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.     

Levels of 209Po and 211Po populated in one-neutron stripping and pickup from 210Po

Energy levels of ²⁰⁹Po have been populated with the ²¹⁰Po(d, t)²⁰⁹Po and ²¹⁰Po(p, d)²⁰⁹Po reactions at bombarding energies of 17.0 and 17.8 MeV respectively. Fifteen levels were observed below 2.7 MeV of excitation. Energy levels of ²¹¹Po were populated with the ²¹⁰Po(d, p)²¹¹Po reaction, also at 17.0 MeV. Thirty-five levels, almost all new, were observed below 3.9 MeV of excitation. Comparison of experimental angular distributions with DWBA calculations allowed l-value assignments and extraction of spectroscopic factors for many levels. In ²⁰⁹Po the observed level structure is well described in terms of a simple particle-vibration coupling model. In ²¹¹Po the level structure is more complex and the simple model is not adequate.     

Variation of total kinetic energy and mass distributions in the fusion-fission reaction of 16O,19F + 209Bi at near barrier energies

Fission fragment mass and energy distributions and their correlations have been measured for the ¹⁶O and ¹⁹F + ²⁰⁹Bi reactions over a wide range of excitation energies ( E ^* = 30-50 MeV). It is observed that in the case of ¹⁶O + ²⁰⁹Bi reaction, the average total fragment kinetic energy, <TKE> is nearly independent of the bombarding energy. However, in the case of ¹⁹F + ²⁰⁹Bi reaction, the average total kinetic energy of the fission fragments shows a peaking behaviour near the barrier. The variation in <TKE> at near barrier energies in the ¹⁹F + ²⁰⁹Bi system seems to be correlated with corresponding strong variation in the variance of the fragment mass distribution. The present results may imply certain dynamical effects leading to compact scission configurations in the fission of ¹⁹F + ²⁰⁹Bi system at near barrier bombarding energies. Received: 9 April 2001 / Accepted: 26 May 2001     

Proton-hole states in Bi From the Po (t, α) reaction

The ²¹⁰Po(t, α)²⁰⁹Bi reaction has been studied at 20 MeV triton energy with an overall resolution of 20 keV FWHM. Absolute cross sections were established by a comparison of the (t, α) intensities to elastic triton scattering. The three lowest single-proton states in ²⁰⁹Bi were observed and spectroscopic factors were extracted from the measured cross sections by DWBA analysis. Starting at 2.43 MeV, six levels were strongly excited. These states have large 2p-1h proton components. The spectroscopic factors were extracted by a comparison of ²¹⁰Po(t, α) cross sections to ²⁰⁸Pb(t, α)²⁰⁷T1 single-proton-hole cross sections measured previously at the same bombarding energy. The observed states and their properties were analyzed within the framework of the particle- (or hole) vibration coupling model. Surface vibrational states in ²⁰⁸Pb coupled to single protons as well as the proton pair addition modes (states in ²¹⁰Po) coupled to a proton-hole were included in the analysis.     

Study of the out-of-plane emission of protons and light fragments in symmetric heavy-ion collisions

Midrapidity protons from²⁰⁹Bi+²⁰⁹Bi collisions were measured with the Kaon Spectrometer at SIS at incident energies of E _Lab /A=400, 700 and 1000 MeV. Additionally, light fragments were analysed at 400 MeV. We have investigated the azimuthal emission pattern of the particles relative to the reaction plane as function of transverse momentum, bombarding energy and impact parameter. We observe an enhanced emission of particles perpendicular to the reaction plane at all bombarding energies. The ratio of the number of particles emitted out-of-plane/in-plane increases strongly with the particles transverse momentum. The anisotropy decreases with increasing beam energy. Composite particles show a much stronger effect than protons.     

Target mass dependence of the average linear momentum transfer

The average transferred linear momentum in nuclear reactions induced by ³He, ⁴He, ¹⁴N, and ²⁰Ne projectiles on Co, Cu and Ag targets was determined by thick-target-thick-catcher recoil range techniques for bombarding energies between 10 AMeV and 90 AMeV. A comparison with results obtained from fission fragment angular correlations indicates a strong target mass dependence of the linear momentum transfer.     

Two-proton multiplets in Po

The ²⁰⁹Bi(α,t)²¹⁰Po reaction at 45 MeV has been used to study ²¹⁰Po. Many new levels have been observed and proton configurations are suggested for the lowest three multiplets.     

The γ-Decays of 210Po-Levels from the (3He, d*γγ)-Reaction

An in-beam experiment with the subcoulomb reaction ²⁰⁹Bi(³He, d^*γγ)²¹⁰Po at 20.5 MeV was performed with two Euroball Cluster detectors in Cologne. It closed the gap between the low energy levels of the level-scheme and the high energy levels found in ²⁰⁹Bi(³He, d)²¹⁰Po and ²⁰⁸Pb(⁴He, t)²¹⁰Po particle experiments. New branchings have been found and the (³He, d^*γγ) reaction below the coulomb-barrier has been used successfully. Received: 17 December 1998 / Revised version: 27 January 1999     

The (t,p) reaction on the even isotopes of neodymium

The Aldermaston multi-angle magnetic spectrograph and tandem Van de Graaff accelerator have been used to study the (t, p) reactions leading to the neodymium isotopes ^{144,146,148,150, 152}Nd bombarding energy of 13 MeV. The Q-values were measured for final states with excitation energies up to about 3 MeV.     

Gamma radiation from the209Bi(3He, 3nγ)209at reaction

Gamma-radiation from the reactions of 28 MeV and 26 MeV³He-ions on the²⁰⁹Bi target has been studied by means of Ge(Li) detectors. Individual observed prompt -ray transitions have been assigned to the corresponding reactions. The results of previous studies of the²⁰⁹At nucleus have been verified and completed. Two new levels at energies of 640 keV and 1270·5 keV have been introduced. Additionally, 15 new transitions have been observed which could belong to²⁰⁹At. The²⁰⁹At level structure is discussed in terms of nuclear models.     

On the fission width of isobaric analogue states

The experimental cross sections of the reactions ²⁰⁹Bi(p, f) and ²⁰⁶Pb(α, f) are analyzed to estimate the ratio Γ_f/Γ_tot for the IAS. From the comparison of the values Γ_f for the nuclear states with different isospins it follows that strong isospin effects exist in the nuclear fission process involving ²¹⁰Po.     

Fission fragment anisotropies for 19F +209Bi system

The fission fragment angular distributions have been measured for the system ¹⁹F +²⁰⁹Bi over a range of bombarding energies from 88.0 MeV to 125.6 MeV. The measured fission fragment anisotropies are in agreement with the saddle point statistical model calculations in the above energy range. Combining these data with those available for ¹¹B, ¹²C, ¹⁴N, ¹⁶O and ¹⁸O +²⁰⁹Bi, ²⁰⁸Pb systems, it is concluded that the spherical target plus projectile systems behave “normal” from near- to above – barrier energies. This observation is in contrast to the “anomalous” anisotropies exhibited by the deformed actinide target – projectile systems at near – barrier energies. Received: 5 May 1999 / Revised version: 25 July 1999     

Scattemng of 14.1 MeV neutrons from 209Bi

Differential cross sections for the elastic and inelastic scattering of 14.1 MeV neutrons from ²⁰⁹Bi have been measured with a time-of-flight system which had an energy resolution of 650 keV. For elastic scattering from ²⁰⁹Bi, an optical-model analysis gave the best-fit potential parameters. The absolute cross sections for excitation of collective (2.66 and 4.36 MeV) states are reproduced by the results of distorted-wave calculations under the assumption of a core (²⁰⁸Pb) excitation model using deformation parameters obtained from (p, p') reactions.     

Isotopic effect in the energy dependence of the total fission cross section of lead and 209Bi nuclei for protons with energies up to 1 GeV

The results from measurements of the total fission cross sections of ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb nuclei for protons with energies from 207 to 1000 MeV and a step of ≈100 MeV. These results complemented by our data for ²⁰⁹Bi and the data of the EXFOR world base allow determination of absolute and relative contributions of the isotopic effect.     

Nuclear structure of 216 Ra at high spin

High-spin states of ²¹⁶Ra ( $\emph{Z}=88$ , $\emph{N}=128$ ) have been investigated through ²⁰⁹Bi(¹⁰B, 3n) reaction at an incident beam energy of 55?MeV and ²⁰⁹Bi(¹¹B, 4n) reaction at incident beam energies ranging from 65 to 78?MeV. Based on ??? coincidence data, the level scheme for ²¹⁶Ra has been considerably extended up to ~ 33 $\hbar$ spin and 7.2 MeV excitation energy in the present experiment with placement of 28 new ??-transitions over what has been reported earlier. Tentative spin-parity assignments are done for the newly proposed levels on the basis of the DCO ratios corresponding to strong gates. Empirical shell model calculations were carried out to provide an understanding of the underlying nuclear structure.     

Neutron-proton final state interaction in the 3H(3He,pα)n reaction at low bombarding energies

The ³H(³He, pα)n reaction has been studied in a kinematically complete experiment at ³He bombarding energies between 0.31 and 2.80 MeV in order to detect a possible resonant energy dependence of the spin singlet (T = 1) neutron-proton final state interaction (FSI). The data in the region of low n-p relative energies have been analyzed in terms of the Watson-Migdal formalism. Only a slight relative increase of the ¹S₀ n-p FSI is observed with increasing bombarding energy. This result is not easily related to the strong isospin violations found in the two-body reaction ³H(³He, d)⁴He at similar ³He energies.     

Yrast and side bands in the odd-odd160, 162Tm nuclei

High-spin states in²⁰⁵Bi, populated in the²⁰⁵Tl(α, 4n) reaction with bombarding energies around 50 MeV have been studied usingγ-ray spectroscopic techniques. Spins and parities have been assigned on the basis of angular distribution andγ-ray linear polarization measurements. States with spins up to 39/2⁺ and excitation energies around 6 MeV have been identified. Calculations using the empirical shell model have been performed in order to assign configurations.     

Cross sections and thermonuclear reaction rates of proton-induced reactions on 37Cl

The yields of γ-rays from the reactions ³⁷Cl(p, γ)³⁸Ar and ³⁷Cl(p, αγ)³⁴S have been measured as a function of bombarding energy over the ranges 0.65–2.15 MeV and 1.25–2.15 MeV respectively, and the yield of neutrons from ³⁷Cl(p, n)³⁷Ar from threshold to 2.50 MeV. The results are compared with global statistical-model calculations and thermonuclear reaction rates are calculated for the temperature range 5 × 10⁸–10¹⁰ K. The significance of these thermonuclear reaction rates for stellar nucleosynthesis calculations is discussed.     

Measurement of photo-fission yields and photo-neutron cross-sections in 209Bi with 50 and 65 MeV bremsstrahlung

The photo-fission yields and photo-neutron cross-sections of ( $ \gamma$ , 3n) and ( $ \gamma$ , 4n) on ²⁰⁹Bi induced by 50 and 65MeV bremsstrahlung have been measured by using a recoil catcher and an off-line $ \gamma$ -ray spectrometric technique. The mass-yield distribution of fission products in ²⁰⁹Bi induced by bremsstrahlung photons from the present work and literature data in the energy range 28-85MeV is symmetric around 103 mass units. However, the full width at half maximum of the yields distribution increases from 19 mass units at 28-40MeV to 23 mass units at 85MeV. The ( $ \gamma$ , 3n) reaction cross-section in the 50MeV and the ( $ \gamma$ , 4n) reaction cross-section in the 50 and 65MeV bremsstrahlung-induced reaction of ²⁰⁹Bi were determined for the first time.     

Polarization of protons from the C(d,p)C reaction

The polarization of protons from the ¹²C(d, p)¹³C reaction has been measured for deuteron bombarding energies between 3.9 and 5.8 MeV. Angular distributions of the polarization for protons leaving ¹³C in its ground state have been measured for laboratory angles between 10° and 135° at several energies in the vicinity of a resonance at 4 MeV in order to determine the effect of that resonance on the polarization. Additional measurements were made at 0.4 MeV intervals throughout the energy range. These measurements demonstrate that the compound-nuclear resonances observed in the cross section do not significantly alter the polarization angular distributions except at back angles. The shape of the observed polarization angular distributions at all energies is quite similar to previous results obtained for deuteron bombarding energies up to 15 MeV.