Photoyields and secondary electron emission (SEE) characteristics were determined under UHV conditions for a group of insulating materials used in spacecraft applications. The SEE studies were carried out with a pulsed primary beam while photoyields were obtained with a chopped photon beam from a Kr resonance source with major emission at 123.6 nm. This provides a photon flux close to that of the Lyman alpha in the space environment. Yields per incident photon are obtained relative to those from a freshly evaporated and air oxidized Al surface. Results are presented for Kapton, FEP Teflon, the borosilicate glass covering of a shuttle tile, and spacesuit outer fabric

Gordon, W. L.

Hoffman, R. W.

Yang, K.

English

NASA Technical Reports Server

qN 8 5 - 2 2 5 1 2 ..... :
i _+"
i-Nii ELEC-TRONYI ELDS.-TROH-SPACEcRAFTMAT_.IALS_...................
_}i!:: K. Yang, id. L. Gordon, and R. W. Hoffman
_T_ Case ides.tern Reserve Univer-st.ty
F._,:, ,
_,,_;, Cleveland, Ohto 44106
determinedunder UHV conditionsfor a group of insulatingmaterialsused in space-
_:__i_;_. craft appl.ications.The SEE studieswere carriedout with a.pulsedprimarybeam
• while photoyieldswere obtained-witha chopped,photonbeam from a Kr resonancesource
ill' with major emissionat 123.6 nm. This providesa photonflux close to that of the 11
• Lyman_ in the Space envi4_onment.Yieldsper incidentphoton are obtainedrelative
_:= to those from a freshlyevaporatedand air oxidizedA1 surface. Samson'svalueof
_:" _: _ 2,4% is taken for the A1 yield. Resultsare presentedfor Kapton,FEP Teflon,the
"_"_'_'_" '__'_:,) ._m_ borosilicateglasscoveringof a shuttletile, and spacesuitouter fabric" l
:_:::_L INTRODUC_ON ....
_._:_ In the use of NASCAP (ref. 11, a computercode which simulateschargingof a
_'!" three-dimensio_al object in space, _-t is _portant to have data on electron yields
fron.the variousmateria4scomprisingthe spacecraftsurface. In an on-going:program
°_4_;_: (ref,2) pulsedprimaryelectronbeam methodshave bee_ developedto avoid charging
_"_'_ effectsin yield measurementsfrom insulatingsurfaces. These studiesare carried
_:__ out in an ultra high vacuum system,employinga commercialdoublepass CMA which per-
"_,, mits sequentialAuger analysisof the surfaceand targetcurrentmeasurementsof
=_°,,,:> electronyield data as a functionof the primaryenergy,EP. This pulsedbeam tech-
:=,_. nique has been extendedto permit vacuumultraviolet(VUV)photoyieldmeasurementsof
_., these insulatingsurfaces:Kapton,Teflon,the borosilicateglass surfaceof shuttle
-_'/ tile, and the outer fabricof spacesuitmaterialu.', •
•,_'_' EXPERIMENTALTECHNIQUES
).,._ Targetcurrentmeasurementsof secondaryyield by pulsedelectronbeam methods,
L_-'_: introducedin a study of insulatingmaterials,have alreadybeen discussedin refer-
_;, ence 2. We describehere the adaptationof this approachto obtainphotoyields. The
_ spectralrange of interestis restrictedto the VUV becauseof the photoemisSion
!,) ': thresholdof most materials• We have chosen to carry out these preliminaryexperi-
i....... ments with an OpthosVUV kryptonSourcepoweredby a Kiva Model MPG 4 microwave
i_:_i. generatorproducingresonancelines at I16.6 and 123,6nm with a relativeintensity
. .... of approximate4y 1 to 7 respectively (or 1 to 15.after transmission through the MgF2
i_ . windows). This providesa reasonableappt._ximat_onto the relevantportionof the
_ ;; ,_ solar spectrumwith its intenseLyman H_ line.i_,
_) Work performedunder NASA Grant No. NSG-3197
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https://ntrs.nasa.gov/search.jsp?R=19850014201 2020-03-22T18:44:22+00:00Z
!As illustratedin figure I, the Kr sourc_i_ mountedon the end of a cyllndrica]
housingcontaininga camera shutterto permit Choppingof the 1.ightbeam, The b_am
travel.s_n an argon atmosphereto avoid air absorptionand entersthe UH_._ystem
througha MgF2 window. The incidentbeam intensityis determinedby irradiatinga
freshlyevaporatedand air oxidizedA1 film of _ 150 nm thicknes_deposite_on a
glass substrateai_ using Samson'svalue (ref, 3) of 2.4% yield per incidentphoton
at the Lyman H_. The yield trom freshlyevaporatedsamplesis typiCally50% greater
than that from samplesexposedto ai_ for about 10 minutes,which cOrrespondSto an
essentiallysaturatedvalue. It is this resul_which we assumecorrespondsto Sam-
son'syield but have not placedthis on an absolutebasis as yet. Samplesto be
studiedand the Al detectorare mountedon the faces of the six-sidedrotatable
carousel.
A cylindricalcup collectorelectrodewas mountedfacing and surroundingthe
sampleon the target as shown in figure 1. The collimatedUV beam i_radiatesthe
sampleby passingthroughan apertureon the axis of the collector. Our usual pro-
" cedurewas to measurecollectorcurrentwith the collectorbiased+ 22.5 V relative
to the groundedtarget. Typicalcurrentsrangedfrom 0.5 nA for relativelyhigh
;J yield materialsdown to 5 to I0 pA for the lowestyields. Althoughthe shutteris
i capableof I millisecondpulse lengths we have been able to avoid chargingwith
;. pulses as long as I second. This has permitteduse of a fast responsechart recorder
i_.:-_ to obtaina plateauvalue for the collectorcurrentduringeach pulse.
i:
MATERIALS
Only insulatingmaterialswere studiedin this investigationand all samples
were obtainedfrom NASA LeRC. Table I summarizesthe raaterialstudiedand includes
the preparationof rear surfacessince good eleCtricalContactto the target is
important. Approximately2 cm x 2 cm sampleswere u_ed in the photoyieldstudies
_ with approximatelyl cm x l cm SiZesemployedin electronyield work. DuSt particles
were removedby blowingdry nitrogenacrossthe surfacebut no other cleaningsteps
were used.
Squaresampleswere cut from the _ .5 mm thick borosilicateglass surfacecoat-
ing of the shuttletilewith a thin rotatingdisk. The silicafiber backingmaterial
was brushedaway to permit good contactwith indiumfoil in whiCh the samplewas
embedded. An AI backingwas evaporatedon the rear surfaceof the outer fabricof
the spacesuitlater to providebetterelectricalcontactwith the target. AS dis-
cusSed later,this fabric and to a lesserextentthe shuttletile surfacematerial,
exhibitedq_alitat_velystrongerchargingeffectsthan did the Kapton and Teflon
sheet. The greateraverage"thickness"of the cloth (_ I mm) means a _educedcapaci-
tance and thus an increasedchargingrate, per currentpulse.
RESULTSAND DISCUSSION
- Photoyields
ResultS,expressedas yields per incidentphoton,are summarizedin table II
with uncertainties based on the scatter of repeated measurements. Systematic uncer-
taintieS, such as the assumptionthat Samson'svalue of 2.4% yield appliesto a
53B
.......... ° "'....... 00000006-TSF 12
il_ _i" !
, freshly ewporated.._nd air oxidized A1 _urface or the effects o_ surfac_ Contaminao
tion, if present, are not _lcluded. Light pulses used in obtaining these results
= " were generally 1 second in duration. A test for charge accumulation was done by
:;_ repeated_rradiationin the same locationon tilesample. He_e we discoveredthat for
,=_- the intensitiesemployed,several(up to 10) pulses could be _Hvered to Kapton
before the yield wou_ begin to drop - such a drop is taken as our operatia_aldefi_
niti_nof charging. For the white shuttletile, however,chargingbegan after _nly
one or twe pulses a_d$or the spacesuitfabricon the first pu1_.
"_": We experiencedsome difficultiesin measuringtl_ehighlyinsulatingmaterials
_.T_,_ having relativelylow photoyields. This is due, in part, to low currentvaluescor_
_' ' respondingto the smallyields such that the valuesare comparableto the noise. In
_i . the low yield insulatingmaterialswe also found a trend towardseven lowervalues
when we chose a modificationof figure1consistingof groundingthe collecter,bias-
ing the target - 22.5 V relativeto ground and measuringthe targetcurrent. This
effect has not been explored_I any detail as yet and may be spurioussince it did
not appearfor the higheryield materials. We concludethat the resultsfor both
" Teflon and the spacesuitfabric are preliminaryand will requirefurtherstudy.
_o_ _ SecondaryElectronYields
_. Results,using targetcurrentmeasurementswith pulse beam methodsas described
in reference2, were obtainedfor the materialsin table I and are presentedin
figures2 through8 where only the total SEE coefficient,a, is displayed. The pri-
mary electronbeam was slightlydefocussedto _ 2 mm diameterand was moved to
, variouslocationson the sampleduring a series _f measurementsto reduce surface
__ chargingeffects. Kaptonand Teflon are di-splayedin figures2 and 3 respectively, i
,.- In figure4, figures2 and 3 are comparedusing a normalizedscale on which a/amax is
' plottedvs. EP/EPm an_ it is clear that they are in close agreement. This is in
.::_ contrastwith the _rly work of Willis and Skinner(ref.4) in which the Teflondata
....: are well above the Kaptonresultson a similarnormalizedplot. Since very different
-_ sampleswere measuredin these two investigationsand differentsurfacecleaning
_ techniqueswere employed,we merely note these differences. Figures5 and 6 present
the SEE coefficientsfor the borosilicateglass shuttletile surfacesin both as-
receivedand sputteredcondition. Some difficultieswere experiencedwith charging
_ . of these samplesbecauseof their thickness (_0.5 mm) but we feel the results,
while preliminary,are representativefor reasonsdiscussedlater. Resultsfrom a
sampleof microscopecover glass are also includedfor comparisonin figure7. In
each of these cases, Sputteringsufficientto removethe nominalsurfacecontamina-
<_ tion has reducedthe SEE coefficientssubstantialy.
!
_" Figure8 containspreliminaryresultsfor the outer fabricof the space suit. I
As noted earlier,the large samplethicknessincreasedthe tendencyto charge. SEE J
-_ resultsfor all materialsreportedhere are summarizedin table Ill to providea
generalcomparison. EP! and EPII are the primarybeam energiesfor which the total
SEE coefficientis unity.
It is importantto note that our usual methodof obtainingSEE data for insulat-
ing materialsappearstO reducechargingproblems, in biasingthe targetnegatively
then positivelyto obtain both the SEE coefficent,a and _, at a given primaryenery
EP, the surfacecharge is reducedin the positivebiasedsituationby electrons
attractedback to the surface. This is illustratedby the record of multiplepulses
- (at EP = 500 eV) deliveredto the same locationi_:the samplein figure9. Here, tile
seriesof dots, althoughthey show substantialscatte_,tend to drop away much less
' 539
I
"_
O0000006-TSF13
Vi=mF ,'
,_les Of + pointsfor wllichthe target remained,- _ from the initial_ value than do tile e_"
negati-velybiased, The latterpointsapproacha unity value mu(_hmore closely,indi-
: caringthat charginghas broughtthe surfacepotential,cl_e to the EPII, the secmld
_z_,cC cross_overvalue. Tilei_11ediatp,effectof low energyelectronsfrom a flood gun is
_, also illustratedand appearsomewhatmore clearlyin the highlych_qed case than in
_,- the alternatingbias mode. One can concludefrom these re3ult_tl)at,tlioughtiledata
_: show substantial_c_tter,alternatingth_ targethia_ helps to reducesurfacecharg-
:--_ ino _ff_cts,
CONCLU510N$
i--/ '
- We have demonstratedreasonable.resultsfor both ele._tronand photoyieldsfront
_= highlyinsuiatingmaterialswhere suchdata are not usuallyavailable. Pulsedit-
" radiationmethodswere used to minimizechargingeffects.
-_" :
-: REFERENCES
-,_ •
I-C I. Katz,K., Cassidy,J.J., Handell,M.,l.,Schnuelle,R.W., Steen, P.G., and Roche,
-_,.::., J.C.:,TheCapabilitiesof the NASA ChargingAnalyzerProgram. Spacecraft
-_i_i' ChargingTechnology- 19'18,NASA Conf. Publ. 2071, 1979, p. lOl.
,_. 2. K_ainsky,I., Lundin,W,, Gordon,W.L., and Hofi_man,R.W.: SecondaryElectron
;-_-_: EmiSsionYield,SpacecraftChargingTechnology- 1980. NASA Conf. Publ. 2182,
1981,p. 179.
_ _,. 3. Cairns,R.B. and Samson,J.A.R.:Metal PhotocathodeSas SecondaryStandardsfor
AbsoluteIntensityMeasurementsin the VacuumUltraviOlet. O. Opt, Soc. Am.
56, 1966, p. 1568.
3. Willis, R.F.-and Skinner, D.K.: Secondary Electron Emission Yield Behavior of
.....,- Polymers Sol St. Comm. 13, 1973, p, 685
TABLE I - HATERIALSSTUDIES
o.. Material Back Surface
ThermalBlanket
.... Kapton,Z mil A1
:': FEP Teflon2 mil AglInconel
.: ShuttleTile ,
White borosilicateglass outer coating In Foil
, Blackborosilicateglass outer coating In foil
_ Space Suit COmponents
Outer fabric EvaporatedAI
(ST11G041.=OI) Iayer
Embeddedin 0.5 mm in foil withoutcoveringsamplesurface.
' 540
..... ' O0000006-TS F14
i._i_ C
' ' TABLE II - PHOTOYIELDS
i-_ i
Material Phot_ield I%)
_!i Kapton 0.65 + 0.03
_= FEP Teflon 0.04 T 0.03
i-_ White ShuttleTile surface 0.2 _ 0.03i/
Z_ . Black Shuttle Tile surface 0.3 _ 0.03
)a=_i Outer-fabricof space suit 0.06_ 0.03
!_;; TABLE Ill - SUMMARYOF.SEE RESULTS
i _ "
,=_._ Sampl-e %ax EPmax(KeY) Ei(KEY) E2(KeY)
_=_i,_ Kapton I.8 O.2 0.05 O.7
_,_ FEP Teflon 2.4 0.3 0.05 1 9
.... Black Space ShuttteTile 2.3 0.5 0.06 2.7
"_J:':'._ White Space Shuttl.eTile 2.3 0.5 0.06 3.5
_ Microscope Cover Glass 3.8 0.5 0.05 5.0
:_;i!. ST11 G041-01...................................2 3 O 3 O.08 3.2
i .
_ H-F2
_ ,-_-,._._ "'_",-,-.-, "_",-t_°"llr-- ni '%
i__!_ U IG
i 2" T
PlCOAMMETER
i.. :
_-i _ rnr_
V- .
i ' Fig. _. Schematic diagram _t the arrangement fc," measuring photoyields.
i=
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" ++';* la.SB I I , I I I I I , , I I , I ,I
_.B8 1,.gB 2. gE 3. isls 4. 8B - 5. 8B 5. m_ _3
. .',_'/' EP (EV)
Fig. 2. Total SEE coefficient for Kapton, as. received surface. Single pulse method was
_;*_:: used wtth Ip • 60 nA.
.._.'_"+,
L
-::5 .i
I_': 3"B_I I I u' I I I_'] I I I I ' I
II
_}':: E _'¢/
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+.
+_+++. - +
+
t-4
_ 1. lab -
," 4.
_. EE 1. I_B _. BB _,, E_ 4. _la _. lab G.BlaE_3 ........................
EP (EV)
Ftg. 3. Total SEE coefficient for FEP Teflon, as-received surface. The single pulse
method _aS used _tth [p = 60 hA,
,+
,+
, 542r
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ic,f,j-'--,r
;_.,..,,+ 00000006-TSG0,

:_i!_
i__
L
• _'_P _-"'T' ! 1 s k.... "°*_ _ - I _ ': t • ;
3,_J_ " ,t AR, PeoaJ,_,ad- --
i , .;_ X SpuLF, e,_Qd
2.813 , •
• <:
(,3 |.UO _,
X t
.
i- 4,
x x
]
X _ •
; X
.__ a. BB I i I I I • I I ! ._ t _,.L-._
iP. 813-- I. IDB ,_. I_la 3. _IB 4. 8_ 5. nO G, J_E _3
_ -- Fig.6. TotalSEEcoefficientforblackborosi|icatesurfaceoftheshuttletile.The
.:!. single pulse method was used with I : 60-70 nA.
:,m_j
!_j_,
4.08
i I I I I I I I I I I
i f 3. 5g -- • . • U,.pukF.e_-.d -
. xX
2. 5B -
e
'< X
X(3 ;Z.g_ - •
i_: +
I. 5_ -- X
X
1. I_i_ - X " -
x
8. BB I. 8B 2. B_ 3. Bg 4. B_ 5. 8B 6. 8gE IP3
EP(EV)
Fig. 7. Total SkE coefficient for microscope Slide cover g]ass, as-received surface.
The sing}e pulse n_thod was used with ]p = 70 hA.
i-_;_ _
544
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'_'_' _'_ .... I I ..... '"'l ' I ! " " I "' I I '" --I .... I I
2. 5_
._ l ?..,,.ID_ e "
i_ e'e 0
;,_ Ui
: t. ol_
_j
-_i':, 1a.58 *
_2 _'. EP (EV)
_ii Fig. 8. Total SEE coefficient fo_ the outer fabric of spacesutt material, as-received
. _ surface. The stngle poise methodwas used with lp = 60 nA.
_3. .
%*
• A],c..ernail;,lL_.o -SOY, 4,56Y T,m,_j,et Bsee
2. 50 --
F,-G. On 3oeo
--; 2. IDB -
; I
•.........-., i.;;±,=:::*- t. Ei_ - ,_ ee •*e_. Ouo
' : Flood G.. O. 5.,_0
- ": I_. 5JD --
# OF ELECTRON BEAH PULSES
F19. 9. Changestn tota] SEE coefftcte.t Of the sputte,'ed surface of the whtte boros_|_-
cate gla_s shuttie ttle co&ti,_ wtth reoea_ed DUlS_nOtO illustrate charqinq
effects _n two operating mode_. The single pulse method was used w_th Ip = 40 nAw_th [P = 500 eV. ....................... '......................
;i_. 545
00000006-TSG05


Electron yields from spacecraft materials

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