arXiv subjects

S. Carmichael

Publications and source records attributed to S. Carmichael.

6 recordsLinked to original sources

arXiv2026

Direct measurement of $^{18}$F($\alpha,p$)$^{21}$Ne cross sections with ANASEN

The $^{18}$F($\alpha,p$)$^{21}$Ne reaction may impact Asymptotic Giant Branch nucleosynthesis and helium burning on accreting white dwarfs. This reaction has never been directly measured, and constraints from a previous time-inverse measurement leave large uncertainties in the reaction rate. We measured $^{18}$F($\alpha,p$)$^{21}$Ne cross sections directly for the first time covering from $2 \leq E_{cm} \leq 4$ MeV. Combining these results with previous work and comparisons to statistical model calculations results in a substantial improvement in uncertainties in the reaction rate. Cross sections were measured in inverse kinematics at TRIUMF-ISAC using a radioactive $^{18}$F beam and ANASEN with an extended $^{4}$He gas target. Protons were detected in arrays of silicon-strip detectors with the measured trajectories and energies allowing reconstruction of the center-of-mass energy and final state in $^{21}$Ne populated. We found the total cross section is in good agreement with statistical calculations, though population of the second excited state in $^{21}$Ne is greater than predicted. This direct measurement is combined with the previous time-inverse measurement and a new reaction rate is calculated for 0.1--3 GK. The uncertainties in the $^{18}$F($\alpha,p$)$^{21}$Ne reaction rate have been reduced to a sufficient level to allow robust predictions from AGB models. The new recommended rate leads to a 45\% increase in $^{19}$F production compared to the previous rate.

nucl-ex↗

arXiv2025

Triaxiality and shape dynamics in $^{70}$Ge

The electromagnetic properties of low-lying states in $^{70}$Ge were investigated via multi-step Coulomb excitation of a $^{70}$Ge beam impinging on a $^{208}$Pb target at the ATLAS facility of the Argonne National Laboratory. A total of 27 transitional elements and six diagonal matrix elements coupling 11 low-lying states, were extracted from the measured cross sections. These were used to calculate reduced transition probabilities, spectroscopic quadrupole moments, and rotational invariant shape parameters, providing enhanced precision and expanding on previous studies. The experimental data were compared within several theoretical frameworks, including the generalized triaxial rotor model, configuration interaction shell-model calculations, and computations within the combined frameworks of relativistic density functional theory and the five-dimensional collective Hamiltonian. The results demonstrate a good agreement with the experimental data and, in conjunction with calculations using a two-state mixing model, support significant triaxiality and strong mixing between the $0^+_1$ and $0^+_2$ states. This results in the magnitudes of their respective quadrupole deformations $[\beta_\text{rms}(0^+_1) = 0.228\,(3),\,\beta_\text{rms}(0^+_2) = 0.273\,(1)]$ being more similar than previously observed. The implications of these results for understanding the complex shape coexistence phenomena, the role of triaxiality, and shape evolution along the Ge isotopic chain are discussed.

nucl-ex↗

arXiv2024

Energy, strength, and alpha width measurements of $E_{\rm{c.m.}} = 1323$ and $1487$ keV resonances in $^{15}$N($\alpha,\gamma$)$^{19}$F

The $^{15}$N($\alpha,\gamma$)$^{19}$F reaction produces $^{19}$F in asymptotic giant branch (AGB) stars, where the low energy tails of two resonances at $E_{\rm{c.m.}} = 1323 \pm 2$ and $1487 \pm 1.7$ keV are estimated to contribute about $30\%$ of the total reaction rate in these environments. However, recent measurements have shown discrepancies in the energies, the strengths, and the corresponding alpha widths of these two resonances, resulting in an increase in the systematic uncertainty of the extrapolated cross section to helium burning energies. With this motivation, we have undertaken new measurements of the $^{15}$N$(\alpha,\gamma)^{19}$F at the University of Notre Dame Nuclear Science Laboratory. The setup consisted of an alpha particle beam impinged on a solid Ti$^{15}$N target with gamma-ray spectroscopy accomplished using a high purity germanium detector. Using the Doppler corrected gamma-ray energies, we confirmed the lower resonance energy to be $1321.6 \pm 0.6$ keV and found a value for the higher one of $1479.4 \pm 0.6$ keV that is more consistent with those found from previous elastic scattering studies. We found that the resonance strengths for both were consistent with most values found in the literature, but a larger alpha width has been recommended for the $E_{\rm{c.m.}} = 1487$ keV resonance. The larger alpha width suggests a reaction rate increase of about $15\%$ at temperatures $T < 0.1$ GK relevant to low mass AGB stars. The impact of the increased reaction rate requires further investigations.

nucl-ex↗

arXiv2023

Obtaining high resolution excitation functions with an active thick-target approach and validating them with mirror nuclei

Measurement of fusion excitation functions for stable nuclei has largely been restricted to nuclei with significant natural abundance. Typically, to investigate neighboring nuclei with low natural abundance has required obtaining isotopically enriched material. This restriction often limits the ability to perform such measurements. We report the measurement of a high quality fusion excitation function for a $^{17}$O beam produced from unenriched material with 0.038\% natural abundance. The measurement is enabled by using an active thick-target approach and the accuracy of the result is validated using its mirror nucleus $^{17}$F and resonances. The result provides important information about the average fusion cross-section for the oxygen isotopic chain as a function of neutron excess.

nucl-ex↗

arXiv2021

MuSIC@Indiana: an effective tool for accurate measurement of fusion with low-intensity radioactive beams

The design, construction, and characterization of the Multi-Sampling Ionization Chamber, MuSIC@Indiana, are described. This detector provides efficient and accurate measurement of the fusion cross-section at near-barrier energies. The response of the detector to low-intensity beams of $^{17,18}$O, $^{19}$F, $^{23}$Na, $^{24,26}$Mg, $^{27}$Al, and $^{28}$Si at E$_{lab}$ = 50-60 MeV was examined. MuSIC@Indiana was commissioned by measuring the $^{18}$O+$^{12}$C fusion excitation function for 11 $<$ E$_{cm}$ $<$ 20 MeV using CH$_{4}$ gas. A simple, effective analysis cleanly distinguishes proton capture and two-body scattering events from fusion on carbon. With MuSIC@Indiana, measurement of 15 points on the excitation function for a single incident beam energy is achieved. The resulting excitation function is shown to be in good agreement with literature data

physics.ins-det↗

arXiv2009

A Blind Search for Magnetospheric Emissions from Planetary Companions to Nearby Solar-type Stars

This paper reports a blind search for magnetospheric emissions from planets around nearby stars. Young stars are likely to have much stronger stellar winds than the Sun, and because planetary magnetospheric emissions are powered by stellar winds, stronger stellar winds may enhance the radio luminosity of any orbiting planets. Using various stellar catalogs, we selected nearby stars (<~ 30 pc) with relatively young age estimates (< 3 Gyr). We constructed different samples from the stellar catalogs, finding between 100 and several hundred stars. We stacked images from the 74-MHz (4-m wavelength) VLA Low-frequency Sky Survey (VLSS), obtaining 3σlimits on planetary emission in the stacked images of between 10 and 33 mJy. These flux density limits correspond to average planetary luminosities less than 5--10 x 10^{23} erg/s. Using recent models for the scaling of stellar wind velocity, density, and magnetic field with stellar age, we estimate scaling factors for the strength of stellar winds, relative to the Sun, in our samples. The typical kinetic energy carried by the stellar winds in our samples is 15--50 times larger than that of the Sun, and the typical magnetic energy is 5--10 times larger. If we assume that every star is orbited by a Jupiter-like planet with a luminosity larger than that of the Jovian decametric radiation by the above factors, our limits on planetary luminosities from the stacking analysis are likely to be a factor of 10--100 above what would be required to detect the planets in a statistical sense. Similar statistical analyses with observations by future instruments, such as the Low Frequency Array (LOFAR) and the Long Wavelength Array (LWA), offer the promise of improvements by factors of 10--100.

astro-ph.EP↗