Yield monitors

Early measurements of " Th were on seawater samples and Th was co-precipitated from 20-30 L of seawater with iron hydroxide (Bhat et al. 1969). This procedure may not recover all of the " Th in the sample, and an alpha emitting Th isotope (e g., °Th or Th) is added as a yield monitor. Following chemical purification of the Th fraction by ion exchange chromatography, the Th is electrodeposited onto platinum or stainless steel planchets. The planchets are then counted in a low background gas-flow beta detector to measure the beta activity and subsequently with a silicon surface barrier detector to determine the alpha activity of the yield monitor. The " Th activity is thus determined as ... 

Measurements of " Th in sediment samples (Aller and Cochran 1976 Cochran and Aller 1979) used much the same approach as outlined above. In this case, the dried sediment sample ( 10 g) was leached with strong mineral acid (HCl) in the presence of a yield monitor (generally Th, an artificial Th isotope resulting from the decay of Th that is produced by neutron capture on Th). Thorium was separated from U and purified by ion exchange chromatography, and electrodeposited onto stainless steel planchets. Counting and determination of " Th activity followed the procedure outlined above. 

Carbon turnover in production fields can be determined, using non-isotopic techniques, by combining historical soil samples, current soil samples, and whole field yield monitor data. Sensitivity analysis of such data shows that the amount of above-ground biomass that could be harvested decreases with root to shoot ratio (Table 8.1). For example, if root biomass is ignored, analysis suggests that only 20-30% of the above-ground biomass can be harvested, whereas if the root to shoot ratio is 1.0, then between 40% and 70% of the residue could be harvested. 

When establishing test pressures for a test section, the maximum test pressure shall be determined by the operator to prevent damage to the pipeline and its components. Consideration must be given to the effect of test section elevation differences on the test pressure. Whenever test pressure will cause a hoop stress in excess of 100% of the specified minimum yield strength (SMYS), refer to Nonmandatory Appendix C for guidance on yield monitoring. The minimum test pressure shall be as required by (a) through (c) below. 

Diethyl sodiomalonate is an example of type (i). Reaction with [(fluorobenzene)Cr(CO)3] proceeds to completion after 20 h at 50 C in HMPA to give the diethyl phenylmalonate complex in over 95% yield. Monitoring the reaction by NMR gave no evidence for an intermediate (e.g. the cyclohexadienyl anion complex) interruption of the reaction by addition of iodine at less than 20 h gave significant amounts of unreacted fluorobenzene. A satisfactory picture is the simple one, that the anion adds reversibly and unfavorably (k < k-i, as in Scheme 3), slowly finding itself at the ipso position then irreversible loss of fluoride gives the substitution product.5152... 

Oscillation was not essentially observed in the oxygen yields when a flash sequence was preceeded by weak continuous illumination [188,190]. This result has been taken as evidence that the steady-state concentrations of the S states are very similar, and consequently the quantum yields of the photosteps must be equal. If this conclusion is correct, one quantum is involved in each transition, since in nonsaturating flashing light the Oj yield, monitoring the final transition 84 80, depends linearly upon the flash intensity [189]. 

A full account of the problems considered in collecting, storing, and processing marine samples for transuranic analysis is given in the above-mentioned review (4). The specific methods discussed here were foimd effective at least for the transuranic analyses of seawater and sediments contaminated by global fallout, nuclear fuel reprocessing wastes, or nuclear power plant operation waste. In these cases, a preliminary acid treatment of the sample in the presence of suitable yield monitors seems to solubilize the transuranic elements and achieves isotopic equilibration between the yield monitor and sample. The yield monitors used were either Pu or sep qj. 238,239,240,24ip whereas Am was used for Am, 2 Cm, and by inference, Cf. In addition, it was convenient to use 50 mg of a lanthanide (neodymium) as a carrier for americium to purify the separated americium fraction. 

Seawater. Plutonium and americium analyses were made using 55-1. seawater samples. The seawater sample is acidified to 0.03M with respect to hydrochloric acid, and yield monitors and carriers were added. Trans-... 

The eflBciency of these methods of co-precipitation was studied for plutonium, only, by comparing the chemical recovery of the original yield monitor with that of a second (and different) plutonium isotope yield monitor added to the acid solution of the co-precipitated hydroxide or oxalate (6). This second monitor shows all losses following the coprecipitation step. The eflBciency of the hydroxide precipitation for plutonium extraction was in the range 70-80%. That for the oxalate coprecipitation was typically 75-85%. 

These drawbacks are overcome when the standard procedure is developed in the presence of a metallation agent in excess. Thus, when the reaction between dialkyl alkylphosphonates and carboxylic acid esters is allowed to proceed in the presence of LDA in excess (2 eq) in THE at low temperature, the resulting lithium phosphonoenolate is obtained in quantitative yield. Monitoring of the reaction by P-NMR shows the appearance of one signal from the lithium phosphonoenolate... 

RYDMR offers a sensitive method to trace the EPR-spectrum of the paramagnetic RP (P+H ), which escapes from an observation by time-resolved EPR techniques due to its short lifetime in the nanosecond range. The effect of microwaves applied to the sample under resonance conditions is detected in a change of the triplet (or singlet) product yield monitored in time-resolved absorption spectroscopy [7,8]. By varying the static magnetic... 

The extent to which a digestion procedure brings the uranium into solution cannot be assessed through spiked experiments or addition of a yield monitor, as the spike is not representative of the actual sample. However, there are standard procedures published by the American Society for Testing and Materials (ASTM) and the Environmental Protection Agency (EPA) of the USA. Certified reference materials are also available from the International Atomic Energy Agency/NIST. 

A yield monitor is an isotope that is added to a sample prior to radiochemical separation and analyzed... 

The yield monitor must be measurable without spectral overlap with the uranium in the sample, and so artificial isotopes are used. These are prepared with minimal contamination of natural or environmentally relevant uranium isotopes. Uranium-232 is a suitable candidate for radiochemical preparation for analysis by alpha spectrometry as it has a suitable half-life and an alpha energy that does not interfere with the peaks from the other isotopes. Uranium-233 is preferred for mass spectrometric techniques because it has a long half-life and has few spectral interferences, whereas U is isobaric with Th. Both these yield monitors also function as internal standards for the measurement technique, which lowers the overall uncertainty associated with yield determination and measurement. 

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