Production of heavy and superheavy neutron-rich nuclei in neutron capture processes

The neutron capture process is considered as an alternative method for production of superheavy (SH) nuclei. Strong neutron fluxes might be provided by nuclear reactors and nuclear explosions in the laboratory frame and by supernova explosions in nature. All these cases are discussed in the paper. There are two gaps of short-lived nuclei (one is the well-known fermium gap and the other one is located in the region of $Z=106$–108 and $N\sim 170)$ which impede the formation of SH nuclei by rather weak neutron fluxes realized at available nuclear reactors. We find that in the course of multiple (rather “soft”) nuclear explosions these gaps may be easily bypassed, and thus, a measurable amount of the neutron-rich long-living SH nuclei located at the island of stability may be synthesized. Existing pulsed reactors do not allow one to bypass these gaps. We formulate requirements for the pulsed reactors of the next generation that could be used for production of long-living SH nuclei. Natural formation of SH nuclei (in supernova explosions) is also discussed. The yield of SH nuclei relative to lead is estimated to be about ${10}^{-12}$, which is not beyond the experimental sensitivity for a search of SH elements in cosmic rays.

Figure 1

Calculated preferable decay modes of nuclei in the upper part of the nuclear map [14]. Known nuclei are denoted by rectangles. The solid line shows the border of short-living nuclei, with half-lives of <1 $\mu$m. The two circles mark the locations of the isotopes that could be synthesized in fusion reactions of ${}^{50}\mathrm{Ti}+{}^{249}$Cf (3$n$ evaporation channel) and ${}^{54}\mathrm{Cr}+{}^{248}$Cm (4$n$ evaporation channel).

Figure 2

Half-lives of nuclei in the upper part of the nuclear map. Schematic views of slow (terminated at the short-living fission fermium isotopes) and fast neutron capture processes with subsequent ${\beta }^{-}$ decays are shown by arrows.

Figure 3

Experimental (open circles) and calculated relative yields of heavy nuclei in the test nuclear explosion “Mike”[34].

Figure 4

Schematic of multiple-neutron irradiation of initial ${}^{238}$U material (top) and probability of formation of heavy nuclei (bottom) in such processes (1, 3, and 10 subsequent explosions). The dotted line denotes the level of few atoms.

Figure 5

Relative yields of heavy elements in an ordinary pulsed reactor (${10}^8$ pulses of ${10}^{16}$ neutrons/cm${}^2$ neutron fluence each; dashed line) and in a high-intensity pulsed reactor (${10}^4$ pulses of ${10}^{20}$ neutrons/cm${}^2$ neutron fluence each; solid line). Total neutron fluence is ${10}^{24}$ neutrons/cm${}^2$ for both cases.

Figure 6

(a) Initial abundance relative to lead of nuclei [41]. The position of rather stable SH copernicium isotopes is indicated by the arrow. (b) Yields relative to lead of thorium (dashed curve) and uranium (solid curve) nuclei depending on the total neutron fluence in the astrophysical $r$ process. Horizontal bars show experimental values of thorium and uranium abundances relative to lead. (c) The same for the relative yield of long-living SH copernicium isotopes ${}^{291}$Cn and ${}^{293}$Cn ($Z=112$). Dashed curves in (b) and (c) show the yields of uranium and SH elements in the case of a zero initial abundance of thorium and uranium at the beginning of the $r$ process (see text).