arXiv subjects

Z. H. Li

Publications and source records attributed to Z. H. Li.

At least 55 records · Page 3Linked to original sources

arXiv2016

The decay characteristic of $^{22}$Si and its ground-state mass significantly affected by three-nucleon forces

The decay of the proton-rich nucleus $^{22}$Si was studied by a silicon array coupled with germanium clover detectors. Nine charged-particle groups are observed and most of them are recognized as $β$-delayed proton emission. A charged-particle group at 5600 keV is identified experimentally as $β$-delayed two-proton emission from the isobaric analog state of $^{22}$Al. Another charged-particle emission without any $β$ particle at the low energy less than 300 keV is observed. The half-life of $^{22}$Si is determined as 27.5 (18) ms. The experimental results of $β$-decay of $^{22}$Si are compared and in nice agreement with shell-model calculations. The mass excess of the ground state of $^{22}$Si deduced from the experimental data shows that three-nucleon (3N) forces with repulsive contributions have significant effects on nuclei near the proton drip line.

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arXiv2016

Beta-decay study of $T_z=-2$ proton-rich nucleus $^{20}$Mg

The $β$ decay of the drip-line nucleus $^{20}$Mg gives important information on resonances in $^{20}$Na, which are relevant for the astrophysical $rp$-process. A detailed $β$ decay spectroscopic study of $^{20}$Mg was performed by a continuous-implantation method. A detection system was specially developed for charged-particle decay studies, giving improved spectroscopic information including the half-life of $^{20}$Mg, the excitation energies, the branching ratios, and the log $ft$ values for the states in $^{20}$Na populated in the $β$ decay of $^{20}$Mg. A new proton branch was observed and the corresponding excited state in $^{20}$Na was proposed. The large isospin asymmetry for the mirror decays of $^{20}$Mg and $^{20}$O was reproduced, as well. However, no conclusive conclusion can be draw about the astrophysically interesting 2645~keV resonance in $^{20}$Na due to the limited statistics.

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arXiv2016

Cluster Decay of the High-lying excited states in $^{14}$C

A cluster-transfer experiment of $^9\rm{Be}(^9\rm{Be},^{14}\rm{C}\rightarrowα+^{10}\rm{Be})α$ at an incident energy of 45 MeV was carried out in order to investigate the molecular structure in high-lying resonant states in $^{14}$C. This reaction is of extremely large $Q$-value, making it an excellent case to select the reaction mechanism and the final states in outgoing nuclei. The high-lying resonances in $^{14}$C are reconstructed for three sets of well discriminated final states in $^{10}$Be. The results confirm the previous decay measurements with clearly improved decay-channel selections and show also a new state at 23.5(1) MeV. The resonant states at 22.4(3) and 24.0(3) MeV decay primarily into the typical molecular states at about 6 MeV in $^{10}$Be, indicating a well developed cluster structure in these high-lying states in $^{14}$C. Further measurements of more states of this kind are suggested.

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arXiv2014

Neutron occupancy of the 0d5/2 orbital and the N=16 shell closure in 24O

One-neutron knockout from 24O leading to the first excited state in 23O has been measured for a proton target at a beam energy of 62 MeV/nucleon. The decay energy spectrum of the neutron unbound state of 23O was reconstructed from the measured four momenta of the 22O fragment and emitted neutron. A sharp peak was found at Edecay=50$\pm$3 keV, corresponding to an excited state in 23O at 2.78$\pm$0.11 MeV, as observed in previous measurements. The longitudinal momentum distribution for this state was consistent with d -wave neutron knockout, providing support for a Jπ assignment of 5/2+. The associated spectroscopic factor was deduced to be C2S(0d5/2)=4.1$\pm$0.4 by comparing the measured cross section (View the MathML source) with a distorted wave impulse approximation calculation. Such a large occupancy for the neutron 0d5/2 orbital is in line with the N=16 shell closure in 24O.

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arXiv2014

Proton widths of the low-lying 16F states from the 15N(7Li, 6Li)16N reaction

All the 16F levels are unbound by proton emission. To date the four low-lying 16F levels below 1 MeV have been experimentally identified with well established spin-parity values and excitation energies with an accuracy of 4 - 6 keV. However, there are still considerable discrepancies for their level widths. The present work aims to explore these level widths through an independent method. The angular distributions of the 15N(7Li, 6Li)16N reaction leading to the first four states in 16N were measured using a high-precision Q3D magnetic spectrograph. The neutron spectroscopic factors and the asymptotic normalization coefficients for these states in 16N were then derived based on distorted wave Born approximation analysis. The proton widths of the four low-lying resonant states in 16F were obtained according to charge symmetry of strong interaction.

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arXiv2013

Spectroscopic factors for low-lying 16N levels and the astrophysical 15N(n,gamma)16N reaction rate

Fluorine is a key element for nucleosynthetic studies since it is extremely sensitive to the physical conditions within stars. The astrophysical site to produce fluorine is suggested to be asymptotic giant branch (AGB) stars. In these stars the 15N(n, g)16N reaction could affect the abundance of fluorine by competing with 15N(a, g)19F. The 15N(n, g)16N reaction rate depends directly on the neutron spectroscopic factors of the low-lying states in 16N. The angular distributions of the 15N(7Li, 6Li)16N reaction populating the ground state and the first three excited states in 16N are measured using a Q3D magnetic spectrograph and are used to derive the spectroscopic factors of these states based on distorted wave Born approximation (DWBA) analysis. The spectroscopic factors of these four states are extracted to be 0.96+-0.09, 0.69+-0.09, 0.84+-0.08 and 0.65+-0.08, respectively. Based on the new spectroscopic factors we derive the 15N(n,g)16N reaction rate. The accuracy and precision of the spectroscopic factors are enhanced due to the first application of high-precision magnetic spectrograph for resolving the closely-spaced 16N levels which can not be achieved in most recent measurement. The present result demonstrates that two levels corresponding to neutron transfers to the 2s1/2 orbit in 16N are not so good single-particle levels although 15N is a closed neutron-shell nucleus. This finding is contrary to the shell model expectation. The present work also provides an independent examination to shed some light on the existing discrepancies in the spectroscopic factors and the 15N(n, g)16N rate.

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arXiv2013

Constraining nova observables: direct measurements of resonance strengths in 33S(p,γ)34Cl

The 33S(p,γ)34Cl reaction is important for constraining predictions of certain isotopic abundances in oxygen-neon novae. Models currently predict as much as 150 times the solar abundance of 33S in oxygen-neon nova ejecta. This overproduction factor may, however, vary by orders of magnitude due to uncertainties in the 33S(p,γ)34Cl reaction rate at nova peak temperatures. Depending on this rate, 33S could potentially be used as a diagnostic tool for classifying certain types of presolar grains. Better knowledge of the 33S(p,γ)34Cl rate would also aid in interpreting nova observations over the S-Ca mass region and contribute to the firm establishment of the maximum endpoint of nova nucleosynthesis. Additionally, the total S elemental abundance which is affected by this reaction has been proposed as a thermometer to study the peak temperatures of novae. Previously, the 33S(p,γ)34Cl reaction rate had only been studied directly down to resonance energies of 432 keV. However, for nova peak temperatures of 0.2-0.4 GK there are 7 known states in 34Cl both below the 432 keV resonance and within the Gamow window that could play a dominant role. Direct measurements of the resonance strengths of these states were performed using the DRAGON recoil separator at TRIUMF. Additionally two new states within this energy region are reported. Several hydrodynamic simulations have been performed, using all available experimental information for the 33S(p,γ)34Cl rate, to explore the impact of the remaining uncertainty in this rate on nucleosynthesis in nova explosions. These calculations give a range of ~ 20-150 for the expected 33S overproduction factor, and a range of ~ 100-450 for the 32S/33S ratio expected in ONe novae.

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arXiv2012

The electromagnetic calorimeter of the AMS-02 experiment

The electromagnetic calorimeter (ECAL) of the AMS-02 experiment is a 3-dimensional sampling calorimeter, made of lead and scintillating fibers. The detector allows for a high granularity, with 18 samplings in the longitudinal direction, and 72 sampling in the lateral direction. The ECAL primary goal is to measure the energy of cosmic rays up to few TeV, however, thanks to the fine grained structure, it can also provide the separation of positrons from protons, in the GeV to TeV region. A direct measurement of high energy photons with accurate energy and direction determination can also be provided.

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arXiv2012

New determination of the 13C(a, n)16O reaction rate and its influence on the s-process nucleosynthesis in AGB stars

We present a new measurement of the $α$-spectroscopic factor ($S_α$) and the asymptotic normalization coefficient (ANC) for the 6.356 MeV 1/2$^+$ subthreshold state of $^{17}$O through the $^{13}$C($^{11}$B, $^{7}$Li)$^{17}$O transfer reaction and we determine the $α$-width of this state. This is believed to have a strong effect on the rate of the $^{13}$C($α$, $n$)$^{16}$O reaction, the main neutron source for {\it slow} neutron captures (the $s$-process) in asymptotic giant branch (AGB) stars. Based on the new width we derive the astrophysical S-factor and the stellar rate of the $^{13}$C($α$, $n$)$^{16}$O reaction. At a temperature of 100 MK our rate is roughly two times larger than that by \citet{cau88} and two times smaller than that recommended by the NACRE compilation. We use the new rate and different rates available in the literature as input in simulations of AGB stars to study their influence on the abundances of selected $s$-process elements and isotopic ratios. There are no changes in the final results using the different rates for the $^{13}$C($α$, $n$)$^{16}$O reaction when the $^{13}$C burns completely in radiative conditions. When the $^{13}$C burns in convective conditions, as in stars of initial mass lower than $\sim$2 $M_\sun$ and in post-AGB stars, some changes are to be expected, e.g., of up to 25% for Pb in our models. These variations will have to be carefully analyzed when more accurate stellar mixing models and more precise observational constraints are available.

astro-ph.SR↗

arXiv2012

The N = 16 spherical shell closure in 24O

The unbound excited states of the neutron drip-line isotope 24O have been investigated via the 24O(p,p')23O+n reaction in inverse kinematics at a beam energy of 62 MeV/nucleon. The decay energy spectrum of 24O* was reconstructed from the momenta of 23O and the neutron. The spin-parity of the first excited state, observed at Ex = 4.65 +/- 0.14 MeV, was determined to be Jpi = 2+ from the angular distribution of the cross section. Higher lying states were also observed. The quadrupole transition parameter beta2 of the 2+ state was deduced, for the first time, to be 0.15 +/- 0.04. The relatively high excitation energy and small beta2 value are indicative of the N = 16 shell closure in 24O.

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arXiv2011

Phase transitions of hadronic to quark matter at finite T and μ_B

The phase transition of hadronic to quark matter and the boundaries of the mixed hadron-quark coexistence phase are studied within the two Equation of State (EoS) model. The relativistic effective mean field approach with constant and density dependent meson-nucleon couplings is used to describe hadronic matter, and the MIT Bag model is adopted to describe quark matter. The boundaries of the mixed phase for different Bag constants are obtained solving the Gibbs equations. We notice that the dependence on the Bag parameter of the critical temperatures (at zero chemical potential) can be well reproduced by a fermion ultrarelativistic quark gas model, without contribution from the hadron part. At variance the critical chemical potentials (at zero temperature) are very sensitive to the EoS of the hadron sector. Hence the study of the hadronic EoS is much more relevant for the determination of the transition to the quark-gluon-plasma at finite baryon density and low-T. Moreover in the low temperature and finite chemical potential region no solutions of the Gibbs conditions are existing for small Bag constant values, B < (135 MeV)^4. Isospin effects in asymmetric matter appear relevant in the high chemical potential regions at lower temperatures, of interest for the inner core properties of neutron stars and for heavy ion collisions at intermediate energies.

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arXiv2009

Asymptotic normalization coefficient from the 12C(7Li,6He)13N reaction and the astrophysical 12C(p,g)13N reaction rate

Angular distribution of the 12C(7Li,6He)13N reaction at E(7Li) = 44.0 MeV was measured at the HI-13 tandem accelerator of Beijing, China. Asymptotic normalization coefficient (ANC) of 13N = 12C + p was derived to be 1.64 $\pm$ 0.11 fm$^{-1/2}$ through distorted wave Born approximation (DWBA) analysis. The ANC was then used to deduce the astrophysical $S(E)$ factors and reaction rates for direct capture in 12C(p,g)13N at energies of astrophysical relevance.

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arXiv2009

Proton Spectroscopic Factor in 7Li from 2H(6He,7Li)n

The angular distribution of the 2H(6He,7Li)n reaction was measured with a secondary 6He beam of 36.4 MeV for the first time. The proton spectroscopic factor of 7Li ground state was extracted to be 0.41 +- 0.05 by the normalization of the calculational differential cross sections with the distorted-wave Born approximation to the experimental data. It was found that the uncertainty of extracted spectroscopic factors from the one-nucleon transfer reactions induced by deuteron may be reduced by constraining the volume integrals of imaginary optical potentials.

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arXiv2009

The 8Li(d,p)9Li Reaction and the Astrophysical 8Li(n,g)9Li Reaction Rate

The $^8$Li($n,γ$)$^9$Li reaction plays an important role in both the r-process nucleosynthesis and the inhomogeneous big bang models, its direct capture rates can be extracted from the $^8$Li($d, p$)$^9$Li reaction, indirectly. We have measured the angular distribution of the $^8$Li($d, p$)$^9$Li$_{\textrm{g.s.}}$ reaction at $E_{\textrm{c.m.}}$ = 7.8 MeV in inverse kinematics using coincidence detection of $^{9}\textrm{Li}$ and recoil proton, for the first time. Based on Distorted Wave Born Approximation (DWBA) analysis, the $^{8}\textrm{Li}(d, p)^{9}\textrm{Li}_{\textrm{g.s.}}$ cross section was determined to be 7.9 $\pm$ 2.0 mb. The single particle spectroscopic factor, $S_{1,3/2}$, for the ground state of $^{9}\textrm{Li}$ = $^{8}\textrm{Li} \otimes n$ was derived to be $0.68\pm 0.14$, and then used to calculate the direct capture cross sections for the $^{8}\textrm{Li}(n, γ)^{9}\textrm{Li}_{\textrm{g.s.}}$ reaction at energies of astrophysical interest. The astrophysical $^{8}\textrm{Li}(n, γ)^{9}\textrm{Li}_{\textrm{g.s.}}$ reaction rate for the direct capture was found to be 3970 $\pm$ 950 $\textrm{cm}^{3}\textrm{mole}^{-1}s^{-1}$ at $T_9$ = 1. This presents the first experimental constraint for the $^{8}\textrm{Li}(n, γ)^{9}\textrm{Li}$ reaction rates of astrophysical relevance.

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arXiv2009

The 13N(d,n)14O Reaction and the Astrophysical 13N(p,g)14O Reaction Rate

$^{13}$N($p,γ$)$^{14}$O is one of the key reactions in the hot CNO cycle which occurs at stellar temperatures around $T_9$ $\geq$ 0.1. Up to now, some uncertainties still exist for the direct capture component in this reaction, thus an independent measurement is of importance. In present work, the angular distribution of the $^{13}$N($d,n$)$^{14}$O reaction at $E_{\rm{c.m.}}$ = 8.9 MeV has been measured in inverse kinematics, for the first time. Based on the distorted wave Born approximation (DWBA) analysis, the nuclear asymptotic normalization coefficient (ANC), $C^{^{14}O}_{1,1/2}$, for the ground state of $^{14}$O $\to$ $^{13}$N + $p$ is derived to be $5.42 \pm 0.48$ fm$^{-1/2}$. The $^{13}$N($p,γ$)$^{14}$O reaction is analyzed with the R-matrix approach, its astrophysical S-factors and reaction rates at energies of astrophysical relevance are then determined with the ANC. The implications of the present reaction rates on the evolution of novae are then discussed with the reaction network calculations.

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arXiv2008

Temperature dependence of single-particle properties in nuclear matter

The single-nucleon potential in hot nuclear matter is investigated in the framework of the Brueckner theory by adopting the realistic Argonne V18 or Nijmegen 93 two-body nucleon-nucleon interaction supplemented by a microscopic three-body force. The rearrangement contribution to the single-particle potential induced by the ground state correlations is calculated in terms of the hole-line expansion of the mass operator and provides a significant repulsive contribution in the low-momentum region around and below the Fermi surface. Increasing temperature leads to a reduction of the effect, while increasing density makes it become stronger. The three-body force suppresses somewhat the ground state correlations due to its strong short-range repulsion, increasing with density. Inclusion of the three-body force contribution results in a quite different temperature dependence of the single-particle potential at high enough densities as compared to that adopting the pure two-body force. The effects of three-body force and ground state correlations on the nucleon effective mass are also discussed.

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arXiv2004

Effect of Three-body Interaction on Phase Transition of Hot Asymmetric Nuclear Matter

The properties and the isospin dependence of the liquid-gas phase transition in hot asymmetric nuclear matter have been investigated within the framework of the finite temperature Brueckner-Hartree-Fock approach extended to include the contribution of a microscopic three-body force. A typical Van der Waals structure has been observed in the calculated isotherms (of pressure) for symmetric nuclear matter implying the presence of the liquid-gas phase transition. The critical temperature of the phase transition is calculated and its dependence on the proton-to-neutron ratio is discussed. It is shown that the three-body force gives a repulsive contribution to the nuclear equation of state and reduces appreciably the critical temperature and the mechanical instable region. At fixed temperature and density the pressure of asymmetric nuclear matter increases monotonically as a function of isospin asymmetry. In addition, it turns out that the domain of mechanical instability for hot asymmetric nuclear matter gradually shrinks with increasing asymmetry and temperature. We have compared our results with the predictions of other theoretical models especially the Dirac Brueckner approach. A possible explanation for the discrepancy between the values of the critical temperature predicted by the present non-relativistic Brueckner calculations including the three-body force and the relativistic Dirac-Brueckner method is given.

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arXiv2004

Hot Nuclear Matter Equation of State with a Three-body Force

The finite temperature Brueckner-Hartree-Fock approach is extended by introducing a microscopic three-body force. In the framework of the extended model, the equation of state of hot asymmetric nuclear matter and its isospin dependence have been investigated. The critical temperature of liquid-gas phase transition for symmetric nuclear matter has been calculated and compared with other predictions. It turns out that the three-body force gives a repulsive contribution to the equation of state which is stronger at higher density and as a consequence reduces the critical temperature of liquid-gas phase transition. The calculated energy per nucleon of hot asymmetric nuclear matter is shown to satisfy a simple quadratic dependence on asymmetric parameter $β$ as in the zero-temperature case. The symmetry energy and its density dependence have been obtained and discussed. Our results show that the three-body force affects strongly the high-density behavior of the symmetry energy and makes the symmetry energy more sensitive to the variation of temperature. The temperature dependence and the isospin dependence of other physical quantities, such as the proton and neutron single particle potentials and effective masses are also studied. Due to the additional repulsion produced by the three-body force contribution, the proton and neutron single particle potentials are correspondingly enhanced as similar to the zero-temperature case.

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