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C. L. Jiang

Publications and source records attributed to C. L. Jiang.

At least 19 recordsLinked to original sources

First direct measurement of the $^{13}$N($α$,$p$)$^{16}$O reaction relevant for core-collapse supernovae nucleosynthesis

Understanding the explosion mechanism of a core-collapse supernova (CCSN) is important to accurately model CCSN scenarios for different progenitor stars using model-observation comparisons. The uncertainties of various nuclear reaction rates relevant for CCSN scenarios strongly affect the accuracy of these stellar models. Out of these reactions, the $^{13}$N($α$,$p$)$^{16}$O reaction has been found to affect various stages of a CCSN at varying temperatures. This work presents the first direct measurement of the $^{13}$N($α$,$p$)$^{16}$O reaction performed using a 34.6 MeV beam of radioactive $^{13}$N ions and the active-target detector MUSIC (MUlti-Sampling Ionization Chamber) at Argonne National Laboratory. The resulting total $^{13}$N($α$,$p$)$^{16}$O reaction cross sections from this measurement in the center-of-mass energy range of 3.26 - 6.02 MeV are presented and compared with calculations using the Hauser-Feshbach formalism. Uncertainties in the reaction rate have been dramatically reduced at CCSN temperatures.

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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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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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Probing the single-particle character of rotational states in $^{19}$F using a short-lived isomeric beam

A beam containing a substantial component of both the $J^π=5^+$, $T_{1/2}=162$ ns isomeric state of $^{18}$F and its $1^+$, 109.77-min ground state has been utilized to study members of the ground-state rotational band in $^{19}$F through the neutron transfer reaction $(d$,$p)$ in inverse kinematics. The resulting spectroscopic strengths confirm the single-particle nature of the 13/2$^+$ band-terminating state. The agreement between shell-model calculations, using an interaction constructed within the $sd$ shell, and our experimental results reinforces the idea of a single-particle/collective duality in the descriptions of the structure of atomic nuclei.

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Study of $\boldsymbol{(α,p)}$ and $\boldsymbol{(α,n)}$ reactions with a Multi-Sampling Ionization Chamber

A large number of $(α,p)$ and $(α,n)$ reactions are known to play a fundamental role in nuclear astrophysics. This work presents a novel technique to study these reactions with the active target system MUSIC whose segmented anode allows the investigation of a large energy range of the excitation function with a single beam energy. In order to verify the method, we performed a direct measurements of the previously measured reactions $^{17}$O$(α,n)^{20}$Ne, $^{23}$Na$(α,p)^{26}$Mg, and $^{23}$Na$(α,n)^{26}$Al. These reactions were investigated in inverse kinematics using $^{4}$He gas in the detector to study the excitation function in the range of about 2 to 6 MeV in the center of mass. We found good agreement between the cross sections of the $^{17}$O$(α,n)^{20}$Ne reaction measured in this work and previous measurements. Furthermore we have successfully performed a simultaneous measurement of the $^{23}$Na$(α,p)^{26}$Mg and $^{23}$Na$(α,n)^{26}$Al reactions.

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Study of the astrophysically important $\boldsymbol{^{23}\mathrm{Na}(α,p)^{26}\mathrm{Mg}}$ and $\boldsymbol{^{23}\mathrm{Na}(α,n)^{26}\mathrm{Al}}$ reactions

The $^{23}$Na$(α,p)^{26}$Mg and $^{23}$Na$(α,n)^{26}$Al reactions are important for our understanding of the $^{26}$Al abundance in massive stars. The aim of this work is to report on a direct and simultaneous measurement of these astrophysically important reactions using an active target system. The reactions were investigated in inverse kinematics using $^{4}$He as the active target gas in the detector. We measured the excitation functions in the energy range of about 2 to 6 MeV in the center of mass. We have found that the cross sections of the $^{23}$Na$(α,p)^{26}$Mg and the $^{23}$Na$(α,n)^{26}$Al reactions are in good agreement with previous experiments, and with statistical model calculations.

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Independent measurement of the Hoyle state $β$ feeding from 12B using Gammasphere

Using an array of high-purity Compton-suppressed germanium detectors, we performed an independent measurement of the $β$-decay branching ratio from $^{12}\mathrm{B}$ to the second-excited (Hoyle) state in $^{12}\mathrm{C}$. Our result is $0.64(11)\%$, which is a factor $\sim 2$ smaller than the previously established literature value, but is in agreement with another recent measurement. This could indicate that the Hoyle state is more clustered than previously believed. The angular correlation of the Hoyle state $γ$ cascade has also been measured for the first time. It is consistent with theoretical predictions.

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High-Sensitivity Measurement of 3He-4He Isotopic Ratios for Ultracold Neutron Experiments

Research efforts ranging from studies of solid helium to searches for a neutron electric dipole moment require isotopically purified helium with a ratio of 3He to 4He at levels below that which can be measured using traditional mass spectroscopy techniques. We demonstrate an approach to such a measurement using accelerator mass spectroscopy, reaching the 10e-14 level of sensitivity, several orders of magnitude more sensitive than other techniques. Measurements of 3He/4He in samples relevant to the measurement of the neutron lifetime indicate the need for substantial corrections. We also argue that there is a clear path forward to sensitivity increases of at least another order of magnitude.

physics.ins-det

Oscillations above the barrier in the fusion of 28Si + 28Si

Fusion cross sections of 28Si + 28Si have been measured in a range above the barrier with a very small energy step (DeltaElab = 0.5 MeV). Regular oscillations have been observed, best evidenced in the first derivative of the energy-weighted excitation function. For the first time, quite different behaviors (the appearance of oscillations and the trend of sub-barrier cross sections) have been reproduced within the same theoretical frame, i.e., the coupled-channel model using the shallow M3Y+repulsion potential. The calculations suggest that channel couplings play an important role in the appearance of the oscillations, and that the simple relation between a peak in the derivative of the energy-weighted cross section and the height of a centrifugal barrier is lost, and so is the interpretation of the second derivative of the excitation function as a barrier distribution for this system, at energies above the Coulomb barrier.

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A Shorter 146Sm Half-Life Measured and Implications for 146Sm-142Nd Chronology in the Solar System

The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its alpha-decay daughter 142Nd. Based on analyses of 146Sm/147Sm alpha-activity and atom ratios, we determined the half-life of 146Sm to be 68 \pm 7 (1sigma) million years (Ma), which is shorter than the currently used value of 103 \pm 5 Ma. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm_0 = 0.0094\pm0.0005 (2sigma), than previously estimated. Terrestrial, Lunar and Martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)_0 values.

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Effects of mutual excitations in the fusion of carbon isotopes

Fusion data for $^{13}$C+$^{13}$C, $^{12}$C+$^{13}$C and $^{12}$C+$^{12}$C are analyzed by coupled-channels calculations that are based on the M3Y+repulsion, double-folding potential. The fusion is determined by ingoing-wave-boundary conditions (IWBC) that are imposed at the minimum of the pocket in the entrance channel potential. Quadrupole and octupole transitions to low-lying states in projectile and target are included in the calculations, as well as mutual excitations of these states. The effect of one-neutron transfer is also considered but the effect is small in the measured energy regime. It is shown that mutual excitations to high-lying states play a very important role in developing a comprehensive and consistent description of the measurements. Thus the shapes of the calculated cross sections for $^{12}$C+$^{13}$C and $^{13}$C+$^{13}$C are in good agreement with the data. The fusion cross sections for $^{12}$C+$^{12}$C determined by the IWBC are generally larger than the measured cross sections but they are consistent with the maxima of some of the observed peak cross sections. They are therefore expected to provide an upper limit for the extrapolation into the low-energy regime of interest to astrophysics.

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Upper Limit on the molecular resonance strengths in the ${}^{12}$C+${}^{12}$C fusion reaction

Carbon burning is a crucial process for a number of important astrophysical scenarios. The lowest measured energy is around E$_{\rm c.m.}$=2.1 MeV, only partially overlapping with the energy range of astrophysical interest. The currently adopted reaction rates are based on an extrapolation which is highly uncertain because of potential resonances existing in the unmeasured energy range and the complication of the effective nuclear potential. By comparing the cross sections of the three carbon isotope fusion reactions, ${}^{12}$C+${}^{12}$C, ${}^{12}$C+${}^{13}$C and ${}^{13}$C+${}^{13}$C, we have established an upper limit on the molecular resonance strengths in ${}^{12}$C+${}^{12}$C fusion reaction. The preliminary results are presented and the impact on nuclear astrophysics is discussed.

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Fusion reactions with the one-neutron halo nucleus 15C

The structure of 15C, with an s1/2 neutron weakly bound to a closed-neutron shell nucleus 14C, makes it a prime candidate for a one-neutron halo nucleus. We have for the first time studied the cross section for the fusion-fission reaction 15C + 232Th at energies in the vicinity of the Coulomb barrier and compared it to the yield of the neighboring 14C + 232Th system measured in the same experiment. At sub-barrier energies, an enhancement of the fusion yield by factors of 2-5 was observed for 15C, while the cross sections for 14C match the trends measured for 12,13C.

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Hindrance in the fusion of $^{48}$Ca+$^{48}$Ca

The coupled-channels technique is applied to analyze recent fusion data for $^{48}$Ca+$^{48}$Ca. The calculations include the excitations of the low-lying $2^+$, $3^-$ and $5^-$ states in projectile and target, and the influence of mutual excitations as well as the two-phonon quadrupole excitations is also investigated. The ion-ion potential is obtained by double-folding the nuclear densities of the reacting nuclei with the M3Y+repulsion effective interaction but a standard Woods-Saxon potential is also applied. The data exhibit a strong hindrance at low energy compared to calculations that are based on a standard Woods-Saxon potential but they can be reproduced quite well by applying the M3Y+repulsion potential with an adjusted radius of the nuclear density. The influence of the polarization of high-lying states on the extracted radius is discussed.

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Study of the Hindrance Effect in Sub-barrier Fusion Reactions

We have measured the fusion cross sections of the 12C(13C, p)24Na reaction through off-line measurement of the beta-decay of 24Na using the beta-gamma coincidence method. Our new measurements in the energy range of Ec.m. = 2.6-3.0 MeV do not show an obvious S-factor maximum but a plateau. Comparison between this work and various models is presented.

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Indications of a shallow potential in $^{48}$Ca+$^{96}$Zr fusion reactions

Fusion data for $^{48}$Ca+$^{96}$Zr are analyzed by coupled-channels calculations. Puzzling features of a previous analysis are eliminated by applying a potential that has a shallow pocket in the entrance channel. Thus the observed $S$ factor for fusion, which develops a maximum at low energy, can be reproduced fairly well. The high-energy data can also be accounted for but that requires the use of a weak, short-ranged imaginary potential that absorbs the incoming flux near the location of the minimum of the potential pocket. Predictions of the fusion hindrance in other Ca+Zr systems are made and are compared with the systematics that has been developed previously.

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Nuclear Structure Relevant to Neutrinoless Double Beta Decay: 76Ge and 76Se

The possibility of observing neutrinoless double beta decay offers the opportunity of determining the neutrino mass IF the nuclear matrix element were known. Theoretical calculations are uncertain and measurements of the occupations of valence orbits by nucleons active in the decay can be important. The occupation of valence neutron orbits in the ground states of 76Ge and 76Se were determined by precisely measuring cross sections for both neutron-adding and removing transfer reactions. Our results indicate that the Fermi surface is much more diffuse than in theoretical (QRPA) calculations. We find that the populations of at least three orbits change significantly between these two ground states while in the calculations the changes are confined primarily to one orbit.

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Pair correlations in nuclei involved in neutrinoless double beta decay: 76Ge and 76Se

Precision measurements were carried out to test the similarities between the ground states of 76Ge and 76Se. The extent to which these two nuclei can be characterized as consisting of correlated pairs of neutrons in a BCS-like ground state was studied. The pair removal (p,t) reaction was measured at the far forward angle of 3 degrees. The relative cross sections are consistent (at the 5% level) with the description of these nuclei in terms of a correlated pairing state outside the N=28 closed shells with no pairing vibrations. Data were also obtained for 74Ge and 78Se.

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