scholarly journals A Computational Model for Oxygen Depletion Hypothesis in FLASH Effect

2021 ◽  
Author(s):  
Ankang Hu ◽  
Rui Qiu ◽  
Zhen Wu ◽  
Hui Zhang ◽  
Wei Bo Li ◽  
...  
Keyword(s):  
Computational Model ◽  
Dose Rate ◽  
Oxygen Concentration ◽  
Microvessel Density ◽  
Oxygen Depletion ◽  
Oxygen Distribution ◽  
Resistance Level ◽  
Negative Exponential Function ◽  
Normal Tissues ◽  
Negative Exponential

Experiments have reported low normal tissue toxicities during FLASH irradiation, but the mechanism has not been elaborated. Several hypotheses have been proposed to explain the mechanism. One hypothesis is oxygen depletion. We analyze the time-dependent change of oxygen concentration in the tissue to study the oxygen depletion hypothesis using a computational model. The effects of physical, chemical and physiological parameters on oxygen depletion were explored. The kinetic equation of the model is solved numerically using the finite difference method with rational boundary conditions. Results of oxygen distribution is supported by the experiments of oxygen-sensitivity electrodes and experiments on the expression and distribution of the hypoxia-inducible factors. The analysis of parameters shows that the steady-state oxygen distribution before irradiation is determined by the oxygen consumption rate of the tissue and the microvessel density. The change of oxygen concentration after irradiation has been found to follow a negative exponential function, and the time constant is mainly determined by the microvessel density. The change of oxygen during exposure increases with dose rate and tends to be saturated because of oxygen diffusion. When the dose rate is high enough, the same dose results in the same reduction of oxygen concentration regardless of dose rate. The analysis of the FLASH effect in the brain tissue based on this model does not support the explanation of the oxygen depletion hypothesis. The oxygen depletion hypothesis remains controversial because the oxygen in most normal tissues cannot be depleted to radiation resistance level by FLASH irradiation.

Patterns of roof runoff contamination and their potential implications on practice and regulation of treatment and local infiltration

1996 ◽  
Vol 33 (6) ◽  
pp. 39-48 ◽  
Author(s):  
Jürgen Förster
Keyword(s):  
Heavy Metals ◽  
Metal Surfaces ◽  
Precipitation Event ◽  
Pollution Level ◽  
Runoff Water ◽  
Roof Runoff ◽  
Negative Exponential Function ◽  
Subsequent Decrease ◽  
First Flush Effect ◽  
Negative Exponential

Roof runoff water was sampled from an experimental roof system and from house roofs in the city of Bayreuth, Germany. Samples were analysed for organic micropollutants, heavy metals and sum parameters. The pollution level and the shape of the runoff profiles are dependent on the individual properties of the precipitation event and the roof, but patterns with high concentrations at the beginning of the event and a subsequent decrease (first flush effect) are very typical. For dissolved substances, the profile can often be well described by a negative exponential function. Metal surfaces on the roofs cause extreme runoff pollution with heavy metals (Cu, Zn) that constitutes an environmental hazard. It is concluded that there is a need for the development of flexible drainage strategies for surface runoff and that metal surfaces should be avoided on roofs.

scholarly journals Modeling Epidemics in Seed Systems and Landscapes To Guide Management Strategies: The Case of Sweet Potato in Northern Uganda

2019 ◽  
Vol 109 (9) ◽  
pp. 1519-1532 ◽  
Author(s):  
K. F. Andersen ◽  
C. E. Buddenhagen ◽  
P. Rachkara ◽  
R. Gibson ◽  
S. Kalule ◽  
...  
Keyword(s):  
Sweet Potato ◽  
Power Law ◽  
Inverse Power ◽  
Centrality Measures ◽  
Northern Uganda ◽  
Seed Systems ◽  
Negative Exponential Function ◽  
Improved Varieties ◽  
Inverse Power Law ◽  
Negative Exponential

Seed systems are critical for deployment of improved varieties but also can serve as major conduits for the spread of seedborne pathogens. As in many other epidemic systems, epidemic risk in seed systems often depends on the structure of networks of trade, social interactions, and landscape connectivity. In a case study, we evaluated the structure of an informal sweet potato seed system in the Gulu region of northern Uganda for its vulnerability to the spread of emerging epidemics and its utility for disseminating improved varieties. Seed transaction data were collected by surveying vine sellers weekly during the 2014 growing season. We combined data from these observed seed transactions with estimated dispersal risk based on village-to-village proximity to create a multilayer network or “supranetwork.” Both the inverse power law function and negative exponential function, common models for dispersal kernels, were evaluated in a sensitivity analysis/uncertainty quantification across a range of parameters chosen to represent spread based on proximity in the landscape. In a set of simulation experiments, we modeled the introduction of a novel pathogen and evaluated the influence of spread parameters on the selection of villages for surveillance and management. We found that the starting position in the network was critical for epidemic progress and final epidemic outcomes, largely driven by node out-degree. The efficacy of node centrality measures was evaluated for utility in identifying villages in the network to manage and limit disease spread. Node degree often performed as well as other, more complicated centrality measures for the networks where village-to-village spread was modeled by the inverse power law, whereas betweenness centrality was often more effective for negative exponential dispersal. This analysis framework can be applied to provide recommendations for a wide variety of seed systems.[Formula: see text] Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

scholarly journals The FLASH effect depends on oxygen concentration

2020 ◽  
Vol 93 (1106) ◽  
pp. 20190702 ◽  
Author(s):  
Gabriel Adrian ◽  
Elise Konradsson ◽  
Michael Lempart ◽  
Sven Bäck ◽  
Crister Ceberg ◽  
...  
Keyword(s):  
Oxygen Concentration ◽  
Underlying Mechanism ◽  
Oxygen Depletion ◽  
Therapeutic Window ◽  
High Dose ◽  
Dose Rates ◽  
Conventional Dose ◽  
Oxygen Concentrations

Objective: Recent in vivo results have shown prominent tissue sparing effect of radiotherapy with ultra-high dose rates (FLASH) compared to conventional dose rates (CONV). Oxygen depletion has been proposed as the underlying mechanism, but in vitro data to support this have been lacking. The aim of the current study was to compare FLASH to CONV irradiation under different oxygen concentrations in vitro. Methods: Prostate cancer cells were irradiated at different oxygen concentrations (relative partial pressure ranging between 1.6 and 20%) with a 10 MeV electron beam at a dose rate of either 600 Gy/s (FLASH) or 14 Gy/min (CONV), using a modified clinical linear accelerator. We evaluated the surviving fraction of cells using clonogenic assays after irradiation with doses ranging from 0 to 25 Gy. Results: Under normoxic conditions, no differences between FLASH and CONV irradiation were found. For hypoxic cells (1.6%), the radiation response was similar up to a dose of about 5–10 Gy, above which increased survival was shown for FLASH compared to CONV irradiation. The increased survival was shown to be significant at 18 Gy, and the effect was shown to depend on oxygen concentration. Conclusion: The in vitro FLASH effect depends on oxygen concentration. Further studies to characterize and optimize the use of FLASH in order to widen the therapeutic window are indicated. Advances in knowledge: This paper shows in vitro evidence for the role of oxygen concentration underlying the difference between FLASH and CONV irradiation.

scholarly journals Measuring the Destination Accessibility of Cycling Transfer Trips in Metro Station Areas: A Big Data Approach

2019 ◽  
Vol 16 (15) ◽  
pp. 2641 ◽  
Author(s):  
Xueying Wu ◽  
Yi Lu ◽  
Yaoyu Lin ◽  
Yiyang Yang
Keyword(s):  
Positive Association ◽  
Distance Decay ◽  
Decay Function ◽  
Metro Station ◽  
Negative Exponential Function ◽  
Transfer Mode ◽  
Last Mile ◽  
Trip Distance ◽  
Using Data ◽  
Negative Exponential

Cycling is a green, sustainable, and healthy choice for transportation that has been widely advocated worldwide in recent years. It can also encourage the use of public transit by solving the “last-mile” issue, because transit passengers can cycle to and from transit stations to achieve a combination of speed and flexibility. Cycling as a transfer mode has been shown to be affected by various built environment characteristics, such as the urban density, land-use mix, and destination accessibility, that is, the ease with which cyclists can reach their destinations. However, cycling destination accessibility is loosely defined in the literature and the methods of assessing cycling accessibility is often assumed to be equivalent to walking accessibility using the same decay curves, such as the negative exponential function, which ignores the competitive relationship between cycling and walking within a short distance range around transit stations. In this study, we aim to fill the above gap by measuring the cycling destination accessibility of metro station areas using data from more than three million bicycle-metro transfer trips from a dockless bicycle-sharing program in Shenzhen, China. We found that the frequency of bicycle-metro trips has a positive association with a trip distance of 500 m or less and a negative association with a trip distance beyond 500 m. A new cycling accessibility metric with a lognormal distribution decay curve was developed by considering the distance decay characteristics and cycling’s competition with walking. The new accessibility model outperformed the traditional model with an exponential decay function, or that without a distance decay function, in predicting the frequency of bicycle-metro trips. Hence, to promote bicycle-metro integration, urban planners and government agencies should carefully consider the destination accessibility of metro station areas.

scholarly journals Response to fertilizer nitrogen and water of post-rainy season sorghum on a Vertisol. 2. Biomass and water extraction

1998 ◽  
Vol 131 (4) ◽  
pp. 429-438 ◽  
Author(s):  
PIARA SINGH ◽  
J. L. MONTEITH ◽  
K. K. LEE ◽  
T. J. REGO ◽  
S. P. WANI
Keyword(s):  
Water Content ◽  
Exponential Function ◽  
Root Length ◽  
Dry Matter ◽  
Root Length Density ◽  
Vapour Pressure Deficit ◽  
Negative Exponential Function ◽  
Neutron Probe ◽  
Negative Exponential ◽  
N Rates

During rainless weather following a monsoon, sorghum (Sorghum bicolor cv. SPH–280) was grown on a Vertisol either unirrigated throughout growth or irrigated for 7 weeks after emergence and rainfed thereafter. Before sowing, ammonium sulphate was applied at six rates from 0 to 150 kg/ha N. Roots were sampled every 2 weeks to determine biomass and root length density as a function of depth. Every week, soil water content in all treatments was measured gravimetrically to a depth of 0·23 m and with a neutron probe from 0·3 to 1·5 m.Below 0·45 m, volumetric water content was a negative exponential function of time after roots arrived and the maximum depth of extraction moved downwards at 2–5 cm per day. In the dry treatment, the extraction ‘front’ lagged behind the deepest roots by c. 12 days initially but the two fronts eventually converged. Irrigation delayed the descent of the extraction front by c. 20 days but thereafter it appeared to descend faster than without irrigation. Averaged over N rates, the time constant of the exponential function was inversely related to the root length density, lv, decreasing with depth from about 20 to 10 days as lv increased from 2·5 to 4·0 km/m3.The biomass[ratio ]water ratio was almost independent of N but increased from a mean of 5·3 g dry matter per kg water in the dry treatments to 6·9 g/kg with irrigation. When normalized by the seasonal mean difference in vapour pressure deficit within irrigated and unirrigated plots, the ratios were 13·1 and 13·3 kPa g per kg water, respectively.

Decomposition rate constants of Picea abies logs in southeastern Norway

1999 ◽  
Vol 29 (3) ◽  
pp. 372-381 ◽  
Author(s):  
Erik Næsset
Keyword(s):  
Picea Abies ◽  
Rate Constant ◽  
Cross Sections ◽  
Decomposition Rate ◽  
Rate Constants ◽  
Initial Density ◽  
Decomposition Rate Constant ◽  
Negative Exponential Function ◽  
Standing Trees ◽  
Negative Exponential

Decomposition rate constants were estimated from 384 cross sections of Norway spruce (Picea abies (L.) Karst.) logs with base diameter >7.0 cm collected in open areas at five different study sites in southeastern Norway. Fresh wood core samples were taken from 95 standing trees adjacent to the logs to estimate the initial density of these cross sections. Based on this chronosequence, a simple negative exponential function of time showed an average decomposition rate constant for all cross sections of 0.033 per year. Cross-section diameter, ground contact, soil moisture, and aspect were all found to have significant impacts on the decomposition rate constant. For different combinations of these characteristics the decomposition rate constant ranged from a minimum of 0.0165 per year to a maximum of 0.0488 per year.

Waveforms and velocities for non-nearest-neighbour contact distributions

1979 ◽  
Vol 16 (1) ◽  
pp. 1-11 ◽  
Author(s):  
Eric Renshaw
Keyword(s):  
Exponential Function ◽  
The Other ◽  
Nearest Neighbour ◽  
Negative Exponential Function ◽  
Specific Contact ◽  
Negative Exponential

This paper examines a model for ecological and epidemiological spread. Expressions are derived for mean waveforms and expectation velocities for two specific contact distributions. Whilst one distribution may be bounded above by a negative exponential function the other may not, and these two situations respectively give rise to finite and infinite asymptotic expectation velocities.

A finite mixture of two Weibull distributions for modeling the diameter distributions of rotated-sigmoid, uneven-aged stands

2001 ◽  
Vol 31 (9) ◽  
pp. 1654-1659 ◽  
Author(s):  
Lianjun Zhang ◽  
Jeffrey H Gove ◽  
Chuangmin Liu ◽  
William B Leak
Keyword(s):  
Model Fitting ◽  
Mixture Distribution ◽  
Finite Mixture ◽  
Diameter Distribution ◽  
Weibull Distributions ◽  
Forest Stands ◽  
Negative Exponential Function ◽  
Diameter Frequency Distribution ◽  
Negative Exponential ◽  
Diameter Distributions

The rotated-sigmoid form is a characteristic of old-growth, uneven-aged forest stands caused by past disturbances such as cutting, fire, disease, and insect attacks. The diameter frequency distribution of the rotated-sigmoid form is bimodal with the second rounded peak in the midsized classes, rather than a smooth, steeply descending, monotonic curve. In this study a finite mixture of two Weibull distributions is used to describe the diameter distributions of the rotated-sigmoid, uneven-aged forest stands. Four example stands are selected to demonstrate model fitting and comparison. Compared with a single Weibull or negative exponential function, the finite finite mixture model is the only one that fits the diameter distributions well and produces root mean square error at least four times smaller than the other two. The results show that the finite mixture distribution is a better alternative method for modeling the diameter distribution of the rotated-sigmoid, uneven-aged forest stands.

scholarly journals Cut-Through Fractured Seepage Properties and Numerical Simulation of Sandstone after Different Temperature Treatments

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-13
Author(s):  
Haopeng Jiang ◽  
Annan Jiang
Keyword(s):  
Fracture Surface ◽  
Shear Failure ◽  
Failure Surface ◽  
Confining Pressure ◽  
Theoretical Formula ◽  
Negative Exponential Function ◽  
Damage Degree ◽  
Different Temperatures ◽  
Negative Exponential ◽  
Fractured Sandstone

To explore the seepage characteristics of cut-through fractured rocks after different temperatures, sandstone in the Hunan area was selected as the research object. First, the influence degree of different temperatures on the permeability of fractured sandstone was studied, and the permeability variation of fractured sandstone with net confining pressure was revealed. The test data was nonlinearly fitted to prove that the relationship between permeability and net confining pressure conforms to the characteristics of the negative exponential function. Second, the macroscopic fractured state of sandstone after different temperature treatments was analyzed, and it is concluded that the inclination angle of the fracture surface decreases with the applied thermal temperature, the fracture surface gradually develops into a single shear failure surface, and the damage degree becomes more and more serious. Finally, the theoretical formula for the calculation of fractured seepage was introduced, and the FLAC3D embedded fish language was used to compile the seepage-stress coupling calculation program of the fractured sandstone after different temperature treatments. Numerical calculations were carried out based on samples with different fracture angles of fractured sandstone, and the calculated values were in good agreement with the test results. The research results can provide guiding significance for the research on the influence of high temperature in fire tunnel on the evolution of permeability of surrounding rock fissures.

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