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To cite this version:
RECENT MEASUREMENTS OFSPHERICAL AND
DEFORMED ISOMERSUSING THE LOHENGRIN
FISSION{FRAGEMENTSPECTROMETER
G.S.SIMPSON,J.C.ANGELIQUE,J.GENEVEYandJ.A.PINSTON LPSC,UniversiteJosephFourierGrenoble1,CNRS/IN2P3,InstitutNational
Polyte hniquedeGrenoble,F-38026 GrenobleCedex,Fran e A.COVELLOandA.GARGANO
DipartimentodiS ienzeFisi he,UniversitadegliStudidiNapoliFederi oIIand INFN,ComplessoUniversitariodiMonteSant'Angelo,ViaCintia,I-80126Napoli,
Italy U.K
OSTER
InstitutLaue-Langevin,B.P.156, F-38042GrenobleCedex9,Fran e R.ORLANDI
INFN,LaboratoriNazionalidiLegnaro,Legnaro, Italy A.SCHERILLO
RutherfordAppletonLaboratory,Chilton,Did otOX110QX,UnitedKingdom W.URBAN
Fa ultyofPhysi s,WarsawUniversity,ul.Ho_za69,PL-00-681Warszawa, Poland
Newexperimentalinformationhasbeenobtainedonthesisomeri as- ade intheveryneutron-ri h
136
Sbusing -rayand onversion-ele tron spe -tros opyat the Lohengrin ssion-produ tspe trometerofthe Institut Laue-Langevin,Grenoble.Twonewtransitionshavebeenobservedandtheir multi-polaritiesdetermined,resolvingthequestionoftheoriginoftheisomerism.The newlevels hemeisingoodagreementwithpredi tionsofarealisti shell-model al ulation. Mi rose ondisomers in deformedneutron-ri h ssion fragments havealso been studied in
107 Mo and
107
T withthe Lohengrin spe trome-ter.These studieshavebeen omplementedbyprompt -rayspe tros opyof thesenu lei,plustheneighboring
105
Mo,followingthespontaneous ssionof a
248
perimental level s hemes of Sb [1,2℄.The Sb isotopesare most ap-propriatefortestingthematrixelementsoftheproton-neutronintera tion betweenvalen enu leonsindi erentmajorshells.These al ulationsshow thatthere isnoneedtoinvokeshell-stru turemodi ationstoexplainthe presentlyavailabledata onneutron-ri hnu leibeyond
132
Sn.Thenu leus 136
Sb,withanN/Zratioof1.67,isatpresentthemostexoti open-shell nu- leusbeyond
132
Snforwhi haspe tros opi studyhasbeenperformed[3℄. Inthisstudyitwas on ludedthatthesisomerobservedmostlikely orig-inatesfromtheI
=6 stateoftheg 7=2 (f 7=2 ) 3
on guration.Onlyone -raytransition of 173 keV wasobserved in oin iden e with
136
Sb ions, whi halonewasnotenoughtoexplaintheoriginofsisomerism.InRef.[3℄ anunseenlow-energyE2transitionwaspostulatedtoberesponsibleforthe slifetime.Inthepresentwork thisnu leus hasbeenstudied withan ex-perimental setup apableofdete tinglow-energy onversionele tronsand rays.
1.2. Experimental Method
Delayed raysand onversionele tronsfrom 136
Sbwereobservedusingthe Lohengrinmassspe trometeratthehigh- uxrea toroftheInstitut Laue-Langevin, Grenoble. Mass 136 nu lei were produ ed by thermal-neutron indu ed ssionof a thin 7 0.5 m
2 , 1mg
241
rays dete tedin theGe orSi dete tors up to 20 s after the arrival of an ion were re orded by the the data a quisition system. Measurements ofdelayed raysand onversionele tronsfrom
136
Sbwerean experimen-tal hallenge astheionization hamberwasunabletoresolvethedi erent isobarsin theA =136mass hain. Anothers isomerexists in this mass hain,the2.95s6
+ state
136
Xe[4℄,whose ssionyieldisafa tor25higher thanthat of
136
Sb.The previously measuredshort lifetime of the isomer (565(50)ns)[3℄,meantalsothatmu hoftheintensityoftheisomeri state de ayedduringthe2.3s ighttimethroughthespe trometer.Hen e, de-layed -rayand onversion-ele tronspe traweredominatedbytransitions fromthe 136 Xeisomer. 1.3. Results
0
50
100
150
200
250
Energy (keV)
0
50
100
150
Energy (keV)
0
50
100
150
200
250
Energy (keV)
0
50
100
150
Energy (keV)
87.3 keV
136
I
53.4 keV
136
Sb
Sb X-K
α
Xe X-K
α
Xe X-K
β
background
135.4 keV
136
I
173.0 keV
136
Sb
backscattering (197 keV -Xe K
α
)
197.0 keV
136
Xe
Fig.1. raysobservedintheGedete torsin oin iden ewithSbK Xraysmeasured intheSidete tors.Ba kgroundtransitionsarepresentfromthestronglyprodu edisomer in
136
Xe[4℄andfrom 136
I,produ edby de ay
Inadditiontothepreviouslyreporteddelayed173keV rayfrom 136
Sb [3℄,twonewdelayedtransitions of53.4(3)keVand51.4(5)keVhavebeen observed.The53.4keV raywasobservedin oin iden ewithSbK
Fig.3,in agreementwith565(50)nsmeasuredin Ref.[3℄.
0
20
40
60
80
100
120
140
160
180
Energy (keV)
0
25
50
75
100
0
5000
10000
15000
Sb X-K
α
Xe X-K
α, β
L+M 51 (
136
Sb)
K 197 (
136
Xe)
Xe X-K
α, β
K 88 (
138
I)
Number of Counts
K 197 (
136
Xe)
Gate on 173 keV
γ
ray
3 - 6
µ
s after arrival of an A=136 ion
K 51+53 (
136
Sb)
Fig. 2. Conversion ele trons and X-rays observed in the Sidete tors, in the upper spe trum3 to 6safterthe arrivalof an A= 136ionand inthe lowerspe trumin oin iden ewiththe173.0keV rayintheGedete tors,0to1.5safterthearrivalof anA=136ion.A onversion-ele tron lineof47.1 keVisobservedandisidenti edas theL+Mlinesofa51.4keVtransitionfrom
136 Sb.
1.4. Dis ussion
Ashell-model al ulationhasbeenperformedwiththesamerealisti inter-a tionusedfor
134 Sband
135
Sb[1,2℄andisdes ribedinthepro eedingsof A. Covello at this onferen eand in [5℄. Theproposed experimental level s hemeof
136
-2
-1
0
1
2
3
4
5
6
Time (
µ
s)
-200
0
200
400
600
800
1000
Number of Counts
136
Sb E
γ
=173.0 keV + K
α
T
1/2
=480(100) ns
Fig.3. Summedtimespe trumofgatesonthe173.0keV -raytransitiondete tedin theCloverGedete torsandtheSbK
X-raysintheSidete tors.
0
53.4
226.4
277.8
173.0 E2
53.4 M1
51.4 E2
0
21
179
210
158
21
31
6
-4
-2
-1
-(6
-
)
(4
-
)
(2
-
)
(1
-
)
T
1/2
= 480(100) ns
B(E2) = 131 e
2
fm
4
EXP
THEORY
136
Sb
Fig.4. Proposedde ays hemeofthe 136
Sbisomer.
The four observed levels in 136
Sb are all identi ed with states whi h aredominated bythe on gurationg
7=2 (f 7=2 ) 3 . It isworthmentioning thattheper entages of on gurationsotherthanthedominantonein the onsideredstatesof
136
Sbareratherlarge,rangingfrom24%to32%. Animportantpie eof informationisprovidedbythemeasuredhal ife ofthe6 state,fromwhi haB(E2;6 !4 )valueof170(40)e
2 fm
4 =4.2 W.u.isextra ted.Usinge e tiveprotonandneutron hargesof1:55eand
neutron-ri hnu leus 136
Sb,whi hrepresentsafurtherstepinthe far-from-stabilityregionbeyondN=82.Themultipolaritiesofthesetransitionshave beendetermined and alevel s heme onstru tedby omparison with the results of a realisti shell-model al ulation. The predi ted energies and B(E2;6 ! 4 ) are in verygood agreementwith measuredvalues. This showsthata onsistentshell-modeldes ription anbegivenofthepresently knownnu leiinthis region.
2. 105;107 Mo, 107 T 2.1. Introdu tion
Therapidshape hangeofthelowlyingstatesintheSrandZrnu lei,from spheri alto deformed, when theneutron numberin reases from 58 to 60 neutronsiswellknown.Inthenearbyeven-even
104 108
Moanewsituation o urs.Thesenu leiarestronglydeformed[8{10℄but,atthesametime,the levelsoftheK
=2
+
; -band des reaseinenergywithin reasingneutron number,suggestingthatthetriaxialdegreeof freedomplaysanimportant role in these isotopes. However, the nature of this triaxiality is not well understood.Cal ulationsbySkalskietal. [11℄predi tgroundstatetriaxial minima,with =19-21
Æ
forthesenu lei.In onstrastSmithetal.suggest that thetriaxialityis dynami in nature [8℄,with 0
Æ
at lowspinsand arotation-indu ed hangeo urringaroundJ 10 12~,thelatterbeing supportedbytheexperimentalobservationofade reaseinthequadrupole moment.Tobetterunderstandthenature ofthisnon-axialdeformationit is alsoimportantto study thestru ture ofthe oddMo nu lei. Thenu lei 105
Mo and 107
Mo have been revisited experimentallyin the presentwork and the stru ture of these nu lei was analyzed in the framework of the parti le-rotor ouplingmodel. Similarlythein uen e ofanodd-protonon theeven-even orewasexaminedbystudying
107 T .
2.2. Experimental Method and Results Thenu lei
105;107
Moand 107
ously, four di erent on gurationswere proposed in 105
Mo however only onewell-developed bandwasobservedwhi h fed thegroundstate[10℄.In thepresentworkfourwelldevelopedbandswere observedin
105 Mo.
105
Mo
(4)
(3)
(2)
(1)
0.0
5/2
–
396.5 (3/2
+
)
662.8
(7/2
+
)
1078.0 (11/2
+
)
1617.8 (15/2
+
)
2276
(19/2
+
)
(1/2
+
) 331.5
(5/2
+
)
524.2
(9/2
+
)
880.5
(13/2
+
) 1364.0
(17/2
+
)
1947
(21/2
+
)
2554
246.4
3/2
+
348.3
5/2
+
507.3
7/2
+
718.1
9/2
+
964.7
11/2
+
246.4
309.9
5/2
+
463.9
7/2
+
649.5
9/2
+
857.3
11/2
+
1118.1
13/2
+
1385.4
(15/2
+
)
1718
(17/2
+
)
2036
(19/2
+
)
2426
(21/2
+
)
2792
(23/2
+
)
232.5
9/2
–
623.0
13/2
–
1175.3
17/2
–
1879
21/2
–
2734
(25/2
–
)
94.7
7/2
–
377.4
11/2
–
795.5
15/2
–
1352.5
19/2
–
2046.0
23/2
–
2872
(27/2
–
)
192.7
356.3
483.5
583
607
266.3
415.1
539.8
658
65
127.8
396.5
85.1
48.3
149.8
314.6
277.8175.9
261.1
457.4
101.8
159.0
210.8
246.6
369.8
246.4
348.4
253.7
274.7 197.3
63.5
115.5
301
339.6
468.4
600
708
154.0
185.6
207.9
260.5
393.3
528.1
651
756
309.9
215.3463.9
369.2
416
232.5
390.5
552.3
704
855
94.7
137.8
144.8
245.6
172.5
379.8
177.2
526
157
282.7
418.0
557.0
694
826
Fig.5. Partiallevels hemeof 105
Moasobtainedinthiswork.
Threewelldevelopedbandswere previouslyreported in 107
Mofrom -ray measurements of the spontaneous ssion of
248
Cm [12℄. To omplete thelevels heme,theLohengrinmassspe trometerhasbeenusedtosear h fordelayed -raysand onversionele tronsinthis nu leususingthesetup des ribedinSe tion1.2.Anewisomerwitha420nshalf lifewasobserved in
107
Mo. It was tentatively assigned to be a1=2 +
state deex iting by a 65.4keV E2 transition to the5=2
+
ground state.A 30 keV, E1, 3.85 s transitionfromthe3/2
+
levelofthe1/2 +
[431℄intruderbandtotheground statewasobservedin
107 T .
2.3. Dis ussion
The ex ited states of the bands in 105;107
Mo and 107
ad ho \Coriolisattenuation"parameter,.Ane e tivevalueof=0:7is ommonin these al ulations. Finally, thenumberof variable parameters was minimized by imposing that all bands of the samenu leus have the sameparametervalues.
440 ns
200 ns
66
1/2
+
66
165
3/2
+
5/2
+
320
492
7/2
+
9/2
+
730
970
11/2
+
13/2
+
1287
15/2
+
0
152
5/2
+
7/2
+
341
567
9/2
+
11/2
+
820
1118
13/2
+
15/2
+
158
223
1/2
+
3/2
+
110
225
3/2
+
5/2
+
384
584
7/2
+
9/2
+
817
11/2
+
1094
1350
13/2
+
15/2
+
0
5/2
+
150
336
7/2
+
9/2
+
567
816
11/2
+
13/2
+
1129
15/2
+
1/2
+
3/2
+
5/2
+
1/2
+
3/2
+
5/2
+
107
Mo
THEORY
EXPERIMENT
γ
= 16.5
ε
2
= 0.32
°
Fig.6. Comparisonofexperimentaland al ulatedpositive-paritybandsin 107
Mo.
The al ulationsare ableto orre tlyreprodu ethe four bands ofthe experimental level s hemesonly when deformationsof
2
=0:32 and 17
Æ
areusedthroughout.Moredetailsof theresultsofthe al ulation an be found in Ref. [13℄. These values were ompared to the deformations extra tedfortheneighboringeven-evenMonu lei,usingthetheDavydov andFilippovtheory, andfoundtobeverysimilarat lowspins,suggesting that all these nu lei have similar ore deformations. The isomeri state in
107
T and level s heme of 105
notgreatlye e t thedeformationofthe ore.
2.4. Summary
Newinformationonprompt -rayhasbeen olle tedon 105;107
Mo, ompris-ingoffour well-developedbands in
105
Mo. Newisomeri stateshavebeen reportedfor
107 Moand
107
T .Thesedatahavebeensu essfullyinterpreted usingsimplequasiparti le-rotormodel al ulations,wheredeformationsof
2
=0:32and 17 Æ
were foundto orre tlyreprodu etheexperimental levels hemes..
Referen es
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6. P.Ho ,J.P.Omtvedt,B.Fogelberg,H.Ma handM.Hellstrom,Phys.Rev. C56,2865(Nov1997).
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11. J.Skalski,S.MizutoryandW.Nazarewi z,Nu l.Phys.A.617,p.282(1997). 12. W.Urban,T.Rza a-Urban,J.A.Pinston,J.L.Durell,W.R.Phillips,A.G. Smith, B. J. Varley,I. Ahmadand N. S hulz, Physi alReview C (Nu lear Physi s) 72,p.027302(2005).
13. J.A.Pinston,W.Urban,C.Droste,T.Rza a-Urban,J.Genevey,G. Simp-son,J.L.Durell,A.G.Smith,B.J.VarleyandI.Ahmad,Physi alReview C(Nu learPhysi s)74,p.064304 (2006).
