Measurements of interaction cross sections and radii of He isotopes

Secondary beams of ³He, ⁴He, ⁶He, and ⁸He were produced through the projectile fragmentation of an 800 MeV/nucleon ¹¹B primary beam. Interaction cross sections (σ_I) of all He isotopes of 790 MeV/nucleon on Be, C, and Al targets were measured by a transmission-type experiment. The interaction nuclear radii of He isotopes R_I(He) = (σ_I/π)^1/2 − R _I(T) where R_I(T) is the radius of the target nucleus, have been deduced to be R_I(³He) = 1.59 ± 0.06 fm, R_I(⁴He) = 1.40 ± 0.05 fm, R_I(⁶He) = 2.21 ± 0.06 fm, and R_I(⁸He) = 2.52 ± 0.06 fm.     

Total charge-changing cross sections for neutron-deficient isotopes from58Ni fragmentation

At the projectile-fragment separator FRS of GSI, relativistic secondary beams of about 520 MeV/nucleon were produced by fragmentation of a primary beam of⁵⁸Ni at 650 MeV/nucleon in a beryllium target. By means of aΔE—Bρ—TOF measurement, the fragments have been identified and their charge-changing probabilities in targets of CH₂, C, Al, and Pb have been determined. We describe the results for the total charge-changing cross sections in this first paper, whereas a second article deals with the partial charge-changing cross sections. At the drip line, the measured charge-changing cross sections exhaust close to 100% of the total interaction cross sections as calculated with semiempirical models. The measurements at the proton drip line with low-Z targets indicate that only a very small increase of the cross sections may be observed, whereas the measurements with a lead target show that no significant increase of the total charge-changing cross sections is present which would be a hint for low-lying dipole strength. Our experimental data are compared to Glauber-type calculations.     

Charge-changing cross sections of the neutron-rich isotopes8,9,11Li

Total charge-changing cross sections have been measured for⁸Li on C and Pb targets, for⁹Li on C, Al, Cu, Sn and Pb targets, as well as for¹¹Li on C, Sn and Pb targets at about 80 MeV/nucleon. These data are compared to measured total reaction cross sections and Glauber-type calculations using Hartree-Fock density distributions. These comparisons allow to draw conclusions on the proton density distribution of the neutronrich lithium isotopes. The results show that even for the most exotic nucleus¹¹Li the proton distribution is only very weakly influenced by the long tail in the neutron density distribution already established in several experiments.     

Photoneutron cross sections for unstable neutron-rich oxygen isotopes.

The dipole response of stable and unstable neutron-rich oxygen nuclei of masses A = 17 to A = 22 has been investigated experimentally utilizing electromagnetic excitation in heavy-ion collisions at beam energies about 600 MeV/nucleon. A kinematically complete measurement of the neutron decay channel in inelastic scattering of the secondary beam projectiles from a Pb target was performed. Differential electromagnetic excitation cross sections d sigma/dE were derived up to 30 MeV excitation energy. In contrast to stable nuclei, the deduced dipole strength distribution appears to be strongly fragmented and systematically exhibits a considerable fraction of low-lying strength.     

Partial charge-changing cross sections for neutron-deficient isotopes from58Ni fragmentation

At the projectile-fragment separator FRS of GSI, relativistic secondary beams of about 520 MeV/nucleon were produced by fragmentation of a primary beam of⁵⁸Ni at 650 MeV/nucleon in a beryllium target. By means of aE—B—TOF measurement, the fragments were identified and their charge-changing probabilities in targets of (CH₂) _n , C, Al, and Pb placed at the exit of the FRS were determined. Whereas a first article dealt with the total charge-changing cross sections, we describe in this second article the element distributions of these secondary fragments, which are found to depend strongly on the isospin of the secondary projectile as well as on the target material. In the case of the lead target, the influence of the electromagnetic dissociation is clearly visible in the one-proton and two-proton removal channels. The preference for the formation of even-Z fragments is much more pronounced for exotic secondary projectiles than for projectiles close to stability. Calculations with a geometrical abrasion-ablation model allow to understand the global features of the experimental data. However, far from stability, the discrepancies between calculations and experimental data increase.On leave of absence from Institute of Experimental Physics, University of Warsaw, PL-00-681 Warszawa, Hoa 69, Poland     

Odd-even staggering in nuclear charge radii of neutron-rich europium isotopes

Nuclear magnetic dipole and electric quadrupole moments and changes in mean square charge radii for the neutron-rich^155–159Eu isotopes have been measured using resonance ionization spectroscopy at the IRIS facility. It has been found that the isotopes withN>92, unlike the isotopes with 89≦N≦92, have an ordinary character of odd-even staggering in nuclear charge radii. This means that the octupole deformation attributed previously to the europium nuclei around¹⁵⁴Eu does not display itself in the charge radii of heavier europium isotopes.     

Structure studies of neutron-rich beryllium and carbon isotopes

The structure of neutron-rich beryllium isotopes has been investigated using different heavy-ion induced transfer reactions. A strong sensitivity to the chosen core nucleus is found for the cores of ⁹Be and ¹⁰Be, respectively. With a ⁹Be core, molecular rotational bands up to high excitation energies are observed due to the pronounced 2α-cluster structure, whereas only few states at low excitation energies are populated with a ¹⁰Be core. For ¹¹Be a detailed investigation has been performed for the three states at 3.41 MeV, 3.89 MeV and 3.96 MeV, which resulted in the most probable spin-parity assignments 3/2⁺, 5/2^? and 3/2^?, respectively. Furthermore we have studied particlehole states of ¹⁶C using the ¹³C(¹²C,⁹C)¹⁶C reaction and found 14 previously unknown states.     

Scaling of inelastic differential cross sections

Scaling (in the average sense) of differential cross sections is generalised to include inelastic (non-diffractive) reactions. The “derivation” uses, as in earlier work, the Ascoli-Arzelà theorem. Data on π^?p→π⁰n, ηn, over large s and t ranges, are seen to scale very well.     

Nuclear moments and charge radii of neutron-rich Rb isotopes by fast-beam laser spectroscopy

A systematic study of nuclear moments and rms charge radii in Rb was made possible by extending high-resolution cw laser spectroscopy into the deep blue. Measurements of optical isotope shifts and hyperfine structure of fission-produced ^89?93Rb are reported.     

The total charge-changing cross sections and the partial cross sections of 56Fe fragmentation on Al,C and CH2 targets

The total charge-changing cross sections and the partial cross sections of ⁵⁶Fe fragmentation on polyethylene, carbon and aluminum targets at the highest energy of 496 A MeV are investigated using CR-39 plastic nuclear track detector. The total charge-changing cross-section for hydrogen target is calculated based on the results of polyethylene and carbon targets. It is found that the total charge-changing cross sections for fragmentation of ⁵⁶Fe on hydrogen, carbon and aluminum targets are independent on the beam energy in our studied energies, and are consistent with the predictions of Bradt–Peter semi-empirical formula, Nilsen parameterized formula, NUCFRG2 and QMSFRG theoretical simulation codes. The partial cross sections for projectile fragment production are independent on beam energy in our studied energies for each targets and do not show a significant even-odd effect.     

Production cross section of neutron-rich calcium isotopes in heavy ion collisions

Based on the isospin-dependent quantum molecular dynamics model along with the GEMINI model, heavy-ion collisions at intermediate energies are studied. We calculate the production cross sections of different fragments for reactions of ¹¹²Sn+¹¹²Sn and ¹²⁴Sn+¹²⁴Sn at different beam energies. The species and production cross sections of neutron-rich isotopes are generally dependent on the isospin of the system and the incident energies. The isotopes ⁴⁸Ca and ⁵⁴Ca are more productive for the neutron-rich system at 30 to 150 MeV/nucleon.     

Enhanced fusion-evaporation cross sections in neutron-rich 132Sn on 64Ni

Evaporation residue cross sections have been measured with neutron-rich radioactive 132Sn beams on 64Ni in the vicinity of the Coulomb barrier. The average beam intensity was 2 x 10(4) particles per second and the smallest cross section measured was less than 5 mb. Large sub-barrier fusion enhancement was observed. Coupled-channel calculations taking into account inelastic excitation significantly underpredict the measured cross sections below the barrier. The presence of several neutron transfer channels with large positive Q values suggests that multinucleon transfer may play an important role in enhancing the fusion of 132Sn and 64Ni.     

Measurement of interaction cross sections using isotope beams of Be and B and isospin dependence of the nuclear radii

Interaction radii for ^11,12,14Be and ^{8,12,13,14,15}B have been determined by the measurement of interaction cross sections at 790 MeV/nucleon. Effective root-mean-square (RMS) radii of nucleon distribution of these nuclei have been deduced using a Glauber-model calculation. Isospin dependence of nuclear radii are presented for isobars of mass numbers from 6 to 12, and compared with theoretical predictions by the droplet model and by a Hartree-Fock (HF) model. The HF model with the Skyrme-III potential, which includes a strong density-dependent force, reproduce the observed isospin dependence.     

Neutron scattering lengths and cross sections of the barium isotopes

Coherent neutron scattering lengths and total cross sections have been measured for barium compounds and on isotopically enriched samples for neutron energies from 0.5 meV up to 132 eV using different techniques. From the experimental data the following quantities were obtained:

the coherent scattering lengths (in fm) of Ba (5.07±0.03) and of the isotopes with mass numbers 130 (?3.6±0.6); 132 (7.8±0.3); 134 (5.7±0.1); 135 (4.66±0.10); 136 (4.90 ±0.08); 137 (6.82±0.10); and 138 (4.83±0.08).

the absorption cross sections σ_γ (at 0.025 eV in barn): of Ba (1.1±0.1) and of the isotopes: 130 (30±5); 136 (0.68±0.17); 137 (3.6±0.2); and 138 (0.27±0.14).

zero-energy scattering cross sections for Ba and the isotopes 136, 137 and 138. On the basis of these data, the isotopic- and spin-incoherent cross sections and the (s)-resonance contributions to the coherent scattering lengths have been determined and discussed.