Two-source Model Research Articles

Comparison of analytic source models for head scatter factor calculation and planar dose calculation for IMRT

The purpose of this study was to choose an appropriate head scatter source model for the fast and accurate independent planar dose calculation for intensity-modulated radiation therapy (IMRT) with MLC. The performance of three different head scatter source models regarding their ability to model head scatter and facilitate planar dose calculation was evaluated. A three-source model, a two-source model and a single-source model were compared in this study. In the planar dose calculation algorithm, in-air fluence distribution was derived from each of the head scatter source models while considering the combination of Jaw and MLC opening. Fluence perturbations due to tongue-and-groove effect, rounded leaf end and leaf transmission were taken into account explicitly. The dose distribution was calculated by convolving the in-air fluence distribution with an experimentally determined pencil-beam kernel. The results were compared with measurements using a diode array and passing rates with 2%/2 mm and 3%/3 mm criteria were reported. It was found that the two-source model achieved the best agreement on head scatter factor calculation. The three-source model and single-source model underestimated head scatter factors for certain symmetric rectangular fields and asymmetric fields, but similar good agreement could be achieved when monitor back scatter effect was incorporated explicitly. All the three source models resulted in comparable average passing rates (>97%) when the 3%/3 mm criterion was selected. The calculation with the single-source model and two-source model was slightly faster than the three-source model due to their simplicity.

Integral frequency distribution of projectile alpha fragments in nuclear multifragmentations at intermediate and high energies

The integral frequency distributions of the numbers of projectile alpha fragments produced in nuclear multifragmentations at intermediate and high energies are studied by a simple two-source model. The calculated results are approximately in agreement with the experimental data of 16 O, 40 Ar, 56 Fe, 84 Kr, 139 La, and 197 Au multifragmentations in nuclear emulsions at 2.00 A , 1.80 A , 1.80 A , 0.08–1.70 A , 0.60–0.99 A , and 0.60–0.99 A GeV, respectively. The initial energy of nuclear limiting fragmentation is expected to be at or lower than 0.08 A GeV. Two correlations between the mean multiplicity of projectile alpha fragments ( 〈 n α 〉 ) and the projectile mass number, as well as 〈 n α 〉 and the maximum multiplicity of projectile alpha fragments are observed.

Relativistic proton production at the Sun in the 20 January 2005 solar event

Based on the concept of multiple acceleration of solar energetic particles (SEP) we analyzed the super-event of 20 January 2005 by the data of ground level, balloon and spacecraft observations. The main characteristics of relativistic solar protons (energy spectra, anisotropy directions and pitch-angle distributions) are derived and their dynamics during the event is studied. It is shown that the flux of relativistic solar protons may consist of two distinct components, the so-called prompt and delayed ones. Within a two-source model of particle generation, one of which is associated with an expanding magnetic loop, we solved the transport equation in energy phase space, including adiabatic losses simultaneously with the stochastic acceleration process, and calculate the expected spectra of the delayed component at the source. The confrontation of experimental spectra with theoretical ones shows that the delayed component may be correctly described by stochastic acceleration, but not the prompt component. The required acceleration efficiencies turned out to be rather high, so that, for this particular event, adiabatic cooling is practically negligible. Our results provide a new support to the existence of two populations of relativistic solar protons in some SEP events.

Site-specific methane production and subsequent midge mediation within Esthwaite Water, UK

Previous analyses of larval chironomid δ13C have suggested that methanotrophic contribution to biomass is site-specific within lakes. We determined larval biomass, larval and methane gas stable carbon isotopes, and potential methane production from the sediments underlying different water column depths in Esthwaite Water, UK. Methane production increased whereas larval δ13C values typically became lighter with increasing lake depth. Reduced methane production at 15 m depth and correspondingly less13C-depleted larvae in the second year of study suggests that the rate of methane production influenced larval assimilation of methane-derived biomass. Larval distribution and other site-specific parameters, combined with two-source mixing models, were used to estimate potential methane-mediation via the abundant chironomid biomass to higher predators.

Aggregation Efficiency-Aware Greedy Incremental Tree Routing for Wireless Sensor Networks

SUMMARY In most research work for sensor network routings, perfect aggregation has been assumed. Such an assumption might limit the application of the wireless sensor networks. We address the impact of aggregation efficiency on energy consumption in the context of GIT routing. Our questions are how the most efficient aggregation point changes according to aggregation efficiency and the extent to which energy consumption can decrease compared to the original GIT routing and opportunistic routing. To answer these questions, we analyze a two-source model, which yields results that lend insight into the impact of aggregation efficiency. Based on analytical results, we propose an improved GIT: “aggregation efficiency-aware GIT,” or AGIT. We also consider a suppression scheme for exploratory messages: “hop exploratory.” Our simulation results show that the AGIT routing saves the energy consumption of the data transmission compared to the original GIT routing and opportunistic routing.

Utility of Radiometric–aerodynamic Temperature Relations for Heat Flux Estimation

In many land-surface models using bulk transfer (one-source) approaches, the application of radiometric surface temperature observations in energy flux computations has given mixed results. This is due in part to the non-unique relationship between the so-called aerodynamic temperature, which relates to the efficiency of heat exchange between the land surface and overlying atmosphere, and a surface temperature measurement from a thermal-infrared radiometer, which largely corresponds to a weighted soil and canopy temperature as a function of radiometer viewing angle. A number of studies over the past several years using multi-source canopy models and/or experimental data have developed simplified methods to accommodate radiometric-aerodynamic temperature differences in one-source approaches. A recent investigation related the variability in the radiometric-aerodynamic relation to solar radiation using experimental data from a variety of landscapes, while another used a multi-source canopy model combined with measurements over a wide range in vegetation density to derive a relationship based on leaf area index. In this study, simulations by a detailed multi-source soil-plant-environment model, Cupid, which considers both radiative and turbulent exchanges across the soil-canopy-air interface, are used to explore the radiometric-aerodynamic temperature relations for a semi-arid shrubland ecosystem under a range of leaf area/canopy cover, soil moisture and meteorological conditions. The simulated radiometric-aerodynamic temperatures indicate that, while solar radiation and leaf area both strongly affect the magnitude of this temperature difference, the relationships are non-unique, having significant variability depending on local conditions. These simulations also show that soil-canopy temperature differences are highly correlated with variations in the radiometric-aerodynamic temperature differences, with the slope being primarily a function of leaf area. This result suggests that two-source schemes with reliable estimates of component soil and canopy temperatures and associated resistances may be better able to accommodate variability in the radiometric-aerodynamic relation for a wider range in vegetated canopy cover conditions than is possible with one-source schemes. However, comparisons of sensible heat flux estimates with Cupid using a simplified two-source model and a one-source model accommodating variability in the radiometric-aerodynamic relation based on vegetation density gave similar scatter. On the other hand, with experimental data from the shrubland site, the two-source model generally outperformed the one-source scheme. Clearly, vegetation density/leaf area has a major effect on the radiometric-aerodynamic temperature relation and must be considered in either one-source or two-source formulations. Hence these adjusted one-source models require similar inputs as in two-source approaches, but provide as output only bulk heat fluxes; this is not as useful for monitoring vegetation conditions.

Semi-Empirical Noise Models for Forced Mixer Jet Noise Predictions

In recent years there has been a significant interest in the development of jet noise prediction models due to increased restrictions on aircraft noise near airports. However, currently there are no industry tools that can be used to predict the noise from the complex flows of modern jet engines that include internal forced mixers. The four-source method is a noise prediction tool applicable to simple coplanar, coaxial jets. In this study the fundamental components of the four-source model are used as the building blocks for a noise prediction method which would be applicable to coaxial jets with internal forced mixers. First, the four-source model is applied to an internally mixed jet with a confluent or axisymmetric mixer. Then, a two-source noise model is proposed to predict the noise for cases with an internal forced mixer. Three variable parameters in this two-source model are optimized to match the forced mixer experimental noise data. It is shown that the two-source noise model is capable of accurately modeling the noise spectra of internally mixed jets with forced mixers.

Comparing the utility of microwave and thermal remote-sensing constraints in two-source energy balance modeling over an agricultural landscape

A two-source (soil + vegetation) energy balance model using microwave-derived near-surface soil moisture as a key boundary condition (TSM SM) and another scheme using thermal-infrared (radiometric) surface temperature (TSM TH) were applied to remote sensing data collected over a corn and soybean production region in central Iowa during the Soil Moisture Atmosphere Coupling Experiment (SMACEX)/Soil Moisture Experiment of 2002 (SMEX02). The TSM SM was run using fields of near-surface soil moisture from microwave imagery collected by aircraft on six days during the experiment, yielding a root mean square difference (RMSD) between model estimates and tower measurements of net radiation (Rn) and soil heat flux ( G) of approximately 20 W m − 2 , and 45 W m − 2 for sensible ( H) and latent heating (LE). Similar results for H and LE were obtained at landscape/regional scales when comparing model output with transect-average aircraft flux measurements. Flux predictions from the TSM SM and TSM TH models were compared for two days when both airborne microwave-derived soil moisture and radiometric surface temperature ( T R) data from Landsat were available. These two days represented contrasting conditions of moderate crop cover/dry soil surface and dense crop cover/moist soil surface. Surface temperature diagnosed by the TSM SM was also compared directly to the remotely sensed T R fields as an additional means of model validation. The TSM SM performed well under moderate crop cover/dry soil surface conditions, but yielded larger discrepancies with observed heat fluxes and T R under the high crop cover/moist soil surface conditions. Flux predictions from the thermal-based two-source model typically showed biases of opposite sign, suggesting that an average of the flux output from both modeling schemes may improve overall accuracy in flux predictions, in effect incorporating multiple remote-sensing constraints on canopy and soil fluxes.

Deflation of the Askja volcanic system: Constraints on the deformation source from combined inversion of satellite radar interferograms and GPS measurements

The Askja central volcano in northern Iceland has been continuously subsiding at least since 1983. GPS and optical leveling tilt measurements show subsidence of at least 75 cm from 1983 to 1998 in the center of the Askja caldera, without any eruptive activity. We have performed an interferometric analysis of Synthetic Aperture Radar images (InSAR) of the area, utilizing data from the ERS satellites. We observe subsidence of the Askja caldera and its fissure swarm, up to a distance of 25 km from the volcano. We evaluate the geometry of the magma chamber at Askja, from a combined inversion of satellite radar interferograms and GPS measurements. Several models were tested, including a Mogi point source as well as an ellipsoidal source. The use of an ellipsoidal source instead of a Mogi source gives an estimate of the dimensions of the magma chamber and its deflating pressure, whereas these parameters are not independently resolved if a Mogi source is used. Two-source models were also considered in order to explain the additional subsidence observed along the Askja fissure swarm. We tested a model using two Mogi sources at different depths, a shallow ellipsoidal cavity with a deeper Mogi source, and then a shallow Mogi source with a deeper elongated ellipsoid, oriented along the fissure swarm. Results indicate that an ellipsoidal source at about 3 km depth can accommodate most of the deflation occurring in the caldera. Residual subsidence occurs along the Askja fissure swarm suggesting the existence of a deeper source of contraction. We interpret this signal in terms of subsidence of the plate boundary.

Measuring carbon isotope ratios of microphytobenthos using compound‐specific stable isotope analysis of phytol

Carbon stable isotope analysis is one of the most common methods for validating theories about the role microphytobenthos (MPB) plays in estuarine food webs. However, difficulties extracting MPB from sediments to determine a pure isotope signature have hampered such studies. We have developed compound‐specific isotope analysis (CSIA) of phytol δ13C (δ13Cphytol) as an accurate proxy for δ13C of bulk MPB (δ13Cbulk). In most circumstances MPB is the dominant source of phytol in estuarine sediments, particularly in shallow water environments, so our method circumvents the need to extract MPB. We have demonstrated in the laboratory the relationship between δ13Cphytol and δ13Cbulk, and could predict δ13Cbulk within 1.8‰ (95% CI) of observed values at moderate replication (n = 5). If water temperature is included, δ13Cphytol predicts δ13Cbulk within 1.3‰. With greater replication (n = 10), predictions are extremely good (within 1.0‰). A two‐source mixing model assessed the usefulness of each method for estimating MPB contribution to consumer nutrition. At moderate replication (n = 5), for a gap between MPB and another producer of 6.0‰, estimates were more precise (i.e., 95% CIs were smaller) when both δ13Cphytol and temperature were used (95% CI 0.40) rather than only δ13Cphytol (0.46). The greatest difference in precision was for a gap of 6.0‰, close to the average gap in the literature (5.5‰). Given the difficulty of extracting MPB from sediment, carbon isotope studies of food webs could benefit by using CSIA of phytol, especially in conjunction with simple measurements of water temperature. The advance in methodology will allow a reevaluation of the trophic importance of MPB.

Utility of Remote Sensing–Based Two-Source Energy Balance Model under Low- and High-Vegetation Cover Conditions

Abstract Two resistance network formulations that are used in a two-source model for parameterizing soil and canopy energy exchanges are evaluated for a wide range of soybean and corn crop cover and soil moisture conditions during the Soil Moisture–Atmosphere Coupling Experiment (SMACEX). The parallel resistance formulation does not consider interaction between the soil and canopy fluxes, whereas the series resistance algorithms provide interaction via the computation of a within-air canopy temperature. Land surface temperatures were derived from high-resolution Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM) scenes and aircraft imagery. These data, along with tower-based meteorological data, provided inputs for the two-source energy balance model. Comparison of the local model output with tower-based flux observations indicated that both the parallel and series resistance formulations produced basically similar estimates with root-mean-square difference (RMSD) values ranging from approximately 20 to 50 W m−2 for net radiation and latent heat fluxes, respectively. The largest relative difference in percentage [mean absolute percent difference (MAPD)] was for sensible heat flux, which was ≈35%, followed by a MAPD ≈ 25% for soil heat flux, ≈10% for latent heat flux, and a MAPD < 5% for net radiation. Although both series and parallel versions gave similar results, the parallel resistance formulation was found to be more sensitive to model parameter specification, particularly in accounting for the effects of vegetation clumping resulting from row crop planting on flux partitioning. A sensitivity and model stability analysis for a key model input variable, that is, fractional vegetation cover, also show that the parallel resistance network is more sensitive to the errors vegetation cover estimates. Furthermore, it is shown that for a much narrower range in vegetation cover fraction, compared to the series resistance network, the parallel resistance scheme is able to achieve a balance in both the radiative temperature and convective heat fluxes between the soil and canopy components. This result appears to be related to the moderating effects of the air temperature in the canopy air space computed in the series resistance scheme, which represents the effective source height for turbulent energy exchange across the soil–canopy–atmosphere system.

Ground deformation of Nisyros Volcano (Greece) for the period 1995–2002: Results from DInSAR and DGPS observations

Differential GPS (DGPS) and Differential Interferometric Synthetic Aperture Radar (DInSAR) analyses were applied to the Kos-Yali-Nisyros Volcanic Field (SE Hellenic Volcanic Arc) to quantify the ground deformation of Nisyros Volcano. After intense seismic activity in 1996, a GPS network was installed in June 1997 and re-occupied annually up to 2002. A general uplift ranging from 14 to 140 mm was determined at all stations of the network. The corresponding horizontal displacements ranged from 13 to 53 mm. The displacement vectors indicate that the island is undergoing extension towards the East, West and South. A two-source “Mogi” model combined with assumed motion along the Mandraki Fault was constructed to fit the observed deformation. The best-fit model assumes sources at a depth of 5500 m NW of the centre of the island and at 6500 m offshore ESE of Yali Island. DInSAR analysis using four pairs of images taken between May 1995 and September 2000 suggests that deformation was occurring during 1995 before the start of the seismic crisis. An amplitude of at least 56 mm along the slant range appeared for the period 1996 through 1999. This deformation is consistent with the two-source model invoked in DGPS modelling. Surface evidence of ground deformation is expressed in the contemporaneous reactivation of the Mandraki Fault. In addition, a 600 m long N-S trending irregular rupture in the caldera floor was formed between 2001 and 2002. This rupture is interpreted as the release of surface stress in the consolidated epiclastic and hydrothermal sediments of the caldera floor.

Evaluation of isotopic fractionation error on calculations of marine-derived nitrogen in terrestrial ecosystems

Pacific salmon (Oncorhynchus spp.) transport nitrogen (N) from oceans to inland ecosystems. Salmon δ15N is higher than δ15N expected in terrestrial plants, so linear two-source mixing models have commonly been used to quantify contributions of marine-derived N (MDN) to riparian ecosystems based on riparian plant δ15N. However, isotopic fractionation potentially contributes to error in MDN estimates by changing δ15N of salmon-derived N appearing in soil and plants. We used a simulation model to examine potential effects of fractionation on MDN estimates. We also measured changes in δ15N and δ13C as N and carbon (C) moved from bear feces into soil, and compared MDN estimates using three different estimates for the marine endmember of a linear mixing model. Simulation demonstrated that fractionation during soil N losses could lead to large overestimations of MDN when δ15N of salmon tissue is used as the marine endmember. δ15N of bear feces was significantly enriched (by 1.9‰) relative to salmon tissue, but did not change during movement of feces-derived N into soil. In contrast, δ13C decreased by 1.9‰ between salmon and bear feces and declined an additional 4.2‰ during movement into soil. We propose a new method for estimating the δ15N of the marine endmember that accounts for isotope fractionation occurring as marine N is cycled in soil. This method uses the proportional difference in soil 15N content between reference and spawning sites to calculate the marine endmember δ15N.

The AGASA and SUGAR Anisotropies and TeV Gamma Rays from the Galactic Center: A Possible Signature of Extremely High Energy Neutrons

Recent analysis of data sets from two extensive air shower cosmic-ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays toward the Galactic center region that turns on around 1018 eV. We demonstrate that the anisotropy could be due to neutrons created in the Galactic center region through charge exchange in proton-proton collisions, where the incident, high-energy protons obey a ~E-2 power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746-2851 (ascribed to p-p-induced neutral pion decay at GeV energies), together with a very reasonable spectral index of 2.2, predicts a neutron flux at ~1018 eV fully consistent with the extremely high energy cosmic-ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air Cerenkov telescope at ~TeV energies is almost equally compatible with the ~1018 eV cosmic-ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We find a plausible resolution of this discrepancy in an effective two-source model. Finally, we argue that the shock acceleration is probably occurring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic center. In support of this contention we note that (1) the extended shell of this object could provide both of the sources suggested by the gamma-ray data and (2) the unusually strong magnetic field at this remnant, together with a perpendicular shock geometry, allow for acceleration of protons up to the extreme energies required to explain the cosmic-ray anisotropy. If the connection between the anisotropy and Sagittarius A East could be firmly established, it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near 1019 eV.

Relativistic proton production at the sun in the October 28th, 2003 solar event

In order to infer about the origin of solar relativistic particles (SRP) from the particle event of October 28th, 2003, we proceed to do a confrontation of the experimental energy spectra with the theoretical spectra derived from a transport equation for stochastic acceleration. On basis to a two-source model of particle generation, one of which is associated with an expanding magnetic loop, we solve the transport equation including adiabatic losses simultaneously with the stochastic acceleration process. The confrontation shows that there are two different populations during this event, one of which, the so-called “delayed component” may be correctly described by stochastic acceleration, but not the so-called “prompt component”. We found that the required acceleration efficiencies turn to be very high, so that for this particular event, adiabatic cooling is practically negligible as far as the energy spectrum is concerned. Qualitative inferences point toward a dominated Alfven accelerating turbulence. Our results provide a new support to the existence of two relativistic particle populations in some solar relativistic particle events.

Derivation of the distribution of extrafocal radiation for head scatter factor calculation

Head scatter factors for high energy photon beams from linear accelerators can be modeled using a two-source model consisting of focal and extrafocal radiation. The focal radiation can be approximated as a point source, and the distribution of the extrafocal radiation is a two-dimensional (2D) radial symmetric function. Various methods, including analytical, Monte Carlo, and empirical trial functions, have been used to determine the radial symmetric function of extrafocal radiation distribution. This article describes a method for directly determining the extrafocal radiation distribution without assuming any empirical trial function. The extrafocal radiation distribution is determined with measured head scatter factors for rectangular fields defined by the lower jaw (X) fixed at 40 cm and the upper jaw (Y) varying from 3 to 40 cm. The derivatives of the measured head scatter factors, with respect to the Y jaw position projected in the plane of extrafocal radiation, are proportional to the one-dimensional (1D) projection (also called the line spread function) of the extrafocal radiation distribution. Two methods are used to solve the radial function of extrafocal radiation from the 1D projection. The first method uses a 2D filtered backprojection algorithm, originally developed for parallel beam computed tomography reconstruction, to directly derive the radial dependence of the extrafocal radiation distribution. The method has been applied to 6 and 18 MV photon beams from a Siemens linear accelerator and has been tested by comparing measured and calculated head scatter factors for square and rectangular fields. The second method uses a Fourier transform followed by a Fourier-Bessel transform to solve the problem. The distributions of extrafocal radiation derived from these two methods are virtually identical.

A semi-distributed, physics-based hydrologic model using remotely sensed and Digital Terrain Elevation Data for semi-arid catchments

A practical, hydrologic model (DPHM-RS) is developed for the semi-arid climate of the Canadian Prairies that could adequately account for a river basin's terrain features by sub-dividing it to sub-basins of uneven shapes and sizes (semi-distributed) based on topographic information derived from the digital terrain elevation (DTED) data. Even though computationally modest, DPHM-RS is scientifically vigorous, can effectively assimilate remotely sensed (RS) data, and has most of its parameters determined through RS data and measurements. The hydrologic processes are estimated for each land cover and then aggregated according to percentage of each land cover present within each sub-basin. As evapotranspiration (ET) usually dominates the hydrology of the Canadian Prairies, ET from each land cover is estimated at three levels by the two-source model that separately considers evaporation from soil and plants. The soil moisture at the top active and the transmission zones are estimated by a water budget approach, while the groundwater dynamics by the topographic soil index obtained from DTED. The surface runoff from each sub-basin is routed to the channel network by a kinematic wave response function, and then routed to the basin outlet by the Muskingum-Cunge model. DPHM-RS, is applied to the Paddle River Basin (265 km2) of Central Alberta divided to five sub-basins. It was calibrated with hourly hydroclimatic and RS data collected in the summer of 1996 and validated with data of 1997 and 1998. In both stages, there are good agreements between simulated runoff at the basin outlet with the observed, between simulated surface temperature and net radiation with the observed, between soil moisture and that retrieved from Radarsat-SAR data, and between simulated ET and that estimated by water balance. Encouraging results from these multi-criteria assessments demonstrate the feasibility of semi-distributed, physics-based hydrologic modelling in the dry climate of Canadian Prairies, and the usefulness of RS and DTED data in basin hydrology.

Modelling of evaporation in a sparse millet crop using a two-source model including sensible heat advection within the canopy

During two successive growing seasons meteorological measurements were made in a pearl millet field in the Sahel to investigate the evaporation process in relation to crop growth. The evaporation was measured by eddy correlation and simulated using the Shuttleworth Wallace (SW) model [Q. J. R. Meteorol. Soc. 111 (1985) 839–855]. To take sun height and multi-layer scattering into account a radiation balance model was formulated. The model indicates that partitioning of the net radiation between the vegetation and the soil may be estimated ( r 2=0.94) from the fraction of diffuse radiation, the leaf area index and an attenuation coefficient, but that the attenuation coefficient may not be similar in different locations. To solve the SW-model with respect to the soil resistance an iterative solution was employed with the total evaporation estimated from the Bowen-ratio calculated from eddy correlation measurements. The procedure made it possible to derive stable estimates of soil resistance at soil evaporation rates down to 25 W m −2. The soil resistance was found to be in accordance with evaporation through a dry surface layer. The SW-model indicates, that advection of sensible heat from the dry soil to the plants, increases transpiration considerably. This will cause management techniques, such as mulching and dry farming, aimed at reducing soil evaporation to be less effective than might be anticipated. The effects of raising the leaf area index to improve the microclimate is discussed in relation to management of the available water and crop security.

Multiple internal reflections in the cochlea and their effect on DPOAE fine structure.

In recent years, evidence has accumulated in support of a two-source model of distortion product otoacoustic emissions (DPOAEs). According to such models DPOAEs recorded in the ear canal are associated with two separate sources of cochlear origin. It is the interference between the contributions from the two sources that gives rise to the DPOAE fine structure (a pseudoperiodic change in DPOAE level or group delay with frequency). Multiple internal reflections between the base of the cochlea (oval window) and the DP tonotopic place can add additional significant components for certain stimulus conditions and thus modify the DPOAE fine structure. DPOAEs, at frequency increments between 4 and 8 Hz, were recorded at fixed f2/f1 ratios of 1.053, 1.065, 1.08, 1.11, 1.14, 1.18, 1.22, 1.26, 1.30, 1.32, 1.34, and 1.36 from four subjects. The resulting patterns of DPOAE amplitude and group delay (the negative of the slope of phase) revealed several previously unreported patterns in addition to the commonly reported log sine variation with frequency. These observed "exotic" patterns are predicted in computational simulations when multiple internal reflections are included. An inverse FFT algorithm was used to convert DPOAE data from the frequency to the "time" domain. Comparison of data in the time and frequency domains confirmed the occurrence of these "exotic" patterns in conjunction with the presence of multiple internal reflections. Multiple internal reflections were observed more commonly for high primary ratios (f2/f1 > or = 1.3). These results indicate that a full interpretation of the DPOAE level and phase (group delay) must include not only the two generation sources, but also multiple internal reflections.

Analysis of particle production in ultrarelativistic heavy-ion collisions within a two-source statistical model

The experimental data on hadron yields and ratios in central lead-lead and gold-gold collisions at $158A\mathrm{GeV}/c$ (SPS) and $\sqrt{s}=130A\mathrm{GeV}$ (RHIC), respectively, are analyzed within a two-source statistical model of an ideal hadron gas. A comparison with the standard thermal model is given. The two sources, which can reach the chemical and thermal equilibrium separately and may have different temperatures, particle and strangeness densities, and other thermodynamic characteristics, represent the expanding system of colliding heavy ions, where the hot central fireball is embedded in a larger but cooler fireball. The volume of the central source increases with rising bombarding energy. Results of the two-source model fit to RHIC experimental data at midrapidity coincide with the results of the one-source thermal model fit, indicating the formation of an extended fireball, which is three times larger than the corresponding core at SPS.