Pulsive Radiation Induced Current in Electric Insulators (I)

Shigeru Or.rnn, Iirrnihilio'lsrnronl. l'lLtsuo 1-,rn,tta,

I(eistrke K.cl.lB:'l' \, Slrigclii N rri,rlrutrr and'l,rslrio Yost ttnr

Reprintecl from

Ar.rnual Report of the Radiation'Center of Osaka Prefecture

irol. 8 1967

Railirlirm Center oi Ostka Prefeclrirc

Sakei. C'is:rkr. .laprir

55

Pulsive Radiation Induced Current in Electric Insulators (I)

Shigeru Oxam, Kunihiko TsuvoRt, 'latsuo 'leeare,

Keisuke K,cwA.s,c.re, Shigeki N.qx.qN,IuRA, and Toshio Yosuto,q'

(Receivecl December 25, f967)

The radiation induced current (RIC) in several kinds of insulator s.as measured at the beginningof irradiation and its pulse shape was observed rvi-h an oscilloscope. The radiation used was produced

by an 18 \{eV electron linear accelerator. \,'ariations of the RIC pulse sl'rape *,ith time and rvitl.r

temperature showed aimost the same feature in some cases. The pulse shape rras found to be depend-

ent on direction of the crystai axis of the sampie against the radiation. Some cliscussions are triecl on

the pulse shape b1, using the image o{ traps and positive holes.

Bflfuffi4^tzTrbtl6t\)r7ffiriBifi,f,Ef;€Erh(I) .rlli'i: ii, ii.''t'|\fr, ';,,,n&:1)<, ,rtll.:j:fi, ,i,fiitiii, 5!r,.-..Lr(=l,,,O,iiiiiEi*lrlil:-rt.(, &qfi',rt;r't,+iiifr,:-.r),1:.'!,iffi1!o,"/it;I'iJtZljiil.,lrr;!iljl:tt(J:lIj)3,.-":.

v., z01t:tvTit.lrJ.'t*rj,*!-t. >t1 ^=-t:itr1'FL, -,:ii:ji:{Ajl$lHo,i4rEt,t,.:ty,,U-at..(:!&_F_,Ji1,l:. ;it

rri+liiii+'Eiiio,:t>zilir2Dl[.ltoH14]iBf;iF]friji&lnlLlilil,Eii-Hi.Loj'fji:ilFlDi:Itfi.Jir;-*Zit,7,t]ij, f.<-rr'rl:;;1+1:-:ir-C_hilid?t/:" hkhll,fra^N)jti,llfi,l;/il,o)L,hljr)il,jl:iJ++iii:/iBllfio$7.:lil::t;tj-.f,..i16a'):llrtil)!i.lri Lr, | 1 .y 7'trtrjLlr Do,Fl{iirit --1:E;'!l/y;fit}i7.

I. INTRODUCTION

In the previous papert) we reported the result ofa studl' on the radiation induced current (RIC' ,

caused b1.- X-ra-vs and electrons, in electric insulatorssucir as fuseri quartz, mica. some kinds ol ceramicand sapphire. The measurements in that rvork rvererestricted to the RIC after the irradiation and theone rvhen it became steadl during the irradiation.

In tl.ris paper measurements of the RIC at the be-ginnig of irradiation and the pulsive RIC ale descri-bed. \iariations of the RIC pulse shape rvith timeand .,vith temperature l-rave been investigated. Some

discussions are tried b-v using available tl-reories onthe interaction of radiations rvith matter2,3,'1,s).

II. METHOD

The sampie used was 10mmX20mm area by 3mmthickness and botl-r of its surfaces were painted rvithsilver paint baked at 600'C. The side of the samplervas also coated rvith silver paint to get good ther-mal conduction and to eliminate surface leakage ofthe electric current.

Sample holders {or }reating and cooiing are shorvr.r

in Fig. 1. In order to heat the sample thermal con-cluction from electric heater was usei and cooling

u.as made by liquid r.ritrogen. 'I'he signal i-rom a

thermocouple attached to the base of the saml;le u,as

recorded by a pen-recorcler to monitor the ternllera-

tr,rre of the sample. The vacuum chamber arrd theconditions of electron beams used for irradiation rverethe same as described in the previous paper. 'lhemeasuring circuit also u,as almost the same rviththat clescribed previousll,: the pulse sirape observatiur-r

rvas made with a oscilloscope by making use of a

voltage drop across a 100O resistor connected inseries to the signal 1ine.

Fig. 1. Sample holders usecl for the measurement oi theRIC with a vacuum of 10-umm Hg.

56

Table 1. Average current (A) o{ the llIC measured rrith picoammeter at the beginning of irradiation before

being sufferecl by radiation darnage.

Bias

Voltage

,1000v -- 4 )< 10-6

RockCrystal

I

RockCr1'51n1

lliusecl

QrartzCaF, LiF 'leflon Ceramic

- 2>..L0-7 I :, to 'ELECTRON(10 pA)13 )Ie\r r 1000v :l )< 10-6 7 x 10-7

TxLo-E 1 z*ro- ]

9t110-i: 4x10-' 2x10-? 2><70 7

1x10{ i I z*rc-

3 x 10-?

-6 x 10-?

-1000 \- I" x 10 7

.\.l(AY

(30pA) I o 3x1o-'g

13\IeV -1000V i^rO:;

4x 10 10 5)< -2 x 10-e 2.5'/.70 tr 1 x10 1' 7 x 10-1

I ).:10-1' I x 10-1 4 x 10-10 3 )< 10- 7x10 1

2 )i l0-, I )( 10-10 6 x 10-, 6 x 10-10 7 x 10-10 Bx10-

iil. \'IEASUREN,{ENT AND RESLILT

A. -\i'trttge RIC at tlrc be g'irning' of the irraclidtiottThe average RIC u.as measured altet 7-2 sec {rom

the beglnning of irradiation to get the RIC, not af-fecteci br- t1-re temperattLre rise ancl racliation clamage.

The results obserr-ec1 rvith a picoammeter at room

temp€rature zrnd liqr,rid-:-rltrogen temperature rvitir dif -

{erent llias voltages ale listed in Tairle l. \-alues

for tht zero bias voltage are considered to be the

sum c-ri secondar-"- emission current and inner clisplace-

ment cur:rent.'lfie vair,res for -1000V have nonsystematic'ap-

pearance. This featr:re can be explained by theresult of oscilloscope observation of the RIC pulse

shapes irrod,.rced b-v singie pulse operation of the

Linac.Figure 2 shows a typicai pulse sl.rape of the RIC:

the part A is the prompt component of ti're RIC clur-

ing the radiation pulse of se\.eral ps rviclth, and thepart B is the dela]'ec1 component rvith a c1eca1' cons-

tant oi: a ferv tens to a ferv hr,rndreds ,as. \{easure-ment rvith current meters like the picoamn'reter means

to get average reading oi the sum o{ A and B, so

tlrat not onl1 the absolr-rte value but also the sense

o{ the current may var}- according to properties ofthe sample. N{oreover, the value of the currentshorv-c r-ariation ."vith irradiation time and temperature.

B. \iridtiott oJ' thc ptrlse shape dut'ittg'corttittztotr.si n'ad iatiott at t'ooilL lcl1l?eraho'e.

During a fer,v to some tens minutes of continuous

irracliation u-ith electrons, the RIC pulse shape varieclgradr.raliy and reachecl the final shape as shou'n inFig. 11. Usually, the RIC pulse shapes {or rock-crystal, CaF:, and fused quartz varied rvithin a fervminutes, u'hile the variation of those for LiF and

I:ig. !. ,-\ -.cheil:rric pri:e-.1::.pe or ihe R1C at the be-

girrnirrg o: irra,iir.tion.

BiosVcltoge-lc,oov

oun n f-i n [1

+IOOOV

Fig. Li. \-:rriation oi the RiC puL,.e'shape -rith time clur'

ing irradiation.

ceramic took rather long time. Variation of the pulse

shape {or Teflon could not be observed because its

l-reatproof cl-raracteristic did not permit the use of in-

ter.rse radiation. At any time during the observation

of pulses as shown in Fig. 3, the pulse shapes forother bias voltages could be observed interchangeablS

on cl-rarlging the bias voltage.

9C603C

Rock crystoi ( perpendiculor io oxis )

l2t] aa-

3 )l 10-7 - 2 >( 10-i 1.7 x 10-7

0 1 x 10-?

57

C. Elfcct of tlrc crystal aris ort tlrc puls<t slupe'utriat iott

Effect oi the crystal axis on the variation ol: the

RIC prilse shape was investiga.ted {or rock-crystal'

Figure .tr shorvs typical pulse s1-rapes observed after

tu,o minutes from the beginning of the irracliation''lhe le{t group represents the result observed jn the

rock-cr1'stal irradiated perpendicuiar to the optical

axis, the midclie shor,l's the pulses from the crystal

irradiateci parallel to the axis' and the rigl-rt s1.ron's

the pr:1se,s from fused quartz rvithout optical axis.

i)iffererces due to the orientation of the cr1*stai

axis can 1.re seen in this case though there u-ere some

negative report on it.6'?) Other cr1'sta1s may also

shor'r' the sirnilar effect.

B,os Rocl. cr lsTol R)ck c,,slc Fused ouorlz\o1cqt l:E'Ee'd,-u''orloo/rs Lpo'or.e "oo,isl

*"-, -- , -- ]--lla-,)-\\/

vt /t i'\*/!

+1000 ij'

Fig. 4. Difference o{ the RlC pulse-shapes due to the

change of crystal axis, observed a{ter ferv minu-

te- irradiation.

D. Effect of the tenrpertitut'e on the RIC.

Three type of phenomena due to temperature

change .r'ere observed.

(1,r Decr.ase of the RTC.

In Fig. 5, the relation between temperature and

RIC is shorvn. Gradual decrease of the RIC w-ith

increasing temperature for the fused quartz and cera-

mic may partly be ascribed to the temperature de-

pendence of the secondarl' electron emissionsr. Con-

siderably large cha:rge of the RIC in CaFz will be

discussed later.

(2) Disappearance of the taii.

The long tail observed in all the samples as de-

scribecl in Sec. JIIA almost disappeared at liquid-nitro-gen ternperature. This fact will be considered in thenext section relating to fr-rndamental processes of the

RIC.

(3) Variation of pulse shape due to temperature rise.

The RIC pulse shapes of CaFz observed with singlepulse operation under heating condition are shown

in Fig. 6. The tendency o{ the variation of thepulse shape with the temperature looks like the var'

Co F''-r BtosVatoge-IOOOV

\t Deformed\\

Fused quorlz

,0"5 roo 200 300 400temp, i Ul

l-ig. 5. Temperature dependence of the RIC' Irradia-

tion r,vas nacle only a fet' seconcls during each

rneasurement. Large decrease in the case of

CaFz tras clue to pulse-shape variation as shou'n

in Fig. 6.

iation lr,ith tin're in the case of continuous irradiation

at room temperature. Though details of variations

w-ere not the same, this is a very remarkable pheno-

mena. The same tendency was observed also in cera-

mics.

E. Contribution of the secondary electron emission'

Contribution o{ the secondary electron emission tothe RIC r,vas investigaJed b1, placing a grid aror:nd

the sample. Figure 7 shorvs the relation betu'een

bias voltage of the grid and RIC- -\s can be seen

{rom this f,gure, complete elimination of the second-

ar-v emission contribution is impcssible rvith a bias

voltage of a ierv thousards volts. becar:se there ishigh energ.,' component of secondar-,- electrons rr,'hich

car-rnot be repelled ri-ith the zrbove voltage. Ho',r'ever,

the f ieid of the 1ou- and high energ-v components ofsecondarl' electrons are considered to be constant dur-

ing the beam pu1se, so that they do not play impor-

tant p?rts in the qualitative expianation of the pheno-

mena. The contribution of the secondary electron

emission has been found to be of the same order of

magnitr-rde with the other component of RIC observa-

b1e witl-r bias voltages of about 1000 V.

1V. CONCLUSION

According to available theories on the interaction

of electrons with matter, most of the energy lost

will be used to the eievation of eiectrons from the

valence band into the conduction band. The produc-

tion of these internal electrons rvi11 cause the crea-

tion of positive holes are expected to be caught b-v

traps rvhich have been produced by lattice defects,

such as vacant lattice point, interstitial ions, domain

to'

5lu

Ceromic

t

58

a

dIo

o

a

!

o

!n>

U

E

F

r::1

i

l:'

;:rl

t;ti:i!+

triiltl)ttl

,+;i:::i:*.i4't. .".i!

r il{

":.1

::l:i.i::iit:l::i;idi,

::rx;Ei:;t:l i i

::;tiiFii:,.:.IjM:e

1i.:;1-iliE:::;:.i:d!i

I',ii

l"r

!t

{:ti:;i n

qsiiti;.;i

:1):ii$ij

xiirEr

<4UE

t3cl- lt-)co- oE!

59

capture to r'leeper traps. l)isappearance of the longtail at liqLrid nitrogen temperature was possibly be

caused b--v much delayed release of trapped charges.Noll', the RIC observed can be decomposed as fo1-

1or,rs:

llIC = (secondary electrons) * (displacement currents)* (conduction electron) * (positive holes)

* (released charges from traps)Mcasurement of the pusive RlC is considerecl to be

a possible nerv method of studying electron solid inter.actions. N{ore investigations to get qllantitative re-sults and to extend the application of the r,tbove me-

t}rod are nor.v in progress.

ACKNOWI,EDGE}IENTS

'lhe autl'rors rvoLrlcl like to ti-r:rnk the Science andTechnologl- Agencl' oi the Government for fi-naIlcial aicl to this u-orli. Thev gratefully acknou,le-clge to Dr. S. Sano for his sr,rppling most oI ceramictest pieces arrcl I)r. \'I. Kittrgarva for his use{ul disr:ns-

-sion.

liEI.'EIi EN(]I]S

1) S. Okabe, K. 'lsr-imtiri. 'I'. Tabata and S.

Nakamura. Ann. Ilep. of ORC. 7, 31 (1966)

2) I1. Pethe. .\lrn. cl. Phl-sik. 5, l-125 (191101

ll., D. E. \f,lr1dridge. Ph1s. Rev. 56, 5ti2 (1939)1. \\-. -qircclilei', J. .\pp. Ph)-s. 9, 635 (1938)5 K. Hecht. Zeits. f. Physik, 77, 235 (1932')

tr, I-. Pensak, Phys. Rev. 75, 472 (L949)t') D.\I.J. Compton, J. App. Phys. 36, 2434 (1965)8) \1. Kno11, O. Hachenberg and J. Randmer, Z.

Plrys, I22, 137 0944)9) I(. G. N{cKay. Ph-vs. Rer,. 77, 816 (1950)

r ^^l: ^.1

|

o* ' "3OOO r' O -IOOO O-rooo o tooo 2000

Bios Volioge (V)

Fig. 7. Relation betrveen bias voltage of tl-re gricl andthe RIC, shou.ing the contribution o{ 1ol'-energvseconclarv emissior.r.

boundaries, cr1'stal boundaries, or by impurities. Theclensity of these tralx has been estimated to be 1016

-1017/66s s;. The variation oI pulse shapes for 1000

V bias voltage u,'ith time, as shorvn in !'ig. 3, is ex-plained as foilolr.s: At the beginning of irraciiatior.rmost of the excited electrons ri-ill be rrairired at thesame rate in the rvhole pulse rvidth of the beam,and the sum oI the concluction current and the se-

condary emission current proclnces flat pulse shapes

as shorvn at the left end of Fig. 3.

L)uring the irradiation traps are graclually filled rvithcharges and the number o{ charges being released u,'illincrease. The inclined top of pulses appearing in therniclclle of Fig. i:i is consic'1erecl to be concerned ,"vith

filling of traps ancl releasing from sl'raliorv trag;.The last pulse shapes o{ Fig. i3 ma1. lre explainecl

rvith deelxr traps. a1reacl1- fill..d rvith charges, ancl

shallorv traps u-hich rep€e-ts trapping ar-rd relea-.ing oIelectrons and l.roles. Similarit-; oi ihe p-rlse shalrsobserved in the samples r-rncler high ter-r-rperatures rvitl-r

last ones in lrig. 3 may have been caused bv the con-linuorrs tlrermal release of the traptleil charges. ancl

llrt: inclinrrtion ol pulse t()p rrir)' lrrve lrt-en rluc to the