Nuclear Reactions.

  1. Reactions with light nuclei.
  2. Study of light exotic nuclei using radiactive beams and few-nucleon transfer reactions.
  3. Low energy reactions in few nucleon systems.
  4. Study of light induced drift applications.
  5. Study of radiation-induced mutagenesis.

Nuclear Reactions.

The fundamental research during the period 1990-1994 covers five topics as: study of elastic and inelastic scattering of 3-He ions, study of the nucleon transfer reactions in some p and f-p shell nuclei, investigation of light exotic nuclei using the radioactive beams and few nucleon transfer reaction, few-nucleon systems, and application of lasers in nuclear and atomic physics. Some results of these studies are briefly presented below.

1. Reactions with light nuclei

Spectroscopic information from (d, p) reactions.

The accurate and precise (d, p) reaction Q- values as well as excitation energies corresponding to 98 states of the nuclei 13-C, 14-C, 17-O, 29-31-Si, 33-S, 38-Cl and odd isotopes 111-117-Cd were obtained as a result of high resolution experiments. The measurements were performed with the 12.3 MeV momentum separated beam of deuterons from the cyclotron of Nuclear Physics Institute in Rez. Protons from the (d, p) reaction were analysed by multi-angle magnetic spectrograph.

Angular distributions of protons from the reaction 37-Cl(d, p)38-Cl were analysed using DWBA calculations and revised values of transferred orbital momenta and spectroscopic factors for 25 states of 38-Cl were given. Single particle strengths for 1f and 2p orbitals were compared with respective strengths from the reaction 36-S (d, p) 37-S which was also investigated in our laboratory. Sum rules indicated that substantial parts of 1f and 2p single-particle strengths were exhausted by states observed in this experiment [1.1, 1.2]. Our programme for the near future is to finish the analysis of the extensive experimental material concerning the high-resolution study of 30-Si(d, p) 31-Si reaction, in which the unbound states of the final nucleus are excited. The analysis uses the standard DWBA theory modified for the particle transfer to such states.

Study of the neutron transfer reactions mechanism.

The mechanism of (p, d), (d, t) and (3He, alpha) reactions in several p-shell nuclei was studied. Angular distributions in these reactions on 7-Li, 9-Be, 10-B, 13C and 14-C were measured partly on the isochronous cyclotron U--120M of NPI at Rez and partly on cyclotron of collaboratoring laboratory of NPI in Tashkent. Absolute values of differential cross sections in the region of the main maximum were measured with about 5 % accuracy. The energy of protons was 18.6 MeV, deuterons 18 MeV and 3-He 38 MeV. The reaction cross sections were analysed in the frame of DWBA, modified by using independent information about nuclear vertex constants which correspond to the synthesis of target nuclei. It was shown that this method, proposed in [1.3] and worked out in our laboratory, provides not only the extraction of consistent values of the spectroscopic coefficients but also clears up the question which kind of the structural information may be obtained from nuclear reactions in dependence on their mechanism.

For the 13-C target nucleus, it was demonstrated that the spectroscopic factors may be extracted from reactions in which the internal region of nucleus gives important contribution to the amplitude of reaction. This is the case of (p, d) reactions for our energies of the projectiles, where good agreement with theoretical values of spectroscopic factors was achieved.

The reactions (3-He, alpha) and (d, t) are localized in the peripheral region and their cross sections are calibrated using corresponding vertex constant rather than by spectroscopic factors. For the reliable extraction of these quantities, it is necessary to use other independent additional information or different energies of particles. The results of this analysis is published in [1.4]. Similar analysis for the 14-C(3-He, alpha)13-C reaction was also performed. The angular distributions of the elastic scattering and (3He, alpha) cross section for 37.9 MeV 3He incident on 13C and 14-C were also analysed taking into account possible presence of the refractive effects in one-nucleon transfer reactions. The analysis of experimental data was performed in the framework of the distorted wave Born approximation using external microscopically calculated form factors. Presence of the rainbow-like mechanism in both elastic and reaction channels was observed. The effect is apparent mainly for transitions where strong orbital mismatching occurs. It was shown that the optical potential which describes the rainbow mechanism of the elastic scattering optimally is very sensitive to the internal region and could be further selected according to the shape of the reaction differential cross section, and thus the well-known ambiguity of the optical-model potentials may be practically eliminated.

The first observation of the refractive mechanism in transfer reactions was made in our laboratory [1.5, 1.6]. A more extensive study of the refractive processes in transfer reactions is very desirable because these processes can give valuable information on the interactions at small distances in the region of the nuclear formfactors.

Inelastic scattering of 3-He on 14-C at energy 37.9 MeV.

The inelastic scattering of charged particles belongs to useful methods for investigation of excited states of nuclei. There are not much data available on the inelastic scattering of particles on the 14-C nucleus, and the data on the scattering of 3-He ions are not available at all. We measured angular distributions of elastically and inelastically scattered 3-He ions by 14-C leading to the several lowest levels, which are relatively strongly excited.

The experiment was performed on the isochronous cyclotron U-120M at the Nuclear Physics Institute in Rez with the energy E(3-He) = 37.9 MeV. The 14-C target was a self-supporting foil enriched to 80 % in 14-C with a thickness 180 micro g/cm^2. The detection of emitted particles was made simultaneously by two detector telescopes delta E-E placed in the target chamber.

For the analysis of angular distributions, the optical model was used. The parameters of the optical model were deduced from the best fit of the angular distribution of the elastically scattered 3-He ions on the 14-C nucleus at 37.9 MeV.

In the inelastic scattering, the collective states of a target nucleus are predominantly excited. From the differential cross sections for the levels 6.73(3-), 7.01(2+) and 8.32(2+) we determined the vibration parameters of the 14-C nucleus as 0.27, 0.25 and 0.17, respectively. These values are somewhat lower than the so far available parameters beta from inelastic scattering of alpha -particles on the 14-C nucleus (R.J. Peterson, et al.: Nucl. Phys. A425(1984)469). These values beta were then employed in calculations by the method of coupled channels. A very good agreement was achieved between calculated and measured differential cross sections for levels 6.73(3-) and 7.01(2+) MeV. The angular distribution of the level 8.32(2+) MeV has somewhat different behaviour, which is, probably, connected with an opened channel for neutron emission.

Levels 6.09(1-) and 7.32(2-) MeV with the negative parity cannot be reached by a collective excitation. Therefore, we used, in both cases, the microscopic model for the calculations of angular distributions. This model uses shell-model wave functions for the description of initial and final states of a nucleus and takes a sum of two-body forces between projectile nucleons and target nucleons for the interaction causing the transition. In the case of the 3-He inelastic scattering, the effective interaction is the sum of nucleon-nucleon interactions between the bound nucleons in the 3-He projectile and bound nucleons in the target nucleus.

[1.1] S. Piskor et al.: Nucl. Phys. A510(1990)301.
[1.2] S. Piskor et al.: Nucl. Phys. A481(1988)269.
[1.3] S.A. Goncharov et al.: Sov. J. Nucl. Phys. 35 (1982)662.
[1.4] I.R. Gulamov et al.: Czech. J. Phys. B40(1990)875.
[1.5] A.M. Mukhamedzhanov et al.: Sov. J. Nucl. Phys. 52 (1990)704.
[1.6] V. Burjan et al.: Phys. Rev. C49(1994)977.

2. Study of light exotic nuclei using radioactive beams and few-nucleon transfer reactions.

Most of the progress in nuclear physics today is expected from the study of extreme states of nuclear matter. This may concern the temperature, the pressure, the deformation, or the isospin degree of freedom, which is very essential for a good understanding of the structure of the nucleus and has not been extensively investigated yet.

From this point of view it seems likely that the experimental investigation of both the lightest nuclei and the quasistable nuclear systems having an anomalous N/Z ratio and lying near the boundary of the nuclear stability may shed light on these problems since the extreme cases of a nucleon configuration should be more sensitive to the choice of the nuclear potential parameter. Nuclear properties that depend upon isospin will be enhanced and clearly observed in high isospin states.

During the last decade studies using radioactive beams have been performed in the collaboration of the NPI Rez with the Flerov Laboratory of Nuclear Reactions, JINR, Dubna. After 1990, the collaboration has extended to the GANIL-Orsay-Dubna-Rez-Bucharest-Warsaw cooperation.

The study of elastic and quasielastic scattering of exotic nuclei on different targets at energies of 10-100 MeV/A.

Experimental investigations of the elastic scattering of exotic nuclei make possible to obtain important data on the structure of the interacting nuclei. Experimental data and the optical potentials of the interaction of exotic nuclei obtained from their analysis is one of the few sources of information on the neutron and proton density distributions. Mainly, the subjects of interest are neutron-rich nuclei such as 6,8-He, 11-Li, 11-Be as well as the neutron deficient nuclei 7-Be, 8-B and 9-C. The interest in neutron-rich nuclei was derived for the most part from the discovery of the so-called neutron ``halo'', whereby the very weak binding of the last one or two neutrons may lead to the formation of an extended distribution well beyond that expected on the basis of systematics. The matter distribution of 11-Li has been studied since the pioneering radioactive-beam experiment of Tanihata (see I. Tanihata et al.: Nucl. Phys. A488(1988)113) at Berkeley, which showed that it has an unusually large interaction cross-section at high energies.

In the frame of the existing cooperation, it was proposed to use the GANIL facility for completing the 11-Li interaction cross section data by the measurement of the elastic scattering of 11-Li on the 28-Si target, advancing the idea of ``nondestructive'' study of this nucleus. The secondary beam of 11-Li (29MeV/A) was produced at GANIL in the reaction of 18-O (76MeV/A) primary beam bombarding a thick 9-Be target. The LISE3 spectrometer consisting of two dipoles and a Wien velocity filter were used to collect, separate, purify and direct the secondary 11-Li beam on the 28-Si target. The incident angle was measured using position-sensitive PPAC detectors and the mass and the charge of scattered particles were identified by bidimensional spectra obtained by various combinations of measured parameters, e.g. their time of flight, energy losses in Si transmission detectors and residual energy in the BGO detector. The BGO detector consisting of seven small BGO crystals was prepared in NPI Rez [2.1]. The elastic scattering of a secondary 11-Li beam (29 MeV/nucleon) on a 28-Si target was measured for the first time [2.2, 2.3]. The data were treated using a phenomenological analysis and by coupled-channel calculations with a double-folding optical potential, with energy- and density-dependent effective interaction and realistic densities. Using coupled-channel calculations with the folding potential, a better description is achieved when the neutron halo of 11-Li is taken into account.

On the other hand, in the proton-rich nuclei it seems quite natural to expect a proton ``halo'', because of the charge symmetry of the nuclear force. However, the Coulomb force between the protons acting besides the nuclear force may prevent the formation of a halo. From an experimental point of view, it is difficult to detect the thin halo by interaction cross-section measurements because the method is mainly designed to observe matter distributions and therefore the effect due to the proton halo could be only a small fraction of the cross section.

During the last year, new experiments have been prepared to measure the elastic scattering of secondary 7-Be and 8-B beams (40 MeV/A) on a 12-C target. The secondary 7-Be and 8-B beams have been produced by fragmentation of a 13-C (60MeV/A) beam on a 9-Be target. To compensate for the low intensity of the secondary beam, a new more efficient detecting system has been built.

For 8-B nucleus, there is a number of similarities with 11-Li as resulted from the combining effect of proton skin and the low threshold for breakup. The CC calculation for this nucleus does not explain the quasielastic scattering perfectly. As in the 11-Li case, it is difficult to explain the small angle cross section by a simple optical model. The missing amplitudes could be attributed to the virtual excitation of the quasibound states above the threshold.

The results obtained in our experiments at GANIL provided further support for the ``neutron halo'' hypothesis and shed more light on the origin of the ``proton halo''.

In the near future, we plan to obtain the detailed information on the 6-He and 8-He nuclei in which the existence of one or two neutron halo or the effect of a neutron skin is also possible. It is of particular interest to study the elastic scattering of nuclear isobars to find out the dependence on isospin. We propose study of the isobaric pair 6-He - 6-Li that will be obtained as the secondary and the primary beam at GANIL with energies 20-100 MeV/A and at the U-400 cyclotron at Dubna with the energies of 10-20 MeV/A. The other nuclei which will be studied are 14-Be and 17-B, for which the existence of a neutron halo has also been predicted. For these experiments, the new BGO detector of 70 mm in diameter and CsI(Tl) detector has been prepared [2.4].

The investigation of proton- and neutron-halo nuclei using the dissociation and fission reaction.

It has been theoretically predicted and experimentally observed that the neutron(s) removal cross section for 11-Li and 11-Be rises significantly as the bombarding energy decreases. However, the existing data are taken only at three energies (800 MeV/A, 60 MeV/A and 30 MeV/A). An increase in cross section of about five times is observed in some cases. Therefore, the question arises whether the increase will continue with decreasing energy and what one can learn from this behaviour about the halo size and structure.

In the experiment performed at GANIL, a multicomponent silicon telescope and a neutron wall were used. The incident halo nuclei of energies (20-30 MeV/A) such as 7-Be, 8-B and 11-Be enter the 28-Si-telescope consisting of 12 layers, which serves as an energy absorber and a target as well. The nucleus gradually loses its energy along the telescope up to the end of its range. If it undergoes a breakup reaction on silicon, then the neutrons or protons escape the detection system and the residual nucleus will stop at a smaller depth inside the telescope because of its smaller total energy as compared with the incident nucleus. This energy ``defect'' as well as the depth at which it stopped can be used to find the point at which the reaction took place and therefore the energy of incident nucleus at that place. The method permits then to measure the whole excitation function from the maximal incident energy down to 8-5 MeV in a single run.

Our preliminary results have shown that this method can be successfully applied to measure the excitation function of neutron and proton removal cross section for loosely bound halo nuclei 7-Be, 8-B and 11-Be. Further data reduction is in progress. The measurement of the removal cross section down to low energies in the neighbourhood of the Coulomb barrier might place strenght constraints on the existing theories and/or representations of halo nuclei.

According to the calculations, the 6-He nuclei should exhibit a strongly pronounced neutron halo structure. An attempt has been made to see whether the effect due to the neutron halo could be seen in the fission reaction.

In the experiments carried out at the Dubna U-400 cyclotron we have measured the excitation function of the fission cross section of 209-Bi induced by 6-He and alpha-particles in the energy range 30-100 MeV/A. The secondary 6-He (55MeV) beam with an energy resolution of about 1% and of hight purity (98%) has been obtained using the 11-B+9-Be reaction. As a result the fission cross sections of 209-Bi exceed the fission cross sections induced by alpha-particles. One of the possible explanation of this effect can be existence of the deformation of the 6-He nucleus.

Determination of the stability and the measurement of the masses of ground state and excited states of some neutron--rich nuclei using the few nucleon transfer reactions.

Heavy-ion physics has opened new possibilities for the study of the ground and low energy excitation states of light neutron-rich nuclei far from beta-stability using few-nucleon transfer reactions. Reactions with two nuclei in the exit channel allow us to define the properties of one of the partners (the energy of the ground and excited states, the energy of resonances) from the energy spectrum of the other one. This method can be applied even in the case when the investigated nucleus is unbound.

The reactions leading to the formation of the lightest exotic nuclei are characterized, as a rule, by the great negative Q value which determines a rather low reaction cross section. The angular distributions of the reaction products have a maximum yield at small angles where the elastic cross section is very high. These conditions set special requirements to the choice of the experimental method. The use of magnetic spectrometers makes possible at the same time to separate a significant portion of the background reaction products and to carry out precise measurements of the energy spectra. Such spectrometers are avaible at GANIL (SPEG), HMI (Q3D) and at Dubna (MSP-144).

For almost a decade the systematic investigations of the light neutron-rich nuclei have been carried out at Dubna using the U-300 and U-400 cyclotrons that provide heavy ions beams of energies 8-20 MeV/A. The reaction products emitted from the targets at an angle of (4.0 +- 0.7) degrees have been analysed by a magnetic spectrometer. The position-sensitive ionization chamber with four sections (dE1, dE2, E, Veto) served as a focal plane detector. The detector system has been built in a cooperation Rez - Dubna.

The hydrogen isotopes 4,5,6-H were studied in the (11-B+9-Be) reaction at energies E(lab)(11-B) = 88 MeV [2.5] and 120 MeV by measuring the energy spectra of the complementary particles 16-O, 15-O and 18-O. Both the measurements have agreed one with another, confirmed the instability of hydrogen isotopes and revealed the existence of virtual unbound state in 4-H at 3.5 MeV. The 7-H nucleus was studied in the reaction 9-Be(11-B, 13-O)7-H at a beam energy of 188 MeV. Only the upper limit for the production of $^{7}$H in its ground state was obtained.

The helium isotopes were studied in the reactions (11-B+7-Li), (11-B+9-Be) as well as (18-O+9-Be) with beam energies of the 11-B and 18-O ions being 120.3 MeV and 260.5 MeV, respectively. The data reduction is in progress.

The energy spectra of the mirror nuclei 15-N and 15-O produced in the multi-nucleon transfer reactions have been measured. The intensive enhancement of the transition probability was observed in the region at the 9-12 MeV excitation in the energy spectra of the 15-N nucleus produced in the reactions 7-Li(11-B, 15-N)3-H and as well as 9-Be(11B, 15-N)5-He. The total cross section of these transitions was estimated to be about 1mb/sr. The same character of the spectra is due to the fact that the same mechanism (i.e. the alpha-cluster pick-up from the target) takes place in the both reactions. On the other hand, in reactions 7-Li(11-B, 15-O)3-n and 9-Be(11-B, 15-O)5-H where the pick-up of the (3p+1n) cluster is present, no enhancements have been observed.

The light exotic nuclei which bear the neutron halo may exhibit further exotic properties originated from the halo structure. One can expect new kinds of collective motion such as that the outer one or two neutrons in halo move against the remaining core nucleus. The frequencies of this kind of collective mode are expected to be low and their amplitudes to be large, compared with corresponding collective motions in stable nuclei (or core nuclei). This behaviour can manifest itself in low energy excitations - soft mode resonances.

Therefore, the reaction 14-C(11-B, 14-O)11-Li has been studied to estimate the contribution of the soft mode resonance in 11-Li. In the measured spectrum, the peak corresponding to the ground state of 11-Li (Q-value = -37.26 MeV) is dominant. No narrow excited state has been found and only the broad bump at the excitation energy of about 5 MeV has appeared.

Studying another exit channel in the same reaction 14-C(11-B, 12-N)13-Be, we were able to investigate the unbound nucleus 13-Be under the same experimental conditions. This reaction is very suitable for the 13Be mass spectroscopy because the masses of the nuclei involved are known with accuracy better than 1 keV and 12-N has no bound excited states. The mass excess (M.E.= 33.95(9) MeV) has been found as corresponding to the lowest observed spectral line. The state is unstable with respect to one-neutron emission by 0.80(9) MeV. The population of the first excited state, which lies at about 1.2MeV higher, is about 10 times higher than that of the ground state. We should note that in our former experiment performed at Viksi (HMI) using the 13-C(14-C, 14-O)13-Be reaction we have found that the lowest peak lies at M.E.= 35.16(5)MeV. Now, it seems that it is the first excited state. The relative intensities of other peaks found in both measurements support this point of view.

In the same reaction 13-C(14-C, 17-F)10-Li as well as the reaction 9-Be(13-C, 12-N)10-Li the mass excess of 10-Li has been measured and found to be 33.445(50)MeV. This means that 10-Li is particle-unstable with respect to one-neutron emission by 0.42(5)MeV [2.6].

In future we plan to use the energies of the GANIL beams and the SPEG spectrometer to extend this type of studies for higher energies what in many cases should lead to the higher reaction cross sections.

[2.1] Z. Dlouhy et al.: Nucl. Instr. Meth. A317(1992)604.
[2.2] M. Lewitowicz et al.: Nucl. Phys. A562(1993)301.
[2.3] R. Anne et al.: Izv. RAN, ser.fiz., 57(1993)127.
[2.4] A.S. Fomichev et al.: Nucl. Instr. Meth. A344(1994)378.
[2.5] A.V. Belozyorov et al.: Nucl. Phys. A460(1986)352. [2.6] H.-G. Bohlen et al.: Z. Physik A344(1993)381.

3. Low energy reactions in few nucleon systems.

The study of fusion reactions has been based on 200 keV polarized deuteron facility, upgraded at the end of the eighties. The initial programme was motivated by some open questions of d+3-H interaction in the muon catalyzed fusion, initiated in the proposal of L.I.Ponomarev (JINR, Dubna, Russia). The program was lately supplemented by the study of d+d reactions in details, important in the polarized fusion concept.

4. Study of light induced drift applications.

The laser group has been established in the Department of Nuclear Reactions of NPI in 1993. The goal of the group is to search and investigate possible applications of the Light Induced Drift (LID) effect and other laser applications in nuclear science and technology. LID is isotopically selective and it can be used for an isotope and nuclear isomer separation. The radioactive isotope separation using LID is an important topic of the laser group investigation. The first experiments with radioactive sodium isotopes 22,24-Na were performed at the Joint Institute for Nuclear Research (JINR) in Dubna in cooperation with the Institute of Automation and Electrometry, Novosibirsk, Russia [4.1, 4.2]. The isotope enrichment of 25 times with the efficiency of about 50 % was achieved after experimental parameter optimization [4.3], further improvement of the isotope enrichment is in progress.

LID can keep a sharp density gradient of atoms of a selected nuclide in the equilibrium with the corresponding diffusion flux and in such a way creates a ``stopper'' for the atoms. It enables to realize an optical trap for free atoms with the trapping time of several hours [4.4]. The trapped atoms can be optically polarized.

The laser group also observed the first LID on atoms with intermediate metastable level (Ba atoms).

The laser measurements of short lived nuclei parameters (radius, spin, electromagnetic moments, deformation and so on) on a cyclotron beam line are considered in the cooperation with Dubna as well as with the Rez cyclotron [4.5].

[4.1] C. Hradecny, J. Slovak, T. Tethal, I.M. Yermolajev, A.M. Shalagin: Appl. Radiat. Isot. 43(1992)1259.
[4.2] Yu.P. Gangrsky, C. Hradecny, J. Slovak, T. Tethal, I.M. Yermolajev: Phys. Lett. A168(1992)230.
[4.3] C. Hradecny, T. Tethal, I.M. Yermolajev, S.G. Zemlyanoi, P. Zuzaan: Appl. Radiat. Isot. 45(1994)257.
[4.4] Yu.P. Gangrsky et al}.: Phys. Lett. A180(1993)353.
[4.5] Yu.P. Gangrsky, C. Hradecny, B.N. Markov: Hyperfine Interaction 61(1990)1411.

5. Study of radiation-induced mutagenesis.

On the external cyclotron beams, the radiation-induced lesions of the cellular and subcellular systems were studied in BRD (Koln) - CR(Brno, Rez) - Russia(Dubna) collaboration for the study of genetic effects of accelerated heavy ions. The biological experiments were performed with deuteron beams of energy 10-18 MeV and alpha-particle beams with energies 14-24 MeV. A special problem connected with the monitoring of the ultra-low particle intensity was solved.

The fast neutron source using the (d, n) reaction on the beryllium target has been built at the U-120M cyclotron. Intensity of neutron flux has reached about 10E10 neutrons/sec cm^2 per 1 microA 20MeV deutron beam. The source was used for biological experiments and the effect of neutrons on cellular tissue was systematically studied.

In the next period, biological research will continue using both neutron and charged particle beams, namely, using Li and C ions. For such experiments, a new facility is being prepared. Also, use of the microbeams of accelerated particles for cellular investigations is planned for the near future.