Low energy radioactive ion beams in Dubna

Abstract

The Dubna radioactive beam factory (DRIBs) [1] will make use of two possibilities for producing secondary beams of radioactive nuclei. During the first phase of the project (Phase I) the possibility for obtaining radioactive nuclei in fragmentation reactions of stable nuclei, accelerated by the cyclotron U400M to intermediate energies (∼ 50 MeVA−1), will be realized. The fragmentation products, obtained in a special ion source, will be converted into single-charged ions, which after transportation to the second cyclotron (U400) will be accelerated up to energies of ∼ 20–25 MeVA−1. This method will allow obtaining mainly beams of light radioactive nuclei with Z 30 with intensities up to 108 pps (e.g., nuclei such as 6He). In the second phase of the project (Phase II) it is supposed to produce and accelerate radioactive neutron-rich nuclei in the mass region 80 A 140. This mass region corresponds to the fragments of low-energy fission of heavy nuclei. For this reason use will be made of the photofission of 238U. The γ -quanta will be produced by the electron accelerator (the microtron MT-25), where the electron beam with an intensity of about 20 μA has an energy of 25 MeV. With the help of a special converter the electron beam is transformed into a beam of γ -quanta with up to 25 MeV energy and a flux of 1014 s−1. This beam, focused into a narrow angle, will fall onto a 238U-target weighing 100 g. It is wellknown that the photofission cross section has a maximum corresponding to the giant dipole resonance at an energy of the γ -quanta equal to Eγ = 13.5–14 MeV [2]. This brings forth an increase of the photofission probability. The yield of the fission fragments will increase as a result of the interactions of the secondary neutrons (γ n and fission neutrons) with the U-target. When the mentioned above parameters of the beam and target are realized, one can get as much as 1011 fission fragments/second. Taking into account the widths of the mass and charge distributions of the fragments, the yields of definite fission fragments can be estimated. When the efficiency of the ion source and the transport system amounts to some 20–30% it is possible to obtain beams of fission fragments in the region of Kr and Xe with an intensity of up to 109 s−1. Simultaneously with the fragments situated close to the maxima of the mass distribution (A = 90 and 130), asymmetric fission fragments are formed with a rather high yield. For the isotopes situated at the “tails” of the fragment