Cold binary and ternary fragmentations in spontaneous fission of
252Cf
( ) A. V. RAMAYYA( 1 ), J. H. HAMILTON( 1 ), J. K. HWANG( 1 ), L. K. PEKER( 1 ), J. KORMICKI( 1 )( ), B.R.S. BABU( 1 )( )T. N. GINTER( 1 ), A. SANDULESCU( 1 )( 2 )( 3 )( 4 ), A. FLORESCU( 1 )( 2 )( 3 ), F. CARSTOIU( 3 ), W. GREINER( 1 )( 2 )( 4 ), G. M. TER-AKOPIAN( 1 )( 2 )( 5 ), YU. TS. OGANESSIAN( 5 ), A. V. DANIEL( 1 )( 2 )( 5 ), W. C. MA( 6 ), P. G. VARMETTE( 6 ), J. O. RASMUSSEN( 7 ), S. J. ASZTALOS( 7 ), S. Y. CHU( 7 ), K. E. GREGORICH( 7 ), A. O. MACCHIAVELLI( 7 ), R. W. MACLEOD( 7 ), J. D. COLE( 8 ), R. ARYAEINEJAD( 8 ), K. BUTLER-MOORE( 8 )( ), M. W. DRIGERT( 8 ), M. A. STOYER( 9 ), L. A. BERNSTEIN( 9 ), R. W. LOUGHEED( 9 ), K. J. MOODY( 9 ), S. G. PRUSSIN( 10 ), S. J. ZHU( 1 )( 2 )( 11 ), H. C. GRIFFIN( 12 ), R. DONANGELO( 13 )(1) Physics Department, Vanderbilt University - Nashville TN 37235, U.S.A. (2) Joint Institute for Heavy Ion Research - Oak Ridge, Tennessee, TN 37835, U.S.A. (3) Institute for Atomic Physics - Bucharest, P.O. Box MG-6, Romania
(4) Institute f¨ur Theoretische Physik der J.W. Goethe Universit¨at
D-60054 Frankfurt am Main, Germany
(5) Joint Institute for Nuclear Research - Dubna 141980, Russia
(6) Department of Physics, Mississippi State University - MS 39762, U.S.A. (7) Lawrence Berkeley National Laboratory - Berkeley CA 94720, U.S.A. (8) Idaho National Engineering Laboratory - Idaho Falls ID 83415-2114, U.S.A. (9) Lawrence Livermore National Laboratory - Livermore CA 94550, U.S.A.
(10) Nuclear Engineering Department, University of California - Berkeley CA 94720, U.S.A. (11) Physics Department, Tsinghua University - Beijing, P.R. China
(12) University of Michigan - Ann Arbor MI 48104, U.S.A.
(13) University Federal do Rio de Janeiro - Rio de Janeiro, CP 68528, RG Brazil
(ricevuto il 3 Luglio 1997; approvato il 15 Ottobre 1997)
Summary. — The phenomenon of cold (neutronless) binary and ternary fission in
spon-taneous fission of252Cf was experimentally observed by triple gamma coincidence tech-nique with Gammasphere with 72 gamma-ray detectors. Many correlated pairs for both binary and ternary (4He,10Be) fission were observed in the spontaneous fission of
252Cf. Yields of cold ternary and cold binary fission were extracted. These results are
compared with the theoretical calculations using M3Y nucleon-nucleon potential and WKB approximation for barrier penetration.
PACS 25.85.Ca – Spontaneous fission. PACS 01.30.Cc – Conference proceedings.
(
)Paper presented at the 174. WE-Heraeus-Seminar “New Ideas on Clustering in Nuclear and
Atomic Physics”, Rauischholzhausen (Germany), 9-13 June 1997.
(
)Also at UNISOR, ORISE, Oak Ridge, TN 37831; on leave from Institute of Nuclear Physics,
Cracow, Poland
(
)National Accelerator Center, P.O. Box 72, Faure, 7131 South Africa (
)Present address: Department of Physics, Western Kentucky University, Bowling Green, KY
42104, U.S.A.
The spontaneous emission of light nuclei (cluster radioactivity) is now a widely ob-served phenomenon, starting from the emission of alpha-particles to heavier clusters such as34
Si [1, 2]. It is very important to establish theoretically and experimentally if cold (neu-tronless) binary and ternary fragmentations are existing in nature. This new phenomenon will be equivalent to cluster radioactivity during the fission. Such cold decays will produce all the fragments with very low or even zero internal excitation energy and consequently with very high kinetic energies. Their total kinetic energy TKE =Q
t
,TXE will be close to the corresponding decay energyQor even equal to it.
The first direct observation of cold (neutronless) binary fragmentations in the spon-taneous fission of252
Cf was made [3, 4], by using the multiple Ge-detector Compact Ball facility at Oak Ridge National Laboratory, and more recently with the early implementa-tion of Gammasphere [4, 5]. We present here the evidence for cold (neutronless) binary and ternary fission. These data were obtained in studying the spontaneous fission of252
Cf with Gammasphere by using the triple coincidence technique. We were able to identify uniquely several correlated pairs equal to binary and ternary fission. We also present the comparison of the experimental results with the theoretical calculations for the cold (neutronless) binary and ternary fission.
2. – Experiment and results
In order to study the fission of252
Cf, a 25Ci 252
Cf source was sandwiched between two Ni foils of thickness 11.3 mg/cm2
and then sandwiched between 13.7 mg/cm2 thick Al foils and was placed at the center of Gammasphere with 72 Compton suppressed Ge detectors at Lawrence Berkeley National Laboratory. A total of9:810
9
triple or higher fold coincidence events were recorded. A - - cube was built using the RADWARE soft-ware [6]. The complex -ray spectra obtained in SF were analyzed by the triple gamma coincidence method. In this method a double gate can be set on two -rays in any partic-ular isotope or in two different isotopes which are partners of each other.
For a given fission fragment there can be several partner isotopes because 0 to10 neutrons can be emitted from its primary partners following scission. The -rays emitted by the partners during deexcitation will be in coincidence with each other. If the tran-sitions in one of the partner nuclei are known, one can identify the -rays belonging to other partner nuclei uniquely because one can compare the fission yields with calculated ones from Wahl’s tables [7]. In the fission of252
Cf, about 100 different final fragments are produced. During the fission process two primary fragments along with several neutrons or light clusters are emitted. The primary fragments may also emit several neutrons until the deexcitation energy is below the neutron binding energy of6 MeV. Then the secondary fragments decay to their ground states by the emission of -rays. Also cold (neutronless) fragmentations are possible. The excited fragments produced in SF are too neutron-rich to emit charged particles such as protons or alpha particles which would take them farther away from stability.
The data to obtain yields per 100 fission events were analyzed by two different methods.
Method 1: This method is described in our earlier paper [5]. In this method the cold (neutronless) binary fragmentations and 1–6 neutron channels were studied by setting double gates on2 + !0 + and4 + !2 +
transitions in an even-even fragment (e.g. heavy or light) and then measuring the intensities of the2
+ !0
+
Fig. 1. – Coincidence spectrum obtained by double gating on the 180.9 keV and 332.0 keV transitions in146Ba.
fragments(e.g. light or heavy). The relative counts for all the pairs of correlated nuclei is determined. Correcting these relative counts for the detector efficiency of the -ray and for internal conversion, we obtain the relative yields for a set of particular pairs. Normalizing the total yields to the values in Wahl’s table[19] or to the experimental total yields, if known, one obtains the yields per 100 fission events.
Method 2: In this method one can determine the yields by setting the first gate on the light or heavy fragment and the second gate on its correlated partner. Determining the intensities of transitions in both fragments and knowing the branching ratios between different transitions we can determine again the relative binary and ternary yields [4, 5] which are normalized to Wahl’s table [7].
In the case of cold (neutronless) alpha ternary fission of252
Cf, we look at the correla-tion between two even-Z fragments with the sum of chargesZ =96and sum of masses A = 248. For example,
146 56
Ba and102 40
Zr are the partners when a ternary -particle is involved. The Mo nuclei are partners of Ba when binary fragmentations with and with-out neutron emission are involved. The -ray spectrum observed in coincidence with the 180.9(2 + ! 0 + ) and 332.0 (4 + ! 2 + ) keV transitions in146
Ba is shown in fig. 1. The transitions belonging to various Mo partners are shown along with the appropriate num-bers of neutrons emitted during the fission. In this spectrum, one can see the (2
+ !0
+ ) transition of energy 171.6 keV of106
Mo, which corresponds to the zero neutron channel. One also sees the 151.8 keV (2
+ !0
+
) transition in102
Zr, which corresponds to cold -ternary fission. Even though this peak is not as clear as some of the other peaks because of background fluctuations, one can see this peak very clearly in fig. 2, where one gate is set on the4
+ !2
+
transition in102
Zr and the other gate on the2 +
!0 +
transition in 146
Ba. In this spectrum one observes also -rays corresponding to Mo isotopes because the gate 326 keV also contains the7
, !5
,
transition of energy 324.2 keV of146 Ba. We also see the yrast band and octupole band in146
Ba because102
Zr is the partner in the -ternary fission. These and similar spectra, which are not shown here, clearly prove that we are observing a new phenomenon, cold -ternary fission with no neutron emis-sion. The fraction of248
Cm in our sample was experimentally determined and found to be too small to give the observed yields. In the following paper of these proceedings by
Fig. 2. – The triple -coincidence spectrum obtained with a double gate set on the2+ ! 0+
transition at 180.9 keV in146Ba and on the4+ ! 2+ transition at 326.2 keV in102Zr. Note the
clear peak at 151.8 MeV which corresponds to another transition in102Zr. The peaks at 203.2, 307.6, 332.0, 428.2, 444.5, 510.8, 514.7, 524.1 and 583.8 keV correspond to different transitions in146Ba.
Sandulescu et al., we present the results of our analysis and comparison with theory. In table I, the relative binary isotopic yields obtained by using methods 1 and 2 for Mo-Ba pairs are shown. The yields in column 2 a,b were obtained by double gating on the 2 + !0 + and4 + !2 + transitions in146
Ba and analyzing the intensities of the transitions to the ground states in Mo isotopes, and normalizing the total Mo yield to the values in Wahl’s table[7]. The102
Zr is yield extracted from its2 +
!0 +
transition intensity relative to the Mo transition intensities and yields. The yields in column 3 a,b were obtained by double gating on the2
+ ! 0 + transition in146 Ba and2 + ! 0 + transitions in the Mo
TABLEI. – Binary(106Moand146Ba)and cold ternary(146Baand102Zr)fission yields of252Cf.
a. Binary (A
Mo and146Ba) yields normalized to Wahl’s table[19]
Nuclei 146Ba/146Ba 146Ba/Mo
106Mo(0n) 0.029 0.029 105Mo(1n) 0.10 0.11 104Mo(2n) 0.30 0.30 103Mo(3n) 0.35 0.31 102Mo(4n) 0.15 0.16 101Mo(5n) 0.079 0.088 100Mo(6n) 0.0049 0.016 Total(Wahl) 1.017 1.017
b. Cold ternary (146Ba and102Zr) fission yields of252Cf.
Ratio 1 =(cold -ternary)/binary-zero-neutron, and Ratio 2 = (cold -ternary)/Total-n-channel.
Nuclei 146Ba/146Ba 146Ba/102Zr
102Zr( ) 0.0054 0.0052
Ratio 1 18.4% 17.6%
TABLEII. – The cold alpha ternary isotopic yieldsYexpobtained per 100 fission events are given.
The predicted relative isotopic yieldsYtheorare compared to the renormalized experimental yields Yrenorm(whereYrenorm=85Yexp) which are also given. Note the discrepancy for the last
fragmen-tation which involves the double magic132Sn.
Yexp(%) Ytheor(%) Yrenorm 92 36Kr 15660 Nd 0.0020.001 0.081 0.17 96 38Sr 15258 Ce 0.0080.003 0.54 0.68 98 38Sr 15058 Ce 0.0140.006 1.24 1.19 99 38Sr 14958 Ce 0.0180.009 0.33 1.53 100 38 Sr 14858 Ce 0.0210.010 0.93 1.79 101 38 Sr 14758 Ce 0.0140.011 0.37 1.19 100 40 Zr 14856 Ba 0.0380.012 1.07 3.23 101 40 Zr 14756 Ba 0.0820.010 2.55 6.97 102 40 Zr 14656 Ba 0.0090.004 2.36 0.85 103 40 Zr 14556 Ba 0.0840.029 8.77 7.14 104 40 Zr 14456 Ba 0.0170.008 6.01 1.45 106 42 Mo 14254 Xe 0.0180.007 2.39 1.53 107 42 Mo 14154 Xe 0.0300.014 1.15 2.55 108 42 Mo 14054 Xe 0.0070.003 1.05 0.60 112 44 Ru 13652 Te 0.0110.006 0.57 0.94 116 46 Pd 13250 Sn 0.0060.003 2.92 0.51 isotopes and102
Zr and using the4 +
!2 +
transition in146
Ba as a measure of the relative yields. In table II, we present the neutronless alpha ternary fission yields observed in the spontaneous fission of252
Cf. The highest experimental yields are found for the Zr + Ba isotopes. Significant yields also are observed for the Mo + Xe and Sr + Ce ternary fragmentations, too. One can observe that their corresponding experimental yields are comparable or even higher than the yields for the even-even neighbours. Further we also observed ternary fragmentations where the ternary particle is 10
Be. We mention here that we found enhanced experimental yields for the cold -ternary fission in which heavier partners are Ba isotopes with masses ranging from 144 to 147. Their higher yields could be attributed to the static octupole deformation observed in this region [8], since octupole shapes at the scission configuration could significantly lower the Coulomb barrier and increase the penetrability between the final fragments. Similar enhanced yields have already been detected in the cold binary fission of252
Cf [5].
We should stress here that the experimentally determined isotopic yields are inte-grated yields. In the spontaneous fission experiment of252
Cf the majority of the binary and ternary splittings lead to highly excited final nuclei which after neutron evaporation are decaying to the lowest states by gamma cascades. Less frequently, there are also cold fragmentations which leave the final nuclei in their ground or first excited states. We define these cold fission experimental yields as integrated yields since they collect the con-tributions of all (neutronless) transitions over a whole range of TXE’s from zero up to at least the neutron binding energy, from which the evaporation of a first neutron becomes possible. These are the first identification and determination of yields of the particular correlated pairs associated with cold ternary fission. The theoretical model used for cal-culating the relative yields is discussed in detail in ref. [5] and the paper by Sandulescu et
retical calculations. We would like to stress that the penetration factors are very sensitive to deformation and an accurate knowledge of them is very important. In conclusion, we have observed very definitely the binary and -ternary fragmentation in
252 Cf.
The works at Vanderbilt University, Mississipi state University, were supported in part by the U.S. Department of Energy under grant No. 88ER40407, DE-FG05-95ER40939 and the works at Idaho National Engineering Laboratory, Lawrence Berke-ley National Laboratory and Lawrence Livermore National Laboratory were supported by contract Nos. DE-AC07-761DO1570, DE-AC03-76SF00098 and W-7405-ENG48, re-spectively. The Joint Institute for Heavy Ion Research has member institutions the Uni-versity of Tennessee, Vanderbilt UniUni-versity and Oak Ridge National Laboratory. It is sup-ported by its members and by the U.S. Department of Energy through contract No. DE-FG05-87ER40361 with the University of Tennessee. Work at Tsinghua University was partially supported by the National Natural Science Foundation of China. A.Sandulescu, A.Florescu, W. Greiner, G.M. Ter-Akopian and A.V. Daniel would like to acknowledge the hospitality of Vanderbilt University and the Joint Institute for Heavy Ion Research dur-ing the completion of this work. We would like to acknowledge the grant received under Twinning Program from National Research Council.
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