On the alpha- and cluster-decays of the double giant
dipole resonance of fissioning nuclei (*)
G. MOUZE(1) and R. A. RICCI(2)
(1) Faculté des Sciences - 06108 Nice Cedex 2, France
(2) INFN, Laboratori Nazionali di Legnaro - Legnaro, Padova, Italy
(ricevuto il 18 Luglio 1997; approvato il 15 Ottobre 1997)
Summary. — Alpha- and light charged particle-accompanied fissions can be
understood as decay modes of a double giant dipole resonance (DGDR). We show that the extra energy needed by a fissioning nucleus for emitting an a-particle is equal to the energy of the DGDR. An attempt is made to interpret observations of Hongyin et al. on the prompt neutron emission in ternary fission as showing the existence of a coincidence between the DGDRs of complementary fragments and the role of a DGDR in the emission of prompt neutrons.
PACS 25.85 – Fission reactions.
PACS 01.30.Cc – Conference proceedings.
1. – Introduction
The phenomenon of a-particle accompanied fission, discovered in 1946, is still unexplained. Still unexplained too is the phenomenon of cluster-accompanied fission, also called “light charged particle (LCP)-accompanied fission”.
The aim of this paper is to show how it is possible to evaluate the energy that has to be found by a fissioning nucleus for emitting a ternary particle, in particular an
a-particle, or a heavier cluster, and further, to show that this energy is about the same
as the energy of the double giant dipole resonance (DGDR) of the fissioning nucleus (sect. 2). This stimulated emission of an a-particle or of a heavier cluster, has to be considered as a new decay mode of the DGDR, which can be compared with the recently discovered 3n-decay of the DGDR. Thus, each fissioning nucleus has a double giant dipole resonance, and the origin of this DGDR has to be found in energy-rich
processes, such as “ionization” processes or rearrangement reactions (sect. 3). It can be
shown that the observed kinetic-energy distribution of the a-accompanied fission of
(*) Paper presented at the 174. WE-Heraeus-Seminar “New Ideas on Clustering in Nuclear and Atomic Physics”, Rauischholzhausen (Germany), 9-13 June 1997.
252Cf is in fact the superposition of a great number of single distributions, each of them corresponding to the stimulation of the a-decay of a given fragment by the DGDR of the fissioning nucleus. Some of these processes can lead to the emission of a-particles with particularly large kinetic energy. An hypothesis concerning an eventual
coincidence of the DGDRs of complementary fragments is presented. It could explain
observations made by Hongyin et al. [1] on the decrease of the prompt neutron multiplicity of 252Cf as a function of the kinetic energy of the a-particles emitted in coincidence (sect. 4).
2. – The evaluation of the missing energy of ternary processes
One of the most striking features of a-accompanied fission is that the kinetic energy distribution of the equatorial a-particles is almost the same for all the fissioning actinide nuclei. First, the distributions peak at the same energy, the mean value of Ea
being always 15.9 MeV for all systems, from (231
Pa 1nth) to 257Fm (s.f.) [2]. Secondly, the width of the kinetic energy distributions is almost the same, its value remaining comprised between 9.6 MeV for (235
U1n), and 10.9 MeV for 252Cf (s.f.) [2]. Figure 1 shows the result of a measurement of this distribution made by Heeg [3] for252Cf.
Fig. 1. – Kinetic energy distribution of the equatorial a-particles emitted by252Cf, after Heeg [3].
This distribution could result from the superposition of single contributions of DGDR-stimulated a-decays of fission fragments as, for example, those of150Ce* (23.38 MeV),106Mo* (18.72 MeV) or 132Sn* (14.20 MeV) (see text).
This almost universal distribution law could have long been recognized as the signature of the role of one and the same mechanism for the emission of a-particles in the ternary fission of the actinides.
In order to demonstrate that this mechanism is the stimulation of the a-emission by the double giant dipole resonance (DGDR) of the fissioning nucleus, let us consider another striking feature of this a-accompanied fission.
This striking feature is that never the Qavalues observed in the natural a-decay of
the nuclei are formed as fragment are compatible with the Qa corresponding to the
mean Ea value of the observed kinetic energy distribution of 15.9 MeV [2]. One can
speak of missing energy: where is the extra energy coming from?
A new indication of the existence of such a unique mechanism has been observed by Mouze and analysed in ref. [4]. At the time of this work, the mass distributions of the binary and ternary fission of 252Cf were known essentially from the data reported by Vandenbosch and Hulzenga [5] and from the work of Theobald et al. [6]. These data showed that binary and ternary distributions almost coincide for AHA 132 , and coincide only for this value. This situation offered an opportunity for evaluating the energy furnished to a fissioning nucleus by such a universal mechanism. Indeed,
AH( ternary ) 4AH( binary ) 4132 means that the correlated AL(ternary) is equal to AL (binary) minus 4 a.m.u., i.e. to 120 24 4116. In other words, the situation could correspond to the following binary (B) and ternary (T) splits:
B ) 252Cf K132Sn 1120Cd , T) 252Cf K132Sn 1116Pd 14He
and the a-particle could be considered as formed from the matter of the light 120Cd fragment. Interestingly, the Qa value of 120Cd is negative and equal to 26.45 MeV,
whereas the “effective” Qavalue of the a-emitter of the ternary process, if its Eamay be
taken equal to Ea4 15 .9 MeV [2], is equal to 15 .9 ( 1 1 4 O116 ) 4 16 .45 MeV . These
considerations showed that the 120Cd fragment has benefited by an energy Qa eff2
Qa theor4 16 .45 2 (2 6 .45 ) 4 22 .90 MeV .
This value is not very far from the energy expected for the double giant dipole resonance [7] of the fissioning252Cf. Indeed, the energy of the giant dipole resonance is given by EDGDR(252Cf ) 4 (31.2A21 O31 20 .6 A21 O6) 413.2 MeV, according to Bermann and Fultz [8], whereas the energy of the DGDR is given by EDGDR4 2 3 EGDR4 26 .4 MeV , according to Emling [9].
An even better agreement between the missing energy and the energy of the DGDR has recently been obtained [10], using very new data on the mass distributions of binary and ternary fission. According to Mutterer et al. [11], these distributions coincide for A 4126–128 rather than for A4132. Furthermore, a more precise value of
Ea of 252Cf has been obtained: Ea4 15 .7 MeV . If the binary process is written as
252
Cf K126Sn 1126Cd, the ternary process corresponding to AH ( ternary )4 AH ( binary )4 126 is 252
Cf K126 Sn 1122
Pd 14He; with Q
a(126Cd ) 429.36 MeV [12] and Qa eff4 15 .7( 1 1
4 O122) 416.21 MeV the missing energy becomes 16.212 (29.36) 425.57 MeV, differing from EDGDRby only 0.83 MeV.
Similar arguments could have been obtained from the consideration of the 3H accompanied fission of 252Cf. According to [11], the binary and ternary mass distribu-tion coincide for A A126–128, and E3 HA 8 .2 MeV . With Q3 H4 2 17 .32 MeV [12], one finds Qeff(3H ) 2Qtheor(3H ) 425.72 MeV, a value which differs only by 0.68 MeV from the DGDR energy.
3. – The a-decay of the DGDR of a fissioning nucleus
The considerations of sect. 2 confirm that the a-accompanied fission is nothing else than the stimulation of the a-decay of fission fragments by the double giant dipole resonance of the fissioning nucleus. Analogously, the triton-accompanied fission can be considered as the stimulation of the triton-decay of fission fragments by the DGDR.
The things can be seen from another point of view. Giant dipole resonances and double giant dipole resonances are very probably excited in all fissioning nuclei, and even in spontaneously fissioning nuclei, as will be discussed below. In such a situation, it seems preferable to say that ternary fission is nothing else than one of the decay modes of the DGDR of a fissioning nucleus, and in particular that a-accompanied
fission is nothing else than the a-decay of the DGDR.
As argument for this new point of view, let us recall that another decay mode of the DGDR has already been discovered; it is the 3n-decay of the DGDR, recently observed in hevy-ion reactions at relativistic energies [13].
However, the appearance, even in a spontaneously fissioning nucleus, of giant dipole resonances, and even of double giant dipole resonances, needs an explanation. How can, in such a nucleus, a great number of protons suddenly vibrate, out of phase, against a great number of neutrons? How is it possible to excite coherently the motion of these protons? New ideas on the dynamics of the fission process are absolutely necessary.
Such new hypotheses have been recently proposed by one of us: first, asymmetric fission can be explained only as the result of the formation of a light cluster from the valence nucleons of its208Pb core in each fissile nucleus [14]. This formation can release an enormous energy, which can be stored as vibration—or (vibration 1 rotation )—energy in the “dicluster molecule”. It can happen that this energy is so great, that it is even greater than the rest energy of a pion [14], and, in this case, a symmetric “ideal” fission can occur. This dicluster molecule, formed in such a first step of the fission process, can be the seat of a kind of collision between its constituents, a core and a light cluster, at high vibration quantum numbers n , and this collision can be viewed as an ionization of the core, because the core can release a very large number of nucleons (76, or less), which are picked up by the light cluster in a kind of rearrangement reaction of the dicluster primordial molecule.
Certainly, such energy-rich processes can be the origin of the coherent motion of a large number of protons and neutrons at least of those which form the most external
shells of the core and of the light cluster.
From this point of view, giant dipole resonances and double giant dipole resonances appear as a very probable excitation form of a fissioning nucleus and of fission
fragments. This raises the question of a possible difference of the DGDRs of light and
heavy fragments, and of effects of coincidences between these resonances (see sect. 4). Let us consider the kinetic energy distribution of the equatorial a-particles emitted by 252Cf measured by Heeg [3] (fig. 1). According to the present model of ternary fission, this distribution results from the superposition of the kinetic energy distributions given by each elementary a-decay stimulated by the DGDR either in the light or in the heavy fragments.
Among these single contributions to the global distribution of fig. 1, let us consider three particular elementary a-decay processes:
1) 150Ce, from the mass split
gives146Ba. This correspond to the ternary split:
a8) 148Ba-4He-120Zr.
2) 106Mo, from the mass split
b) 146Ba-106Mo,
gives102Zr. This correspond to the ternary split:
b8) 146Ba-102Zr-4He. 3) 132Sn, from the mass split
c) 132Sn-120Cd,
gives128Cd. This correspond to the ternary split:
c8) 128Cd-4He-120Cd.
The ternary splits a8) and b8) correspond to the same mass split,146Ba-102Zr, but we will show that the expected a-particle energies are very different. It is noteworthy that both processes 1) and 2) have been observed [15]: a-particles can be formed either from
a light fragment or from a heavy fragment. This observation of [15] thus confirms the
recent work of Mutterer et al. [11].
Let us now evaluate the most probable kinetic energy of the ternary a-particles of the processes 1)-3), according to our hypothesis of a stimulation of the emission by a DGDR of 26.4 MeV mean energy.
The normal Qa-values of 150Ce, 106Mo and 132Sn are negative [12] and respectively
equal to: 22380 (140) keV, 26940 (60) keV and 211750 (200) keV. However, thanks to the stimulation by the DGDR, the effective Qa’s become, respectively: 26 .40 22.38 4
24 .02 MeV , 26 .40 26.94 419.46 MeV and 26.40211.75 414.65 MeV. Thus, the expected kinetic energies are: 24 .02 3146O150 423.38 MeV, 19.463102O106 4 18 .72 MeV and 14 .65 3128O132 414.20 MeV. Figure 1 shows the three Ea values in
the global spectrum.
4. – On the competition between prompt neutron emission and a-particle emission in ternary fission
Hongyin et al. [1] have measured the mean value of the prompt neutron multiplicity of the a-accompanied fission of252Cf. They found: n 43.1360.02, a value which differs from the value for the binary fission, n 43.7676 (0.0047).
It is noteworthy that the n values of the binary and ternary processes would be compatible with the hypothesis of a prompt neutron decay of the DGDR of 262Cf, because the separation energy of 3.13 or 3.76 neutrons from the (light or heavy) fission fragments could reasonably be compatible with the energy of the DGDR.
But Hongyin et al. have also measured the multiplicity n of the ternary process of 252Cf as a function of the kinetic energy of the coincident a-particle. They found that the neutron multiplicity n varies linearly as a function of the kinetic energy Ea, and
that the slope of this line is given by
dnOdEa4 2 0 .037( 0 .003 ) MeV21[ 1 ] .
This observation of Hongyin et al. proves the existence of a correlation between
prompt neutron emission and a-particle emission in the ternary process.
If the emission of a-particles in fission results from the a-decay of a DGDR, the prompt neutron emission in ternary fission could have been stimulated too by a DGDR.
Thus the correlation found by Hongyin et al. raises the question: can prompt neutron emission and a-particle emission supervene in one and the same, light or heavy, fission fragment? But the a-particle emission requires a great energy expense, which has to be furnished by the DGDR; an extra expense of DGDR energy of the same fragment seems rather improbable, even if only one neutron, instead of 3.13 neutrons, or more, were emitted. Furthermore, it can be demonstrated [ 15 , 11 ] that a-particles can be emitted as well by heavy fragments as by light fragments: this means that both kinds of fragments are the seat of DGDRs. Thus, prompt neutrons may be emitted by a fragment, complementary of that emitting the a-particles and in coincidence with it, as observed by Hongyin et al.
The existence of DGDRs in complementary fission fragments constitutes a
supplementary argument in favour of the existence of rearrangement reactions during the fission process, since these reactions are necessary for inducing a coherent motion of valence protons against valence neutrons.
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