FLNR series of works recognized with JINR Encouragement Prize 2019

News, 07 July 2020

“Features of the population of isomeric states in reactions with weakly bound nuclei”

(a series of works by the team of the G. N. Flerov Laboratory of Nuclear Reactions JINR, recognized with the JINR Encouragement Prize 2019 for contribution to the field of experimental physics).

An isomeric state was discovered by O. Hahn in 1921 in naturally occurring radioactive nuclei (234Pa formed following the β-decay of 234U). In 1935 the researchers of the I.V. Kurchatov Laboratory discovered an isomer of the 80Br isotope synthesized in the 79Br(n, γ) reaction. The first theoretical explanation of this phenomenon was proposed by C. Weizsäcker in 1936.

Isomeric states of nuclei differ from other excited states in that the probability of their transition to any low-lying state is strongly inhibited by the selection rules of spin and parity changes. In particular, high-multipolarity transitions (i.e. transitions with large spin changes) and low-energy transitions are suppressed. Thus, two half-lives are observed when the nucleus undergoes radioactive decay: one involving spin and transition energy changes and allowed by selection rules and the other hindered. More than 100 isomeric states with lifetimes over 1 s are presently known.

The groups of isomers close to magic numbers form the so-called “islands of isomerism”. This phenomenon was qualitatively explained within the nuclear shell model. For odd nuclei in which the number of protons or neutrons is close to magic numbers, the shell model predicted the existence of nuclear levels that are similar in energy but substantially different in spin.

Isomers turned out subsequently to be of several types: shape isomers (decay is forbidden due to the shape mismatch), spin isomers (spin mismatch), and K-isomers (change in spin orientation with respect to the nucleus symmetry axis).

In their works submitted for JINR competition 2019, the authors studied the population of isomers related to “traditional” spin isomers in reactions involving halo, weakly bound, and cluster nuclei. The series of works was performed in collaboration with researchers from the Institute of Nuclear Physics of the Academy of Sciences of the Czech Republic, Řež.

The population probability ratio of ground and isomeric states in the atomic nucleus, defined as σm / σg and called the isomeric ratio (IR), depends on a number of factors, in particular, transferred angular momentum. Not only does the measurement of IRs in various nuclear reactions provide important information on the structure of the nucleus and the mechanism of its formation, but it also provides insights into the degree of its excitation, including the distribution of nuclear level densities and spins of excited nuclear states.

The production of beams of weakly bound radioactive nuclei opened new avenues for studying the structure and interaction mechanism of these nuclei, which is substantially different from that of stable nuclei: sub-barrier reactions, cluster transfer reactions, a higher probability of population of isomeric states, etc. One of the topics that aroused a great deal of interest among researchers are mechanisms of reactions induced by weakly bound radioactive nuclei characterized by an unusual distribution of proton and neutron densities that markedly affects nuclei interaction. Unlike in reactions with densely packed nuclei(α-particles, etc.), an increase in the mean angular momentum transfer is expected in reactions involving weakly bound and halo nuclei in view of a larger radius of nuclear matter distribution in the projectile nucleus. Compared with the population of isomeric and ground states in nuclei produced in reactions with ordinary, neighbouring stable nuclei, this distribution can in turn affect the population probability of isomeric and ground states of complete and incomplete fusion and transfer reaction products. In such reactions, it seems likely that the population of high-spin isomeric levels will differ from the population probabilities of the given states produced in reactions induced by light stable particles, for example, by protons and 4He.

The population of ground and isomeric states in nuclei originating from fusion reactions depends strongly on the excitation energy of the compound nucleus, distribution of angular momentum, and type of particles emitted in the de-excitation of the nucleus that are able to carry away various energies and angular momenta.

In the series of works, a study was conducted on the effect of fusion and transfer reaction mechanisms in collisions of 3He weakly bound and 6He halo nuclei with light and heavy target nuclei upon the excitation of newly formed nuclei, which leads to the population of isomeric and ground states both in residual nuclei and reaction products when transferring single nucleons and clusters to the target and projectile nuclei.

Nuclei were studied near the closed shells Z = 82 and N = 126, which could be produced in reactions with 194Pt and 197Au in ground and isomeric states.

The investigations were performed with a 6He beam produced at the DRIBs accelerator complex of JINR’s Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna and a 3He beam delivered from the U-120M cyclotron of the Institute of Nuclear Physics of the Czech Republic in Řež. The experiments were carried out using the activation analysis method. Highly efficient germanium detectors were used to measure induced γ-activity. Gamma-ray energy and half-lives characteristic for the process were used for the identification of nuclei with relatively long-lived ground and isomeric states and for the measurement of their formation cross-sections. On the basis of the measured values of typical γ-activity during the decay of the nucleus under consideration, the excitation functions for the formation of these nuclei in ground and isomeric states were measured.

In first experiments with 6He and 197A, the measured cross-sections of fusion and transfer reactions near the Coulomb barrier were shown to be much higher compared to those obtained previously in reactions with α-particles (4He). Yields of a number of isotopes produced both in fusion and transfer reactions were observed with a higher probability.

Excitation functions were measured for the production of the 198Tl and 196Tl isotopes in the ground and isomeric states via the complete fusion reactions 197Au (6He, 5n 7n) 198,196Tl.

IRs measured for the 198Tl and 196Tl isotopes turned out to be close to the average statistical weight ratio (2Jm + 1) / (2Jg + 1), where Jg and Jm are spins in ground and isomeric states. The IR calculations demonstrated that the imparted angular momentum in reactions with 6He was close to that in fusion reactions induced by α-particles at higher energies. The IRs for 198Tl in the sub-barrier fusion reaction 197Au (3He, 2n)198Tl reached 1.1-1.5, which was close to the IR values for 198Tl produced in the fusion of α-particles with 6He at the energy above the Coulomb barrier.

IRs for 195Hg were measured at a higher 6He energy. The nucleus was produced in the 197Au (6He, p7n) 195Hg fusion reaction accompanied by the emission of neutrons and a charged particle. The IR for this isotope formed in the fusion reaction followed by the evaporation of a proton and seven neutrons also exceeded unity and remained rather unaffected by the increase in the 6He energy up to 20 MeV/nucleon.

It was of particular interest to compare the population of ground and isomeric states in nuclei formed in nucleon transfer reactions, which should be more sensitive to angular momentum. The excitation functions were measured for the production of the 196Au and 198Au isotopes in ground and isomeric states in the 197Au + 6He reaction accompanied by neutron transfer to the target nucleus or incident particle (stripping and pickup mechanisms). The IRs for 196Au and 198Au were shown to be lower than those in complete fusion reactions and exhibited marked differences in behaviour with change of the energy of the bombarding particle. It was found that the IRs for 196Au were essentially independent of the 6He energy within the energy range under consideration up to 20 MeV/nucleon and fell close to the value of 3 ∙ 10–1. With increasing 6He energy and in the energy range from 20 to 60 MeV, the IRs for 198Au increase and vary from 10-3 to 10-2.

The graph below illustrates the behavior of IRs for various 197Au + 6He reaction channels. It is to be noted that for 196Au, the dependence of IR on the sup>4He energy is quite different from that for the 198Au isotope, both being produced in the 197Au + sup>4He reaction: the IR for 196Au is seen to increase as the 4He energy varies but remains almost unaffected for 198Au.

The charge-exchange reaction 197Au (3He, t) 197Hg was studied using the 3He beam. IRs and cross sections were measured for the 197Hg isotopes in the ground and isomeric states. The IRs were shown to be fairly low (~ 0.1) and essentially independent of the 3He energy.

Owning to a specially designed and successfully employed technique for irradiating targets and measuring nuclear production cross sections in ground and isomeric states using the unique 6He beam emitted by the JINR acceleration complex DRIBs and the 3He beam delivered from the U-120M cyclotron of the Nuclear Physics Institute (Řež, Czech Republic), some interesting results were obtained. They allowed us to determine regularities in the production and behavior of excitation functions of various reaction channels and draw the following conclusions:

  1. Production cross sections of the isotopes of Hg, Au, and Tl were measured for the first time in ground and isomeric states in the 194Pt + 3He and 197Au + 6He reactions using beams of weakly bound nuclei (3He and 6He). Excitation functions of various reaction channels were measured (fusion, transfer, and charge exchange reactions).
  2. IRs were measured for the first time for the following isotopes:

    195Hg, 196Tl, 198Tl produced in the 194Pt + 3He
    and 197Au + 3He (6He) fusion reactions;

    196Au 198Au formed in transfer reactions with 197Au using 6He and 3He;

    197Hg produced in the charge exchange reaction 197Au(3He,t)197Hg.
  3. The IR of fusion reaction products formed with the beams of halo nucleus 6He and weakly bound nucleus 3He was shown to be higher than that of nucleon transfer and charge exchange reactions, indicating different population mechanisms of excited states in these reactions. The shift of the IR maximum to lower energies in complete fusion reactions can be attributed to a larger amount of angular momentum added by the 3He and 6He nuclei.

The revealed dependence of IRs (in stripping and pickup reactions) and clusters on the energy of the bombarding particle can be explained by the difference in selective population of single-particle and collective states in reactions with beams of weakly bound and stable nuclei.

Thus, reactions with beams of weakly bound radioactive nuclei can be used to populate high-lying states of nuclei, including those at the neutron drip line.

The main results of the work were tested and reported at international conferences: International Conference “Isomers in Nuclear and Interdisciplinary Research” (July 2011, Peterhof, Russia); International conferences on nuclear spectroscopy and the structure of the atomic nucleus: 64 International Conference “NUCLE-2014” (Minsk, Belarus), 65 International Conference “NUCLE-2015” (Peterhof, Russia), 68 International Conference “NUCLE-2018” (Voronezh, Russia ); International symposium: EXON 2016 (September 2016, Kazan); Zakopane Conference on Nuclear Physics “Extremes of the Nuclear Landscape” (September 2018, Zakopane, Poland) and others.

Skobelev N.K., Senior Researcher, FLNR