arXiv subjects

W. P. Tan

Publications and source records attributed to W. P. Tan.

At least 19 recordsLinked to original sources

arXiv2020

Observation of T=3/2 Isobaric Analog States in 9Be using p+8Li resonance scattering

Background: Resonance scattering has been extensively used to study the structure of exotic, neutron-deficient nuclei. Extension of the resonance scattering technique to neutron-rich nuclei was suggested more than 20 years ago. This development is based on the isospin conservation law. In spite of broad field of the application, it has never gained a wide-spread acceptance. Purpose: To benchmark the experimental approach to study the structure of exotic neutron-rich nuclei through resonance scattering on a proton target. Method: The excitation function for p+8Li resonance scattering is measured using a thick target by recording coincidence between light and heavy recoils, populating T=3/2 isobaric analog states (IAS) in 9Be. Results: A good fit of the 8Li(p,p)8Li resonance elastic scattering excitation function was obtained using previously tentatively known 5/2- T=3/2 state at 18.65 MeV in 9Be and a new broad T=3/2 s-wave state - the 5/2+ at 18.5 MeV. These results fit the expected iso-mirror properties for the T=3/2 A=9 iso-quartet. Conclusions: Our analysis confirmed isospin as a good quantum number for the investigated highly excited T=3/2 states and demonstrated that studying the structure of neutron-rich exotic nuclei through IAS is a promising approach.

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arXiv2020

New measurement of $^{12}$C+$^{12}$C fusion reaction at astrophysical energies

Carbon and oxygen burning reactions, in particular, $^{12}$C+$^{12}$C fusion, are important for the understanding and interpretation of the late phases of stellar evolution as well as the ignition and nucleosynthesis in cataclysmic binary systems such as type Ia supernovae and x-ray superbursts. A new measurement of this reaction has been performed at the University of Notre Dame using particle-$γ$ coincidence techniques with SAND (a silicon detector array) at the high-intensity 5U Pelletron accelerator. New results for $^{12}$C+$^{12}$C fusion at low energies relevant to nuclear astrophysics are reported. They show strong disagreement with a recent measurement using the indirect Trojan Horse method. The impact on the carbon burning process under astrophysical scenarios will be discussed.

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arXiv2019

New $γ$-ray Transitions Observed in $^{19}$Ne with Implications for the $^{15}$O($α$,$γ$)$^{19}$Ne Reaction Rate

The $^{15}$O($α$,$γ$)$^{19}$Ne reaction is responsible for breakout from the hot CNO cycle in Type I x-ray bursts. Understanding the properties of resonances between $E_x = 4$ and 5 MeV in $^{19}$Ne is crucial in the calculation of this reaction rate. The spins and parities of these states are well known, with the exception of the 4.14- and 4.20-MeV states, which have adopted spin-parities of 9/2$^-$ and 7/2$^-$, respectively. Gamma-ray transitions from these states were studied using triton-$γ$-$γ$ coincidences from the $^{19}$F($^{3}$He,$tγ$)$^{19}$Ne reaction measured with GODDESS (Gammasphere ORRUBA Dual Detectors for Experimental Structure Studies) at Argonne National Laboratory. The observed transitions from the 4.14- and 4.20-MeV states provide strong evidence that the $J^π$ values are actually 7/2$^-$ and 9/2$^-$, respectively. These assignments are consistent with the values in the $^{19}$F mirror nucleus and in contrast to previously accepted assignments.

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arXiv2019

Key $^{19}$Ne states identified affecting $γ$-ray emission from $^{18}$F in novae

Detection of nuclear-decay $γ$ rays provides a sensitive thermometer of nova nucleosynthesis. The most intense $γ$-ray flux is thought to be annihilation radiation from the $β^+$ decay of $^{18}$F, which is destroyed prior to decay by the $^{18}$F($p$,$α$)$^{15}$O reaction. Estimates of $^{18}$F production had been uncertain, however, because key near-threshold levels in the compound nucleus, $^{19}$Ne, had yet to be identified. This Letter reports the first measurement of the $^{19}$F($^{3}$He,$tγ$)$^{19}$Ne reaction, in which the placement of two long-sought 3/2$^+$ levels is suggested via triton-$γ$-$γ$ coincidences. The precise determination of their resonance energies reduces the upper limit of the rate by a factor of $1.5-17$ at nova temperatures and reduces the average uncertainty on the nova detection probability by a factor of 2.1.

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arXiv2015

The first direct measurement of 12C(12C,n)23Mg at stellar energies

Neutrons produced by the carbon fusion reaction 12C(12C,n)23Mg play an important role in stellar nucleosynthesis. However, past studies have shown large discrepancies between experimental data and theory, leading to an uncertain cross section extrapolation at astrophysical energies. We present the first direct measurement that extends deep into the astrophysical energy range along with a new and improved extrapolation technique based on experimental data from the mirror reaction 12C(12C,p)23Na. The new reaction rate has been determined with a well-defined uncertainty that exceeds the precision required by astrophysics models. Using our constrained rate, we find that 12C(12C,n)23Mg is crucial to the production of Na and Al in Pop-III Pair Instability Supernovae. It also plays a non-negligible role in the production of weak s-process elements as well as in the production of the important galactic gamma-ray emitter 60Fe.

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arXiv2014

Measurement of the $^{58}$Ni($α$,$γ$)$^{62}$Zn reaction and its astrophysical impact

Cross section measurements of the $^{58}$Ni($α$,$γ$)$^{62}$Zn reaction were performed in the energy range $E_α=5.5-9.5$ MeV at the Nuclear Science Laboratory of the University of Notre Dame, using the NSCL Summing NaI(Tl) detector and the $γ$-summing technique. The measurements are compared to predictions in the statistical Hauser-Feshbach model of nuclear reactions using the SMARAGD code. It is found that the energy dependence of the cross section is reproduced well but the absolute value is overestimated by the prediction. This can be remedied by rescaling the $α$ width by a factor of 0.45. Stellar reactivities were calculated with the rescaled $α$ width and their impact on nucleosynthesis in type Ia supernovae has been studied. It is found that the resulting abundances change by up to 5\% when using the new reactivities.

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arXiv2013

Measurement of the $^{90, 92}$Zr(p,$γ$)$^{91,93}$Nb reactions for the nucleosynthesis of elements around A=90

Cross section measurements of the reactions $^{90, 92}$Zr(p,$γ$)$^{91,93}$Nb were performed using the NSCL SuN detector at the University of Notre Dame. These reactions are part of the nuclear reaction flow for the synthesis of the light p nuclei. For the $^{90}$Zr(p,$γ$)$^{91}$Nb reaction the new measurement resolves the disagreement between previous results. For the $^{92}$Zr(p,$γ$)$^{93}$Nb reaction the present work reports the first measurement of this reaction cross section. Both reaction cross sections are compared to theoretical calculations and a very good agreement with the standard NON-SMOKER model is observed.

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arXiv2013

Systematic study of (p,γ) reactions on Ni isotopes

A systematic study of the radiative proton capture reaction for all stable nickel isotopes is presented. The results were obtained using 2.0 - 6.0 MeV protons from the 11 MV tandem Van de Graaff accelerator at the University of Notre Dame. The γ-rays were detected by the NSCL SuN detector utilising the γ-summing technique. The results are compared to a compilation of earlier measurements and discrepancies between the previous data are resolved. The experimental results are also compared to the theoretical predictions obtained using the NON-SMOKER and SMARAGD codes. Based on these comparisons an improved set of astrophysical reaction rates is proposed for the (p,γ) reactions on the stable nickel isotopes as well as for the 56Ni(p,γ)57Cu reaction.

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arXiv2012

Isospin observables from fragment energy spectra

The energy spectra of light charged particles and intermediate mass fragments from 112Sn+112Sn and 124Sn+124Sn collisions at an incident energy of E/A=50 MeV have been measured with a large array of Silicon strip detectors. We used charged particle multiplicities detected in an array with nearly 4-pi coverage to select data from the central collision events. We study isospin observables analogous to ratios of neutron and proton spectra, including double ratios and yield ratios of t/3He and of asymmetries constructed from fragments with Z=3 to Z=8. Using the energy spectra, we can construct these observables as functions of kinetic energy. Most of the fragment asymmetry observables have a large sensitivity to sequential decays.

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arXiv2009

Measurements of proton induced reaction cross sections on 120Te for the astrophysical p-process

The total cross sections for the 120Te(p,gamma)121I and 120Te(p,n)120I reactions have been measured by the activation method in the effective center-of-mass energies between 2.47 MeV and 7.93 MeV. The targets were prepared by evaporation of 99.4 % isotopically enriched 120Te on Aluminum and Carbon backing foils, and bombarded with proton beams provided by the FN tandem accelerator at the University of Notre Dame. The cross sections and $S$ factors were deduced from the observed gamma ray activity, which was detected off-line by two Clover HPGe detectors mounted in close geometry. The results are presented and compared with the predictions of statistical model calculations using the codes NON-SMOKER and TALYS.

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arXiv2007

d-alpha Correlation functions and collective motion in Xe+Au collisions at E/A=50 MeV

The interplay of the effects of geometry and collective motion on d-$α$ correlation functions is investigated for central Xe+Au collisions at E/A=50 MeV. The data cannot be explained without collective motion, which could be partly along the beam axis. A semi-quantitative description of the data can be obtained using a Monte-Carlo model, where thermal emission is superimposed on collective motion. Both the emission volume and the competition between the thermal and collective motion influence significantly the shape of the correlation function, motivating new strategies for extending intensity interferometry studies to massive particles.

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arXiv2007

The $^{15}$O($α$,$γ$)$^{19}$Ne Breakout Reaction and Impact on X-Ray Bursts

The breakout reaction $^{15}$O($α,γ$)$^{19}$Ne, which regulates the flow between the hot CNO cycle and the rp-process, is critical for the explanation of the burst amplitude and periodicity of X-ray bursters. We report on the first successful measurement of the critical $α$-decay branching ratios of relevant states in $^{19}$Ne populated via $^{19}$F($^3$He,t)$^{19}$Ne. Based on the experimental results and our previous lifetime measurements of these states, we derive the first experimental rate of $^{15}$O($α,γ$)$^{19}$Ne. The impact of our experimental results on the burst pattern and periodicity for a range of accretion rates is analyzed.

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arXiv2006

Isospin Diffusion Observables in heavy ion reactions

Collisions of 112Sn and 124Sn nuclei, which differ in their isospin asymmetry, provide information about the rate of isospin diffusion and equilibration. While several different probes can provide accurate diffusion measurements, the ratios of the mirror nuclei may be the simplest and most promising one. Ratios of the mass seven mirror nuclei yields are analyzed to show the rapidity, transverse momentum and impact parameter dependence of isospin diffusion.

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arXiv2006

Cooling Dynamics in Multi-fragmentation processes

Fragment energy spectra of neutron deficient isotopes are significantly more energetic than those of neutron rich isotopes of the same element. This trend is well beyond what can be expected for the bulk multi-fragmentation of an equilibrated system. It can be explained, however, if some of these fragments are emitted earlier through the surface of the system while it is expanding and cooling.

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arXiv2005

Comparison of mid-velocity fragment formation with projectile-like decay

The characteristics of intermediate mass fragments (IMFs: 3<=Z<=20) produced in mid-peripheral and central collisions are compared. We compare IMFs detected at mid-velocity with those evaporated from the excited projectile-like fragment (PLF*). On average, the IMFs produced at mid-velocity are larger in atomic number, exhibit broader transverse velocity distributions, and are more neutron-rich as compared to IMFs evaporated from the PLF*. In contrast, comparison of mid-velocity fragments associated with mid-peripheral and central collisions reveals that their characteristics are remarkably similar despite the difference in impact parameter. The characteristics of mid-velocity fragments are consistent with low-density formation of the fragments. Neutron deficient isotopes of even Z elements manifest higher kinetic energies than heavier isotopes of the same element for both PLF* and mid-velocity emission. This result may be due to the decay of long-lived excited states in the field of the emitting system.

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arXiv2004

Similarity of fragment characteristics at mid-velocity for mid-peripheral and central collisions

The characteristics, in particular the isotopic composition (N/Z), of intermediate mass fragments (IMF : 3<=Z<=20) produced near the center-of-mass in mid-peripheral and central collisions of 114Cd ions with 92Mo target nuclei at E/A=50 MeV are compared to that of IMFs emitted from the projectile-like fragment (PLF) in mid-peripheral collisions. IMFs produced at mid-velocities are on average larger in atomic number, more energetic, and more neutron-rich as compared to IMFs emitted from the PLF. In contrast, the characteristics of mid-velocity IMFs in central collisions and mid-peripheral collisions are comparable.

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arXiv2004

Interplay of initial deformation and Coulomb proximity on nuclear decay

Alpha particles emitted from an excited projectile-like fragment (PLF*) formed in a peripheral collision of two intermediate-energy heavy ions exhibit a strong preference for emission towards the target-like fragment (TLF). The interplay of the initial deformation of the PLF* caused by the reaction, Coulomb proximity, and the rotation of the PLF* results in the observed anisotropic angular distribution. Changes in the shape of the angular distribution with excitation energy are interpreted as being the result of forming more elongated initial geometries in the more peripheral collisions.

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arXiv2003

Isospin Effects in Nuclear Multifragmentation

We develop an improved Statistical Multifragmentation Model that provides the capability to calculate calorimetric and isotopic observables with precision. With this new model we examine the influence of nuclear isospin on the fragment elemental and isotopic distributions. We show that the proposed improvements on the model are essential for studying isospin effects in nuclear multifragmentation. In particular, these calculations show that accurate comparisons to experimental data require that the nuclear masses, free energies and secondary decay must be handled with higher precision than many current models accord.

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