Teflon electret radiation dosimeter

M.A. Parada, A. de Almeida *

Departamento de F�ıısica e Matem�aatica, FFCLRP-USP, Av. Bandeirantes 3900, CEP 14040-901 Ribeir~aao Preto, Brazil

Abstract

Electrets are insulating materials with a quasi-permanent electric polarization. Since charge compensation occurs

when subjected to ionizing radiation, electrets may be used for a, b, c, X, e� and neutron radiation dosimetry. The

compensating charge may either be produced in the electret material itself or by interaction by the radiation field with

surrounding insulating material. We report the results of investigations of electrets produced from Teflon� polytet-

rafluoroethylene (PTFE), tetrafluoroethylene-hexa-fluoropropylene (FEP) and tetrafluoroethylene-per-fluorometh-

oxyethylene (PFA) films. The electret state was produced uniformly on one surface of the films by a corona method with

a negative 15 kV potential applied to multiple pointed electrodes facing the film on a grounded plate. After polarization,

the films were exposed to a known X radiation and the uncompensated charge was nondestructively measured with

capacitive probes. The area of the probe was designed in accordance with the spatial resolution desired. The resolution

and sensitivity of such probe is ultimately limited by the Paschen discharge between the electret and the probe. Re-

sponse curves compared the ratio of uncompensated charge density after and before exposures to the radiation as a

function of exposure. The linearity of these curves shows that the films may be used as a radiation dosimeter. The slope

of the response curves indicates the sensitivity to the ionizing radiation. The PFA film displays two linear regions which

correspond to two electron trap levels. We demonstrate that the image forming nature of these planar electret dosi-

meters has a millimeter of spatial resolution. We also report the development of innovative electret geometry for

measurements of the directional dependence of the radiation and by choice of the surrounding insulating materials, an

almost complete selectivity in mixed radiation fields. � 2002 Elsevier Science B.V. All rights reserved.

Keywords: Dosimetry; Electrets; Probes

1. Introduction

Electrets are insulating materials (electricalconductivity lower than 10�8 (X m)�1), with aquasi-permanent electric polarization, once theincorporated charge persists for �109 s [1]. In thiswork the electret state was obtained by a coronaprocess [2] and the radiation dosimetry application

was accomplished using the external electric fieldas a collector of the ions resulting from the inter-action between the radiation beam and the airnear the surface of the electret, as in an ionizingchamber [3]. The ions compensate the incorpo-rated charges and this compensation is propor-tional to the dose. Depending on the type ofmeasurements, dosimeters with different geome-tries may be used, such as cylindrical electretdosimeter and planar electret dosimeter [4]. Afterthe irradiation the potential from the uncompen-sated charge may be measured by an induction

Nuclear Instruments and Methods in Physics Research B 191 (2002) 820–824

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* Corresponding author. Tel.: +55-16-6023780.

E-mail address: dalmeida@ffclrp.usp.br (A. de Almeida).

0168-583X/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

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process with probes especially designed for eachgeometry.

The following investigations were performed:first, determination of the most adequate poly-meric film for an X-ray radiation dosimeter, sec-ond, resolution and sensitivity evaluation forcharge measurements made with point probes, andthird, angular dependency analysis of an innova-tive cylindrical dosimeter for fast neutrons.

In the first study we evaluated the polymericfilms polytetrafluoroethylene (PTFE), tetrafluoro-ethylene-hexa-fluoropropylene (FEP) and tetra-fluoroethylene-per-fluoromethoxyethylene (PFA)[5]. PFA is the most sensitive to X-ray radiationalthough not the best one suited for a dosimeter.For this study we used a corona process with anegative 15 kV potential applied to 130 pointedelectrodes facing the film on a grounded plate toestablish an electret state on the films. Subse-quently, the test films were irradiated with 16 keVX radiation in a volume of 10 � 10 cm2 by 1 cmdeep layer of air which permitted ion productionfor charge compensation. The electret charge be-fore irradiation and the uncompensated chargeafter irradiation were measured with a planar probe.

In the second study, the resolution and sensi-tivity for point probes were determined for po-tential measurements on the electret films [6] forimaging applications. The sensitivity of the probeswere limited only by the Paschen discharge fromhighly charged films. For simulating an image anU shaped metallic phantom was designed whichdefined image lines with 1 mm spacing.

The third study is related to a 1/3 sector cylin-drical geometry probe [7] for angular dependencymeasurements of the fast neutrons from an Am–Besource. After exposure to the source, it was pos-sible to observe that the charge was more com-pensated on the side of a cylindrical dosimeter thatfaced the neutron source. This study also demon-strated that almost total selectivity for any oneradiation in a mixed field of fast neutrons, slowneutrons and electromagnetic radiation could beobtained by choice of insulating materials whichinteract only with specific radiation to produce thecompensating charge.

As results of these studies, the point probewith the smaller reading diameter presents the best

spatial resolution, while the best sensitivity is ob-tained with the minimum distance between theprobe and the electret, limited ultimately by Pas-chen discharge [8]. The cylindrical 1/3 sector geo-metry is an innovative probe for determining theuncompensated charge and the direction to theradiation source.

2. Materials and methods

In the first study, the films used to obtain theplanar electrets were the PTFE, FEP and PFApolymers, each 25 lm thick. These films wereplaced on acrylic frames in order to guaranteethe geometry, and corona discharge was madewith a multiple point electrode device (130 points).The films were submitted to charge compensatingX radiation, with an energy of 16 keV and theinitial and remaining charges were determined witha planar probe.

In the second study, the point probe charac-teristics and design are presented, respectively inTable 1 and Fig. 1. Three probes with differentdiameters were constructed. An U-shaped metallicphantom (with parallel bars of 1 mm width spaced2 mm) was made for spatial resolution measure-ments. This phantom was brought to a negativepotential to simulate a typical electret image ofuncompensated charge. Each probe was connectedto an electrometer for potential readings andmoved by an XY manipulator such that potentialmeasurements were made for a selected directionperpendicular to the arms of the phantom. Thepotential profile of the probe obtained with eachprobe is presented in Section 3.

In the third study, a 120� sector cylindricalprobe was constructed and is presented in Fig. 2.

Table 1

Probe parameters

Probe

parameters

Probe 1 Probe 2 Probe 3

£1 (mm) 1.1 2.0 3.9

£2 (mm) 4.0 8.2 14.3

C ½F� � 10�12 4.08 3.78 4.33

£1: central electrode diameter, £2: probe internal diameter,

C: calculated probe capacitance.

M.A. Parada, A. de Almeida / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 820–824 821

The FEP polymer was selected for a cylindricaldosimeter (CED) and was charged by corona dis-charge. Subsequently, the outer electrode of thedosimeter was lined with a polyethylene sleeve thatproduced recoil protons which ionize the air be-tween the sleeve and the central electret electrode.

The CED was exposed in a perpendicular di-rection to a fast neutron source and the uncom-pensated charge was measured with the 1/3 sectorcylindrical probe. The cylindrical dosimeter wasconnected coaxially to the probe and the uncom-pensated charge determined for each 40� rotationaround a complete turn.

3. Results and discussion

The results of the studies with PTFE, FEP andPFA films that correlate the charge compensationof the electret as a function of the exposition timeare presented in Figs. 3–5. In each figure one cannotice linear regions, one each for PTFE and FEPand two linear regions for PFA (region I and re-gion II in Fig. 5), indicating two electron traplevels. PFA is more sensitive than PTFE and bothare more sensitive than FEP. The first trap regionof PFA is the most sensitive to ionizing radia-tion, but presents low charge storage stability. Thesecond level trap could be used for measurements,but it is impractical once the first level ‘‘memory’’has to be erased. The films most appropriate foruse as dosimeters are the FEP and the PTFE, thelatter being more sensitive.

For the sensitivity and resolution studies of thepoint probes, a spatially varying potential wasproduced by a negatively biased metal phantom.The probes were connected to an electrometer andmoved by an XY scanning device. The resolutionsfor probes 1, 2 and 3 in Table 1 were demonstratedfor a fixed probe–electret distance and the resultsare shown in Fig. 6, where probe 1 with the

Fig. 3. Uncompensated charge measurements on an PTFE

electret film after successive time intervals in a constant X ra-

diation field. The time spent making the measurement after

each interval with the electrometer and a planar probe is irrel-

evant and not shown.

Fig. 1. Plan views of a point probe showing internal and ex-

ternal diameters, that determine the spatial resolution when

measuring electret charge density. The coaxial fitting shown on

the right connects to an electrometer which reads the charge

when the capacitance is known (see Table 1 for the diameters

and capacitances for the three geometries used).

Fig. 2. 1/3 sector cylindrical probe. The outer cover of the cy-

lindrical dosimeter completed the coaxial geometry. The probe

connected on the right directly to the electrometer input such

that the electrometer indicated the potential between the central

electret and the outer cover.

822 M.A. Parada, A. de Almeida / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 820–824

smallest diameter gave the best resolution. Themeasurements indicate that if the diameter ofthe probe is doubled, the sensitivity triples and theresolution decreases by a factor 2. In Fig. 7, rep-resenting measurements with the high resolutionprobe 1, one can notice that the sensitivity dependson the probe–electret distance, eventually limitedby the occurrence of electrical discharge.

The cylindrical dosimeter, after irradiated by anAm–Be fast neutron source, was connected to anelectrometer with the 1/3 sector cylindrical probeand the initial and residual charges, after threeintervals exposed to the radiation source, weremeasured. Fig. 8 shows that the uncompensatedcharge on the electret is larger on the side notfacing the source, dramatically demonstrating that

Fig. 7. Transverse measurements of the potential near the

phantom made with the 1.1 mm diameter probe used for the

measurement shown in Fig. 6 at vertical distances 1.1, 1.52 and

2.01 mm. The signal is greatest when the probe is close to the

phantom.

Fig. 4. Measurements of uncompensated charge on an FEP

electret film versus accumulated radiation dose, as described for

Fig. 3.

Fig. 5. Measurements for PFA as described in Figs. 3 and 4.

This material is able to retain a much higher charge density

than FEP and PTFE and therefore is more sensitive to incre-

mental dose than the others. However, the presence of two

levels of charge compensation, makes it less practical as a

dosimeter.

Fig. 6. Transverse measurements of the potential at a constant

1.5 mm vertical distance, above a simulated image (phantom)

made with three different probes whose diameters were 1.1, 2.0

and 3.9 mm as shown in Table 1. The probe with the largest

area gives the largest signal but with less spatial resolution.

M.A. Parada, A. de Almeida / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 820–824 823

the innovative 120� probe permits angular depen-dency measurements.

4. Conclusions

The polymeric films PTFE and FEP are ade-quate for dosimetry. PFA is not, owing to theexistence of two level traps.

Point probes have greater resolution for smallercentral electrode diameter and smaller probe–electret distance. The sensitivity of point probes ishigher for larger central electrode diameter andsmaller probe–electret distance.

Our innovative 1/3 sector cylindrical geometryprobe permits angular dependency measurementswith a cylindrical electret.

Acknowledgements

M.A. Parada was supported by grants fromCAPES/ CNPq (Brazil).

References

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Fig. 8. Angular dependence of the uncompensated charge on a

cylindrical electret exposed to fast neutrons incident from the

left. The measurements were made with the 1/3 sector probe

shown in Fig. 2. The initial charge density on the central electret

was not exactly symmetric. Even so, the direction of the inci-

dent radiation is clearly revealed by the measurements of the

accumulated dose received during three equal time intervals.

824 M.A. Parada, A. de Almeida / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 820–824