Production of Heavy Clusters with an Expanded Coalescence Model in CEM

0.48 MB·English

by  Leslie M. Kerby

ProductionofHeavyClusterswithanExpandedCoalescenceModelinCEM
LeslieM.Kerby,1,2StepanG.Mashnik1
1LosAlamosNationalLaboratory,LosAlamos,NewMexicoUSA
2UniversityofIdaho,IdahoFalls,IdahoUSA
[email protected]
INTRODUCTION 300MeV<T <1000MeV
Theproductionofheavyclustersinnuclearreactionsis
importantinawidevarietyofapplications:radiationshielding, p (d) = 150MeV/c;
c
space engineering and design, medical physics, accelerator p (t) = p (3He)=175MeV/c; (2)
5 design,andmore. AccordingtotheCascadeExcitonModel c c
1 (CEM) [1, 2], there are three ways high-energy heavy clus- pc(4He) = 175MeV/c.
0 ters can be produced. The first way is via coalescence of
2
nucleonsproducedintheIntraNuclearCascade(INC).The
Ifseveralcascadenucleonsarechosentocoalesceinto
r second way is via the preequilibrium model. The last way
a compositeparticles,theyareremovedfromthedistributions
isviaFermibreakup. PreviousworkinCEMexaminesthe
M of nucleons and do not contribute further to such nucleon
impactofexpansionsofthepreequilibriummodelandFermi
characteristicsasspectra,multiplicities,etc.
breakupmodelonheavyclusterproduction[3–7]. Thepresent
2
workstudiestheimpactofexpandingthecoalescencemodel
COALESCENCEEXPANSION
onheavyclusterspectra. CEM03.03[2],thedefaulteventgen-
]
h eratorintheMonteCarloN-Particletransportcodeversion6
TheCoalescenceModelinCEM03.03allowsforcoales-
-t (MCNP6)[8]forintermediate-energynuclearreactions,isca- cenceupto4He. Wehaveexpandedthistoadditionallyallow
cl pableofproducinglightfragmentsupto4Heinitscoalescence forthecoalescenceof6He,6Li,7Li,and7Be.
model.Inthepresentstudy,wehaveexpandedthecoalescence
u CEM03.03usesthesimplestversionofthecoalescence
n modeltobeabletoproduceupto7Be. Preliminaryresultsare model[9,10]andchecksonlythemomentaofnucleonsemit-
[ promising. tedduringthecascadestageofreactions, withoutchecking

theircoordinates.
2
BACKGROUND
Themomentum, p,ofeachnucleoniscalculatedrelativis-
v
8 Whenthecascadestageofareactioniscompleted,CEM ticallyfromitskineticenergy,T (CEM03.03providesinits
8 output files the energy of particles, but not their momenta).
usesthecoalescencemodeldescribedinRef. [9,10]to“cre-
4 Coalescenceoccursifeachnucleoninthegrouphas|∆p|≤ p ,
ate”high-energyd,t,3He,and4Hebyfinal-stateinteractions c
6 where ∆p is defined as the difference between the nucleon
amongemittedcascadenucleonsoutsideofthetargetnucleus.
0 momentumandtheaveragemomentumofallnucleonsinthe
ThecoalescencemodelusedinCEMissimilartoothercoa-
1. lescencemodelsforheavy-ion-inducedreactions. Themain group.
0 differenceisthatinsteadofcomplex-particlespectrabeingesti- ThecoalescencemodelofCEM03.03firstchecksallnu-
5 cleonstoform2-nucleonpairs,theirmomentapermitting. It
matedsimplybyconvolvingthemeasuredorcalculatedinclu-
1 sivespectraofnucleonswithcorrespondingfittedcoefficients, then checks if an alpha particle can be formed from two 2-
: nucleonpairs(eitherfrom2n-ppairsorfroman-nandp-p
v CEM03.03usesinitssimulationsofparticlecoalescencereal
pair). Afterthisitcheckstoseeifanyofthe2-nucleonpairs
i informationaboutallemittedcascadenucleonsanddoesnot
X leftcancombinewithanothernucleontoformeithertritium
useintegratedspectra. (Notethatthecoalescenceintroduced
r or3He. Andlastly,itcheckstoseeifanyofthese3-nucleon
recentlyintheLiègeintranuclearcascade(INCL)[11-14],is
a groups(tritiumor3He)cancoalescewithanothernucleonto
in a way similar to the coalescence considered by CEM as
form4He.
proposed in Ref. [9, 10], with the main contrast being that
The expanded coalescence model then takes these 2-
INCLconsiderscoalescenceofINCnucleonsontheborderof
nucleon pairs, 3-nucleon (tritium or 3He only) groups, and
anucleus,insidethetarget-nucleus,whileCEMcoalescesINC
4Hetoseeiftheycancoalescetoformheavyclusters. 4He
nucleonsoutsidethenucleus.) Weassumethatallthecascade
nucleonshavingdifferencesintheirmomentasmallerthan p can coalesce with a 3-nucleon group to form either 7Be or
c 7Li. Two 3-nucleon groups can coalesce to form either 6Li
andthecorrectisotopiccontentformanappropriatecomposite
or6He. And4Hecancoalescewitha2-nucleonpairtoform
particle. Thecoalescenceradii p asusedinCEM03.03are:
c either6Lior6He. Allcoalescednucleonsareremovedfrom
Incidentenergy,T,<300MeVor>1000MeV
thedistributionsofnucleonssothatourcoalescencemodel
conservesbothatomic-andmass-numbers.Inaddition,thisex-
p (d) = 90MeV/c; pansionrequiresaninsignificantamount(2–3%)ofincreased
c computationtime.
pc(t) = pc(3He)=108MeV/c; (1) ForadditionaldetailsoftheCoalescenceModelexpan-
p (4He) = 115MeV/c. sion,seeRef.[15].
c
CoalescenceParameter p low- and moderate-energy, thus improving agreement with
c
experimentaldataintheseenergyregionsaswell.
Asmentionedabove, pc determineshowdissimilarthe Fig.3displaysexperimentalresultsofthereaction480
momenta of nucleons can be and still coalesce. pc was ex- MeV p + natAg → 6Li by Green et al. [17] (green circles),
panded to also include a value for heavy clusters, or light compared with simulations from results from CEM03.03F
fragments(LF): pc(LF). Ournew pc’sforincidentenergies, withoutcoalescenceexpansion(bluesolidlines),CEM03.03F
T,lessthan300MeVorgreaterthan1000MeVare: withcoalescenceexpansion(reddashedlines),andtheorig-
inal CEM03.03 (brown dashed-dotted lines). Even without
pc(d) = 90MeV/c; the coalescence expansion, CEM03.03F (which contains a
p (t) = p (3He)=108MeV/c; (3) preequilibriumexpansionandatotalreactioncrosssectionim-
c c
p (4He) = 130MeV/c. provement)yieldsmuchbetterresultsthanCEM03.03without
c
anyoftheseimprovements.Addingthecoalescenceexpansion
p (LF) = 175MeV/c.
c producesevenbetterresults.
Similarresultsformanyotherreactionsinducedbypro-
Andfor300MeV<T <1000MeV
tons, neutrons, and heavy ions (the last are simulated with
LAQGSM03.03[2],butwithanextendedcoalescencemodel
p (d) = 150MeV/c;
c asdescribedinRef. [18])andfurtherdiscussionscanbefind
pc(t) = pc(3He)=175MeV/c; (4) inRefs.[15,18].
p (4He) = 205MeV/c.
c
p (LF) = 250MeV/c.
c
Note that the p (4He) was also increased compared to
c
the old p values. Too many alpha particles were lost (coa-
c
lescedintoheavyclusters),andthereforewecompensatedby
increasingthecoalescenceof4He.
RESULTSANDANALYSIS
Examples of results of our coalescence expansion are
displayedinFigs.1–3. TheupgradedCEM03.03Fwithout
coalescence expansion (blue solid lines) and the upgraded
CEM03.03Fwithcoalescenceexpansion(reddashedlines)
are compared with experimental data (green circles). This
coalescenceexpansionanalysisispartofalargerproject[3–7]
aimedatproducinghigh-energylightfragmentsinspallation
reactions.CEM03.03FreferstotheupgradedCEM03.03code,
whichhasbeenupgradedwithaModified-Exciton-Modelex-
pansionandatotalreactioncrosssectionmodelimprovement.
Thebluesolidlinescontainbothoftheseimprovementsover
theoriginalCEM03.03,andthereddashedlinescontainboth
oftheseimprovementsplusthecoalescenceexpansion.
Fig.1displaysfragmentproductionspectraof6Heand
6Liforthereaction1200MeVp+197Au. Experimentaldata
byBudzanowskietal. [16](greencircles)arecomparedwith
resultsfromCEM03.03Fwithoutcoalescenceexpansion(blue
solidlines)andCEM03.03Fwithcoalescenceexpansion(red
dashedlines). Thecoalescenceexpansionincreasesthepro-
ductionofhigh-energy6Heand6Li,andimprovesagreement
withexperimentaldata.
Fig. 2 displays the fragment production spectra of 7Li
and7Beforthereaction480MeVp+natAg. Experimental
databyGreenetal. [17](greencircles)arecomparedwith
Fig.1. ComparisonofexperimentaldatabyBudzanowski,et
resultsfromCEM03.03Fwithoutcoalescenceexpansion(blue
solidlines)andCEM03.03Fwithcoalescenceexpansion(red al. [16](greencircles)fortheproductionof6Heand6Liat
dashedlines). Again,thecoalescenceexpansionincreasesthe anangleof20◦fromthereaction1200MeVp+197Au,with
productionofheavyclusters,andimprovesagreementwith resultsfromCEM03.03Fwithoutcoalescenceexpansion(blue
experimentaldata. solidlines)andCEM03.03Fwithcoalescenceexpansion(red
Thesereactionsalsohighlighthowthecoalescencecan dashedlines).
produce heavy clusters not just of high-energy, but also of
opportunitytostudywithsomeoftheworld’sgreatestex-
pertsinnuclearphysics,particularlyhigh-energyphysics.
3. Dr. AkiraTokuhiro,forhiscontinuedsupportandexper-
tiseinservingasmythesisadvisor.
ThisstudywascarriedoutundertheauspicesoftheNa-
tionalNuclearSecurityAdministrationoftheU.S.Department
ofEnergyatLosAlamosNationalLaboratoryunderContract
No. DE-AC52-06NA25396.
This work is supported in part (for L.M.K.) by the M.
HildredBlewettFellowshipoftheAmericanPhysicalSociety,
www.aps.org.
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