Low energy radioactive ion beams in Dubna

Transcription

1 Nuclear Physics A 701 (2002) 104c 108c Low energy radioactive ion beams in Dubna Yu.Ts. Oganessian, Yu.E. Penionzhkevich Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, Russia 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 MeV A 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 MeV A 1. This method will allow obtaining mainly beams of light radioactive nuclei with Z 30 with intensities up to 10 8 pps (e.g., nuclei such as 6 He). 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 238 U. 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 s 1.This beam, focused into a narrow angle, will fall onto a 238 U-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 E γ = 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 (γnand fission neutrons) with the U-target. When the mentioned above parameters of the beam and target are realized, one can get as much as 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 10 9 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 * Corresponding author. address: oganessian@flnr.jinr.ru (Yu.Ts. Oganessian) /02/$ see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S (01)

2 Yu.Ts. Oganessian, Yu.E. Penionzhkevich / Nuclear Physics A 701 (2002) 104c 108c 105c mass distribution (for instance, for 77 Ni, 78 Ni) the expected yields are s 1.This allows producing them in sufficiently large amounts to permit precise measurement of the characteristics of their decay. The problem lies only in the possibility to realize a relatively fast ion source having a diffusion time for these elements not longer than s. The obtained ions, corresponding to the exotic nuclei with energies up to 10 kev, will be transported to a special experimental hall, where investigations will be carried with low energy radioactive nuclear beams. Low energy radioactive beams of light elements, obtained in fragmentation reactions at the U400M cyclotron, also will be transported to the same area. In this laboratory investigations of the properties of exotic nuclei will be performed in the following directions. 1. Nuclear structure The data on the structure of neutron-rich nuclei with 30 Z 60 is rather scarce in spite of the fact that many interesting features have been predicted for them new deformation regions, inversion of the energy levels, change of the sign of the deformation when going from neutron-deficient to neutron-rich nuclei [3], shape isomerism [4], etc. Fig. 1 presents the systematics of the low-energy levels of the Sn isotopes. A sharp change in the energy is observed in the transition to the neutron-rich isotopes. Thus for the 2 + states of the isotopes Sn this value amounts to 1.2 MeV, while for the isotope 132 Sn it is already about 4 MeV. This can be explained by different factors, including the deformation close to the closed shell N = 82. In the given project it is suggested to Fig. 1. The gamma-quanta energies (E) for different Sn-isotopes (A).

3 106c Yu.Ts. Oganessian, Yu.E. Penionzhkevich / Nuclear Physics A 701 (2002) 104c 108c study energy level schemes using 4π γ -spectroscopy (gamma-balls), the deformation and the root-mean-square radii using the methods of collinear laser spectroscopy [5]. From this point of view interesting are also nuclei with neutron magic numbers, e.g., N = 82, such as 131 In, 130 Cd and 129 Ag, which will also be formed with yields sufficient to allow investigating them with the above-mentioned methods. As it has been mentioned in some articles (e.g., Ref. [6]) because of the large deformation of the fission fragments, high spin isomers can be formed. It is interesting to study isomers such as 99m Nb (I = 5 + ), 125m Cd (I = 11/2 + ), 128m In (I = 8 ), 130m In (I = 10 ), 131m Sn (I = 11/2 ), 131m Sn (I = 19/2 ) and others. It is of additional interest within the DRIBs project to produce also isomeric beams. The perspective of studying oriented nuclei using the methods of low-temperature nuclear orientation [7] are also promising. These methods allow high precision determination of the quadrupole moments of nuclei, including those of isomers. For odd-neutron nuclei the LMR-method has proven to be highly efficient [8]. It should be also mentioned that the low-temperature nuclear orientation method could be used for the production of polarized beams of fission fragments. It is interesting to study also the neutron decay of fission fragments. Because of the relatively high β-decay energy of neutron-rich nuclei in this mass region, possible are β- delayed 2n- and3n-decay modes of nuclei such as 100,102 Rb, Cd, Sn etc. By investigating the correlation between the two neutrons emitted in the decay of such nuclei it is possible to look for possible di-neutron configurations in these nuclei. Estimation shows that for neutron-rich fission fragments exotic decay modes such as cluster decay are also energetically possible. Moreover, after β-decay (for these nuclei Q β 15 MeV) the excitation of giant resonances at energies E GR 15 MeV is also possible. Thus the beams of fission fragments open new perspectives in the investigation of neutron-rich nuclei with Z in the range 30 <Z<60, which are practically very weakly studied. Low energy beams of fission fragments are expected to be very useful for high precision measurements of nuclear masses. The exact determination of the mass excess of nuclei far from the line of β-stability is a very important issue of nuclear physics. On the basis of the mass values conclusions can be drawn about the stability of nuclei close to the nucleon driplines as well as about the location of the driplines themselves. The particles of low energy could be used in special high-frequency traps [8]. Also, the nuclei stored in such traps can be used for the determination of the charge radii and quadrupole moments using the methods of nuclear spectroscopy. Fig. 2 presents the quadrupole deformation of different Zr isotopes. The experimental points obtained by means of nuclear spectroscopic methods are for isotopes up to 101 Zr. It can be seen that for the two neutron-rich isotopes 100,101 Zr the value of the deformation rapidly increases up to β 0.4. The theoretical calculations within the shell model predict for the Zr isotopes with A>101 an abrupt change in the deformation and, moreover, a change in its sign. In order to reveal this interesting effect it is necessary to investigate all Zr isotopes including 106 Zr. All of them are produced with a rather high yield as fission fragments.

4 Yu.Ts. Oganessian, Yu.E. Penionzhkevich / Nuclear Physics A 701 (2002) 104c 108c 107c Fig. 2. The dependence of β 2 on the neutron number of Zr isotopes. 2. Applied research with low-energy fission-fragment beams When using low-energy radioactive beams for investigations in the field of condensed matter physics and biology, we should stress on one very important advantage, namely, that they can be implanted in the studied object without causing its radiation damage, as is usually the case in the interaction of high-energy beams, where a large part of the imported energy is deposited in the end of the Bragg curve. In the investigation of the structure of a crystal, the implanted radioactive nuclei emitting penetrating radiation (e.g., γ -radiation) can experience the blocking effect inside the crystal lattice [9]. This radiation can be measured with special position-sensitive detectors, which allows determining with high resolution the structure of the crystal and its changes as a result of the radiation effects. The use of low-energy radioactive beams opens new perspectives in radiobiological investigations. The usual methods of studying neurons and neuron meshes consist of exciting and measuring the signals using special microelements. In the case of implantation of a radioactive nucleus in the region of the studied neuron the latter is subjected to excitation by the radiation, e.g., by α-particles emitted by the implanted nucleus. Such a method allows excitation of a definite group of neurons without affecting or destroying the adjacent regions of biological tissues. A fundamental problem of biology is the investigation of chromosomal aberrations, which can bring forth different illnesses, including oncological ones. The chromosomes fission into two types and further they form new chromosomes. At this, stable and unstable aberrations can be realized. The unstable aberrations can lead to the creation of new degenerated cells, which are the cause of different illnesses at a genetic level. The modeling of such processes can be carried out efficiently by implanting radioactive nuclei in the region of the investigated cells [10].

5 108c Yu.Ts. Oganessian, Yu.E. Penionzhkevich / Nuclear Physics A 701 (2002) 104c 108c References [1] Yu.Ts. Oganessian, Brief Description of the Dubna Radioactive Ion Beams Project DRIBs, Dubna, [2] J.T. Caldwall et al., Phys. Rev. C 21 (1980) [3] J.L. Wood et al., Phys. Rep. 102 (3-4) (1992). [4] K. Neergard et al., Nucl. Phys. A 262 (1976) 61. [5] Yu.P. Gangrsky, Sov. J. Part. Nucl. 23 (1992) [6] H. Kudo et al., in: Yu.Ts. Oganessian, R. Kalpakchieva (Eds.), Proceedings of the VI International Schoolseminar on Heavy Ion Physics, Dubna 1997, World Scientific, 1998, p [7] M. Finger (Ed.), Proceedings of the International Workshop on Symmetry and Spin, Prague 1998, Czech J. Phys. 49 (1999). [8] N. Hermanspahn et al., Acta Phys. Pol. B 27 (1996) 357. [9] M. Morjean et al., in: Yu.Ts. Oganessian, R. Kalpakchieva (Eds.), Proceedings of the VI International School-seminar on Heavy Ion Physics, Dubna 1997, World Scientific, 1998, p [10] E.A. Krasavin, S. Kozulek, Mutagenetic Influence of Radiation with Different Linear Energy Transfer, Energoizdat, Moscow, 1991 (in Russian).