Neutron occupancy of the 0d5/2 orbital and the N = 16 shell closure in 24O

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Neutron occupancy of the 0d5/2 orbital and the N = 16

shell closure in 24O

K. Tshoo, Y. Satou, C.A. Bertulani, H. Bhang, S. Choi, T. Nakamura, Y.

Kondo, S. Deguchi, Y. Kawada, Y. Nakayama, et al.

To cite this version:

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Neutron

occupancy

of

the

0d

₅

/

2

orbital

and

the

N

=

16 shell

closure

in

24

O

K. Tshoo

a

,

∗

,

Y. Satou

b

,

C.A. Bertulani

c

,

H. Bhang

b

,

S. Choi

b

,

T. Nakamura

d

,

Y. Kondo

d

,

S. Deguchi

d

,

Y. Kawada

d

,

Y. Nakayama

d

,

K.N. Tanaka

d

,

N. Tanaka

d

,

Y. Togano

d

,

N. Kobayashi

e

,

N. Aoi

f

,

M. Ishihara

g

,

T. Motobayashi

g

,

H. Otsu

g

,

H. Sakurai

g

,

S. Takeuchi

g

,

K. Yoneda

g

,

F. Delaunay

h

,

J. Gibelin

h

,

F.M. Marqués

h

,

N.A. Orr

h

,

T. Honda

i

,

T. Kobayashi

j

,

T. Sumikama

j

,

Y. Miyashita

k

,

K. Yoshinaga

k

,

M. Matsushita

l

,

S. Shimoura

l

,

D. Sohler

m

,

J.W. Hwang

b

,

T. Zheng

n

,

Z.H. Li

n

,

Z.X. Cao

n

a_{RareIsotopeScienceProject,InstituteforBasicScience,Daejeon305-811,RepublicofKorea} b_{DepartmentofPhysicsandAstronomy,SeoulNationalUniversity,Seoul151-742,RepublicofKorea} c_{TexasA&MUniversity-Commerce,POBox3011,Commerce,Texas75429,USA}

d_{DepartmentofPhysics,TokyoInstituteofTechnology,Tokyo152-8551,Japan} e_{DepartmentofPhysics,UniversityofTokyo,Tokyo113-0033,Japan} f_{ResearchCenterforNuclearPhysics,OsakaUniversity,Osaka567-0047,Japan} g_{RIKENNishinaCenter,Saitama351-0198,Japan}

h_{LPC-Caen,ENSICAEN,UniversitédeCaen,CNRS/IN2P3,14050Caencedex,France} i_{DepartmentofPhysics,RikkyoUniversity,Tokyo171-8501,Japan}

j_{DepartmentofPhysics,TohokuUniversity,Miyagi980-8578,Japan} k_{DepartmentofPhysics,TokyoUniversityofScience,Chiba278-8510,Japan} l_{CenterforNuclearStudy,UniversityofTokyo,Saitama351-0198,Japan}

m_{InstituteforNuclearResearchoftheHungarianAcademyofSciences,POBox51,H-4001Debrecen,Hungary} n_{SchoolofPhysicsandStateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing100871,China}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received17June2014

Receivedinrevisedform9October2014 Accepted13October2014

Availableonline18October2014 Editor:D.F.Geesaman

One-neutron knockout from 24_{O leading to the first excited state in} 23_{O has been measured for a}

protontargetatabeamenergyof62MeV/nucleon.Thedecayenergyspectrumoftheneutronunbound state of 23_{O was reconstructed fromthe measured fourmomenta of the} 22_{O fragment and emitted}

neutron. A sharppeakwas foundat Edecay=50±3 keV, corresponding toan excitedstatein23Oat

2.78±0.11 MeV,asobserved inpreviousmeasurements.The longitudinalmomentumdistributionfor thisstatewasconsistentwithd-waveneutronknockout,providingsupportfora Jπ assignmentof5/2+. TheassociatedspectroscopicfactorwasdeducedtobeC2_S(0d

5/2) =4.1±0.4 bycomparingthemeasured

crosssection(

σ

₋exp1n=61±6 mb)withadistortedwaveimpulseapproximationcalculation.Suchalarge

occupancyfortheneutron0d5/2orbitalisinlinewiththeN=16 shellclosurein24O.

©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP3_.

The neutron drip-line nucleus 24O, which has been studied extensively in recent years [1–14], is now considered a doubly-closed-shell nucleus. In particular, the high excitation energy of the first 2+ state [7] andsmall quadrupole transition parameter

[14]are strongindicatorsofan N

=

16 sphericalshellclosure. In addition, Kanungoet al. [8] found a large spectroscopic factor –

*

Correspondingauthor.

E-mailaddress:tshoo@ibs.re.kr(K. Tshoo).

C2S

(

1s1/2

)

=

1

.

74

±

0

.

19 –forone-neutronknockoutfrom24O re-flectinganalmostcompleteoccupancyofthe1s1/2orbital.

In this Letter we report on the spectroscopic factor for d5/2 neutronremovalfrom24O. Thefirstexcitedstateof23O,whichis neutronunbound[15,13,16],waspopulatedbyone-neutron knock-out from 24O with a proton target. The excitation energy, cross section,andlongitudinalmomentumdistributionweredetermined andallowedthe0d5/2neutron-holenatureofthisstatetobe iden-tified. The large spectroscopic factordeduced forthis state is in linewiththe

N

=

16 shellclosurein24O.

http://dx.doi.org/10.1016/j.physletb.2014.10.033

20 K. Tshoo et al. / Physics Letters B 739 (2014) 19–22

Fig. 1. (Color online.) Schematic view of the experimental setup. The experiment was performed at the RIPS facility [17] at RIKEN.TheexperimentalsetuphasbeendescribedinRefs.[14,18], andisdepictedinFig. 1.Thesecondary 24_{Obeamwas produced} using a 1.5 mm-thick Be production target and a 95 MeV/nu-cleon 40Ar primary beam of

∼

40 pnA. The average intensity of the 24_{O beam was}

_∼

_{4 particles/sec.The momentum of the} sec-ondary beam was determined particle-by-particle by measuring thepositionatthedispersivefocusF1ofRIPSwithaparallelplate avalanche counter.The energyloss (



E) andtime-of-flight (TOF) weremeasuredusing350μm-thicksiliconand500μm-thick plas-ticscintillator detectors, respectively, atthe achromatic focus F2. The liquid-hydrogen(LH2) target [19] was installed atthe achro-matic focus F3.The effective target thicknessandthe mid-target energyof24Owere159

±

3 mg

/

cm2 and62 MeV/nucleon, respec-tively.The 24_{Obeamincident onthetarget was tracked} particle-by-particle by using two multi-wiredrift chambers installed just upstream of the target. The target was surrounded by an array consistingof48 NaI(Tl)crystals (DALI) todetect

γ

rays from de-excitationofthefragments.

The

B

ρ

-TOF-



E method wasemployedtodeterminemassand charge ofthefragment followingreactions ofthe 24Obeamwith the LH2 target. The magnetic rigidity (B

ρ

) was determined from thepositionandangleinformationmeasuredwithtwomulti-wire driftchambersplacedattheentranceandexitofthedipole mag-net.TheTOFofthefragmentwasmeasured withtheplastic scin-tillator charged particle hodoscope which also gave energy loss information.Thebeamvelocitydecayneutronwasdetectedusing aplasticscintillatorcounterarrayplacedsome4.7 mdownstream ofthetarget,equippedwithachargedparticlevetocounter.A neu-trondetectionefficiencyof25

±

1% at64 MeVwasmeasuredfora 2 MeVeethresholdinaseparate7_Li

₍

_p

_,

_n

₎

_{measurementperformed} duringtheexperiment.

Thedecayenergyspectrumof23O∗ wasreconstructedfromthe measuredfourmomentaof22Oandtheemittedneutron.The de-cayenergyEdecay isexpressedas:

Edecay

=



(

Ef

+

En

)

2

− |

pf

+

pn

|

2

− (

Mf

+

Mn

),

(1) where

E

f

(

En

)

,pf

(p

n

)

,and

M

f

(

Mn

)

arethetotalenergy, momen-tum, and mass of 22_{O (neutron), respectively. Fig. 2 shows the} decayenergyspectrumintermsofcrosssection (d

σ

/

dEdecay) af-tercorrecting forthe detection efficiencies andacceptances. The backgroundcontributionwassubtracted hereby usingdatataken withan empty target. The errorbars are statisticalonly. The ge-ometrical acceptancewas estimated usinga MonteCarlo simula-tiontakingintoaccount thebeamprofile,geometryofthesetup, experimental resolutions, and multiple scattering of the charged particles.Theexperimentalenergyresolutionwas estimatedtobe



Edecay

≈

0

.

52



Edecay(MeV)inFWHM.

Thedecayenergyspectrumwasdescribedusingoneresonance forthepeakat Edecay

=

50

±

3 keV andaMaxwelliandistribution forthe non-resonant continuum [20]. The peak observed has an

Fig. 2. Thecrosssection,dσ/dEdecay,obtainedbymeasuringaneutronin

coinci-dencewith22_{O.Theerrorbarsarestatisticalonly.Thedottedanddot-dashedlines}

aretheresultsofthefitsforaresonanceatEdecay=50±3 keV andanon-resonant

continuumcomponent[20],respectively.Thedecaypathfromthefirstexcitedstate of23_{Otothegroundstateof}22_{Oisrepresentedintheinset.}

Table 1

Theexcitationenergiesofthefirsttwostatesin23_{Oandthespectroscopicfactors}

forneutronremovalfrom24_{OarecomparedwiththeresultsoftheUSDBandWBT}

shell-modelcalculations.

Jπ _{Energy (MeV) Spectroscopic factor C}2_S

Exp. USDB WBT Exp. USDB WBT

1/2+ 0.0 0.0 0.0 1.74±0.19a _{1.81 1.73}

5/2+ 2.78±0.11 2.59 2.72 4.1±0.4 5.67 5.52

a _FromRef.[8].

asymmetriclineshapeowing toitsproximitytothreshold,andits widthisfullydominatedby theexperimentalresolution.The cor-responding excitation energy is Ex

=

2

.

78

±

0

.

11 MeV,given the separationenergyof

S

n

(

23O

)

=

2

.

73

±

0

.

11 MeV[21–23].Sinceno

γ

-ray lines were observed for the 22_O

₋

_{n channel, we assumed} that 22Oisinthegroundstate.As such,thelocation ofthe reso-nanceisingoodagreementwithpreviousexperiments[15,16].

The one-neutron knockout cross section to the resonance at Edecay

=

50 keV wasdeterminedtobe

σ

₋exp1n

=

61

±

6 mb,after sub-tractingthenon-resonantcontinuum.Thequotederrorarisesfrom theuncertaintyinthechoiceofthefunctionalformdescribingthe non-resonant continuum(8%),statisticaluncertainty(4%),neutron detectionefficiency(3%),andthetargetthickness(2%).

Thesingle-particleconfigurationsforthegroundstatesof23,24_O are expectedtobe

ν

(

0d5/2

)

6

ν

(

1s1/2

)

1 and

ν

(

0d5/2

)

6

ν

(

1s1/2

)

2, re-spectively, in the shell-model picture (see, e.g., Refs. [8,24]), and suggestthat thefirstexcited stateof23_{Opresentedhere(neutron} hole state) is very likely to be populated by the 0d5/2 neutron knockout. Calculations using the USDB interaction [25] in the sd modelspace,aswellasWBT[26]interactioninthe

spsdp f model

space, predictthat thespin-parityofthisstate is J π

=

5

/

2+ ( Ta-ble 1).

The one-neutron knockout cross section, above a few tens MeV/nucleon energy, can be decomposed intothe single-particle cross section (

σ

sp) relatedto the reaction and the spectroscopic factor(C2_{S) reflectingthenucleonoccupancy[27],asfollows:}

σ

−1n

=



nlj



A A

−

1



Λ C2S



Jπ

,

nlj



σ

sp



nlj

,

Seff_n



,

(2)

where J π is thespin-parityofthefinalstateoftheresidue(core); nls denote thequantumnumbersoftheknocked-outneutron;

S

eff n is the effective separation energywhich corresponds the sumof the neutron separation energy of the projectile and the excita-tion energyof thecore; A is the massnumberof theprojectile;

Fig. 3. Longitudinalmomentumdistributionfor23_O∗_{(seetext).Theerrorbarsare} statisticalonly.Thesolidanddashedlinesrepresentthecalculatedlongitudinal mo-mentumdistributionsnormalizedtothepeakofthemeasureddistributionforthe knockoutofneutronsinthe0d5/2and1s1/2orbitals,respectively.Thedistributions

wereconvolutedwiththeexperimentalresolution.

Thesingle-particle crosssection assuming aspectroscopic fac-torofunity was calculatedusingthe distortedwave impulse ap-proximation(DWIA) calculation forthe quasifree (p

,

pn)

reaction as described in Ref. [29]. The calculation employed the eikonal approximation for the distorted wave functions of incoming and outgoingreactionchannels.Theeffectivenucleon–nucleon interac-tionM3Y [30]along withtheCoulomb potential wasadoptedfor the real partof the optical potential. The nucleon–nucleus cross sectioninnuclearmatterdevelopedinRefs. [31–33]andthe den-sitiesoftheprojectile,orcore, foldedwiththe matterdensityof protonwere employedtointroducetheimaginarypartofthe op-ticalpotential.

Thewavefunctionsofsingle-particleneutronorbitsaroundthe corewerecalculatedusingtheWoods–Saxonpotentialinthe man-nerasdescribedinRef.[34].TheSkyrme(SkX)Hartree–Fock(HF) calculation [35] was employed to deduce the root-mean-squared (rms)radius

r

HF ofeach single-particleorbitintheprojectile.The code nushell[36]wasusedforthiscalculation.TheHFrmsradius of24Owas

r

HF

=

3

.

434 fm forthe0d5/2orbital.Thereducedradius r0wasdeterminedsothatthecalculatedsingle-particlewave func-tionwithinthepotentialwelladoptingadiffusenessof

a

0

=

0

.

7 fm satisfies

r

sp

=

√

A

/(

A

−

1

)

rHF atthe HFpredictedbindingenergy. Thereducedradiuswascalculatedtobe1.171fmforthe0d5/2 or-bital.Withthis

r

0,thedepthofthepotentialwellwasfurther ad-justedtoreproduce Seff

n .The

S

effn wascalculatedforthe2.78-MeV statetobe6.97 MeVbyadopting Sn

(

24O

)

=

4

.

19

±

0

.

14 MeV[21]. The single-particle cross section was calculated to be

σ

sp

=

13

.

5 mb, from which we deduced the spectroscopic factor C2_Sexp

₍

_0d5

/2

)

=

4

.

1

±

0

.

4 byusingEq.(2).Theone-neutron knock-out crosssection was calculated to be

σ

th

−1n

(

0d5/2

)

=

83

.

1 mb by employing the shell-model spectroscopic factor of C2Sth

=

5

.

67 obtainedusingtheUSDBinteraction.The reductionfactordefined as Rs

=

σ

₋exp_1n

/

σ

₋th_1n was derived tobe Rs

=

0

.

73

±

0

.

07 whichis consistent with the systematics in Refs. [37,34]. Moreover, a re-duction factorof Rs

=

0

.

84

±

0

.

10 may be deduced forinclusive one-neutron knockout at 51 MeV/nucleon from the neighboring N

=

14 closedsub-shellnucleus22_O[38].

Inthecore

(

23O

)

+

n system description,thewidthofthe longi-tudinalmomentum(p_) distributionofthecoreis directlylinked to the single-particle wave function of the knocked-outneutron, andcanbe utilized to identifyits orbital angularmomentum (l). Wehavereconstructedherethelongitudinalmomentum distribu-tionof23O∗fromthemeasuredmomentaofthe22Ofragmentand neutron.

The p_ distribution for the first excited state of 23_{O in the} laboratory frame is displayed in Fig. 3. The error bars are sta-tisticalonly. The widthofthe distribution was determined to be 356

±

28 MeV

/

c (FWHM) byaGaussianfit.Theexperimental

dis-Fig. 4. (Coloronline.) Spectroscopicfactorsofthe lowest1/2+ and 5/2+ states, C2_S(1s

1/2)(a)andC2S(0d5/2)(b),ineven–evenN=16 isotones.The

experimen-tallydeterminedspectroscopicfactors(datapoints)takenfromRefs.[8,40,41]were derivedfrom theneutronknockoutreactionsonaberylliumtarget. Thepresent resultisshownbythered-filledcircle.TheresultsoftheUSDBshell-model calcu-lationarerepresentedbythedashedlines.Thegraybandsindicatetheresultsof thecalculationsincludingareductionfactor Rs,wheretheupperandlowerlimits

ofthebandscorrespondto0.9and0.7,respectively.

tribution is compared in Fig. 3 with the DWIA calculations. The resultsofthecalculationfortheremovalof0d5/2 (solidline)and 1s1/2 (dashed line) neutrons are shownin Fig. 3, where the cal-culateddistributionswereconvolutedwiththemomentum resolu-tion.Themomentumresolutionwasdeterminedtobe195 MeV

/

c (FWHM)usingtheMonteCarlosimulationtakingintoaccountthe momentum spread of the 24_{O beam, range difference of} incom-ingandoutgoingparticlesinsidetheLH2target,Coulombmultiple scatteringeffects,andthedetectorresolutions. Thewidthsof the calculateddistributions forthe0d5/2 and1s1/2 knocked-out neu-trons were to be 334 and 270MeV/c (FWHM), respectively. The resultfor0d5/2 neutronknockoutreproduceswellthemomentum distribution,supportingthespin-parity assignmentof J π

=

5

/

2+. Itshouldbenotedthatthisstatehasalsobeenmeasuredvia two-protonandone-neutronremoval[15] andprotoninelastic scatter-ing[16].

Theexcitation energyandthespectroscopicfactorforneutron removalfrom24Oarecomparedwiththeresultsoftheshell-model calculationsusingtheUSDBandWBTinteractionsinTable 1.Both calculationsreproducethe energyandalsothe spectroscopic fac-torassumingareductionfactorof

R

s

≈

0

.

7–0

.

9 suggestedbyGade etal.[37,34].We note thatthe USDBinteractionsuccessfully de-scribestheenergies ofthefirstexcitedstatesandthe quadrupole transitionparametersof22,24OreportedinRefs.[14,39].

Fig. 4showstheprotonnumberdependencyofthe experimen-tally determined and calculated (dashed lines) spectroscopic fac-tors for the even–even N

=

16 isotones. While the spectroscopic factor for the

ν

1s1/2 orbital gradually increases in moving from Z

=

12 to 8,thatforthe

ν

0d5/2orbitaldramaticallyrisesatZ

=

8. ThistrendisreasonablywellreproducedbytheUSDBshellmodel calculations. Significantly, the C2S

(

0d5/2

)

for 24O is much larger thanthoseofotherisotones,indicatingalargeneutronoccupancy ofthe0d5/2 orbital.Theseresultsareinlinewithan N

=

16 sub-shellclosurein24_O.

22 K. Tshoo et al. / Physics Letters B 739 (2014) 19–22

behaviourmayprovideabenchmarktotestmoresophisticated nu-clearstructure models,aswell asreaction mechanisms.Recently, forexample,Grinyeretal.[42]haveinvestigatedthequenchingof thespectroscopic factorsin 10_{Be and}10_{Cfor one-neutron} knock-out andfound that an ab initio structure calculation, takingboth 3-body forcesandcontinuum effects intoaccount, well describes thereduction.

In summary,we haveinvestigated the occupancy of the neu-tron0d5/2 orbital in24Oviaone-neutronknockoutfrom24Owith a proton target. The excitation energy of the first excited state of23O was measured tobe 2

.

78

±

0

.

11 MeV and thespin-parity was assigned, based on the longitudinalmomentum distribution, tobe J π

=

5

/

2+.The large correspondingspectroscopicfactor of C2_Sexp

₍

_0d

5/2

)

=

4

.

1

±

0

.

4 supportsthepictureofan

N

=

16 spher-icalshellclosurein24_O.

Acknowledgements

We would like to thank the accelerator operations staff of RIKEN for providing the 40_{Ar beam. This work is supported by} the Grant-in-Aid for Scientific Research (No. 19740133) from MEXT Japan, the WCU program and Grant 2010-0024521 of the NRF Korea, the Rare Isotope Science Project of Institute for Ba-sic Science funded by Ministry of Science and NRF of Korea (2013M7A1A1075765), DOE grants No. DE-FG02-08ER41533 and No. DE-FG02-10ER41706, and the French–Japanese LIA (IN2P3-RIKEN).

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