arXiv subjects

S. Upadhyayula

Publications and source records attributed to S. Upadhyayula.

At least 19 recordsLinked to original sources

arXiv2025

Direct measurement of $^{59}$Cu($p$,$\alpha$)$^{56}$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($p$,$\gamma$)$^{60}$Zn and $^{59}$Cu($p$,$\alpha$)$^{56}$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($p$,$\alpha$)$^{56}$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↗

arXiv2025

Potential absence of observed $π^2$ linear-chain structures in $^{14}$O via $^{10}$C($α,α$) resonant scattering

Background: The preference for light nuclear systems to coagulate into $α$-particle clusters has been well-studied. The possibility of a linear chain configuration of $α$-particles would allow for a new way to study this phenomenon. Purpose: A rotational band of states in $^{14}$C has been claimed showing a $π^2$ linear chain structure. The mirror system, $^{14}$O, has been studied here to examine how this linear chain structure is affected by replacing the valence neutrons with protons. Method: A beam of $^{10}$C was incident into a chamber filled with He:CO$_2$ gas with the tracks recorded inside the TexAT Time Projection Chamber and the recoil $α$-particles detected by a silicon detector array to measure the $^{10}\mathrm{C}(α,α)$ cross section. Results: The experimental cross section was compared with previous studies and fit using R-Matrix theory with the previously-observed $^{14}$O states being transformed to the $^{14}$C using mirror symmetry. The measured cross section does not replicate the claimed states, with the predicted cross section exceeding that observed at several energies and angles. Conclusion: A series of possibilities are highlighted with the most likely being that the originally-seen $^{14}$C states did not constitute a $π^2$ rotational band with a potentially incorrect spin assignment due to the limitations of the angular correlation method with non-zero spin particles. The work highlights the difficulties in measuring broad resonances corresponding to a linear chain state in a high level density.

nucl-ex↗

arXiv2025

First In-Beam Demonstration of a hybrid LaBr3/CeBr3/BGO array to measure radiative capture resonance energies in an extended gas target using a novel time of flight technique

We have deployed a new hybrid array of LaBr3, CeBr3, and BGO scintillators for detecting $γ$ rays at the DRAGON recoil separator at TRIUMF. The array was developed to improve $γ$-ray timing resolution over the existing BGO array. This allows the average position of resonant capture in an extended gas target to be determined with $\sim$15 mm precision or better, even with five or fewer detected capture events. This, in turn, allows determination of resonant capture energies with statistical uncertainties below ${\sim} 1\%$. Here we report the results of a first in-beam demonstration of the array, measuring the $E_{cm} = 0.4906(3)$ MeV resonance in the ${}^{23}\mathrm{Na}(p,γ){}^{24}\mathrm{Mg}$ reaction, focusing on the timing properties of the array and its anticipated performance in future experiments with radioactive beams.

physics.ins-det↗

arXiv2024

Study of the $in ^{34}$Ar($\alpha,p$)$^{37}$K reaction rate via proton scattering on $^{37}$K, and its impact on properties of modeled X-Ray bursts

Background: Type I X-Ray bursts (XRBs) are energetic stellar explosions that occur on the surface of a neutron star in an accreting binary system with a low-mass H/He-rich companion. The rate of the $^{34}$Ar($\alpha,p$)$^{37}$K reaction may influence features of the light curve that results from the underlying thermonuclear runaway, as shown in recent XRB stellar modelling studies. Purpose: In order to reduce the uncertainty of the rate of this reaction, properties of resonances in the compound nucleus $^{38}$Ca, such as resonance energies, spins, and particle widths, must be well constrained. Method: This work discusses a study of resonances in the $^{38}$Ca compound nucleus produced in the $^{34}$Ar($\alpha,p$) reaction. The experiment was performed at the National Superconducting Cyclotron Laboratory, with the ReA3 facility by measuring proton scattering using an unstable $^{37}$K beam. The kinematics were designed specifically to identify and characterize resonances in the Gamow energy window for the temperature regime relevant to XRBs. Results: The spins and proton widths of newly identified and previously known states in $^{38}$Ca in the energy region of interest for the $^{34}$Ar($\alpha,p$)$^{37}$K reaction have been constrained through an R-Matrix analysis of the scattering data. Conclusions: Using these constraints, a newly estimated rate is applied to an XRB model built using Modules for Experiments in Stellar Astrophysics (MESA), to examine its impact on observables, including the light curve. It is found that the newly determined reaction rate does not substantially affect the features of the light curve.

nucl-ex↗

arXiv2022

alpha-cluster structure of 18Ne

In this work we study alpha-clustering in 18Ne and compare it with what is known about clustering in the mirror nucleus 18O. The excitation function of 18Ne was measured in inverse kinematics from the resonant elastic scattering reaction of 14O on 4He in the excitation energy range from 8 to 17 MeV, using the active target TexAT. The analysis was performed using a multi-channel R-matrix approach. Detailed spectroscopic information is obtained from the R-matrix analysis: excitation energy of the states, spin and parity as well as partial alpha and total widths. This information is compared with theoretical models and previous data. Clustering structures appear to be robust and mostly isospin symmetric. A good correspondence was found between the levels in 18O and 18Ne. We carried out an extensive shell model analysis of the experimental data. This comparison suggests that strongly clustered states remain organized in relation to the corresponding reaction channel identified by the number of nodes in the relative alpha plus core wave function. The agreement between theory and experiment is very good and especially useful when it comes to understanding the clustering strength distribution. The comparison of the experimental data with theory shows that certain states, especially at high excitation energies, are significantly more clustered than predicted. This indicates that the structure of these states is collective and is aligned towards the corresponding alpha reaction channel.

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↗

arXiv2021

Study of ($^6$Li, $d$) and ($^6$Li, $t$) reactions on $^{22}$Ne and implications for $s$-process nucleosynthesis

We studied $α$ cluster states in $^{26}$Mg via the $^{22}$Ne($^{6}$Li,$dγ$)$^{26}$Mg reaction in inverse kinematics at an energy of $7$ MeV/nucleon. States between $E_x$ = 4 - 12 MeV in $^{26}$Mg were populated and relative $α$ spectroscopic factors were determined. Some of these states correspond to resonances in the Gamow window of the $^{22}$Ne($α$,n)$^{25}$Mg reaction, which is one of the main neutron sources in the astrophysical $s$-process. We show that $α$-cluster strength of the states analyzed in this work have critical impact on s-process abundances. Using our new $^{22}$Ne($α$,n)$^{25}$Mg and $^{22}$Ne($α$,$γ$)$^{26}$Mg reaction rates, we performed new s-process calculations for massive stars and Asymptotic Giant Branch stars and compared the resulting yields with the yields obtained using other $^{22}$Ne+$α$ rates from the literature. We observe an impact on the s-process abundances up to a factor of three for intermediate-mass AGB stars and up to a factor of ten for massive stars. Additionally, states in $^{25}$Mg at $E_x$ $<$ 5 MeV are identified via the $^{22}$Ne($^{6}$Li,$t$)$^{25}$Mg reaction for the first time. We present the ($^6$Li, $t$) spectroscopic factors of these states and note similarities to the $(d,p$) reaction in terms of reaction selectivity.

nucl-ex↗

arXiv2021

Evidence against the Efimov effect in $^{12}\mathrm{C}$ from spectroscopy and astrophysics

Background: The Efimov effect is a universal phenomenon in physics whereby three-body systems are stabilized via the interaction of an unbound two-body sub-systems. A hypothetical state in $^{12}\mathrm{C}$ at 7.458 MeV excitation energy, comprising of a loose structure of three $α$-particles in mutual two-body resonance, has been suggested in the literature to correspond to an Efimov state in nuclear physics. The existence of such a state has not been demonstrated experimentally. Method: Using the combined data sets from two recent experiments, one with the TexAT TPC to measure $α$-decay and the other with Gammasphere to measure $γ$-decay of states in $^{12}\mathrm{C}$ populated by $^{12}\mathrm{N}$ and $^{12}\mathrm{B}$ $β$-decay respectively, we achieve high sensitivity to states in close-proximity to the $α$-threshold in $^{12}\mathrm{C}$. Results: No evidence of a state at 7.458 MeV is seen in either data set. Using a likelihood method, the 95\% C.L. $γ$-decay branching ratio is determined as a function of the $β$-decay feeding strength relative to the Hoyle state. In parallel, calculations of the triple-alpha reaction rate show the inclusion of the Efimov corresponds to a large increase in the reaction rate around $5 \times 10^{7}$ K. Conclusion: From decay spectroscopy - at the 95\% C.L., the Efimov state cannot exist at 7.458 MeV with any $γ$-decay branching ratio unless the $β$-strength is less than 0.7\% of the Hoyle state. This limit is evaluated for a range of different excitation energies and the results are not favorable for existence of the hypothetical Efimov state in $^{12}\mathrm{C}$. Furthermore, the triple-alpha reaction rate with the inclusion of a state between 7.43 and 7.53 MeV exceeds the rate required for stars to undergo the red giant phase.

nucl-ex↗

arXiv2020

Almost medium-free measurement of the Hoyle state direct-decay component with a TPC

Background: The structure of the Hoyle state, a highly $α$-clustered state at 7.65 MeV in $^{12}\mathrm{C}$, has long been the subject of debate. Understanding if the system comprises of three weakly-interacting $α$-particles in the 0s orbital, known as an $α$-condensate state, is possible by studying the decay branches of the Hoyle state. Purpose: The direct decay of the Hoyle state into three $α$-particles, rather than through the $^{8}\mathrm{Be}$ ground state, can be identified by studying the energy partition of the 3 $α$-particles arising from the decay. This paper provides details on the break-up mechanism of the Hoyle stating using a new experimental technique. Method: By using beta-delayed charged-particle spectroscopy of $^{12}\mathrm{N}$ using the TexAT (Texas Active Target) TPC, a high-sensitivity measurement of the direct 3 $α$ decay ratio can be performed without contributions from pile-up events. Results: A Bayesian approach to understanding the contribution of the direct components via a likelihood function shows that the direct component is $<0.043\%$ at the 95\% confidence level (C.L.). This value is in agreement with several other studies and here we can demonstrate that a small non-sequential component with a decay fraction of about $10^{-4}$ is most likely. Conclusion: The measurement of the non-sequential component of the Hoyle state decay is performed in an almost medium-free reaction for the first time. The derived upper-limit is in agreement with previous studies and demonstrates sensitivity to the absolute branching ratio. Further experimental studies would need to be combined with robust microscopic theoretical understanding of the decay dynamics to provide additional insight into the idea of the Hoyle state as an $α$-condensate.

nucl-ex↗

arXiv2020

Texas Active Target (TexAT) detector for experiments with rare isotope beams

The TexAT (Texas Active Target) detector is a new active-target time projection chamber (TPC) that was built at the Cyclotron Institute Texas A$\&$M University. The detector is designed to be of general use for nuclear structure and nuclear astrophysics experiments with rare isotope beams. TexAT combines a highly segmented Time Projection Chamber (TPC) with two layers of solid state detectors. It provides high efficiency and flexibility for experiments with low intensity exotic beams, allowing for the 3D track reconstruction of the incoming and outgoing particles involved in nuclear reactions and decays.

physics.ins-det↗

arXiv2020

Constraining the $^{22}$Ne($α$,$γ$)$^{26}$Mg and $^{22}$Ne($α$,n)$^{25}$Mg reaction rates using sub-Coulomb $α$-transfer reactions

The $^{22}$Ne($α$,$γ$)$^{26}$Mg and $^{22}$Ne($α$,n)$^{25}$Mg reactions play an important role in astrophysics because they have significant influence on the neutron flux during the weak branch of the s-process. We constrain the astrophysical rates for these reactions by measuring partial $α$-widths of resonances in $^{26}$Mg located in the Gamow window for the $^{22}$Ne+$α$ capture. These resonances were populated using $^{22}$Ne($^6$Li,d)$^{26}$Mg and $^{22}$Ne($^7$Li,t)$^{26}$Mg reactions at energies near the Coulomb barrier. At these low energies $α$-transfer reactions favor population of low spin states and the extracted partial $α$-widths for the observed resonances exhibit only minor dependence on the model parameters. The astrophysical rates for both the $^{22}$Ne($α$,$γ$)$^{26}$Mg and the $^{22}$Ne($α$,n)$^{25}$Mg reactions are shown to be significantly different than the previously suggested values.

nucl-ex↗

arXiv2020

Decay properties of $^{22}\mathrm{Ne} + α$ resonances and their impact on $s$-process nucleosynthesis

The astrophysical $s$-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding $s$-process nucleosynthesis is the neutron flux generated by the ${}^{22}\mathrm{Ne}(α, n){}^{25}\mathrm{Mg}$ reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing ${}^{22}\mathrm{Ne}(α, γ){}^{26}\mathrm{Mg}$ reaction, is not well constrained in the important temperature regime from ${\sim} 0.2$--$0.4$~GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the ${}^{22}\mathrm{Ne}({}^{6}\mathrm{Li}, d){}^{26}\mathrm{Mg}$ reaction in inverse kinematics, detecting the outgoing deuterons and ${}^{25,26}\mathrm{Mg}$ recoils in coincidence. We have established a new $n / γ$ decay branching ratio of $1.14(26)$ for the key $E_x = 11.32$ MeV resonance in $^{26}\mathrm{Mg}$, which results in a new $(α, n)$ strength for this resonance of $42(11)~μ$eV when combined with the well-established $(α, γ)$ strength of this resonance. We have also determined new upper limits on the $α$ partial widths of neutron-unbound resonances at $E_x = 11.112,$ $11.163$, $11.169$, and $11.171$ MeV. Monte-Carlo calculations of the stellar ${}^{22}\mathrm{Ne}(α, n){}^{25}\mathrm{Mg}$ and ${}^{22}\mathrm{Ne}(α, γ){}^{26}\mathrm{Mg}$ rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ${\sim} 0.2$--$0.4$~GK.

nucl-ex↗

arXiv2019

Structure of 9C through proton resonance scattering with Texas Active Target detector

Background: Level structure of the most neutron deficient nucleon-bound carbon isotope, 9C, is not well known. Definitive spin-parity assignments are only available for two excited states. No positive parity states have been conclusively identified so far and the location of the sd-shell in A=9 T=3/2 isospin quadruplet is not known. Purpose: We have studied the level structure of exotic nucleus 9C at excitation energies below 6.4 MeV. Methods: Excited states in 9C were populated in 8B+p resonance elastic scattering and excitation functions were measured using active target approach. Results: Two excited states in 9C were conclusively observed, and R-matrix analysis of the excitation functions was performed to make the spin-parity assignments. The first positive parity state in A=9 T=3/2 nuclear system, the 5/2+ resonance at 4.3 MeV, has been identified. Conclusions: The new 5/2+ state at 4.3 MeV in 9C is a single-particle L=0 broad resonance and it determines the energy of the 2s shell. The 2s shell in this exotic nucleus appears well within the region dominated by the p-shell states.

nucl-ex↗

arXiv2017

Structure of 10N in 9C+p resonance scattering

The structure of exotic nucleus 10N was studied using 9C+p resonance scattering. Two L=0 resonances were found to be the lowest states in 10N. The ground state of 10N is unbound with respect to proton decay by 2.2(2) or 1.9(2) MeV depending on the 2- or 1- spin-parity assignment, and the first excited state is unbound by 2.8(2) MeV.

nucl-ex↗

arXiv2016

Dark matter effective field theory scattering in direct detection experiments

We examine the consequences of the effective field theory (EFT) of dark matter-nucleon scattering for current and proposed direct detection experiments. Exclusion limits on EFT coupling constants computed using the optimum interval method are presented for SuperCDMS Soudan, CDMS II, and LUX, and the necessity of combining results from multiple experiments in order to determine dark matter parameters is discussed. We demonstrate that spectral differences between the standard dark matter model and a general EFT interaction can produce a bias when calculating exclusion limits and when developing signal models for likelihood and machine learning techniques. We also discuss the implications of the EFT for the next-generation (G2) direct detection experiments and point out regions of complementarity in the EFT parameter space.

astro-ph.CO↗

arXiv2016

WIMP-Search Results from the Second CDMSlite Run

The CDMS low ionization threshold experiment (CDMSlite) uses cryogenic germanium detectors operated at a relatively high bias voltage to amplify the phonon signal in the search for weakly interacting massive particles (WIMPs). Results are presented from the second CDMSlite run with an exposure of 70 kg days, which reached an energy threshold for electron recoils as low as 56 eV. A fiducialization cut reduces backgrounds below those previously reported by CDMSlite. New parameter space for the WIMP-nucleon spin-independent cross section is excluded for WIMP masses between 1.6 and 5.5 GeV/$c^2$.

astro-ph.CO↗

arXiv2015

Improved WIMP-search reach of the CDMS II germanium data

CDMS II data from the 5-tower runs at the Soudan Underground Laboratory were reprocessed with an improved charge-pulse fitting algorithm. Two new analysis techniques to reject surface-event backgrounds were applied to the 612 kg days germanium-detector WIMP-search exposure. An extended analysis was also completed by decreasing the 10 keV analysis threshold to $\sim$5 keV, to increase sensitivity near a WIMP mass of 8 GeV/$c^2$. After unblinding, there were zero candidate events above a deposited energy of 10 keV and 6 events in the lower-threshold analysis. This yielded minimum WIMP-nucleon spin-independent scattering cross-section limits of $1.8 \times 10^{-44}$ and $1.18 \times 10 ^{-41}$ cm$^2$ at 90\% confidence for 60 and 8.6 GeV/$c^2$ WIMPs, respectively. This improves the previous CDMS II result by a factor of 2.4 (2.7) for 60 (8.6) GeV/$c^2$ WIMPs.

hep-ex↗

arXiv2015

First direct limits on Lightly Ionizing Particles with electric charge less than $e/6$

While the Standard Model of particle physics does not include free particles with fractional charge, experimental searches have not ruled out their existence. We report results from the Cryogenic Dark Matter Search (CDMS II) experiment that give the first direct-detection limits for cosmogenically-produced relativistic particles with electric charge lower than $e$/6. A search for tracks in the six stacked detectors of each of two of the CDMS II towers found no candidates, thereby excluding new parameter space for particles with electric charges between $e$/6 and $e$/200.

hep-ex↗