We study the sequential breakup of E/A=24.0 MeV ^{7}Li projectiles excited through inelastic interactions with C, Be, and Al target nuclei. For peripheral events that do not excite the target, we find very large spin alignment of the excited ^{7}Li projectiles longitudinal to the beam axis. This spin alignment is independent of the target used, and we propose a simple alignment mechanism that arises from an angular-momentum-excitation-energy mismatch. This mechanism is independent of the potential used for scattering and should be present in many scattering experiments.
The structure of the extremely proton-rich nucleus $^{11}_{~8}$O$_3$, the mirror of the two-neutron halo nucleus $^{11}_{~3}$Li$_8$, has been studied experimentally for the first time. Following two-neutron knockout reactions with a $^{13}$O beam, the $^{11}$O decay products were detected after two-proton emission and used to construct an invariant-mass spectrum. A broad peak of width $\sim$3\,MeV was observed. Within the Gamow coupled-channel approach, it was concluded that this peak is a multiplet with contributions from the four-lowest $^{11}$O resonant states: $J^{\pi}$=3/2$^-_1$, 3/2$^-_2$, 5/2$^+_1$, and 5/2$^+_2$. The widths and configurations of these states show strong, non-monotonic dependencies on the depth of the $p$-$^9$C potential. This unusual behavior is due to the presence of a broad threshold resonant state in $^{10}$N, which is an analog of the virtual state in $^{10}$Li in the presence of the Coulomb potential. After optimizing the model to the data, only a moderate isospin asymmetry between ground states of $^{11}$O and $^{11}$Li was found.
Proton radiotherapy has the potential to provide clinically effective treatment and highly conformal dose delivery when the rapid dose falloff at the end of its proton-beam range is correctly aligned to the distal margin of the clinical target volume. However, in current clinical practice an additional 2-3.5% safety margin must be added to the proton range to account for uncertainties in the estimation of proton-beam range when using stopping-power ratios (SPRs) derived from single-energy CT scans. Several approaches have been proposed to estimate stopping power by using dual-energy CT (DECT) and have been shown through theoretical analysis to outperform single-energy CT (SECT) under the presence of tissue composition and density variations. Our lab previously proposed a joint statistical image reconstruction (JSIR) method built on a basis-vector model (BVM) tissue parameterization for SPR estimation, which was shown to perform comparatively better than other DECT image- and sinogram-domain decomposition approaches on simulated as well as experimental data. This comparison, however, assumed theoretical SPR values calculated from the samples' known compositions and densities as ground truth and used the mean excitation energy and effective electron density from ICRU reports along with a simplified version of the Bethe-Bloch equation to determine SPR reference values. Furthermore, CT scans were acquired with an assumed ideal point source at a narrow beam collimation; thus, the signal formation assumed by our JSIR process neglected scatter and off-focal radiation. In this paper, we verify the accuracy of our method by comparing the SPR values derived from JSIR-BVM to direct measurements of relative SPR, as well as present a preliminary study on the impact of fan-beam scatter radiation on JSIR-BVM SPR prediction accuracy.
Particle-decaying states of the light nuclei $^{11,12}\mathrm{N}$ and $^{12}\mathrm{O}$ were studied using the invariant-mass method. The decay energies and intrinsic widths of a number of states were measured, and the momentum correlations of three-body decaying states were considered. A second $2p$-decaying ${2}^{+}$ state of $^{12}\mathrm{O}$ was observed for the first time, and a higher-energy $^{12}\mathrm{O}$ state was observed in the $4p+2\ensuremath{\alpha}$ decay channel. This $4p+2\ensuremath{\alpha}$ channel also contains contributions from fissionlike decay paths, including $^{6}\mathrm{Be}_{\mathrm{g}.\mathrm{s}.}+^{6}\mathrm{Be}_{\mathrm{g}.\mathrm{s}.}$. Analogs to these states in $^{12}\mathrm{O}$ were found in $^{12}\mathrm{N}$ in the $2p+^{10}\mathrm{B}$ and $2p+\ensuremath{\alpha}+^{6}\mathrm{Li}$ channels. The momentum correlations for the prompt $2p$ decay of $^{12}\mathrm{O}_{\mathrm{g}.\mathrm{s}.}$ were found to be nearly identical to those of $^{16}\mathrm{Ne}_{\mathrm{g}.\mathrm{s}.}$, and the correlations for the new ${2}^{+}$ state were found to be consistent with sequential decay through excited states in $^{11}\mathrm{N}$. The momentum correlations for the ${2}_{1}^{+}$ state in $^{12}\mathrm{O}$ provide a new value for the $^{11}\mathrm{N}$ ground-state energy. The states in $^{12}\mathrm{N}/^{12}\mathrm{O}$ that belong to the $A=12$ isobaric sextet do not deviate from the quadratic isobaric multiplet mass equation form.
Large longitudinal spin alignment of $E/A=24\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}\phantom{\rule{4pt}{0ex}}^{7}\mathrm{Li}$ projectiles inelastically excited by Be, C, and Al targets was observed when the latter remain in their ground state. This alignment is a consequence of an angular-momentum-excitation-energy mismatch, which is well described by a DWBA cluster-model ($\ensuremath{\alpha}+t$). The longitudinal alignment of several other systems is also well described by DWBA calculations, including one where a cluster model is inappropriate, demonstrating that the alignment mechanism is a more general phenomenon. Predictions are made for inelastic excitation of $^{12}\mathrm{C}$ for beam energies above and below the mismatch threshold.
The invariant-mass distribution of $^{11}\mathrm{O}$ fragments formed in two-neutron-knockout reactions with a $^{13}\mathrm{O}$ projectile and $^{9}\mathrm{Be}$ target has been further examined. Gating on events where the $^{9}\mathrm{C}$ decay fragments produced following $2p$ emission are recoiled transversely in the projectile's frame improves the overall invariant-mass resolution. The observed peak is now shown to have contributions from at least two $^{11}\mathrm{O}$ levels, in contradiction with the suggestion of Fortune [Phys. Rev. C 99, 051302(R) (2019)]. The data, however, do not differentiate between the ground-state properties obtained by the Gamow coupled-channels calculation and the prompt $2p$-decay model of Fortune. The ground-state $2p$-decay energy is 4.25(6) MeV in a two-level fit to the data, but lower values are possible if more states contribute to the observed spectrum, as suggested in the previous analysis.
The invariant-mass method is used to study the structure of a number of light proton-rich isotopes utilizing fast beams. Reactions where the projectile picks up a proton have been used to study $d$-wave resonances in $^{14}\mathrm{F}, ^{16}\mathrm{F}$, and $^{18}\mathrm{Na}$. While the $^{16}\mathrm{F}$ and $^{18}\mathrm{Na}$ results are consistent with previous studies, the $^{14}\mathrm{F}$ results are not consistent with the only previous work. We have tentatively identified the ${4}^{+}$ member of a rotational band in $^{10}\mathrm{B}$, which is the analog of well-known states with strong $\ensuremath{\alpha}$-cluster structure in $^{10}\mathrm{Be}$ and $^{10}\mathrm{C}$. Finally, spin and parities of newly observed states in $^{11}\mathrm{C}$ which decay sequentially into three-body exit channels have been determined or restricted.
To assess the potential of a joint dual-energy computerized tomography (CT) reconstruction process (statistical image reconstruction method built on a basis vector model (JSIR-BVM)) implemented on a 16-slice commercial CT scanner to measure high spatial resolution stopping-power ratio (SPR) maps with uncertainties of less than 1%.JSIR-BVM was used to reconstruct images of effective electron density and mean excitation energy from dual-energy CT (DECT) sinograms for 10 high-purity samples of known density and atomic composition inserted into head and body phantoms. The measured DECT data consisted of 90 and 140 kVp axial sinograms serially acquired on a Philips Brilliance Big Bore CT scanner without beam-hardening corrections. The corresponding SPRs were subsequently measured directly via ion chamber measurements on a MEVION S250 superconducting synchrocyclotron and evaluated theoretically from the known sample compositions and densities. Deviations of JSIR-BVM SPR values from their theoretically calculated and directly measured ground-truth values were evaluated for our JSIR-BVM method and our implementation of the Hünemohr-Saito (H-S) DECT image-domain decomposition technique for SPR imaging. A thorough uncertainty analysis was then performed for five different scenarios (comparison of JSIR-BVM stopping-power ratio/stopping power (SPR/SP) to International Commission on Radiation Measurements and Units benchmarks; comparison of JSIR-BVM SPR to measured benchmarks; and uncertainties in JSIR-BVM SPR/SP maps for patients of unknown composition) per the Joint Committee for Guides in Metrology and the Guide to Expression of Uncertainty in Measurement, including the impact of uncertainties in measured photon spectra, sample composition and density, photon cross section and I-value models, and random measurement uncertainty. Estimated SPR uncertainty for three main tissue groups in patients of unknown composition and the weighted proportion of each tissue type for three proton treatment sites were then used to derive a composite range uncertainty for our method.Mean JSIR-BVM SPR estimates deviated by less than 1% from their theoretical and directly measured ground-truth values for most inserts and phantom geometries except for high-density Delrin and Teflon samples with SPR error relative to proton measurements of 1.1% and -1.0% (head phantom) and 1.1% and -1.1% (body phantom). The overall root-mean-square (RMS) deviations over all samples were 0.39% and 0.52% (head phantom) and 0.43% and 0.57% (body phantom) relative to theoretical and directly measured ground-truth SPRs, respectively. The corresponding RMS (maximum) errors for the image-domain decomposition method were 2.68% and 2.73% (4.68% and 4.99%) for the head phantom and 0.71% and 0.87% (1.37% and 1.66%) for the body phantom. Compared to H-S SPR maps, JSIR-BVM yielded 30% sharper and twofold sharper images for soft tissues and bone-like surrogates, respectively, while reducing noise by factors of 6 and 3, respectively. The uncertainty (coverage factor k = 1) of the DECT-to-benchmark values comparison ranged from 0.5% to 1.5% and is dominated by scanning-beam photon-spectra uncertainties. An analysis of the SPR uncertainty for patients of unknown composition showed a JSIR-BVM uncertainty of 0.65%, 1.21%, and 0.77% for soft-, lung-, and bony-tissue groups which led to a composite range uncertainty of 0.6-0.9%.Observed JSIR-BVM SPR estimation errors were all less than 50% of the estimated k = 1 total uncertainty of our benchmarking experiment, demonstrating that JSIR-BVM high spatial resolution, low-noise SPR mapping is feasible and is robust to variations in the geometry of the scanned object. In contrast, the much larger H-S SPR estimation errors are dominated by imaging noise and residual beam-hardening artifacts. While the uncertainties characteristic of our current JSIR-BVM implementation can be as large as 1.5%, achieving < 1% total uncertainty is feasible by improving the accuracy of scanner-specific scatter-profile and photon-spectrum estimates. With its robustness to beam-hardening artifact, image noise, and variations in phantom size and geometry, JSIR-BVM has the potential to achieve high spatial-resolution SPR mapping with subpercentage accuracy and estimated uncertainty in the clinical setting.
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