Study of non-fusion products in the 50Ti+249Cf reaction

University of Groningen

Study of non-fusion products in the 50Ti+249Cf reaction

Di Nitto, A.; Khuyagbaatar, J.; Ackermann, D.; Andersson, L.-L.; Badura, E.; Block, M.; Brand,

H.; Conrad, I.; Cox, D. M.; Düllmann, Ch. E.

Published in:

Physics Letters B

DOI:

10.1016/j.physletb.2018.07.058

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Di Nitto, A., Khuyagbaatar, J., Ackermann, D., Andersson, L-L., Badura, E., Block, M., Brand, H., Conrad,

I., Cox, D. M., Düllmann, C. E., Dvorak, J., Eberhardt, K., Ellison, P. A., Esker, N. E., Even, J., Fahlander,

C., Forsberg, U., Gates, J. M., Golubev, P., ... Schädel, M. (2018). Study of non-fusion products in the

50Ti+249Cf reaction. Physics Letters B, 784, 199-205. https://doi.org/10.1016/j.physletb.2018.07.058

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Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Study

of

non-fusion

products

in

the

50

Ti

+

249

Cf reaction

A. Di Nitto

a

,

b

,

J. Khuyagbaatar

b

,

c

,

∗

,

D. Ackermann

b

,

1

,

L.-L. Andersson

c

,

d

,

E. Badura

b

,

M. Block

a

,

b

,

c

,

H. Brand

b

,

I. Conrad

b

,

D.M. Cox

d

,

Ch.E. Düllmann

a

,

b

,

c

,

J. Dvorak

c

,

K. Eberhardt

a

,

c

,

P.A. Ellison

e

,

f

,

N.E. Esker

e

,

f

,

J. Even

a

,

c

,

2

,

C. Fahlander

g

,

U. Forsberg

g

,

J.M. Gates

e

,

P. Golubev

g

,

O. Gothe

e

,

f

,

K.E. Gregorich

e

,

W. Hartmann

b

,

R.D. Herzberg

d

,

F.P. Heßberger

b

,

c

,

J. Hoffmann

b

,

R. Hollinger

b

,

A. Hübner

b

,

E. Jäger

b

,

B. Kindler

b

,

S. Klein

a

,

I. Kojouharov

b

,

J.V. Kratz

a

,

J. Krier

b

,

N. Kurz

b

,

S. Lahiri

h

,

B. Lommel

b

,

M. Maiti

h

,

3

_,

_{R. Mändl}

b

_,

_{E. Merchán}

b

_,

_{S. Minami}

b

_,

_{A.K. Mistry}

d

_,

_{C. Mokry}

a

,

c

_,

_{H. Nitsche}

e

,

f

_,

J.P. Omtvedt

i

,

G.K. Pang

e

,

D. Renisch

a

,

D. Rudolph

g

,

J. Runke

b

,

L.G. Sarmiento

j

,

4

,

M. Schädel

b

,

k

,

H. Schaffner

b

,

B. Schausten

b

,

A. Semchenkov

i

,

J. Steiner

b

,

P. Thörle-Pospiech

a

,

c

,

N. Trautmann

a

,

A. Türler

l

,

m

,

J. Uusitalo

n

,

D. Ward

g

,

M. Wegrzecki

o

,

P. Wieczorek

b

,

N. Wiehl

a

,

A. Yakushev

b

,

V. Yakusheva

c

a_{JohannesGutenbergUniversityMainz,55099Mainz,Germany}

b_{GSIHelmholtzzentrumfürSchwerionenforschungGmbH,64291Darmstadt,Germany} c_{HelmholtzInstituteMainz,55099Mainz,Germany}

d_{UniversityofLiverpool,Liverpool,L697ZE,UK}

e_{LawrenceBerkeleyNationalLaboratory,Berkeley,CA 94720-8169,USA} f_{UniversityofCalifornia,Berkeley,CA 94720-1460,USA}

g_{LundUniversity,22100Lund,Sweden}

h_{SahaInstituteofNuclearPhysics,Kolkata-700064,India} i_{UniversityofOslo,0315Oslo,Norway}

j_{UniversidadNacionaldeColombia,BogotáD.C.111321,Colombia} k_{JapanAtomicEnergyAgency,Tokai,Ibaraki319-1195,Japan} l_{UniversityofBern,3012Bern,Switzerland}

m_{PaulScherrerInstitute,5232Villigen,Switzerland} n_{UniversityofJyväskylä,40014Jyväskylä,Finland} o_{InstituteofElectronTechnology,02-668Warsaw,Poland}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received29March2018

Receivedinrevisedform27July2018 Accepted31July2018

Availableonline2August2018 Editor: V.Metag

Keywords:

Productionofradioactivenuclei

αdecay

Multi-nucleontransferreactions

Theisotopicdistributionofnucleiproducedinthe50_{Ti +}249_{Cfreactionhasbeenstudiedatthe}

gas-filledrecoilseparatorTASCAatGSIDarmstadt,whichseparatesionsaccordingtodifferencesinmagnetic rigidity. The bombardmentwas performedatanenergy aroundthe Bassbarrier and withthe TASCA magneticfieldssetforcollectingfusion-evaporationreactionproducts.Fifty-threeisotopeslocated “north-east”of208_{Pbwereidentifiedasrecoilingproductsformedinnon-fusionchannelsofthereaction.These}

recoilswereimplantedwithenergiesintwodistinctranges;besidesonewithhigherenergy,asignificant low-energycontributionwasidentified.Thelatterobservationwas notexpectedtooccuraccording to kinematicsoftheknowntypesofreactions,namelyquasi-elastic,multi-nucleontransfer,deep-inelastic

*

Correspondingauthorat:GSIHelmholtzzentrumfürSchwerionenforschungGmbH,64291Darmstadt,Germany. E-mailaddress:j.khuyagbaatar@gsi.de(J. Khuyagbaatar).

1 _{Presentaddress:GrandAccélérateurNationald’IonLourds- GANIL,CEA/DSM-CNRS/IN2P3,Bd.HenriBecquerel,BP.55027,14076CaenCedex5,France.} 2 _{Presentaddress:KVI-CART,UniversityofGroningen,9747AAGroningen,TheNetherlands.}

3 _{Presentaddress:DepartmentofPhysics,IndianInstituteofTechnologyRoorkee,Roorkee-247667,India.} 4 _{Presentaddress:LundUniversity,22100Lund,Sweden.}

https://doi.org/10.1016/j.physletb.2018.07.058

0370-2693/©2018TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

200 A. Di Nitto et al. / Physics Letters B 784 (2018) 199–205

Quasifission collisions or quasifission.The present observations are discussed withinthe framework of two-body

kinematicspassingthroughtheformationofacompositesystem.

©2018TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

0. Introduction

During the last decades, heavy-ion induced reactions were largely exploited for various applications aiming to explore the entire chart of nuclei [1]. Especially, great success has been achieved in the region of superheavy elements (SHE) by discov-eringelementsupto Z

=

118 (Og)inheavy-ioninducedcomplete fusion reactions with subsequent emission of neutrons (fusion-evaporation)[2].

Complete fusion is the final result of the two colliding nu-clei,whichformacompositesystemaftertheyhaveovercomethe Coulomb repulsion [3]. However, the probability for fusion, lead-ingtofurtherfissionand/orevaporationoflightparticlesfromthe compoundnucleus, maystronglybe reduceddueto thebreaking ofthe compositesystem, oftenreferred alsoas dinuclearsystem, and depends on the properties of the reactants [4–10]. This in-creases the probability of the process complementary to fusion denotedasquasifission(QF)[4].QFbecomespredominantin reac-tionshavingahighCoulombforceattheentrancechannel,which istypicallyquantified asthechargeproduct ofprojectileand tar-getnuclei, ZpZt [11].Accordingly,thefusionprobabilityisstrongly

hinderedandthusreducesfusion-evaporationcrosssectionsof re-actionsforthesynthesisofSHE,wheretheheavyionscollidewith massive target nuclei [5,7,12]. Therefore, an alternative pathway featuring higher productionyields for the synthesis of SHE,thus reducingthesometimesverylongexperimental duration[13] and allowingthesynthesis ofmoreneutron-rich isotopesthanare ac-cessibleviafusionreactions,hasbeensoughtfordecades[14,15].

RecenttheoreticalcalculationssuggesttheproductionofSHEin multi-nucleon transfer reactions [16] often referred also asdeep inelastic transfer or strongly damped collisions [17]. Evidently, a classification ofthesetypes ofreactions, oftenbased on overlap-ping experimental observables [4,18], does not always properly reflecttheevolutionofthenuclearreaction.Similarly,no compre-hensive theoretical description of these reactions still exists due toa lackofexhaustivedata ontheobservablesdespite many ex-perimental efforts[19–22]. Regardless different namings and ab-senceofthe comprehensivetheory,thecommonfeature ofthese reactionsconcernstheir outcomeproducts,whichare well distin-guishablefromtheonesofelastic-typesofscatteringand fusion-evaporationreactions.

Pioneering studies on such types of reaction have been per-formed in the late 1970s by applying chemical separation tech-niques to gain access to relatively long-lived nuclei (T1/2

≥

1 h)

[23,24]. Recently, in 48_Ca

₊

248_{Cm reactions [25,26], new}

short-lived(T1/2

≈

1 ms) neutron-deficient isotopes ofheavy elements

(216U,219Np,223Am, 229Am and233Bk) havebeen synthesized at thevelocity filterSHIPatGSIDarmstadt.These andother known experimental results [27–29] demonstrate that reactions, whose recoiling products havemaximum yields atgrazingangles (often

>

0◦) and wide angular distributions, can be studied at forward anglesaround0◦.

To further elucidate the origin of nuclear reaction products emitted at near-zero-degree angles, we employed the gas-filled recoil TransActinide Separator and Chemistry Apparatus (TASCA) [30], whichexploits adifferent principle forionseparation com-paredtoSHIP.Thus,thepresentresultsobtainedbyselectingions fromthe50Ti

+

249Cf reaction,basedontheir magnetic rigidities, complementthedatafromSHIPfor48_Ca

₊

248_{Cm,whereproducts}

were selectedbytheir velocity. Inthisletter,we reportthestudy ontheidentificationofvariousnucleiproducedinthe50_Ti

₊

249_Cf

reaction, andthe observation of two components of their recoil energies. The latter result has been obtained for the first time inreactions withdeformedactinidetargets. Fusionproductsfrom suchreactions arecurrentlytheonlywaytogiveaccesstothe is-landofthestability.

Experimental datawereaccumulatedduring a longrunaimed to synthesizeelement Z

=

120 inthefusion-evaporation reaction [31].Since,thenucleiidentifiedinthisworkdonotbelongto de-caysofeither Z

=

120 nucleiorfusion-fissionproducts(thenuclei with massesaround A

=

150 and around the lineofbeta stabil-ity,[12]),andthusmayoriginateeitherfromQFormulti-nucleon transfer reactions. Therefore, we prefer to refer them as “non-fusion”ratherthan“transfer”or“target-like”products[27,28]. 1. Experimentalsetup

The experimentwas performed atthe gas-filled recoilTASCA [30], GSI Darmstadt.5 ms-long 50_Ti12+ _{beam pulseswith a}

rep-etition rate of50 Hz and an energy of 306MeV were provided by theUniversal LinearAccelerator.Fourarc-shaped 249Cf2O3

tar-getswithanaveragethicknessof(565

±

6) μg/cm2wereprepared by electro-depositiononto (2

.

2

±

0

.

2) μm-thickTi foils [32]. The targetsweremountedonawheel,whichrotatessynchronouslyto the beampulsestructure[33].The beamenergyinthe centerof the target was estimated as 288 MeV [34] at which the largest fusion-evaporation crosssectionforsynthesisof Z

=

120 element isexpected[31].

Inthisexperiment,TASCAwasfilledwith0.8mbarheliumand its magneticsettings wereadjusted tocollect ionswithmagnetic rigidity, B

ρ

=

2

.

14 Tm in thecenterof thefocal plane [31]. This valuecorrespondstotheexpectedB

ρ

ofevaporationresidueswith

Z

=

120 andmassnumberA

≈

295 [35].

Nucleiemergingfromthetargetfirsttraversedamultiwire pro-portionalcounter (MWPC) andwere subsequentlyimplantedinto two Double Sided siliconStrip Detectors (DSSD’s). Each300 μm-thick DSSDhaving anarea of 72

×

48 mm2 _{resultedinto totalof}

144vertical(X-axis)and48horizontal(Y-axis)strips. Thus, prod-ucts withmagnetic rigiditiesin therange ofB

ρ

=

1

.

97–2

.

31 Tm werecollectedintheDSSD.Twosingle-sidedsiliconstripdetectors were mounted behind the DSSD, withthe samedimensions, and wereusedtoregisterpunching-throughchargedparticles.A Com-bined ANalog andDIgital (CANDI)acquisition system[36,37] was usedtoprocessthesignalsfromalldetectors.Signalsfromthe144 vertical stripswere connectedto theanalog branch,where every signalwasduplicatedandprocessedintwodynamicalrangesupto 20MeV and200MeV.Signalsfromthe48horizontalstripswere connectedtothedigitalbranchofCANDIwheretheirpulseshapes were storedin50 μs-longtraces.Thedynamicalrangeofthe dig-ital branch, used to read out the horizontal strips, was limited to 35MeV in orderto optimizethe

α

-particleenergyresolution. The energy resolution (FWHM) of individual Y-strips was about 40 keVfor8–9 MeV

α

particlesregisteredassingleeventsinthe traces, andabout110keVformultiple8–9MeV

α

-particleevents storedinasingle tracewithtimedifferencesontheorderof1 μs. Fora detaileddescription ofTASCAandits detectionsystemssee [28,30,36,37].

Fig. 1. (Coloronline.) (a)Energy spectraofbeam-offα-likeeventsdetectedwithin anenergyrangeof6–9MeVwithoutfurtherconditions.Thespectrumwithinan energyrangeof8–18MeVisshownasinset.(b)αparticlescorrelatedtotheirRI signalswithin0.5s.First(c)andsecond(d)αparticlesfromRI-α(≤100 s)-α(≤10 s) correlations.

2. Experimentalresults

AnenergyspectrummeasuredwiththeDSSDduring beam-off periods at an accumulated beam dose of about 4

.

5

×

1018 _part

is shown in Fig. 1(a). Energies were extracted from the traces collected withthe digitalbranches(48 Y-strips), by adopting the single-signalamplitudeestimationprocedure.Severalpeaks, origi-natingfrom

α

decays,areevidentontherelativelyenhanced back-ground, whose originis dueto the summing of the 48different Y-stripsdataandthepresenceofvariouspile-upevents.Thelatter correspondtothedetectionofsubsequent

α

particleswithinshort times(



μs),whoseenergiesaresummedupandbecomemore ev-identinthehigherenergyrange,E

=

8–18 MeV (shownasinsetin Fig.1(a)). Correspondingtracesof thesemulti-signalevents were storedandenergiesextractedwithmulti-signalamplitude estima-tionprocedure.Therefore,each

α

-likeevent,i.e.,occurringduring beam-offwithE

<

18 MeV,wasconsideredasoriginatingfromthe decayof an implantednucleus. We note that in thisexperiment thousandsoffission-likeevents(DSSD signalswith E

>

100 MeV andin anti-coincidence with MWPC) were detected. However, it wasnotpossibletoidentifytheirisotopicorigin.Identificationsof

α

linesandpile-upswereachievedinthespatial ( X and Y axes) andtimecorrelationanalyses,betweenimplantationsignalsand/or

α

-likeevents,andusingliteraturedata[38].

Accordingtoknownpropertiesofnon-fusionreactions[26,28], theenergiesofrecoilimplantations(RI)wereselectedtobewithin therangeERI

=

30–110 MeV.The RI-likeeventswere requiredto

occurincoincidencewithMWPCsignalsandduringbeam-on pe-riods.

ThefirststepwastheidentificationofimplantednucleiinRI-

α

correlations with a search time up to

≤

0.5 s due to the count-ingrateforRI-likeevents.Anenergyspectrumofsuch correlated

α

particles, with E

=

6–9 MeV is shownin Fig. 1(b).Comparing ourexperimentally determinedhalf-livesand

α

energieswith lit-erature data, 215Ra, 214,214m_Fr, 213_Rn, 212,212m_{At, and}211_{Po were}

identified.

Furthermore, RI-

α

–

α

, RI-

α

(7–18 MeV)-

α

, RI-

α

-

α

(7–18 MeV), RI(pile-up)-

α

(anoteonthiscorrelationwillbegivenbelow),

α

–

α

and

α

–

α

–

α

correlationanalyseswereperformedtoidentifythe re-mainingpeaksandthepile-ups(cf.Figs.1(a)and(b)).Correlation search times betweenthe members have been varied depending onthecorrelation types.In

α

–

α

–

α

correlations, forinstance, iso-topeswithT1/2 intheorderofminuteswere possibletobe

iden-tified.InFigs.1(c)and1(d),energyspectraofthefirstandsecond

α

membersof RI-

α

–

α

correlationsare shownas exampleof the abovementionedanalyses. Theidentificationprocedure was com-pleted withthe analysisof tracesstored inthe digital branch of CANDItoresolvethepile-upevents[36,37].Asaresultofthispart of theanalysis, 37isotopes were identified asbeingdirectly im-plantedand11asdaughterorgranddaughterofRI’s.

In total, 53 isotopes with mass numbers 210

≤

A

≤

226 and atomic numbers 83

≤

Z

≤

90 were identified; 42 of them were directlyimplanted.

Finally,thenumbersofeventscorresponding topeaksat8.70, 8.48, 8.43, 8.09 MeV in the beam-off spectrum were compared withthededucednumbersof215Ra,214mFr,214Fr,213Rnfromthe correlationanalysis(cf.Fig.1(a)and(b))toensurethestatistical agreement. Only 85% of the events were found to be correlated with RI. The remaining 15%, then, were searched in the corre-lations (e.g., RI-

α

) where the ERI range was expanded to lower

energies.

Energy spectraof RIcorrelated with

α

decaysof 213_Rn, 215_Ra

and223_{ThareshowninFig.2(a).Twowellseparatedcomponents}

are visible for all cases. The RI withenergies

≤

30 MeV account well for those missing 15% mentioned above. The high-energy component(HEC),withERI

>

30 MeV,featuresapeak-like

distribu-tioncenteredataround70 MeV,whilethelow-energycomponent (LEC)hasatail-likeshape.Accordingly,partoftheLECmaybenot detected duetothe thresholdsofthedetectionsystem. Time dis-tributions ofRI-

α

(215Ra)correlationsforHEC andLECeventsare showninFig. 2(b).Both distributions,showingthe samehalf-life of215Ra,indicatetheproperassignmentoflowenergeticrecoils.

TheLECrecoilscouldpossiblybegeneratedbyHECones pass-ing throughthickerlayers (duetoinhomogeneities) ofthetarget, MWPC-windowsordead-layersofthe DSSD.Thespectrum ofthe

249_Cf–

_α

_{-particles passingthrough TASCA andtheMWPCas}

mea-sured with the DSSD shows a single well-defined peak. There-fore,significantinhomogeneitiesofthetarget,MWPC-windowsor dead-layers oftheDSSD arenegligible. Consequently,we exclude that theLECcanbe artificiallygeneratedby HECpassingthrough thickerlayers.

ThepossibilitythatLECrecoilsoriginatefromthescatteringof HECrecoilsisexcluded.Theamountofsuch scatteredHECrecoils isnegligibletakingintoaccountthenumbersofobservedHECand thecrosssectionsfortheelasticscattering.Therefore,theLECare mostlikely associated withthe reaction itself. This conclusion is supported from the observed energy losses of LEC and HEC re-coilsintheMWPC(



E)showninFig.2(c).Ithasbeenfoundthat the HEC and LEC recoils havelost different amounts of energies inthe MWPC,asshownin thecaseof215Ra.Such behaviorwell agreeswiththesimulatedenergylossesofrecoilswithtwo differ-entkineticenergies showninFig.2(d).Detailsonsimulationsare discussedlater.

202 A. Di Nitto et al. / Physics Letters B 784 (2018) 199–205

Fig. 2. (Coloronline.) (a)Implantation-energy distributions of215_Ra, 213_Rn,and 223_{Thisotopes.(b)Timedistributionsofα(}215_{Ra)eventsinbeam-offperiods} corre-latedwithHEC(multipliedbyafactor0.16)andLECrecoils.Thehalf-livesobtained byfitting experimentaldata (circles)with universal radioactive decay functions (lines)areconsistentwiththeliteraturevalue(T1/2=1.67(1)ms[38]).(c)Estimated experimentaland(d)SRIMsimulatedenergylossesintheMWPC(E)andDSSD (ERI)detectorsofRIof215Ra events.(e)Relativeyieldsofsameeventsasfunction ofDSSDX-stripofHECandLECrecoils.

Furthermore,thankstothesmallamplitudeofRIsignalsofthe LEC events,RI and

α

signalsoccurring ina short time (downto a fraction of μs)were recorded withfull shapesin single traces withinthedynamicalrangeoftheuseddigitalelectronicssystem, [39].Therefore,by analyzingtheRI(LEC)traceswithmultiple sig-nalswecouldidentifythedecaysofshort-lived219_Th,218_Ac,217_Ra, 216_Frand215_{Rnnucleiforthefirsttimeinthistypeofexperiment.}

Thedistributions ofHECandLECrecoils alongthe X -strips of the DSSDare shownin Fig.2(e). The ratesof both HECandLEC recoilsincreasedonthelow-B

ρ

side.Evidently,suchdistributions pointtowardsapartialcollectionofrecoilswithwidelyspreadand muchloweraverage B

ρ

(

<

1.97 Tm)thanthesetvalue(2.14Tm). TheaverageB

ρ

ofHECandLECrecoilsof215_{Ra canbeestimatedif}

their kineticenergies(EK)andmeancharge statesareknown.We

deducedtheEK fromtheenergymeasured intheDSSDbytaking

intoaccountpulseheightdefects[40] andenergylosses[34].Then, the HEC-peak at 70MeV corresponds to 150MeV, and10 MeV, takenasarepresentativevaluefortheLEC,correspondto45MeV. Considering the EK of 150and 45 MeV forHEC andLEC recoils,

respectively,andthemeanchargestatesaccordingtotheformulas in Ref. [41], we calculated the corresponding B

ρ

=

1

.

76 Tm and 1.67Tm. Forthe simulationsof theFig. 2(d),we used the above valuesforkineticenergies.

Therefore,themainpartofrecoilswithsuchvaluesofB

ρ

will misstheDSSD,whichisinagreementwiththeobservedHECand LECdistributions(seeFig.2(e))andwiththeabovediscussion.

The absolute cross sections for the identified nuclei cannot be given due to the unknown efficiencies for their transmission through TASCA and the efficiency for the DSSD coverage (see Fig.2(e)).Tostill arriveataroughvalue,wetakeadetection effi-ciencyforfull-energy

α

particlesintheDSSDas55%,andabeam intensityof3

×

1012_{part/s.Undertheseassumptions,the}

implan-tationratesof213_{Rn HECand}223_{Th LEC,representativesof}

identi-fiednucleiwithhighandlowratesintheDSSD,respectively,were estimatedas8

.

1

×

10−3 and1

.

3

×

10−4 eventspersecond.These valueswouldcorrespondtotheproductofefficiencyandcross sec-tion (

σ ε

,where

ε

isthe overallefficiencysimilar toRef. [26])of 4.4and0.07nb,respectively.Thelattervalueisourlower

σ ε

limit forthenucleiidentifiedinthepresentexperiment.

3. Discussionandsummary

All fifty-three identified isotopes are located “north-east” of

208_{Pb, on the N vs. Z plane as shown in Fig. 3(a). The}

popu-lation of nuclei in this region, far from the target, is similar to known experimental findings for reactions with actinide targets measured over a wide angular range[18]. Onlydata obtainedat SHIP for the 48Ca

+

248Cm reaction [26] can be compared with thepresentresults.Inbothworkstheimplantednucleihavebeen identified via their characteristicradioactive decay properties. At SHIP,theseparationofheavyionswithparticularmassandcharge state isperformedaccordingtoionvelocity.Incontrast,the sepa-ration inTASCA isbased ontheaverage magneticrigidity, where noparticularlyselectedmass,velocityandchargestateofionscan be isolated. Moreover, the gas-filled separators often have short lengths(forexample3.5mofTASCAand12mofSHIP)compared to thoseoperatinginthe vacuummode thuspotentially result to thehighertransmissionandtheshortflightpathfortheproducts, seeRef. [42].Studiesatbothtypesofseparators,potentially com-pleted by radiochemical studies [20], appear best suited to shed further lightonthe reactionmechanismleadingto theformation oftheseproducts.

In Fig. 3(a) the isotopic distribution of the 48Ca

+

248Cm re-action products is showntogether withthe presentresults.Both distributions overlapwidely,despitesome deviations,such as rel-atively narrow distribution of isotopes along the N and Z axes in thepresentdata.A comparisonofdirectly implantednucleiis showninFig.3(b).Inthe presentdatathe gapbetweenN

=

127 and130,aregioncontainingpredominatelyveryshort-livednuclei (mostlydecayingduringtheflightthroughTASCA),issignificantly reducedcompared tothe 48_Ca

₊

248_{Cm data,likely thanksto the}

implementation ofa multi-pixelizedDSSD andCANDI,aswell as totheshorterflighttime,aboutafactor3,betweenthetargetand theimplantation detectoratTASCAcomparedto SHIP.Itisworth notingthat nucleiaround/abovethe 249Cf-targetwere not identi-fiedduetothelimitedsensitivityofthecorrelationtechniqueused here.

Fig. 3. (Coloronline.) Cutoutofthechartofnucleiintherelevantregion[38].(a) Theisotopesidentifiedbytheirαdecayandgeneticcorrelationsin50_Ti+249_Cf re-actions(bluecircles)arecomparedwiththoseidentifiedin64_Ni+207_{Pb [43] and} 48_Ca+248_{Cm [26] reactionsmarkedingreenandorange,respectively.(b)Isotopes} directlyimplantedintothefocalplanedetector.Thefilled(empty)bluecircles cor-respondto 50_Ti+249_{Cf reaction productsidentifiedincorrelation analyseswith} (without)RI-likeevents.Theorangeframesrelateto48_Ca+248_{Cm reactionproducts} observedincorrelationwithRI-likeevents[26,25].

The isotopic distribution ofthe 50_Ti

₊

249_{Cf reaction products}

canbeaffectedifthetargetcomprisedaPbcontamination,which canalsoproducesimilarproducts in50Ti

+

Pb reactions.However, thiscontributionislikelynegligibletakingintoaccounttheupper limitofleadimpurity(

<<

1%)andtheexpectedN– Z distributions ofreactions withPb, cf.thedataforthe64_Ni

₊

207_{Pb reaction}

in-cludedinFig.3(a)[27].

Onecanalsocomparethe

σ ε

valuesofdirectlyimplanted nu-cleiinthe50Ti

+

249Cf and48Ca

+

248Cm reactions[26].For quanti-tativeaspectsofthisdiscussionitisimportanttobearinmindthat theTASCAsettingswere settomaximize theefficiencyforfusion products,whereasSHIPwas operatedatsettingsbettersuitedfor non-fusionproducts.InFig.4the

σ ε

valuesforTh,AcandRa iso-topesareshownasfunctionofN.Ingeneral,asimilarbehavior of thepresentdatatothoseofthe48Ca

+

248Cm reaction[26] canbe noticedmostpronouncedforThisotopesdespitethedifferencesin termsofreactionsandseparationtechniques.Nevertheless, devia-tionsintheabsolutevaluesexistandcan beattributedtofactors including the different overall efficiencies of the two separators andreactions. For instance about4 times lower values, on aver-ageforTh isotopes,obtainedatTASCAmaybe duetothepartial collectionbytheDSSDofrecoilsreachingthefocalplaneofTASCA (seeFig.2(e))and/ortothedifferencesinthepopulationofthem through nucleon exchange process. By assuming that the

σ ε

for

Fig. 4. (Coloronline.) Productofcrosssectionandefficiency,σ ε,fordirectly im-plantedTh,AcandRaisotopes.The50_Ti+249_{Cf reactionvalues(solidsymbols)are} comparedwiththe48_Ca+248_{Cm reactionones(opensymbols)[26].The}225_{Uσ ε} (opendiamond)from[26] wasalsoincludedinthelowerpanelasrepresentativeof productswithZ>90.Thedashedlinesshowthe1nblevelandthedottedlinethe

σ εlowerlimitforthenon-fusionproductsidentificationinthepresentexperiment.

Uisotopesis onaveragereducedby afactor4compared toSHIP ones[26],similarly totheThcase,one canexpect todetect225U atTASCAwitha

σ ε

of0.03nb.Thisvalueisbelowthe experimen-talsensitivityfornon-fusionproductsinthiswork,thusthislatter can be the reason for the non-observation of 225U. Instead, the non-observation of other Uisotopes cannot be explained by the lower cross section limit argument, having been measured with crosssectionssignificantlyhigherthan225UatSHIP.However,we avoidtoprovideaninterpretationonthe

σ ε

ofheavierUisotopes, notincludedinFig.4,becausethey wereextractedinananalysis thatdidnotincludeRI-likeevents.

Thenucleiobservedinthe50Ti

+

249Cf and48Ca

+

248Cm reac-tions originate fromaprocess wherea largenumberofnucleons flowsinthedirectionoftheprojectilenucleuswithcrosssections significantly higher than the fusion-evaporation ones. Significant massexchangefromtargettoprojectileoftenindicatesnuclear re-actions occurringon reaction timescales that are longenough to allowtheformationofadinuclearsystem,butshortenoughforit tobreakbeforecompletemassequilibrationisachieved,i.e., favor-ingQFand/ormulti-nucleontransfer,whichareknowntoproduce theobservednuclei.

The abovearguments aresupported by theknown features of fusion systematics. According to this, a strong hindrance for fu-sioninthe 50Ti

+

249Cf fusionisexpected[12].Thisfollowsfrom the criteriaof the charge numbers ofprojectile and target prod-ucts, where the present reaction has ZpZt

=

2156, which is far

abovethelower limit(

≈

1600),beyondwhichthefastQFprocess isknowntosetin,accordingtoRef. [11].

ItiswellknownthattheheavyfragmentsfromQFofheavy-ion induced reactions with actinide targets reveal mass distributions withmaximaaround A

=

215–220 [12,5,7,8],duetotheinfluence of thedoubly magic 208Pb.Thus, nuclei observed inthis work – andalsoin48_Ca

₊

248_{Cm –couldoriginatefromQF.}

Theobservedtwo componentsinimplantation energiesof nu-clei fromthe50_Ti

₊

249_{Cf reaction,thus,could be explainedwith}

dynamical propertiesoftheQFprocess. SinceQFtypically occurs during the rotation of the dinuclear system, the heavy fragment withmass A emittedwithavelocityvcmatdifferentanglesinthe

centerofmassframe

θ

cm canbedetectedinthelaboratoryframe

within wide angular and velocity ranges. According to Ref. [27], thetwo energycomponents ofheavy fragmentscanbe explained by emission of them at

θ

cm

=

0◦ and 180◦, i.e. along the beam

axis,withvelocitiesofvHEC

=

vcm

+

vCNandvLEC

=

vcm

−

vCN,

re-spectively, wherevCN isthevelocity ofthe compoundnucleusin

204 A. Di Nitto et al. / Physics Letters B 784 (2018) 199–205

Table 1

Experimentalandcalculatedkineticenergies,EK,(inMeV)andemissiondirections (forward(→)orbackward(←)inthelaboratoryframe)ofheavyfragments(RI) fromthethreereactions.Twocasesoftheiremissionsinthecenterofmassframe (θcm=0◦andθcm=180◦ correspondingtoHECandLEC,respectively)aregiven forthreereactions,whosebeamenergies(ElabinMeV)areindicated.Theisotope 215_{Ra wasconsideredinthefirstreaction,}215_{Ra and}223_{Th forthesecond,and}226_Ac forthethird.Allexperimentaldataweremeasuredintheforwarddirectioninthe laboratorysystem.

Reaction Elab RI

HEC (θcm=0◦) LEC (θcm=180◦)

EK(Dir.) EK(Dir.)

Exp. Calc. Exp. Calc.

64_Ni207_{Pb 360} 215_{Ra 194}a_{(→) 193 (→) 10}a_{(→) 8.5 (→)}

50_Ti+249_{Cf 288}

 215_{Ra 150 (→) 182 (→) 45}*_{(→) 2.9 (←)}

223_{Th 142 (→) 176 (→) 44}*_{(→) 1.7 (←)}

48_Ca+248_{Cm 256} 226_{Ac 110}b_{(→) 151 (→) – 1.2 (←)}

a _{Deduced frommeasuredvelocitiesinRef. [27].} b _{Deduced frommeasuredvelocitiesinRef. [26].} * _{Representative valuefortheLEC.}

emittedat

θ

cm

=

180◦ wouldnotbedetected atbackwardangles,

butinsteadinforwarddirectioninthelaboratoryframeasinthe caseof

θ

cm

=

0◦.Insuch cases,heavy fragmentswithtwo

differ-ent velocities will be observed at forward angles, similar to the presentfindingsshowinga HECanda LEC.Onlyonesuch exper-imental case is known forthe 64_Ni

₊

207_{Pb reaction[27], where}

twovelocitycomponentsfornon-fusionproducts(showninFig.3) have been measured. Measured velocities for both components havebeencalculatedwithintheabovementionedapproach,where vcm was deduced fromthe TKE accordingto theViola

systemat-icsfor QFwith an asymmetric mass split [44]. A fairagreement betweenthe estimatedand experimental velocities was reached, supportingthe suggestedscenariofortheexplanationofthe HEC andLEC[27].Therefore,within thisapproach,aninterpretationof thepresentdatacan be given.Moreover, we calculatedthe ener-giesoftheLECandHECoftheseidentifiedproducts,namely215_Ra

and223Th.Togain insighton thisapproach wehavegivenin Ta-ble1thevelocitiesoftheLECandHEC,expressedintermsofEK,

andemissiondirectionsinthelaboratoryframeresultingfrom cal-culationstogetherwiththeexperimentallydeducedones.

The results for 215_{Ra fromthe} 64_Ni

₊

207_{Pb reaction are also}

showninTable1,wherebothHECandLEChavebeendetectedin theforwarddirection.

Atfirst,wecalculatetheenergiesfor215_{Ra forwhichbothHEC}

andLEChavebeenobservedinthe64Ni

+

207Pb and50Ti

+

249Cf re-actions.InthecaseoftheLEC,thecalculatedvcmwaslargerthan

the vCN (cf. Table 1) for the 50Ti

+

249Cf reaction, which means

that 215Ra isemitted in backward directions,i.e.no detection of theLECin forwarddirectionis expected.Similarly, thecalculated LECvelocity of theheavier isotope 223Th alsoprevents thisfrom entering intothe TASCA acceptance angle.At thesame time, ex-perimentalEK ofHECare overestimatedinboth cases,215Ra and 223_{Th. Therefore, the present results have poor numerical}

agree-ments withthe calculationsin both theHEC andthe LEC. These deviations could be due to non-directly measured experimental energies of 215Ra and 223Th, forwhich only rough estimates for bothHECandLECaregiven.However,thevelocityofHECof226_Ac

fromthe48Ca

+

248Cm reactionhavedirectlybeenmeasuredatthe velocityfilterSHIP,thus,one cancompareitwiththecalculation. Despitethe measurement technique forthe HEC-energy, 226_{Ac is}

stilloverestimatedby asimilaramount(

≈

25–35%) asinthecase of the presentdata, cf.Table 1. Accordingly, deviations could be associated withthe overestimationof the QF TKE and/or dueto thekinetic-energydissipation inthe heavyfragment. Observation oftheLECof226_{Ac in}48_Ca

₊

248_{Cm reactionsisnotpredictedby}

thecalculations,asonecouldexpect.Wenotethatduetothelack ofexperimentaldata,thiscannotpresentlybeconfirmed.

It could be inferred that a quantitative description of the ki-netic energies of non-fusion products from the 50Ti

+

249Cf and

48_Ca

₊

248_{Cm reactionscannotbegivenwithinthecurrent}

knowl-edgeontheTKEoftheQF.However,aqualitative explanationvia the QFscenarioisstill possible.This,though,wouldsubstantially profit from further dedicated experiments on the kinematics of non-fusionproductsfrom50_Ti

₊

249_{Cf and/orsimilarreactions.}

Finally, inthis letterwe demonstratedthat theforward-angle gas-filled separator can be used forthe studyof the non-fusion reaction mechanisms and, consequently, TASCA can be suitable for the synthesis of the exotic nuclei originating in other reac-tion channelsthanfusion-evaporation. TheobservationsatTASCA reportedhereprovideastimulusforfutureexplorationswith op-timizedoperation ofgas-filledseparatorsfortheisolation of non-fusionproductstotapthefullpotentialofthismethodforthe in-vestigationofnucleiwithlow-productionyield.Fifty-threeisotopes ofelementsfromBitoTh,identifiedwithhelpoftheadvanced de-tection system, showsthe sudden disappearanceofthe detection yield oftheelements Z

>

90 inthenon-fusion reactionchannels that issomehowdifferentfromtheresultsobtainedatthe veloc-ityfilterSHIP.Presently,wecannotdrawafinalconclusiononthe reasonfordeviationsintheisotopicaldistributions,because differ-ent experimentaltechniques,detectionsetupsandreactionswere adopted inthetwo experiments.Thereforeto disentanglethe in-fluence of the differentaspects responsible of such deviations, it appears ideal to investigate the same target/projectile combina-tion at differenttypes ofseparators, like SHIPand TASCA, under identical reaction conditions. Nevertheless, we note that produc-tion yields of U isotopes suddenly decreases compared to those of Thonesaccordingto thefusion-evaporation reaction data.For instance, in the 48Ca

+

176Yb reaction Th isotopes are produced withcrosssectionvaluesintheorderof100 μb[45],whereasthe cross sectionsofUisotopes inthe 50_Ti

₊

176_{Yb reactionare only}

fewnb[37].Furthermoretodefinethelimitsoftheisotopical dis-tributions, one should consider the measurements of

β

-decaying and/or long-livednuclei for which differentdetection techniques arerequiredcomparedtothosepresentlyemployed.Recently,such asetupwhichincludesthecombinationofthechemicalseparation andhighefficient measurements ofALpha, BEtaandGAmma de-cays ALBEGAisunderfinalconstruction [46].The mostintriguing result of the present work is the observation of recoils emitted at forward angle with energy distribution showing two compo-nents which seem to be associated with reaction mechanism of the particular non-fusion channel. By considering the significant nucleon flow populating the nuclei located “north-east” of 208_Pb

andthe observationoftwo energycomponents, thepossible ori-gin oftheidentifiedisotopeswas suggestedtobequasifissionfor whichforward-angledatascarcelyexist.Theresultsofthepresent work motivate further studies on nuclear reactions aimed at ex-amining the presence of a low-energy componentand providing wide-rangeisotopicdistributions thatcouldshedlightonthe un-derstandingofthedifferentreactionmechanisms.

Acknowledgements

We are grateful to GSI’s ion-sourceand UNILAC staff. Twoof us(A. DiNittoandJ. Khuyagbaatar)thankDr.S. Heinzforfruitful discussions. Thisworkwas inpartfinanciallysupported byBMBF contract-No. 06MZ7164,theSwedishResearchCouncil under con-tractsVR2008-4240andVR2011-5253.Oneofus(D. Ackermann) is supported by the European Commission in the framework of CEA-EUROTALENT2014-2018(noPCOFUND–GA–2013- 600382). WethanktheLBNLNuclearScienceDivision’sR.F.FairchildII,N.E. Reeves, J.A. Van Wart andthe Radiation Protection Group of the

Environmental Health andSafety Division for their support with thepreparationandexecutionofthe249CfshipmenttoGermany. References

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