Multi-nucleon transfer reactions for production and study of heavy neutron rich nuclei

The present limits of the upper part of the nuclear map are very close to stability while the unexplored area of heavy neutron-rich nuclides along the neutron closed shell N = 126 is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleosynthesis. A new way was recently proposed for the production of these nuclei via low-energy multi-nucleon transfer reactions. The estimated yields of neutron-rich nuclei are found to be rather high in such reactions and several tens of new nuclides can be produced, for example, in the near-barrier collision of 136Xe with 208Pb. A new setup is proposed to produce and investigate heavy neutron-rich nuclei located along the neutron closed shell N=126.


Introduction
Properties of light and medium mass nuclei located far from the stability line have been studied already for many years. As a rule, the light exotic nuclei are produced in fragmentation processes and the medium mass neutron rich nuclei with А~100 -in fision reactions. Production and study of heavier neutron rich nuclei with A>160 encounters both physical and technical problems. Due to the "curvature" of the stability line (its increasing bending to the neutron axis), fusion reactions of stable nuclei produce only proton rich isotopes of heavy elements. For example, in the fusion of rather neutron rich 18 O and 186 W isotopes one may get only the neutron deficient 204 Pb excited compound nucleus, which after evaporation of several neutrons shifts even more to the proton rich side. As a result, on the nuclear map there are, for example, 19 known neutron rich isotopes of cesium (Z = 55) and only 4 of platinum (Z = 78). For elements with Z > 100 only neutron deficient isotopes (located to the left of the stability line) have been synthesized so far. That is the main reason also for the impossibility to reach the center of the "island of stability" (Z~114 and N~184) in the superheavy mass region, and neutron deficient isotopes of elements with Z>120 (being synthesized in fusion reactions) should have very short half-lives (less than one microsecond), insufficient for their separation and identification. At the same time, the unexplored area of heavy neutron rich nuclei is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleogenesis (a sequence of neutron-capture and β --decay processes). Just in this region the closed neutron shell N=126 is located which is the last "waiting point" in the r-process (see Fig. 1). The halflives and other characteristics of these nuclei feed directly into the r-process scenario of supernovae explosions. Study of the structural properties of nuclei along the neutron shell N = 126 could also contribute to the present discussion of the quenching of shell gaps in nuclei with large neutron excess.

Physics objectives
Three possibilities for the production of heavy neutron rich nuclei are known. These are the multinucleon transfer reactions [1], fusion reactions with extremely neutron rich radioactive nuclei and rapid neutron capture processes. Today the two last methods look unrealizable because of low intensity of radioactive beams and low neutron fluxes in existing nuclear reactors. On the contrary, the low-energy multi-nucleon transfer reactions, as well as the quasi-fission processes [2] (which are similar) are quite practical. They can be used for the production of new neutron rich isotopes not only in the region of Z~80 but also in the superheavy mass area [3]. Theoretical estimations show that several tens of new nuclides in the region of N=126 and Z~75 can be produced, for example, in the near-barrier collision of 136 Xe with 208 Pb (see Fig. 2). fermium isotopes (A > 260) are extremely interesting for several reasons. Firstly, as mentioned above, all known isotopes of fermium (and of more heavy elements) are located to the left side of the betastability line (see Fig. 1). Secondly, the well known "fermium gap" (isotopes 258−260 Fm with very short half-lives for spontaneous fission) impedes formation of nuclei with Z>100 by the weak neutron fluxes realized in existing nuclear reactors. In this connection, it is extremely interesting to know what is the first β --decaying fermium isotope and how long is its half-life. This is important not only for reactorbased physics but also for explosive nucleosynthesis in which this fermium gap might be bypassed. Unfortunately, the neutron rich heavy nuclei with Z>70 formed in the multi-nucleon transfer reactions cannot be separated and studied at available setups created quite recently just for studying the products of deep inelastic scattering (such as VAMOS, PRISMA and others). These fragment separators (as well as other setups) cannot separate heavy nuclei with Z>70 by their atomic number.

Scheme of setup
During the last several years a combined method of Z and A separation has been intensively studied and developed based on stopping nuclei in gas and subsequent resonance laser ionization [5][6]. This method was used up to now for separation and study of light exotic nuclei and fission fragments. Such techniques allows one to extract nuclei with a given atomic number, while a separation of the singleionized isotopes by their mass number can be done rather easily by a magnetic field. Half-lives of heavy neutron rich nuclei, which we are interested in (as a rule, β --decaying), are much longer than the extraction time of ions at such a setup. The scheme of this setup is shown in Fig. 3. Neutron rich isotopes of heavy elements are produced in multi-nucleon transfer reactions with heavy ions accelerated up to 5-10 MeV/nucleon (depending on projectile-target combination). The target, a foil of about 300 µg/cm 2 thickness (or larger), is placed at the window of gas cell (or inside it). Nuclear reaction products recoiling from the target as multicharged ions, are decelerated and neutralized due to collisions with the electrons and atoms of buffer gas in the gas cell filled with pure argon or helium. Then the desired atoms (with a given Z number) are ionized by means of two or three-step resonance laser irradiation and extracted through the exit hole or supersonic nozzle into the vacuum volume as positive charged ions (Q=+1) with slow energies of about 0.2 eV. In vacuum the ions are transported through the sextupole or quadrupole ion-guide system, which helps to pump away the residual buffer gas and keep the ions. Then the ions are accelerated up to 30-60 keV and separated by the magnet of the mass-separator. A detailed description of the technique of stopping and cooling the ion beams in buffer gas, laser ionization and separation can be found in [4][5][6][7][8][9][10][11][12][13][14]. In this way a low-energy beam of singly-ionized ions is produced with an extremely small emittance. Atomic nuclei of this beam have a definite value of charge number and a given (chosen) mass value, there are neither isobars nor isotopes. This allows one to perform subsequent high sensitive analysis of spectroscopic and decay properties of these nuclei, as well as a measurement of spins, magnetic dipole and electric quadrupole moments and charge radii of these nuclei by means of laser spectroscopy.

Conclusions
The use of the designed setup for separation and study of heavy neutron rich nuclei does not require any specific beams of accelerated ions. The ion beams available at FLNR fully satisfy our requirements which look as follows: the ions -from 16,18 О to 238 U (i.e., quite different depending on the problem to be solved), beam energies 4.5 -9 MeV/nucleon (slightly above the Coulomb barrier), beam intensity up to 10 13 pps with beam size at the target 3-10 mm and beam emittance 20π mm mrad. The different heavy targets, including actinides it is expected to use.
The proposed method allows one to extract not only heavy transfer reaction products but also any other nuclei with half-lives longer than a few tens of milliseconds including neutron rich fission fragments, fusion reaction products and light exotic nuclei. Such studies are already performed at the similar setups, for example, in Finland and Belgium, and at some different with a hot cavity facility in CERN. Estimated efficiency of such setups is about 10% and depends on the half-life or the extracted ion. This is quite sufficient for the study of nuclear reaction products formed with cross sections of about 1 microbarn at a beam intensity of 0.1 pµA. This method allows one to study the structure and decay properties of new exotic nuclei as well as spectroscopic properties of the corresponding atoms. At higher beam intensity this setup could be used also for separation and study of neutron rich long living superheavy nuclei produced in multi-nucleon transfer reactions with actinide targets (see above). This setup could definitely open a new opportunity in the studies at heavy-ion facilities and will have significant impact on future experiments.