arXiv subjects

B. A. Brown

Publications and source records attributed to B. A. Brown.

At least 163 records · Page 9Linked to original sources

arXiv2006

Shell structure underlying the evolution of quadrupole collectivity in S-38 and S-40 probed by transient-field g-factor measurements on fast radioactive beams

The shell structure underlying shape changes in neutron-rich nuclei between N=20 and N=28 has been investigated by a novel application of the transient field technique to measure the first-excited state g factors in S-38 and S-40 produced as fast radioactive beams. Details of the new methodology are presented. In both S-38 and S-40 there is a fine balance between the proton and neutron contributions to the magnetic moments. Shell model calculations which describe the level schemes and quadrupole properties of these nuclei also give a satisfactory explanation of the g factors. In S-38 the g factor is extremely sensitive to the occupation of the neutron p3/2 orbit above the N=28 shell gap as occupation of this orbit strongly affects the proton configuration. The g factor of deformed S-40 does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.

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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

Effects of deformation in the three-body structure of 11Li

11Li is studied within a three-body model 9Li+n+n where the core is allowed to be deformed and/or excite. In particular, we include reorientation couplings and couplings between the two bound states of 9Li. Contrary to the other examples studied within this model, we find that core excitation does not affect the structure of 11Li significantly. Reorientation couplings of the deformed 9Li can change the ground state of 11Li from a predominantly two neutron s1/2^2 configuration into a p1/2^2. In addition, we see no evidence for the existence of significant d-wave strength in its ground state, as opposed to the prediction by shell model. A comparison with shell model is presented.

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arXiv2006

Half-life and spin of 60Mn^g

A value of 0.28 +/- 0.02 s has been deduced for the half-life of the ground state of 60Mn, in sharp contrast to the previously adopted value of 51 +/- 6 s. Access to the low-spin 60Mn ground state was accomplished via beta decay of the 0+ 60Cr parent nuclide. New, low-energy states in 60Mn have been identified from beta-delayed gamma-ray spectroscopy. The new, shorter half-life of 60Mn^g is not suggestive of isospin forbidden beta decay, and new spin and parity assignments of 1+ and 4+ have been adopted for the ground and isomeric beta-decaying states, respectively, of 60Mn.

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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

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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arXiv2006

Probing shell structure and shape changes in neutron-rich sulfur isotopes through transient-field g factor measurements on fast radioactive beams of 38S and 40S

The shell structure underlying shape changes in neutron-rich nuclei near N=28 has been investigated by a novel application of the transient field technique to measure the first-excited state g factors in 38S and 40S produced as fast radioactive beams. There is a fine balance between proton and neutron contributions to the magnetic moments in both nuclei. The g factor of deformed 40S does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.

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arXiv2006

Reduced neutron spectroscopic factors when using potential geometries constrained by Hartree-Fock calculations

We carry out a systematic analysis of angular distribution measurements for selected ground-state to ground-state (d,p) and (p,d) neutron transfer reactions, including the calcium isotopes. We propose a consistent three-body model reaction methodology in which we constrain the transferred-neutron bound state and nucleon-target optical potential geometries using modern Hartree-Fock calculations. Our deduced neutron spectroscopic factors are found to be suppressed by ~30% relative to independent-particle shell-model values, from 40Ca through 49Ca. The other nuclei studied, ranging from B to Ti, show similar average suppressions with respect to large-basis shell-model expectations. Our results are consistent with deduced spectroscopic strengths for neutrons and protons from intermediate energy nucleon knockout reactions, and for protons from (e,e'p) reactions, on well-bound nuclei. PACS: 24.50.+g, 24.10.Eq, 25.40.-h, 25.45.-z

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arXiv2005

New $^{32}$Cl(p,$γ$)$^{33}$Ar reaction rate for astrophysical rp-process calculations

The $^{32}$Cl(p,$γ$)$^{33}$Ar reaction rate is of potential importance in the rp-process powering type I X-ray bursts. Recently Clement et al. \cite{CBB04} presented new data on excitation energies for low lying proton unbound states in $^{33}$Ar obtained with a new method developed at the National Superconducting Cyclotron Laboratory. We use their data, together with a direct capture model and a USD shell model calculation to derive a new reaction rate for use in astrophysical model calculations. In particular, we take into account capture on the first excited state in $^{32}$Cl, and also present a realistic estimate of the remaining uncertainties. We find that the $^{32}$Cl(p,$γ$)$^{33}$Ar reaction rate is dominated entirely by capture on the first excited state in $^{32}$Cl over the whole temperature range relevant in X-ray bursts. In the temperature range from 0.2 to 1 GK the rate is up to a factor of 70 larger than the previously recommended rate based on shell model calculations only. The uncertainty is now reduced from up to a factor of 1000 to a factor of 3 at 0.3-0.7 GK and a factor of 6 at 1.5 GK.

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arXiv2005

Indirect techniques in nuclear astrophysics. Asymptotic Normalization Coefficient and Trojan Horse

Owing to the presence of the Coulomb barrier at astrophysically relevant kinetic energies it is very difficult, or sometimes impossible, to measure astrophysical reaction rates in the laboratory. That is why different indirect techniques are being used along with direct measurements. Here we address two important indirect techniques, the asymptotic normalization coefficient (ANC) and the Trojan Horse (TH) methods. We discuss the application of the ANC technique for calculation of the astrophysical processes in the presence of subthreshold bound states, in particular, two different mechanisms are discussed: direct capture to the subthreshold state and capture to the low-lying bound states through the subthreshold state, which plays the role of the subthreshold resonance. The ANC technique can also be used to determine the interference sign of the resonant and nonresonant (direct) terms of the reaction amplitude. The TH method is unique indirect technique allowing one to measure astrophysical rearrangement reactions down to astrophysically relevant energies. We explain why there is no Coulomb barrier in the sub-process amplitudes extracted from the TH reaction. The expressions for the TH amplitude for direct and resonant cases are presented.

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arXiv2005

First observation of 54Zn and its decay by two-proton emission

The nucleus 54Zn has been observed for the first time in an experiment at the SISSI/LISE3 facility of GANIL in the quasi-fragmentation of a 58Ni beam at 74.5 MeV/nucleon in a natNi target. The fragments were analysed by means of the ALPHA-LISE3 separator and implanted in a silicon-strip detector where correlations in space and time between implantation and subsequent decay events allowed us to generate almost background free decay spectra for about 25 different nuclei at the same time. Eight 54Zn implantation events were observed. From the correlated decay events, the half-life of 54Zn is determined to be 3.2 +1.8/-0.8 ms. Seven of the eight implantations are followed by two-proton emission with a decay energy of 1.48(2) MeV. The decay energy and the partial half-life are compared to model predictions and allow for a test of these two-proton decay models.

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arXiv2004

Magnetic moments of the 2${^+_1}$ states around $^{132}$Sn

Development of neutron-rich radioactive beams at the HRIBF facility has stimulated experimental and theoretical activity in heavy Sn and Te isotopes. Recently, the g-factor of the first 2$^+$ state in $^{132}$Te has been measured. We report here new shell-model calculation of magnetic moments for selected Sn and Te isotopes. The residual interaction is based on the CD-Bonn renormalized $ G $-matrix. Single-particle spin and orbital effective g-factors are evaluated microscopically including core polarization and meson exchange currents effects.

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