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Cold binary and ternary fission

) Physikalisches Institut der Universit¨at T¨ubingen, T¨ubingen, Germany (

2

) Institut Laue-Langevin, Grenoble, France

(ricevuto il 29 Agosto 1997; approvato il 15 Ottobre 1997)

Summary. — Cold binary fission has been studied in the spontaneous decay of252 Cf (sf). Cold fission for the mass split 132/120 is especially interesting because here it is the doubly magic132

Sn which is dominating the yield for a range of total excitation energies 0<TXE<10 MeV. For the reaction

242

Am(n,f) induced by thermal neutrons the yields of the heaviest clusters being emitted as ternary particles have been investigated at the mass spectrometer Lohengrin of the Institut Laue-Langevin. The emission of the heaviest clusters (33;34;35

Si) is a cold process with all three reaction partners being born in or very close to their ground states.

PACS 25.85 – Fission reactions.

PACS 01.30.Cc – Conference proceedings.

1. – Introduction

In the general case of nuclear fission the energy liberated in the process is eventu-ally shared between the kinetic and the excitation energies of the fission fragments. The excitation energy shows up in the neutron and gamma emission from the fragments. A certain, albeit small fraction of all fission reactions rather surprisingly proceeds with no neutron evaporation at all. Among these events may be even cases where also gamma emission is virtually absent. The fragments emerge then from the fission process directly in their respective ground states. This limiting case of nuclear fission has been termed cold fission. It has, however, become customary to speak of cold fission in a slightly more loose sense, namely whenever the total excitation energy TXE being left to the fragments is small compared to the neutron binding energy, i.e. a couple of MeV. In an even more liberal approach cold fission may be identified with neutronless fission. On the other hand,

( 

)Paper presented at the 174. WE-Heraeus-Seminar “New Ideas on Clustering in Nuclear and Atomic Physics”, Rauischholzhausen (Germany), 9-13 June 1997.

1090 F. G¨ONNENWEIN A. M¨OLLER, M. CR ¨ONI, M. HESSE ETC.

the notion of cold fission lends itself to generalisation. Instead of requiring the total ex-citation energy to vanish for the fragments being infinitely apart, interesting situations may also occur when right at scission the nascent fragments carry no intrinsic excitation energy. In these cases the energy stored in deformation at scission may relax when the fragments separate and eventually neutrons will again be evaporated. However, it may also happen that the deformation energy is consumed by the emission of a third charged particle at scission and the question arises with respect to the existence of cold ternary fission.

Experimental studies of cold fission started in the early 80’s [1, 2]. The technique, still in use nowadays, is to search for those events with the highest total kinetic energies TKE and to check whether this energy is fully exhausting theQ-value of the reaction. Energy

conservation then tells that no energy is left to deform or excite the fragments. More re-cently considerable progress has been achieved in the study of neutronless spontaneous fission reactions by analysing in multiple coincidence the gammas from correlated frag-ment pairs with arrays of high resolution Ge-detectors [3].

Obviously, cold fission and cluster radioactivity have several features in common. This similarity has been stressed from a theoretical point of view in numerous papers surveyed in [4]. Also a close link between cold fission in the lighter actinides and bimodal fission [5] of some nuclei in the vicinity of the doubly magic264

Fm has been established.

In the present paper we report on a study of cold binary fission in the spontaneous decay of252

Cf and of cold ternary fission in the reaction242

Am(n,f) induced by thermal neutrons.

2. – Cold binary fission of252

Cf

Back-to-back ionization chambers sharing a common cathode are ideally adapted to the study of binary fission. They have in fact been in use since the early days of fission. Operated with ultrapure hydrocarbons as counting gas and equipped with ultrathin and homogeneous targets the energy resolution for fission fragments approaches 100 keV. For neutronless fission events this resolution allows to unambiguously identify the fragment masses from mass and momentum conservation. In addition, also the nuclear charges of ions entering the chamber can be found from a closer inspection of the Bragg curve.

Results for the reaction252

Cf(sf) are presented in fig. 1, a scatter plot of the total kinetic energy TKE vs. the mass of the heavy fragment. Also included in the figure are the maximumQ-values of the binary decay of

252

Cf for a given fragment mass chain (heavy dots). Neutronless fission is immediately recognised by the individual mass lines showing only up in case that momentum conservation is not spoiled by the evaporation of neutrons. It is evident from the figure that, for many masses, the high energy tail of TKE merges with the respectiveQ-value and, hence, cold fission is attained. However, it has to

be stressed that, even for the very large statistics of more than10 9

events accumulated in the present experiment, no cold fission is to be seen for symmetric mass splits nor for very asymmetric splits. This means that cold fission is not just a more or less trivial limiting case of nuclear fission with phase space being explored up to its frontiers everywhere. Models of cold fission have to reproduce this feature [4, 6]. It is further remarkable that, for the very asymmetric mass splits with heavy fragment masses beyond 158, mass lines are visible indicating neutronless fission, while the high TKE events stay away by several MeVs from theQ-values and, hence, there is no true cold fission.

Cold fission is observed in fig. 1 to be attained for two different mass regions. In a first region, the bulk of cold events encompass a range of masses with heavy/light mass

Fig. 1. – Scatter plot of total kinetic energy vs. fragment mass.

ratios of 138/114 up to 156/96. In this mass range the yields drop steeply upon reaching theQ-value. It is also interesting to note that neutronless events are to be spotted far

away from theQ-value, i.e. at rather large total excitation energies TXE of about 12 MeV.

This observation is readily understood from the fact that there are two fission fragments and that it takes at least some 6 MeV to evaporate a neutron per fragment. A second mass region with cold fission comprises only a narrow mass range around the mass split 132/120. Here the yields at the highest TKE gently fade away but, nevertheless, there are events within 1–2 MeV of theQ-value, especially for the even heavy masses 132, 134 and

136.

The high-energy part of the TKE distribution is shown in fig. 2 for a characteris-tic mass fragmentation from each of the two mass regions outlined in the foregoing. The points represent the experimental yields in an energy window with a width of 200 keV. The vertical bars indicate theQ-values for different splits of nuclear charges with the charge

of the heavy fragment being specified by the labels attached to the bars. As already dis-cussed in connection with fig. 1, the shape of the TKE distribution is quite different for the two cold fission regions. The more detailed display in fig. 2 reveals that for the mass ratio 143/109 the TKE yield decreases not only steeply, as already observed, but also very smoothly upon approaching theQ-value. By contrast, for the mass ratio 132/120 the

de-crease in yield is much gentler, but more surprisingly, a structure shows up. The structure may be described as a shoulder with even some fine structure being superimposed. This structure is only seen for three or four mass pairs around the fragmentation 132/120 on display in fig. 2.

1092 F. G¨ONNENWEIN A. M¨OLLER, M. CR ¨ONI, M. HESSE ETC.

Fig. 2. – High-energy tail of TKE for fixed mass ratio.

The cause for the puzzling appearance of a shoulder in fig. 2 becomes transparent when not only the masses but also the nuclear charges of the fission fragments are iden-tified. In fig. 2 the contributions of different charge splits to a given mass fragmentation are disentangled and the respective contributions are indicated by broken lines. The fig-ures attached to the lines correspond, similarly to those for theQ-values, to the charges

of the heavy partner of the charge pair. For the standard cold fission region with the mass fragmentation 143/109 as a typical representative, it is evident from fig. 2 that all charges contribute in fairly comparable amounts to the total yield, the main effect being the dis-placement of the individual yields by the shifts in theQ-values. By contrast, for the mass

pair 132/120, obviously only one single charge split 50/48, i.e. Sn/Cd, is responsible for the shoulder in the TKE distribution, the neighboring charge pair 51/47 contributing only a few percent of the total yield up to 10 MeV in excitation energy. Unfortunately, the data cannot be analysed further to larger excitation energies because neutron emission sets in and destroys the mass resolution of the present experimental approach. It appears that the yield for the charge pair 50/48 saturates and the yield of the pair 51/47 takes over for larger excitation energies.

The present study of the spontaneous fission of252

Cf thus provides direct evidence that for fragment mass pairs with the ratio 132/120 a very special situation occurs at the high-est total kinetic energies when the doubly magic132

Sn and the complementary120

Cd are formed. The strange but well pronounced structure of the TKE distribution being traced to the formation of the magic cluster132

Sn lends itself to the interpretation that a distinct cluster decay mode is present in252

Cf. This cluster mode may either be considered as a precursor to bimodal fission in the heavier actinides [3], or as cluster radioactivity with the doubly magic208

Pb of standard cluster radioactivity being replaced by the doubly magic

132

Sn. Of course, it should be tempting to correlate the fine structure in the shoulder of fig. 2 to specific patterns of the level structure in both,132

Sn and120

Fig. 3. – Yields of heavy ternary particles and model calculations.

3. – Cold ternary fission of243

Am*

Cold ternary fission has been investigated for the compound nucleus243

Am* at the Lohengrin spectrometer of the Institut Laue-Langevin. The high neutron flux at the target position of this instrument allows to breed242

Am from241

Am in situ and to study fission of243

Am* following thermal neutron capture in242

Am. The interest was focused on the measurement of the yields of ternary charged particles emitted in the course of fission. The hydrogen and helium isotopes carry 97% of the total yield, but it is the yield of the heaviest ternary particles which should be of special interest in the context of cold fission. This surmise is due to the observational fact that the total excitation energy left to the fissioning system is the smaller, the larger the mass and/or charge number of the ternary particle is [7].

The yields for the heaviest ternary particles are plotted in fig. 3. Triangles pointing upwards correspond to nuclides having actually been spotted, while triangles pointing downwards are the upper bounds of yields for nuclides having been searched for but not observed. All yields are normalized to the fictitious yield10

4

for4

He as the ternary par-ticle. It should be noted that all of the heavier ternary particles are neutronrich and unstable. The lines in fig. 3 result from a fit to the data in the framework of the Halpern model of ternary fission [8]. It is assumed in this model that ternary yields are correlated to the energy costs required to first create a ternary particle and then place it in between the two main fission fragments. As demonstrated in fig. 4, the correlation is well estab-lished. Based on this correlation the model would predict that still much heavier ternary particles than the observed Si-isotopes33

Si,34

Si and35

Si should come into view for the sensitivity achieved in the present experiment.

1094 F. G¨ONNENWEIN A. M¨OLLER, M. CR ¨ONI, M. HESSE ETC.

Fig. 4. – Correlations between yield and energy costs.

The failure to detect ternary particles beyond the Si-isotopes in the242

Am(n,f) reaction is intriguing. More insight can be gained by considering the energies involved. The en-ergy costs for creating the heaviest particles are read from fig. 4 to be quite large, though it has to be stressed that the energies are calculated from a model for the scission config-uration. Anyhow, when very heavy ternary particles are formed out of the neck nucleons between the two fission fragments, they will in any case require considerable space and, therefore, they can only be created from very stretched out scission configurations. The fission prone nucleus may then either undergo binary fission with the large deformation energy being converted into a large excitation energy, or ternary fission with the defor-mation being consumed for the creation of a ternary particle. In the latter case virtually no excitation energy should be left to the system, i.e. a cold ternary fission process is an-ticipated. Unfortunately,the uncertainties inherent to the model calculations do not allow to give reliable figures for the ternary excitation energies. It can be stated safely, how-ever, that the range of excitation energies known from binary fission is sufficiently large to cover the energy costs required for ternary fission though, except perhaps for a few MeV, all of the excitation energy is squeezed out. A more direct experimental proof that cold ternary fission is attained for the Si-isotopes observed has come from a recent study of ternary fission of252

Cf [9]. There it has been shown that the total excitation energy de-creases from 26 MeV down to 14 MeV when instead of4

He a ternary C-isotope is emitted. Extrapolating this trend to the heaviest ternary particles reported here, the remaining excitation energy appears to be a few MeV at best. The limit of detectability of heavy ternary particles, hence, coincides with the limit of phase space being reached.

4. – Conclusion

The cold decay mode of252

Cf where the doubly magic nucleus132

Sn plays such a dom-inant role is interesting because it may be viewed as being intermediate between cluster radioactivity and fission. Conceptually, in cluster radioactivity the clusters are thought to

be preformed well before decay, while in fission the fragments are formed in the course of the process, usually well after the fission barrier has been passed. A special situation is present for the132

Sn mode in252

Cf. The high kinetic energies involved correspond to very compact scission configurations with large Coulomb repulsion and, therefore, the

132

Sn cluster must already have taken shape close to the earliest distinct stage of fission,

i.e. close to the saddle point. The existence of this mode in252

Cf(sf) has been predicted by theory [10, 11] some years ago, but it is seen here in experiment for the first time. In the decay of heavier actinides this mode is at the origin of bimodal fission [5].

By contrast, cold ternary fission as explored here for the decay of243

Am* with heavy Si-isotopes as the ternary particles proceeds from very stretched scission configurations. The process may, hence, be considered as a variant of cold deformed fission having been already described before [12].

  

The present work has been supported by a grant from the BMBF, Bonn, under con-tract no. 06T¨u669.

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