Variation Of Decay Rate Research Articles

The variation of short-period comet size and decay rate with perihelion distance

Out of over 200 known short-period comets, we analyse a self-consistent list of 105 comets which have accurately estimated nuclei radii. It is found that both the median size and the size distribution index of these comets vary as a function of the perihelion distance, q, of the cometary orbit. A value of q ≈ 2.7 au divides the comets into an outer solar system group which are hardly affected by decay, and an inner solar system group which are decaying quickly. It is estimated that 10, 20 and 30 per cent of the 105 comets will have decayed away after 1000, 2000 and 3000 yr, respectively.

The thermodynamics of magnetic aftereffect

Thermal magnetic aftereffect is due to the interaction between magnons (spin waves) and the change of equilibrium magnetization states. This non-equilibrium process is controlled by (i) the height of the energy barriers between magnetization states and (ii) by the chemical potential that determines how many magnons (thermal quanta) are available. Non-Arrhenius behavior is observed in a non-monotonic variation of decay rate with temperature observed in nanoparticulate media at low temperature. We attribute this behavior to quantum statistics and the nonuniform density of states in energy space. At low temperatures, Bose–Einstein condensation of the magnons is observed in nanoparticulate systems. In these materials, the decay rate increases to a maximum at low temperatures above the condensation temperature. A standard method of evaluating permanent magnet materials is to accelerate the decay rate, by applying a reverse holding field of the order of coercivity. We show that the problem with this technique is the ratio of the decay rate in zero field to the decay rate at the coercivity is a function of the material properties. This model shows excellent agreement with measurements of the variation of the thermal magnetic aftereffect at low temperatures in both nanograin iron powders and MP tapes.

Decomposition rates of coarse woody debris—A review with particular emphasis on Australian tree species

We reviewed the decay patterns and lifetimes (time to reach 95% mass loss) of coarse woody debris (CWD) on the forest floor. The objectives were to identify the factors influencing the decomposition process of CWD and to provide estimates of lifetimes for CWD from Australian tree species. This information is required for greenhouse accounting of forestry activities and land use change as well as the sustainable management of CWD in forest ecosystems. The analysis of a global data set on decay rates of CWD showed that, in particular, the mean annual temperature was a main driver of decomposition, accounting for 34% of the variation in decay rates. The Q10, the factor by which biological processes accelerate when temperature increases by 10�C, was 2.53. Additional determinants of CWD decay rates were the initial density of wood and the diameter of logs. Median and average lifetimes derived from 184 decay rates were 49 and 92 years, respectively, which is considerably higher than the 10-year default for all litter proposed by the Intergovernmental Panel on Climate Change. The pattern of decay in most cases followed a negative exponential curve. To overcome the paucity of information on decomposition of CWD in Australian forests and woodlands, decay rates for a large number of species were derived from wood durability and decay resistance studies. For native Australian species, lifetimes ranged from 7 years in Eucalyptus regnans to 375 years in E. camaldulensis. The lifetimes for timber durability Classes 1–4 were 54, 39, 26 and 11 years, respectively, below 30� latitude and without the influence of termites. However, the experimental conditions under which durability and decay resistance are commonly determined are substantially different from the situation under which CWD decomposes in the field. These estimates must therefore be regarded as minimum lifetimes for CWD of most species. To determine decay rates of CWD with greater certainty, long-term field experiments in a wide range of ecosystems are required.

Grainfall processes in the lee of transverse dunes, Silver Peak, Nevada

Grainfall deposition and associated grainflows in the lee of aeolian dunes are important in that they are preserved as cross‐beds in the geological record and provide a key to the interpretation of the aeolian rock record. Despite their recognized importance, there have been very few field, laboratory or numerical simulation studies of leeside depositional processes on aeolian dunes. As part of an ongoing study, the relationships among grainfall, wind (speed and direction), stoss sand transport rates and dune morphometry (height and aspect ratio) were investigated on four relatively small, straight‐crested transverse dunes at Silver Peak, Nevada. Between 55% and 95% of the total grainfall was found to be deposited within 1 m of the crest, and 84–99% within 2 m, depending primarily on dune size and shape. Grainfall decay rates on high dunes of large aspect ratio were observed to be very consistent, with a weak positive dependence on wind speed. For small dunes with low aspect ratios, grainfall deposition was more varied and decreased rapidly within 1 m of the dune crest, whereas at increased distance from the dune crest, it eventually approached the smaller decay rates observed on the large dunes. No dependence of grainfall on wind speed was observed for these small dunes. Comparison of field data with predictions from Anderson's (1988) saltation model of grainfall, based on the computation of saltation path lengths, indicates lack of agreement in the following areas: (1) deposition rate magnitude; (2) variation in decay rate with wind speed; and (3) the magnitude and location of the localized lee‐slope depositional maxima. The Silver Peak field results demonstrate the importance of dune aspect ratio and related wake effects in determining the rate and pattern of grainfall. This work confirms earlier speculation by McDonald & Anderson (1995) that temporary, turbulent suspension (or `modified saltation') of relatively large grains does occur within the dune wake, so that transport distances generally are larger than predicted by numerical simulations of `true' saltation.

Spin Selection Rules in Radiative and Nonradiative Recombination Processes in Bulk Crystals and in Thin Films of II-VI Compounds

The origin of a fast component of Mn 2+ intra-shell photoluminescence (PL) decay is discussed. We demonstrate that efficient spin-dependent interactions between localised spins of Mn ions and spins of free carriers result in a variation of formation and decay rates of PL emissions in diluted magnetic semiconductors and affect spin selection rules for radiative recombination transitions.

The experimental evaluation of decay rate variations of 57Co by the Δλ/λ method

The measurement of the relative changes Δλ/λ of the radioactive decay by the electron capture process is a direct experimental way to determine the relative changes of the electron charge density on the nucleus. Here we present the results of the experiment with 57Co, where the differences of the decay rate were determined for different chemical states of 57Co. We analyzed the experimental time dependences of the activity of 57Co by means of the direct approximation method and the specialized optimization tool – a genetic algorithm. Using these methods improved the accuracy of the final Δλ/λ values between 57Co and 57Co(Pd), 57CoCl2,57CoS, 57CoSO4⋅7H2O, which are in good agreement with theoretical considerations reflecting the electron charge rearrangement and screening effects.

Seasonality in elephant dung decay and implications for censusing and population monitoring in south‐western Cameroon

AbstractDung decay rates have been used in the past to correct elephant dung counts to estimates of population numbers. These assume that the dung is in a ‘steady state’ where production rates are equal to disappearance rates. The inter‐seasonal variation in decay rates, however, has never been fully investigated for elephant dung. This study, in the Banyang‐Mbo Wildlife Sanctuary in SW Cameroon, investigated whether a single dung decay rate is really appropriate when converting dung counts to population density estimates. A total of 870 fresh elephant dung‐piles were monitored until they had completely decayed, with at least 60 fresh dung piles located each month of the year between July 1994 to June 1995. The elephant dung decay rate obtained was 55.6% to 66.7% lower than those obtained for the rainforests of Gabon. This has implications for using decay rates obtained elsewhere to compute elephant densities from dung counts. Statistically significant differences were found in the mean duration of dung‐piles between different months and different seasons. Environmental variables, with the exception of rainfall, did not aid in predicting the duration of elephant dung‐piles. The model of Plumptre & Harris was used to estimate the standing crop of dung at any particular time of the year. The results show that, if there was no emigration of elephants from the population, then there was only a three‐month period when the dung was in a ‘steady state’. If emigration occurred during the wet season as is thought to occur then there was no period of ‘steady state’.

Environment-induced modification of spontaneous emission: Single-molecule near-field probe

The modification of lifetime experienced by a fluorescent molecule placed in an arbitrary environment is investigated theoretically within the framework of linear response theory. We present a complete description of the interaction of the particle with arbitrary structures on a plane substrate or inside a cavity. The theory is based on a self-consistent scattering procedure in which retardation effects and contributions from both homogeneous and evanescent modes of the electromagnetic field are included. The decay rate variations are computed and the concept of single-molecule near-field probe is discussed.

Dependence of the Nuclear Excitation by Electron Transition on the Decay Rate

Nuclear excitation by electron transition (NEET) is understood as a resonance phenomenon between the excited atomic state and the nucleus. In the situation where the strong external radiation field transfers energy to a nucleus through the excitation of the atomic shell, e.g., the laser-assisted inverse electron bridge process, the decay rate variation of the initial or final state in the NEET process plays an essential role in the NEET probability. The dependence of the NEET probability on this variation is analyzed in detail with the solution of the equation of motion in the NEET process, and the calculation using 197Au is shown as an example.

Photochemical Transformation of Polycyclic Aromatic Hydrocarbons on Solid Particles

Photochemical transformation and adsorption of some polycyclic aromatic hydrocarbons, PAH, e.g., phenanthrene, anthracene and pyrene, on the different particulate matter: alumina, silica and oil-shale fly ash were investigated. Fly ash from oil-shale thermal power plants is characterised by a small specific surface area and a low adsorption capacity in comparison to commercial adsorbents. The surface on which PAH were adsorbed have a strong effect on their decay. The phototransformation efficiency of the PAH studied was largest for silica, followed by alumina. PAH were more resistant to photochemical transformation on fly ash than on alumina and silica. Variation in decay rates between the different PAH on any given matrix was found to be significant. Phenanthrene was much less photochemically reactive than pyrene, and especially anthracene, on any particulate matter used in this study. Nonphotochemical degradation (autoxidation) of PAH on alumina, silica and fly ash was found to be negligible. Stability of carcinogenic pollutants adsorbed on fly ash may cause a significant public health concern.

Surface imaging in near-field optical microscopy by using the fluorescence decay rate: a theoretical study.

In this paper, we study the fluorescence decay rate of a molecule above a corrugated interface, and particularly the variations of the decay rate as a function of the lateral position of the molecule. As a first step, one has to determine the field diffracted by a corrugated interface when the incident field is the field emitted by a dipole. For this purpose, we have used a perturbative Rayleigh method, and we show that the decay rate variations can be connected to the surface profile via a transfer function. Some numerical calculations of this transfer function and of decay rate variation images are presented for dielectric and metallic samples. The visibility of the theoretical images is up to 20% and, moreover, resolution of the images is good enough to use the fluorescence lifetime of molecules as signal in a life-time scanning near-field optical microscope. The technical problems are discussed briefly.

Rate and state of background stress estimated from the aftershocks of the 1989 Loma Prieta, California, earthquake

Estimates of the tectonic stress state including loading rate and magnitude of background stress are derived from the spatial and temporal distribution of Loma Prieta aftershocks. This technique was previously applied to the Landers aftershock sequence [Gross and Kisslinger, 1997] and is based upon the seismicity model of Dieterich [1994]. Dieterich's theory suggests that background seismicity should be proportional to stress rate and the number of aftershocks in an area should be proportional to the stress step experienced in that area. We used two independently derived source models to compute the stress step from the mainshock and to determine how effective that stress step was in triggering aftershocks. A background stress state is then chosen which makes the stress steps at the aftershock locations most distinct from the stress steps at hypocenters of background seismicity. The best fitting background stress state has its greatest compressive stress plunging ≈ 17° to N13°E, and an intermediate stress very close in magnitude to the least principal stress. The small shear stresses at depth and low coefficient of friction suggest that high‐pressure pore fluids may be present inside active faults. The estimated stress rate of ∼ 70 Pa/d (0.25 bar/yr) is comparable to the stress rate found for the southern San Andreas Fault system. We found a best fitting “effective” coefficient of friction μ′ ≈ 0.2 for the Loma Prieta area, significantly less than μ′ ≈ 0.6 estimated for Landers in previous work. Variations in aftershock decay rate within the Loma Prieta aftershock zone are correlated with static stresses caused by postseismic slip as modeled by Bürgmann et al. [1997]. Some of the postseismic slip occurred on structures that did not slip during the mainshock, so the postseismic and coseismic stress step fields have different spatial distributions. The effectiveness of slowly accumulated postseismic static stresses in triggering aftershocks is especially interesting because dynamic stresses are insignificant in this case.

A STUDY OF THE INITIAL DECAY RATE OF TWO-DIMENSIONAL VIBRATING STRUCTURES IN RELATION TO ESTIMATES OF LOSS FACTOR

The aim of this study was to obtain a better understanding of the process of the initial decay of energy in relation to the estimates of damping loss factor; to investigate the spatial variation of the initial decay rate in order to obtain more reliable estimates of damping loss factor from the decay rate method; and to compare the spatial averages and spatial variation of damping loss factor between the decay rate method and the power input method. The initial decay rate of energy was experimentally investigated on uniform plates. The energy mean free path time was introduced as a factor to characterize the lower limit of a decay interval for fitting an initial decay slope, and to determine the initial point of a decay curve. It is concluded that the initial decay slopes can sometimes be determined within a very short decay interval (e.g., less than 10 dB) provided that the corresponding time interval is larger or much larger than the energy mean free path time. Additionally, the effect of the direct field on the initial decay can be ignored provided that the response point is far enough from the drive point. An analysis of the spatial variation of initial decay rate showed that an increase in the number of modes per band, and light and moderate damping can reduce the spatial variation. High damping may increase the spatial variation. For a frequency band with more modes, fewer response points need to be used to obtain a stable estimation of spatial variance. It was confirmed that the damping loss factor determined from the decay rate method was in general in good agreement with the power input method. The comparison of the spatial variances of damping loss factor between the two methods showed that the decay rate method gives a more reliable estimate of the damping loss factor of the plates. It is concluded that the decay rate method is to be preferred to the power input method when determining the damping loss factor of a system.

Verification of isomerism and direct measurement of half-lives in184Au

The existence of two decaying isomers in ${}^{184}\mathrm{Au}$ was confirmed through direct observation of the variation of decay rates with time. The ordering of the low- and high-spin isomers was verified by the decay studies. The isomers are identified as a low-spin (probably ${2}^{+}$) state at an energy of 69 keV with a half-life of $48\ifmmode\pm\else\textpm\fi{}1\mathrm{sec}$ and a high-spin (probably ${5}^{+}$) ground state with a half-life of $21\ifmmode\pm\else\textpm\fi{}1\mathrm{sec}.$

Temporal variations of the coda decay rate on Kamchatka: Are they real and precursory?

Temporal variations of the decay rate of the coda of local earthquakes are studied for four Kamchatka stations. To suppress any bias caused by the coda Q versus lapse time dependence, I use the deviation of an individual coda decay function from the empirical reference decay function, and the analysis is done over a fixed lapse time window. For each individual coda record, a decay rate parameter “α” is determined by the following procedure. First, the reference log coda decay function is subtracted from an individual one giving a residual function; second, the slope of this residual versus lapse time is determined over the fixed lapse time window giving an α estimate. The data used are records of 150–250 Kamchatka earthquakes of 1967–1990 (ML=3.5–5) obtained by four stations employing three‐component 1‐s seismographs. The processed data show apparent temporal variation of α with high statistical significance. Several possibilities are then investigated of mimicking the genuine temporal variation of α by systematic variation of other parameters, first of all epicenter wandering and nodal plane rotation. This analysis does not reveal any strong bias and thus suggests that the observed variation is genuine. Then I show the significance of five apparently precursory anomalies identified retroactively. Also, I describe briefly the real‐time prediction experiment conducted on Kamchatka for 1982–1990 using coda decay, aimed at the intermediate‐term forecasting of large Benioff zone earthquakes. This experiment resulted in the successful forecast of the August 17, 1983, Mw=7.0 event with an accuracy of 2 months in time and 100–200 km in location; magnitude was overestimated by 0.5. Later, a false alarm was also issued. Thus the experiment confirms the reality of the coda decay rate precursor but also shows that it needs further improvement to become reliable. Physically, the variations of the coda decay rate are associated with time‐dependent local variations of scatterer density in the lithosphere.

Spontaneous emission rate and level shift of an atom inside a dielectric microsphere

Abstract The transition frequency shift and the spontaneous decay rate (the radiative linewidth) variation of an atom placed in a transparent dielectric microsphere are presented. It is demonstrated that with the refractive index of the microsphere material (e.g. diamond) being high enough, the size of the microsphere can be selected such that the spontaneous decay rate decreases (or increases) several times, depending on the position of the atom. At the same time, the atomic transition frequency shift proves to depend more strongly on the position of the atom, reaching high values near the surface of the microsphere.

Spontaneous emission rate and level shift of an atom inside a dielectric microsphere

Abstract The transition frequency shift and the spontaneous decay rate (the radiative linewidth) variation of an atom placed in a transparent dielectric microsphere are presented. It is demonstrated that with the refractive index of the microsphere material (e.g. diamond) being high enough, the size of the microsphere can be selected such that the spontaneous decay rate decreases (or increases) several times, depending on the position of the atom. At the same time, the atomic transition frequency shift proves to depend more strongly on the position of the atom, reaching high values near the surface of the microsphere.

Magnetic Field Effects on the Solute Luminescence of Alkane Solutions Irradiated with Heavy Ions

The effects of track structure on the luminescence decays in cyclohexane and in 2,2,4-trimethylpentane (isoocatane) solutions of 2,5-diphenyloxazole (PPO) have been determined in the presence and in the absence of an external magnetic field. Irradiations were performed with protons of 1-15 MeV and with helium ions of 2-20 MeV energy. Companion studies were performed with {sup 90}Sr-{sup 90}Y {Beta}-radiolysis. The magnetic field effect is due to the hyperfine interaction of nuclear spins in the geminate pair of solvent radical ions produced. In both solvents, the effect of the magnetic field on luminescence decreases with increasing linear energy transfer (LET) from about 40% for {Beta}-particles to only a few percent with helium ions. Magnetic field effects with protons decrease in time whereas they are constant with {Beta}-particles. This result is attributed to the overlap of initially isolated spurs during the evolution of the proton track; the probability of nongeminate recombination increases with the number of neighboring ion pairs. The total luminescence intensity per incident particle remains constant with proton energy but increases slightly with increasing helium ion energy. At a given particle energy, the intensity is greater in cyclohexane than in isoocatane. The pulse shapes of the luminescence decays reflect themore » distributions in ion recombination times, and very little variation in luminescence decay rates is observed with increasing LET. 37 refs., 7 figs., 1 tab.« less

Effects of tidally mediated litter moisture content on decomposition of Spartina alterniflora and S. patens

Relationships between flooding frequency, flooding duration, litter moisture levels, and litter decay rates were investigated across the natural hydrologic gradient common to intertidal salt marshes. The effects on litter decay of natural and experimental alterations of litter moisture content were assessed in both field litterbag experiments (conducted in a southern New Jersey salt marsh from 1989 to 1990) and laboratory incubations (1990). Overall, tidally mediated litter moisture content was the dominant factor controlling litter decay throughout the vegetated marsh. Rates of carbon loss were most closely related to litter moisture levels (r=0.84), which were directly related to flooding frequency (r=0.66) and duration (r=0.63). Litter moisture levels were related to elevation within the tidal range due to increasing surface levation from creekbank to high marsh (ca. 54 cm) and height of litter above the sediment surface. Carbon losses from litter of short and tall form Spartina alterniflora Loisel. and S. patens (Aiton) Muhl. along the marsh elevation gradient indicate that while some of the variations in decay rates may be due to litter type, litter moisture accounted for most of the observed variation between marsh zones and within each litter type. Mousture levels are also affected by the water retention capacity of each litter type, which may also secondarily influence decay rates. Short-term incubations of litter indicated that CO2 evolution was positively related to moisture content with negligible respiration at moisture levels below 15% (fresh mass), increasing to a maximum between 65 and 75% depending upon litter type. Since most Spartina spp. litter remains above the marsh surface where it maintains a lower moisture content than surface litter, the use of surface litterbags may overestimate rates of carbon loss in some systems. In addition, since changes in elevation of only a few centimeters had significant effects on both litter moisture levels and decay rates, slight changes in tidal regime may have important consequences for organic matter cycling in salt marshes by affecting litter decomposition processes.

Identifying Water-Flow Problems

Summary The most important element of conformance control in fields where water injection is used to enhance oil production is identifying the nature and source of the problem. Chemical analysis and water-flow logging can help operators isolate water-flow, invasion, and annular leaks that destroy casing integrity and decrease profitability. This article presents a case history that demonstrates use of analytical techniques to identify conformance-control problems and solutions. Conditions In a west Texas waterflood field, an injection well was being used to sweep oil toward offsetting wells (Fig. 1a). Wells 1 and 2 are producers, Well 3 is the injection well, Well 4 is the water source, and Well 5 is on an adjacent lease that belongs to another operator. The waterflood zone for Well 3 is between 4,000 and 4,900 ft and between 2,500 and 3,100 ft for Wells 1, 2, and 5. Well 3 was drilled and completed to serve as an injector; however, Well 3 showed 325 psi pressure between its intermediate and surface strings soon after completion. The pressure could be relieved by opening the surface valve but would recur within 24 hours. Two remedial cement jobs were performed on the well to halt the gas flow that was causing the pressure. About 1 month after the squeeze cement job on Well 3, a wet spot (Fig. 1b) appeared on the ground around Well 4, 150 ft from Well 3. Chemical analysis of the water showed that it had not come from Well 3 or from Well 4, the injection-water source. Salt concentration in the water had increased from 15,000 to 170,000 mg/L chlorides. Additionally, the production annulus pressure in Wells 1 and 2 had increased, and shutting off Well 3 did not affect the annulus pressure of Wells 1 or 2 or the chloride content. The produced water was routed to tanks and allowed to flow so the 350- to 400-psi shut-in casing pressures could be replaced. Log Analysis A water-flow analysis log was run on Well 3 to determine water-flow velocity and direction behind two strings of casing at Well 3. This log was an oxygen-activation device that uses (1) statistical variations in isotope decay rates, through measurement of gamma ray emission counts, and (2) log movement to determine water velocity within several inches of the wellbore. Analysis of the log showed water entering the annulus at 2,807 ft, exiting at 1,980 ft, re-entering the annulus at 980 ft, exiting below the surface casing shoe at 306 ft, and rising to the surface (Fig. 2). Water-flow analysis logs appeared to show that water was coming from Well 5 and taking a route to the surface through the Well 3 annulus and a naturally occurring or induced fracture network. The water traveled through one or more brackish sands, gaining salinity by dissolving anhydrides along the way. With a most likely flow pattern established, injection into offset wells (four) was halted, including injection from Well 5, even though it belonged to another operator. Within minutes of shut-in, most annular flow had ceased. All wells remained shut in for 3 days. To isolate the source of the flow, injection was temporarily conducted into each surrounding well, one well at a time. This pumping method isolated the problem to Well 5. A foam squeeze cement job was designed and pumped into 10 new perforations between 2,692 and 2,694 ft in Well 3, successfully shutting off injection flow from Well 5. The foam cement also effectively filled and squeezed the uncemented annulus of Well 3 from 2,700 to 1,900 ft, which should help prevent future cement-sheath leaks. The offset producing wells are all back on pump and producing at presqueeze levels, and no sign of underground water escape to the surface is evident. Well 5 is now being injected at 1,300 psi, with no influence on Wells 1 through 4. P. 699