Nuclear c1usters in cold fission: long-living three

REVISTA MEXICANA DE FislCA 45 SUI'LE~IENTO
2.120-124
OCTUBRE 1999
Nuclear c1usters in cold fission: long-living three-c1uster nuclear molecules
A. V. Ramayyal, J.H. Hamilton, I J.K. Hwangl, ami GANDS95 Collaboralions
(JI Physics, Vmuierbill Universiry. Nashvil/e, TCllfleSSee 37235, USA
1 Department
Recibido el J O de fehrero de 1999; aceptado el 25 de fehrero de 1999
Thc phcnomcnoll
of cold (neutronless)
binary and ternary fission in spontancous tission nf
2Cf was cxpcrimcntally
:.!!i
observed by triple
garlllna coincidcIll.::ctcchnique wilh Garnmasphere with 72 dctcctors. Many corrclated pairs for oo(h binary and ternary fission were observed
in the sponlancolls fission of 252Cf. Yiclds of cald lernary and ca Id hinary lission were cxtractcd from intensitics 01',-[ay transitions originating frolll the dc-cxcitation of primary/sccondary fragrncnts. Thc, spectrum for \JGSr.IOBe.14GBaternary fission indicates a very long life
for Ihis triple nuclear ll1olecule.
KevYo'ord.\: Spontaneous I1ssion; binary and ternary; 252Cf
Los fenómcnos de fisión fría binaria y ternaria ha sido observada experimentalmente en la fisión expontánea de 252Cf usando la técnica de
triple coincidencia de rayos gama utilizando los 72 detectores de la Gammaspherc. Muchos pares tanto para la lisión binaria y ternaria que se
observaron estahan correlacionados. procedentes de la fisión cxpontánca de 252Cf. Conteos de la fisión fría binaria y ternaria fueron extraídos
de Ins intensidades de las transiciones gama, originadas de la des-excitación de fr¡¡gmenlos primarios y/o secundarios. Los espectros de la
fisión lernarin \JoSr_lOBe_J4GBa indican una larga vida de esta lTlolecula Iluclear triple.
Descriptores:
Fisión exponlánea; binaria y ternaria; 252Cf
rACS: 27.90+h: 24.75+i: 25.85.Ca
1. Introduction
Sincc Ihe discovery 01' nuclear fission, invesligation 01' low
cllcrgy tission has remained as a dynamical field of nuclear
physics [1-3]. Delailed studies 01' fission fragll1ent charge,
J11assand energy distrihutions for a nUll1her of fissile sys.
tems cOl1tinue to he an importanl source 01' information to
understand the dynall1ics of fission [4]. The total kinetic energy(TKE) al' tission fragll1ents is mainly defined hy the
Coulomh (plus nuclear) potential jusI aner scission. Therefore, TKE values, together with rragll1cnt rnass asymmetry,
providc informalion on sorne essential characteristics ol' seission configurations. Prornpt fission neulrons and 'Y.rays carry
informalion ahout the exeitation energy 01' lhe fission fragmcnts. and are an important source of infonnation ahout lowenergy lIssion [5].
The well known asymmetric mass distrihlltions of f1ssion
fragments is generally recognized to he lhe reslIlt (JI'shell cffects (see Re!".6 and references in). An extreme case is that 01"
(he himodal f1ssion observed ror the Fm ano Md isotopes 171
where lhe predominant fragmentations are close to lhe double magic 1.12Sn nuclci. Here, two distinct fission channels
are observed, one with very high TKE, which practically exhausts the disintegration energy (q-value), associated. wilh
two spherical fragments and lhe second one at much lower
TKE which proceeu through the usual fission valley with enlongatcJ shapes. The previous techniqllcs of measuring lhe
l11asscs. charges and TKEs 01' the fragment's [8] have allowed one to draw qualitative conclusions. In addition, there
are very interesting. hut litlle studieJ new cold I1ssion modes
(neutronless) which are only a small part 01' the bulk 01' the
f1ssion events.
In recent times. many new experimental data conceflling the spontaneous cold fragmentations of nuclei have been
dctcrmincd. These include exotic decays with emission 01'
heavy c1usters having masses from AL = 12 to 34 [9]. The
existence of the so-called fusion vallcys or "cold" valleys on
the potential encrgy surfaces al' fissionable heavy nllclei {9]
pro ved lo be a key ingredient ror the interpretation 01'c1ustcr
decays (the 20HPh, 100Sn and 132Sn vallcys). In thcse situations, the final fragmcnts have compact shapes al the scission
points and almost loero cxcitation encrgy.
Recently, a new expcrimental tcchnique has heen applied
to sludy flssion processes 11,2]. By ll1easuring with large
Gc detector arrays he intensities and encrgies 01' prompt ,rays cmittcd by fission fragments (both primary and secondary) in (he triple 'Ycoincidence mode, one can extracl the
indcpendent yields of differcnt charge splits for sccondary
f\ssion fragments fonned after ncutron evaporation and of
cold hinary and ternary tlssion modes where no neutrons are
emitted, prompt neutron multiplicily dislrihutions, and angular momcnta of (he f1ssion fragments for specific fragment
pairs [1,2,10-13]. Thcse ncw types of information give significant new insights ¡nto the fission process.
2, Experimental
techniques
In our cxpcrimellts, a 25 JlCi SOllrce of 252Cf (3.1 % sponlaneous lission) covcreu with 11.3 mg/cm2 Ni and 13.7 mg/cll12
Al foils \Vas placcd at lhe center 01'Gammasphere. The primary fragments are fonned gcnerally in highly excited sta tes
(;::, 30 McV) and cmit sevcral (1-10) ncutrons, leading to seeondary fragments. Thc 'Y-rays cmitted hy these secondary
r--.;UCLEAR CLUSTER
T,\BI.E
1. Average
S IN COLD
,\10/ Ba
LONG-LIVING
ca Id binar)' lISSiollyiclJs from gafes on
AI,IAII
Zr / Ce
FISSION:
} ~'xl'
light fr:lglllclll anú
two
121
MOLECULES
heavy fragmcnllransilions.
Al, I _-\11
)~'xp
y{l"enj
tbl'
](XI1I5:!
O.OJl)(:!1
()(XI.j
Te / es
1(NII.j3
0090( 18)
0.128
1021150
0.20(41
0.1133
Ru / Xc
1101142
O.060( 12)
0.044
0.10(2)
0.083
103/149
0030(6)
O.O-N
111/141
I0411.j8
0.010(2)
0.012
112/140
0.020(4)
0.007
10411.j8
0.010(2)
0.(XI5
114/138
0.020(4)
0.014
1051147
OO.jO(8)
0.063
1161136
0050(20)
0.027
1Of>!1.j6
0.(J40(8)
0.007
1071145
0070( 14)
O.())(l
108/1.\4
0.030(6)
0.087
cluster rat1ioacliviry,
In :.!;¡:!cr
approximalely
sr
duccd
t\Vo
NUCLEAR.
} ,( "",,)
11",
Pd/Tc
rr,lgmcnls are delcctcd
in triple coincidence.
The ,~~(¡MoAi:hBa pair cOlTesponds lo Ihe situation where no neutrons
are emitted and is eallcd eold hinary fragmcnlalion,
a type
(Jf
TIIREE-CLUSTER
and lile ~i-Spcctrulll
100 dilTcrcnt
is very
complex.
nuclci are proMorcover
il"
\~liJ\l(} is Ihe lluclcus of inlcrest,
ils 'Y rays are in coincidellce
with aH thosc in its cOlllplilllclltary t'í;Ba fraglllcnts. Hcnce a
single galc un a 2+ -+0+ transition may not he sumcicllt lo
c!illlinate the complexitics
arising frolll trallsitions of similar
cllcrgies in dillerent isotopcs or in lhe samc isolope and its
p¡lrlners. i\ -'-~I-)coincidcnec
euhe \\'as huilt hy using lhe
RADW¡\RE software. Frolll Ihe cuhe one can sel two gates
(AND con<iilion) ami extracI lhe ~l-ray speclrulll. in coincidence willl bOlh gate transiliolls silllu!taneollsly,
rOl' cxample,
like tlle 2+ -+ ()+ and 4+ --t 2+ lransitions in ¡he same iS(llope, or t\\'o lransitions
in lwo partner
wherc Ihe tolal yicld was normalizcd
to Wahl's tahle (IGI.
Presenlly, many 01' lhe speclra nf odd-Z l1uclei are not suHiciently knO\\i'1lso thal olle <.:ould Ilot delermine experimclllally
most odd-Z isotopk yields. The colo hinary flssioll yields are
shown in Tahle I alollg with the lheorclical values preoieted
by Sandu Ieseu el al. I j ji. \n Tablc \. lbe !irs! repor! 01' lbe eold
hinary lissioll 01"an (ldd-Z -ood-Z fragmenlation
is showll ror
lile Te and Cs pair.
The lrends. (',g. variatiolls with A and Z, of lhe theoretical1y cakulatcd yields are in good agrcemenl with the experimcntal yields, Uf particular nole, for even Z the odd A-ood A
pairs are prcdkted to be larger Ihan lhe evcn A-cven A pairs
and this is observcd in cvcry casc. Morcovcr the ¡argest theoretical yield predicled is rol' the odd Z-odd Z 1O!JTc_113CS
and this is lrue \vithin lhe experimcntal
results generally
arc in remarkahle
error. Overall
lhese
agrcerncnt.
isolopes.
4. Cold n ternary spontaneous fission
3. Cold binary fission
No dircct measurelllenls
nI' yiclds of correlalcd pairs in l'old
hinary lission have heen made prior 10 our work. Sincc the
Ilclltmnless binary cvenls are much sm<lller than Ihose \Vilh
llcutmns emillcd, doublc gating techniques
have heen elllphlyúl to extracl (he yields rOl' lhe eold hinary lission. Earlier
\Ve reported lhe firsl resulls rOl" lhe correlalcd pairs in cold
binary lission in :.!ti2Cf[11-1,1] and 2'12pU [l,j].
Subsequenlly
we extraclcl! more delailed yields nI" c(lld
hinary tission 111] As <In examrk
rOl' colL! binary
llssion
(Fig. 1), a douhlc gate was sel on lhe I XO.() and JJ2,() keV
lransitiollS in 1'lhUa. Une can ckady
sce lhe 171.6 keV
2+ -+ ()+ transilioll in JOEif\10, the /,ero neulron emissioll partHer or 1 ,l(iHa alllilransitiollS
in olher ]\;10 partncrs or in HhBa.
Thc 11lll11her 01' nculrons evaporated
corresponding
10 each
channel is lIlarked 011 Ihe speclrum. The yiclds rOl' cold al~
pila lernary. and cold IOnc lernary llssioll also were cxtracled
rrom ¡he ~¡"-~l-'""dala.
By detertllining
lile inlensitics 01" /' Irallsitions in hoth
l'ragmenls ¡¡nd kllowing the hranching ratios hel\Vcen dillerClll ¡ransilions.
rclative hinary yields 11] 1 \Vere dctermillcd
HOI lernary lission has heen known ror a long time, with the
Ihird particle generally heing an n parliclc. However, cold
ternal")' fission has not heen dire<.:lly ohscrvcd. In the case of
cold (neulronless)
alpha lernary f1ssion of2.':'2Cf, one looks at
lhe corrclatioll helwcen lwo even-Z rragmel1ls wilh lhe surn
ur chargcs Z
0G ;¡nd sum 01"masscs A
248. For exampie, Aí\(;Ba amI ~g2Zr are the partners ror an (t -partiele in cold
lernary lissiol1. In Fig. 1, olle sces the 2+ -+ 0+ lransition of
energy 171.ú keV 01" 101;1\10, which eorrcsponds
lo the zero
=
=
neulron cllallllel. One also sees lhe 151.8 keV (2+ -+ 0+)
transition in IIUZr, which corresponos
lo colo u-lernary
lission. In Fig, 1, rhe 151.X keV transition is nOI as clcar as some
01"tlle other peaks hccause 01"highcr hackground and lhe transilions fmm all Mo partncr nuclei is coincidenec
with l-1GBa,
Howevcr, rcstricling to Olle (t~channel
2Zr) and I XO.9 kcV
gatc on 32ú,2
e0
hy sclting a double
as shown in
e46Ba)
Fig, 2, one can reduce the hackground.
In Pig. 2 and in its
insel, one clcarly sees the 151.8 keV (2+ --t- 0+) Iransition
in IO:!Zr as highlig.hled in Ihe inset. Ir we 1110VCcither gale hy
5-1 () keV lo either side, Ihe 151.H keV transition
to show ils prcscn<.:e is not I"mm sOllle hackground
Re". !vlc.\". h~\.. 45 S2 (199tJ) 120--124
disappcars
circe!.
122
AV RAMAYYA. IH. HAMILTON. J.K. HWANG. AND GANDS95 COLLABORATIONS
10
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Double Cote on 160.9 ond 332.0 keV Tronsitions
::;
~ S
~ ""
::;
w ~
15'0
,1",,,,,,,
r--g~5
..;
!"
'"
300
200
~OO
...
400
E,(keV)
FIGURE l. Coincidcncc spcctmm obtained
by
double gating on Ihe 180.9 kcV nnd 332.0 keV transitions in
146Ba.
TABLE 11.Thc alpha ternary isotopic yiclds Yexp oblained pcr lOO fission cvcnts.
y"n"l\
the
},r"n
(\ Partncr llucJci
}~'x¡> (%)
~¡¿Kr-ágúNd
0.002(1 )
0002
j~Sr-~~~Ce
0.008(3)
0.Q10
~~~Sr-J,~oCc
0.014(6)
0.017
~~Sr-A~9Cc
0.018(9)
0.016
:~~OSr-A~8Cc
0.021( 10)
0.027
A~lSr-U; Ce
0014(11)
0.010
.\goZr-A(~8Ba
0.038( 12)
0.017
.~glZr-At,7Ba
0082( I O)
0.058
18 Zr-A;\5Ba
0084(29)
0.050
.\glzr-AJ Ba
O()(~)(4)
0.017
,~g.lZr-J,t,l
0.017(8)
0.016
0030(14)
0.017
0.011(6)
0.028
ú
_~~7
Ba
f\lo-!,:¡ I Xc
U2Ru_gGTc
In Tahle 11, lhe cold neulronless alpha lernary f1ssion
yields ohservcd in lhe spontancous f1ssion of 252Cf are prcsClltetl [22]. The highesl experimental yiclds were found for
thc Zr + Ba isotopes. Significant yields also were ohserved
for Ihe Mo + Xe and Sr + Ce lernary fragmentalions. The
avcrage valucs of the n ternary fission yields ohtaincd hy
double-gating on the heavy fragments or lighl fragmcnts are
lislcd in Tahle 11.
In (he case of cold ternary tission, Santlulcscu el l/l. [17J
("ollsidcred that close lo the scission configuration a few of
(he nucleons fonn a short neck. Al a givcn value of lhe ncck
radius a t10uble scission takcs place, a third Iighl fragmcnl
is formed nelwcen lhe two heavicr ones and from this point
slarts the ternary nssion harricr corresponding to a givcn mass
antl charge splitting [17J. Thc widlh of the potential harrier
hClwccn the heavicr fragments is only 2.5 lo 3 fm with an exit
point at lip distance between the fragmenls 01"3.5 lo 4 fm,
indicating that iniliJlly the two hcavier fragments are penelrating their relalive potential harrier [17J. Later on as the
hcavier fragmcnls move away, lhe potenlial well for lhe light
fragment disappears (for tir distances of about 7 to X fm), il
n Partrler nuclei
3
1~6~1o-U1Xe
\~8M(}_A~oXc
,~áüPd-l.J2 Sn
}~,xp (%)
tillo'
0.018(7)
0.031
O.(XJ7(3)
0.014
0.(XJ6(3)
0.048
hecomes free bcing repellcd mostly along the y axis hy lhe
two hcavier fragments which have already acquired large kinetic cnergics. Consequently, due to the presence of lhe shorl
Ilcck the po(cntial harricrs hetwcen lhe Iwo hcavier fragments
are slightly ch<mged, Ihe ca1culated cold o-ternary yields of
2r:i2Cf heing similar lo lhe calculaled cold binary yiclds of
24HCm 117] ami ilS calculatcd cold IOBc-lernary yields being
similar to lhe calculated cold binary yields 01"242PU.
Thc Ihcorclical values [17] are shown in columns 3 and 6
ofTable 11.Since a diffcrenl normalization is used for the theoretical calculations, the (otal theoretical yield is nonnalized
lo the lotal experimental yicld, and lhe reported values are
Ihe renormalized relative Iheoretical yiclds. In general, there
is good agrccment helwcen the relative thcoretical and experimental yiclds. The experimenlal yiclds ror odd-odd splittings are highcr than the yiclds rOl'the even-evcn ncighhours,
as foulld in the theoretical calculalions also. Thcsc enhanced
yiclds in these cases Illay he hecause 01' Ihe differcnces in
level dCllsities near the ground Slales in o-o and the e-e nuclci.
Rev. Mex. Fís. 45 S2 (1999) 120--124
NUCLEAR
CLUSTERS
IN COLIJ FISSION,
LONG-LIVING
"'so)
Double gole on 180,9(
ond 326.2( 102Z,) keV
V 5000
e
e
2
4000
U
va. jOOO
e'" 2000
:l
o
o
¡
1000
T1IREE-CLUSTER
NUCLEAR
900
¡
¡
500
'~
"
~
100
j~
123
MOLECULES
N
~¡ j
! !r~
¡~ ~
!j'{ !i~l-l --}1
M
'9'
~~~l~ 1'15'5
~
N
'15
r;= :rJ
~ - -~",¡¡
'!1
D'
~N
500
400
300
700
900
E, (keV)
-+ 0+ IXO.9 kcV transilion in 1468a and the 4+ -+ 2+
--+ 0+ transition in I02Zr. Thc pcaks al 203.2, 307.6.
146
3.'2.0. 42S.2. -t-W.5. 510.8, 51-l.7. 52-1..1and 583.8 kcV are lransilions in ¡'¡üBa hccausc (Ir lhe 324.2 kcY (7- --t .) -) transition of
83 in
f-1(iURE 2. Thc coincidcn(.'c
~pcctrum
ohtaincd
with a oouhlc
gatc
011
326.2 kcV lrallsilion in J02Zr. Thc c1cnr peak al 151.8 McV corrcspomh
(he 2+
10 lhe 2+
lhe gateo
The cnhanccJ experimental yiclds for the cold (\ .lcrnary
J
o) Ooo.ble 90Ie en 'lBto( 1OlIBo)cn:I
6\4,1(
Iission in \vhich heavicr partncrs are Ba ¡SOlopes could he al.
Irihulctl lo lhe static octuplllc dcformatioll ohscrvcd in this region [2], sinec oetupolc
shapes at lhe seissiol1 conligurarion L'ould signiticantly
lower lhe Coulomh harrier and increase lhe pcnclrabilily
hetwcen the final fragmenls.
Note
thal the cxperimentally
determined
isotopic yields <lre illtegralcd yields. In lhe spontancous
fission experiment 01' :!;'"':!cr
Ihe majorily
01' lhe hinary amllernary
splittings
silions over a wholc rtlnge 01' TXE's fmm zcm up 10 al Ieasl
lhe neulroJl hinding energy. These are the !irsl identilicatioll
alld determinaliol1
nI' yields 01' lhe panicular corrclaled pairs
wilh eold n lernary
lissiol1.
5. Cold lOUc Icrnary sponlancous
b) Do.ble 9'Jl.e en 116(1£) ond
1l\4.1(-sr)
keY tronsitions
!Cad 10 highly
exciteu f1lw.1nuclei whieh afIer neutron evaporalion are decaying 10 lhe lowesl states hy gamma cascadcs. Less frctllIenlly, lherc are also cold fragmenlalions
which leave lhe
hnal nuclei in thcir ground 01' lirst rew cxcited states. \Ve detine these (old tission experimental
yiclds as inh:gralcd yiclds
sincc lhey eolleel the contrihuliollS of all (nclIlt"Onless) lran-
associaletl
-sr) keY Ironsitions
lission
The (olal yield of ternary spol1taneous lission \vith lhe lhinl
particlc heing IOlk is expecled (o he ::::::I()()liHIt:S 100\'L'rthan
to(al (l lernal")' tission yield IISJ. Nexl \ve scarchcd
nClllronless lernar)' lission wilh the third parliclc
hcavier cluster such as IOBe. Contrary lo n-temary
ror cold
hcing a
Sr: ""¡Ih
o
JOOO
"00
J200
J300
E7 (keV)
F((;URE J. a) COillCidcllCC ¡.ray spCClrUIllohtaincd
by doublc gat.
til1~ (In thl' XI-t.7. 2' ---¡. 0+ trallsilion in 9651" and 181.0 kcV.
:2+ ---¡. O' Ir<lnsilioll in I \l)Ba. h) Coincidl'llcc
lailll'd hy douhlL' gatling
un Ihe SI4.7.
2+
-t
¡'.ray
spcclrum
0+ transition
amI on lhe hackground al 176.0 keV ncar lhe 181.1 kcV 2+
transilioll in l'lliBa. BG : Background
ob-
in 9GSr
-t
0+
The coincidcnce spcclra sl1mvs lhe 977.5 keV, .1+ ---t 2+ transili(J1l in !H;Sr and lhe IX 1.0 ant..l 332.6 keV peaks il1 146Ba.
\Vilh;¡ douhle gate on Ihe 3362 eOBe) and 181.0 et.t6Ba) kcV
peaks, lhe X14.7 keV (2+ ---t 0+) transilion
This dala eSlablish
corrclaled
lhe IOBc lernary
in 90Sr is sccn.
SF for 965r_146Ba
pairo The yield lo the lIrsl excitcd
state 01' IOBe
n c.\Ciled slale energics > 20 f\1cV so -, clIlissioll \\'ould
nol he scell. IOBe has all eXl:iled (2+) slale al an cnergy 01"
1'0,. ''''Sr_"uiBa panner is '" ~(2) X 10-4 per lOO SF and
lo lhe ground s(alc ::::::O. This yiclLl is 4.4% of lhe col ti (\-
J .J6X f\ le V. \\'!len sClli ng a t..Iouhle gale 011 lhe X14.7, and
IXI.O keV 2+ --+ 0+, lransilions
in %Sr allll]l(¡Ba
rcspcl:t¡vely, we see the 977 keV ,1+ ---t 2+ transilion in ~1¡;Sr(Fig. J)
ami a peak al ::::::.,36R keV (ohscrvcd al 3362 kcV) wilh no
Dorrler hroadening.
Assurning thal the peak at .1362 kcV is
lile 2+ -t 0+ lransitioll in IOBe. \Ve sel one gate on lhis lransilion ant..llhe other gale on the 2+ --+ ü+ Iransition in HliSr.
(crnary lission yield 01' 0.009(4)
per 100 events for 146Ba and
""Zr 1121 and 1,O';{,01' the eold hinary [ission yield [111 01'
O,O~ (1) per lOO evenls for I4"Ba and IO"Mo, Thc yicld 01'
this corrclaled pair with IOBe is 8 ::l: 4% ol' the total IORe
yicld [18J. Thc major queslion is \vhy is the 3362 keV line
no! Doppler hroad~neJ sincc the 2+ slate Iifctime is 125 fs
antl lhe stopping lime 01' a 17 McV IOBe is 1250 fs. A 11011-
Rl'\'. Ml'x. F¡.\' . .t5 S2 (1<)99) I 20-1
2-t
124
A.v. RAMAYYA, J.H. HAMILTON, J.K. HWANG, AND GANDS95 COLLABORATIONS
Dopplcr shincd peak is <1lsoobscrvcd by Muttcrcr et al. (1 X)
with NaI uctcclors in JOBe ternary ¡¡ssion. Onc possihlc cxplanatinn is Ihat after formation 01' 10Be in the ncck !J(j SrJOBc_I.1üSa form a nuclear Illolcculc with lOSe hcld in lhe
potcntial
\Vell ror a time;::::: 10-12 s during
which il l' dccays
bcfofC lhe lluclci break up into ternary f¡ssion.
particlc detectors
LCP ami') rays.
so we (,;an galc on Ihe
insidc Gmnmasphcrc
Acknowledgments
Ir Ihis is IfUC,
Ill<lny cxciling new possibilities
for molecular configuratiolls
ami nuclear structures are apencd up. Wc will gain furthcr insight ¡ntn Ihis qucslion in our cxpcrirncl1t with ¡¡ght chargcd
Thc work al Vanucrhilt
thc U.S. Dcpartmcnl
H8ER40407.
Univcrsity
01' Encrgy
was supported in part by
undcr grant No. DE-FG05-
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