Analysis of laser-generated plasma ionizing radiation by synthetic single crystal diamond detectors

ContentslistsavailableatSciVerseScienceDirect

Applied

Surface

Science

j o ur na l ho me p age :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Analysis

of

laser-generated

plasma

ionizing

radiation

by

synthetic

single

crystal

diamond

detectors

M.

Marinelli

a

_,

_E.

_Milani

a

_,

_G.

_Prestopino

a

_,

_C.

_Verona

a,∗

_,

_G.

_{Verona-Rinati}

a

_,

_M.

_Cutroneo

b

_,

_L.

_Torrisi

b

_,

D.

Margarone

c

_,

_A.

_Velyhan

c

_,

_J.

_Krasa

c

_,

_E.

_Krousky

c a_{Dip.diIng.Meccanica,UniversitàdiRoma“TorVergata,”Roma00133,Italy} b_{Dip.diFisica,UniversitàdiMessina,S.Agata98166,Italy}

c_{InstituteofPhysicsofASCR,Prague,CzechRepublic}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Available online 15 June 2012 Keywords:

Singlecrystaldiamond Diamonddetector Laser-generatedplasma Ionizingradiation Time-of-fightspectrometer

a

b

s

t

r

a

c

t

Diamondbaseddetectorshavebeenusedinordertoanalyzetheionizingradiationemittedfromthe laser-generatedplasma.Highenergyproton/ionbeamsweregeneratedatPragueAsterixLaserSystem (PALS)Centrebythesub-nanosecondkJ-classlaseratintensitiesabove1016_W/cm2_.

Thetesteddetectorsconsistedofaphotoconductivedevicebasedonhighqualitychemicalvapor depo-sition(CVD)singlecrystaldiamond,producedatRome“TorVergata”University.Theyhavebeenoperated inplanarconfiguration,havinginter-digitizedelectrodes.

TheproposeddiamonddetectorswereabletomeasureUV,X-rays,electronsandions.Theyhavebeen employedintime-of-flight(TOF)configurationandtheirreliabilitywascheckedbycomparisonwith standardioncollectors(mostlyusedatPALS).Boththeforwardandbackwardexpandingplasmawas characterizedintheexperiment.Theresultsindicatethatdiamonddetectorsareverypromisingforthe characterizationoffastprotonandionbeamsproducedbyhighpowerlasersystems.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Theinvestigation of fastion emission from laser plasmasis crucialfor future applicationsof laser-acceleratedionbeams in differentareas,suchaswarmdensemattergeneration,probing ofhighelectricandmagneticfields,nuclearapplications(compact neutronsources,isotopeproduction,fissionreactions),medicine, astrophysics,etc.Recently,thegenerationofionswithenergyof fewMeV/uandcurrentdensitiesoffewA/cm2_{bytheuseofthe}

sub-nanosecond,kJ-classlaseratPragueAsterixLaserSystem(PALS) laboratoryhasbeenreported[1–3].Thebeamparametersare esti-matedwithapplicationofvariousdevicesandtechniquesusually operatedin“singleshot”mode.Inparticular,time-of-flight(TOF) spectrometryisaneffectivediagnostictool[4]sinceit provides time-resolvedmeasurementswhicharefundamentalforadetailed studyofionbeamparameters,suchasspeciesandcharge-states, theirkineticenergy,andtotalcharge.However,thereexistvarious difficultiesinsuchdiagnostictechniqueswhenhigh-energylaser pulsesareused,e.g.highelectromagneticnoise,overlappingofthe signalduetophotopeak,difficultiesinseparationoftheionsignals fromit,breakdownsatveryhighioncurrents,lowsensitivityto fastelectronsandprotonsortolowchargestateions.Toovercome

∗ Correspondingauthor.

E-mailaddress:[email protected](C.Verona).

the above measurement difficulties, the development of new detectorsisincontinuousgrowing.Inparticular,semiconductor detectorsseemtobeverypromisingfordetectionoffastprotons andionbeamsproducedbyhighpowerlasersystems[5–7].Among them,theuseofdiamonddetectors,producingsignalsproportional totheenergydepositedbytheincidentradiation,isveryattractive becausethesedetectorsarenotsensitivetothevisiblelight(band gap5.48eV).Moreover,highthermalconductivity,highresistivity, lowdielectricconstant,highcarriermobilityandradiation hard-ness[8,9]suggestthatdiamondisanidealmaterialformonitoring theradiationemissionfromlaser-generatedplasmas.Thanksto thefastresponsetime,detectorsbasedonnaturalandsynthetic diamondhavebeenemployedinTOFtechniquesinseveral exper-imentsreported in literature, in particular for thedetection of electrons[10]andheavyions[11].Detectorsbasedonhighquality chemicalvapordeposition(CVD)singlecrystaldiamondhavebeen developedattheDepartmentofMechanicalEngineeringof Univer-sityofRome“TorVergata”forbothnuclearparticles(thermaland fastneutrons,highenergyions,chargedparticles)[12]andphoton radiations(X-rays,UV,etc.)[13].Suchdiamonddetectorshavebeen permanentlyinstalled atJointEuropeanTorus(JET)tomeasure UV/X-rayradiationandneutronsproducedbyJETplasma demon-stratingafasttimeresponse,goodsensitivity,highsignal-to-noise ratioandagoodstabilityandreliabilityoftheresponse[14].

In this work, single-crystal CVD diamond detectors are employed in TOF technique to monitor the different plasma

0169-4332/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.142

products(i.e.photons,electronandions)generatedbyhighpower pulsesfocalizedondifferenttargetsatPALSlaboratoryinPrague. Bothforwardandbackwardemissionoftheplasmawas character-ized.Inparticular,differentthinhydrogenatedtargetswereused intheexperimentinordertogenerateforwardprotons,i.e.,proton emissionfromthereartargetsides.Adifferentelectrode configu-rationwasadoptedforthesinglecrystaldiamonddetectorswith respecttoRefs.[12–14]inordertoimprovethetimeresolutionand betterseparatethedifferentreactionproducts.

Inthiswork,single-crystalCVDdiamonddetectorsoperatingin planarconfigurationareemployedinTOFtechniquetomonitorthe emissionofphotons,electronandionsejectedfrombothforward andbackwardemissionoftheplasmaproducedatPALSlaboratory inPrague.

2. Materialandmethods

The diamond detectors were fabricated in a planar inter-digitizedelectrodeconfiguration.Anominallyintrinsicdiamond layer, which operates as the detecting region, was homoepi-taxiallygrownby MicrowavePlasmaEnhanced ChemicalVapor Deposition(MWPECVD)ona4mm×4mm×0.5mmcommercial low-costsyntheticHighPressureHighTemperature(HPHT) dia-mondsubstrate.Thethicknessoftheintrinsic layerwaschosen tobeapproximately50␮m.Thehydrogenterminationoftheas grown diamond surface was removed by isothermal annealing at 500◦C for 1h in air. Finally, 50nm thick aluminum fingers werepatternedbyastandardlift-offphoto-lithographictechnique and bythermal evaporation.The metalfingers were processed to5␮minwidthwithspacingbetweentheelectrodesof20␮m. Thedetectoractiveareawasapproximately8mm2_.Sucha

struc-ture,operating in planar configuration, acts as a two terminal metal-diamond-metal(MDM)device.Anexternalvoltageof50V wasappliedbetween themetal contactsgeneratingan electric fieldperpendiculartotheincominglightacrossthedevice.Mobile chargesproducedbytheabsorbedradiationdriftinthiselectric fieldandgenerateacurrentintheexternalcircuit.From simula-tionsbasedontheShockley–Ramo–Gunntheorywiththehelpof ComsolMultiphysicsfiniteelementsoftware[15],thecharge col-lectionefficiencyoftheMDMdetectorissuchtoallowcollecting electron–holepairsgenerateduptoabout25␮mbelowthe super-ficialmetallicelectrodes.Sothatthediamondsubstrate,whichis poorinelectricalquality,doesnotcontributetothedetection pro-cess.Thesensitivelayeristhickenoughtocollectmostoftheenergy oftheemittedionsand,inparticular,protonsofenergyupto2MeV arefullycapturedinthislayer(SRIMMonteCarlocode[16]).

Inordertoreduceelectromagneticnoise,thediamonddetectors wereplaced inashieldedholder havinga pin-holetocollimate theradiationonlyonthedetectorsensitivearea.Theoutputsignal wasrecordedwitha1GHz,5GS/sfastoscilloscopethrougha bias-T.Allinputstoscopewereterminatedin50.Fig.1showsthe schematicstructure oftheMDMdevicewiththeschemeofthe usedelectroniccircuit(a),aphotoofAlinter-digitizedelectrodes (b)anditsassemblyintheexperiment(c).Thediamonddetectors wereemployedinTOFmeasurementstodetectallthecomponents oftheradiationemittedfromthelaser-generatedplasma.

Theexperiment hasbeen performed atthe PALS facility by focusingthefirstharmonics(0=1315nm)ofthe300pspulsed

laser at a maximum laser energy of 600J and laser intensity above1016_W/cm2_{.Thickandthinmetallayersorpolymerswere}

employedastargets.Moreover,multilayeredfilms,mostofthem constitutedby hydrogenatedpolymerscoupledtothin metallic films(Mylar/Al,Al/Mylar/Al,Au/polyethylene,polymers contain-ingnanoparticlesand/ornanostructures,etc.),wereusedinorder toincreasetheenergyofejectedions.Investigationofprotonand

Fig.1.SchematicstructureoftheMDMdevicewiththeschemeoftheused elec-troniccircuit(R=3.3MandC=0.68␮F)(a),aphotoofAlinter-digitizedelectrodes (b)anditsassemblyintheexperiment(c).

ionemissionsinbackwardandforwarddirections(thickand thin-nertargets,respectively)oftheplasmaswasanalyzed.Forward emissionofions(mainlyprotons)wasmeasuredfromthe rear-side(targetbackside)ofthintargetsalongthenormaltothetarget surface,besidesbackwardemissionoftheplasmaisfromthefront sideofthetarget.

TheMDMdetectors wereplacedata distanceof101cmand 123cmfromthetargetsurfaceinforwardandbackwarddirections, respectively,andat30◦ anglewithrespecttothetargetnormal directioninbothcases.Asimplesketchoftheexperimentalsetup isshowninFig.2.

Finally,aFaradayCupwasemployedasanioncollector(IC)ata distanceof100cmfromthetargetinforwarddirectionandat30◦ angleinordertocomparetheexperimentaldataobtainedfrom DiamondwiththatfromIC.

3. Resultsanddiscussion

First,thetimeresolutionanalysisoftheMDMdetectorshasbeen performedinourlaboratories.Then,theresultsobtainedbytwo photodetectors,oneplacedinforwardandanotheronein back-warddirectionoftheplasmaatPALSlaboratory,arereportedin thefollowing.

Fig.2. PhotoandschemeoftheexperimentalirradiationvacuumchamberatPALS laboratory.

3.1. Timeresponse

Thehighsaturationvelocityandhighmobilityofcharge carri-ersindiamondmakestheMDMdetector’sresponseveryfast.In thecaseofhomogeneouselectricfieldEdconditionand

neglect-inganyrecombinationandcapture(trapping),theeffectivecharge collectiontime()ofelectron–holepairsgeneratedinthediamond acrosstheelectrodesisgivenby[17]

= e·e+h·h e+h e/h= _d e/hEd ·



1+

_v

e/hEd sat(e,h)



(1)

wheredisthedistancebetweentheelectrodes,e/histhecharge

carriermobilityand

v

sat(e,h) isthesaturationvelocity.Usingthe

values d=20␮m, e=1714cm2V−1s−1, h=2064cm2V−1s−1,

Ed=2×104Vm−1 (drift field) and

v

sat(e)=9.6×106cms−1,

v

sat(h)=1.4×107cms−1 [18,19],Eq.(1)gives ≈200ps.Inorder

tostudytheeffectivetimeresponse oftheMDMdetectors,the devices are have been irradiated with 5.5MeV alpha particles (13␮m penetration depthin diamond [16]). The detector was placedatapproximately1cminfrontofthe241_{Amsource.Forsuch}

measurement,alownoiseDiamondBroadbandAmplifier(DBA-III)

[20]wasusedasfastfront-endelectronicscoupledtoadigitalfast scope(2.3GHz bandwidthand 50GHz sampling rate). Charged ␣particlespassingthroughthediamonddetectorproduceshort electricalpulses.Apulsefrom5.5MeValphaparticlesmeasured bydiamonddetectorisshowninFig.3varyingthebiasappliedto thedetector.FromtheGaussianfitofthepulses,about350psfull widthathalfmaximum (FWHM)is observedwhen theapplied biasishigherthan40Vdemonstratinga highintrinsicresponse velocityoftheMDMdetectors.However,thediscrepancybetween thetheoreticalvaluecalculatedaboveandtheexperimentalone is probably to beascribed to the approximations, e.g. the non uniformity of the electric field in the bulk of MDM detector, trapping,etc. 2 1 0 -1 -2 0.00 0.05 0.10 0.15 0.20 0.25

FWHM=352 ps

FWHM=389 ps

Amplitude (V)

Time (ns)

V_bias=20V Gauss Fit V_bias=40V Gauss Fit V_bias=60V Gauss Fit FWHM=355 ps

Fig.3. Apulsefrom5.5MeValphaparticlesobtainedwiththeDBA-IIIpreamplifier andrecordedwithadigitalfastoscilloscope.

3.2. Time-of-flightmeasurementsatPals

AtypicalTOFspectrummeasuredbythediamonddetectorin backwardemissionoftheplasmaisshowninFig.4.Thisspectrum isobtainedbyirradiatingthepolyethyleneterephthalate(PET) tar-getmaterialat1064nm,494mJpulseenergyandwiththelaser focusexactlyonthetargetsurface.TheMDMdetectorisableto discriminatedifferentcomponentsoftheradiationemittedfrom thetarget.Inparticular,thefollowingpeakscanbeseenvs.time: atthebeginningofthescale,thephoto-peakarisingfromUVand X-rayradiation(0–20ns),thepeaksarisingfromfastplasma elec-trons(20–100ns),thepeakarisingfromfastprotons(120ns),the peaksfromdifferentionspeciesandionizationstages(190–400ns) andthepeaksarisingfromslowions(0.4–1␮s)emittedfromthe hottargetmaterialsurroundingtheplasmaspot.Thesedifferent plasmaproducts(radiationand particles)wereassigned toeach peakaccordingtopreviousresults[7,21]andalsothanksto simul-taneousmeasurementsbyparticlespectrometersemployedinthe experimentbutnotreportedinthiswork.TheconversionofTOF intothekineticenergyEpisdeterminedbythefollowingequation

usingtherelativistickinematics Ep= 1₂M0c2·



1



1−ˇ2 −1



ˇ= d c·TOF (2) 1.0 0.8 0.6 0.4 0.2 0.0 0 5 10 15 20

Cn+ Slow carbon ions Cn+ Fast carbon ions

H+protons e electrons -Yield (V) TOF (μs) MDM detector Target: PET EL= 494 J Fp= 0 μm Shot #40457 Photopeak

Fig.4. TypicalTOFspectrameasuredbydiamonddetectorinbackwardemissionof theplasma.

20 15 10 5 0 0 5 10 15

Yield (V)

TOF (ns)

MDM detector Zoom Shot #40457

Photo-Peak

Rise Time ~1ns 6.3 ns

Fig.5.Enlargedviewofthephoto-peakofthespectrumreportedinFig.4.

whereM0istherestmassofaproton,electronorion,cisthelight

velocityand dis theflight path(i.e.thetarget-diamond detec-tordistance). Thespectrum indicatesa proton energyof about 280keVandacarbonionenergyrangingbetweenabout295keV and11MeV.Intheportionofthespectrumbetween50and100ns, thephotodetectorresponseisnotaffectedbynoisebutan addi-tionalandoppositepolaritysignalduetocontributionofsecondary electron emission arising from the metallic interdigitated con-tactsinterfereswhenlowenergeticelectronsareincomingtothe diamondsurface.Suchelectrons,haveanenergyoftheorderof 2.8keV–500eV,confirmingthatthehottergeneratedplasmamay reachatemperatureoftheorderof2keV,inagreementwith liter-ature[21].AnenlargedviewoftheTOFspectrumdiscussedabove isreportedinFig.5.Becausethediamondisnotsensitivetothe plasmavisibleandsoft-UVradiationupto225nm,ashort photo-peakwithatemporaldecayofabout15nsandarisetimeof1nscan beobserved.Asecondpeakabout6nsafteriswellvisible, demon-stratingthehightimeresolutionofthedetector.Suchapeakcan possiblybeascribedtoafastplasmaelectroncomponentwithan averageenergyofabout50keV.

TwotypicalTOFspectraobtainedatPALSintheforward direc-tion(about 500J laser energy, thin Mylar and Al/Mylar/Al) are reported in Fig. 6(a) and (b). The TOF diamond spectra were comparedwithtraditional ion collectorspectra.Thex-axiswas normalizedatadistanceof100cminordertoperformadirect com-parisonoftheTOFspectrasimultaneouslyrecordedwiththetwo differentdetectors.Inallcases,theICspectrashowahuge photo-peakwithataillastingabout0.5␮ssothatthesignalrelatedtofast ionsandprotonsiscompletelyhiddenbysuchpeak.However,it showsahighefficiencyforthelow-energyionsbetween1and4␮s oftheTOFspectra.TheseslowCandAlionsatdifferentchargestate andenergyarealsomeasuredbythediamonddetectorasclearly seeninFig.6(a)andintheinsetofFig.6(b).Infact,theslowions donotloseenergyinthefrontelectrodesincetheMDMdetectors, operatinginaplanarconfiguration,haveitsactivediamondvolume directlyexposedtoimpingingradiation.TheCn+_meanenergy

mea-suredbytheICR(30keV)isslightlylowerthantheonemeasured bythediamonddetector(40keV),sinceitsannulargeometrydoes notallowtomeasurethefasterionstreampassingonaxiswhere thediamonddetectorisplaced.Infact,inalaserplasmathehigher energyionstreamsaretypicallymorecollimated(smalleraperture cone)[22,23].Moreover,theprotonpeakiswellseparatedfrom thephoto-peak,whichisquitenarrow,andgivethepossibilityof measuringverywellfastprotonsandions.Finally,MDMdetector revealsalsoelectrons,whosepeaksarebetweenthephoto-peak and proton peak. It is possible to conclude that by using the

2.5

2.0

1.5

1.0

0.5

0.0

0

3

6

Yield [V]

TOF@1m [

μ

s]

4

3

2

1

0

0

1

2

3

4

TOF@1m [

μ

s]

Yield [V]

a

b

Fig.6.TypicaldiamondTOFspectrafordiamondandioncollectorobtainedby ablatingdifferentthintargetsatPALSlaboratoryinforwarddirection.

diamonddetectorsinTOFconfiguration,itiseasiertoemphasize thefastionpeakandelectronsatexpensesoftherelativelyshort XUVphotopeakrespecttotheICdetectorwhichisinsteadmore efficienttomeasureslowions,sinceithasalowersensitivitytothe highenergy,lowchargestatparticles,andalsobecauseitisvery sensitivetothesoft-UVplasmaradiationwhichoverlapswiththe fastionpeak[24].

4. Conclusion

Single-crystalCVD diamonddetectors withaninterdigitated metalelectrodehavebeenfabricatedandtestedat“TorVergata” RomeUniversity.Suchdiamonddetectorshavebeenemployedin time-of-flight(TOF)configurationtomonitortheemissionplasma obtainedatPALSlaboratoryinPraguebyusingalaserpulse inten-sityofabout1016_W/cm2_.

The achieved results demonstrated that diamond detectors showimportant advantages withrespect toother semiconduc-tordetectors.Theseincludetheabsenceofaresponsetovisible lightandthepossibilitytosimultaneouslydetectenergetic pho-tons,electrons,andionsejectedfromtheplasmabothinforward andbackwarddirections.Finally,diamonddetectorshaveagood sensitivitytofastionsandhighenergyresolution(fastresponse time)whichisagoodadvantagewhenitisnecessaryto distin-guishbetweentheacceleratedfastprotonsandtherelativelylong plasmaphotopeak.

Acknowledgements

ThisworkissupportedbyLIANA-INFNExperimentandpartially bytheFondazioneRoma.

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