Volume 78A, number1 PHYSICSLETTERS 7 July 1980

MEASUREMENT OF THE DECAY AND TIME-RESOLVED SPECTRA OF THE EMISSIONFROM iv-EXC1TONS IN KBr IN THE NANOSECOND RANGE,PRODUCEDBY HEAVY-ION IRRADIATION

K. KIMURA, K. MOCHIZUKI, T. FUJISAWA and M. IMAMURATheInstituteofPhysicalandChemicalResearch,Wako,Saitama351, Japan

Received27 June1979Revisedmanuscriptreceived18 March 1980

While energeticheavyions arecurrentlyin increasingusein solid statephysics,radiationchemistryandbiology, thereisstifi little experimentalinformationregardingtheprimaryphysicochemicalprocesses.Thesehigh LET (linearenergytransfer)ion particlesinjectedinto condensedmatterproduceshort life-intermediatessuchasexcitedstates,ions,andradicals,at highdensityalongthetrack.Thedensityeffectof theseintermediatesmay causedamage-formationandchemicalreactionto yielddifferentresultscomparedwith low LET radiation(~‘-, X-ray) or photo-irradiation.Electronicallyexcitedstates,amongtheaboveintermediates,arethoughtto beespeciallyimportantprecursors.Measurementsof thedecayandtime-resolvedspectraofthe emissionareexpectedto give usefulinformationregardingtheradiationactionof heavyions.Themeasurementsweredoneusinga KBr single crystalat 4.2K. We reportedpreviously[1] thatheavyion irradiatedKBr resultsin anextraordinarilylargeratio of a-emissionto IT-emission1 intensity(-~5 timeslargerthanin theX- or electron-irradiation).This largeratio is,aswasascertainedpreviously,not dueto apparentprocessessuchastemperatureincreaseof atrackbut to intrinsic processeswhichwill bediscussedhere.

Theemissiondecayprocesseswere studiedby means Crystalof a coincidencetechniqueusingsingle ion irradiation, - <,cintdlator yostat_~i)photoncountingand a TAC (time-to-amplitudeconver- Ions Crtor), asshown in fig. 1. A thin plasticscintillator,about 10-pmthick, was placedin theheavy-ionbeam ~~-St0P ______________

to generate“stop pulses”astimingsignalsfor a TAC. I TW.4E P!~KOFF [ MONOCHR0MA~R1

The scintillatoremitsa few tensof photonsfor each TAC ..—startheavyion passingthrougl~.Thephotonswere detected ~ITIME PICKOFF I

I __witha HamamatsuR647.photomultiplier(darkcount: _____ COINCIDENCE ISAabout20 cps.).Theionswereirradiatedat suchlow AO~NVERTERjcurrents(oneion per 100 beambunches)thatthewidths C~MPU~ERof the beambunchmaybe regardedasnegligibly small.The stoppulseswere alsogeneratedby shapingandfre- Fig. 1. Experimentalarrangementandcoincidencecircuits forquencydividing circuitsof the cyclotronRF frequency. measurementof emissiondecaycurves.

“Start pulses”weregeneratedby outputpulsesfromthe photomultiplierwhich detectsemissionfrom thetargetat 4.2 K. Thenumberof start pulseswasmuch excitedstatescouldbemeasuredexactly.The timelowerthanthe stoppulses,sothat the lifetime of the interval betweenthe stopandstartpulsesweremeasured

usingthe TAC with the experimentalarrangementshown

a- andiT-emission,havingpeaksat280and512nmrespectively, in fig. 1. Accidentalpulseswereomittedby LinearGateweredefinedastheemissionfrom o-(’ E,~)andir-(

3Eti) ex- Stretchersanda UniversalCoincidence.Theindividualcitons. outputpulsesweredigitized by anAD-convertorand

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Volume78A, number1 PHYSICSLET’FERS 7 July 1980

accumulatedusing anon-line computer.Theresultingplot of countversuspulseheightgives thedecaycurveof theexcitedstatesat a givenwavelength.The timescalewascalibratedusingan RF frequencyobtainedfrom the cyclotronanddelaylines. -

U,Figure2 showsa decaycurve obtainedat 280nm z

for the a-excitoncreatedin a KBr singlecrystal,at o -

4.2K, using irradiationwith 85..MeVC-ions. Two peaks ~correspondto iterativebunchesof a cyclotronRF fre-quency,andthe interval is precisely130 ns. A logarith-mic plot of thedecaycurveis shownin fig. 3, whichsuggeststhat the decaycurve consistsof two exponentialparts.The lifetime of theintense,short-livedcomponentandof the weak,long-livedonewere foundto be3.8 _____________________________________and16 ns,respectively.Blair et al.reportedtheappear- 10.0 20.0 30.0 ~0.0 50.0 60.0anceof two componentswith lifetimesof 3.3,and17 NSnsusinga low-LET proton irradiation [2]. Irradiation Fig. 3. A semilogarithmicplot of thedatain fig. 2. Peakposi-effectsof suchlow-LET protonsmaybe regardedas tion in fig. 2 wasdefinedastimezero.beingsimilar to thoseproducedby X-rays or electrons,whencomparedwith thoseproducedby heavy-ions, confinedto the following: (1) enhancementof radia-The goodagreementbetweentheobservedvaluesfrom tionlessrelaxationof ir-excitons,(2)enhancementofheavy-ionsandprotonirradiationaswell asthe straight radiativedecayof a-excitonsand(3) theenhancedyieldline in fig. 3 indicatethat a-excitonscreatedby heavy- of the a-exciton,5 timeshigherthanthat of the ir-ion irradiationdecayspontaneously,andthat neither exciton,in heavy-ionirradiation.The first alreadyhasinteractionof the a-excitonswith colorcentersnor beenruledout, asdescribedin ref. [1] ,by theresultsinteractionbetweena-excitonstakeplace.Theresults obtainedfrom measurementsof the doseeffect ofalsoindicatethatno enhancementof the radiative ‘a”IT andof a decaytimeof the ir-exciton. Thesecondprocessof a-excitonsoccurs. isalso ruledoutfromthe presentresultsmentioned

Sinceheavy-ions(or high LET) irradiationcanyield above:stimulatedemissionof a-excitonsis impossible,excitonsand colorcentersat highdensityalongits sinceneitherinteractionof the a-excitonswith eachtrack,onemayexpectthat the largevalueof the inten- othernorwith color-centerswere recognized.Thesesity ratio,i,,/i~is in responseto the densityeffect. resultssuggestthat onemustconsiderearlier stagesProbableprocessesgiving the largevalueof ‘a”IT are thanself-trappedstates,suchas a-and ir-excitons.They

are knownto be createdby self-trappingof free exci-non,

tons, asin theschemeshownin fig. 4.Thethird processisthereforemorereliable.Enhan-

cementof theyield of a-excitonsmaybe explainedasfollows. Freeexcitonsarecreatedso denselyin the

1001)I

KBr ~ HOLE * eionizationj 500 l\ ~direct excitation FREE EXCITON

TI-EXCITON o-EXCITON COLOR- hvFig. 2. A plot of photonintensityvs. channelsof pulseheights CENTERSfor KBr irradiatedat 4.2 K. C6~ion currentsof approximately10 pA wereused.130 ns correspondsto iterativebeambunches. Fig. 4. Schematicdiagramof a- and ir-excitonformation.

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Volume78A, number1 PHYSICSLETTERS 7 July 1980

Figure4 showsthe time-dependentspectraof a-emissionat 0, 2, 4, 7 and9 ns after ion impact.The spec-

5.0X 102o o tra appearto bethe samein spiteof thetwo components,

which appearin fig. 3. The existenceof anothercompo-4.OX 102 nent whichhasthe sameemissionspectra,but occurring

U, at a different decaytime from thatassociatedwith the~ 3.OX1O’ a-excitonis not concludedin thepublishedexperimental=a-

andtheoreticalresultsavailablefor alkali halides.2.0X102

The presenttechniqueis alsoapplicableto targetswith low emissionefficiencyandto particleswith low

1.ox10,intensityandenergy,e.g. radioisotopes.Shorteningofstoppulsesanduseof low energyparticlesare possible

620 480 by measuringelectronsemittedfrom athink carbonEV film (100A is possible)usingmultichannelplates(lOp

Fig. 5. Time-resolvedspectraof a-emissionof KBr; Symbols secresolution)insteadof the presentscintillator film.o, A, +, x, ando imply 0, 2, 4, 7 and9 nsafter ion-impact. It is,however,as yet inadequateto observedirectly the

decayof freeexcitonsin alkali halidesbecauseof veryshortlifetimes (a few p sec)andweakintensities.Theo-

vicinity of a heavy-iontrackthat theycoffide with reticalexplanationof disproportionatereactionof theeachother.As a result,the a-and ir-excitonsarepro- free excitonsis also too difficult, sinceeventhe existenceducedin a ratio different from thosein X- or electron- of the freeexcitonsin alkali halidesis nottheoreticallyirradiationwherespontaneousintersystemcrossing explained.mustbedominant.Otherwise,the creationof a-andir-excitonsthroughdirect excitation(seefig. 4) by The authorsare gratefulfor stimulativeandhelpfulsecondaryelectrons(subexcitationelectrons[4]) may discussionswith ProfessorN. Itoh andM . Hirai. Theyalsobetakeninto consideration,althoughX-ray or shouldalso like to acknowledgethe technicalassistanceelectron-irradiationare usuallythoughtto give up energy of Dr. I. Kohno andthe operationgroupat the IPCRexclusivelyby ionization inalkali halides.For instance, cyclotron.Satoet al. calculatedthat the ratio of totalexcitationto total ionizationis 0.74for Ne gasirradiatedwith References100 keV electrons[5]. Differencesin spatialandenergydistribution of secondaryelectronsaswell as differ- [1] K. Kimura andM. lmamura,Phys.Letters67A (1978)159.encesin the yield for heavyion-andelectron-irradia- [2] LM. Blair, D. Pooley andI). Smith, AERE-R6090(1971).tion mayinfluencethe relativeyield of a-andir-exci- [3] H. Nishimura,J.Phys.Soc.Japan43 (1977)157.

[4] R.L. Platzman,mt. J.Appi. RadiationandIsotopes10tons.If thisis the case,onemay expecta largervalueof (1961)116.

‘a”,r for electron-irradiationthan that for photo- ~sI S. Sato,K. OkazakiandS. Ohno,Bull. Chem.Soc.Japanirradiation. The factthat the valueOf ‘O/’IT calculated 47 (1974)2174.from theresult for electron-irradiatedKBr by Pooley [6] 11 PooleyandW.A. Runciman,I. Phys.C: Solid St. Phys.et a!. [61andfor 7.7eV photo-irradiatedKBr by I~esawa 3 (1970) 1815.

[7] M. IkezawaandT. Kojima, J. Phys.Soc.Japan27 (1969)et al. [7] are 2.5and1.3, respectively,is in accordwith 1551.the explanationgivenabove.

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