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The one-step decay ($A$ → $a_1$ + $A_1$) of the ground state of the resting parent nucleus A with the formation of real states of nucleus $A_1$ and particle $a_1$ is impossible, if the heat $Q_1$ of this decay, defined as $Q_1$ = $E_A$ − $E_{A_1}$ −$E_{a_1}$, where $E_A$, $E_{A_1}$ and $E_{a_1}$ are the internal energies of nuclei $A$, $A_1$ and particle $a_1$ , and connected at the implementation of the energy conservation law with the kinetic energies $T_{A_1}$ and $T_{a_1}$ of nucleus $A_1$ and particle $a_1$ as $Q_1$ = $T_{A_1}$ + $T_{a_1}$ has a negative value. At the same time, the two-step decay of the same nucleus ($A$ → $a_1$ + $A_1$ → $a_1$ + $a_2$ + $A_2$ ) described by the Feynman diagram, including at the first step the flight of the real particle $A_1$ from the nucleus $A$ with the formation of the virtual state of the intermediate nucleus $A_1$, described by the Green function $G_{A_1}$ of this nucleus whose pole lies in the region of negative values of the kinetic energy $T_{A_1}$ of the nucleus $A_1$, and at the second step the decay of the nucleus $A_1$ with the formation of the real particle $a_2$ and the real daughter nucleus $A_2$, is possible if the heat $Q_2$ of the decay of the nucleus $A_1$ satisfies the condition $Q_2$ >│$Q_1$│, when the total heat $Q$ of the analyzed decay, defined as $Q$ = $Q_1$ + $Q_2$, is positive.
For the first time, the named above virtual mechanism of the two-step decay was successfully used [1] to describe the true double β-decay of an even-even parent nucleus ($A$, $Z$) with the formation of a daughter nucleus ($A$, $Z ± 2$) and flight of two electrons and two electron antineutrinos or of two positrons and two electron neutrinos, when one-step β-decay of nucleus ($A$, $Z$) with the formation of an intermediate nucleus ($A$, $Z ± 1$) is forbidden. This situation was realized because of the influence of Cooper pairing of two protons and two neutrons in the parent nucleus ($A$, $Z$), when the heats $Q_1$ and $Q_2$ of the single β-decays of the nucleus ($A$, $Z$) and the virtual state of the intermediate nucleus-isobar ($A$, $Z ± 1$) satisfy the conditions $Q_1$ < 0, $Q_2$ >│$Q_1$│and $Q$ = $Q_1$ + $Q_2$ > 0.
In [2], the phenomenon of two-proton radioactivity also caused by the proton pairing effects was predicted for series of neutron-deficient nuclei ($A$, $Z$) even in $Z$. In [3], the presentation about virtual two-step mechanism for the two-proton decay of nuclei was first used unlike to the earlier proposed concept [4] of two-proton decay of the nucleus ($A$, $Z$) with the simultaneous flight of the daughter nucleus ($A-2$ , $Z-2$) and two protons. This allowed to describe the two-proton widths and the energy and angular distributions of two emitted protons in the superfluid model of atomic nucleus in a consistent manner.
Finally, it can be shown that the appearance of long-range $α$-particles emitted as third (fourth) particles in the true ternary (quaternary) fission of compound nuclei ($A$ , $Z$) produced in nuclear reactions with slow neutrons can be described successively with usage the stationary mechanism of named above reactions associated with the appearance of virtual states of intermediate nuclei formed in the two-step (three-step) nuclear decay of compound nuclei ($A$ , $Z$) in contrast to the nonstationary nonadiabatic mechanism of [5].
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