arXiv subjects

D. Bazin

Publications and source records attributed to D. Bazin.

At least 145 records · Page 8Linked to original sources

arXiv2007

Shape and structure of N=Z 64Ge; Electromagnetic transition rates from the application of the Recoil Distance Method to knock-out reaction

Transition rate measurements are reported for the first and the second 2+ states in N=Z 64Ge. The experimental results are in excellent agreement with large-scale Shell Model calculations applying the recently developed GXPF1A interactions. Theoretical analysis suggests that 64Ge is a collective gamma-soft anharmonic vibrator. 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 knock-out reaction. RDM studies of knock-out 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.

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arXiv2006

Z=50 shell gap near $^{100}$Sn from intermediate-energy Coulomb excitations in even-mass $^{106--112}$Sn isotopes

Rare isotope beams of neutron-deficient $^{106,108,110}$Sn nuclei from the fragmentation of $^{124}$Xe were employed in an intermediate-energy Coulomb excitation experiment yielding $B(E2, 0^+_1 \to 2^+_1)$ transition strengths. The results indicate that these $B(E2,0^+_1 \to 2^+_1)$ values are much larger than predicted by current state-of-the-art shell model calculations. This discrepancy can be explained if protons from within the Z = 50 shell are contributing to the structure of low-energy excited states in this region. Such contributions imply a breaking of the doubly-magic $^{100}$Sn core in the light Sn isotopes.

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arXiv2006

One-neutron knockout in the vicinity of the N=32 sub-shell closure: 9Be(57Cr,56Cr+ gamma)X

The one-neutron knockout reaction 9Be(57Cr,56Cr + gamma)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 gamma-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.

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arXiv2006

Measurement of excited states in 40Si and evidence for weakening of the N=28 shell gap

Excited states in 40Si have been established by detecting gamma-rays coincident with inelastic scattering and nucleon removal reactions on a liquid hydrogen target. The low excitation energy, 986(5) keV, of the 2+[1] state provides evidence of a weakening in the N=28 shell closure in a neutron-rich nucleus devoid of deformation-driving proton collectivity.

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arXiv2006

Shell structure at N=28 near the dripline: spectroscopy of $^{42}$Si, $^{43}$P and $^{44}$S

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 gamma-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.

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arXiv2006

Evolution of the $E(1/2^+_1)-E(3/2^+_1)$ energy spacing in odd-mass K, Cl and P isotopes for $N=20-28$

The energy of the first excited state in the neutron-rich N=28 nucleus 45Cl has been established via in-beam gamma-ray spectroscopy following proton removal. This energy value completes the systematics of the E(1/2^+_1)-E(3/2^+_1) 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 pi + rho meson exchange potentials. A composite model for the proton 0d_{3/2}-1s_{1/2} single-particle energy shift is presented.

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arXiv2006

Two-neutron knockout from neutron-deficient $^{34}$Ar, $^{30}$S, and $^{26}$Si

Two-neutron knockout reactions from nuclei in the proximity of the proton dripline have been studied using intermediate-energy beams of neutron-deficient $^{34}$Ar, $^{30}$S, and $^{26}$Si. The inclusive cross sections, and also the partial cross sections for the population of individual bound final states of the $^{32}$Ar, $^{28}$S and $^{24}$Si 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.

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arXiv2006

Cross-shell excitation in two-proton knockout: Structure of $^{52}$Ca

The two-proton knockout reaction $^9$Be($^{54}$Ti,$^{52}$Ca$ + γ$) has been studied at 72 MeV/nucleon. Besides the strong feeding of the $^{52}$Ca 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 $^{52}$Ca.

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arXiv2006

One-neutron knockout from $^{57}$Ni

The single-particle structure of $^{57}$Ni and level structure of $^{56}$Ni were investigated with the \mbox{$^{9}$Be ($^{57} $Ni,$^{56}$Ni+$γ$)$\it{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 $^{56}$Ni were deduced.

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arXiv2006

The (t,3He) and (3He,t) reactions as probes of Gamow-Teller strength

Charge-exchange reactions are an important tool for determining weak-interaction rates. They provide stringent tests for nuclear structure models necessary for modeling astrophysical environments such as neutron stars and core-collapse supernovae. In anticipation of (t,3He) experiments at 115 MeV/nucleon on nuclei of relevance (A~40-120) in the late evolution of stars, it is shown via a study of the 26Mg(t,3He) reaction that this probe is an accurate tool 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 which 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. A second goal of this work is to improve the understanding of the (t,3He) and (3He,t) reaction mechanisms at intermediate energies since 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 due to interference between Gamow-Teller dL=0, dS=1 and dL=2, dS=1 amplitudes that both contribute to transitions from 0+ to 1+ states.

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arXiv2006

Measurement of the Gamow-Teller Strength Distribution in 58Co via the 58Ni(t,3He) reaction at 115 MeV/nucleon

Electron capture and beta decay play important roles in the evolution of pre-supernovae 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 ~10^6 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 non-negligible due to large interferences between the dL=0, dS=1 Gamow-Teller amplitude and the dL=2, dS=1 amplitude.

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arXiv2006

Population of bound excited states in intermediate-energy fragmentation reactions

Fragmentation reactions with intermediate-energy heavy-ion beams exhibit a wide range of reaction mechanisms, ranging from direct reactions to statistical processes. We examine this transition by measuring the relative population of excited states in several sd-shell nuclei produced by fragmentation with the number of removed nucleons ranging from two to sixteen. The two-nucleon removal is consistent with a non-dissipative process whereas the removal of more than five nucleons appears to be mainly statistical.

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arXiv2006

Spectroscopy of the odd-odd fp-shell nucleus 52Sc from secondary fragmentation

The odd-odd fp-shell nucleus 52Sc was investigated using in-beam gamma-ray spectroscopy following secondary fragmentation of a 55V and 57Cr cocktail beam. Aside from the known gamma-ray transition at 674(5)keV, a new decay at E_gamma=212(3) keV was observed. It is attributed to the depopulation of a low-lying excited level. This new state is discussed in the framework of shell-model calculations with the GXPF1, GXPF1A, and KB3G effective interactions. These calculations are found to be fairly robust for the low-lying level scheme of 52Sc irrespective of the choice of the effective interaction. In addition, the frequency of spin values predicted by the shell model is successfully modeled by a spin distribution formulated in a statistical approach with an empirical, energy-independent spin-cutoff parameter.

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arXiv2005

Application of the Time of Flight Technique for Lifetime Measurements with Relativistic Beams of Heavy Nuclei

A novel method for picosecond lifetime measurements of excited gamma-ray emitting nuclear states has been developed for fast beams from fragmentation reactions. A test measurement was carried out with a beam of 124Xe at an energy of ~55 MeV/u. The beam ions were Coulomb excited to the first 2+ state on a movable target. Excited nuclei emerged from the target and decayed in flight after a distance related to the lifetime. A stationary degrader positioned downstream with respect to the target was used to further reduce the velocity of the excited nuclei. As a consequence, the gamma-ray decays from the 2+ excited state that occurred before or after traversing the degrader were measured at a different Doppler shift. The gamma-ray spectra were analyzed from the forward ring of the Segmented Germanium Array; this ring positioned at 37 deg. simultaneously provides the largest sensitivity to changes in velocity and the best energy resolution. The ratio of intensities in the peaks at different Doppler shifts gives information about the lifetime if the velocity is measured. The results and range of the application of the method are discussed.

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arXiv2003

Half-life Limit of 19Mg

A search for 19Mg was performed using projectile fragmentation of a 150 MeV/nucleon 36Ar beam. No events of 19Mg were observed. From the time-of-flight through the fragment separator an upper limit of 22 ns for the half-life of 19Mg was established.

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arXiv2001

Electromagnetic Dissociation of 8B and the Rate of the 7Be(p,gamma)8B Reaction in the Sun

In an effort to better determine the 7Be(p,gamma)8B reaction rate, we have performed inclusive and exclusive measurements of the Coulomb dissociation of 8B. The former was a study of longitudinal momentum distributions of 7Be fragments emitted in the Coulomb breakup of intermediate energy 8B beams on Pb and Ag targets. Analysis of these data yielded the E2 contribution to the breakup cross section. In the exclusive measurement, we determined the cross section for the Coulomb breakup of 8B on Pb at low relative energies in order to infer the astrophysical S factor for the 7Be(p,gamma)8B reaction. Interpreting the measurements with 1st-order perturbation theory, we obtained SE2/SE1 = 4.7 (+ 2.0,- 1.3) times 10^-4 at Erel = 0.6 MeV, and S17(0) = 17.8 (+ 1.4,- 1.2) eV b. Semiclassical 1st-order perturbation theory and fully quantum mechanical continuum-discretized coupled channels analyses yield nearly identical results for the E1 strength relevant to solar neutrino flux calculations, suggesting that theoretical reaction mechanism uncertainties need not limit the precision of Coulomb breakup determinations of the 7Be(p,gamma)8B S factor. A recommended value of S17(0) based on a weighted average of this and other measurements is presented.

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arXiv2001

S17(0) Determined from the Coulomb Breakup of 83 MeV/nucleon 8B

A kinematically complete measurement was made of the Coulomb dissociation of 8B nuclei on a Pb target at 83 MeV/nucleon. The cross section was measured at low relative energies in order to infer the astrophysical S factor for the 7Be(p,gamma)8B reaction. A first-order perturbation theory analysis of the reaction dynamics including E1, E2, and M1 transitions was employed to extract the E1 strength relevant to neutrino-producing reactions in the solar interior. By fitting the measured cross section from Erel = 130 keV to 400 keV, we find S17(0) = 17.8 (+1.4, -1.2) eV b.

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arXiv2000

Spectroscopy of 13,14B via the one-neutron knockout reaction

The single-nucleon knockout reactions 9Be(14B, 13B + gamma)X and 197Au(14B, 13B + gamma)X, at an incident energy of 60 MeV per nucleon, have been used to probe the structure of 14B and of the core fragment 13B. A dominant 2s configuration is deduced for the neutron in the ground state of 14B. The longitudinal momentum distribution for this state is consistent with "neutron halo" structure. Spin assignments for 13B excited states at 3.48 and 3.68 MeV are proposed based on the observed spectroscopic factors for one-neutron removal.

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