arXiv subjects

I. Wiedenhöver

Publications and source records attributed to I. Wiedenhöver.

16 recordsLinked to original sources

arXiv2026

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

The $^{18}$F($α,p$)$^{21}$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($α,p$)$^{21}$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($α,p$)$^{21}$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.

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arXiv2026

The ICESPICE demonstrator for particle/$γ$-$e^{-}$ coincidence experiments at Florida State University

The Internal Conversion Electron SPectrometer In Coincidence Experiments (ICESPICE) demonstrator has been developed at Florida State University to enable particle/gamma-electron coincidence measurements in low-energy nuclear structure studies. ICESPICE is based on the mini-orange spectrometer concept and features a modular design using commercially available permanent magnets arranged in toroidal configurations to transport internal conversion electrons to room-temperature PIPS detectors while suppressing background from undesired particles. The system was optimized through SolidWorks modeling, COMSOL magnetic field simulations, and Geant4 particle tracking to maximize the magnetic transmission probability for electrons around 1 MeV. Commissioning tests using a calibrated 207Bi source demonstrated the performance of multiple spectrometer-detector configurations. Coincidence measurements between CeBr3 detectors from the CeBrA array and PIPS detectors revealed clear gamma-electron correlations. The first in-beam particle-electron measurements using ICESPICE were performed with the Super-Enge Split-Pole Spectrograph (SE-SPS) in the 208Pb(d,t)207Pb reaction. Prompt coincidences between tritons detected with the SE-SPS and electrons detected with ICESPICE were observed. The presented results show that ICESPICE is a promising ancillary detector system for in-beam internal conversion electron spectroscopy at the FSU SE-SPS.

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arXiv2026

Properties of states in \textsuperscript{19}Ne important for the \textsuperscript{18}F$(p,α)$\textsuperscript{15}O reaction rate

Observation of the 511-keV positron-annihilation line would be a powerful probe of classical novae, with the primary source of positrons likely from the $β^+$ decay of \textsuperscript{18}F. We have determined the properties of important resonances in $^{19}$Ne which govern the \textsuperscript{18}F($p,α$)\textsuperscript{15}O reaction rate and the production of \textsuperscript{18}F in novae. Measured $α$ and proton angular distributions from states populated in the \textsuperscript{19}F(\textsuperscript{3}He,$t$)\textsuperscript{19}Ne reaction identified six near-threshold proton $s$-wave \textsuperscript{18}F$+p$ ($L_p=0$) states, and the asymptotic normalization of these states was studied using the symmetry-adapted no-core shell model. We have improved our understanding of states contributing to the \textsuperscript{18}F($p,α$)\textsuperscript{15}O reaction rate and show that earlier studies significantly underestimated the uncertainties.

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arXiv2026

Study of the $in ^{34}$Ar($α,p$)$^{37}$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($α,p$)$^{37}$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($α,p$) 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($α,p$)$^{37}$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.

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arXiv2024

$^{11}$B states above the $α$-decay threshold studied via $^{10}$B$(d,p){}^{11}$B

The resonance region of $^{11}$B covering excitation energies from 8.4 MeV to 13.6 MeV was investigated with the $(d,p)$ reaction performed on an enriched $^{10}$B target at the Florida State University Super-Enge Split-Pole Spectrograph of the John D. Fox Superconducting Linear Accelerator Laboratory. Complementary measurements were performed with a target enriched in $^{11}$B to identify possible $^{12}$B contaminants in the $(d,p)$ reaction. Four strongly populated $^{11}$B states were observed above the $α$-decay threshold. Angular distributions were measured and compared to DWBA calculations to extract angular momentum transfers and $^{10}\mathrm{B}\left(3^+\right)+n$ spectroscopic factors. The recently observed and heavily discussed resonance at 11.4 MeV in $^{11}$B was not observed in this work. This result is consistent with the interpretation that it is predominantly a $^{10}\mathrm{Be}\left(0^+\right)+p$ resonance with a possible additional $^{7}\mathrm{Li}+α$ contribution. The predicted $^{10}\mathrm{B}\left(3^+\right)+n$ resonance at 11.6 MeV, analogous to the 11.4-MeV proton resonance, was not observed either. Upper limits for the $^{10}\mathrm{B}\left(3^+\right)+n$ spectroscopic factors of the 11.4-MeV and 11.6-MeV states were determined. In addition, supporting configuration interaction shell model calculations with the effective WBP interaction are presented.

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arXiv2024

Suppressed Electric Quadrupole Collectivity in $^{49}$Ti

Single-step Coulomb excitation of $^{46,48,49,50}$Ti is presented. A complete set of $E2$ matrix elements for the quintuplet of states in $^{49}$Ti, centered on the $2^+$ core excitation, was measured for the first time. A total of nine $E2$ matrix elements are reported, four of which were previously unknown. $^{49}_{22}$Ti$_{27}$ shows a $20\%$ quenching in electric quadrupole transition strength as compared to its semi-magic $^{50}_{22}$Ti$_{28}$ neighbour. This $20\%$ quenching, while empirically unprecedented, can be explained with a remarkably simple two-state mixing model, which is also consistent with other ground-state properties such as the magnetic dipole moment and electric quadrupole moment. A connection to nucleon transfer data and the quenching of single-particle strength is also demonstrated. The simplicity of the $^{49}$Ti-$^{50}$Ti pair (i.e., approximate single-$j$ $0f_{7/2}$ valence space and isolation of yrast states from non-yrast states) provides a unique opportunity to disentangle otherwise competing effects in the ground-state properties of atomic nuclei, the emergence of collectivity, and the role of proton-neutron interactions.

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arXiv2023

The CeBrA demonstrator for particle-$γ$ coincidence experiments at the FSU Super-Enge Split-Pole Spectrograph

We report on a highly selective experimental setup for particle-$γ$ coincidence experiments at the Super-Enge Split-Pole Spectrograph (SE-SPS) of the John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University (FSU) using fast CeBr$_3$ scintillators for $γ$-ray detection. Specifically, we report on the results of characterization tests for the first five CeBr$_3$ scintillation detectors of the CeBr$_3$ Array (CeBrA) with respect to energy resolution and timing characteristics. We also present results from the first particle-$γ$ coincidence experiments successfully performed with the CeBrA demonstrator and the FSU SE-SPS. We show that with the new setup, $γ$-decay branching ratios and particle-$γ$ angular correlations can be measured very selectively using narrow excitation energy gates, which are possible thanks to the excellent particle energy resolution of the SE-SPS. In addition, we highlight that nuclear level lifetimes in the nanoseconds regime can be determined by measuring the time difference between particle detection with the SE-SPS focal-plane scintillator and $γ$-ray detection with the fast CeBrA detectors. Selective excitation energy gates with the SE-SPS exclude any feeding contributions to these lifetimes.

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arXiv2023

$g_{9/2}$ neutron strength in the $N=29$ isotones and the $^{52}$Cr($d,p$)$^{53}$Cr reaction

We performed a measurement of the $^{52}$Cr$(d,p)^{53}$Cr reaction at 16 MeV using the Florida State University Super-Enge Split-Pole Spectrograph (SE-SPS) and observed 26 states. While all of the states observed here had been seen in previous $(d,p)$ experiments, we changed five $L$ assignments from those reported previously and determined $L$ values for nine states that had not had such assignments made previously. The $g_{9/2}$ neutron strength observed in $^{53}$Cr in the present work and in the $N=29$ isotones $^{49}$Ca, $^{51}$Ti, and $^{55}$Fe via $(d,p)$ reactions is much smaller than the sum rule for this strength. Most of the observed $L=4$ strength in these nuclei is located in states near 4 MeV excitation energy. The remaining $g_{9/2}$ strength may be located in the continuum or may be fragmented among many bound states. A covariant density functional theory calculation provides support for the hypothesis that the $g_{9/2}$ neutron orbit is unbound in $^{53}$Cr. The ($α,^3$He) reaction may provide a more sensitive probe for the missing $g_{9/2}$ neutron strength. In addition, particle-$γ$ coincidence experiments may help resolve some remaining questions in this nucleus.

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arXiv2022

$^{54}$Fe($d$,$p$)$^{55}$Fe and the evolution of single neutron energies in the $N=29$ isotones

A measurement of the $^{54}$Fe($d$,$p$)$^{55}$Fe reaction at 16 MeV was performed using the Florida State University Super-Enge Split-Pole Spectrograph to determine single-neutron energies for the $2p_{3/2}$, $2p_{1/2}$, $1f_{5/2}$, $1g_{9/2}$ and $2d_{5/2}$ orbits. Two states were observed that had not been observed in previous (d, p) measurements. In addition, we made angular momentum transfer, \textit{L}, assignments to four states and changed \textit{L} assignments from previous ($d$, $p$) measurements for nine more states. The spin-orbit splitting between the $2p_{3/2}$ and $2p_{1/2}$ orbits is similar to that in the other $N=29$ isotones and not close to zero as a previous measurement suggested. While the $1f_{5/2}$ single neutron energy is significantly lower in $^{55}$Fe than in $^{51}$Ti, as predicted by a covariant density functional theory calculation, the single-neutron energy for this orbit in $^{55}$Fe is more than 1 MeV higher than the calculation suggests, although it is only 400 keV above the $2p_{1/2}$ orbit. The summed spectroscopic strength we observed for the $1g_{9/2}$ orbit up to the single-neutron separation energy of 9.3 MeV is only 0.3. This is surprising because the $1g_{9/2}$ orbit is predicted by Togashi \textit{et al.} to be located only 5.5 MeV above the $2p_{3/2}$ orbit.

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arXiv2021

Measurement of the $^{18}$Ne($α,p$)$^{21}$Na reaction with ANASEN at $E_{c.m.} = $ 2.5-4 MeV

The $^{18}$Ne($α,p$)$^{21}$Na reaction plays a significant role in Type-I X-ray bursts. It is a major path in the breakout from the hot-CNO cycles to the synthesis of heavier elements in the $αp$-- and $rp$-processes. An experiment to determine the cross section of this reaction was performed with the ANASEN active-target detector system, determining the cross section at energies between 2.5 and 4 MeV in the center-of-mass frame. The measured cross sections for reactions populating the ground state in $^{21}$Na are consistent with results obtained from the time-inverse reaction, but significantly lower than the previously published experimental data of direct measurements. The total cross sections are also compared with those derived from indirect methods and statistical-model calculations. This experiment establishes a new experimental data set on the excitation function of the $^{18}$Ne($α,p$)$^{21}$Na reaction, revealing the significance of the excited states' contributions to the total reaction cross section and allowing to separate the contribution of the $(α,2p)$ reaction. The impact of the measured cross section on thermal reaction rates is discussed.

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arXiv2021

$^{50}$Ti($d$,$p$)$^{51}$Ti, single neutron energies in the $N=29$ isotones and the $N=32$ subshell closure

A measurement of the $^{50}$Ti($d$,$p$)$^{51}$Ti reaction at 16 MeV was performed using a Super Enge Split Pole Spectrograph to measure the magnitude of the $N=32$ subshell gap in Ti. Seven states were observed that had not been observed in previous ($d$,$p$) measurements, and the \textit{L} transfer values for six previously measured states were either changed or measured for the first time. The results were used to determine single neutron energies for the $p_{3/2}$, $p_{1/2}$ and $f_{5/2}$ orbitals. The resulting single neutron energies in $^{51}$Ti confirm the existence of the $N=32$ gap in Ti. These single neutron energies and those from previous measurements in $^{49}$Ca, $^{53}$Cr and $^{55}$Fe are compared to values from a covariant density functional theory calculation.

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arXiv2021

Experimental study of the $^{17}$F+ $^{12}$C fusion reaction and its implications for fusion of proton-halo systems

The halo nature of the low-lying $1/2$+ first excited state of the exotic weakly-bound proton drip-line nucleus $^{17}$F has long been hypothesized. The structure of such a halo nucleus would imply special nuclear properties including, possibly, an enhancement in its fusion cross section above the barrier. The total fusion cross section of $^{17}$F + $^{12}$C near the Coulomb barrier was studied using the newly developed "Encore" active-target detector at Florida State University. Total fusion cross sections for the stable counterpart systems $^{16}$O + $^{12}$C and $^{19}$F + $^{12}$C were also measured to enable a systematic comparison. No influence of the halo nature of the $^{17}$F $1/2$+ first excited state on its fusion excitation function was observed when compared with the stable counterpart systems.

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arXiv2018

Measurement of $d + ^7$Be cross sections for Big-Bang nucleosynthesis

The cross sections of nuclear reactions between the radioisotope $^7$Be and deuterium, a possible mechanism of reducing the production of mass-7 nuclides in Big-Bang nucleosynthesis, were measured at center-of-mass energies between 0.2 MeV and 1.5 MeV. The measured cross sections are dominated by the $(d,α)$ reaction channel, towards which prior experiments were mostly insensitive. A new resonance at 0.36(5)~MeV with a strength of $ωγ$ = 1.7(5)~keV was observed inside the relevant Gamow window. Calculations of nucleosynthesis outcomes based on the experimental cross section show that the resonance reduces the predicted abundance of primordial $^7$Li, but not sufficiently to solve the primordial lithium problem.

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arXiv2014

Sub-barrier enhancement of fusion as compared to a microscopic method in 18O+12C

Measurement of the energy dependence of the fusion cross-sec on at sub-barrier energies provides an important test for theoretical models of fusion. To extend the measurement of fusion cross-sections in the sub-barrier domain for the 18O+12C system. Use the new experimental data to confront microscopic calculations of fusion. Evaporation residues produced in fusion of 18O ions with 12C target nuclei were detected with good geometric efficiency and identified by measuring their energy and time-of-flight. Theoretical calculations with a density constrained time dependent Hartree-Fock (DC-TDHF) theory include for the first time the effect of pairing on the fusion cross-section. Comparison of the measured fusion excitation function with the predictions of the DC-TDHF calculations reveal that the experimental data exhibits a smaller decrease in cross-section with decreasing energy than is theoretically predicted. The larger cross-sections observed at the lowest energies measured indicate a larger tunneling probability for the fusion process. This larger probability can be associated with a smaller, narrower fusion barrier than presently included in the theoretical calculations.

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arXiv2009

Structure of $^{240}$Pu: Evidence for Octupole Phonon Condensation?

The expanded level structure of $^{240}$Pu available from the present study highlights the role of strong octupole correlations in this nucleus. Besides a delayed alignment in the yrast band, the observations include the presence of both $I^{+}{\to}(I-1)^{-}$ and $I^{-}{\to}(I-1)^{+}$ E1 transitions linking states of the yrast and negative-parity bands at high spin and the presence of an additional even-spin, positive-parity band deexciting exclusively to the negative parity sequence. The observations appear to be consistent with expectations based on the recently proposed concept of octupole phonon condensation.

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arXiv2004

$p$-$sd$ shell gap reduction in neutron-rich systems and cross-shell excitations in $^{20}$O

Excited states in $^{20}$O were populated in the reaction $^{10}$Be($^{14}$C,$α$) at Florida State University. Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and $γ$ radiation was detected with the FSU $γ$ detector array. Five new states were observed below 6 MeV from the $α$-$γ$ and $α$-$γ$-$γ$ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited 1p-1h excitations across the $N = Z = 8$ shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the $p$-$sd$ shell gap as neutrons are added.

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