IEEE Transactions on Electrical insulation Vol. EI-17 No.4, August 1982

RADIATION-INDUCED CONDUCTIVITY IN POLYMERIC INSULATINGMATERIALS DEGRADED UNDER SPECIFIED CONDITIONS

Yoshiaki Nakase* and Isamu Kuriyama*

Japan Atomic Energy Research InstituteTakasaki Radiation Chemistry Establishment

Watanuki-machi, Takasaki-shi, Japan

and

Tohru Takahashi and Setsuya IsshikiThe Fujikura Cable Works Ltd.Kiba, KWto-ku, Tokyo, 135, Japan

ABSTRACT

Various polymeric insulating materials for cableswere degraded by simulated irradiation and environ-mental conditions for normal operation and underaccident at a nuclear power reactor.

Thermally stimulated currents were observed onlyin the crystalline samples, and the higher thecrystallinity, the larger the amounts of detrappedcarriers. The change of fine structure of the de-graded sample was investigated by the change ofX-ray crystallinity, melting behavior, and glasstransition temperature. The radiation-inducedconductivity was studied during irradiation anda decay curve was measured after the irradiation.

Analysis of the conductivity decay curve enabledus to detect at most four kinds of carriers withdifferent time constants. Long-lived carriers werehardly observed in the non-crystalline samples,while many were seen in the crystalline samples.With the decrease of crystallinity by degradation,only short-lived carriers were observed, indicatingthe existence of trapping sites for the long-livedcarriers in or around the polymer crystallites.Treatment of samples with high temperature steamand chemicals showed no special effect on thesamples except for polyimide which dissolved inalkaline solution.

I NTRODUCTI ON

The cables used in the primary containment vesselof a nuclear power generating station are required to

keep their integrity and reliability even during an

accident, for example, a loss-of-coolant accident(LOCA), as well as under normal operation.

It is well known that radiation-induced currents

are observed in polymeric materials during irradia-tion. The induced currents in a control cable are

analogous to a noise which stops the nuclear plant.Therefore, it becomes important to investigate theseinduced currents during irradiation, and also theirdecay after the irradiation in various polymericmaterials used for electrical insulation.

Several works [1-5] have been reported on the elec-trical conductivity of polymeric insulators. Theamount of carriers induced by the irradiation andtheir characteristics are strongly dependent on thechemical composition and also on the fine structureof the materials [3,5]. Theoretical considerationsof the radiation-induced currents have been reportedin the temperature regions above [1] and below [2]room temperature.

In the preceding paper [5], we investigated the de-pendence of the radiation-induced conductivity upontemperature (above room temperature), dose rate, andthe influence of ambient condition during measure-ment. In this paper, the radiation-induced conduc-tivity of various insulating materials is investi-

0U18_9-_67/8f /2/O8Q00C_zX0'60'0. 75 _ 982 IEEE

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Nakase. Kurivama, Takahashim and Isshiki: Radiation-induced conductivity in polymers

gated in samples degraded under circumstances speci-fied both by aging during the promised life (ca. 40years) of the plant and also by an accident. Here,an absorbed dose of 150 Mrad and a high temperaturesteam environment were considered.

Table 1. Samples with X-ray Crystallinity.

Materials and Chemical unit brevi- Crystal-ation linity(%)

High density HDPE 73

Polyethylene Low density -CH2-CH LDPE 60

Crosslinked XLPE 59

Polypropylene PP 52

-CH2-CH(CH3)-

Ethylene-Propylene Rubber EPR -O

-(CH2-CH2)n (CH CH)m

CH3

Polystyrene PS -10

-CH2-CH )-

Polyimide PI O

| N _CocIC l

Table 2. Specified conditions for Degradation ofInsulating Materials.

Items Abbrevi- Treatment Conditionation

Original 0 no treatment (as prepared)

High Temperature H 121°C, 7 days.(Thermal Aging) (Aging during Installed

life)

Y-ray Irradiation y 1 x 10 R/h, 150 hours,in air, at room tempera-ture.

y' 1 x 106 R/h, 186 hours.

Exposure to Steam S 155°C (Saturated Steamand Chemical at 0.4 MPaG), 7 day; in

Chemical Solution forfirst 24 h.(Aging during Accident)

EXPER I MENTAL

Sarnp Les

The samples studied were high density polyethylene(HDPE), low density polyethylene (LDPE), polypropylene(PP), ethylenepropylene rubber (EPR), polystyrene(PS), and polyimide (PIm). The chemical formulas ofthese materials are presented in Table 1. The X-raycrystallinities of these samples [6] are also demon-strated in order to partially characterize thesamples.

Sheets of 1 mm thickness of samples were preparedfrom the fundamental compoundings except for PIm. Inthe case of PIm, a sheet of 30X\50 um thickness wasprepared by a solvent casting process.

The conditions for the sample treatment, or for thedegradation, are listed in Table 2. The originalsample (0) was degraded by a single or a combinedtreatment by the conditions specified in Table 2,where high temperature (H) is assumed to simulate theaging during the installation life of the plant (40years), and the irradiation with y-rays at a dose of150 Mrad (y) to simulate exposure during accident,and the exposure to high-temperature steam and chemi-cal solution (S) to simulate the LOCA environment ofa pressurized water reactor. These conditions arespecified in the IEEE standard [7]. In the irradia-tion, 50 Mrad during the normal operation period waseliminated, since degradation of samples was so muchat 200 Mrad that conductivity measurement was notpossible in many samples. The samples degraded underthese conditions are shown by the symbols 0,H,y, andS as shown in Table 2. In the case of degradationby a combined condition, the symbols are connectedwith a (+) sign in the order applied to the sample,for example, H + y means y-ray irradiation (y) afterthermal aging (H).

Apparatus and Measurements

The radiation-induced currents were measured withthe circuit illustrated in Fig. 1. The sample sheetwith evaporated gold electrodes was placed in anapparatus shown schematically in Fig. 2, where theevaporated gold electrode was prepared prior to fur-ther treatments. This electrode was repaired, ifnecessary, after treatments.

The sample system was exposed to 60Co y-rays at adose rate of 105 R/h at room temperature under re-duced pressure.

The applied voltage during conductivity measure-ments was +500 V for 1 mm sheet and 200 V for 50 pmsheet, the electric field was less than 4x104 V/cm,in which range Ohm's law was satisfied. Each conduc-tivity measurement was performed with two or threesheets of the same type to confirm the reproducibility.

Dark currents (before irradiation) and Comptoncurrents (without electric field) were evaluated asshown in the preceding paper [5]. These measurementsof the degraded sample have been done more than 2hours after the treatment shown in Table 2. Duringthe (y) treatment, the electrodes of the sampleswere grounded.

* Composition of Chemical0.064 mol of Na2S203 inNaOH to make pH 10.5 at

Solution; 0.28 mol of H BO3demineralized water, anm250C.

The induced conductivity showed an equilibriumvalue after an irradiation of 30 s, so the samplewas irradiated for only 3 min during measurementsas shown in Fig. 3 to avoid radiation degradation.

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IEEE Transactions on Electrical Insulation Vol. EI-17 No.4, August 1982

DryBattery Irradiation Room

Electromagnetic _Oscilographyra

D.C. Anp. ->

Vibrating-readElectrometer L _

Specimen

Fig. 1: Block diagram of measuring circuit forinduced conductivity.

y-ray

Vacuum Pump

Thermally stimulated currents were also measuredin the irradiated sample at the last stage of thetreatment in order to evaluate the amounts of re-leased charge. Here the heating rate was 20C/minunder atmospheric pressure.

The change of the fine structure of the degradedsamples was evaluated by measurements of x-ray cry-stallinity, enthalpy of melting, melting temperature,and glass transition temperature.

RESULTS

1. ThermalZy StimuZated Currents (TSC)

It is well known that a stimulated current can beobserved on heating the irradiated sample [8]. Inthis work, the currents were measured also in thesample degraded by (y) treatment. It is, therefore,expected that carriers must be trapped at sites pro-duced by radiative degradation as well as those pre-viously present. Such trapped carriers were releasedby heating the samples. The stimulated conductionwas observed in samples heated at a constant rate of2°C/min up to the temperature just before melting.The samples were kept for half an hour at a maximumtemperature, i.e. 1200C in HDPE and PP, and 1000C inLDPE in order to release the trapped carriers as muchas possible. The conductivities of the sample werealso measured on cooling from the maximum to roomtemperature. Example of TSC measurements are shownin Fig. 4. It is clearly demonstrated that the con-ductivity decreased during half an hour at a maximumtemperature, so almost all carriers are released ex-cept those removed to deeper traps.

The amount of the released charge is calculatedsimply from the area surrounded by heating and cool-ing curves, after taking care of sample thickness.In the lower temperature region, heating and coolingcurves do not coincide with each other. However, weevaluated the area by connecting simply two curvesof the starting point of the heating and the endpoint of the cooling.

radiation-induced conductivity

>14.)

ad(t)

4.)

0

0 3

Time (min)

Fig. 3: Definition of 00, aT,, and ad (t), in the

radiation induced conductivity and the irradiationperiod.

The TSC results obtained in the samples degradedunder various circumstances where irradiation is atthe last stage are summarized in Table 3. Severalunknown factors are included in the calculated area,but a relative comparison between samples is possible.

In non-crystalline samples such as EPR, PS and PIm,TSC was not observed even after 150 Mrad irradiation.

2. Radiation-induced Conductivity

The radiation-induced conductivities of the samplesdegraded by a single or a combined environment des-cribed in Table 2 were measured at room temperature.

Fig. 5 shows the examples of radiation-induced con-ductivity in the samples as prepared (original) anddegraded. The induced conductivity increases con-siderably just after the irradiation to reach anequilibrium state (aeq). After the end of the irra-diation, the conductivity decreases rapidly in thenoncrystalline samples, and rather slowly in thecrystalline samples, though in a different way depend-ing on the degradation treatment. The characteristicsof the decay curve seem to correlate with the chemicalstructure of the material.

Fig. 2: Apparatus formeasurement.

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Nakase, Kuriyama, Takahashi, and Isshiki: Radiation-induced conductivity in polymers

.'4-14 X1 '4-16 /16 1. t

1i0l/ 101

10 HDPE 0- HDPE

10 -ol

0

60 80 100 120 40 60 8001

HDPE HDPE

Fig. 4: ThermZZ stmZae codleHveyo

10

2°C/m* inar :hatn uv, -

_0/

H -12- 102-4.

4)

*0

106080/ 0 120 40 6/ 0 2

10g 4 Theray ystm ae cndcivteso

cooling curve. SamplZe notations are indlicatedin the Figure.

Table 4 summarizes the radiation-induced conductiv-ity Aa defined as the difference between aeq and theconductivity before irradiation (due to dark current)ao i.e. Aa a-ceao. When ao is negligibly small

eq/1

(less than lxlO- 7Q-1-cm-l), we obtain Aa-eq., butwhen ao becomes rather larger due to the degradation,reflecting the change of fine structure of thesamples, then Aa decreases comparatively.

The decay curves of conductivity after irradiationhave been investigated theoretically and emypirically[3,9,10]. When the carriers were estimated to beelectronic, the following equation was presented basedon the band model [1, 3,9] .

1t0 a = 1 + t (1)

0d

where aeq and ao are the equilibrium conductivityduring irradiation and the conductivity before irra-diation, respectively, ad(t), the conductivity attime t after the end of irradiation (t-O), T, abso-lute temperature, and k, a constant.

The decay curves of equilibrium induced conductivityas shown in Fig. 4 did not follow the Eq. (1). Thekinds of carriers, therefore, have not been clarifiedin this way. Harrison's method [10] was applied toexpress ad(t) as a superposition of the conductivitiesdue to several carriers with different time constants,irrespective of the kind of carriers.

TABLE 3Amounts of Detrapped Charge (Q) in the CrystallineSamples Irradiated up to 150 Mrad, Calculatedfrom the Area Surrounded by Heating and CoolingCurves of Thermally Stimulated Conductivity.

Heating Rate: 2°C/min.

ad(t) a = y a. exp(-t/T )

i=1(2)

Here t is time from the end of irradiation (t = 0),and Od(O) aO = aeq - ao = Aa.

The results of this analysis for ad(t) obtained bythe measurement at room temperature are summarizedin Table 5. ad(t) was a superposition of at mostfour components with time constants which were in theorder of 5 to 15 sec (T1), 20 to 50 sec (T2), 70 to130 sec (T3) and 800 to 1500 sec (C4). In the cry-stalline samples such as PE and PP, all four compon-ents are obtained, but in the non-crystalline samplessuch as EPR, PS and PIm, only one or two componentsare observed.

The changes of the fine structure of the crystallinesamples were examined by the following methos; cry-stallinity k by the X-ray diffraction method, meltingbehavior as enthalpy of meltingAHm and as the start-ing temperature Ts of melting, and glass transitiontemperature Tg by torsional braid analysis.

Samples Treatment Q

(165 C/cm3)

Y 44

HDPEH+y74

HDPE | S + y 63

S + H+y 80

y 2.4LDPE S + Y 0.5

H + y 0.7XLPE S + H + y - 1.5

.p y 3.6P H + y 1.6

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IEEE Transactions on Electrical Insulation Vol. EI-17 No.4. August 1982

Table 4. Radiation-Induced Conductivity (ha) defined as the Difference between

the Equilibrium Value (aeq) during Irradiation and the Value (o ) due to the

Dark Current (before irradiation) in the degraded samples.

Dose rate: 1 x 10 R/h, at room temperature.

Treat- Sample namesment HDPE LDPE XLPE PP EPR | PS | PImEabbre- -15 -1- -1

viation. 00 ~ AO Conductivity ( x 10 XL- cm)(viation,) AG a0 ha aO Au °a ha ao Aa C AG ao ha0 ~~0 0 0 0 0 0

0 < 3.0 < 2.5 < 3.2 < 0.9 < 1.8 < 0.3 < 0.1

H < 1.1 o*0.4 1.7 < 0.9 < 1.8 < 0.8 < 0.2

Y 0.3 0.6 0.4 0.3 - < 0.4 < 1.2 < 0.8 < 0.1

H + Y 0.2 0.5 < 1.8 0.05 1.1 < 1.1 < 1.2 < 0.1

Y + H 4.0 8.0 22 4.0* _ 0.08 1.6 < 1.4 < 0.8 < 0.2

S < 10* < 6.1* < 4.7 < 8.5* < 11* **

S + Y 0.1 1.8 < 1.3 < 2.9 < 2.9 _

S + H < 0.9 < 1.3* < 1.9 < 1.3 < 1.3 -

S + H + Y 0.05 0.5 < 1.1 0.02 0.9 0.02 0.9 _

Note: < : less than 1 x 10 17 (1cm 1

* : Sheet prepared from the degraded sample after the treatment.

** : Sheet was not prepared.

: Sheet was destroyed during the treatment.

: Measurements were not done.

These results are summarized in Table 6. The HDPEsample after (H + y) treatment shows a little highercrystallinity than that of the original, while Ts,AHm and T remain almost constant. On the otherhand, HDPg after (y + H) treatment shows quite low kand Tg, and moreover, no clear melting endotherm inthe heating curve to give either T. or AHm.

DISCUSS ION

The amount of the released charge Q of HDPE is re-

markably large compared with that of other samplesas seen in Table 3. The crystallinity of HDPE ishigher than other samples, so samples with high cry-

stallinity may give large Q, suggesting the existenceof trapping sites in or around polymer crystallites.This suggestion can be supported by the followinginvestigations: 1) the liberation of trapped carrierswith increasing temperature takes place mainly fromthe amorphous part and the irregular part on the sur-face of crystalline regions in polyethylene [3],2) long-lived carriers are released by melting thepolyethylene sample irradiated by e-beam [11], and3) two peaks of thermoluminescence in irradiated poly-ethylene on heating are observed at about 50°C andabout 70°C, the former peak is due to the trappedcarriers in the amorphous region and the latter tothe crystalline region [12].

The thermal aging, i.e. (H) treatment, was performedat 121°C, which is higher than the melting point(103°C) of LDPE, so the sheets of LDPE could not keeptheir form during the treatment. Accordingly, we

were unable to measure the induced conductivity inLDPE degraded by (H) and hence (H + y) treatmentsas shown in Table 4.

The same situation of sample deformation was ob-served in the other samples, except XLPE, degraded by(S) treatment. However, in the cases of HDPE, LDPE,PP and EPR, it was possible to make a sheet again

from the degraded samples by pressing at 120°C for1 min and then to measure the induced conductivities.

In the cases of PS and PIm, sheets were not preparedagain. The sample sheets of PIm were strongly hydro-lized and lost their shape, so the conductivitymeasurement was not possible. Therefore, we examinedthe hydrolysis of the sample produced by condensationpolymerization (PIm and other polymers) in alkalinesolution at 170°C for 2 hours. In PIm, a weight lossof 37% was observed, 27% in polyamideimide, more than50% in polyesterimide and complete loss, i.e. 100%,in polyester. No weight loss was observed in PE, PP,EPR, and PS.

It is well known that PIm is the insulating materialwith superior heat-and radiation-resistant properties,but the test mentioned above indicates that the weakpoint of PIm is when the cable suffers a LOCA condi-tion.

The samples degraded by (S) treatment were reformedinto a sheet except for XLPE, PS, and PIm as men-tioned above. These samples showed remarkably largeinduced conductivities Au, but these conductivitiesdecreased after (H) treatment (S + H).

It has already been reported that water in PE in-creased the number of carriers under strong electricfield [13]. So in order to clarify the reason forlarge induced conductivity in the sample degraded by(S) treatment, the degraded samples were heated at80°C for 2 howrs, then aeq at room temperature wasmeasured and observed to be quite similar to that ofthe original samples. Since the heating at 80°Cshowed no fine structure change even after 10 hours[14], the large induced conductivities of the sampledegraded by (S) treatment can only be due to thepresence of water in the sample but not due to anystructure change. It is, therefore, supposed thatthe sample degraded by further treatment after (S)shows mainly the characteristics due to the degrada-tion by the subsequent treatments.

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Nakase, Kuriyama, Takahashi, and Isshiki: Radiation-induced conductivity in polymers

10-o'.-mX

10

3

ca

4m I -16

EPR

-(yX

fHyh5+S4T

S+H5I L * L -O 5 10

Time t/win) Time (t/min) Time (t/min)

Fig. 5: Radiation-induced conductivities duringirradiation, and their decays after irradiationin samples degraded under various conditions.Dose rate: 1 x 105 R/h at room temperature.The samp le name and the conditions are indicatedin the Figures.

Original, a:y, O:H, X:S, e:y+H,

O:H+y, L&:S+y, *:S+H, A:S+H+y

In the crystalline samples, i.e. PE and PP, the Auvalues after (y + H) treatment are quite high, proba-bly due to the trapped carriers which have not beenreleased during (H) treatment. In the non-crystallinesamples, no special increase of Aa in those treatedby (y + H) was observed. This indicates the absenceof trapped carriers.

The induced conductivity in the samples of LDPE andHDPE degraded by (y + H) treatment was of same orderbefore, during and after irradiation as shown inFig. S. It is supposed that carrier implantation canoccur to the same order or higher than production ofthe radiation-induced carriers. However, further in-vestigation must be performed in order to understandthese phenomena.

TABLE 5

Analysis of Decay Curves of Equilibrium InducedConductivity of Samples Degraded Under

Various Circumstancos. Equation (2) is used.

unit:ifC!cm/Samples Treat- Aa 01 12 C03 o+

ment x1015 X10o15 X10o15 X10 15 x10o15

O 3.0 1.0 1.2 0.5 0.3H 1.1 0.9 - 0.1 0.1

HDPE y 0.6 0.5 - 0.1 -

H +y 0.5 0.5 - - -y + H 8.0 - 4.2 3.8 _

O 2.5 - 0.8 0.9 0.8LDPE H (1. 7j 0.7 0.4 0.3 0.3

y 0.3 0.1 - 0.2 -(XLPE ) H + Y (1.8r 0.9 0.3 0.3 0.3

y + H, 4.0 2.0 2.0 - _

o 0.9 0.6 0.1 0.1 0.1H 0.9 0.7 0.1 0.1 -

PP y 0.4 0.4 - - -H + y 1.1 1.0 0.1 _y + H. 1.6 1.6 - - -

O 1.8 1.4 0.1 0.1 0.2H 1.8 1.7 0.05 - 0.0 5

EPR y 1.2 1.2 - - -

H + y 1.1 1.1 -. - -

y+Hi1.4 1.4 -

o 0.3 0.3 -

H 0.8 0.7 -0.1 - -PS y 0.8 0.8 -- -

H :V ya 1.2 1.0 0.2 - Xy + H 0.8 0.8 -- -

o o.i 0.1 -- -H 0.2 0.2 -- -

Pi y 0.1 0.1 -- -HH+y 0.1 0.1 - - -

y H, 0.2 02 -- -

*:Values obtained in XLPE.

As the degradation proceeds in the crystallinesamples, short-lived carriers become predominant.This is typical in the non-crystalline samples. Asmentioned above, long-lived carriers might be trappedat or around the crystalline region. In the case ofEPR, long-lived carriers were observed in the ori-ginal and (H) treated. A small amount of microcry-stallites in EPR might be produced by a comparativelylong methylene sequence in the main chain, which wewere unable to detect by X-ray diffraction [15].So long-lived carriers might exist in the originaland (H) treated EPR, but not after y-irradiation,since the amount of these microcrystallites mightbe decreased.

In PS and PIm, only a short- lived carrier is pre-dominant even in the original material. These poly-mers contain aromatic groups, or conjugated doublebonds, in the molecular chain, which show the reson-ance stabilization of the excited states [16]. Inother words, conjugated double bonds may localize thecarriers with a short time constant.

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IEEE Transactions on Electrical Insulation Vol. EI-17 No.4, August 1982

TABLE 6

Changes of X-ray Crystallinity (K), Starting Temper-ature of Melting (TS), Enthalpy of Melting (AHm) andGlass Transition Temperature (Tg) of CrystallineMaterials for Electrical Insulation

Samples Treatment K (%)T (OC) AH ( J/g) T (OC)

O 73 123.5 96 -18

HDPE H + y 79 122.0 88 -15

y + H 49 no endotherm -37

o 60 103.0 54 -45LDPE y+H 4 3y+H 48 no endothermn -3

O 52 153.0 71 -20

'1 50 121.0 113 -*PP H + y 57 130.5 113 - 8

y'+ H 36 130.0 71 *

y: 150 Mrady': 186 Mrad* : Unable to measure because of brittleness.

Error estimation; K: ±5%, TS: t0.50C, Mm: +5%, andTg: l0c.

It is clearly noticed from Table 6 that the crystal-lites in HDPE are destroyed to a large extent by(Y + H) treatment as seen in the decrease of crystal-linity, in the disappearance of the endothermic peak of(Ts and AHm) the heating curve, and in the shift ofthe glass transition temperature to a lower tempera-ture. By using (H + y) treatment, there is lessdestruction of crystallites in HDPE.

In LDPE, the destruction of crystallites by (y + H)treatment is again inferred, since the decrease ofcrystallinity and no endothermic peak are observed,similar to the case of HDPE.

On the other hand, in the case of PP, no disappear-ance of the endothermic peak was observed, but agreat decrease of crystallinity in the sample de-graded by (y' + H) treatment was seen, while a smallincrease of crystallinity in the (H + y) treatedsample was observed. Small changes in Ts, AHm, andT occurred in the sample degraded by the (H+y) or( + H) treatment. These facts indicate theexistence of small size crystallites in the degradedsample. The wide-angle X-ray scattering patternof degraded PP showed the characteristics of 6-form,while the original showed the a-form as describedalready [17], indicating drastic change of the finestructure by (y') and (H + y) treatments. Thea-form was also observed in the sample degraded by(H) and (y' + H) treatments.

It is suggested that the order of applying the de-gradation steps plays an important role in the finestructure change of crystalline samples. The activespecies produced by irradiation accelerate the de-gradation of samples during thermal aging aftery- irradiation.

It can be concluded that the large increase of aoand also of Au in the sample degraded by (y + H)

treatment may be due tc the presence of small-sizecrystallites. The short-lived carriers may perhapsbe trapped on the surface or interface of such asmall-size crystallite, while long-lived carriersmay be associated with a crystallite itself.

CONCLUSION

1. Thermally stimulated corductivity was observedin samples irradiated up to 150 Mrad, reflecting therelease of trapped carriers. The amounts of carrierswere calculated from the area surrounded by the con-ductivity curves. It was noticed that the higherthe crystallinity of the sample, the larger theamounts of the released carriers.

2. Radiation-induced conductivity was quite highin the crystalline sample degraded by the combinedcircumstances of heat and radiation. Here, higherconductivity was observed in the sample degraded bythermal aging after irradiation than the sample de-graded by irradiation after thermal aging. The orderof applying degradation conditions to the sampleshad a serious influence on the changes of their finestructure, which was confirmed by crystallinitymeasurement, melting behavior and change of glasstransition temperature.

3. In the non-crystalline samples, a small in-crease of induced conductivity was observed afterdegradation.

4. By the analysis of decay curves from equili-brium induced conductivity, it was suggested thatlong-lived carriers must be trapped in the crystal-lites. By decreasing the crystallinity of thesample, the amounts of long-lived carriers were de-creased, while those of short-lived carriers in-creased. That is, the increase of the induced con-ductivity of the degraded sample must be due to theshort-lived carriers.

S. In the non-crystalline samples, long-livedcarriers are hardly trapped during irradiation, butshort-lived ones are trapped. The presence of con-jugated double bonds may localize the short-livedcarriers.

6. Materials prepared by condensation polymeriza-tion such as polyimide are easy to hydrolize underalkaline conditions even if they are superior forheat and radiation resistance.

REFERENCES

[1] J. F. Fowler, "X-Ray Induced Conductivity in In-sulating Materials" Proc. Roy. Soc. London, AVol. 236, 464 (1956).

[2] K. Yahagi and A. Danno, "Gamma-ray Induced Con-ductivity in Polyethylene and Teflon underRadiation at High Dose Rate", J. Appl. Phys.,Vol. 34, 804 (1963).

[3] K. Yahagi and K. Shinohara, "Effect of CarrierTraps in Polyethylene under Gamma-Ray Irradia-tion", J. Appl. Phys., Vol. 37, 310 (1966).

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Nakase, Kuriyama, Takahashi, and Isshiki: Radiation-induced conductivity in polymers

[4] T. J. Ahrens and F. Wooten, "Electrical Conduc-tivity in Insulating Materials", IEEE Trans.Nucl. Sci., Vol. NS-23, 1268 (1976).

[5] T. Takahashi, T. Shinmoto, S. Isshiki, Y. Nakaseand I. Kuriyama, "Radiation-Induced Currents inVarious Polymeric Materials", Trans. IEE ofJapan, Vol. 101-A 167 (1981).

[6] A. Weindinger and P. H. Hermans, "On the Deter-mination of the Crystalline Fraction of Iso-tactic Polyethylene from X-Ray Diffraction"Makromol. Chem. Vol. 50, 98 (1961).

[7] IEEE Standard 323-1974, "IEEE Standard for TypeTests of Class 1E Electric Equipment for NuclearPower Generating Stations", The Institute ofElectrical and Electronics Engineers, Inc.,New York, N.Y., 1974.

[8] T. Mizutani, K. Ishii, Y. Takai and M. Ieda,"Carrier Traps in Poly-p-Xylene Films", Japan.J. Appl. Phys. Vol. 19, 1073 (1980).

[9] K. Yahagi, K. Shinohara and I. Mori, "InsulationResistance in Polyethylene and Teflon underGamma-Ray and Electron Beam Irradiation", DenkiGakkai Zasshi, Vol. 83, 292 (1963).

[10] S. E. Harrison, F. N. Coppage and A. W. Snyder,"Gamma-Ray and Neutron-induced Conductivity inInsulating Materials", IEEE Trans. Nucl. Sci.,Vol. NS-10, 118 (1963).

[11] M. Ieda, M. Kosaki and U. Shinohara, "ElectricConduction of Irradiated Polyethylene" J. Phys.Soc. Japan, Vol. 18 1103 (1963).

[12] A. E. Blake, A. Charlesby and R. J. Randle,"High Temperature Thermoluminescence in Poly-ethylene" J. Polym. Sci. Polym. Lett. Vol. 11165 (1973).

[13] K. Yahagi and K. Shinohara, "On the ElectricalConduction in Polyethylene in Air and in Vacuumat Room Temperature", Denki Gakkai Zasshi (J.Inst. Elect. Eng. Japan), Vol. 90, 741 (1970).

[14] Y. Nakase, H. Arai and I. Kuriyama, "DTA ofPolyethylene obtained by Radiation-inducedPolymerization", J. Macromol. Sci. Phys., Vol.B10, 41 (1974).

[15] I. Iwai, S. Inoue and H. Inaba, "The Crystal-linity of Ethylene-Propylene Terpolymers",Nippon Gomu Kyokaishi, Vol. 42 101 (1969).

[16] G. Friedlander and J. W. Kennedy, "Nuclear andRadiochemistry", Wiley, New York 1955.

*Present Address: Japan Atomic Energy ResearchInstitute, Osaka Laboratory forRadiation Chemistry, Osaka, Japan

Manuscript was received 24 October 1980, in finaZform 7 January 1982.

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