Decay Threshold Research Articles

Candidate Resonant Tetraneutron State Populated by the ^{4}He(^{8}He,^{8}Be) Reaction.

A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186 MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst) MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

Nuclear structure beyond the neutron drip line: The lowest energy states in 9 He via their T = 5/2 isobaric analogs in 9 Li

The level structure of the very neutron rich and unbound $^9$He nucleus has been the subject of significant experimental and theoretical study. Many recent works have claimed that the two lowest energy $^9$He states exist with spins $J^\pi=1/2^+$ and $J^\pi=1/2^-$ and widths on the order of hundreds of keV. These findings cannot be reconciled with our contemporary understanding of nuclear structure. The present work is the first high-resolution study with low statistical uncertainty of the relevant excitation energy range in the $^8$He$+n$ system, performed via a search for the T=5/2 isobaric analog states in $^9$Li populated through $^8$He+p elastic scattering. The present data show no indication of any narrow structures. Instead, we find evidence for a broad $J^{\pi}=1/2^+$ state in $^9$He located approximately 3 MeV above the neutron decay threshold.

B10+αstates with chain-like structures inN14

I investigate $^{10}$B+$\alpha$ cluster states of $^{14}$N with a $^{10}$B+$\alpha$ cluster model. Near the $\alpha$-decay threshold energy, I obtain $K^\pi=3^+$ and $K^\pi=1^+$ rotational bands having $^{10}$B($3^+$)+$\alpha$ and $^{10}$B($1^+$)+$\alpha$ components, respectively. I assign the band-head state of the $K^\pi=3^+$ band to the experimental $3^+$ at $E_x$=13.19 MeV of $^{14}$N observed in $\alpha$ scattering reactions by $^{10}$B and show that the calculated $\alpha$-decay width is consistent with the experimental data. I discuss an $\alpha$-cluster motion around the $^{10}$B cluster and show that $^{10}$B+$\alpha$ cluster states contain significant components of a linear-chain 3$\alpha$ configuration, in which an $\alpha$ cluster is localized in the longitudinal direction around the deformed $^{10}$B cluster.

Neutrino mass hierarchy and precision physics with medium-baseline reactors: Impact of energy-scale and flux-shape uncertainties

Nuclear reactors provide intense sources of electron antineutrinos, characterized by few-MeV energy E and unoscillated spectral shape Phi(E). High-statistics observations of reactor neutrino oscillations over medium-baseline distances L ~ O(50) km would provide unprecedented opportunities to probe both the long-wavelength mass-mixing parameters (delta m^2 and theta_12) and the short-wavelength ones (Delta m^2 and theta_13), together with the subtle interference effects associated with the neutrino mass hierarchy (either normal or inverted). In a given experimental setting - here taken as in the JUNO project for definiteness - the achievable hierarchy sensitivity and parameter accuracy depend not only on the accumulated statistics but also on systematic uncertainties, which include (but are not limited to) the mass-mixing priors and the normalizations of signals and backgrounds. We examine, in addition, the effect of introducing smooth deformations of the detector energy scale, E -> E'(E), and of the reactor flux shape, Phi(E) -> Phi'(E), within reasonable error bands inspired by state-of-the-art estimates. It turns out that energy-scale and flux-shape systematics can noticeably affect the performance of a JUNO-like experiment, both on the hierarchy discrimination and on precision oscillation physics. It is shown that a significant reduction of the assumed energy-scale and flux-shape uncertainties (by, say, a factor of 2) would be highly beneficial to the physics program of medium-baseline reactor projects. Our results also shed some light on the role of the inverse-beta decay threshold, of geoneutrino backgrounds, and of matter effects in the analysis of future reactor oscillation data.

Impact of isotopic effect on density limit and LHCD efficiency in the FT-2 experiments

Current drive by lower hybrid waves (LHCD) is the most effective method to sustain the plasma current, but it is feasible only at the plasma density not exceeding some density limit nDL. In the present work the main attention is paid to the investigation of this effect on the FT-2 (R = 0.55 m, a = 0.08 m, BT ⩽ 3 T, Ipl = 19–40 kA, f0 = 920 MHz) tokamak. The dependence of LHCD efficiency on isotopic plasma content (hydrogen/deuterium) is studied. Characteristic features of such an experiment are a strong influence of the isotope plasma composition on the LH resonance density nLH. For hydrogen plasma nLH ≈ 3.5 × 1019 m−3, while for deuterium plasma nLH ≈ 2 × 1020 m−3. The suppression of the LHCD and beginning of the interaction of LH waves with ions are determined by the hydrogen/deuterium plasma density rise. In the hot hydrogen plasma (Te(r = 0 cm) ≈ 700 eV) the density limit nDL of LHCD is approximately equal to the resonance value nLH ≈ nLC, where nLC is the point of linear conversion. In the hot deuterium plasma one could expect an increase of nDL because of a much higher value of nLH ⩾ nLC ≈ 1020 m−3. However it appeared that the observed density limit for LHCD generation nDL ≈ (3.5–4) × 1019 m−3 is not determined by nLH. The role of parametric instabilities in CD switch-off is considered in both cases. The cooling of the plasma column and density rise could lead to a reduction of the threshold for the parametric decay of f0 and result in early suppression of LHCD. In both cases the LHCD was inversely proportional to the density, which corresponds to the theoretical predictions. In order to analyse the experimentally observed LHCD efficiency the GRILL3D and FRTC codes have been used.

Association mapping and favourable allele exploration for plant architecture traits in upland cotton (Gossypium hirsutumL.) accessions

SUMMARYAssociation mapping based on linkage disequilibrium (LD) is a promising tool to identify genes responsible for quantitative variations underlying complex traits. The present paper presents an association mapping panel consisting of 172 upland cotton (Gossypium hirsutumL.) accessions. The panel was phenotyped for five cotton plant architecture traits across multiple environments and genotyped using 386 simple sequence repeat (SSR) markers. Of these markers, 101 polymorphic SSR markers were used in the final analysis. There were abundant phenotypic variations within this germplasm panel and a total of 267 alleles ranging from two to seven per locus were identified in all collections. The threshold of LD decay was set tor2= 0·1 and 0·2, and the genome-wide LD extended up to about 13–14 and 6–7 cM, respectively, providing the potential for association mapping of agronomically important traits in upland cotton. A total of 66 marker–trait associations were detected based on a mixed linear model, of which 35 were found in more than one environment. The favourable alleles from 35 marker loci can be used in marker-assisted selection of target traits. Both the synergistic alleles and the negative alleles for some traits, especially plant height and fruit branch angle, can be utilized in plant architecture breeding programmes according to specific breeding objectives.

Modified energy–momentum conservation laws and vacuum Cherenkov radiation

We present a general parametrization for the leading order terms in a momentum power expansion of a non-universal Lorentz-violating, but rotational invariant, kinematics and its implications for two-body decay thresholds. The considered framework includes not only modified dispersion relations for particles, but also modified energy–momentum conservation laws, something which goes beyond effective field theory. As a particular and relevant example, bounds on the departures from special relativistic kinematics from the non-observation of vacuum Cherenkov radiation are discussed and compared with those obtained within the effective field theory scenario.

Highly excited states of 6He

Highly excited states of 6He are sought in the reactions involved in the absorption of stopped pions by 10,11B nuclei. Several levels in 6He are observed for the first time. A state with Ex ≈ 9.3(2) MeV and Γ ≈ 1.0(4) MeV is seen to form in two reactions: 10B(π−, pt)X and 11B(π−, dt)X. Two states are seen to form in the 10B(π−, pt)X reaction channel at energies above the threshold of 6He decay into two tritons (Ex ≥ 12.3 MeV): one state with Ex = 22(1) MeV, Γ = 2.7(1.4) MeV and the other with Ex = 27.0(8) MeV, Γ = 2.5(1.1) MeV.

On the possibility of reducing the instability threshold of a parametric decay of an extraordinary wave into two upper hybrid waves in an inhomogeneous plasma

A parametric decay instability (PDI) of an extraordinary wave leading to excitation of two upper hybrid (UH) plasmons at frequencies close to half the pump wave frequency is analyzed. It is shown that the two-plasmon PDI power threshold can be significantly reduced under conditions of electron cyclotron resonance heating (ECRH) experiments in toroidal magnetic devices, where the plasma density profile is often nonmonotonic, which leads to the localization of UH waves.

Structure of $^{26}$Na via a Novel Technique Using ($d,p\gamma $) with a Radioactive $^{25}$Na Beam

States in 26Na were populated in the (d, pγ) reaction, induced by bombarding deuterium target nuclei with an intense reaccelerated beam of 25Na ions from the ISAC2 accelerator at TRIUMF. Gamma-rays were recorded in coincidence with protons and used to extract differential cross sections for 21 states up to the neutron decay threshold of 5 MeV. Results for levels below 3 MeV are discussed in detail and compared with shell model calculations and with previous work. The angular distributions of decay gamma-rays were measured for individual states and are compared to theoretically calculated distributions, highlighting some issues for future work.

Many-body resonances and continuum states above many-body decay thresholds

We study the resonance spectroscopy of the He isotopes and their mirror, proton- rich nuclei with an α + N + N + N + N cluster model. Many-body resonances are treated with the correct boundary condition as the Gamow states using the complex scaling method. We obtain the resonances up to the five-body decaying states in 8He and 8 C. The spectrum agrees with the recent experiment systematically for energies and decay widths and we predict several resonances. We also discuss the mirror symmetry breaking of He isotopes and their mirror nuclei for the configurations and the spatial distributions of the valence nucleons.

Development of MAIKo: the active target with μ-PIC for RI beam experiments

An active target system MAIKo (μ-PIC based active target for inverse kinematics.) is under development at RCNP. MAIKo is designed to investigate inelastic scattering at forward angles with a radio isotope (RI) beam. The active target is a promising device to study excited states above particle decay thresholds in unstable nuclei. In the present paper, the detailed design of the detector system is described. The first experiment using an accelerated beam was performed to study the detector performance under high counting rate. Preliminary results of the experiment are also discussed.

Giant dipole resonance as a fingerprint of α clustering configurations in 12C and 16O.

It is studied how the α cluster degrees of freedom, such as α clustering configurations close to the α decay threshold in (12)C and (16)O, including the linear chain, triangle, square, kite, and tetrahedron, affect nuclear collective vibrations with a microscopic dynamical approach, which can describe properties of nuclear ground states well across the nuclide chart and reproduce the standard giant dipole resonance (GDR) of (16)O quite nicely. It is found that the GDR spectrum is highly fragmented into several apparent peaks due to the α structure. The different α cluster configurations in (12)C and (16)O have corresponding characteristic spectra of GDR. The number and centroid energies of peaks in the GDR spectra can be reasonably explained by the geometrical and dynamical symmetries of α clustering configurations. Therefore, the GDR can be regarded as a very effective probe to diagnose the different α cluster configurations in light nuclei.

Low initial aspect-ratio direct-drive target designs for shock- or self-ignition in the context of the laser Megajoule

Analysis of low initial aspect ratio direct-drive target designs is carried out by varying the implosion velocity and the fuel mass. Starting from two different spherical targets with a given 300 µg-DT mass, optimization of laser pulse and drive power allows to obtain a set of target seeds referenced by their peak implosion velocities and initial aspect ratio (A = 3 and A = 5). Self-ignition is achieved with higher implosion velocity for A = 5-design than for A = 3-design. Then, rescaling is done to extend the set of designs to a huge amount of mass, peak kinetic energies and peak areal densities. Self-ignition kinetic energy threshold Ek is characterized by a dependance of Ek ∼ vβ with β-values which depart from self-ignition models. Nevertheless, self-ignition energy is seen lower for smaller initial aspect ratio. An analysis of Two-Plasmons Decay threshold and Rayleigh–Taylor instability e-folding is carried out and it is shown that two-plasmon decay threshold is always overpassed for all designs. The hydrodynamic stability analysis is performed by embedded models to deal with linear and non-linear regime. It is found that the A = 5-designs are always at the limit of disruption of the shell.

Non-statistical decay and α-correlations in the 12C+12C fusion–evaporation reaction at 95 MeV

Multiple alpha coincidences and correlations are studied in the reaction 12C+12C at 95 MeV for fusion–evaporation events completely detected in charge. Two specific channels with carbon and oxygen residues in coincidence with α −particles are addressed, which are associated with anomalously high branching ratios with respect to the predictions of Hauser–Feshbach calculations. Triple alpha emission appears kinematically compatible with a sequential emission from a highly excited Mg. The phase space distribution of α − α coincidences suggests a correlated emission from a Mg compound, leaving an oxygen residue excited above the threshold for neutron decay. These observations indicate a preferential α emission of 24Mg at excitation energies well above the threshold for 6 − α decay.

It takes a village to raise a tide: non-linear multiple-mode coupling and mode identification in KOI-54

We explore the tidal excitation of stellar modes in binary systems using Kepler observations of the remarkable eccentric binary KOI-54 (HD 187091; KIC 8112039), which displays strong ellipsoidal variation as well as a variety of linear and non-linear pulsations. We report the amplitude and phase of over 120 harmonic and anharmonic pulsations in the system. We use pulsation phases to determine that the two largest amplitude pulsations, the 90th and 91st harmonics, most likely correspond to axisymmetric m = 0 modes in both stars, and thus cannot be responsible for resonance locks as had been recently proposed. We find evidence that the amplitude of at least one of these two pulsations is decreasing with a characteristic time-scale of ∼100 yr. We also use the pulsations’ phases to confirm the onset of the travelling wave regime for harmonic pulsations with frequencies ≲50 Ωorb, in agreement with theoretical expectations. We present evidence that many pulsations that are not harmonics of the orbital frequency correspond to modes undergoing simultaneous non-linear coupling to multiple linearly driven parent modes. Since coupling among multiple modes can lower the threshold for non-linear interactions, non-linear phenomena may be easier to observe in highly eccentric systems, where broader arrays of driving frequencies are available. This may help to explain why the observed amplitudes of the linear pulsations are much smaller than the theoretical threshold for decay via three-mode coupling.

Dynamical evidence for nonlinear Langmuir wave processes in type III solar radio bursts

AbstractThe nonlinear processes and evolution of Langmuir waves in the source regions of type III solar radio bursts are explored in detail. Langmuir waves recorded by the Time Domain Sampler of the STEREO/WAVES instrument can be roughly classified into six groups based on the waveform, power spectra, and field strength perpendicular to the local magnetic field. It is argued that these groups correspond to either different stages of the evolution of Langmuir waves generated by electron beams or differ due to the direction of the magnetic field relative to the solar wind velocity. Approximately half of the observed Langmuir waves have strong perpendicular fields, meaning that understanding how these fields are produced is crucial for understanding type III sources. Most events recorded are either localized waveforms consistent with Langmuir eigenmodes or have two or more spectral peaks consistent with electrostatic (ES) decay of Langmuir/z mode waves. The remaining events appear to correspond to either earlier or later stages of Langmuir wave evolution or are decay events for which the Doppler shift is insufficient to distinguish the beam‐driven and product Langmuir waves. This is supported by the fact that most events exceed the threshold for ES decay even though their spectra show no evidence for decay and some of the events are observed when the solar wind flow is approximately perpendicular to the magnetic field, minimizing Doppler shifting. Low‐frequency fields produced by intense Langmuir waves are quantitatively consistent with density perturbations produced by the ponderomotive force, ion‐acoustic waves produced by ES decay, or sheath rectification. Above the observed nonlinear threshold, quantitative analysis suggests that the observed low‐frequency signals are consistent with perturbations produced by ponderomotive effects and ion‐acoustic waves produced by ES decay, but effects of sheath rectification may also contribute.

Search for [formula omitted] hypernucleus by the [formula omitted] reaction at [formula omitted

We have carried out an experiment to search for a neutron-rich hypernucleus, HΛ6, by the Li6(π−,K+) reaction at pπ−=1.2 GeV/c. The obtained missing-mass spectrum with an estimated energy resolution of 3.2 MeV (FWHM) showed no peak structure corresponding to the HΛ6 hypernucleus neither below nor above the HΛ4+2n particle decay threshold. An upper limit of the production cross section for the bound HΛ6 hypernucleus was estimated to be 1.2 nb/sr at 90% confidence level.

Computational Antioxidant Capacity Simulation (CAOCS): A Novel Framework of Antioxidant Capacity Profiling

Abstract Inconsistent ranking is a well-known drawback of antioxidant capacity (AOC) profiling methodologies that use free-radical species as oxidant. This problem leads to assay results that are not biorelevant. Linear free energy relationships (LFER) theory predicts proton transfer (PT) kinetics as a surrogate for biorelevant hydrogen atom transfer (HAT) kinetics. Computational antioxidant capacity simulation (CAOCS), based on real-time proton transfer kinetics modeling (PTKM) of polyphenols and phenol-like small molecules, inspired a novel AOC profiling methodology. Kinetic data acquired by incremental addition of resorcinol to an oxidized probe (phenol red), was fitted to mono-exponential decay equation (MED). Absorbance decay data from strongly antioxidant phenol-like molecules (e.g. ascorbic acid) and a new chromogenic probe (phenolphthalein) was fitted to MED and bi-exponential decay equation. The preferred model and corresponding best-fit rate constant (K ptt) was identified by comparison of fits, using Akaike’s Information Criterion (AICc). Photometric phenolphthalein assay (PPA)-derived metric was normalized with photometric phenol red assay (PPRA) results by using a function developed from proton concentration differential between phenolphthalein and phenol red, with respect to decay threshold to plateau (assay endpoint) interval. pKa dependence of the CAOCS’ metric is a signature of structure–function relationships, and hence, biorelevance. It is shown, unambiguously, that a combination of two phenolic probe molecules, an analytical system devoid of free radicals, and statistical identification of preferred exponential decay fit to PT kinetics data, constitutes a novel algorithm for AOC profiling of polyphenols and phenol-like molecules. This methodology holds a promise of utility in quality assurance of dietary supplements.

Microscopic study of the string breaking in QCD

Theory of strong decays defines in addition to decay widths, also the channel coupling and the mass shifts of the levels above the decay thresholds. In the standard decay models of the 3P0 type the decay vertex is taken to be a phenomenological constant "gamma" and such a choice leads to large mass shifts of all meson levels due to real and virtual decays, the latter giving a divergent contribution. Here we show that taking the microscopic details of decay vertex into account, one obtains new string width coefficient, which strongly suppresses virtual decay contribution. In addition for a realistic space structure of the decay vertex of highly excited states, the decay matrix elements appear to be strongly different from those, where the constant "gamma" is used. From our analysis also follows that so-called flattening potential can imitate the effects of intermediate decay channels.