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Polymer electret dosimetry Chang, Charles 1974

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POLYMER ELECTRET DOSIMETRY by CHARLES CHANG B.Sc, Queen's University 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the department of Elect r i c a l Engineering We accept this thesis as conforming to the required standard Research Supervisor Members of Committee. Head of Department, Members of the Department of Electrical Engineering THE UNIVERSITY OF BRITISH COLUMBIA July, 1974 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o lumbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada ABSTRACT The work reported i n this thesis investigates th.e possible use of thin film teflon electrets as X-ray dosimeters. Electrets were prepared by corona- and breakdown field-charging and found to exhibit a number of properties that would render them suitable for personnel dosimetry. The s t a b i l i t y of the residual electret charge as a function of environment was also studied and i t was found that hot, humid or unshielded conditions led to rapid charge decay. i TABLE OF CONTENTS Page 11 v i i 1 5 5 7 10 12 13 ABSTRACT TABLE OF CONTENTS . . . . . . . . . • • • • .• • • •.• LIST OF ILLUSTRATIONS i v LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . v i ACKNOWLEDGEMENT . 1. INTRODUCTION • • 2.. ELECTRET CHARGE STORAGE AND STABILITY . . . . . . . . . . 2.1 Trapping Mechanisms. . 2.2 Stability of Electrets ... . . . . . . 2.3 Charge Decay by X-ray Irradiation . . . 3. EXPERIMENTAL APPARATUS AND PROCEDURES 12 ° 1 E l c ^ ^ e t o-ri °1 . . . . . . . . . . . . . . . . . . 3.2 Charging Equipment . 3.2.1 Corona-Charging . . . . . . . . . . 13 3.2.2 Breakdown Field-Charging . . . . . . . . . . 16 3.3 Charge Measurement 21 3.3.1 Introduction . - 21 3.3.2 Theory of Operation of Vibrating Electrode Method of Charge Measurement 22 3.3.3 Construction 3.3.4 Operation. . 3.4 X-ray Apparatus and Measurements- . . 3.4.1 X-ray Equipment . . . . . . . . 3.4.2 Exposure Measurement 4.. RESULTS 24 26 28 28 29 32 i i Page 4.1 Charge Decay of Unirradiated Electrets as a Function of.Storage Conditions' . ..• 32 • 4.2 .' Charge Decay of Irradiated Electrets 36 4.3 Residual Charge Versus Exposure as a Function of Storage Conditions . . . . . . . . . . . ^ 41 5. DISCUSSION ' 49 6. CONCLUSIONS 52 BIBLIOGRAPHY ; '53 i i i LIST OF ILLUSTRATIONS ' Page Fig. 3.1 Electret ring holder 12 Fig. 3.2 Corona charging apparatus 14 Fig. 3.3 The dependence of i n i t i a l charge density on voltage for different pass'es and air "gaps 15 Fig. 3.4 Schematic cross-sections of setup for charging teflon f o i l s through application of breakdown fields and f o i l recovery after charging 17 'Fig. 3.5 Breakdown f i e l d charging apparatus . 19 Fig. 3.6 The dependence of charge density on voltage for different charging-periods and thicknesses of soda lime glass 20 Fig. 3.7 Schematic cross-section of vibrating capacitor. . . 22 Fig. 3.8 The vibrating capacitor 25 Fig. 3.9 The dependence of charge density on frequency for differently charged electrets at various a i r gaps . 27 Fig. 4.1 Charge decay for teflon electrets, corona-charged, stored under different conditions . . . . . . . . . 34 Fig. 4.2 Charge decay for teflon electret, breakdown f i e l d -charged, stored under different conditions . . . . . 35 Fig. 4.3 Dosimetric response for teflon electrets,corona-charged and irradiated without Al shielding . . . . . . . . 37 Fig. 4.4 Dosimetric response for teflon electrets,breakdown field-charged and irradiated without Al shielding . 38 Fig. 4.5 Dosimetric response for corona-charged teflon electrets irradiated through Al shielding 39 Fig. 4.6 Dosimetric response for breakdown field-charged teflon electrets irradiated through Al shielding . 40 Fig. 4.7 Dosimetric response for teflon electrets, corona-charged and irradiated through Al shielding. After every irradiation electret stored i n Al box at lab. atm. for 1 day 42 i v Page Fig. 4.8 Dosimetric response for teflon electrets charged with breakdown f i e l d and irradiated through. Al shielding. After every irradiation electret stored in Al box at lab. atm. for 1 day 43 Fig. 4.9 Dosimetric response for teflon electrets, corona-. charged and irradiated through. Al shielding. After every irradiation electrets- stored in Al box at ' . 36° C for 1 day 44 Fig.- 4.10 Dosimetric response for teflon electrets,corona-charged and irradiated through Al shielding. After every irradiation electret stored in Al box at 100% R.tt. at 23°C for 1 day. 45 Fig. 4.11 Dosimetric response for teflon electrets,charged with breakdown f i e l d and irradiated through Al shielding. After every irradiation electret stored in Al box at 36°C for 1 day . . . * 46 Fig. 4.12 Dosimetric response for teflon electrets, charged with breakdown f i e l d and irradiated through Al shielding. After every irradiation electret stored in Al box at 100% R.H. at 23°C for 1 day 47 v LIST OF TABLES Page Table 1. Table 2. Table 3. The dependence of exposure rate on X-ray tube parameters The' thicknesses of Al f i l t e r s used for different IX-ray tube voltages and currents Hie dependence of exposure rate on x-ray parameters when using the different thicknesses of Al shielding referred to i n table 2 29 31 32 v i ACKNOWLEDGEMENT The author wishes' to express his sincere gratitude to Dr. D.L. Pulfrey for his patient guidance and advice throughout this investigation. The author wishes to thank the National Research Council of Canada for the grant which supported this work. The author is indebted to Jack Stuber for building the equipment and apparatus and grateful acknowledgement i s given to W. Rachuk, Ken Humphrey, Chris Shiffer and Bob Butters for their technical assistance and to Donna Lam for typing this thesis. v i i 1 I INTRODUCTION An electret is a piece of material which is permanently ele c t r i c a l l y polarized and can be thought of as the electrostatic analogue of a permanent magnet i.e. i t produces an electric f i e l d external to i t s e l f . The f i e l d arises from the distribution of charge within the electret and the charge can be introduced into the would-be electret in a variety of ways that have recently been reviewed by Wintle."^ We can distinguish between:-(i) dipolar electrets - formed i n materials which have permanent dipole moments, the latter being forced (by a combination of heating and ele c t r i c f i e l d application) to orient i n one direction; ( i i ) internal space charge electrets - formed, i n a manner similar to (i) above, i n materials i n which free charges exist; ( i i i ) injected space charge electrets - formed by injecting charge into a material by application of an electric f i e l d approaching the breakdown strength. (iv) deposited charge electrets - formed by bombarding the material with electric charge e.g. a corona discharge or an electron beam. The most common type of electret formed by methods (i) and ( i i ) is the thermoelectret and the charge type can either be heteropolar or homopolar. In method ( i i i ) the charge has the same sign as that of the adjacent electrode and is thus called homopolar. In method (iv) the electrets so formed are monopolar. This thesis is concerned with application of electrets to radiation dosimetry, a topic which has come under recent investigation 2 3 and seems to offer high promise as a cheap, sensitive method ' for 2 soli d state dosimetry with an easy read-out procedure. In principle, any property of a solid material which can be related to the radiation dosage received can be u t i l i z e d as a solid state dosimetric method and 4 at present, only two types of solid state dosimeter have been extensively used. One type makes use of the fact that before and after ionizing radiation there can be a change of optical density of a visual color change and this change can be related to the dose received. However, fading (which can actually be positive or negative), due to .; thermal or ultra-violet bleaching, can take place and this i s highly undesirable in a device i n which s t a b i l i t y under a l l conditions except the ionizing environment being monitored i s required. The other type of solid state dosimeter is concerned with luminescence. In fact there are two types of luminescence relavant .here namely, radio-photoluminescence and thenno-luminescence. The former process relies on the fact that after subjection to the ionizing irradiation, there would be a creation of color centres i n the dosimetric material which would be luminescent, under subsequent UV or X-ray excitation. The concentration of newly-created color centres is proportional to the dose received and, hence, the luminescence output can be employed as a measure of dose. However, the severe energy dependence, complex readout procedure and fading have limited the practical use of dosimeters of this type. If a thermo-luminescence s o l i d , at a sufficiently low temperature, i s exposed to ionizing radiation, many of the freed electrons (or holes) become trapped at la t t i c e imperfections in the crystalline s o l i d . They remain trapped for long periods of time when stored at the requisite low temperature. If the temperature i s raised, the electrons (or holes) are released from the traps, subsequently returning to stable energy states 3 with, emission of l i g h t . The t o t a l i n t e n s i t y of the emitted l i g h t can, i n c e r t a i n circumstances, c o r r e l a t e with the dose received. Fading v a r i e s from one material to another. Lithium f l u o r i d e shows l i t t l e fading (less than 5% per year), i s f a i r l y s e n s i t i v e (0.1 rad i s detectable), and has been used as an X-ray dosimeter i n s p i t e of i t s complicated des t r u c t i v e readout procedure. The property of i n t e r e s t here i s the decay of stored charge i n the e l e c t r e t under the influence of i o n i z i n g r a d i a t i o n . A c o r r e l a t i o n between destroyed charge and r a d i a t i o n dose has been noted i n UV-irradiated 5 2 ZnS e l e c t r e t s , X - r a y - i r r a d i a t e d polymer e l e c t r e t s and X - r a y - i r r a d i a t e d 3 CaF^ e l e c t r e t s . The use of polymer e l e c t r e t s seems p a r t i c u l a r l y a t t r a c t i v e i n view of t h e i r low cost, long charge re t e n t i o n time^ and close r e l a t i o n of t h e i r X-ray and neutron absorption spectra to that of the human body. 7 On account of these factors the use of polymer e l e c t r e t s as X-ray dosimeters was investigated i n t h i s present work. An exploratory i n v e s t i g a t i o n i n t h i s area by Fabel and Heniscn n i Q i u a t t u LnaL my^ar, polystyrene and t e f l o n could be s u i t a b l e s t a r t i n g materials and t e f l o n 8 was selected f o r t h i s work because of i t s d e t a i l e d c h a r a c t e r i z a t i o n as 2 an e l e c t r e t material. Fabel and Henisch prepared thermoelectrets but t h i s charging method ( c i t e d as ( i ) e a r l i e r ) may not be the most s u i t e d to dosimetric a p p l i c a t i o n on account of the p a r t i a l recovery of stored charge during "dormant" periods between i r r a d i a t i o n s . The cause of this p a r t i a l recovery i s not known but i t may p o s s i b l y be r e l a t e d to the d i p o l a r nature of the stored charge. In t h i s study, therefore, a l t e r n a t i v e charging methods were i n v e s t i g a t e d , namely:- breakdown-field charging, gi v i n g homopolar e l e c t r e t s and corona-charging giving monopolar e l e c t r e t s . 9 With corona-charged e l e c t r e t s , the charges reside on the surface rather than i n the bulk of the e l e c t r e t which i s case i n d i p o l a r e l e c t r e t s and t h i s 4 may possibly be related to the above-mentioned recovery process. The long charge s t a b i l i t y of electrets formed by both of the proposed 6 10 methods is well-established. ' Also investigated, i n this' thesis-was the effect of storage environment on the charge decay and from these measurements some conclusions about the physical mechanisms of charge decay can be.drawn, • The next chapter of this thesis discusses charge trapping and s t a b i l i t y in general and, in particular, deals with, the effect of X-ray irradiation. In chapter III the experimental procedures and equipment used are described. Results are presented i n Chapter IV and are discussed i n Chapter V. II. ELECTRET. CHARGE STORAGE AND STABILITY In this chapter information on the trapping and retention of charge is briefly reviewed with particular emphasis on homo- and mono- polar systems. The effect on the trapped charge of ionizing radiation i s then discussed, with attention being given to X-ray irradiation of polymers. 2.1 TRAPPING MECHANISMS. •In corona-charged, polymer electrets three possible storage sites have been suggested"'""'" and termed primary, secondary and tertitary levels of trapping. The primary level of trapping occurs at the particular individual atom bonds within the molecules. Trapping at the secondary level includes caging of the decelerated electrons between adjacent main C chains, due to the electron affinity of the neighbouring group (e.g. H, Cl, F). Trapping at a tertitary level accommodates trapping of the injected electrons in cavities (structural defects) within the crystalline or amorphous region, or at the crystalline-amorphous interfaces of the polymer. Impurities in polymers could also be directly responsible for the g trapping. However, work by the Bell-Northern group on a series" of corona-charged substituted polyolefins suggested the charge traps in the polymers do not arise from impurities but that the chemical change caused by the corona i t s e l f may be important. In particular, corona discharges in a i r can give rise to double bond (^TC=Cd) and carbonyl group (Z^0=0 ) formation, both of which may act as electron traps. Experiments using different ambients for corona support might prove useful in yielding more information on this p o s s i b i l i t y . Some 9 experiments of this nature were carried out-by Cross and Blake who investigated charging of a FEP teflon sample held between two elec-trodes in ambients of both medium vacuum and SF^ (highly electronegative) at atmospheric pressure. No differences i n the level of retained charge were detected thus indicating that the most l i k e l y mechanism appears to be electron injection from the metal electrode aided by the applied electric f i e l d . The charging method referred to here might more properly be termed breakdown field-charging but was equated to corona-charging by Cross and Blake by the suggestion that the small, but f i n i t e , gaseous gap between the metal electrodes and the electret sample would allow a "corona" to form.. However, because of the insensitivity of charging to the ambient the conclusion i s that i n breakdown f i e l d charging (as defined in this thesis)charge i s injected directly from the contacting electrode and not via an intermediate gas discharge. However the fin a l site of the trapped charge is not identified except that i t is thought to be very close to the surface, e.g. within a depth equivalent to about 5% of film thickness. The charging mechanism involved in the breakdown f i e l d charging method^ has not been investigated, although, as mentioned 9 above, the work of Cross and Blake i s probably relevant here. Charging is usually accomplished"*"^ by applying a large voltage to a metal / polymer / glass dielectric / metal system and thus charges can be driven into the polymer from either the adjacent metal or the d i e l e c t r i c . By the polarity of the voltage recorded in an induction charge measurement method^, i t appears that homocharge electrets always result. The charge must be injected from the dielectric because as 22 Sessler and West have shown , no voltage would result, using their measurement technique, for the case of homopolar charge injected from the metal. 7 2.2 STABILITY OF ELECTRETS The s t a b i l i t y of electrets is of major concern when possible applications are being considered. A.series'of negatively corona-charged 8 ' electrets has been investigated by Creswell and co-workers for the s t a b i l i t y of charges. Total halogen substitution improved electret s t a b i l i t y whereas partial substitution, forming less symmetric polymers, caused large decreases in s t a b i l i t y . So the most stable electrets were polytetrafluoroethylene(teflon)and linear polyethylene. It appears that the more electronegative halogen substituent(F i n teflon) provides a more stable charge storage site for negatively charged electrets. However, even though teflon has been found to be the most stable material for retaining i t s charge, i t s t i l l exhibits some charge decay. The charges accumulating on the insulator surface can diminish by travelling through any of three paths, v i z : (1) the surrounding a i r ; (2) the electret surface; and (3) the interior of the insulator. Surface charges could be "lost" to the air either by dispersion into the air caused by the electret's Own f i e l d or due to recombination with ions existing i n the a i r . Ieda and his 12 -collaborators showed that i f the capacitance between a probe (used to measure surface potential) and surface of the electret i s independent of time, then the time derivative of potential would be inversely proportional to the capacitance and directly proportional to the decay of charge into the ambient.. Since the capacitance is inversely proportional to the distance, the rate of change of potential would be larger on increasing the distance from the probe to the surface of the electret i f dispersion was the main decay mode. However, Ieda et al's results show the opposite effect. Therefore charge diminution by this mechanism is very probably negligibly small. The other alternative i s that charge compensation occurs at the surface and i s due to ions in the atmosphere attracted to and adsorbed upon the electret surface. This mechanism is favoured by Anderson and co-workers who also mentioned that charge compensation can be proved by washing in ionic solutions containing a wetting agent, IGEPAL Co-630 (a non-ionic surfactant). This wash reduced the charge level to 6% of Its i n i t i a l level and on subsequent rinsing i n d i s t i l l e d water the charge recovered to 89% of i t s i n i t i a l value. Thus, charge compensation could be responsible for the decay of charges. Decay across the surface i s thought to be insignificant by 12 Ieda et al who showed that the potential decay curves for different surface resistances and sample thicknesses were identical. For internal charge decay there are 3 p o s s i b i l i t i e s i.e. diffusion, ohmic conduction and d r i f t through the electret's own f i e l d . , 12 Again, Ieda et als finding that the potential decay curves are independent of specimen thickness suggests that diminishing of charges due to diffusion i s insignificant. Ohmic conduction has been shown to be insignificant by van Turnhout''"4 in an experiment which consisted of vibrating an electrode (for measuring charge density) over the electret which was situated in a compartment in which the temperature could be raised uniformly (at a rate 1° C/min.). Van Turnhout found that in his 2nd run of heating, the charge remained remarkably constant, namely right up to the f i n a l temperature of the f i r s t heating run. If ohmic conduction had been the main decay mode then the charge-temperature curves would bend downward at the same temperature as i n the f i r s t heating run. So ohmic conduction does not appear to be the main decay mode. In mono-charged electrets the free excess charges are 9 .believed to d r i f t i n the'electret's.own. internal f i e l d towards the image charges and . this .motion is. space-charge-limited and therefore called SCL d r i f t . Charges in trap sites are believed by Perlman and 15 TJnger to be untrapped by both a lowering of trap depth with molecular motion plus thermal excitation of the charge out of the trap, followed by d r i f t in the bulk of the material with the pos s i b i l i t y of retrapping 16 17 as well. Wintle ' has tried to propose mathematical models which combine the equation of continuity and Poisson's equation under the condition that in the system.under consideration, the total current > density is zero. In his early model"^ surface potential was related to the mobility of the carrier and to the .time after charging. According to this model surface charge should i n i t i a l l y f a l l linearly to half of i t s i n i t i a l value and thereafter f a l l hyperbo'lically. This model could then explain the "cross-over effect" that had been noticed i n 12 28 various charge-time curves ' . However the observed trend of a higher i n i t i a l f i e l d due to higher electret charge density giving a higher mobility cannot be explained with this model. Wintle tried to incorporate an explanation of this phenomenon in a later model"'"7 i n which a power-law dependence of mobility on electric f i e l d was proposed. However, the resulting decay curves, even though presenting a better f i t to the i n i t i a l decay data, s t i l l did not predict a cross-over effect. In the above models the assumption that a l l charges reside uniformly on the surface of the electret and can be represented by a delta function at the very surface has been made. Although i n i t i a l 9 charges can reside on the surface the charge could also penetrate 18 into the bulk depending on the charging conditions (e.g. applied voltage i n breakdown f i e l d charging). For corona-charged electrets, i n i t i a l charges are believed.to reside on the surface with l i t t l e p o s s i b i l i t y of penetration into the bulk as well. Thus i n these electrets there could either be an insufficient charge deposited to f i l l a l l the deep traps in the material or a much higher concentration of such traps could exist adjacent to the surface than the bulk. The observation of considerable late r a l charge motion within J 8 a few minutes of deposition" suggests that the f i r s t alternative i s unlikely, although i t cannot be ruled out entirely. The second alternative seems to be a more likely- condition particularly as deep traps adjacent to the surface could arise from damage during the charging process or from prior oxidation and mechanical handling. Although none of the models completely f i t the experimental results yet, the approach seems a reasonable way i n which to attempt to char-acterize the decay u i e c i i a u i c > i u § . The .only possible conclusion:: at present seem to be that the decay of charge in mono-charged electrets • could either be due to surface compensation, SCL d r i f t , or both. 2.3 CHARGE DECAY BY X-RAY IRRADIATION In general the effect of ionizing radiation on insulators can 19 result i n four p o s s i b i l i t i e s , namely (i) ionization-induced conduction, ( i i ) charge transfer ( i i i ) a i r ionization (iv) space charge build-up. In ionization-induced conduction in insulator materials, the primary effect of a single Ionizing particle traversing an insulating material is to create a "track" along which electrons could be ejected from their bound states, or moved to an orbit of higher energy (excitation). In the former case a positively charged atom or molecule (ion) and a free electron would.be l e f t and the positive ion may react with neighbouring electrons or ions. The 11 free electron may either .'return . to . i t s parent molecule . to give a highly excited molecule, or i t may.Be captured elsewhere, giving a negative ion. Nuclear reactions would not occur u n t i l energy levels reached millions 20 of electron volts . Charge in the traps might be released by X-rays. The charges may be retrapped again or move along to neutralize the positive ions. Charge transfer i n general i s caused by incident radiation either because a secondary electron has received enough kinetic energy to move significantly in a preferential direction or because an electron was created close enough to the surface of a material to escape into the ai r . The basic processes i n air ionization are identical to those in insulators, namely electron-ion production, thermal!zation and motion under the electric f i e l d and density gradient. Radiation-induced space charge can result from either direct injection or defect creation and subsequent charge trapping. Considering the above four p o s s i b i l i t i e s in relation to the observed charge decay on X-ray irradiating polymer electrets i t appears that space-charge formation can be ruled out as X-ray photon absorption would lead to creation of equal amounts of positive and negative charge. Air ionization would appear to be insignificant as the size of air molecules i s large and penetration into the teflon polymer i s improbable, although some surface recombination may take place. Indeed, the presence on the surface of ions of polarity opposite to that of the i n i t i a l charge on the surface could explain the recovery process noted » in dipolar electret X-ray dosimeters. Ionization-induced conduction i s believed to take place in teflon material and charge transfer could also occur as there are charges residing on the surface of the electret. 12 H I EXPERIMENTAL APPARATUS AND PROCEDURES 3.1 ELECTRET MATERIAL As outlined in Chapter 1 the'material chosen for this work g was teflon, which is known to be a very stable electret . The teflon film used was 0.004 cm thick and was obtained, complete with, an evaporated aluminum film backing, from Dielectrrx Corp. This material was essentially charge-free. In order to handle the material without contaminating the surface, a double brass ring holder was designed. Csee figure 3.1). The holder consisted of two interlocking rings with the teflon film stretched li k e a drumskin across the inner ring Cdiameter l 1 / 4 in.) and then clamped tightly between the rings. The uncoated side of the teflon film formed the top surface and was thus surface was in contact with the inner ring but insulated from the outer ring by the teflon film i t s e l f . A teflon rod could be screwed into the side of the outer ring to form a frying-pan-type structure very suitable for handling and transporting of the electret. teflon (ay e r Figure 3.1: electret ring holder 3.2 CHARGING'EQUIPMENT 3.2.1'CORONA-CHARGING The corona charging apparatus is shown in figure 3.2 and consisted of a knife edge electrode (razor blade) which could be moved vertically by a micrometer screw insulated from the electrode by a polymethylmethacryalate (PMMA) block. The razor blade corners were trimmed to prevent air breakdown at the edges. The would-be electret was positioned between the brass rings as described i n section 3.1 and placed on a movable PMMA bed. By rotating an insulated handle the • electret could be passed under the knife edge. Care had to be taken during charging to ensure that the razor blade would be far enough away from the rim of the outer ring so that the a i r would not breakdown. A piece of thin aluminum f o i l was placed on top of the PMMA bed and was connected to ground. A resistor of high resistance ( > 2 Mohms) was connected to the upper electrode in order to li m i t the current from the voltage source. Corona-charging c a n proceed with either negative or positive 21 applied voltages. Reiser, Lock and Knight have indicated that the rate of decay of positively and negatively corona-charged electrets is about the same for the same i n i t i a l charge, but as the reproducibility of the positively corona-charged electrets was apparently poor, only negatively corona charged electrets were investigated i n this study. Electrets were produced with air gaps of 0.5 mm and 1 mm at different applied voltages (4, 5, 6 kV) and with up to three passes -under the high voltage electrode, with each pass taking about 20 seconds. •Figure 3.3 shows the resulting charge density as a function of voltage witl air gaps of 1 mm and 0.5 mm. For both air gaps the charge densities Figure 3.2: corona charging apparatus 15 VOLTAGE fkV) Figure 3.3: the dependence of i n i t i a l charge density on voltage for different passes, (x-x-x) 1 pass. (A-A-A) 2 or' 3 passes. ( ; ) a i r gap 0.5 mm ( ) a i r gap 1.0 mm seemed to saturate after two passes at a value higher', than that for —10 2 —10 2 one pass by about 4x10 coul/cm for the 1mm gap and 2x10 coul/cm for the 0.5 mm gap. Each point on figure 3.3 i s the average of two or more readings and the spread of results for any given condition was about ± 10%. r~ • 8 Perlman and his collaborators charged polymer films C 25u thick) by a negative corona i n a i r with 0.5 mm gap beneath a knife edge held at 4 to 6 kV. The net charge was measured immediately after ' charging with a biased vibrating electrode arrangement. A maximum — 8 2 charge density of 14x10 coul/cm was obtained which was more than —8 2 twice as much as the maximum values (^5.6x10 coul/cm ) obtained in our experiment. This discrepancy might have been due the presence of the slight differences in the charging systems used, e.g. sharpness of high voltage electrode, suppression of corona from high voltage connectors, or - could be the result of a difference in the charging time used (2 mins. i n this work, unspecified in ref. 8 ). Another relevant fact may be that in Perlman et a l ' case charge was -measured as the teflon electret was passed continuously from the corona apparatus using driven spools, while i n our case the electret had to be manually moved from the charging set-up to the charge-measuring instrument. For X-ray dosimetry and charge s t a b i l i t y i n -—8 2 vestigations electrets of charge density 5.5x10 coul/cm were produced at a voltage of 6 kV with an air gap of 0.5 mm and employing 2 passes. 3.2.2 BREAKDOWN FIELD CHARGING Sessler and West^ were the f i r s t to .investigate electret formation by the breakdown f i e l d charging method and the arrangement 17 described here i s based on their description. A high, f i e l d was applied to the teflon samples (clamped to the rings as shown in figure 3.1) by sandwiching between two electrodes. The arrangement i s shown schematically in figure 3.4 with the actual apparatus being shown i n figure 3.5 (A) (B) power supply electrode eta I layer-~ —-.electret-die lee trie ^support electrode air gap power suppl y. Figure 3.4: (A) schematic cross section of setup for charging teflon f o i l s through application .. of breakdown fields. (B) schematic representation of f o i l recovery after charging. A voltage (several kV) was applied to the metallized side of the teflon,resting on the soda lime glass, for different periods of time (2-20 mins) at room temperature. The glass di e l e c t r i c was required i n order to obtain relatively high charge densities"*"^. The air gap voltage between the surface of the electret and the die l e c t r i c support has been shown^ to be less as the thickness of the di e l e c t r i c support increases. Therefore surface charges should remain high for thicker dielectric supports on account of the smaller air gap voltage producing less charge leakage across the air gap. 18 Different thicknesses of soda lime glass plate were used.to investigate the above argument. After charging, the upper electrode was disconnected and the handling rod was connected to the ring. If the metallized side of the teflon was connected to ground as. suggested by Sessler and West"^, -10 2 a charge density of only 1x10 coul/cm was detected. This presumably r-1 was the charge remaining after electret sxirface leakage, which could have been considerable in our case due to the geometry of the system. Thus in this work, the metallized side of the electret was not grounded during recovery. As shown in figure 3.5, the apparatus consisted of a grounded Stage, the height of which was adjustable by a screw shaft so that different thicknesses of dielectric plate could be accommodated. The teflon and support rings were pressed onto the glass plate during charging by two hooks fastened to the tv?o pegs. After charging the stage could be released by the release handle and would come to rest on a rubber foam bed. The whole apparatus apart from the electrodes, was made of PMMA. The charge densities resulting from different charging periods, voltages and insulator thicknesses are shown in figure 3.6. For soda lime glass of thickness 0.057 cm a voltage of 9 KV was needed to — 8 2 produce a charge density of 5x10 coul/cm ; while for thicknesses of 0.108 cm and 0.156 cm voltages of 1.8 kV and 1.77 kV respectively were required to produce the same charge density (duration of application of voltage was 10 mins.) This confirms that the thicker the d i e l e c t r i c plate the higher the charge density would be for a given charging voltage, although a saturation effect rapidly develops. The spread i n results is about ±10% which is about the same as that reported by Sessler and •release handle T=I. Jo high voltage staqe rubber foam .hook PM MA PP9 Figure 3.5: breakdown f i e l d charging apparatus 70-VOLTA GE (kV) Figure 3.6: the dependence of charge density on voltage for different charging periods, (•-•-o) 2 minutes. (o-o-o) 5 minutes. (A -A -A ) 10 minutes, (x-x-x) 20 minutes. ( ) thickness of soda lime glass = 0.057 cm ( ) thickness of soda lime glass = 0.108 cm (—v—v-—) thickness of soda lime glass = 0.156 cm and i s almost indifferent to time \ 21 West 1 0. For a dielectric thickness of 0.156 cm, an application of 2 kV —8 2 for 10 minutes yielded a charge density of 5.4x10 coul/cm i n the present experiment. This magnitude was about 3 times as low than that obtained by Sessler and West 1 0 under similar conditions. The difference is,again, most l i k e l y attributable to slight differences i n the charging apparatus used. For X-ray dosimetry and investigation of s t a b i l i t y of charge, electrets —8 2 with a charge density of 5.5x10 coul/cm were produced by using a voltage of 2 kV for 10 minutes and employing a soda lime glass, plate of thickness 0.108 cm. 3.3 CHARGE MEASUREMENT 3.3.1 INTRODUCTION In this work i t was necessary to make repeated measurements of the stored charge remaining in an electret after i t had been subjected to various treatments. It was therefore necessary that a non-destructive measurement of charge density be used. There are three possible methods, 22 23 two of which are based on the dissectible capacitor ' , while the other 24 uses optical techniques . The la t t e r method, even though i t i s capable of high resolution and fast response,was deemed not suitable for the present work in view of the necessity to maintain an optically f l a t electret surface. Of the former methods, one of them, the l i f t e d 22 25 electrode method, can be divided into two catagories ' - the small 2 22 potential probe used by Fabel and Henisch and the induction method . The potential probe method, which monitors the surface potential, i s not desirable in this case as the surface potential could vary due to different distributions of the charge density within the d i e l e c t r i c and thus not give a unique reading for a given stored charge. So, the induction method and the vib»ating capacitor method would appear to be the two possible candidates. Instruments u t i l i s i n g each of these methods were constructed but the vibrating capacitor method proved easier to operate and yielded more reproducible data. Accordingly, the vibrating capacitor method was used in this work.. : 3.3.2 THEORY OF OPERATION OF VIBRATING ELECTRODE METHOD OF CHARGE , MEASUREMENT The vibrating capacitor method used i n this work is based 23 on the technique described by Reedyk and Perlman . Essentially i t consisted of two metal electrodes sandwiching the electret, which had i t s metallized side- resting on top of the lower electrode. The upper electrode,which was some distance from the electret,could be vibrated •up and down. Because of -the electric f i e l d produced by the electret, the vibration generated an A.C. signal which could then be measured with either an oscilloscope or voltmeter connected between the electrod es. If a bias of suitable polarity, depending on whether the net charge density of the electret was positive or negative, was then applied to the lower electrode, the A.C. signal could be n u l l i f i e d . The surface charge density i s then related to the bias voltage. Figure 3.7 shows the schematic cross section of the vibrating capacitor. 1 n vibrating electrode guard ring E2i iDZ T r~ -1 Oy-i_ 1 ixed\ -electret lectrode Figure 3.7: schematic cross section of vibrating capacitor To calculate the el e c t r i c f i e l d in the air gap, apply the line integral law to .the two.layer capacitor of figure 3.7 • ' d l E l + d 2 E 2 " 1 C 1 ) where d^ and are the. thicknesses of the electret and air gap respectively, E^ and E^ are the elec t r i c fields i n the electret and the air gap respectively, and "V is the potential difference between the electrodes. Tram Gauss's law we. have ^ - D 2 = a r C2) where and D are the electric displacements in the electret and the air gap respectively, and cr is the real charge on the surface of the electret, By definition \ " P l + e o E l ° ) where P^ is the polarization response of the electret to the applied f i e l d , and e is the permittivity of free space. The pol-arization response P^(t) is resolved into two components ^ ( t ) . = P 1(t) + P s(t) C4) where P g(t) is the component that responds slowly to changes i n the internal f i e l d E^(t), and P^(t) i s the component that responds instantaneously to changes in the internal f i e l d and can be characterized by a dielectric constant K, i.e. P ± ( t ) = e Q ( K - l ) E 1 ( t ) (5) In the air gap, P (t) = 0, and thus combining eqs.(3), (4) and (5) s we have D 2 = e o E 2 (6) and D = Ke E. + P (7) 1 o 1 s Fina l l y , combining eqs. ( 1 ) , C2), C6) and (7), the f i e l d i n the a i r gap where (a- - P ) is the net surface charge" density. If we now apply a bias V = Y B so that = 0, eq.(8) yields C a r - P s) = Ke 0V B/ d ; L C9) Thus the net surface charge density of the sample is simply related to the D.C. bias that produces zero-field i n the air gap, and hence zero A.C. output signal when the upper electrode i s vibrated. The value, of the net surface charge i s theoretically independent of the air gap and frequency of vibration. The polarity of the net surface charge is automatically given by the polarity of V . a 3.3.3 CONSTRUCTION The vibrating capacitor u t i l i s e d a 5 i n . loudspeaker for providing the vibration instead of using the more conventional solenoid arrangement which i s heavy,bulky and prone to generation of harmonics. Since the loudspeaker could drive only a relatively light weight, the upper electrode was made of a thin printed c i r c u i t board in which was etched a circle of diameter 2 i n . and an outer ring, which when grounded served as a guard ring. The circular part of a thermal, foamy cup 1 i n . in length was glued inbetween the printed c i r c u i t board and the speaker. This allowed the upper electrode to be removed in order to f a c i l i t a t e transfer of the electret. The loudspeaker could be raised or lowered by means of a shaft threaded through a sturdy aluminum box (see figure 3.8). The lower electrode, which was a circular brass plate of diameter ^I^ ^n*> w a s glued on top of a circular teflon disc. A small groove of i.d. 3/8 i n . was cut on top Figure 3 . 8 : vibrating capacitor 26 0 of the lower electrode and inside the teflon disc there was a duct of diameter 3/8" connected to the groove so that a vacuum pump could be used to suck the electret tightly onto the top of the: lower electrode to ensure good electrical contact. The circular teflon disc was supported by three replaceable legs which, allowed for considerable length adjustment i f so required i.e. i n excess of that which could be met by adjustment of the upper electrode. A l l e l e c t r i c a l connections used coaxial cables with the upper electrode line being specially made to reduce i t s weight. It was made from a thin insulated wire enclosed by a stranded light steel wire which was connected to ground. 3.3.4 OPERATION .For the driving source of the loudspeaker, a sinusoidal voltage from a-Kavetek (model no.Ill)"function generator was used. The A.C. output signal, obtained by vibrating the upper electrode, was monitored by an oscilloscope. A D.C. bias of negative polarity from a Harrison 6525A D.C. Power Supply with range 0-4000 V and 0-50 mA was then applied so that the resulting oscilloscope signal could be nulled i.e. reduced to less than one tenth of a volt. Theoretically, from section 3.3.2, eq.9 shows.that the measured surface charge density should be independent of the ai r gap length. To test this practically the system was designed to allow 23 variation of the air gap separation. Reedyk and Perlman also made a. harmonic analysis of the electrical.mesh equivalent to the c i r c u i t ~50 60~- JO FREQUENCY (Hz) Figure 3.9: the dependence of charge density on frequency for 3 differently charged electrets at various air gaps d„. (A-A-A) ( x - ^ x - x ) ( A - A — A ) d 2 = 0.37 cm (©-©-©) (Q-CD-Q) (o-o-o) d 2 - 0.42 cm (B-B-B) (V-V-^) (•-•-•) d_ = 0.47 cm 28 shown i n figure 3.7 and related the output voltage VQ to the ci r c u i t parameters. V should be independent .of the frequency- of vibration and this prediction was also investigated using the present apparatus. Three different charge density- levels in a series of electrets were obtained by breakdown f i e l d charging at voltages of 0.5, 1^ 0 and 2.0 kV for 2 mins. with a soda lime glass plate of thickness 0.108 cm." The workable frequency range was found to be from 50 Hz to 70 Hz. Frequencies lower than 50 Hz were not used because the lower l i m i t of frequency response of the loudspeaker was 50 Hz. Frequencies higher than 70 Hz. • tended to cause non-sinusoidal signals presumably because of excessive vibration. Figure 3.9 shows the apparent change i n charge density with change in frequency for air gap separations of 0.37, 0.42 and 0.47 cm. The measured charge density seems to be independent of frequency but does show-a slight dependence on a i r gap. separation. It seems that the closer the separation, the larger the apparent charge density. In spite of . this discrepancy the method of charge measurement was, s t i l l suited to our purposes of X-ray dose measurement, as i t is the sensitivity and relative change in charge density that are important. As a result, -a l l the charge density measurements were taken under a fixed separation of 0.42 cm and at a frequency of 60 Hz. Within the limits imposed by these conditions the overall accuracy was ±5%. 3.4 X-RAY APPARATUS AND MEASUREMENTS 3.4. X-RAY EQUIPMENT The X-ray machine was from North American Philips Co. Inc, model no.42201-42202, and was capable of delivering 60 kV X-rays at target currents of up to 50 rmA. A copper target was used i n a.tube which had four windows from which ejection of X-rays could be controlled 29 by opening shutters; only one window was used i n this work. The area over which the X-rays would irradiate was detected by a fluorescent phosphor detector and marked' on a mount on which the electret to be irradiated could be fixed. Every- time during irradiation the electret was placed, at a fixed distance, 27 cm, away from the window. This distance was chosen as a compromise between being close enough to get sufficient intensity to reduce the charge of the electret and being far enough away to get the required area- uniformity i n the X-ray beam. 3.4.2 EXPOSURE MEASUREMENT Exposure was measured by an ion-chamber X-ray dosimeter, model no.37C made by Electronic Instruments Ltd.' A 35 c c . chamber placed at the site (27 cm away from the source) at which the electrets received irradiation was used to monitor the output of the X-ray machine. This instrument i s capable of measuring the; exposure rates for various X-ray over a period of time. The measured exposure rates for various X-ray tube voltages and currents are shown i n table 1. The figures quoted refer to a sampling time of 10 seconds and are the average of two or more readings. Table 1: the dependence of exposure rate on X-ray tube parameters 20 30 40 5 0.9 6' 1.8 0 2.5 8 10 1.5 6 3.2 0 5.25 The results seems to be reasonable as the X-ray intensity I, which is proportional to the exposure rate measured here, can be described 2(5 by the usual equation for a continuous spectrum T •= BiCV) n where B is the proportionality constant, 7 is the tube voltage, n is a tube parameter and i is the tube current. The' accuracy of the result i is dependent on the control of the timing of opening and shutting the window and on the accuracy of the X-ray dosimeter, which is stated by the manufacturer to be ±10%. : Electrets that had been charged by the breakdown f i e l d method at a voltage of 1 kV for 2 minutes using a soda-lime glass plate of thickness 0.108 cm were tested at different radiation levels and the complete annihilation of the electret's charge was found to occur, even for the lowest energies possible, in times of around 1 minute. This time is somewhat short for the i n i t i a l experimental purpose of determining the electret's charge decay as a function of radiation exposure. Locating the electret significantly further away from the machine was not viable because of constraints imposed by the room boundaries. Thus in order to make, the measurement possible aluminum f i l t e r s , of different thick-nesses according to the X-ray energies employed, were placed in front of the electret so that the complete reduction i n charge density would require times of the order of several minutes. The exposure rate corresponding to irradiation through the various aluminum shields l i s t e d in table 2 were measured by the ion-chamber dosimeter and are recorded in table 3. It can be seen that the use of the f i l t e r s enables similar exposure rates to be obtained for the range of' tube voltages and currents used in this work. Table 2: the thicknesses of Al f i l t e r s used for different X-ray tube voltages and currents 20 30 40 5 0.030 0041 0.077 10 0.03 6 0.07 7 0.14 8 ^. ucjjenuerice or exposure rate on X-ray tube parameters when using the different thicknesses of Al shielding referred to in table 2 20 30 40 5 4.1 0 5.2 4.6 10 5.63 5.2 4.4 32 IV RESULTS. 4 • 1 CHARGE DECAY OE UNIRRADIATED ELECTRETS AS A FUNCTION OF STORAGE CONDITONS Stability of electrets i s of great concern i n the applications f i e l d , whether the electret be used as a microphone, dosimeter, electrostatic lens, air f i l t e r e tc. 1 In .X-ray dosimetry i f the charge of the electret i s not stable enough, then one may confuse the "natural" decrease in charge with that due to the 2-ray dose received. Tests were carried out with electrets l e f t under different storage conditions to see which storage condition gave the most stable'electret. Besides the usual parameters of temperature, l i g h t and humidity, the effect of 13 shielding was also investigated. Figure 4.1 shows the charge decay of corona-charged electrets R t n r p d n n d f t r different conditions. A l l electrets were charged to a — 8 2 charge density of 5.4x10 coul/cm ±10% and the result quoted for any given condition, i s the mean taken from at least two (sometimes three or four) samples. The most stable electret was the one held at room 3 3 1 temperature i n an aluminum box ( 1 / /' x 1 /^" x I^ ) with aluminum thickness 0.42 mm. "After 5 days there was almost 30% (spread of results between similarly-treated samples was ±3%) decrease i n charge 13 density, a result which differs from that quoted by Anderson et a l . 1 whose experiment showed almost no decrease i n charge density after 5 days. The reason for this difference could simply be due to the low i n i t i a l —8 2 charge density (2x10 coul/cm ) of the electrets used by Anderson et a l . or i t may be related to the relative "air-tightne ss" of the two enclosures. The electret stored in the laboratory atmosphere without shielding by the aluminum box showed a fast decay (39%) i n the f i r s t 5 days but a much slower decay for the next 25 oays. After a total of 30 days a decrease 13 of 55% (spread D f results ±8%) was recorded. Anderson -et a l . found a 45% decrease i n charge density densit}' in 30 days, and again this lower figure could be due to the lower i n i t i a l charge density employed. A decrease of 37% (spread of results ±4%) in 30 days was recorded for the electret stored in the aluminum bbx at 36°C, and this was about the same decay rate as that for electrets stored in a 14" x 7" x 7" cardboard box instead. An electret stored in the aluminum box at 100% R.tt. (relative humidity) shows a decrease of 45% over a period of 30 days •Figure 4.2 shows the results for :electrets charged by the breakdown f i e l d method when tested under the same storage conditions as used for the corona-charged electrets. A l l electrets were charged to — 8 2 5.75x10 coul/em ±10%. Again the most stable electret was the one stored i n the aluminum box at laboratory atmosphere. A decrease of only 8% (spread of result ±5%) was recorded after 5 days; after a total of 30 days the charge only decayed by 11% (spread of result ±5%) which was less than that of the best corona-charged samples. The decay observed by Sessler and West"^ was about 50% and this would be most -7 2 lik e l y due to the high i n i t i a l charge density (about 5x10 coul/cm ) they used. Higher charge density electrets were not used here because of the insulation limitation imposed by the cable connectors of the D.C. bias voltage source used in the charge density measurement; also we wished to compare the decay with that of corona-charged electrets. Again the electret stored i n the laboratory atmosphere without the aluminum box showed the fastest decay (36% in 30 days with spread of result ±8%). It seemed that the electret stored i n the dark without benifit of the aluminum box decayed less than the electret stored i n J 30 DAYS Figure 4.1: charge decay for teflon electrets, corona-charged, 3 stored under different conditions. (A-A-A) stored in Al box it lab. atm. (o-o-o) open at lab. atm. (o-n-o) stored in Al box at 36 C. (a-s-e) open in dark (x-x-x) stored in Al box it 100% R.H. Figure 4.2: charge decay for t e f l o n eloctretSjcharged with breakdown DAYS u> f i e l d under d i f f e r e n t cond:.tions. (A-A-A) stored i n Al box al: lab. atm. (o-o-o) open at lab. atm. (•-D-D) stored i n Al box at: 36 C. (•-•-•) open i n dark (x-x-x) stored i n Al box at: 100% R.H. 36 the aluminum box at 36°C. The' electret i n the aluminum box at 100% R.H. decayed 33% (spread of result. ±20%) in 30 days which was 12% less than that shown by the corona-charged electrets over the same period' of time. 4.2 CHARGE DECAY OF'IRRADIATED ELSCTRETS A l l electrets used in the X-ray irradiation studies were l e f t in air for more than 5 days after charging in order to allow the i n i t i a l rapid charge decay to occur and the electrets to "stabilize". Figure 4.3 shows the decrease In charge density versus the exposure received. For a given X-ray exposure, the decrease' in charge density i s slightly greater at the higher values of X-ray energy. Similar results were found for the breakdown field-charged electrets and are shown in figure 4.4 The exposure ranges used for the corona-charged and breakdown-charged electrets were 1 -14 R and 1 - 1 7 R respectively. Figure 4.5 shows the dosimetric response for electrets, corona-charged,when using different thicknesses of aluminum shielding. Because of the thicker aluminum shielding used for higher energy X-rays (see table 3), higher exposure of higher energy X-rays are needed for a given electret charge reduction. The possible exposure range depends, in fact, on the thickness of shielding used. The decay of charge density versus exposure curves shown in figure 4.6 are for the breakdown"field-charged y electrets. Doses about 400 R higher than used with the corona-charged electrets were possible due to the higher i n i t i a l charge of the 2 breakdown-field-charged electrets.. The range of exposure was around 1 - 16x10 2 R and 1 - 20x10 R for the corona- and breakdown f i e l d - charged electrets respectively. In order to test whether the X-ray-induced charge decay of the electrets 13 was due to charge compensation, a test as mentioned by Anderson et a l . (see sec.2.2)was carried out. An electret after X-ray irradiation had i t s IOO r X-RAY EXPO (R) Figure 4.3: dosimetric response for teflon electrets,corona-charged and irradiated without Al shielding. ( A - A - A ) 20 kV 5 mA (o-o-o) 20 kV 10 mA (o-o-o) 30 kV 5 mA (©-©-a) 30 kV 10 mA (A~A-A) 40 kV 5 mA (B-B-B) 40 kV 10 mA loo r r - . 0 5 TO . . X-RAY EXPO (R) Figure 4.4: dosimetric response ::or.teflon electrets,breakdown field-charged and irradiated without Al shielding. ( A - A - A ) 20 kV 5 mA (o-o-o) 20 kV 10 mA ( O - Q - D ) 30 kV 5 mA (•-«-©) 30 kV 10 mA" ( A - A - A ) 40 kV 5 mA (o-o-a) 40 kV 10 mA 5 10 75 X-RAY EXPO/10'2(R) 19 dosimetric response for breakdown field-charged teflon electrets irradiated through Al shielding. (A-A-A) 20 kV 5 mA (o-o-o) 20 *V 10 mA (o-o-a) 30 kV 5 mA (•-o-o) 30 kV 10 mA (A-A-A) 40 fcV 5 mA (B-B-B) 40 kV 10 mA o charge density reduced to 10% of i t s i n i t i a l value when washed by a non-tonic surfactant, monophenoxy-polyoxy-ethanol,, (IGEPAL Co-630) diluted by tap water. After this washing procedure zero charge density was recorded. Then the electret was rinsed twice with'distilled water. A — 8 2 charge density of 1.1x10 coul/cm was recorded i . e . 20% of the i n i t i a l charge density might have been recovered. However subjecting electrets,without X-ray irradiation possessing, a charge density of — 8 2 5.5x10 coul/cm to the same procedure, again yielded the same charge density — 8 2 ' of 1.1x10 coul/cm . In fact teflon, possessing no charge, was tested with the same procedure as above and became charged to the same charge density — 8 2 density of 1.1x10 coul/cm . As a result the test performed by 13 Anderson et a l . is inconclusive and the method -used could i n 13 fact cause charging of the electret. Anderson et al's quote of an 89% recovery prcbHb7.y if? r 1 ,. , p i~n t-he f a r t t h a t their samnles had only a very — 8 2 low i n i t i a l charge density (^2x10 coul/cm ). A l l tests and results were averages of 2 or more samples and the spread of results was ±25%. 4.3 RESIDUAL CHARGE VERSUS DOSE AS A FUNCTION OF STORAGE CONDITIONS: In order to further see whether the teflon electret dosimeter might be of any practical value,a series of tests of residual charge retentivity versus exposure as a function of storage conditions was carried out. The procedure used was to irradiate the electret at a given X-ray energy for 1 minute, measure the residual charge, store the electret under a variety of conditions for 1 day, remeasure the residual charge and then repeat the sequence u n t i l nearly a l l the charge had been destroyed. Different electrets were irradiated at different X-ray energies. The results are s u m m e r i z e d i n figures 4.7-4.12. As was to be 13 expected from the results reported i n section 4.1 and by Anderson et a l . , storage in high temperature (36°C) and / or high humidity (100%) conditions DECREASE IN /o OF CHARGE DENSITY CW C H H-H co a. 3 o 1 1 Cu H cu cn 9 t> Co CU 1 1 "<! Cu H- 9 ® t> • H- t i fD V S V s CO i-i r t rf to H- Cu H-o o O H- O CO rt t i < < fD fD fD pu cn r- 1 U l fD •O O O rt 2 rt 3* 0 H fD t-i cn O fD rt 0 (W Hi ,—, cn 31 O rf r i 1 1 >. o a r-i fcr* 1 1 fD fD > o a. cn t-h V " — ' t- 1 H- O -O N> 3 fD 3 O O M 7? ?? O. fD <3 < 3 fD cr cw o U i t- 1 o • rt O X H fD 3 g CU > £ > > rt t-h cn rt " M fD £L H o a a Cf o i i • fD H E • < Q 1 1 Co fD 3 m a rf l-i CO • 3 V ! | f r - 3* o o r-h fo o H f-i CW < <! fD Cu Ul \* o zv wo r X-RAY EXPO/10' ' (R) Figure 4.8: dosimetric response fcr teflon electrets,charged with breakdown f i e l d and irradiated through Al shielding. After every irradiation electret store in Al box at lab. atm. for 1 day. ( A - A - A ) 20 kV 5 mA (o-o-o) 20 kV 10 mA (a-a-a) 30 kV 5 mA (©-©-©) 30 kV 10 mA U - A - A ) 40 kV 5 mA (B-B-B ) 40 kV 10 mA DECREASE IN % OF CHARGE DENSITY H-00 C H fl> -t> VD © el an do 1 1 fD CO o > O K * : 1 1 r t o > H l - l CD — ' CD H r t , r t tu H • r o CO C u H * 1 O O H - O ] CO Co ? f r t r t H r <1 < O fD CD ; >-t C u cn U l (D • O O r t O H « D * 3 • a H CO O CD C OQ H i l ^— s o • O c r 1 H J 1 O > o X r t I 6 CO H j ' •~— *—' r t - IT y-> ' H - o • P - r o U ) fD O O O A |-> oa. CD ? V o H - r - 1 < CD i - n OQ O U l o • c f i O i - i H CD 9 rj M > r t > l - t i CO C u r t •* CD o • — , i - l o a • 1 I CD O 1 B D <1 1 I CD Co 0 a ri 1 •—^ O cr - p - CO o o ri H ri OQ CU CD <3 <! C u C u H -r - 1 U l CO O r t H -O 3 100r X-RAY EXP0/10 '£ (R Figure 4.10: dosimetric response for teflon electrets,corona-charged, and irradiated through Al shielding. After every irradiation electret store in Al box at 100% R.H. at 23°C for 1 day. ( A - A - A ) 20 kV 5 mA (o-o-o) 20 kV 10 mA (•-.•-o) 30 kV 5 mA ( o - ® - a ) 30 kV 10 mA ( V - A - A . ) 40 kV 5 mA ( a - a - a ) 40 kV 10 mA 10 X-RAY EXPO/10 2 ( R) 16 Figure 4.12: dosimetric response for teflon electrets,charged with breakdown f i e l d , and irradiated through Al shielding. After every irradiation electret store in Al box at 100% R.H. at 23°C for 1 ds.y ( A - A - M 20 kV 5 mA (sD-Q-o) 30 kV. 10 mA (o-o-c) 20 kV 10 mA 40 kV 5 mA (o-o-o) 30 kV 5 mA (e-a- s) 40 kV 10 mA led to rapid charge decay. Storage i n other environments produced charge decays which were small in comparison. For a l l cases, the charge decay on storage decreased greatly as the -magnitude of the residual charge decreased. A l l electrets after the f i n a l irradiation were l e f t i n their respective environments for 6 days and no recovery were found, only slight decays (less than 2%) were observed. : V DISCUSSION The two methods of- electret formation used In this work have proven easy to implement and have yielded.electret. samples of uniform charge density with -magnitudes up to 6x10 coul/cm . Higher charge densities are undoubtedly possible but were" not tried in this work owing to the modifications that would have been necssary to the bias c i r c u i t of the charge measuring instrument in order to measure the charge. •From the results of the charge decay of -unirradiated electrets i t appears that storage in an aluminum box greatly enhances charge 13 retentivity, which agrees with the findings of Anderson et al . The aluminum box presumably prevents atmospheric electromagnetic radiation from interacting with the electret i.e. limits the ion-pairs created and also stops these energy sources from exciting molecules within the teflon material , thus causing detrapping and assisting the charges > to d r i f t at a faster rate. Charge decay curves showed that electrets charged by the breakdown f i e l d method yielded a higher "final"charge density than that of corona-charged electrets when both sets of samples were charged to about the same i n i t i a l charge level; also the charge-decay rates were not the same for the two charging methods. This latter difference would suggest that charge removal due to surface ion compensation was not the sole mechanism of charge decay, providing, of course, that the nature of both the charges and the traps were the same for samples prepared by both charging methods. Again with this latter proviso, i t does not appear that charge decay by SCL d r i f t i s the only mode of charge loss as such a mechanism would lead to "stabilized" charge levels of similar magnitude for samples charged to similar i n i t i a l levels.. Thus, on the basis of the charge-decay measurements 50 It i s not possible to enumerate the relative magnitudes of the contributions from sufface-Ion recombination and SCL d r i f t , nor can i t , be unequivocally stated that the trapped charges and their trapping sites' are the same for both methods. High temperature and high relative humidity both speed up the decay of electret charge. The temperature increase would cause thermal excitation and charge detrapping; i n fact, the technique of TSC (thermally stimulated currents) relies on this mechanism. The mechanism of decay enhancement on account of high relative luimidity is not well-defined even though the effect has been known for a long time. Presumably the effect i s related to water condensation on the electret surface, thus providing short circuiting leakage paths. The electrets fabricated in this work were stored in the open circuit condition; while for dipolar electrets the short circuit condition is invariably used. Short circuiting the homopolar electrets leads to a rapid annihilation of the electret charge, as i s perhaps to be expected from the fact that the charge apparently resides at, or very close to, the surface. The results of the X-ray irradition studies showed that electrets formed by both methods of charging showed a linear response of charge decay to X-ray exposure when the decrease in charge density density was 70% or less. A dependence of the charge decay on X-ray energy cannot conclusively be inferred from the data ( f i g . 4.3 and 4.4) owing to the limited accuracy of the measurements, however some dependence may be expected and this fact would lead to a slight limitation in the event of practical implementation of electret dosimeters,, namely some confusion as to the actual exposure received would result in cases where there was exposure to X-rays of widely different energies. Fabel and Henisch4" found for 50 kV X-rays the working exposure range (governed by the rate of charge decay) was 1 - 10^ mR. Homopolar electrets used in the present work were only studied over the range . 1 - ^ 15R, but presumably are sensitive alsp in the milli-R region. These figures refer to unshielded electrets and the exposure would appear to be of the right magnitude as far as personnel dosimetry i s concerned. For example, the maximum permissible dose (for the skin of the whole body) of ionizing radiation is about 30 rads (-33R)* in any. period of 52 consecutive weeks and for the whole body blood-forming organs and, 27 eyes is about 3 rads (=3.3R)* in any periods of 13 consecutive weeks Extension of the maximum dosage range to atound 1600 to 2000 R was shown to be possible by the use of aluminum shielding. Complete dosimeter fabricated in this fashion would have the advantage of better storage properties but vculd only be accui.ai.cly e f f e c L i v e in monoenergecic X-ray environments. 2 , In Fabel and Henisch's recent work on polymer, dipolar electret dosimeters a charge recovery of 5% after 500 storage hours was found. In the present work no such recovery was ever observed even after storage at 36°C and 100% R.H. for 150 hours. Only a slight charge decay (less than 2%) was observed. Breakdown field-charged electrets were generally slightly more stable than corona-charged electrets, but the former electrets performed very badly in environments of high, humidity. As a result corona charged electrets would be perhaps be more preferable to breakdown f i e l d charged-ones in X-ray dosimeter applications. *The conversion from the stated absorbed dose i n rads to an equivalent air exposure in roentgen was made assuming a value of 0.9 for the 29 conversion factor CONCLUSIONS Corona- and breakdown field-charging have been' found to be simple methods for producing semi-permanent charge densities in metal-backed teflon films. The decay of electret charge i s greatly affected by the storage medium, but can be much reduced by enclosing the electret in a. box. The electrets prepared in this work appear to be useful as possible X-ray dosimeters with attractive features being:-(1) charge decay i s linear with X-ray exposure rate (up to a loss of 70% of the I n i t i a l charge), (2) no recovery in electret charge between radiation exposure (at least over radiation' intervals of 6 days), (3) simple non-destructive read-out procedure, (4) s u i t a b i l i t y to personnel dosimetry in that doses of health hazard proportions are easily detectable and the material used i s compatible with human tissue as regards absorption properties. The undesirable features that need to be remedied or allowed -for in order to permit widespread use are:-(1) possible slight dependence of charge decay on X-ray energy level, (2) rapid decay in hot, humid and / or unshielded environments. 53 BIBLIOGRAPHY 1. H.J. Wintle, J. Acous. Soc. Am. J5, 1578, (1973). 2. G.W. Fabel and H.K, H e n i s c h , Phys. Stat. Sol. 6_, 535, (1971). 3. E.B. Podgorsak, G.E. Fuller and P.R. Moran, "Electrets:-Charge Storage and Transport in Dielectrics", M.M. Perlman ed., Electrochem. Soc. Publ., p.172, (1973). 4. H. A l l i x and W.C. Roesch, "Radiation Dosimetry", Acadamic Press: New York and London, chapter 13, (1966). 5. G.W. Fabel and H.K. Henisch, Sol. State Electron. . 13_, 1207, (1970). 6. R.A. Creswell and M.11. Perlman, J. Appl. Phys. 41, 2365, (1970),. 7. II.A. Smith and R.D. Neff, Health Phys. 19, 59, (1970). 8. R.A. Creswell, M.M. Perlman and M.A. Kabayama, "Dielectric Properties of Polymers", F.E. Karasz ed., Plenum Publishing Corporation: New York, p295, (1972). 9. J.D. Cross and R. Blake, "Electrets: Charge Storage and Transport in Dielectrics",'M.M. Perlman ed.,' "Electro chem. 'Soc. Publ., p. 300, U9 73;. . 10. G.M. Sessler and J.E. West, J. Appl. Phys. 43, 922, (1972). 11. R.A. Creswell, B.I. Gibbon,M.A. Kabayama, and M.M. Perlman, Telesis 2, No.l, p.21, (1971). 12. M. Ieda, G. Sawa and U. Shinohara, E l e c t r i c a l Engineering i n Japan 88, 67, (1968). 13. E.W. Anderson, L.L. Blyler, G.E. Johnson, "Electrets: Charge Storage and Transport in Dielectrics", M.M. Perlman ed., Electrochem. Soc. Publ. p.424, (1973). 14. J. Van Turnhout, "Electrets: Charge Storage and Transport i n Dielectrics", M.M. Perlman ed., Electrochem. Soc. Publ., p.230, (1973). 15. M.M. Perlman and S. Unger, "Electrets: Charge Storage and Transport i n Dielectrics", M.M. Perlman ed. , Electrochem. Soc; Publ., p,105, (1973). 16. H.J. Wintle, J. Appl. Phys. 41, 4004, (1970). 17. H.J. Wintle, J. Appl. Phys. 43, 2927, (1972). 18. G.M. Sessler and J.E. West, "Electrets: Charge Storage and Transport in Dielectrics", M.M. Perlman ed., Electrochem. Soc. Publ., p.292, (1973). 54 19. V.A.J. Van Lint and J.W. Earrity, IEEE Trans. Elc. Ins. 6^  111, (1971). 20. A. Charlesby, "Atomic Radiation and Polymers", Pergamon Press: Oxford, Chapter 3, (I960). 21. A. Reiser, M.W.B. Lock and J. Knight, Trans. Faraday Soc. 65_, 2168, (1971). . 22. G.M. Sessler and J.E. West, Rev. Sci. Instr. 42, 15, (1971). 23.. W. Reedyk and M.M. Perlman, J. Electrochem. Soc. 115, 49, (1968). 24. E.A. Ballik, J. Appl. Phys. 43, 302, (1972). 25. D.K. Davies, J. Sci. Instrum. 44, 521, (1967). 26. B.D. C u l l i t y , "Element of X-ray Diffraction". Addison-Wesley .-. Publishing Co.: London, Chapter 1,' (1959). 27. W. Rachuk, Manual of Radiation Hazards Control at theUniversity of British Columbia", (1972). 28. M. Ieda and G. Sawa, Japan J. Appl. Phys. 6_, 793, (1967). 29. Int. Comm. Radiobiological units and Meas.,Rept. 10b (1962). 


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