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C. Wheldon

Publications and source records attributed to C. Wheldon.

16 recordsLinked to original sources

Differential cross sections for ${{^{12}\mathrm{C}(n,\alpha_{0})}}$, ${{^{16}\mathrm{O}(n,\alpha_{0})}}$ and ${{^{16}\mathrm{O}(n,\alpha_{1,2,3})}}$ between ${{E_n}}$ = 7.2 and 10 MeV with an active-target Time Projection Chamber

Data for the ${{^{12}\mathrm{C}(n,\alpha_{0})}}$, ${{^{16}\mathrm{O}(n,\alpha_{0})}}$ and ${{^{16}\mathrm{O}(n,\alpha_{1,2,3})}}$ differential cross sections are important for several different areas of nuclear physics such as understanding neutron transmutation in nuclear reactors. The TexAT Time Projection Chamber was used to measure the differential and angle-integrated cross sections in active-target mode. The chamber was filled with CO$_2$ gas and used a quasi-monoenergetic neutron beam from the $d(d,n)$ reaction at Edwards Accelerator Lab at Ohio University. A comparison between our current and previous results at overlapping energies and angles which showed good agreement in angular dependence and absolute cross section. A broader angular coverage than previous results demonstrated that the integrated cross section for the \po16 reaction deviates from ENDFVIII.0 evaluations. This first instance of neutron-induced measurements with an active-target Time Projection Chamber demonstrates the use of this method for high-quality differential cross section data across a broad angular range, generating good statistics with a relatively low-intensity beam.

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

Background: The preference for light nuclear systems to coagulate into $\alpha$-particle clusters has been well-studied. The possibility of a linear chain configuration of $\alpha$-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 $\pi^2$ 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 $\alpha$-particles detected by a silicon detector array to measure the $^{10}\mathrm{C}(\alpha,\alpha)$ 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 $\pi^2$ 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.

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Unbound neutron $\nu0d_{3/2}$ strength in $^{17}$C and the N=16 shell gap

Significant continuum strength has been observed to be populated in $^{17}$C produced in the d($^{16}$C,p) reaction at a beam energy of 17.2~MeV/nucleon. The strength appears at greater than $\sim$2~MeV above the single-neutron decay threshold and has been identified as arising from transfer into the neutron $0d_{3/2}$ orbital. Guided by shell model predictions the greater majority of the strength is associated with a 3/2$^+$ state at an excitation energy of 4.40$_{-0.14}^{+0.33}$ MeV and a much weaker 3/2$^+$ level at 5.60$_{-0.45}^{+1.35}$ MeV. The corresponding total widths were determined to be 3.45$_{-0.78}^{+1.82}$ and 1.6$_{-1.4}^{+4.6}$ MeV, respectively. From the backward angle proton differential cross sections and the branching ratios for neutron decay to the $^{16}$C(2$_{1}^{+}$) level, the corresponding spectroscopic factors to the ground state were deduced to be 0.47$\pm{10}$ and $<$0.09. Shell-model calculations employing the phenomenological SFO-tls interaction as well as Gamow Shell-Model calculations including continuum effects are in reasonable agreement with experiment, although the predicted strength lies at somewhat lower energy. The size of the N=16 shell gap ($\varepsilon_{ \nu0d_{3/2}}-\varepsilon _{\nu 1s_{1/2}}$) was estimated to be 5.08$_{-0.33}^{+0.43}$~MeV - some 1.3~MeV larger than found in the SFO-tls shell model calculation.

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$^{179}$Ta(n,$\gamma$) cross-section measurement and the astrophysical origin of $^{180}$Ta isotope

Tantalum-180m is nature's rarest (quasi) stable isotope and its astrophysical origin is an open question. A possible production site of this isotope is the slow neutron capture process in Asymptotic Giant Branch stars, where it can be produced via neutron capture reactions on unstable $^{179}$Ta. We report a new measurement of the $^{179}$Ta($n,\gamma$)$^{180}$Ta cross section at thermal neutron energies via the activation technique. Our results for the thermal and resonance-integral cross-sections are 952 $\pm$ 57 b and 2013 $\pm$ 148 b, respectively. The thermal cross section is in good agreement with the only previous measurement (Phys. Rev C {\bf 60} 025802, 1999), while the resonance integral is different by a factor of $\approx$1.7. While neutron energies in this work are smaller than the energies in a stellar environment, our results may lead to improvements in theoretical predictions of the stellar cross section.

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Cross-shell states in $^{15}$C: a test for p-sd interactions

The low-lying structure of $^{15}$C has been investigated via the neutron-removal $^{16}$C$(d,t)$ reaction. Along with bound neutron sd-shell hole states, unbound p-shell hole states have been firmly confirmed. The excitation energies and the deduced spectroscopic factors of the cross-shell states are an important measure of the $[(p)^{-1}(sd)^{2}]$ neutron configurations in $^{15}$C. Our results show a very good agreement with shell-model calculations using the SFO-tls interaction for $^{15}$C. However, a modification of the $p$-$sd$ and $sd$-$sd$ monopole terms was applied in order to reproduce the $N=9$ isotone $^{17}$O. In addition, the excitation energies and spectroscopic factors have been compared to the first calculations of $^{15}$C with the $ab~ initio$ self-consistent Green's function method employing the NNLO$_{sat}$ interaction. The results show the sensitivity to the size of the $N=8$ shell gap and highlight the need of going beyond the current truncation scheme in the theory.

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Irradiation Facilities and Irradiation Methods for High Power Target

High Power Target systems are key elements in future neutrino and other rare particle production in accelerators. These systems transform an intense source of protons into secondary particles of interest to enable new scientific discoveries. As beam intensity and energies increase, target systems face significant challenges. Radiation damages and thermal shocks in target materials were identified as the leading cross-cutting challenges of high-power target facilities. Target material R&D to address these challenges are essential to enable and ensure reliable operation of future-generation accelerators. Irradiation facilities and alternative methods are critical to provide a full support of material R&D and better address these critical challenges.

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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.

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Low-lying single-particle structure of 17C and the N = 14 sub-shell closure

The first investigation of the single-particle structure of the bound states of 17C, via the d(16C, p) transfer reaction, has been undertaken. The measured angular distributions confirm the spin-parity assignments of 1/2+ and 5/2+ for the excited states located at 217 and 335 keV, respectively. The spectroscopic factors deduced for these states exhibit a marked single-particle character, in agreement with shell model and particle-core model calculations, and combined with their near degeneracy in energy provide clear evidence for the absence of the N = 14 sub-shell closure. The very small spectroscopic factor found for the 3/2+ ground state is consistent with theoretical predictions and indicates that the ν1d3/2 strength is carried by unbound states. With a dominant l = 0 valence neutron configuration and a very low separation energy, the 1/2+ excited state is a one-neutron halo candidate.

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The Hoyle Family: The search for alpha-condensate states in light nuclei

Our present understanding of the structure of the Hoyle state in $^{12}$C and other near-threshold states in $α$-conjugate nuclei is reviewed in the framework of the $α$-condensate model. The $^{12}$C Hoyle state, in particular, is a candidate for $α$-condensation, due to its large radius and $α$-cluster structure. The predicted features of nuclear $α$-particle condensates are reviewed along with a discussion of their experimental indicators, with a focus on precision break-up measurements. Two experiments are discussed in detail, firstly concerning the break-up of $^{12}$C and then the decays of heavier nuclei. With more theoretical input, and increasingly complex detector setups, precision break-up measurements can, in principle, provide insight into the structures of states in $α$-conjugate nuclei. However, the commonly-held belief that the decay of a condensate state will result in $N$ $α$-particles is challenged. We further conclude that unambiguously characterising excited states built on $α$-condensates is difficult, despite improvements in detector technology.

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Clustering in $^{18}$O -- absolute determination of branching ratios via high-resolution particle spectroscopy

The determination of absolute branching ratios for high-energy states in light nuclei is an important and useful tool for probing the underlying nuclear structure of individual resonances: for example, in establishing the tendency of an excited state towards $α$-cluster structure. Difficulty arises in measuring these branching ratios due to similarities in available decay channels, such as ($\mathbf{^{18}}$O,$\mathbf{n}$) and ($\mathbf{^{18}}$O,$\mathbf{2n}$), as well as differences in geometric efficiencies due to population of bound excited levels in daughter nuclei. Methods are presented using Monte Carlo techniques to overcome these issues.

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Strong neutron pairing in core+4n nuclei

The emission of neutron pairs from the neutron-rich $N\!=\!12$ isotones $^{18}$C and $^{20}$O has been studied by high-energy nucleon knockout from $^{19}$N and $^{21}$O secondary beams, populating unbound states of the two isotones up to 15~MeV above their two-neutron emission thresholds. The analysis of triple fragment-$n$-$n$ correlations shows that the decay $^{19}$N$(-1p)^{18}$C$^*\!\rightarrow^{16}$C+$n$+$n$ is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a $^{14}$C core surrounded by four valence neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay $^{21}$O$(-1n)^{20}$O$^*\!\rightarrow^{18}$O+$n$+$n$, attributed to its formation through the knockout of a deeply-bound neutron that breaks the $^{16}$O core and reduces the number of pairs.

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Characterization of the proposed 4-α cluster state candidate in 16O

The $\mathrm{^{16}O}(α, α^{\prime})$ reaction was studied at $θ_{lab} = 0^\circ$ at an incident energy of $\textrm{E}_{lab}$ = 200 MeV using the K600 magnetic spectrometer at iThemba LABS. Proton and $α$-decay from the natural parity states were observed in a large-acceptance silicon-strip detector array at backward angles. The coincident charged particle measurements were used to characterize the decay channels of the $0_{6}^{+}$ state in $\mathrm{^{16}O}$ located at $E_{x} = 15.097(5)$ MeV. This state is identified by several theoretical cluster calculations to be a good candidate for the 4-$α$ cluster state. The results of this work suggest the presence of a previously unidentified resonance at $E_{x}\approx15$ MeV that does not exhibit a $0^{+}$ character. This unresolved resonance may have contaminated previous observations of the $0_{6}^{+}$ state.

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Evidence for Triangular D_3h Symmetry in 12C

We report a measurement of a new high spin Jp = 5- state at 22.4(0.2) MeV in 12C which fits very well to the predicted (ground state) rotational band of an oblate equilateral triangular spinning top with a D_3h symmetry characterized by the sequence 0+, 2+, 3-, 4+/-, 5- with almost degenerate 4+ and 4- (parity doublet) states. Such a D_3h symmetry was observed in triatomic molecules and it is observed here for the first time in nuclear physics. We discuss a classification of other rotation-vibration bands in 12C such as the (0+) Hoyle band and the (1-) bending mode band and suggest measurements in search of the predicted ("missing") states that may shed new light on clustering in 12C and light nuclei. In particular the observation (or non-observation) of the predicted ("missing") states in the Hoyle band will allow us to conclude the geometrical arrangement of the three alpha-particle composing the Hoyle state at 7.654 MeV in 12C.

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Clusters in Light Nuclei

A great deal of research work has been undertaken in the alpha-clustering study since the pioneering discovery, half a century ago, of 12C+12C molecular resonances. Our knowledge of the field of the physics of nuclear molecules has increased considerably and nuclear clustering remains one of the most fruitful domains of nuclear physics, facing some of the greatest challenges and opportunities in the years ahead. In this work, the occurence of "exotic" shapes in light N=Z alpha-like nuclei is investigated. Various approaches of superdeformed and hyperdeformed bands associated with quasimolecular resonant structures are presented. Results on clustering aspects are also discussed for light neutron-rich Oxygen isotopes.

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Binary reaction decays from 24Mg+12C

Charged particle and gamma decays in 24Mg* are investigated for excitation energies where quasimolecular resonances appear in 12C+12C collisions. Various theoretical predictions for the occurence of superdeformed and hyperdeformed bands associated with resonance structures with low spin are discussed within the measured 24Mg* excitation energy region. The inverse kinematics reaction 24Mg+12C is studied at E_lab(24Mg) = 130 MeV, an energy which enables the population of 24Mg states decaying into 12C+12C resonant break-up states. Exclusive data were collected with the Binary Reaction Spectrometer in coincidence with EUROBALL IV installed at the VIVITRON Tandem facility at Strasbourg. Specific structures with large deformation were selectively populated in binary reactions and their associated gamma decays studied. Coincident events associated with inelastic and alpha-transfer channels have been selected by choosing the excitation energy or the entry point via the two-body Q-values. The analysis of the binary reaction channels is presented with a particular emphasis on 24Mg-gamma, 20Ne-gamma and 16O-gamma coincidences. New information (spin and branching ratios) is deduced on high-energy states in 24Mg and 16O, respectively.

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Reaction mechanisms in 24Mg+12C and 32S+24Mg

The occurence of "exotic" shapes in light N=Z alpha-like nuclei is investigated for 24Mg+12C and 32S+24Mg. Various approaches of superdeformed and hyperdeformed bands associated with quasimolecular resonant structures with low spin are presented. For both reactions, exclusive data were collected with the Binary Reaction Spectrometer in coincidence with EUROBALL IV installed at the VIVITRON Tandem facility of Strasbourg. Specific structures with large deformation were selectively populated in binary reactions and their associated $γ$-decays studied. The analysis of the binary and ternary reaction channels is discussed.

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