Search Results (86 total)

Bound excited states in the neutron-deficient nucleus ²¹Mg were probed in the one-neutron knockout reaction ⁹Be(²²Mg,²¹Mg+γ)X at 74 MeV/nucleon projectile energy. The low-lying level scheme of ²¹Mg was investigated for the first time with γ-ray spectroscopy. Contrary to the interpretation of previous particle spectroscopy data, our proposed excitation scheme is in agreement with the mirror nucleus and shell-model calculations in the p-sd shell. Spectroscopic factors for the one-neutron removal from ²²Mg to ²¹Mg are extracted and compared to shell-model calculations using the WBP effective interaction.

In-beam γ-ray spectroscopy of ^66,68Fe following intermediate-energy one- and two-proton knockout from cobalt and nickel secondary beams has been performed at the National Superconducting Cyclotron Laboratory. New transitions have been observed in ⁶⁶Fe and ⁶⁸Fe. This is the first observation of γ-ray transitions in ⁶⁸Fe. In addition, ⁶⁴Cr was produced using the ⁹Be(⁶⁶Fe,⁶⁴Cr)X two-proton knockout reaction. An unexpectedly low inclusive cross section is observed for this reaction, an order of magnitude smaller than for the ⁹Be(⁶⁸Ni,⁶⁶Fe)X reaction. This observation is discussed in terms of a significant structural difference between ⁶⁶Fe and ⁶⁴Cr and considerable admixtures of ν(pf)^n-2(g_9/2)⁺² configurations in the ground and excited states of ⁶⁴Cr at N=40.

The neutron unbound ground state of ²⁵O (Z=8, N=17) was observed for the first time in a proton knockout reaction from a ²⁶F beam. A single resonance was found in the invariant mass spectrum corresponding to a neutron decay energy of 770_-10⁺²⁰  keV with a total width of 172(30) keV. The N=16 shell gap was established to be 4.86(13) MeV by the energy difference between the ν1s_1/2 and ν0d_3/2 orbitals. The neutron separation energies for ²⁵O agree with the calculations of the universal sd shell model interaction. This interaction incorrectly predicts an ²⁶O ground state that is bound to two-neutron decay by 1 MeV, leading to a discrepancy between the theoretical calculations and experiment as to the particle stability of ²⁶O. The observed decay width was found to be on the order of a factor of 2 larger than the calculated single-particle width using a Woods-Saxon potential.

Both one-proton and one-neutron knockout reactions were performed with fast beams of two asymmetric, neutron-deficient rare isotopes produced by projectile fragmentation. The reactions are used to probe the nucleon spectroscopic strengths at both the weakly and strongly bound nucleon Fermi surfaces. The one-proton knockout reactions ⁹Be(²⁸S,²⁷P)X and ⁹Be(²⁴Si,²³Al)X probe the weakly bound valence proton states and the one-neutron knockout reactions and ⁹Be(²⁸S, ²⁷S)X and ⁹Be(²⁴Si, ²³Si)X the strongly bound neutron states in the two systems. The spectroscopic strengths are extracted from the measured cross sections by comparisons with an eikonal reaction theory. The reduction of the experimentally deduced spectroscopic strengths, relative to the predictions of shell-model calculations, is of order 0.8–0.9 in the removal of weakly bound protons and 0.3–0.4 in the knockout of the strongly bound neutrons. These results support previous studies at the extremes of nuclear binding and provide further evidence that in asymmetric nuclear systems the nucleons of the deficient species, at the more-bound Fermi surface are more strongly correlated than those of the more weakly bound excess species.

The breakdown of the N=20 magic number in the so-called island of inversion around ³²Mg is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal- and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of ³⁰Mg and ³²Mg, with the latter consisting nearly purely of 2p-2h N=20 cross-shell configurations. Single-neutron knockout experiments on ^30,32Mg projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions.

The breakup of E/A=50 MeV ¹²Be fragments following inelastic scattering off of hydrogen and carbon target nuclei has been studied. The breakup channels α+⁸He, ⁶He+⁶He, t+⁹Li, and p+¹¹Li were observed. Two doublets at excitation energies of 12.8 and 15.5 MeV were found for the α+⁸He channel. A low-energy shoulder in the excitation-energy spectra at 10.2 MeV indicates one or more additional states. This work could not confirm the presence of ⁶He-⁶He rotational structure reported by Freer [Phys. Rev. C 63, 034301 (2001)], although possible peaks at excitation energies of 13.5 and 14.5 MeV were found for ⁶He+⁶He decay. Significant structure is observed in the excitation-energy spectrum for p+¹¹Li at 25–30 MeV which maybe associated with T=3 analog states.

The possibility of studying particle-like states near the Fermi surfaces of exotic nuclei by using measurements of heavy-ion-induced single-nucleon pickup reactions with in-flight separated rare-isotope beams is discussed. The analysis of an exploratory data set for the intermediate-energy ⁹Be(²⁰Ne,²¹Na)X proton pickup reaction measured using a ²⁰Ne beam at 63 MeV per nucleon is reported. The data are compared with expectations based on model calculations of the transfer reaction cross sections and the ²¹Na residue spectroscopy prediction by the sd-shell model. The measured cross sections are broadly consistent with these expectations.

Rare isotope beams of neutron-deficient ^106,108,110Sn from the fragmentation of ¹²⁴Xe were employed in an intermediate-energy Coulomb excitation experiment. The measured B(E2,0₁⁺→2₁⁺) values for ¹⁰⁸Sn and ¹¹⁰Sn and the results obtained for the ¹⁰⁶Sn show that the transition strengths for these nuclei are larger than predicted by current state-of-the-art shell-model calculations. This discrepancy might be explained by contributions of the protons from within the Z=50 shell to the structure of low-energy excited states in this region.

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of ²³O at 2.8(1) MeV excitation energy which decays into the ground state of ²²O. This interpretation is consistent with theory. The reaction mechanism supports the assignment of the observed state as the 5/2⁺ hole state. This assignment and the recently observed 3/2⁺ particle state advance the understanding of ²³O.

We report on the first determination of the 2₁⁺ energy of ²⁰Mg, the most neutron-deficient Mg isotope known to exist. The result, E(2₁⁺)=1598(10) keV, obtained from in-beam γ-ray spectroscopy following the two-neutron removal from a ²²Mg secondary beam, is discussed in the framework of the isobaric mass multiplet equation (IMME). Resulting predictions for the excitation energies of the T=2,2⁺ states in the ²⁰F and ²⁰Na isobars are presented. The mirror energy difference, E(2₁⁺,²⁰Mg)-E(2₁⁺,²⁰O)=-77(10) keV, is compared to a recent prediction within the nuclear shell model based on the “USD^m - gap Z14<” modification of the universal sd (USD) effective interaction.

We report on the first spectroscopy study of the very neutron-rich nucleus ₁₂³⁶Mg₂₄ using the direct two-proton knockout reaction ⁹Be(³⁸Si,³⁶Mg+γ)X at 83  MeV/nucleon. The energy of the first excited 2⁺ state of ³⁶Mg, E(2₁⁺)=660(6)  keV, was measured. The magnitude of the partial cross sections to the ground state and the 2₁⁺ state is indicative of strong intruder admixtures in the lowest-lying states as suggested by Monte Carlo shell-model calculations.

Transition rate measurements are reported for the 2₁⁺ and 2₂⁺ states in N=Z ⁶⁴Ge. The experimental results are in excellent agreement with large-scale shell-model calculations applying the recently developed GXPF1A interactions. The measurement was done using the recoil distance method (RDM) and a unique combination of state-of-the-art instruments at the National Superconducting Cyclotron Laboratory (NSCL). States of interest were populated via an intermediate-energy single-neutron knockout reaction. RDM studies of knockout and fragmentation reaction products hold the promise of reaching far from stability and providing lifetime information for excited states in a wide range of nuclei.

The results of measurements of the production of neutron-rich nuclei by the fragmentation of a ⁴⁸Ca beam at 142 MeV/nucleon are presented. Evidence was found for the production of a new isotope that is the most neutron-rich silicon nuclide, ⁴⁴Si, in a net neutron pickup process. A simple systematic framework was found to describe the production cross sections based on thermal evaporation from excited prefragments that allows extrapolation to other weak reaction products.

The one-neutron knockout reaction ⁹Be(⁵⁷Cr,⁵⁶Cr+γ)X has been measured in inverse kinematics with an intermediate-energy beam. Cross sections to individual states in ⁵⁶Cr were partially untangled through the detection of the characteristic γ-ray transitions in coincidence with the reaction residues. The experimental inclusive longitudinal momentum distribution and the yields to individual states are compared to calculations that combine spectroscopic factors from the full fp shell model and nucleon-removal cross sections computed in a few-body eikonal approach.

Electron capture and β decay play important roles in the evolution of presupernovae stars and their eventual core collapse. These rates are normally predicted through shell-model calculations. Experimentally determined strength distributions from charge-exchange reactions are needed to test modern shell-model calculations. We report on the measurement of the Gamow-Teller strength distribution in ⁵⁸Co from the ⁵⁸Ni(t,³He) reaction with a secondary triton beam of an intensity of ~10⁶ pps at 115 MeV/nucleon and a resolution of ~250 keV. Previous measurements with the ⁵⁸Ni(n,p) and the ⁵⁸Ni(d,²He) reactions were inconsistent with each other. Our results support the latter. We also compare the results to predictions of large-scale shell-model calculations using the KB3G and GXPF1 interactions and investigate the impact of differences between the various experiments and theories in terms of the weak rates in the stellar environment. Finally, the systematic uncertainties in the normalization of the strength distribution extracted from ⁵⁸Ni(³He, t) are described and turn out to be nonnegligible due to large interferences between the ΔL=0,ΔS=1 Gamow-Teller amplitude and the ΔL=2,ΔS=1 amplitude.

The energy of the first excited state in the neutron-rich N=28 nucleus ⁴⁵Cl has been established via in-beam γ-ray spectroscopy following proton removal. This energy value completes the systematics of the E(1/2₁⁺)-E(3/2₁⁺) level spacing in odd-mass K, Cl, and P isotopes for N=20-28. The results are discussed in the framework of shell-model calculations in the sd-fp model space. The contribution of the central, spin-orbit, and tensor components is discussed from a calculation based on a proton single-hole spectrum from G-matrix and π+ρ meson exchange potentials. A composite model for the proton 0d_3/2-1s_1/2 single-particle energy shift is presented.

Excited states in ⁴⁰Si have been established by detecting γ rays coincident with inelastic scattering and nucleon removal reactions on a liquid hydrogen target. The low excitation energy, 986(5) keV, of the 2₁⁺ state provides evidence of a weakening in the N=28 shell closure in a neutron-rich nucleus devoid of deformation-driving proton collectivity.

Measurements of the N=28 isotones ⁴²Si, ⁴³P, and ⁴⁴S using one- and two-proton knockout reactions from the radioactive beam nuclei ⁴⁴S and ⁴⁶Ar are reported. The knockout reaction cross sections for populating ⁴²Si and ⁴³P and a 184 keV γ-ray observed in ⁴³P establish that the d_3/2 and s_1/2 proton orbits are nearly degenerate in these nuclei and that there is a substantial Z=14 subshell closure separating these two orbits from the d_5/2 orbit. The increase in the inclusive two-proton knockout cross section from ⁴²Si to ⁴⁴S demonstrates the importance of the availability of valence protons for determining the cross section. New calculations of the two-proton knockout reactions that include diffractive effects are presented. In addition, it is proposed that a search for the d_5/2 proton strength in ⁴³P via a higher statistics one-proton knockout experiment could help determine the size of the Z=14 closure.

The inclusive cross sections for 38 different reaction products produced in the interaction of ⁴⁸Ca, ⁴⁰S, and ⁴²S beams around 100 MeV/nucleon with a liquid deuterium target are reported. The cross sections for the ⁴⁸Ca +²H products are compared to those with ⁴⁸Ca incident on the commonly used fragmentation targets ⁹Be and ¹⁸¹Ta and also to global calculations for fragmentation reaction cross sections based on the EPAX parameterization. The sizes of the measured reaction cross sections for the deuterium target were comparable to those of the cross sections measured on the heavier targets, indicating that nucleon removal from a deuterium target can be carried out for single- and multiple-nucleon knockout reaction studies. It was also found that the charge exchange cross sections were large enough that it should be possible to obtain nuclear structure information from these reactions.

Two-neutron knockout reactions from nuclei in the proximity of the proton dripline have been studied using intermediate-energy beams of neutron-deficient ³⁴Ar, ³⁰S, and ²⁶Si. The inclusive cross sections, and also the partial cross sections for the population of individual bound final states of the ³²Ar, ²⁸S and ²⁴Si knockout residues, have been determined using the combination of particle and γ-ray spectroscopy. Similar to the two-proton knockout mechanism on the neutron-rich side of the nuclear chart, these two-neutron removal reactions from already neutron-deficient nuclei are also shown to be consistent with a direct reaction mechanism.

It is shown via a study on a ²⁶Mg target that the (t,³He) reaction at 115 MeV/nucleon reaction is an accurate probe for extracting Gamow-Teller transition strengths. To do so, the data are complemented by results from the ²⁶Mg(³He, t) reaction at 140 MeV/nucleon that allows for a comparison of T=2 analog states excited via the mirror reactions. Extracted Gamow-Teller strengths from ²⁶Mg(t,³He) and ²⁶Mg(³He, t) are compared with those from ²⁶Mg(d,²He) and ²⁶Mg(p,n) studies, respectively. A good correspondence is found, indicating probe independence of the strength extraction. Furthermore, we test shell-model calculations using the new USD-05B interaction in the sd-model space and show that it reproduces the experimental Gamow-Teller strength distributions well. In anticipation of further (t,³He) experiments on medium-heavy nuclei aimed at determining weak-interaction rates of relevance for stellar evolution, a second goal of this work is to improve the understanding of the (t,³He) and (³He, t) reaction mechanisms at intermediate energies because detailed studies are scarce. The distorted-wave Born approximation is employed, taking into account the composite structures of the ³He and triton particles. The reaction model provides the means to explain systematic uncertainties at the 10%–20% level in the extraction of Gamow-Teller strengths as being because of interference between Gamow-Teller ΔL=0,ΔS=1 and ΔL=2,ΔS=1 amplitudes that both contribute to transitions from 0⁺ to 1⁺ states.

The single-particle structure of ⁵⁷Ni and level structure of ⁵⁶Ni were investigated with the ⁹Be (⁵⁷Ni,⁵⁶Ni+γ)X reaction at 73 MeV/nucleon. An inclusive cross section of 41.4(12) mb was obtained for the reaction, compared to a theoretical prediction of 85.4 mb, hence only 48(2)% of the theoretical cross section is exhausted. This reduction in the observed spectroscopic strength is consistent with that found for lighter well-bound nuclei. One-neutron removal spectroscopic factors of 0.58(11) to the ground state and 3.7(2) to all excited states of ⁵⁶Ni were deduced.

The two-proton knockout reaction ⁹Be(⁵⁴Ti,⁵²Ca+γ) has been studied at 72 MeV/nucleon. Besides the strong feeding of the ⁵²Ca ground state, the only other sizeable cross section proceeds to a 3^- level at 3.9 MeV. There is no measurable direct yield to the first excited 2⁺ state at 2.6 MeV. The results illustrate the potential of such direct reactions for exploring cross-shell proton excitations in neutron-rich nuclei and confirms the doubly-magic nature of ⁵²Ca.

An exclusive measurement has been made of the Coulomb dissociation of the two-neutron halo nucleus ¹¹Li at 70  MeV/nucleon at RIKEN. Strong low-energy (soft) E1 excitation is observed, peaked at about E_x=0.6  MeV with B(E1)=1.42(18)  e² fm² for E_rel≤3  MeV, which was largely missed in previous measurements. This excitation represents the strongest E1 transition ever observed at such low excitation energies. The spectrum is reproduced well by a three-body model with a strong two-neutron correlation, which is further supported by the E1 non-energy-weighted cluster sum rule.