NOTE BREVI
On some properties of the a-decay of the double giant
dipole resonance of fissioning nuclei
G. MOUZE(1) and R. A. RICCI(2)
(1) Faculté des Sciences - 6198 Nice Cedex 2, France
(2) INFN, Laboratori Nazionali di Legnaro - Padova, Italy
(ricevuto il 7 Agosto 1997; approvato il 21 Agosto 1997)
Summary. — New arguments are presented in favour of the interpretation of the a-accompanied fission as the a-decay of the double giant dipole resonance (DGDR) of the fissioning nucleus. The kinetic-energy distribution of the a-particles emitted by fissioning actinides results from the superposition of single contributions belonging to DGDR-stimulated a-decays of fission fragments. The correlation between prompt neutron multiplicity and kinetic energy of coincident a-particles, reported by Hongyin et al., is interpreted as suggesting the existence of a neutron decay of the DGDR and of a coincidence between the DGDR’s of light and heavy fragments. PACS 25.85 – Fission reactions.
1. – The hypothesis of an a-decay of the DGDR
In a recent paper [1], we have demonstrated that the missing energy of the ternary
a-particle emission, i.e. the energy that must have been furnished to a fissioning
actinide nucleus for the emission of an a-particle with Ea4 Ea4 15 .9 MeV [2], has
almost the same value as the energy of the double giant dipole resonance (DGDR) of such a fissioning nucleus.
For example, the giant dipole resonance of252
Cf has an energy of A 13.2 MeV [3], and the DGDR [4] has an energy of A 26.4 MeV. But, from the coincidence of the mass distributions of binary and ternary fission of 252Cf for A
H (binary) 4AH (ternary) 4
126 [5], it can be deduced that the correlated mass AL(ternary) is equal to AL(binary)
minus 4 a.m.u., i.e. to 126 24 4122. This situation could correspond to the following binary (B) and ternary (T) splits: (B) 252
Cf K126
Sn1126Cd, (T) 252
Cf K126
Sn1122
Pd1
4He, i.e. correspond in fact to the a-decay of 126Cd. But Q
a (126Cd) 429.36 MeV [6],
whereas the effective Qavalue of the a-emitter of the ternary process—if its Eamay be
taken equal to Ea4 15 .7 MeV , the most recent value of Ea[5]—is equal to 15.7 ( 1 1
4 O122) 416.21 MeV. The “missing energy”, 16.21 2 (29.36) 425.57 MeV, has been compensated by an energy-rich internal process. Where is this compensating energy coming from? We made the hypothesis that this energy is furnished by the DGDR. In other words, a-accompanied fission is nothing else than the DGDR-stimulated
a-emission of the fission fragments.
However, there exists another point of view. One could say that this ternary process is nothing else than the a-decay of the DGDR of the fissioning nucleus, i.e. one of the decay modes of the DGDR. In fact, a 3 n-decay of the DGDR has been already observed in heavy-ion reactions at relativistic energies [7].
The aim of the present paper is to show that further arguments can be found in favour of the existence of an a-decay of the DGDR in fissioning nuclei. We show, in sect. 2, that the kinetic-energy distribution of the a-particles emitted by 252Cf results from the superposition of single contributions belonging to DGDR-stimulated
a-decays of fission fragments. The intriguing correlation between prompt neutron
multiplicity and kinetic energy of coincident a-particles, reported by Hongyin et al. [8], is interpreted in sect. 3 as suggesting the existence of a neutron decay of the DGDR in ternary fission.
2. – The kinetic-energy distribution of equatorial a-particles
In sect. 1, we have shown that the missing energy of ternary processes does not differ very much from the mean energy of the DGDR. Let us now go the reverse way and show that it is possible to justify the observed kinetic energy distribution of the ternary a-particles as resulting from the superposition of single a-energy
Fig. 1. – Kinetic-energy distribution of the equatorial a-particles emitted by252Cf, after Heeg [9].
The calculated energies of the a-particles emitted by150Ce* (23.38 MeV), and106Mo* (18.72 MeV)
are indicated in the figure, together with the calculated energy of the a-particles emitted by a
contributions belonging to the stimulated a-decays of fission fragments. Figure 1 reproduces the experimental energy distribution of the equatorial a-particles of 252Cf measured by Heeg [9]. For the evaluation of these single distributions, we will assume that the DGDR energy is constant and equal to 26.4 MeV.
For the demonstration given in sect. 1, it was important to be sure that the considered nucleus, e.g. 126Cd, could really be responsible for an a-emission. For the
intended demonstration, it is important to choose fission fragments which have been really observed in binary fission as responsible for a ternary process. It is the case for the following decays 1) and 2), recently studied by Ramayya et al. [10]; the decay 3) concerns the doubly magic nucleus 132Sn , known to be formed with high yield in
asymmetric fission [11]:
1) 150Ce, from the mass split a) 150Ce-102Zr
gives146Ba. This correspond to the ternary split: a8) 146Ba-4He-102Zr.
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 according to processes 1) and 2), a-particles can be formed either from a light fragment
or from a heavy fragment. This observation of [10] thus confirms the recent work of
Mutterer et al. [5].
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 [6] 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 position in the global Easpectrum of the three calculated single
Ea-values. It is noteworthy that they fall within the experimental distribution.
We make the assumption that other calculated values also correspond to the experimental spectrum, and that this spectrum can be considered as resulting from the
superposition of all the contributions corresponding to the DGDR-stimulated a-decays
3. – On the competition between prompt neutron emission and a-particle emission in ternary fission
Hongyin et al. [8] have measured the mean value of the prompt neutron multiplicity of the a-accompanied fission of252
Cf. They found: n 43.1360.02, a value which differs from the value for the binary fission, n 43.7676(0.0047) [14]. It is noteworthy that the n value of the binary process would be compatible with the hypothesis of a prompt
neutron decay of the DGDR of252Cf, because the separation energy of 3.7676 neutrons from the (light or heavy) fission fragments could reasonably be equal to about 26.4 MeV.
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 linear law is given by
dnOdEa4 2 0 .037( 0 .003 ) MeV21[ 8 , 12 ] .
( 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, and
suggests that the prompt neutron emission could have been stimulated too by a DGDR. Can prompt neutron emission and a-particle emission supervene in one and the same (light or heavy) fragments? This seems rather improbable, even if only one neutron, instead of 3.13 neutrons or more, were emitted, because a-particle emission alone requires a great expense of DGDR energy.
But if a-particle emission can occur as well in heavy fragments as in light fragments, as shown in sect. 2, both kinds of fragment can be the seat of a GDR, and even of a DGDR. Thus this correlation could result from the coincidence of DGDR’s occurring in two complementary fragments.
4. – Conclusion
The observations of sect. 1-3 confirm that a-accompanied fission can be considered as the a-decay of the DGDR of the fissioning nucleus, and that there exists a prompt neutron decay of the DGDR in ternary fission.
However, the appearance, even in a spontaneously fissioning nucleus, and probably in both fragments, 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 recently been proposed by one of us: first, asym-metric fission can be explained only as the result of the formation of a light cluster from the valence nucleons of its 208Pb core in each fissile nucleus [13]. This formation
can release an enormous amount of energy, which can be stored as vibration—or (vibration1rotation)—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 core-light cluster at high vibration quantum number v, and this collision can be viewed as an ionization of
the core, because the core can release a very great 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 great number of protons, at least of those which form the most external shells of the
core and of the light cluster.
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