Beams In Inverse Kinematics Research Articles

Probing the isospin dependence of the in-medium nucleon-nucleon cross sections with radioactive beams

Within a transport model we search for potential probes of the isospin dependence of the in-medium nucleon-nucleon (NN) cross sections. Traditional measures of the nuclear stopping power are found sensitive to the magnitude but they are ambiguous for determining the isospin dependence of the in-medium NN cross sections. It is shown that isospin tracers, such as the neutron/proton ratio of free nucleons, at backward rapidities/angles in nuclear reactions induced by radioactive beams in inverse kinematics is a sensitive probe of the isospin dependence of the in-medium NN cross sections. At forward rapidities/angles, on the other hand, they are more sensitive to the density dependence of the symmetry energy. Measurements of the rapidity/angular dependence of the isospin transport in nuclear reactions will enable a better understanding of the isospin dependence of in-medium nuclear effective interactions.

Evidence for [formula omitted] core excitation from g factor and B(E2) measurements on the 21+ states of [formula omitted

The g factors of the 21+ states in 42,44Ca have been measured employing Coulomb excitation of isotopically pure calcium beams in inverse kinematics combined with the transient field technique. In addition, the lifetimes were redetermined using the Doppler-shift-attenuation method. While the g factor of 44Ca(21+) has definitely a positive value, g=+0.17(3), for 42Ca a vanishingly small g factor, g(21+)=+0.04(6), was observed in a very first measurement. These are striking results since one expects for an assumed inert 40Ca core with two or four valence neutrons in the fp shell a negative g factor, g≃−0.4, in accordance with experimental data for the 7/2− ground states of the odd neighbouring isotopes. Therefore, the new experimental findings clearly require substantial admixtures of configurations from sd shell core excitations in the wave functions of the nuclear states. A “co-existence” picture, along the lines of Gerace and Green, suggests core-deformed components in the 2+1 state of 55(6)% and 66(4)% in 42Ca and 44Ca, respectively. A separate large scale shell model calculation in the (sd−fp) shell space results in good overall agreement with the experimental data.

A model for consecutive spallation and fragmentation reactions in inverse kinematics at relativistic energies

Secondary reactions induced by relativistic beams in inverse kinematics in a thick target are relevant in several fields of experimental physics and technology, like secondary radioactive beams, production of exotic nuclei close to the proton drip line, and cross-section measurements for applications of spallation reactions for energy production and incineration of nuclear wastes. A general mathematical formulation is presented and successively applied as a tool to disentangle the primary reaction yields from the secondary production in the measurement of fission of a 238U projectile impinging on a proton target at the energy of 1 A GeV.

First measurement and shell model interpretation of the g factor of the 21+ state in self-conjugate radioactive 44Ti

The g factor of the 21+ state in radioactive 44Ti has been measured for the first time with the technique of α transfer to 40Ca beams in inverse kinematics in combination with transient magnetic fields, yielding the value, g(21+)=+0.52(15). In addition, the lifetimes of the 21+, τ=3.97(28) ps, and the 41+ states, τ=0.65(6) ps, were redetermined with higher precision using the Doppler shift attenuation method. The deduced B(E2)'s and the g factor were well explained by a full fp shell model calculation using the FPD6 effective NN interaction. The g factor can also be accounted for by a simple rotational model (g=Z/A). However, if one also considers the B(E2)'s and the E(41+)/E(21+) ratios, then an imperfect vibrator picture gives better agreement with the data.

Probing the halo structure of exotic nuclei by direct reactions with radioactive beams

The investigation of light-ion induced direct reactions using radioactive beams in inverse kinematics gives access to a wide field of nuclear structure studies in the region far off stability. The present contribution is mainly focused on the investigation of light neutron-rich halo nuclei. The method of proton-nucleus elastic scattering at intermediate energies is demonstrated to be an effective means for obtaining accurate and detailed information on the size and radial shape of halo nuclei. Differential cross sections for small-angle scattering were measured at energies near 700 MeV/u for the neutron-rich lithium isotopes 8Li, 9Li and 11Li. The experimental concept and the procedure of the data analysis in terms of the Glauber multiple scattering theory are discussed and the results on the nuclear matter radii, the matter distributions, and the significance of the data for a halo structure are presented. In a very recent experiment data on p 6,8He scattering at higher momentum transfer, covering the region of the first diffraction minimum, were taken. Such data are expected to give more detailed information on the structure of the α-like core in these halo nuclei. First results are presented.

Competition between proton and neutron excitations in 96Zr

The g factors of the 2+1 and 3−1 states in 96Zr have been measured for the first time by Coulomb excitation of 96Zr beams in inverse kinematics using transient magnetic fields. The resulting g factors are g(2+1)=+0.03(7) and g(3−1)=+0.98(15). The lifetime of the 2+1 state, τ=0.82(10) ps, was redetermined via the Doppler Shift Attenuation Method. The 2+1→0+1 transition rate and the g-factor data are in fair agreement with shell-model calculations involving both proton and neutron excitations. The pronounced octupole strength of the 3−1→0+1 transition is not seen in the g factor which exceeds significantly an expected Z/A value.

Nuclear Structure Studies by Direct Reaction Experiments with Stored Radioactive Beams

The experimental conditions at the next generation facilities for stored radioactive ion beams presently proposed or under construction will provide unique opportunities for nuclear structure studies on nuclei far off stability and will enable to explore completely new and unknown regions in the chart of nuclides. In particular, the predicted luminosities will allow for the investigation of direct reactions with stored and cooled radioactive beams in inverse kinematics using internal H, He, etc. targets. Such light-ion induced direct reactions, as for example elastic and inelastic scattering, transfer, charge exchange, or knock-out reactions, have been proved in the past, for the case of stable nuclei, to be powerful tools for obtaining nuclear structure information, and were also applied within the last decade for the investigation of light exotic nuclei with radioactive beams bombarding external targets.A brief overview on the physics program planned at the future NESR storage ring at GSI, the physics questions to be addressed, the dedicated reactions to be used, and the regions of interest in the chart of nuclides is given. The advantage of using stored beams and internal targets for such investigations will depend on the specific exotic nuclei and reactions chosen. This is discussed by a comparison of the conditions for the application of internal and external targets for reaction experiments with exotic beams.For the special case of elastic proton scattering at intermediate energies a feasibility study is presented, including a discussion of the experimental scenario. This method was applied recently for the investigation of nuclear matter distributions of the light neutron-rich halo nuclei 6,8He and 11Li using external targets. A few examples of the results are briefly discussed. The use of internal targets and stored beams will allow in future to extend such investigations to a wide range of medium heavy and heavy nuclei far off stability, and will therefore be one of the key reactions of the experimental program at the future NESR facility.

Development of detection systems for low-energy heavy ions at DRAGON

The new DRAGON facility at TRIUMF is designed to measure alpha and proton capture reactions with radioactive ion beams in inverse kinematics. For nucleo-synthesis in astrophysical scenarios, the relevant energies lie in the 0.15–1 MeV/u range, where very low cross sections are expected. Therefore the separation of the recoil products from the beam particles will be a difficult task. This paper focuses on the end detectors, which will be used to distinguish recoils from beam particles at the end of the DRAGON separator.

First measurements of the g factors of the 2 +1 and 3 −1 states of semi-magic 9040Zr

The g factors of the 2 + 1 and 3 − 1 states in semi-magic 90Zr have been measured for the first time with the technique of Coulomb excitation of 90Zr beams in inverse kinematics in combination with transient magnetic fields. The resulting g factors, g(2 + 1)=+1.25(21) and g(3 − 1)=+0.986(56), are both consistent with a dominant g 9/2 proton configuration and a closed N=50 neutron shell. The lifetime of the 2 + 1 state was measured by the Doppler shift attenuation method, τ=0.126(3) ps.

Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE

As a tool for studying the structure of nuclei far off stability the technique of gamma-ray spectroscopy after low-energy single-nucleon transfer reactions with radioactive nuclear beams in inverse kinematics was investigated. Modules of the MINIBALL germanium array and a thin position-sensitive parallel plate avalanche counter (PPAC) to be employed in future experiments at REX-ISOLDE were used in a test experiment performed with a stable 36S beam on deuteron and 9Be targets. It is demonstrated that the Doppler broadening of gamma lines detected by the MINIBALL modules is considerably reduced by exploiting their segmentation, and that for beam intensities up to 10^6 particles/s the PPAC positioned around zero degrees with respect to the beam axis allows not only to significantly reduce the gamma background by requiring coincidences with the transfer products but also to control the beam and its intensity by single particle counting. The predicted large neutron pickup cross sections of neutron-rich light nuclei on 2H and 9Be targets at REX-ISOLDE energies of 2.2 MeV A are confirmed.

Study of resonant reactions with radioactive ion beams

A fast and efficient method to study (p,p) and (p,α) resonances with radioactive beams in inverse kinematics is described. It is based on the use of thick targets and large area double-sided silicon strip detectors (DSSDs) to detect the recoiling light-charged particles and to determine precisely their scattering angle. The first nuclear physics experiments with the technique have been performed recently at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge with stable beams of 17 O and radioactive beams of 17 F . The high-quality resonance measurements obtained demonstrate the capabilities of the technique. Pure 17 F beams from HRIBF were produced by fully stripping the ions and separating the interfering and more abundant 17 O ions by the beam transport system. The removal of interfering isobars is one of the various common challenges to both accelerator mass spectrometry (AMS) and radioactive ion beam (RIB) production. Experiments done with RIBs will benefit from the use of the most efficient techniques for production, isobar separation, transport and detection.

Spin dependence of d5/2 neutron configurations in the wave functions of 92,94Zr from g-factor measurements

g factors of the 2 + 1 and 4 + 1 states in 92,94Zr have been measured via Coulomb excitation of isotopically pure 92,94Zr beams in inverse kinematics in combination with the transient field technique. The high precision results, g( 92Zr; 2 + 1) = −0.180(10) and g( 94Zr; 2 + 1) = −0.329(15) as well as the new data on the 4 + 1 state, g( 92Zr; 4 + 1) = −0.50(11), provide, for the first time, clear evidence of the dominant role of d 5/2 neutrons in the wave functions of these nuclei just above the N=50 shell closure. Moreover, the data imply increased purity of the d 5/2 neutron component in the wave function with increasing spin.

Nuclear structure studies on halo nuclei by direct reactions with radioactive beams

The investigation of direct reactions with exotic beams in inverse kinematics gives access to a wide field of nuclear structure studies in the region far off stability. The basic concept and the methods involved are briefly discussed. The present contribution will focus on the investigation of light neutron-rich halo nuclei. Such nuclei reveal a new type of nuclear structure, namely an extended neutron distribution surrounding a nuclear core. An overview on this phenomenon, and on the various methods which gave first evidence and qualitative confirmation of our present picture of halo nuclei, is given. To obtain more quantitative information on the radial shape of halo nuclei, elastic proton scattering on neutron-rich light nuclei at intermediate energies was recently investigated for the first time. This method is demonstrated to be an effective means for studying the nuclear matter distributions of such nuclei. The results on the nuclear matter radii of 6He and 8He, the deduced nuclear matter density distributions, and the significance of the data on the halo structure is discussed. The present data allow also a sensitive test of theoretical model calculations on the structure of neutron-rich helium isotopes. A few examples are presented. The investigation of few-nucleon transfer reactions in inverse kinematics may provide new and complementary information on nuclear structure, as well as astrophysical questions. The physics motivation and the experimental concept for such experiments, to be performed due to momentum matching reasons at low incident energies around 5–20 MeV/u at the new generation low energy radioactive beam facilities SPIRAL, PIAFE, etc., is briefly discussed.

Coulomb excitation of radioactive nuclear beams in inverse kinematics

Two techniques for determining B( E2:0 1 + → 2 1 +) values using low-energy Coulomb excitation of Radioactive Nuclear Beams in inverse kinematics are presented. One method, applicable if the lifetime of the excited state is ∼1 ns, uses a linear array of NaI(Tl) detectors to measure the decay curve, and thus the 2 1 + state lifetime, for Coulomb excited radioactive beam nuclei as they decay in flight downstream from a carbon excitation target. In the second method, useful when lifetimes are short, <1 ns, or for low intensity beams, a through-well NaI(Tl) detector is used to measure the Coulomb excitation cross section from which the B( E2:0 1 + → 2 1 +) value is calculated. The results of test experiments with beams of stable Os isotopes are presented.