In the beam pipe of the positron Main Damping Ring (MDR) of the Next Linear Collider (NLC), ionization of residual gases and secondary electron emission give rise to an electron cloud which can cause the loss of the circulating beam. One path to avoid the

Ifweconsider,at rst,thatourSEYmeasurementiserrorless,onecanarguethatouractivationwasnotcomplete.Hence,someinitialairformedoxideisstillpresentandthesurfacechemistryisnotidenticaltoafullyactivatedNEG,henceexplainingourδafteractivationof1.3.InthiscasetheSEYshouldstillnotincreaseabove1.35[8]asthegetterdoesnothavemanypumpingsitesleft.However,thishypothesismighthavetobediscardedastheminimaltemperaturewearecertaintohaveachievedisatleast201 C.Asaresultofapoweroutageofanhour,duringtheelevendaysofmonitoring,aniongauge,aresidualgasanalyserandtheionpumpwereturnedo .Attheswitchonofthegauges,hot lamentsreleasegasesinthesystembeforebeingpumpedawaybytheionpump.Therecordedpressureinthesystemwasattheswitchonofthegauges～6.10 10Torr.Thiskindofincidentalsocanhappeninanaccelerator,anditisinterestingtoseethatthise ectleadstotherecontaminationofthegetter.Betweenthedayselevenanddaystwenty,thesystemwasusedtoXPSothersamples.Duetothetransferofsamplesfromtheloadlockchambertothemeasurementchamber,thepressureroseupmomentarilyto～2.10 9Torr.

IfCOorCO2exposuredonotseemtoa ecttheincreaseofδ,airexposuredoes[8].Itispossiblethatopeningourbakedmeasurementchamber(P～3.10 10Torr)totheloadlockchamber(P～9.10 9Torr),whichisfrequentlyopentoair,canbeconsiderasanairexposure.Hence,thereisnocontradictionbetweenCERN[8]andourresults.

XPSanalysiswascarriedouttoobservetheevolutionofthecarbonchemistryduringthis20daysandcomparedtotheXPSspectrumtakenaftertheendoftheactivation.TheXPSspectrumshowsaslightriseoftheoxygenpeakandadoublevalencycarbon1speak.Theelementalcarbonpeakhasanenergyof～285eV,theoxidizedcarbonpeakhasanenergyof～288eV,andtheatypicalmetal-carbidepeakisaround～283eV.Theoxidizedcarbonpeakaftertheendoftheactivationat288eVwasbarelypresent,Fig.16,blueplot.OnlyTheCpeakandTi-Cpeakispresent,theNEGbeingat180 C.Acleanly-scrapedcarbonsurface,measuredatroomtemperature,willonlypresentapeakat285eV.Aftersixteenhoursofpumping,greenplot,theoxidizedstateofthecarbonshowsup,andthecarbidepeakisgone.Thisstate,oxidized,becomesdominantafterelevendaysofpumpingandkeepincreasingafter20days,graycurve.InthelastcasestheNEGwasatroomtemperature.TheplotspresentedinFig.16area toftheactualdata.ThisincreaseoftheC-OpeakimpliesthattheSEYshouldalsoincrease,asisusuallythecaseforoxidizedmetalsurfaces.

Theroughness(R)oftheTiZrV lmisunknown.Thisroughnessa ectsnotonlytheSEY[4]butalsothepumpingcapacityandspeedoftheNEG.TiZrVdepositedonS/Sisrelativelysmooth(R 1)andismuchrougheronanaluminiumsubstrate[11].Let’sassumethatoursystemwasfortwentydaysat～3.10 10Torr,theCObeingpresentat15%ofthetotalspectrum,andthatthestickingcoe cientσforCOis0.4.Thestickingcoe cientdecreaseswhentheTiZrVNEGhaspumpedalmosthalfofamonolayer(1ML～1015molecules-cm 2forasmoothsurface)byafactor10afterreaching1ML,andbyafactor100after10ML[11].Amonolayerofasurface(MLs)canbede nedas:MLs=ML×R[10].Thefrequencyofcollisionν(molecules.s 1.cm 2)equation4andthetotalamountofCOpercm 2pumpedbytheNEGinonedayisgivenbyequation5.Pν=3.51231022(4)NCO=ν×86400×σ(5)MTwherePisthepressureinTorr,MtheatomicmassandTisthetemperatureinK.AfteronedayofpumpingtheNEGwouldhavepumped～0.6ML.Assumingthat