Competition between $\beta$-delayed proton and $\beta$-delayed $\gamma$ decay of the exotic $T_z$ = -2 nucleus $^{56}$Zn and fragmentation of the IAS

0.56 MB·English

by  B. Rubio

Competition between β-delayed proton and β-delayed γ
decay of the exotic T = -2 nucleus 56Zn and fragmentation
z
of the IAS
5 B.Rubio1,S.E.A.Orrigo1,Y.Fujita2,3,B.Blank4,W.Gelletly5,J.Agramunt1,A.
1
Algora1,P.Ascher4,B.Bilgier6,L.Ca´ceres7,R.B.Cakirli6,H.Fujita3,E.Ganiog˘lu6,
0
2 M.Gerbaux4,J.Giovinazzo4,S.Gre´vy4,O.Kamalou7,H.C.Kozer6,L.Kucuk6,T.
n Kurtukian-Nieto4,F.Molina1,8,L.Popescu9,A.M.Rogers10,G.Susoy6,C.Stodel7,T.
a Suzuki3,A.Tamii3,J.C.Thomas7
J
1InstitutodeF´ısicaCorpuscular,CSIC-UniversidaddeValencia,E-46071Valencia,Spain
4
1 2DepartmentofPhysics,OsakaUniversity,Toyonaka,Osaka560-0043,Japan
3ResearchCenterforNuclearPhysics,OsakaUniversity,Ibaraki,Osaka567-0047,Japan

] 4Centred’EtudesNucle´airesdeBordeauxGradignan,CNRS/IN2P3-Universite´ Bordeaux1,33175
x
e GradignanCedex,France
- 5DepartmentofPhysics,UniversityofSurrey,GuildfordGU27XH,Surrey,UK
l
c 6DepartmentofPhysics,IstanbulUniversity,Istanbul,34134,Turkey
u 7GrandAcce´le´rateurNationald’IonsLourds,BP55027,F-14076Caen,France
n
8Comisio´nChilenadeEnerg´ıaNuclear,Casilla188-D,Santiago,Chile
[
9SCK.CEN,Boeretang200,2400Mol,Belgium

1 10PhysicsDivision,ArgonneNationalLaboratory,Argonne,Illinois60439,USA
v
0 E-mail:berta.rubio@ific.uv.es
9
3 (ReceivedSeptember30,2014)
3
0 A very exotic decay mode at the proton drip-line, β-delayed γ-proton decay, has been observed in
. theβdecayoftheT =-2nucleus56Zn.Threeγ-protonsequenceshavebeenobservedfollowingthe
1 z
0 βdecay.ThefragmentationoftheIASin56Cuhasalsobeenobservedforthefirsttime.Theresults
5 were reported in a recent publication. At the time of publication the authors were puzzled by the
1 competitionbetweenprotonandγ decaysfromthemaincomponentoftheIAS.Hereweoutlinea
:
v possible explanation based on the nuclear structure properties of the three nuclei involved, namely
i 56Zn,56Cuand55Ni,closetothedoublymagicnucleus56Ni.FromthefragmentationoftheFermi
X
strengthandtheexcitationenergyofthetwopopulated0+ stateswecoulddeducetheoff-diagonal
r
a matrix element of the charge-dependent part of the Hamiltonian responsible for the mixing. These
results are compared with the decay of 55Cu with one proton less than 56Zn. For completeness we
summarisetheresultsalreadypublished.
KEYWORDS: β-delayedγ-protondecay,56Zn,Gamow-Tellertransitions,Charge-exchange
reactions,Isospinmixing,Isospinsymmetry,fragmentationoftheIAS,proton-richnuclei.
1. Introduction
The study of the properties of nuclei far from stability is one of the main frontiers of modern
nuclearphysics.Amongmanypossibleobservablesfornuclearstructure,theβ-decaystrengthspro-
vide important testing grounds for nuclear structure theories far from stability. The mechanism of β
decayiswellunderstoodanddominatedbyallowedFermi(F)andGamow-Teller(GT)transitions.A
successfuldescriptionofthenuclearstructureofthestatesinvolvedshouldprovidegoodpredictions
forthecorrespondingtransitionstrengths B(F)and B(GT).
1
Wehavestudiedtheβ+ decayoftheT = −256ZnnucleustotheT = −156Cunucleusandhave
z z
reportedourresultsin[1].Wehaveobservedcompetitionbetweenβ-delayedprotonandγ emission
in states well above the proton separation energy S . Moreover we observed β-delayed γ rays that
p
populate proton-unbound levels that subsequently decay by proton emission. This observation em-
phasizestheneedforGedetectorsinordertodeterminetheB(GT)correctly,evenwhentheβ-feeding
is to proton unbound levels. In the present case, in order to determine B(GT) properly, the intensity
of the proton transitions has to be corrected for the amount of indirect feeding coming from the γ
de-excitation.Althoughsimilarcasesweresuggestedinthe sd-shell[2,3]andobservedinthedecay
of32Ar[4],thisexoticdecaymodehasbeenobservedhereforthefirsttimeinthe fpshell.Moreover
how this new decay mode affects the determination of B(GT) has not been discussed prior to our
paper[1].
The proton decay of the IAS is usually isospin forbidden and consequently the slower γ decay
modecancompeteevenifthestateiswellabovetheS .Suchcaseshavebeenobservedbefore[5].
p
However the case of 56Zn decay is special, we have observed that its IAS in 56Cu is fragmented in
two 0+states. These two states are an admixture of T=1 and T=2 configurations. The proton decay
of the T=2 component is isospin forbidden, but the T=1 is not. In consequence, the de-excitation
+
of both 0 states should be dominated by the fast proton decay. The observed competition between
proton-andgamma-decayforthe3508keVstate(oursensitivitydoesnotallowustoseethisforthe
otherstate),makesthiscasebothexoticandinteresting.Webelievethatthisunusualbehaviourisdue
to the structure of the nuclei involved and to the fact that we are close to the doubly-magic nucleus
56Ni.Herewesummarisetheresultsalreadypublished[1]andalsodiscusstheproton-γcompetition
for the firsttime. Another important difference with respectto the results presented in[1] is that the
mass excess of 56Cu has now been measured. A preliminary value for the mass excess of 56Cu of
-38530(90) keV has been reported [6] which agrees with the value we used in [1] that was derived
fromsystematics[7],ratherthanthevaluegivenin[8].Sincethemeasuredvalueisstillpreliminary,
we have followed the same criteria as in [1] and used [7] to calculate the Q# and S . The present
EC p
experiment was motivated by a comparison with the mirror charge exchange (CE) reaction on 56Fe
[9]. Indeed β decay and CE studies are complementary and, assuming isospin symmetry, they can
be combined to determine the absolute B(GT) values up to high excitation energies [10–12]. Hence
precisedeterminationofthe B(GT)isimportant.
2. Experiment
The β-decay experiment was performed at the LISE3 facility of GANIL [13] in 2010, using a
58Ni26+primarybeamwithanaverageintensityof3.7eµA.Thisbeam,acceleratedto74.5MeV/nucleon,
wasfragmentedona200µmthicknaturalNitarget.ThefragmentswereselectedbytheLISE3sep-
aratorandimplantedintoa300µmthickDouble-SidedSiliconStripDetector(DSSSD),surrounded
by four EXOGAM Ge clovers for γ detection. The DSSSD was used to detect both the implanted
fragments and subsequent charged-particle decays (βs and protons). An implantation event was de-
fined by simultaneous signals in both a silicon ∆E detector located upstream and the DSSSD. The
implantedionswereidentifiedbycombiningtheenergylosssignalinthe∆E detectorandtheTime-
of-Flight (ToF) difference between the cyclotron radio-frequency and the ∆E signal. Decay events
weredefinedasgivingasignalintheDSSSDandnocoincidentsignalinthe∆E detector.
3. AnalysisandResults
The56Znionswereselectedbysettinggatesoff-lineonthe∆E-ToFmatrix.Thecorrelationtime
was defined as the time difference between a decay event in a given pixel of the DSSSD and any
implantation signal that occurred before and after it in the same pixel that satisfied the conditions
2
140
V
e 91 S) (a)
ts / 10 k 110200 1.6 2.661 3.423 3.508 (IA Charged-particle
n
u 80 spectrum from
o
C
60 56Zn b -decay
1 7
9 3
3 5
40 1. 2.
20
0
0 1 2 3 4 5
Decay energy [MeV]
V
ounts / 2.5 ke122505000000 +1.720, 1 +2.729, 1 +527, 0+ (IAS)3.599, 0 56(F4e2(03H Mee,tV)5,6 0C(cid:176) o)(b)
C 3.
+1+1
1000 2, 6,
39
44
3.3.
500 3
3
6
2.
0
1 2 3 4 5
56Co excitation energy [MeV]
Fig. 1. (a) Charged-particle spectrum measured in the DSSSD for decay events correlated with 56Zn im-
plants.Thepeaksarelabeledaccordingtothecorrespondingexcitationenergiesin56Cu.(b)56Fe(3He,t)56Co
reactionspectrum[9].Peaksarelabeledbytheexcitationenergiesin56Co.
requiredtoidentifythenuclearspecies.Theprotondecayswereselectedbysettinganenergythresh-
old above 800 keV and looking for correlated 56Zn implants. The spectrum of the time correlations
between the 56Zn implants and the protons was fitted with a function including the β decay of 56Zn
andaconstantbackground.Ahalf-life(T )of32.9(8)mswasobtainedfor56Zn[1],inagreement
1/2
with[5].Fig.1(a)showsthespectrumofcharged-particlesassociatedwith56Znimplants.
Twokindsofstateareexpectedtobepopulatedintheβdecayof56Znto56Cu:theT =2, Jπ=0+
IAS,andanumberofT =1,1+states.Fromthecomparisonwiththemirrornucleus56Co,allofthese
states will lie above S = 560(140) keV [7] and will decay by protons. Indeed most of the strength
p
in Fig. 1(a) isinterpreted as β-delayed proton decay tothe 55Ni ground state. Weattribute the broad
bump below 800 keV to β particles that are not in coincidence with protons. The proton peaks seen
above800keVarelabeledintermsoftheexcitationenergiesin56Cu.Thelargeuncertaintyof±140
keVinthe56CulevelenergiescomesfromtheuncertaintyintheestimatedS .Theprotondecayof
p
the IAS is identified as the peak at 3508 keV, as in [5]. Figure 1(b) shows the triton spectrum from
the mirror T = +2 → +1, β−-type CE reaction 56Fe(3He,t)56Co recorded at RCNP Osaka [9]. The
z
corresponding56Costatesweredeterminedwith∼ 30keVresolution.Figs.1(a)and1(b)havebeen
alignedinexcitationenergyin56Cuand56Co.Thereisagoodcorrespondencebetweenstatesinthe
mirrornuclei56Cuand56Co.
3
Imposing coincidence conditions on the total proton spectrum or on the various proton peaks in
Fig. 1(a), three proton-γ-ray coincidence peaks were observed (Fig. 2). All of our observations are
discussedin[1]andresultinthe56ZndecayschemeshowninFig.3.The831keVprotontransition
couldbeplacedasde-excitingthe3508or1391keVlevels.Theformerisruledoutbytheobserved
831keVproton-309keVγcoincidences(Fig.2(d)).Threecasesofβ-delayedγ-protonemissionhave
beenestablishedexperimentally.OthercasesofγdecayfromanIASaboveS havebeenobservedin
p
thismassregion[5,14].Theparticularcircumstancehereisthatthefinallevelisalsoproton-unbound,
consequentlytheβ-delayedγ-protondecayhasbeenobservedforthefirsttimeinthe fp-shell.Table
Ishowsthecenter-of-massenergiesoftheprotonandγpeaks,andtheirintensitiesdeducedfromthe
areas of the peaks. For a proper determination of B(F) and B(GT), the β feeding to each 56Cu level
was estimated from the proton and γ intensities, taking into account the amount of indirect feeding
produced by the γ de-excitation. In the case of the IAS we have assumed that the state decays in
a similar way to its counterpart in the mirror therefore another γ-decay branch was added [1]. The
measuredT ,βfeedings I and B togetherwiththe Q# andS from[7]wereusedtodetermine
1/2 β p EC p
the B(F)and B(GT)valuesshowninTableIIandinFig.3.
ThetotalFermitransitionstrengthhastobe|N −Z|=4.The56CuIASat3508keVhasaFermi
strength B(F)=2.7(5).Themissingstrength,1.3(5),hastobehiddeninthebroadpeakat3423keV.
Thisisaconfirmationthatthe56CuIASisfragmentedandthusmostofthefeedingtothe3423keV
level (assuming it contains two or three unresolved levels) corresponds to the Fermi transition and
the remainder to the GT transition. The isospin impurity α2 (defined as in [9]) and the off-diagonal
matrix element of the charge-dependent part of the Hamiltonian (cid:104)H (cid:105), responsible for the isospin
c
mixingofthe3508keVIAS(0+,T = 2)andthe0+ partofthe3423keVlevel(T = 1),werederived
assumingtwo-levelmixing.For56Cuitwasfoundthat(cid:104)H (cid:105)=40(23)keVandα2 =33(10)%,similar
c
to the values obtained in the mirror 56Co [9], (cid:104)H (cid:105) = 32.3(5) keV and α2 = 28(1)%. It is interesting
c
tocompareourresultsonthedecayof56ZnwithT = −2withtheinvestigationonthedecayof55Cu
z
withT = −3/2,onlyoneprotonapart.Inarecentarticle[15]astrongfragmentationoftheIASwas
z
observed.Wenotethatin[15]thestatescouldbefedbybothFandGTtransitions,althoughtheGT
+ +
component is small. Similarly here the second 0 state (fed by F) could not be separated from a 1
state(fedbyGT)althoughthefeedingtothe1+ stateissmall.Inthe55Nicase,(cid:104)H (cid:105)=9(1)keVand
c
α2 =27(7)%,alsoinaccordwithobservationsinthemirror.Inbothcasesthemixingmatrixelement
issmall,butitseemstoincreasewhenmovingawayfromN=Z.
1000 (a) 15 p-peak gated (b)
V V
ts / 8 ke500 511 x10 1835 ts / 4 ke 10 (1691 keV) 1835
n n
u u 5
o o
C C
0 0
0 1000 2000 1600 1800 2000
Gamma energy [keV] Gamma energy [keV]
15 p-peak gated (c) 15 p-peak gated (d)
V V
ke (2661 keV) ke (1391 keV) 9
s / 4 10 861 s / 4 10 30
t t
n n
u 5 u 5
o o
C C
0 0
600 800 1000 200 400 600
Gamma energy [keV] Gamma energy [keV]
Fig. 2. γ-ray spectra in coincidence with (a) all charged particles, and protons from
thelevelsat(b)1691,(c)2661and(d)1391keV.
4
Q# = 12870(300) keV T=2, 0+
EC
56Zn (T =-2)
Bp = 88.5(26) % b + z
T = 32.9(8) ms
1/2
E [keV] B(F) B(GT)
T=2, 0+ x
p 3508(140) 2.7(5)
{5 1 T=1, 0+,1+}3423(140) 1.3(5) £ 0.32
3 6
18 8 T=1, 1+ 2661(140) 0.34(6)
T=1, (1+) 2537(140) 0
T=1, 1+
9 1691(140) 0.30(9)
0 T=1, 0+
3 1391(140) 0
-
T=1/2, 7/2 S = 560(140) keV
b +55Ni (T =-1/2) p
z T=1, 4+
T = 209 ms b +
1/2 56Cu (T =-1)
z
T = 93 ms
1/2
Fig. 3. 56Zn decay scheme deduced from results of the present experiment. Observed proton or γ decays
areindicatedbysolidlines.Transitionscorrespondingtothoseseeninthemirror56Conucleusareshownby
dashedlines.Theerrorof140keVcomesfromtheuncertaintyinS# (seetext).
p
Fig. 4. Aschematicdiagramshowingtwopossibleexplanationsfortheobserveddecayofthe56ZnIAS(see
text).
Aninterestingandpuzzlingopenquestionis,consideringthattheisospinmixingisquitelargein
56Cu,whywearestillobservingtheγde-excitationfromtheIASwhenthepartially-allowedproton
decayis,inprinciple,muchfaster.Themirrorlevelin56Coliesat3600keV.Ithast = 2×10−14 s,
1/2
anddecaysonlybyγde-excitation,withthestrongestofthethreebrancheshavinganenergysimilar
to that of the corresponding gamma-ray observed in the decay of the 56Cu IAS. This allows us to
estimate the 3508 keV partial γ half-life. The proton half-life of the IAS in 56Cu is estimated to be
of the order of 10−18 s, i.e. 104 times faster. One possible answer to this dilemma lies in the nuclear
structureofthelevelsinvolved.InFig.4wesketchthewaythedecaymayproceed.Westartwiththe
assumptionthatthe56Zngroundstate(gs)hasthetwoextraprotonsabovetheZ=28gapintheπp
3/2
orbitalandthetwoneutronholesintheνf .InFig.4athedecayproceedsbytransformingaproton
7/2
in the πp into a neutron in νp . The subsequent proton decay will most probably remove the
3/2 3/2
unpairedprotonfromtheπp orbital.Asshowninthefigure,thefinalstatein55Ni,willconsistof
3/2
aneutronparticle-holeexcitationacrosstheN=28gapontopoftheneutronholeintheνf orbital
7/2
5
Table I. Center-of-Massprotonenergies,γ-ray Table II. β feedings for Fermi and Gamow
energies,andtheirintensities(normalizedto100 Tellertransitionstrengthstolevelsin56Cuinthe
decays)forthedecayof56Zn.*IAS. β+decayof56Zncalculatedusing[7].*IAS.
E (keV) I (%) E (keV) I (%) I (%) E(keV) B(F) B(GT)
p p γ γ β
2948(10)* 18.8(10) 1834.5(10) 16.3(49) 43(5) 3508(140)* 2.7(5)
2863(10) 21.2(10) 861.2(10) 2.9(10) 21(1) 3423(140) 1.3(5) ≤0.32
2101(10) 17.1(9) 309.0(10) - 14(1) 2661(140) 0.34(6)
1977(10) 4.6(8) 0 2537(140) 0
1131(10) 23.8(11) 22(6) 1691(140) 0.30(9)
831(10) 3.0(4) 0 1391(140) 0
whichisthemostprobableconfigurationforthegsin55Ni.Thisneutronparticle-holeexcitationwill
lie at 4 to 5 MeV excitation energy and is therefore outside the available energy gap for the protons
of ∼3 MeV (3508-560 keV, see Fig. 3). In Fig. 4b we assume the same gs for 56Zn but in this case
the decay proceeds through the transformation of a proton from the πf level into one of the two
7/2
available holes in the νf shell. The proton decay will probably remove one of the two available
7/2
protonsintheπp orbitalin56Cugivingrisetoafinalprotonparticle-holeexcitationontopofthe
3/2
νf hole in the 55Ni gs which should again lie at 4 to 5 MeV above the gs in 55Ni. We note that
7/2
thesameargumentappliesifwestartbyassumingthatthetwoprotonsoccupytheπf ortheπp
5/2 1/2
orbitals(moreover,iftheprotondecayoriginatesfromtheπf thesituationisevenlessfavourable
7/2
since the final state will involve two particle-hole excitations across the the N and Z = 28 gaps). In
summary,theprotondecayoftheIASof56Zninto55Niisstronglyhinderedfromthenuclearstructure
point of view. This may explain why the γ-decay can compete with it. Shell model calculations to
confirmtheseideasareinprogress.
4. Acknowledgments
This work was supported by the Spanish MICINN grants FPA2008-06419-C02-01, FPA2011-
24553; CPAN Consolider-Ingenio 2010 Programme CSD2007-00042; MEXT, Japan 18540270 and
22540310; Japan-Spain coll. program of JSPS and CSIC; Istanbul University Scientific Research
Projects, Num. 5808; UK (STFC) Grant No. ST/F012012/1; Region of Aquitaine. R.B.C. acknowl-
edges support by the Alexander von Humboldt foundation and the Max-Planck-Partner Group. We
acknowledgetheEXOGAMcollaborationfortheuseoftheircloverdetectors.WethankI.Hamamoto
forvaluablehelpintheinterpretationofourobservations.
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6