Secondary Electron Yield Measurements of TiN Coating and TiZ(9)

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

Figure10:Spectrumofasec-

ondaryelectronbeamfroma

300eVincidentprimarybeam

impingingonaTiNonAlsub-

stratesample

[13]Figure11:TiNthicknessmea-suredbyx-ray uorescenceoftheTiK-lineoffourBNLsamples

4ResultsandComments

TheTiNcoating,madeatBrookhavenNationalLaboratory(BNL),wasdepositedontoaluminiumalloysubstrates,followingthesamerecipedescribedin[9].FortheSpallationNeutronSource(SNS)project,thecoatingwasdoneonS/S.AccordingtoBNL,theexpected lmthicknessisaround1000 A.WemeasuredtheactualsamplethicknessesusingXRF,Fig.11.

TheprincipleofXRFistocollectsecondaryx-raysgeneratedandexitingthesamplewhenbombardedbyprimaryincidentx-rays.Thesecondary uorescenceyieldishighestsomewhatabovetheK-absorptionedgeofTi,soprimaryx-raysof7keVareusedfortheexcitationoftheTi-Kαline(4.51keV).ATiN lmofsimilarknownthickness(byRutherfordbackscatterspectrometry,performedatanoutsidelab)isusedtocalibratethetechnique.

TheheightofthemeasuredTi-Kαline,usingaSiLi-driftedx-raydetector,islinearlyproportionaltothenumberofTiatomsinthe lm.ResultsareshownforafewsamplesinFig.11.

SEYmeasurementsresultsofsixdi erent”asreceived”TiNsamplesaredisplayedinFig.12andFig.13.Theelectronbeamimpingingontothesurfaceisoftheorderof2nAoveranareaoflessthanamm2.Typicallythebeamsizeisbetween0.2mmto0.4mmindiameter.ThelowcurrentisnecessaryinordertoavoidsurfaceconditioningduringSEYmeasurement.Thesizeofthebeamcanbecheckedbyusinga uorescentscreen,orisinferredfromsecondaryelectronmicroscopicalimaging(availableonthemeasurementsystemandusedtopreciselychoosethepointofSEYmeasurement).

TheSEYofthesamplesvariesfrom1.5to2.5,withthethickest lmsamplesdisplayingthelowerSEY.However,wehavenodataconcerningtheroughness,andtheroughnesscanbeafactorwhichcanchangetheSEY.Thecommonlyacceptedhypothesisisthat,foragivenchemicalsurface,theroughersurfacehasalowerSEYthanasmootherone

[4].TheirregularityintheSEY,atnearmaximum,forsampleTiN#4andTiN#6canbeduetoanonuniformspotemittingsecondarieswithtwodi erentyields.Theresult