D. A. Scheimank, High Performance Polymers, 12, 104 (2000).
• [7] J. A. Denver, K. K. Groh, A. J. Towed, L. L. Wang, AIAA paper, 895, 1 (1998).
• [8] A. L. Evelyn, D. Ila, R. L. Zimmerman, K. Bhat, D. B. Poker and Hensley, Materials Research Symposium Proceedings, 438, 499 (1997).
• [9] C. R. Brundle, C. A. Evans Jr., S. Wilson, Encyclopedia of Materials Characterization: surfaces, interfaces, thin films, Butterworth-Heinemann, Boston, 1992.
• [10] J. Brandrup, E. H. Immergut, Polymer Handbook, third edition, John Wiley and Sons, USA, 1989.

Publication Dates

• Publication in this collection
  26 Oct 2004
• Date of issue
  Sept 2004

History

• Received
  05 Oct 2003