Quasi-fission reactions induced by 365 Mev 63Cu ions on a 197Au target 1975 J. Péter.pdf

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QUASI-FISSIONREACTIONSINDUCEDBY
365MeV63CuIONSONA197AuTARGET
ÉTER,Ô,
ChimieNucléaire,InstitutdePhysiqueNucl&aire,91406Orsay,France
Résumé.2014Lessectionsefficacesdesréactionsdefusioncomplèteetdequasi-fissionontété
mesuréespourlesystème63Cu+197Auàuneénergiedebombardementégaleà1,1foislabarrière
d’éactionsinduitesparkryptonétudiéesprécédemment,la
fusioncomplèteesttrèspeuprobableetlasectionefficacedequasi-fission(250±50mb)représente
lamoitiédelasectionefficacederé-fissionvarieavecl’angle
dedéégers(voisinsducuivre)présenteunmaximum
àunanglelégèrementinférieuràl’angled’ésultatssuggèrentque,pourlesfaibles
paramètresd’impact,unéchangedemasseplusimportantalieuetquel’écartangulaireparrapportà
ladiffusionélastiqueestbeaucoupplusfortquepourlesgrandsparamètresd’’énergie
cinétiquetotaleestconstanteenfonctiondel’angle(oulégèrementcroissanteauxfaiblesangles).
-fissionandcompletefusioncrosssectionshavebeenmeasuredforthesystem
63Cu+
case,veryfewcompletefusioneventsareobserved,andthequasi-fissioncrosssection(250±50mb)

-fissionproductsvarieswiththe
(massaroundthatofcopper)showaclearmaximumatan

correspondtoprojectileswithlowl-valueswhichhaveundergoneagreatermassexchangethan
thehighl-,orslightly
increasingatforwardangles.
Sincethereactionofquasi-fissionhasbeenobserv-tanceofcompletefusionandofquasi-fissionarosefor
ed[1]inbombardmentsofheavynucleiwithg4KrreactionsinducedbyprojectilesbetweenArandKr.
ions,severalfeaturesofthisphenomenahavebeenAnanswertothisquestionwasprovidedbyapre-
studied[2,3]andourpresentexperimentalknowledgeliminaryresultfromanexperimentdonewiththe
canbesummarizedasfollows:twomainfragmentsfirst63CuprojectilesacceleratedattheOrsayheavy
areproduced,
tothemassoftheprojectileandthemassoftheotherincidentona186Wtarget,themainobservedfeatures
:low
fragmentkineticenergiesarecloseto(somewhatfissioncrosssection,highquasi-fissioncrosssection
higherthan)thevaluesexpectedforfissionoftheandanangulardistributionpeakednear(butslightly
compoundnucleus[2]andattheenergieswherelowerthan)thegrazingangle[4].
measurementhavebeenmade(-
theinteractionbarrier)thecrosssectionforquasi-rences[1-4]havebeenobtainedbysimplymeasuring
fissionaccountsformorethanonehalfofthetotalthekineticenergiesandanglesofthetwofinalpro-
reactioncrosssection,
-
feature,however,isthattheangulardistributionduct,bymeasuringitstime-of-flightandenergy.
ofthelightquasi-fissionproductsisstronglypeakedThechoiceofthe19’Autargetandofthebombard-
atanangleslightlylowerthantheprojectilegrazingingenergy(365MeV)arebasedonthefollowing
:
Sincethemajorfractionofthereactioncross
sectiongoesintoacompletefusionprocessforpro-1)Themaximumangularmomentuminvolved
jectilesupto4°Ar,thequestionoftherelativeimpor-inthereactionisaround100~andthecompound
ArticlepublishedonlinebyEDPSciencesandavailableat/jphyslet:0197500360202300
L-24
r

ofthecompletefusionnucleithatareproducedaredistributionoflightproducts(giveninfigure3d),
expectedtoundergobinaryfissionwithasymmetricnoneasymmetriconesareexpectedtobefoundat
massdistribution[5].thisangle(56~).Theycanbeobservedonlyatlower
2)Theresultswillnotincludefissionfollowingangleswheremeasurementsaredifficult.

observedwithArandKrprojectilesontargetsheavierobtainedforproductsofquasi-fissionandfission
thangold[8].thathaveamasslowerthan130(halfofthemass
3)-ofthecompoundnucleus),-
actionbarrier,
quasi-fissionhasbeenobservedpreviouslywithKrthat:
andCuprojectiles.
1)Nocoincidencewithanyotherproductwas
Thetimeofflighthasbeenmeasuredbymeansofrequired.
aplasticscintillator(Ne111145Jlgjcm2)associated2)Themeasuredmassisthatoftheproductafter
withaphotomultiplier,andwithaheavyionsurfacepossibleparticleevaporation.

In2,arethemassdistributions
3amuforamassofN60amuand7amuforamassfiguredisplayed
:
,theresolutionwas
evenworse,andthustheresultspresentedherearea)Alownumberoffragmentswhichcanbe
formasseslowerthan130(,
thanhalfofthemassofthecompoundnucleus).sincethemassdistributionofsuchfragmentsis
Itmustbenotedthatnocoincidencewasrequiredexpectedtopeakaroundmass126.
)Themaximainthedistributionsarearound
Figure1showsatypicaltwo-dimensionalspec-mass60(-excitation)
trumofthemassesofthedetectedproductsversustheatthehighangles,andtheyshifttohighermassesat

.[3]JforthesystemKr+Bi
elasticallyandinelasticallyscattered63CUor197Auat600MeV.-
nucleiaswellasthelightorheavyquasi-elastic
-
-
ratedfromthelightquasi-
notwellseparatedfromtheheavyquasi-elastictransfer
area,duetothelackofmassandenergyresolution
,ifone
calculatestheangulardistributionoftheheavyquasi-
.-Distributionofthereactionproductsreceivedat56~.
Theabscissaisthelaboratorykineticenergyandtheordinatethe
massoftheproduct,deducedfromitstime-of-flightandenergy.
Thelong-dashedrectangleistheareaofquasi-.-Massdistributionsofthelightquasi-fissionandfission
.
L-25
c)Thewidthincreasesasthemassexchangeanditdisappearscompletelyformoreimportantmass
becomesmoreimportant,
distributionshiftstowardmoresymmetricmassofthemechanismasthemasstransferincreases.
,formassesaround130,thedistribution
couldbethatoffissionfragmentsissuedfroma
Theselasttworesultsandseemto
(b)c)above)
indicatethatthereactiontimehasbeenforcomplete-fusion
longerthecaseandalsothatthewidthofthemassdistri-
theproductsdetectedatlowanglesthanforthepro-
butionisaround50-60amu,weobtaintheangular


ofthemassasymmetryoffreedomis
degreetypicallya1/sin0shapeforthisdistribution,inthecenter-of-
about1-2MeV[6]thismeansthatthereactiontime
masssystem(ashapewhichisconsistantwiththe
isoftheorderorlessroughlythantheoscil-
slightlyexperimentalpointreportedhere),wecandeduce
lationtimeofthisoffreedom,whichis
degreeamaximumof30mb:t12mbforfissionfollowing
2-4x10-21s.
completefusion(ourresultsarenottobetakenas
Anotherwaytoshowtheresultis
experimentalevidencefora1/sin0shapebecausewehavenotmade
theevolutionoftheofthedistribution
).
withtheamountofmasstransferredwhichappears
Thefullcurveinfigure3eistheangulardistribution
in3a-
figureregionofthelightquasi-fissioneventswhichresultswhen
projectile,
thecomplete-fusionfissioneventsaresubstracted.
peakisnotassharpforamassregion70M90
Thetotalcrosssectionofthesequasi-fissioneventsis
250±50mb,whichisonehalfofthetotalreaction

verysimilartotheonealreadyobservedinrefe-
rences[2-4].
Infigure3d,thedot-dashedcurveisthecenter-of-

laboratoryangulardistribution(givenbythedashed
curve)byamethodoftransformationthatinvolved
,whichis
slightlysmallerthanthe1100grazinganglefor63CU,
consistantwithothercasesinwhichquasi-fission
hasbeenobserved.
Letusconsidertheextenttowhichthevarious1-
wavescontributetothedifferentprocessesthatmake

ourselvestothesharpcut-
maketheusualassumptionthatfusiontakesplace
forthelowestI-waves,thentheobservedfusioncross
sectionof30mbcorrespondstoacriticalangular
momentum/e~?~25Ii,beyondwhichcomplete
,for
partialwavesofl>100,onlyelasticscatteringoccurs.
Forpartialwaves25I100twoprocessestake
place:quasi-elastictransferforpartialwavesgreater
thanabout75,andquasi-fissionfor25~I~75.
Forelasticscatteringandforquasi-elastictransfer
processestherelationshipbetweenI-valuesandthe
,
theproductsarepeakedattheprojectilegrazing
angle,andforelasticscatteringtheemissionangle
-
fissionweneedtoexplainatthesametimetheshapes
ofboththeangulardistributionandofthecorres-
,b,c,d.-Angulardistributionsofthequasi-
fissionintheforseveralofmassvalues.
productslaboratory,rangesqualitativeconclusionthatweareledtoisthat,
Inaddition,thecenter-of-massangulardistributionisgivenfor
toelastictheemis-
themassrange50-,light-product
-
.-Angulardistributionsofthefragmentswhichmaybeangleincreasing
low1valuestheofthe
attributedtofissionfromafusionnucleus,andtoquasi-fissiontively(near30)penetration
,
2
L-26
whichallowsequilibriuminthemass-degreeof
freedomtobemorenearlyachievedandwhichalso
allowsarelativelylargerotationofthesystembefore

forwardangles(26-36°)andtheirmassdistribution

small,sincepartialcrosssectionsforlowI-waves
-values(near70),the
nuclearpenetrationislessdeep,thetimeofreaction
isshorter,theextentofmasstransferisless,andthe
angleofrotationissmallerbeforere-separation.
Thusthelightproductsareobservedatagreater
emissionangleinthiscase,andthecrosssectionis
higher,duetothegreaterpartialcrosssectionsat
higherI-
distributionpeakedat60amuwhichisobserved
near78~.Sincetheaboveexplanationofourresults
requiresthatapartialrotationofthecomposite
systemtakesplace,itisnotsurprisingtofindthepeak
oftheoverallangulardistributionofthelightquasi-
.-Totalkineticenergyinthecenter-of-massforquasi-
,versustheangle

Thisviewgivenaboveisdifferentfromtheoneofproductpointscorrespond
tothreepossiblehypothesisconcerningthenumbersofneutrons

[3],emittedbythefragments(detailsinthetext).Thearrowindicates
experimentalresultforthevariationofthemassthevaluepredictedforthiscoupleoffragmentsbytheliquiddrop