V.A. Karnaukhov

New neutron-deficient isotopes of barium and rare-earth elements

Abstract This paper deals with an investigation of the short-lived neutron-deficient isotopes of barium and rare-earth elements. By using the BEMS-2 isotope separator with a heavy-ion beam, we succeeded in producing 19 new isotopes with mass numbers ranging from 117 to 138. Five of these ( 117 Ba, 129, 131 Nd and 133, 135 Sm) turned out to be delayed proton emitters. Theβ-decay probabilities for the new isotopes have been analyzed in terms of the β-strength function. An analysis of the proton spectrum shape has been performed using the statistical model for delayed proton

Excitation energy of the spontaneously fissioning isomeric state in 240Am

Abstract The excitation function for the reaction 241 Pu(p, 2n) 240m Am leading to the spontaneously fissioning isomeric state in 240 Am has been measured with proton energies 9.6–13.6 MeV. The excitation function shows a typical threshold behaviour, corresponding to a threshold of 10.7 ± 0.1 MeV. This is 3.15±0.25 MeV higher than the estimated threshold for the ground state reaction, and it is interpreted as the excitation energy of the isomeric state. Possible reasons for the unusual stability towards γ-emission of such a high lying level are discussed.

The BEMS-2 on-line isotope separator at the JINR heavy-ion cyclotron

Abstract The BEMS-2 on-line isotope separator operated in a heavy-ion beam employs a high-temperature surface-ionization ion source. The design features of the ion source with an external target are described. The recoil effect occuring in heavy-ion reactions is used to transport the radioactive isotopes under investigation into the source cavity through a 1.4 μm tantalum entrance foil. The operational characteristics of the ion source for Cs and Ba isotopes are studied. The average hold-up time of the activity in the ion source, τ, which is determined by the diffusion of recoils from the catcher, is measured. The value of τ depends on the temperature and type of the catcher used. For a tantalum catcher τ ≈ 1.0s at T = 2500°C, and the value of τ increases with decreasing temperature. In the case of a tungsten catcher, the hold-up time is several times longer. The ion source efficiency for Ba is equal to 30–50%.

A study of the decay of the delayed proton emitters 117Ba, 119Ba and 121Ba

Abstract The delayed proton emitters 117, 119, 121 Ba have been investigated using the on-line BEMS-2 isotope separator. From measurements of positron-proton coincidences the ( Q 0 - B p ) values for these isotopes (where Q 0 is the K-capture energy and B p is the proton binding energy of the daughter nucleus) have been obtained to be 7.9±0.3, 6.2±0.2 and 4.3±0.3 MeV, respectively. The shapes of the proton spectra have been analysed within the framework of the statistical model. The analysis indicates the presence of a local resonance in the β-strength function at an excitation energy of the daughter nucleus of 5 MeV.

New neutron-deficient isotopes of lanthanum and cerium

The short-lived neutron-deficient isotopes of lanthanum and cerium were produced in the 32S+96, 98Ru reactions and separated according to their masses. Seven isotopes 123–125La and 124–127Ce have been first observed and their half-lives and low-energy γ-ray data are reported. In addition, low-energy γ-ray data are given for the 122–125Ba and 127La decays. The β-decay probabilities for Ce, La and Ba isotopes are discussed in terms of the average β-strength function.

The proton decay of radioactive nuclei

Abstract Experiments on radioactive nuclei proton decay are described. The data on two proton emitters detected from the exposure of Ni to Ne 20 and O 16 beams are given. The first emitter (Ne or Mg light isotopes) has a half life of 85±15 msec and emits protons of 5±0.2 MeV energy, while the second emitter has a 23±4 sec half life and emits protons of 2.5±0.2 MeV energy. After a series of experiments we concluded that the second emitter is one of the Br or Kr light isotopes, i.e., in this case subbarrier proton emission takes place (the Coulomb barrier height is about 8.5 MeV). It is most probable that protons are emitted from a daughter nucleus after positon transition; the half life under measurement is associated with this transition. The process of proton emission with E p = 5 MeV is similar to that for delayed neutron emission. The mechanism of subbarrier proton emission with E p = 2.5 MeV is analogous to that for long-range alpha emission by heavy nuclei. The possibility of configuration isomer proton decay is also considered and is not excluded in the case of a 2.5 MeV proton emitter.

PROTON EMITTERS AMONG Te ISOTOPES

Abstract The delayed proton emitters obtained by irradiating 92 Mo with 20 Ne and 22 Ne ions and 94 Mo with 20 Ne ions have been studied. Proton activities with half-lives 4.2±0.2 sec, 19±0.7 sec, 13±2 sec and 60–80 sec have been observed. The first two emitters are most likely to be Te isotopes with mass numbers 109 and 111, respectively. On the basis of the data obtained, the cross sections for the reactions 98 Mo( 20 Ne, 4−6n) 108−106 Te have been estimated.

On the fine structure in delayed proton spectra.

Abstract The fine structure in delayed-proton spectra is interpreted on the basis of statistical fluctuations associated with the variation of β-decay matrix elements and the relative proton widths of the excited states. Formulae are derived which give the variance of relative proton intensity in terms of the average characteristics of the delayed proton emission and the level density. The experimental data on the variance of the relative proton intensity for 111Te and the calculations using these formulae are in satisfactory agreement. The fluctuations predicted for 109Te considerably exceed the measured ones.

Search for superdense nuclei of Rb and Cs among the products of 8 GeV proton interactions with Ta

Abstract Experimental searches for β-active superdense nuclei of Rb and Cs among the products of 8 GeV proton interactions with Ta are described. The method of selective off-line mass-separation with scanning of the β-activity distribution on the collector was used. According to π-condensation theory, the long-lived anomalous nuclei are expected to be more neutron-deficient than the usual ones. Moreover, their mass numbers may be significantly nonintegral measured on the normal mass scale, because of unusual binding energy. These predictions guided the present search. No anomalous nuclei were found. The upper limit for Rb nuclei in the range 69 ⩽ A ⩽ 78 is 10 −5 with respect to the yield for 81 Rb; for Cs in the range 112 ⩽ A ⩽ 122 the limit is (5–10) × 10 −6 of the yield for 127 Cs. These estimations are based on the assumption that the half-life of superdense nuclei equals 5 h.

On polarization effects in reactions induced by heavy ions

Abstract The decay of a polarized compound nucleus with high angular momentum is considered. The calculations have been performed for the Coulomb potential at the surface of a dynamically deformed rotating nucleus with a sharp edge. The known results on the shape of the rotating nucleus obtained in the framework of the liquid-drop model have been used. The degree of polarization is estimated for neutrons evaporated by the polarized nucleus with high angular momentum. Multinucleon transfer reactions (partial fusion) with light-fragment detection are considered as a tool for the production of high-spin polarized nuclei.