arXiv subjects

A. A. Chen

Publications and source records attributed to A. A. Chen.

At least 19 recordsLinked to original sources

arXiv2025

Direct measurement of $^{59}$Cu($p$,$α$)$^{56}$Ni precludes a strong NiCu cycle in Type-I X-ray bursts

Model-observation comparisons of type-I X-ray bursts (XRBs) can reveal the properties of accreting neutron star systems, including the neutron star compactness. XRBs are powered by nuclear burning and a handful of reactions have been shown to impact the model results. Reactions in the NiCu cycles, featuring a competition between $^{59}$Cu($p$,$γ$)$^{60}$Zn and $^{59}$Cu($p$,$α$)$^{56}$Ni, have been shown to be among the most important reactions as they are a critical checkpoint in $rp$-process flow and significantly impact the light curves and burst ashes. We report a direct measurement of $^{59}$Cu($p$,$α$)$^{56}$Ni bringing stringent constraints on this reaction rate. New results rule out a strong NiCu cycle in XRBs, with a negligible degree of recycling, $\leq$5\% up to 1.5 GK. The new reaction rate, when varied within new uncertainty limits, shows no impact on one-zone XRB model light-curves tailored for clocked-burster $\tt{GS 1826-24}$, hence removing an important nuclear physics uncertainty in the model-observation comparison.

astro-ph.HE↗

arXiv2022

Elastic scattering of 3He+4He with SONIK

Measurements of the elastic scattering cross section of 3He and 4He are important in order to improve constraints on theoretical models of 4He(3He,g)7Be, a key reaction in Big Bang nucleosynthesis and solar neutrino production. The astrophysical S-factor for this reaction is a significant source of uncertainty in the standard solar-model prediction of the 7Be and 8B solar neutrino fluxes. The elastic scattering measurements reported in the literature do not extend to low energies and lack proper uncertainty quantification. A new measurement of the 4He(3He,3He)4He reaction has been made at center-of-mass energies Ec.m. = 0.38-3.13 MeV using the Scattering of Nuclei in Inverse Kinematics (SONIK) scattering chamber: a windowless, extended gas target surrounded by an array of 30 collimated silicon charged particle detectors situated at TRIUMF. This is the first elastic scattering measurement of 3He+4He made below 500 keV and it has greater angular range and better precision than previous measurements. The elastic scattering data were analyzed using both R-matrix and Halo Effective Field Theory (Halo EFT) frameworks, and values of the s-wave scattering length and effective range were extracted. The resulting improvement in knowledge of the s-wave effective-range function at low energies will reduce the overall uncertainty in S34 at solar energies.

nucl-ex↗

arXiv2022

Direct measurement of resonances in $^7$Be($α,γ$)$^{11}$C relevant to $νp$-process nucleosynthesis

We have performed the first direct measurement of two resonances of the $^7$Be($α,γ$)$^{11}$C reaction with unknown strengths using an intense radioactive $^7$Be beam and the DRAGON recoil separator. We report on the first measurement of the 1155 and 1110 keV resonance strengths of $1.73 \pm 0.25(stat.) \pm 0.40(syst.)$ eV and $125 ^{+27}_{-25}(stat.) \pm 15(syst.)$ meV, respectively. The present results have reduced the uncertainty in the $^7$Be($α,γ$)$^{11}$C reaction rate to $\sim$ 9.4-10.7% over T = 1.5-3 GK, which is relevant for nucleosynthesis in the neutrino-driven outflows of core-collapse supernovae ($νp$-process). We find no effect of the new, constrained reaction rate on $νp$-process nucleosynthesis.

nucl-ex↗

arXiv2022

First inverse kinematics measurement of resonances in $^7$Be($α,γ$)$^{11}$C relevant to neutrino-driven wind nucleosynthesis using DRAGON

A possible mechanism to explain the origin of the light $p$-nuclei in the Galaxy is the nucleosynthesis in the proton-rich neutrino-driven wind ejecta of core-collapse supernovae via the $νp$-process. However this production scenario is very sensitive to the underlying supernova dynamics and the nuclear physics input. As far as the nuclear uncertainties are concerned, the breakout from the $pp$-chains via the $^7$Be($α,γ$)$^{11}$C reaction has been identified as an important link which can influence the nuclear flow and therefore the efficiency of the $νp$-process. However its reaction rate is poorly known over the relevant temperature range, T = 1.5-3 GK. We report on the first direct measurement of two resonances of the $^7$Be($α,γ$)$^{11}$C reaction with previously unknown strengths using an intense radioactive $^7$Be beam from the ISAC facility and the DRAGON recoil separator in inverse kinematics. We have decreased the $^7$Be($α,γ$)$^{11}$C reaction rate uncertainty to $\sim$ 9.4-10.7% over the relevant temperature region.

nucl-ex↗

arXiv2022

Constraining the $^{30}$P($p,γ)^{31}$S reaction rate in ONe novae via the weak, low-energy, $β$-delayed proton decay of $^{31}$Cl

The $^{30}$P$(p,γ)^{31}$S reaction plays an important role in understanding nucleosynthesis of $A\geq 30$ nuclides in oxygen-neon novae. The Gaseous Detector with Germanium Tagging was used to measure $^{31}$Cl $β$-delayed proton decay through the key $J^π=3/2^{+}$, 260-keV resonance. The intensity $I^{260}_{βp} = 8.3^{+1.2}_{-0.9} \times 10^{-6}$ represents the weakest $β$-delayed, charged-particle emission ever measured below 400 keV, resulting in a proton branching ratio of $Γ_p / Γ= 2.5^{+0.4}_{-0.3} \times 10^{-4}$. By combining this measurement with shell-model calculations for $Γ_γ$ and past work on other resonances, the total $^{30}$P$(p,γ)^{31}$S rate has been determined with reduced uncertainty. The new rate has been used in hydrodynamic simulations to model the composition of nova ejecta, leading to a concrete prediction of $^{30}$Si/$^{28}$Si excesses in presolar nova grains and the calibration of nuclear thermometers.

nucl-ex↗

arXiv2021

First direct measurement of $^{59}$Cu(p,$α$)$^{56}$Ni: A step towards constraining the Ni-Cu cycle in the Cosmos

Reactions on the proton-rich nuclides drive the nucleosynthesis in Core-Collapse Supernovae (CCSNe) and in X-ray bursts (XRBs). CCSNe eject the nucleosynthesis products to the interstellar medium and hence are a potential inventory of p-nuclei, whereas in XRBs nucleosynthesis powers the light curves. In both astrophysical sites the Ni-Cu cycle, which features a competition between $^{59}$Cu(p,$α$)$^{56}$Ni and $^{59}$Cu(p,$γ$)$^{60}$Zn, could potentially halt the production of heavier elements. Here, we report the first direct measurement of $^{59}$Cu(p,$α$)$^{56}$Ni using a re-accelerated $^{59}$Cu beam and cryogenic solid hydrogen target. Our results show that the reaction proceeds predominantly to the ground state of $^{56}$Ni and the experimental rate has been found to be lower than Hauser-Feshbach-based statistical predictions. New results hint that the $νp$-process could operate at higher temperatures than previously inferred and therefore remains a viable site for synthesizing the heavier elements.

nucl-ex↗

arXiv2020

Beyond the acceptance limit of DRAGON: the case of the $\mathrm{^6Li(α,γ)^{10}B}$ reaction

Radiative capture reactions play a pivotal role for our understanding of the origin of the elements in the cosmos. Recoil separators provide an effective way to study these reactions, in inverse kinematics, and take advantage of the use of radioactive ion beams. However, a limiting factor in the study of radiative capture reactions in inverse kinematics is the momentum spread of the product nuclei, which can result in an angular spread larger than the geometric acceptance of the separator. The DRAGON facility at TRIUMF is a versatile recoil separator, designed to study radiative capture reactions relevant to astrophysics in the $\mathrm{A \sim 10-30}$ region. In this work we present the first attempt to study with DRAGON a reaction, $\mathrm{^6Li(α,γ)^{10}B}$ , for which the recoil angular spread exceeds DRAGON's acceptance. Our result is in good agreement with the literature value, showing that DRAGON can measure resonance strengths of astrophysically important reactions even when not all the recoils enter the separator.

physics.ins-det↗

arXiv2019

First inverse kinematics measurement of key resonances in the ${}^{22}\text{Ne}(p,γ)^{23}\text{Na}$ reaction at stellar temperatures

In this Letter we report on the first inverse kinematics measurement of key resonances in the ${}^{22}\text{Ne}(p,γ)^{23}\text{Na}$ reaction which forms part of the NeNa cycle, and is relevant for ${}^{23}$Na synthesis in asymptotic giant branch (AGB) stars. An anti-correlation in O and Na abundances is seen across all well-studied globular clusters (GC), however, reaction-rate uncertainties limit the precision as to which stellar evolution models can reproduce the observed isotopic abundance patterns. Given the importance of GC observations in testing stellar evolution models and their dependence on NeNa reaction rates, it is critical that the nuclear physics uncertainties on the origin of ${}^{23}$Na be addressed. We present results of direct strengths measurements of four key resonances in ${}^{22}\text{Ne}(p,γ)^{23}\text{Na}$ at E$_{\text c.m.}$ = 149 keV, 181 keV, 248 keV and 458 keV. The strength of the important E$_{\text c.m.}$ = 458 keV reference resonance has been determined independently of other resonance strengths for the first time with an associated strength of $ωγ$ = 0.439(22) eV and with higher precision than previously reported. Our result deviates from the two most recently published results obtained from normal kinematics measurements performed by the LENA and LUNA collaborations but is in agreement with earlier measurements. The impact of our rate on the Na-pocket formation in AGB stars and its relation to the O-Na anti-correlation was assessed via network calculations. Further, the effect on isotopic abundances in CO and ONe novae ejecta with respect to pre-solar grains was investigated.

nucl-ex↗

arXiv2019

A direct measurement of the 17O(a,g)21Ne reaction in inverse kinematics and its impact on heavy element production

During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorb neutrons thereby influencing the final heavy element abundances. At low metallicities, the high neutron capture rate of 16-O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the 17O(a,n)20Ne reaction. The efficiency of this neutron recycling is determined by competition between the 17O(a,n)20Ne and 17O(a,g)21Ne reactions. While some experimental data are available on the former reaction, no data exist for the radiative capture channel at the relevant astrophysical energies. The 17O(a,g)21Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Laboratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV Ecm, reaching into the Gamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and 1.122 MeV Ecm have been extracted. The experimentally based reaction rate calculated represents a lower limit, but suggests that significant s-process nucleosynthesis occurs in low metallicity massive stars.

nucl-ex↗

arXiv2019

Observation of excited states in $^{20}$Mg sheds light on nuclear forces and shell evolution

The exotic Borromean nucleus $^{20}$Mg with $N$ = 8, located at the proton drip-line provides a unique testing ground for nuclear forces and the evolution of shell structure in the neutron-deficient region. We report on the first observation of proton unbound resonances together with bound states in $^{20}$Mg from the $^{20}$Mg($d$,$d'$) reaction performed at TRIUMF. Phenomenological shell-model calculations offer a reasonable description. However, our experimental results present a challenge for current first-principles nuclear structure approaches and point to the need for improved chiral forces and {\it ab initio} calculations. Furthermore, the differential cross section of the first excited state is compared with distorted-wave Born approximation calculations to deduce a neutron quadrupole deformation parameter of $β_n$=0.46$\pm$0.21. This provides the first indication of a possible weakening of the $N$ = 8 shell closure at the proton drip-line.

nucl-ex↗

arXiv2018

First measurement of $^{30}$S+$α$ resonant elastic scattering for the $^{30}$S($α$,p) reaction rate

Background: Type I x-ray bursts are the most frequent thermonuclear explosions in the galaxy, resulting from thermonuclear runaway on the surface of an accreting neutron star. The $^{30}$S($α$,p) reaction plays a critical role in burst models, yet insufficient experimental information is available to calculate a reliable, precise rate for this reaction. Purpose: Our measurement was conducted to search for states in $^{34}$Ar and determine their quantum properties. In particular, natural-parity states with large $α$-decay partial widths should dominate the stellar reaction rate. Method: We performed the first measurement of $^{30}$S+$α$ resonant elastic scattering up to a center-of-mass energy of 5.5 MeV using a radioactive ion beam. The experiment utilized a thick gaseous active target system and silicon detector array in inverse kinematics. Results: We obtained an excitation function for $^{30}$S($α$,$α$) near $150^{\circ}$ in the center-of-mass frame. The experimental data were analyzed with an $R$-Matrix calculation, and we observed three new resonant patterns between 11.1 and 12.1 MeV, extracting their properties of resonance energy, widths, spin, and parity. Conclusions: We calculated the resonant thermonuclear reaction rate of $^{30}$S($α$,p) based on all available experimental data of $^{34}$Ar and found an upper limit about one order of magnitude larger than a rate determined using a statistical model. The astrophysical impact of these two rates has been investigated through one-zone postprocessing type I x-ray burst calculations. We find that our new upper limit for the $^{30}$S($α$,p)$^{33}$Cl rate significantly affects the predicted nuclear energy generation rate during the burst.

nucl-ex↗

arXiv2018

Direct Measurement of Astrophysically Important Resonances in $^{38}\mathrm{K}(p, γ){}^{39}\mathrm{Ca}$

According to sensitivity studies, the $^{38}\mathrm{K}\left( p, γ\right){}^{39}\mathrm{Ca}$ reaction has a significant influence on $\mathrm{Ar}$, $\mathrm{K}$, and $\mathrm{Ca}$ production in classical novae. In order to constrain the rate of this reaction, we have performed a direct measurement of the strengths of three candidate $\ell = 0$ resonances within the Gamow window, at $386 \pm 10~\mathrm{keV}$, $515 \pm 10~\mathrm{keV}$, and $689 \pm 10~\mathrm{keV}$. The experiment was performed in inverse kinematics using a beam of unstable $^{38}\mathrm{K}$ impinged on a windowless $\mathrm{H}_2$ target. The $^{39}\mathrm{Ca}$ recoils and prompt $γ$ rays from $^{38}\mathrm{K}\left( p, γ\right){}^{39}\mathrm{Ca}$ reactions were detected in coincidence using a recoil mass separator and a BGO array, respectively. For the $689$ keV resonance, we observed a clear recoil-$γ$ coincidence signal and extracted resonance strength and energy values of $120^{+50}_{-30}~\mathrm{(stat.)}^{+20}_{-60}~\mathrm{(sys.)}~\mathrm{meV}$ and $679^{+2}_{-1}~\mathrm{(stat.)} \pm 1~\mathrm{(sys.)}~\mathrm{keV}$, respectively. We also performed a singles analysis, extracting a resonance strength of $120 \pm 20~\mathrm{(stat.)} \pm 15~\mathrm{(sys.)}~\mathrm{meV}$, consistent with the coincidence result. For the $386$ keV and $515$ keV resonances, we extract $90\%$ confidence level upper limits of $2.54$ meV and $18.4$ meV, respectively. We have established a new recommended $^{38}\mathrm{K}(p, γ){}^{39}\mathrm{Ca}$ rate based on experimental information, which reduces overall uncertainties near the peak temperatures of nova burning by a factor of ${\sim} 250$. Using the rate obtained in this work in model calculations of the hottest oxygen-neon novae reduces overall uncertainties on $\mathrm{Ar}$, $\mathrm{K}$, and $\mathrm{Ca}$ synthesis to factors of $15$ or less in all cases.

nucl-ex↗

arXiv2017

Confirmation of the isomeric state in 26P

We report the independent experimental confirmation of an isomeric state in the proton drip-line nucleus $^{26}$P. The $γ$-ray energy and half-life determined are 164.4 $\pm$ 0.3 (sys) $\pm$ 0.2 (stat) keV and 104 $\pm$ 14 ns, respectively, which are in agreement with the previously reported values. These values are used to set a semi-empirical limit on the proton separation energy of $^{26}$P, with the conclusion that it can be bound or unbound.

nucl-ex↗

arXiv2016

Isobaric multiplet mass equation in the $A=31$ $T = 3/2$ quartets

The observed mass excesses of analog nuclear states with the same mass number $A$ and isospin $T$ can be used to test the isobaric multiplet mass equation (IMME), which has, in most cases, been validated to a high degree of precision. A recent measurement [Kankainen et al., Phys. Rev. C 93 041304(R) (2016)] of the ground-state mass of $^{31}$Cl led to a substantial breakdown of the IMME for the lowest $A = 31, T = 3/2$ quartet. The second-lowest $A = 31, T = 3/2$ quartet is not complete, due to uncertainties associated with the identity of the $^{31}$S member state. Using a fast $^{31}$Cl beam implanted into a plastic scintillator and a high-purity Ge $γ$-ray detection array, $γ$ rays from the $^{31}$Cl$(βγ)$$^{31}$S sequence were measured. Shell-model calculations using USDB and the recently-developed USDE interactions were performed for comparison. Isospin mixing between the $^{31}$S isobaric analog state (IAS) at 6279.0(6) keV and a nearby state at 6390.2(7) keV was observed. The second $T = 3/2$ state in $^{31}$S was observed at $E_x = 7050.0(8)$ keV. Isospin mixing in $^{31}$S does not by itself explain the IMME breakdown in the lowest quartet, but it likely points to similar isospin mixing in the mirror nucleus $^{31}$P, which would result in a perturbation of the $^{31}$P IAS energy. USDB and USDE calculations both predict candidate $^{31}$P states responsible for the mixing in the energy region slightly above $E_x = 6400$ keV. The second quartet has been completed thanks to the identification of the second $^{31}$S $T = 3/2$ state, and the IMME is validated in this quartet.

nucl-ex↗

arXiv2016

Beta-delayed gamma decay of 26P: Possible evidence of a proton halo

Background: Measurements of $β$ decay provide important nuclear structure information that can be used to probe isospin asymmetries and inform nuclear astrophysics studies. Purpose: To measure the $β$-delayed $γ$ decay of $^{26}$P and compare the results with previous experimental results and shell-model calculations. Method: A $^{26}$P fast beam produced using nuclear fragmentation was implanted into a planar germanium detector. Its $β$-delayed $γ$-ray emission was measured with an array of 16 high-purity germanium detectors. Positrons emitted in the decay were detected in coincidence to reduce the background. Results: The absolute intensities of $^{26}$P $β$-delayed $γ$-rays were determined. A total of six new $β$-decay branches and 15 new $γ$-ray lines have been observed for the first time in $^{26}$P $β$-decay. A complete $β$-decay scheme was built for the allowed transitions to bound excited states of $^{26}$Si. $ft$ values and Gamow-Teller strengths were also determined for these transitions and compared with shell model calculations and the mirror $β$-decay of $^{26}$Na, revealing significant mirror asymmetries. Conclusions: A very good agreement with theoretical predictions based on the USDB shell model is observed. The significant mirror asymmetry observed for the transition to the first excited state ($δ=51(10)\%$) may be evidence for a proton halo in $^{26}$P.

nucl-ex↗

arXiv2016

Isospin Mixing Reveals $^{30}$P($p,γ$)$^{31}$S Resonance Influencing Nova Nucleosynthesis

The thermonuclear $^{30}$P($p,γ$)$^{31}$S reaction rate is critical for modeling the final elemental and isotopic abundances of ONe nova nucleosynthesis, which affect the calibration of proposed nova thermometers and the identification of presolar nova grains, respectively. Unfortunately, the rate of this reaction is essentially unconstrained experimentally, because the strengths of key $^{31}$S proton capture resonance states are not known, largely due to uncertainties in their spins and parities. Using the $β$ decay of $^{31}$Cl, we have observed the $β$-delayed $γ$ decay of a $^{31}$S state at $E_x = 6390.2(7)$ keV, with a $^{30}$P($p,γ$)$^{31}$S resonance energy of $E_r = 259.3(8)$ keV, in the middle of the $^{30}$P($p,γ$)$^{31}$S Gamow window for peak nova temperatures. This state exhibits isospin mixing with the nearby isobaric analog state (IAS) at $E_x = 6279.0(6)$ keV, giving it an unambiguous spin and parity of $3/2^+$ and making it an important $l = 0$ resonance for proton capture on $^{30}$P.

nucl-ex↗

arXiv2015

Observation of Doppler broadening in $β$-delayed proton-$γ$ decay

Background: The Doppler broadening of $γ$-ray peaks due to nuclear recoil from $β$-delayed nucleon emission can be used to measure the energies of the nucleons. This method has never been tested using $β$-delayed proton emission or applied to a recoil heavier than $A=10$. Purpose: To test and apply this Doppler broadening method using $γ$-ray peaks from the $^{26}$P($βpγ$)$^{25}$Al decay sequence. Methods: A fast beam of $^{26}$P was implanted into a planar Ge detector, which was used as a $^{26}$P $β$-decay trigger. The SeGA array of high-purity Ge detectors was used to detect $γ$ rays from the $^{26}$P($βpγ$)$^{25}$Al decay sequence. Results: Radiative Doppler broadening in $β$-delayed proton-$γ$ decay was observed for the first time. The Doppler broadening analysis method was verified using the 1613 keV $γ$-ray line for which the proton energies were previously known. The 1776 keV $γ$ ray de-exciting the 2720 keV $^{25}$Al level was observed in $^{26}$P($βpγ$)$^{25}$Al decay for the first time and used to determine that the center-of-mass energy of the proton emission feeding the 2720-keV level is 5.1 $\pm$ 1.0 (stat.) $\pm$ 0.6 (syst.) MeV, corresponding to a $^{26}$Si excitation energy of 13.3 $\pm$ 1.0 (stat.) $\pm$ 0.6 (syst.) MeV for the proton-emitting level. Conclusions: The Doppler broadening method has been demonstrated to provide practical measurements of the energies for $β$-delayed nucleon emissions populating excited states of nuclear recoils at least as heavy as $A=25$.

nucl-ex↗

arXiv2015

Discovery of $^{34g,m}$Cl$(p,γ)^{35}$Ar resonances activated at classical nova temperatures

Background: The thermonuclear $^{34g,m}$Cl($p,γ$)$^{35}$Ar reaction rates are unknown due to a lack of experimental nuclear physics data. Uncertainties in these rates translate to uncertainties in $^{34}$S production in models of classical novae on oxygen-neon white dwarfs. $^{34}$S abundances have the potential to aid in the classification of presolar grains. Purpose: Determine resonance energies for the $^{34g,m}$Cl($p,γ$)$^{35}$Ar reactions within the region of astrophyical interest for classical novae to a precision of a few keV as an essential first step toward constraining their thermonuclear reaction rates. Method: $^{35}$Ar excited states were populated by the $^{36}$Ar($d,t$)$^{35}$Ar reaction at $E$(d)=22~MeV and reaction products were momentum analyzed by a high resolution quadrupole-dipole-dipole-dipole (Q3D) magnetic spectrograph. Results: Seventeen new $^{35}$Ar levels have been detected at a statistically significant level in the region $E_x\approx$~5.9-6.7~MeV ($E_r$ \textless~800~ keV) and their excitation energies have been determined to typical uncertainties of 3~keV. The uncertainties for five previously known levels have also been reduced substantially. The measured level density was compared to those calculated using the WBMB Hamiltonian within the $sd-pf$ model space. Conclusions: Most of the resonances in the region of astrophyical interest have likely been discovered and their energies have been determined, but the resonance strengths are still unknown, and experimentally constraining the $^{34g,m}$Cl($p,γ$)$^{35}$Ar reaction rates will require further experiments.

nucl-ex↗