Surface radioactivity measurements

The radioactivity measurements of BaCOa allowed us to calculate, according to the law of the isotope dilution, the amount of ethyl groups, fixed on the a-titanium trichloride surface. 

Apparatus and Procedure. Surface Isotherms. The technique for determining the n-A and AV-A curves of the lipid films has been described (6). Briefly, the Wilhelmy plate method was used to measure surface tension, from which the surface pressure was calculated (n = 7h2o—yfiim) The surface potential was measured by means of a radioactive (226Ra) air electrode and a saturated calomel electrode connected to a high impedance model 610 B Keithley electrometer (Keithley Instruments, Cleveland, Ohio). 

Several features of this experiment (5 — 7) as a radiotracer method are as follows (1) since the relative adsorbability has been determined from the ratio of radiocounts of dried samples, it is not affected by errors in the absolute values for radioactivity and (2) specific activity, which has to be large usually for the study of surface phenomena, may be very small and yet the radioactive measurements yield accurate results. The concentrations of solutions examined were all close to but less than the c.m.c. values of respective surfactant solutions. The foams were stable enough to carry the experiment in this concentration range and the mole ratio of counterions in the bulk was equal to the stoichiometric mole ratio —i.e., there was no shift in mole ratio of dispersed counterions owing to the micelle formation at the present study. 

By measuring surface radioactivity of DPL films spread onto aqueous solutions containing the radiotracers 45Ca++ and 36C1", we have established... 

In the present work we studied the interaction of prothrombin with lipid monolayers in the presence of Ca++. The stoichiometric relations of the interacting components were determined by measuring surface radioactivity of 3H-labelled prothrombin and 45Ca. The penetration was inferred from the effect of the interaction on the capacitance of the surface layer. 

Surface Composition. The surface composition after spreading a known amount of PS as to obtain a condensed monolayer ( 0.36 fig/cm2) was obtained by measuring surface radioactivity of prothrombin or Ca++ injected to different concentrations underneath the lipid monolayer. The... 

There are cases, however, including the very common one of an air-water surface, where no ions can possibly pass the boundary thermodynamical equilibrium cannot therefore be set up between the water and air, and adsorption potentials (the surface potentials of Chapters II and III) are permanent. The usual method for measuring surface potentials with a radioactive air-electrode does not appreciably disturb the adsorption potentials the gaseous ions are very few and are attracted into the water by image forces so that no double layer, compensating the double layer in the water due to the dipoles of the molecules in the surface film, can build up in the air. 

Biogenic PFe actually consists of two distinct pools, intracellular or biological Fe and surface-adsorbed or scavenged Fe (Flutchins, 1995). The latter has usually not been considered to be a true component of the cellular Fe quota, and a titanium (Ti) wash method (Hudson and Morel, 1989) has therefore been commonly employed to remove surface-bound Fe in radiotracer studies (Eldridge et al., 2004 Hutchins et al., 1999 Poorvin et al., 2004 Sunda and Huntsman, 1997 Sunda et al., 1991). However, the titanium reagent is itself heavily contaminated with Fe and so its use is questionable for non-radioactive measurements. 

A summary of airborne radioactivity measurements in Monthldry, France is presented in the report by Milles-Lacroix et al. (1994). Radioactive aerosols particles are collected in surface air, using cellulose filters and high output pumps (110 m%). Equipment operates on a permanent, 24-hour a day basis, and filters are replaced daily throughout the year. The volume of filtered air is 2600 m per day. After removal, individual filters are subjected to alpha counting, using a ZnS counter, and to beta... 

Bourlat (1991) reported some results of radioactivity measurements of Sr, Cs and in Polynesian oceanic waters (at the surface and at different depths) and in... 

Figure 13.1 illustrates possible on-site inspection activities, including sampling, search for surface disturbances and underground voids, radioactivity measurements, seismic listening for residual cavity collapse activity and, in the extreme case, drilling down into a suspected test cavity. Inspections will serve as an important supplement to the international data networks. 

The present work investigates the action of pancreatic lipase on spread monolayers of ethylene glycol adipate oligomers. The tritiated hydrolysis products of the uniformly labeled substrate escape into the subphase. Surface radioactivity measurements allow us to determine the substrate concentration at the interface (2, 3) and the extent of the enzymic reaction. The kinetics were investigated as a function of subphase pH and temperature and substrate length and concentration. By... 

Electrocapillary methods, described in Sections 13.2 and 13.3, are very useful in the determination of relative surface excesses of specifically adsorbed species on mercury. As discussed in Section 13.4, such methods are less straightforward with solid electrodes. For electroactive species and products of electrode reactions, the faradaic response can frequently be used to determine the amount of adsorbed species (Section 14.3). Nonelectro-chemical methods can also be applied to both electroactive and electroinactive species. For example, the change in concentration of an adsorbable solution species after immersion of a large-area electrode and application of different potentials can be monitored by a sensitive analytical technique (e.g., spectrophotometry, fluorimetry, chemiluminescence) that can provide a direct measurement of the amount of substance that has left the bulk solution upon adsorption (7, 44). Radioactive tracers can be employed to determine the change in adsorbate concentration in solution (45). Radioactivity measurements can also be applied to electrodes removed from the solution, with suitable corrections applied for bulk solution still wetting the electrode (45). A general problem with such direct methods is the sensitivity and precision required for accurate determinations, since the bulk concentration changes caused by adsorption are usually rather small (see Problem 13.7). 

For measurement purposes, use can be made of all radioactive measurement facilities with suitable detectors, for example a Geiger counter, preferably in the form of a large-surface counter or proportional gas flow counter. 

Testing of corrosion resistance of materials is most commonly done with the use of kryptonates, i.e., solids into the surfaces of which Kr has been incorporated by ion bombardment. Kryptonates of test materials are exposed to corrosive atmosphere and the residual P activity of Kr is measured. Surfaces labeled with a radioactive tracer by isotope exchange, electrodeposition, or vacuum deposition are also employed. 

During any release of radioactive materials, surfaces of objects or equipment may become contaminated. The objective of these measurements is to identify those objects that should be decontaminated, disposed of as waste, or treated in some other controlled fashion. The physical quantity that is generally measured is surface activity. Measurements may be radionuclide-specific or can also be assessed based on dose-rate measurements. The results are generally expressed in Bq/m or Sv/h or equivalent, respectively. 

Let us briefly review which of the listed values and in what manner can be determined. It is possible to calculate the distribution of complex species in the solution, that is, bulk concentrations or activities, by means of the methods discussed in Chapter 1. Besides, the composition of solutions can be changed so that Eqs. (7.8)-(7.10) would become simpler. For example, one can use a series of solutions with constant concentration of free ligand for which i In l = 0- In the series of isopotential solutions (see Section 2.1), the condition dlna = 0 holds. By differentiation of the electrocapillary curve with respect to E for a solution of constant composition, one can obtain the value of the charge density e and use Eq. (7.6) for the calculation of total adsorptions Fj x nd Flx. Thus, electrocapillary measurements, as well as a number of other methods (radioactive indicator, surface stress, piezoelectric, extensometer methods), give information only about the total amounts of adsorbed metal and ligand. Consequently, for further solution of the problem posed, some model images are necessary. 

The casters also mentioned a number of what they considered to be longterm safety risks radiation which might be emitted from a radioactive measuring device, the infrared rays which develop in molten steel and which can affect the eyes, the dust resulting from the use of a powder to cover the surface of the steel in the mold and the draught caused by the ventilator. 

The most widely used experimental method for determining surface excess quantities at the liquid-vapor interface makes use of radioactive tracers. The solute to be studied is labeled with a radioisotope that emits weak beta radiation, such as H, C, or One places a detector close to the surface of the solution and measures the intensity of beta radiation. Since the penetration range of such beta emitters is small (a ut 30 mg/cm for C, with most of the adsorption occurring in the first two-tenths of the range), the measured radioactivity corresponds to the surface region plus only a thin layer of solution (about 0.06 mm for C and even less for H). 

Surface Area. Overall catalyst surface area can be determined by the BET method mentioned eadier, but mote specific techniques are requited to determine a catalyst s active surface area. X-ray diffraction techniques can give data from which the average particle si2e and hence the active surface area may be calculated. Or, it may be necessary to find an appropriate gas or Hquid that will adsorb only on the active surface and to measure the extent of adsorption under controUed conditions. In some cases, it maybe possible to measure the products of reaction between a reactive adsorbent and the active site. Radioactively tagged materials are frequentiy usehil in this appHcation. Once a correlation has been estabHshed between either total or active surface area and catalyst performance (particulady activity), it may be possible to use the less costiy method for quaHty assurance purposes. 

Until the advent of modem physical methods for surface studies and computer control of experiments, our knowledge of electrode processes was derived mostly from electrochemical measurements (Chapter 12). By clever use of these measurements, together with electrocapillary studies, it was possible to derive considerable information on processes in the inner Helmholtz plane. Other important tools were the use of radioactive isotopes to study adsorption processes and the derivation of mechanisms for hydrogen evolution from isotope separation factors. Early on, extensive use was made of optical microscopy and X-ray diffraction (XRD) in the study of electrocrystallization of metals. In the past 30 years enormous progress has been made in the development and application of new physical methods for study of electrode processes at the molecular and atomic level. 

The role of radionuclides as tracer of the chemical transport in river is also reinforced by the fact that each of the U-Th-Ra elements has several isotopes of very different half-lives belonging to the U-Th radioactive series. Thus, these series permit comparison of the behavior of isotopes of the same element which are supposed to have the same chemical properties, but very different lifetimes. These comparisons should be very helpful in constraining time scales of transport in rivers. This was illustrated by Porcelli et al. (2001) who compared ( " Th/ U) and ( °Th/ U) ratios in Kalix river waters and estimated a transit time for Th of 15 10 days in this watershed. The development of such studies in the future should lead to an important progress in understanding and quantifying of transport parameters in surface waters. This information could be crucial for a correct use of U-series radioactive disequilibria measured in river waters to establish weathering budgets at the scale of a watershed. 

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