Effect of nuclear-reaction mechanisms on the population of excited nuclear states and isomeric ratios

Experimental data on the cross sections for channels of fusion and transfer reactions induced by beams of radioactive halo nuclei and clustered and stable loosely bound nuclei were analyzed, and the results of this analysis were summarized. The interplay of the excitation of single-particle states in reaction-product nuclei and direct reaction channels was established for transfer reactions. Respective experiments were performed in stable (⁶Li) and radioactive (⁶Не) beams of the DRIBs accelerator complex at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, and in deuteron and ³Не beams of the U-120M cyclotron at the Nuclear Physics Institute, Academy Sciences of Czech Republic (?e? and Prague, Czech Republic). Data on subbarrier and near-barrier fusion reactions involving clustered and loosely bound light nuclei (⁶Li and ³He) can be described quite reliably within simple evaporation models with allowance for different reaction Q-values and couple channels. In reactions involving halo nuclei, their structure manifests itself most strongly in the region of energies below the Coulomb barrier. Neutron transfer occurs with a high probability in the interactions of all loosely bound nuclei with light and heavy stable nuclei at positive Q-values. The cross sections for such reactions and the respective isomeric ratios differ drastically for nucleon stripping and nucleon pickup mechanisms. This is due to the difference in the population probabilities for excited single-particle states.     

Search for high-spin isomeric states in the rare-earth region

Several new high-spin (I>10) isomeric states in the mass region A=144–164 are located in a systematic search with ¹²C induced reactions, employing a sixteen NaI (Tl)Ge(Li) detector multiplicity filter. The proximity to the N=82 neutron shell closure strongly suggests that shell effects contribute significantly to this isomerism.     

Shape coexistence of high-spin isomeric states in proton-rich A~190 nuclei

High-spin isomeric states in proton-rich A~190 nuclei have been investigated using configuration-constrained calculations of potential-energy surfaces. The calculations reproduce reasonably the experimental data, and predict shape coexistence of high-spin isomeric states in light Po isotopes.     

Delayed autoionization of recoil ions by the decay of high-spin isomeric states

The time dependence of the ionization for isotopically different heavy ion fusion recoil ions has been observed. Delayed nuclear-induced autoionization of recoil ions caused by the decay of high-spin nuclear isomeric states by internal conversion was established. Internal conversion in isolated recoil atoms results in a drastic rearrangement in the atomic cloud with a loss of a great number of orbital electrons. Possibilities for the use of the observed phenomena in atomic and nuclear physics are discussed.     

g-factors of high-spin isomeric states in 216Ra

The g-factors of two isomeric states at E_x = 3763 and 5170 keV in ²¹⁶Ra have been measured to be 0.51 ± 0.03 and 0.63 ± 0.06, respectively, with a TDPAD method. Spin and parity assignments of 19^? for the 3763 keV state and 25^? or 24⁺ for the 5170 keV state are consistent with the measured g-factors. Proposed configurations for the 19^? and 25^? assignments are of the same type as those predicted by a deformed independent-particle model for ²¹⁴Rn, which is an isotone of ²¹⁶Ra.     

Physics of high-spin states in nuclear reactions caused by he isotopes

Isomeric ratio and excitation functions for ¹¹³In(α, n)^116mg Sb and ¹¹³In(⁶He, 3n)^116mg Sb reactions are calculated. The obtained results are compared to refined experimental data in reactions with α particles.     

Searches for effects of fundamental-symmetry violation in isomeric nuclear states

The results of broad searches for schemes that are convenient for observing effects of time-inversion-invariance violation (T violation) simultaneously with parity violation (PT violation) in electromagnetic transitions in nuclei are presented. The main problems in observing such effects are discussed. A scheme that seems one of the most promising for this and which relies on measuring the linear polarization of gamma radiation accompanying the deexcitation of isomeric states of nuclei that were oriented by a magnetic field at ultralow temperatures is highlighted.     

Special features of isomeric ratios in nuclear reactions induced by various projectile particles

Calculations for (p, n) and (α, p3n) reactions were performed with the aid of the TALYS-1.4 code. Reactions in which the mass numbers of target and product nuclei were identical were examined in the range of A = 44–124. Excitation functions were obtained for product nuclei in ground and isomeric states, and isomeric ratios were calculated. The calculated data reflect well the dependence of the isomeric ratios on the projectile type. A comparison of the calculated and experimental data reveals, that, for some nuclei in a high-spin state, the calculated data fall greatly short of their experimental counterparts. These discrepancies may be due to the presence of high-spin yrast states and rotational bands in these nuclei. Calculations involving various level-density models included in the TALYS-1.4 code with allowance for the enhancement of collective effects do not remove the discrepancies in the majority of cases.     

On the ferromagnetism of high-spin states

It is established on the basis of the idea of strong interaction within the unit cell that ferromagnetic instability is possible in a system with hopping between d-electron states of atoms that are in high-spin states and form a bcc lattice. The phase diagram of the existence of ferromagnetic ordering as a function of the degree of filling of the d shell is constructed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 275–280 (25 August 1998)     

Spectroscopy of high-spin states in 211,212,213Fr

The level structures of ²¹¹Fr, ²¹²Fr and ²¹³Fr have been observed to high spins, ~ 28 /kh, (and excitation energies ~ 8 MeV) using a variety of γ-ray spectroscopic techniques. The structure of these nuclides is discussed in terms of couplings of single-particle states through empirical shell-model calculations. Good agreement with experiment is obtained. In ²¹²Fr and ²¹³Fr core-excited configurations are required to explain the properties of the highest states. A number of long-lived states were observed in each nucleus some of which decay by enhanced E3 transitions. The E3 transition strengths are discussed.     

Investigation of isomeric ratios in high-energy reactions

Experimental data on the isomeric ratios of cross sections for products originating from the interaction of ¹²C ions (E 12 _C = 26.4 GeV), protons (E _p = 3.65 GeV), and deuterons (E _d = 7.3 GeV) with targets enriched in the tin isotopes ¹¹²Sn, ¹¹⁸Sn, ¹²⁰Sn, and ¹²⁴Sn are presented. The dependence of the isomeric ratios on the interaction mechanism, projectile type and energy, and nuclear properties of the final-state nucleus are discussed. The dependence of the isomeric ratios on the degree of deformation of the product nucleus is also considered.     

Analysis of isomeric ratios for medium-mass nuclei

Values of the isomeric ratios for product nuclei originating from simple charge-exchange reactions were analyzed. The cross sections for the formation of product nuclei in ground and isomeric states were calculated with the aid of the TALYS 1.4 and EMPIRE 3.2 codes. The calculated values of the isomeric ratios were compared with their experimental counterparts taken from the EXFOR database. For the ^86,87Y, ^94,95,96,99Tc, and ⁴⁴Sc nuclei, the experimental values of the isomeric ratios exceed the respective calculated values. The nuclei in question feature weak deformations and have high-spin yrast lines and rotational bands. The possible reason behind the discrepancy between theoretical and experimental isomeric ratios is that the decay of yrast states leads with a high probability to the formation of isomeric states of detected product nuclei.     

Properties of unbound high-spin states in27Al

Selectively excited high-spin states in²⁷Al have been located by the reaction¹²C(¹⁶O,p)²⁷Al. The excitation functions of these states have been measured at incident energiesE _cm=18.7 to 30.1 MeV at intervals of 150 keV. They exhibit maxima of presumably non-statistical origin. At four different energies the subsequent decay of the²⁷Al states has been studied by detecting the final heavy recoils with the Munich recoil spectrometer in coincidence with the proton emitted in the first stage of the reaction. Using this new method branching ratios of theγ-, α-, andn-decay have been measured. Based on angular correlation arguments spins up to 27/2? have been determined within the experimental accuracy of 1–2?. The observations suggest a superdeformed shape of²⁷Al at least in some of the states.     

Lifetime study of high-spin states in 104,105In

In the neutron deficient isotopes _104,105In lifetimes of high spin states in the range from 0.5 ps to 600 ps have been measured in a coincidence recoil distance Doppler shift (RDDS) experiment. By combining the Doppler shift attenuation (DSA) and RDDS methods at few μm flight distances, the problem of delayed feeding has been avoided and very short lifetimes in the range 0.5–0.8 ps have been determined. Shell model calculations with strong restrictions for the neutron orbitals reveal good agreement with experimental level energies and still fair agreement for most measured B(M1) and B(E2) values. Received: 30 June 1998