Decay Threshold Research Articles

Electrostatic decay of Langmuir/z‐mode waves in type III solar radio bursts

About 40% of the waveforms observed by STEREO during type III solar radio bursts exhibits Langmuir beating and have split spectral peaks, suggestive of decay into product Langmuir and ion‐acoustic waves. For lower electron beam speeds vb/c≲0.1, the spectra of Langmuir events with split spectral peaks are shown to be consistent with electrostatic (ES) decay into Langmuir‐like waves with frequencies above the electron plasma frequency. For faster beam speeds , the spectra are consistent with one or more successive generations of ES decay and an end state of low wave number Langmuir/z‐mode waves with strong electric fields perpendicular to the magnetic field. For many of the split spectral‐peak events, an intense low‐frequency response occurs that is consistent with ion‐acoustic waves produced by ES decay, providing further evidence that these events are ES decay. An observed event is shown to be consistent with three successive backscatter decays, but such events are very rare in type III bursts. About 90% of the split spectral‐peak events occur when Ti/Te≲0.6. Similarly, over 80% of the split peak events have energy densities above the theoretical nonlinear threshold for ES decay (for reasonable Langmuir damping rates ≲10−3ωp). All events have beam speeds and energy densities below the maxima appropriate for ES decay of beam‐driven Langmuir waves.

Seismic data reconstruction based on CS and Fourier theory

Traditional seismic data sampling follows the Nyquist sampling theorem. In this paper, we introduce the theory of compressive sensing (CS), breaking through the limitations of the traditional Nyquist sampling theorem, rendering the coherent aliases of regular undersampling into harmless incoherent random noise using random undersampling, and effectively turning the reconstruction problem into a much simpler denoising problem. We introduce the projections onto convex sets (POCS) algorithm in the data reconstruction process, apply the exponential decay threshold parameter in the iterations, and modify the traditional reconstruction process that performs forward and reverse transforms in the time and space domain. We propose a new method that uses forward and reverse transforms in the space domain. The proposed method uses less computer memory and improves computational speed. We also analyze the antinoise and anti-aliasing ability of the proposed method, and compare the 2D and 3D data reconstruction. Theoretical models and real data show that the proposed method is effective and of practical importance, as it can reconstruct missing traces and reduce the exploration cost of complex data acquisition.

Yield measurements for resonances above the multi-αthreshold in20Ne

The reaction ${}^{16}$O($\ensuremath{\alpha},{}^{20}\text{Ne}$), has been studied with beam energies from 23.0 to 29.0 MeV in 100-keV steps. Resonant states in ${}^{20}$Ne have been populated which are above the multi-$\ensuremath{\alpha}$ decay threshold and that decay via ${}^{8}$Be + ${}^{8}$Be + $\ensuremath{\alpha}$ and ${}^{8}$Be + ${}^{12}$C$({0}_{2}^{+})$. An array of four silicon-strip detectors was used for the detection of four of the five emitted $\ensuremath{\alpha}$ particles, enabling the full reconstruction of the kinematics. The normalized yields have been obtained, with indications of significant strength above 24.5 MeV arising from several possible resonances.

Decay threshold of acetylated rattan (Calamus manan) against soft rot

We investigated the resistance of acetylated rattan against soft rot and other soil inhabiting micro-organisms in comparison with wood of beech and Scots pine. Calamus manan of 10 and 13 years old under rubber tree canopy was acetylated to different levels by reaction times (0.25 to 30 hours) and was tested for soft rot decay for 32 weeks. Acetylated rattan at decay protection thresholds of 15.4% and 16.2% weight gain (WG) were fully protected, as shown by both weight loss and strength loss criteria. The static bending properties of untreated rattan decayed by soft rot were significantly lower than for acetylated rattan.

Study of 24Mg resonances relevant to carbon burning nucleosynthesis

We have studied the decays of the resonances of 24Mg at excitation energies 1–6 MeV above the 12C+12C decay threshold, using the 12C(16O,α)24Mg* reaction at E(16O) = 94 MeV. Some preliminary results are presented and further analysis is in progress.

Correlation investigations of the low-energy 10He spectrum

The spectrum of low-lying states in the 10He nucleus is investigated for the two-neutron transfer reaction 3H(8He, p)10He. The secondary beam of 8He nuclei with the energy 21.5 MeV/nucleon and a cryogenic tritium target are used in the experiment. The 10He ground state is observed in the missing mass spectrum at the energy of 2.1 MeV (Γ ∼ 2 MeV) above the decay threshold. Analysis of the angular correlations of the 10He decay products yields the spin and parity of two excited 10He states, Jπ = 1− in the energy range from 4 to 6 MeV and Jπ = 2+ at energies above 6 MeV.

Search for α-particle condensation in nuclei from the Hoyle state deexcitation

Heavy ion reactions induced by few tens MeV/nucleon beams constitute an ideal framework for producing and studying nuclear excited states close to the decay threshold. The fragmentation of quasi-projectiles from the nuclear reaction 40Ca+12C at 25 MeV/nucleon performed at LNS-Catania was employed in order to populate the Hoyle state of 12C and investigate the theoretically predicted molecular structures. Complete kinematic characterization of individual decay events, made possible by the CHIMERA high-granularity 4π charged particle multi-detector, reveals that 7.5±4.0% of the particle decays of the Hoyle state correspond to direct decays in three equal-energy α-particles and thus fulfill the decay criteria of an α-particle condensate. Moreover, events with increased kinetic energy dispersion in the 12C center of mass, which amount to 9.5±4.0%, point toward the occurrence of a second competing molecular configuration, a linear α-chain type. Both α-particle condensate and linear chain structures have been theoretically predicted but, to our knowledge, not experimentally confirmed so far.

Studies of Light Neutron-Excess Systems from Bounds to Continuum

The generalized two-center cluster model, which can handle various single particle configurations in general two center systems, is applied to the light neutron-rich system, 12Be = α+α+4N. We discuss the change of the neutrons’ configuration around two α-cores as a variation of an excitation energy. We show that the excess neutrons form various chemical-bonding-like configurations around two α cores in the unbound region above the α decay threshold. The possibility of the α cluster formation in the heavier neutron-excess system, 28Ne, is also discussed.

Forecasting Holding Periods for Leveraged ETFs Using Decay Thresholds: Theory and Applications

Due to the interplay between daily rebalancing and volatility, Leveraged Exchange Traded Funds (ETFs) may experience a cumulative return different from the product of their daily leverage ratio and the periodic index return. When the leveraged ETF’s return over a particular time frame minus the leverage ratio of the ETF multiplied by the underlying index return is negative, we refer to it as decay. The quantity of this decay is influenced by the daily leverage ratio, time, index return, and volatility. This study quantifies the tradeoffs these variables have on decay, and derives maximum holding periods given an investor's decay threshold under various volatility environments. While the majority of studies suggest short holding periods for leveraged ETFs, this paper asserts that extended holding periods can be justified depending on volatility levels and an investor's willingness to accept a certain level of decay. For decay thresholds of only -2% and low volatility levels, holding periods beyond 6 months can be justified for some leveraged funds. Empirically, forecasted decay over a 3-month period based on the Russell 1000 and using a GARCH (1, 1) volatility estimate rarely underestimates actual decay. Through 2007, the average justified holding period for all but the -3x fund generally exceeded 4 months. During the financial crisis, justified holding periods decreased dramatically.

Decay threshold of acetylated rattan against white and brown rot fungi

The decay resistance of acetylated rattan was compared to beech and pine wood. Calamus manan grown under rubber tree canopy of different ages (10 and 13 years) was acetylated to different levels by reaction times (0·25–24 h) and was subjected to decay against the white and brown rot fungi, T. versicolor and C. puteana. Protection against decay occurred at 13·4 and 9·0% weight gains (WGs) against T. versicolor and 13·5 and 10·3% WG against C. puteana for 10 and 13 years old rattan respectively.

Non-linear decay of building stones during freeze–thaw weathering processes

This paper studies the resistance of rocks to freeze–thaw and their petrophysical evolution during weathering. Moreover, the accuracy of existing standards regarding frost durability is discussed. A long-term test was established with these purposes, in which 102 samples of six different dimension stone types were tested (carbonates).Samples were divided into five groups and each group was tested after 0, 12, 24, 48 and 96 freeze–thaw cycles. At the end of the cycles several properties were measured: volume loss, open porosity variation, visual damage, mechanical properties evolution (measuring strength and elastic modulus) and ultrasonic propagation (quantifying both P-wave velocity and spatial attenuation). The micro-textural evolution was also studied using SEM in polished samples.Results display that the rocks with the highest open porosity values (>10%) are the least durable. These rocks show a non-linear decay pattern, with long periods of apparent stability followed by rapid and catastrophic decay. Microscopic observation reveals that during the stable period, isolated microcracks appear from where new ones nucleate and grow as the test progresses. When a critical threshold is exceeded, microcracks turn into cracks and grow rapidly, causing rock breakdown after a low number of cycles. Most of the measured petrophysical parameters do not predict the ultimate breakdown of rocks. However, spatial attenuation of ultrasonic waves reveals as the most sensitive parameter, detecting the critical decay threshold of rocks and their imminent breakdown. Results suggest an important review of standardized durability tests since they do not reflect the reality of frost weathering of rocks: to increase the number of freeze–thaw cycles and to monitoring the weathering process of samples by means ultrasonic measurements.

Extraction of Gamow-Teller strength distributions from56Ni and55Co via the(p,n)reaction in inverse kinematics

Background: Gamow-Teller (GT) transition strength distributions in stable and unstable $pf$-shell isotopes are key inputs for estimating electron-capture rates important for stellar evolution. Charge-exchange experiments at intermediate beam energies have long been used to test theoretical predictions for GT strengths, but previous experiments were largely restricted to stable nuclei. Since a large fraction of the nuclei relevant for astrophysical applications (including key nuclei such as ${}^{56}$Ni) are unstable, new methods are needed to perform charge-exchange experiments in inverse kinematics with unstable isotopes.Purpose: The ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were measured in inverse kinematics in order to extract GT strengths for transitions to ${}^{56}$Cu and ${}^{55}$Ni, respectively. The extracted strength distributions were compared with shell-model predictions in the $pf$ shell using the KB3G and GXPF1J interactions. By invoking isospin symmetry, these strength distributions are relevant for electron captures on the ground states of ${}^{56}$Ni and ${}^{55}$Ni to final states in ${}^{56}$Co and ${}^{55}$Co, respectively.Method: Differential cross sections and excitation energy spectra for the ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were determined by measuring neutrons recoiling from a liquid hydrogen target into the Low Energy Neutron Detector Array. GT contributions to the spectra were extracted by using a multipole decomposition analysis and were converted to strengths by employing the proportionality between GT strength and differential cross section at zero linear momentum transfer.Results: GT strengths from ${}^{56}$Ni and ${}^{55}$Co were extracted up to excitation energies of 8 and 15 MeV, respectively. Shell-model calculations performed in the $pf$ shell with the GXPF1J interaction reproduced the experimental GT strength distributions better than calculations with the KB3G interaction.Conclusions: A new technique for measuring ($p,n$) charge-exchange reactions on unstable nuclei was successfully developed. It can be used to study the isovector response of unstable nuclei in any mass region and for excitation energies beyond the particle decay threshold. In the first experiment, ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were studied and GT transition strengths were extracted for the purpose of testing shell-model calculations used to estimate electron-capture rates in simulations of late stellar evolution. The calculation using the GXPF1J interaction was found to best reproduce the experimental strength distribution.

Level structure of18Ne and its importance in the14O(α,p)17F reaction rate

The level structure of ${}^{18}$Ne above the $\ensuremath{\alpha}$-decay threshold has been studied using the ${}^{16}$O(${}^{3}$He,$n$) reaction. A coincidence measurement of neutrons and charged particles decaying from populated states in ${}^{18}$Ne has been made. Decay branching ratios were measured for six resonances and used to calculate the ${}^{14}$O($\ensuremath{\alpha}$,$p$)${}^{17}$F reaction rate which is a measure of one of two breakout paths from the hot CNO cycle. The new experimental information combined with previous experimental and theoretical information provides a more accurate calculation of the reaction rate.

How atomic nuclei cluster

Nucleonic matter displays a quantum-liquid structure, but in some cases finite nuclei behave like molecules composed of clusters of protons and neutrons. Clustering is a recurrent feature in light nuclei, from beryllium to nickel. Cluster structures are typically observed as excited states close to the corresponding decay threshold; the origin of this phenomenon lies in the effective nuclear interaction, but the detailed mechanism of clustering in nuclei has not yet been fully understood. Here we use the theoretical framework of energy-density functionals, encompassing both cluster and quantum liquid-drop aspects of nuclei, to show that conditions for cluster formation can in part be traced back to the depth of the confining nuclear potential. For the illustrative example of neon-20, we show that the depth of the potential determines the energy spacings between single-nucleon orbitals in deformed nuclei, the localization of the corresponding wavefunctions and, therefore, the degree of nucleonic density clustering. Relativistic functionals, in particular, are characterized by deep single-nucleon potentials. When compared to non-relativistic functionals that yield similar ground-state properties (binding energy, deformation, radii), they predict the occurrence of much more pronounced cluster structures. More generally, clustering is considered as a transitional phenomenon between crystalline and quantum-liquid phases of fermionic systems.

Scrape-off layer ion acceleration during fast wave injection in the DIII-D tokamak

Fast wave injection is employed on the DIII-D tokamak as a current drive and electron heating method. Bursts of energetic ions with energy Eo > 20 keV are observed immediately following fast wave injection in experiments featuring the 8th ion cyclotron harmonic near the antenna. Using the energy and pitch angle of the energetic ion burst as measured by a fast-ion loss detector, it is possible to trace the origin of these ions to a particular antenna. The ion trajectories exist entirely within the scrape-off layer. These observations are consistent with the presence of parametric decay instabilities near the antenna strap. It is suggested that the phase space capabilities of the loss detector diagnostic can improve studies of wave injection coupling and efficiency in tokamaks by directly measuring the effects of parametric decay thresholds.

Dynamical structure factor of quasi-two-dimensional antiferromagnet in high fields

We study high-field magnon dynamics and examine the dynamical structure factor in the quasi-2D tetragonal Heisenberg antiferromagnet with interlayer coupling corresponding to realistic materials. Within spin-wave theory, we show that a non-zero interlayer coupling mitigates singular corrections to the excitation spectrum occurring in the high-field regime that would otherwise require a self-consistent approach beyond the 1/S approximation. For the fields between the threshold for decays and saturation field we observe widening of the two-magnon sidebands with significant shifting of the spectral weight away from the quasiparticle peak. We find spectrum broadening throughout large regions of the Brillouin zone, dramatic redistributions of spectral weight to the two-magnon continuum, two-peak structures and other features clearly unlike conventional single-particle peaks.

Studies of Light Neutron-Excess Nuclei from Bounds to Continuum

The generalized two-center cluster model, which can handle various single particle configurations in general two center systems, is applied to the light neutron-rich system, 12Be = α+α+4N. We discuss the change of the neutrons’ configuration around two α-cores as a variation of an excitation energy. We show that the excess neutrons form various chemical-bonding-like configurations around two α cores in the unbound region above the α decay threshold. The possibility of the α cluster formation in the heavier neutron-excess system, 28Ne, is also discussed.

Spectroscopy of the 9Li isotope in stopped-pion absorption reactions

The origin of the 9Li isotope is sought in missing-mass spectra measured in 11B(π−, d)X, 12C(π−, pd)X, and 14C(π−, dt)X stopped-pion absorption reactions. The parameters of the three observed low-lying excited states coincide with the literature data. A state with excitation energy Ex = 9.1 ± 0.1 MeV, lying above the threshold of 9Li decay into 6He + t, is observed for the first time.

Isovector soft dipole mode in 6Be

By using the 1H(6Li, 6Be)n charge-exchange reaction, population of continuum states in 6Be was observed up to ET=16 MeV, ET being the 6Be energy above its three-body decay threshold. In kinematically complete measurements performed by detecting α+p+p coincidences, an ET spectrum of high statistics was obtained, containing approximately ∼5×106 events. The spectrum provides detailed correlation information about the well-known 0+ ground state of 6Be at ET=1.37 MeV and its 2+ state at ET=3.05 MeV. Moreover, a broad structure extending from 4 to 16 MeV was observed. It contains negative parity states populated by ΔL=1 angular momentum transfer without other significant contributions. This structure can be interpreted as a novel phenomenon, i.e. the isovector soft dipole mode associated with the 6Li ground state. The population of this mode in the charge-exchange reaction is a dominant phenomenon for this reaction, being responsible for about 60% of the cross-section obtained in the measured energy range.

Recent results related to excited states of6Be and10He

The 1H(6Li,6Be)n charge-exchange reaction and the 3H(8He, p)10He two-neutron transfer reaction were recently carried out at the ACCULINNA fragment separator (FLNR, Dubna) to populate the ground and excited states of 6Be and 10He nuclei, respectively. The 6Be ET spectrum (ET is the 6Be energy above its 4He+p+p decay threshold) was obtained with high statistics and described by the well-known 0+ ground state of 6Be at ET = 1.37 MeV, the 2+ state at ET = 3.05 MeV and a broad structure extending from 4 to 16 MeV which could be interpreted as the isovector soft dipole mode associated with the 6Li ground state. In the 10He case the 0+ ground state was found at about 2.1(2) MeV (T ∼ 2 MeV) above the 8He+n+n breakup threshold. Angular correlations observed for 10He decay products show prominent interference patterns allowing to draw conclusions about the structure of low-energy excited states: 1- (E T ∼ 5 MeV) and 2+ (ET > 6 MeV).