Core polarisation effect on the 3p 1/2, 3p 3/2, 2f 5/2, 2f 7/2 and 1i 13/2 proton states of209Bi

The weak fragments of the 3p_1/2, 3p_3/2, 2f_5/2, 2f_7/2 and 1i_13/2 proton states of²⁰⁹Bi as seen in the²⁰⁸Pb (³He,d) reaction can be explained with the coupling of the renormalised proton states and the vibrational states of²⁰⁸Pb. The energies of the zerothorder proton shell-model states as well as the collective admixtures of the weak fragmented proton states of²⁰⁹Bi have been deduced from the core-particle coupling model.     

Fragmentation of the high spin discrete neutron and proton states of209Pb and209Bi

The distribution of the discrete 1h _9/2, 2f _7/2, 1i _13/2 and 2f _5/2 proton states of²⁰⁹Bi and 2g _9/2, 1i _11/2, 1j _15/2 and 2g_7/2 neutron states of²⁰⁹Pb have been obtained within the particle-vibration coupling model calculation and compared with the experimental datas baising on the most recent high resolution stripping reaction on²⁰⁸Pb using 480 MeV¹²C projectile. The optimised shell model energies arising from the core-polarisation effect have profound influence for both the study of the structures of the high spin continuum shell model states of²⁰⁸Pb and stability of superheavy nuclei.     

Decay of the 21/2− isomer in211Bi and structure of thevg 9 2/2 πh 9/2 levels

Gamma rays and conversion electrons have been measured in²¹¹Bi populated by the²⁰⁹Bi(t,p) reaction, and the near yrast levels were observed up to 21/2^?. The 21/2^? state is isomeric withT _1/2=70(5) ns. No alpha decaying isomer was found in²¹¹Bi. ObservedM1 transitions reveal mixing of the²¹⁰Pb parent states in the levels of²¹¹Bi. The levels and transitions are well reproduced by the shell model with experimental matrix elements and pure configurations.     

High spin states and neutron multiplicities after pion capture in181Ta and209Bi

The absorption of stoppedπ ^? in¹⁸¹Ta and²⁰⁹Bi has been investigated by studying prompt and delayedγ-ray spectra. Absolute cross-sections for the yield of isotopes per capturedπ ^? in (π ^?, xn) reactions, as well as the relative probability of populating nuclear states of different spins have been measured for the hafnium and lead isotopes, respectively. A spin as high as 20 has been observed in the production of²⁰⁴Pb. The ground-state rotational bands of the hafnium isotopes are excited to spin values up to 16. Neutron multiplicities as large as 15 have been observed for both targets. A neutron multiplicity of ?8 is most probable for both tantalum and bismuth targets. The strong interaction monopole energy shift? ₀ and widthΓ ⁰ for the 4f level are found to beε ₀(¹⁸¹Ta)=540±100eV; ?₀(²⁰⁹Bi)=1790±150 eV;Γ ₀(¹⁸¹Ta)=225±57 eV;Γ ₀(²⁰⁹Bi) =1166±70 eV. The quadrupole moments, determined from the hyperfine splitting of the 4f pionic atom level, areQ=3.30±0.06b andQ=}-0.50±0.08b for¹⁸¹Ta and²⁰⁹Bi, respectively.     

A dependence of inclusive pion double charge exchange

The A dependence of the forward cross section for inclusive pion double charge exchange on nine target nuclei from ⁶Li to ²⁰⁹Bi at T₀ = 0.59 GeV, as well as the cross section for ⁶Li, ⁷Li, and ¹²C nuclei at T₀ = 0.59, 0.75, and 1.1 GeV, was measured with the 3-m magnetic spectrometer of the Institute of Theoretical and Experimental Physics (ITEP, Moscow). The resulting A dependence is well described within the model involving two sequential single charge exchanges and taking into account the renormalization of the amplitude for pion single charge exchange in a nucleus. A relatively weak energy dependence of the cross section for the ⁶Li, ⁷Li, and ¹²C nuclei agrees with the analogous dependence obtained previously for the ¹⁶O nucleus, but it contradicts the predicted sharp decrease in the cross section within the model involving two sequential single charge exchanges. This result provides an additional piece of evidence that the contribution from the mechanism of inelastic Glauber rescattering is significant at T₀ ? 0.6 GeV.     

Fission and particle evaporation following stopped-negative-pion absorption in 209Bi

Fission and particle evaporation following stopped- π⁻ absorption were measured for ²⁰⁹Bi. The fission probability per captured π⁻ was obtained as W_f = (3.01 ± 0.34) × 10⁻³ from the number of fission tracks observed in the mica track detector and that of π⁻ stopped in the Bi target. The latter was derived from the observed absolute intensities of pionic Bi X-rays. The isotopic yield distributions for the (π⁻, xn) and (π⁻, pxn) reactions were also determined by in-beam and activation γ-ray measurements. From the analysis of the fission probability and the product-yield distribution, it was concluded that the thermally equilibrated nuclear states have a wide energy distribution and rather low excitation energies (∼50 MeV) compared to the initially involved energy of 140 MeV. A model was proposed for the formation and decay of ²⁰⁹Bi nuclei excited by the capture of the stopped π⁻. The model well reproduces the experimental results.     

The γ-decay of two-particle states in 210Bi

The γ-decay of excited states in ²¹⁰Bi up to 3330 keV excitation energy has been studied using the ²⁰⁹Bi(d, pγ)²¹⁰Bi reaction. From the observed decay scheme, tentative J^π values are made for 25 states. The excitation energies, (d, p) strengths and decay branching ratios of these states are compared with those calculated using the two-particle shell-model wave functions of Kim and Rasmussen and Kuo and Herling.     

Effect of neutron transfer channels in fusion reactions with weakly bound nuclei at subbarrier energies

The role of neutron transfer in fusion reactions of weakly-bound nuclei at subbarrier energies is studied within the empirical model of channel coupling. The results from calculating the fusion cross sections for the ⁷Li + ²⁰⁹Bi, ^{9, 11}Li + ^{208, 206}Pb, ^{6, 7, 9, 11}Li + ¹⁵²Sm reactions are presented. Good agreement with the available experimental data is shown. Several combinations of colliding nuclei for which the strong enhancement of subbarrier fusion due to the effect of neutron transfer processes are predicted.     

Above-barrier enhancement of fusion in the 6He + 209Bi nuclear reaction

The fission cross sections for the ⁶He + ²⁰⁹Bi reaction as functions of the ⁶He-beam energy are measured. The experimental excitation functions for the reaction ²⁰⁹Bi(⁶He, 4n)²¹¹ At are also presented. The ⁶He secondary ion beam is obtained on the basis of the extracted-beam transport system of the U400-M accelerator (the Q4DQ spectrometer). A comparison of the experimental data obtained with available results for the ⁶He + ²⁰⁹Bi reaction shows that a pronounced enhancement of the fission cross sections in the above-barrier energy region is observed in the case of the reaction with the ⁶He beam. In order to fit the results of theoretical calculations to the experimental data, it is necessary to reduce the Coulomb barrier by 15% (20%). This corresponds to an increase of 1.5 (1.6) fm in the parameter r ₀ of the nuclear potential.     

Photoexcitation of 209Bi and natHg using neutron-capture γ-rays from cobalt

Using neutron-capture γ-rays from cobalt, the ²⁰⁹Bi(γ,γ') and ^natHg(γ,γ') reactions were studied using a 53 cm³ Ge(Li) detector and the reactor facility at the Rhode Island Nuclear Science Center. Resonant scattering was observed at 4.924±0.005 MeV for ^natHg. Resonant scattering was observed at 5.609±0.005 MeV in ²⁰⁹Bi, with an effective cross section of 348±70 mb. The ratio of intensity scattered at 150° to that scattered at 100° was found to be 1.18±0.12 for ²⁰⁹Bi. This is consistent with a dipole transition to a $\text{9}\text{2}$ state but does not eliminate other transitions and spin assignments. No significant inelastic structure was found in the spectra from either sample. The improved energy specification in the present experiments has led to different energy assignments for the resonant states and hence, different incident intensities are producing the observed scattering. These differences lead to higher effective cross sections than those previously reported. Minimum ground state level widths for ²⁰⁹Bi and ^natHg were estimated to be 0.032 eV and 0.43 eV respectively. The need for high-resolution survey data to re-establish systematics is suggested.     

Fission of209Bi induced by6He ions

Secondary beams of⁴He and⁶He were produced through the transfer reactions with the 18.5 MeV/u¹¹B and 35.5 MeV/u⁷Li primary beams. The fusion-fission cross sections have been measured for the^4,6He +²⁰⁹Bi reactions at energies E_cm>1.5*B_fus. In the present experiment it was found that the fission excitation function for the neutron rich nuclei⁶He is significantly higher than for the⁴He nuclei.     

Experimental study of excitation functions and isomer ratios for 211, 212Po

Excitation functions have been measured for ground states and isomers populated in the ²⁰⁸Pb(α, n)²¹¹Po, ²⁰⁹Bi(α, np)²¹¹Po, ²⁰⁹Bi(α, p)²¹²Po and ²⁰⁹Bi(α, n)²¹²At reactions for α-beams ranging from 45 MeV to 172.5 MeV. The experimental results have been compared with theoretical calculations assuming direct and preequilibrium reaction mechanisms, respectively. It is found that the experimental excitation functions can be reproduced satisfactorily by calculations in the framework of the preequilibrium model. Isomer ratios have been extracted from the data as well. Their energy dependence can be reproduced by an optical-model calculation for beam energies larger than about 60 MeV.     

Partial fusion of a weakly bound projectile with heavy target at energies above the Coulomb barrier

Partial-fusion cross-sections for the systems ⁶Li + ²⁰⁸Pb, ⁹Be + ²⁰⁹Bi have been determined. The effect of breakup on fusion for weakly bound projectiles ⁶Li and ⁹Be incident on ²⁰⁸Pb or ²⁰⁹Bi targets has been discussed comparing experimental fusion cross-section excitation functions to those evaluated with a semi-classical approach. It is shown that complete fusion of a weakly bound projectile with heavy target is reduced, whereas the breakup process has very little influence on the total-fusion cross-section for some of the studied systems at energies above the Coulomb barrier.     

Search for the (n th, α) reaction on180Hf,181Ta and209Bi

The (n _th,α) reaction on¹⁸⁰Hf,¹⁸¹Ta and²⁰⁹Bi was studied at the thermal neutron guide of the Grenoble high flux reactor. Upper limits ofσ _α ≦13μb, ≦μb and ≦0.3μb, respectively, were obtained for the a branches to the ground states in contrast to the values of ≈100μb to ≈500 mb reported in the literature. Possible reasons for these discrepancies are discussed. Furthermore, the present upper limits of the cross sections are compared with the values calculated from the statistical model.     

Excitation function for the production of 265108 and 266109

The excitation function for the production of²⁶⁶109 by the reaction ⁵⁸Fe + ²⁰⁹Bi was investigated. Its position and width could be reproduced using the same values (E* = 13.9 MeV; FWHM=4.8 MeV) as inthe case of²⁶⁵108 produced by the reaction ⁵⁸Fe + ²⁰⁸Pb. A cross-section maximum of (7.5 ± 2.7) pb was obtained. Referred to ²⁶⁵108, this value is reduced by a factor of 8.8 ± 3.3, that is more than the average reduction of cross-section by 3.7 per element. The hindrance factor of 2.4 ± 0.9 could be due to a specialization energy in the fusion process by the unpaired proton of ²⁰⁹Bi in the h_9/2 orbital. A total of twelve decay chains of ²⁶⁶109 was measured, resulting in a mean half-life of ( _1.7?0.4 ^+0.6 ) ms. The new data confirm the decay data of the previously measured two decays of ²⁶⁶109.     

Experimental investigation of highly excited states of the 5,6He and 5,6Li nuclei in the (6Li, 7Be) and (6Li, 7Li) one-nucleon pickup reactions

The (⁶Li, ⁷Be) and (⁶Li, ⁷Li) reactions on ⁶Li and ⁷Li nuclei were investigated in the angular interval 0°–20° in the laboratory system at a ⁶Li energy of 93 MeV. In addition to low-lying states of the ^5,6He and ^5,6Li nuclei, broad structures were observed near the t(³He)+d and t(³He)+t thresholds at the excitation energies of 16.75 (3/2⁺) and ～20 MeV (for ⁵He), 16.66 (3/2⁺) and ～20 MeV (⁵Li), 14.0 and 25 MeV (⁶He), and ～20 MeV (⁶Li). The angular distributions measured in the ⁷Li(⁶Li, ⁷Be)⁶He reaction for transitions to the ground state (0⁺) and excited states at E _x=1.8 MeV (2⁺) and 14.0 MeV of the ⁶He nucleus were analyzed by the finite-range distorted-wave method assuming the 1p-and 1s-proton pickup mechanism. The (⁶Li, ⁷Be) and (⁶Li, ⁷Li) reactions were shown to proceed predominantly through the one-step pickup mechanism, and the broad structures observed at high excitation energies are considered as quasimolecular states of the t(³He)+d and t(³He)+t types.     

Hyperfine spectrum of207Bi by absorption spectroscopy: Isotope shift systematics in heavy elements

The hyperfine spectrum of 38?y ²⁰⁷Bi in the 6p ^{3 4} S _3/2?6p ²(³ P ₀)7s ⁴ P _1/2 3,067-Å resonance line was measured by absorption spectroscopy. The²⁰⁷Bi?²⁰⁹Bi isotope shift is 0.0999(20) cm^?1. The relative isotope shifts in bismuth are found to follow the general pattern of the even-neutron number isotopes for the isotones withZ=80, 81, 82 and 87. The magnetic dipole and electric quadrupole hfs interaction constants for²⁰⁷Bi,A(⁴ P _1/2) =0.1630(3) cm^?1 andB(⁴ S _3/2)=?0.016(3) cm^?1, were obtained. Except for the isotope shift, these more precise values are in agreement with our previous results.     

Fusion-fission reactions induced by 6He-ions

The experimentally measured excitation functions for the fission and 4n evaporation channels are presented for the ⁶He + ²⁰⁹Bi reaction. The secondary ⁶He beam was produced using the special beam line (Q4DQ-spectrometer) of the U400M accelerator at FLNR, JINR. The comparison of the obtained experimental data with similar results for the ⁴He + ²⁰⁹Bi reaction shows that in the case of the ⁶He + ²⁰⁹Bi reaction a significant enhancement of the cross-section is observed for energies above the barrier. In order to get an agreement between the experimental data and the theoretical calculations it is necessary to reduce the Coulomb barrier by 15-20% , which corresponds to an increase of the parameter r₀ of the nuclear potential up to 1.5-1.6 fm. Received: 1 May 2001 / Accepted: 4 December 2001     

7 MeV dipole ground state transitions in 207Pb, 208Pb and 209Bi

The Doppler-broadened 7.117 MeV line from the ¹⁹F(p, αγ)¹⁶O reaction has been used to resonantly excite levels in ²⁰⁸Pb at 7071±2 and 7091±2 keV, in ²⁰⁷Pb at 7186±5 and 7206±5 keV and in ²⁰⁹Bi at 7179±5 and 7202±5 keV. On the basis of angular distribution measurements the 7071 and 7091 keV levels of ²⁰⁸Pb are assigned spins of 1 and assuming 100% ground state branching the widths are calculated to be 31±3 and 17±2 eV respectively. It is suggested that the states in ²⁰⁷Pb and ²⁰⁹Bi arise from the weak coupling of a $\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ neutron hole and an $\text{h}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ proton respectively to one or the other of the ²⁰⁸Pb levels. The widths obtained in terms of _gΓ_o²/Γ are 15±4 and 25±5 eV for the 7186 and 7206 keV levels of ²⁰⁷Pb and 24±5 and 30±5 for the levels of ²⁰⁹Bi respectively. These values are consistent with the weak-coupling suggestion.