Field loss

A ring specimen is cut from the tube. Two coils are wrapped around this ring, one exciter coil and one receiver coil. The exciter coil N1 should cover the entire ring so that there are no field losses when ring is saturated. The sinusoidal exciter current which can be measured at... 

The interaction of microwaves with ferrites (qv) has many complicating features. Low field loss mechanism (41), nonlinear effects, and losses at high power levels (41,43) as well as dielectric losses are among these. 

Although IOP is a poor predictor of which patients will have visual field loss, the risk of visual field loss increases with increasing IOP. IOP is not constant it changes with pulse, blood pressure, forced expiration or coughing, neck compression, and posture. 

Open-angle glaucoma is slowly progressive and is usually asymptomatic until the onset of substantial visual field loss. Central visual acuity is maintained, even in late stages. 

LOCKYER, D.R. (1984). A system for the measurement of field losses of ammonia through volatilisation. Journal of the Science of Food and Agriculture 35, 837-848. 

Organic Sensory neuropathy (dysarthia, paresthesia, constriction of visual field, loss of taste, hearing, smell), motor dysfunction (tremors)... 

Vigabatrin Irreversibly inhibits GABA-transaminase 70% bioavailable not bound to plasma proteins not metabolized, ti/2 5-7 h (not relevant because of mechanism of action) Partial seizures, infantile spasms Toxicity Drowsiness, dizziness, psychosis, visual field loss Interactions Minimal... 

Field-loss rates have also been measured after dry deposition of particulate activity. Witherspoon Taylor (1970, 1971) and Peters Witherspoon (1972) used silicaceous particles, labelled with 134Cs, in three size ranges, 1-44 fim, 44-88 nm and 88-175 fim. Millard et al. (1983) used submicrometre particles labelled with 141Ce or 134Cs. All reported field loss as showing an initial rapid phase lasting a few days, followed by one or more slow phases. In the period from a few days to about five weeks after application of the activity, the values of XG were in the range 0.02-0.06 d-1, with no obvious correlation with particle size, or with the incidence of rainfall. 

Table 2.16. Field loss of nuclides from herbage and cereals... 

Pinder etal. (1985) deduced NSA values in the range 20-41 m2d kg-1 from measurements of 238Pu in corn near the Savannah River Plant. Similar values also apply to non-radioactive trace elements. Allcroft et al. (1965) made a series of measurements of fluorine in herbage on a farm near Stoke-on-Trent and also recorded fallout of fluorine in a nearby deposit gauge. During the period of study (1956-61) the productivity of the pasture was improved, and the yearly average NSA declined from 59 to 27 m2d kg-1 (Chamberlain, 1970). When productivity is low, the time T0 before maturity is increased, and so is the NSA (Table 2.17). Also XG may be increased, since the rate of field loss seems to be related to growth rate. 

The slope of the curves in Fig. 2.11, from day 10 onwards, corresponds to a field-loss rate of 0.058 d-1 (compare Table 2.16). Assuming this continues indefinitely, the integrals under the curves are 0.27 m2d l-1 for 90Sr and 2.2 m2d l-1 for 137Cs, values of km which are compared in Table 2.19 with those calculated previously. 

Chadwick, R.C. Chamberlain, A.C. (1970) Field loss of radionuclides from grass. Atmospheric Environment, 4, 51-6. 

Iodine is lost from herbage by the same processes which cause field loss of Sr, 137Cs and other nuclides (Section 2.13). There is also the possibility of revolatilisation of iodine. If XG is the rate constant of field loss (fraction of iodine per unit area of ground lost from vegetation per second) and X1 the rate constant of radioactive decay, the combined apparent or effective loss rate is XE = XG + Xt. The effective half-life is Te = 0.693/A . The use of the term half-life implies that field loss is exponential and TE invariant with time, which is not always true. 

Fig. 7-2. Summary of environmental pathways by which terrestrial plants may become contaminated with radionuclides. In the case of an input from atmosphere, or as a result of the process of resuspension , any external radionuclide burden may be reduced by field loss mechanisms conversely, an initially external radionuclide deposit (Rat) may become internalised (i int) following foliar absorption and translocation. Radioactive contaminants of soils may be derived either from atmospheric inputs or from seepage in ground waters. Partitioning of radionuclides in soil—soil water systems controls their availability for root absorption, which normally occurs exclusively from the liquid phase. The chemical speciation of the nuclide in this phase, however, provides a further control on bioavailability which is highly radionuclide specific.

After initial contamination, the aerial parts of plants lose radioactivity by a process known as field loss . The mechanisms by which this takes place are not fully understood but may involve the following ... 

Fig. 7-10. Loss curves for 85Sr sprayed onto grass swards during (a) winter and (b) summer. The field loss half lives during these periods were 19 and 49 days, respectively (reproduced from Chadwick and Chamberlain, 1970).

The diagnosis of open-angle glaucoma is confirmed by the presence of characteristic optic disk changes and visual field loss, with or without increased lOP. Normal tension glaucoma refers to disk changes, visual field loss, and lOP of less than 21 mm Hg. Ocular hypertension refers to lOP of more than 21 mm Hg without disk changes or visual field loss. 

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