Emanating power

The emanating power or coefficient of rocks or soil is defined as the proportion of the radon activity per unit bulk volume which is in the interstitial gas. The emanating coefficients of rocks and soils vary greatly. Barretto et al. (1972) found values ranging from 0.01 to 0.26 for... 

On the emanating power and the measurement of geologic time. Geol. Foren. i. Stockholm Forh. 64, 465 (1942). 

Emanation techniques are based on the production of radioactive noble gases by decay of mother nuclides or by nuclear reactions. The emanating power has been defined by Hahn as the fraction of radioactive noble gas escaping from a solid relative to the amount produced in the solid. It depends on the composition of Ihe solid, ils lallice structure and its spccihc smTace area. Reactions in the solid have a major inlluence. Further factors affecting the emanating power are the half-life of the noble gas radionuclide, its recoil energy and the temperature. 

In Fig. 18.6 various possibilities of produeing radioactive noble gases, suitable for preparation of emanating sourees and investigation of emanating power, are listed. The isotopes of radon are applied most frequently. They are all produeed by decay of radium isotopes, whieh are formed by deeay of thorium isotopes. Therefore, either the radium isotopes or the thorium isotopes may be incorporated into the solid... 

Samples exhibiting high emanating power (70 to 100%) are prepared for application as emanating sources. Examples are Th or Ra coprecipitated with thorium hydroxide. Rn given off by these emanating sources may be used for chemical or physical investigations with radon. Formerly these sources have also been prepared for application of Rn in medicine. 

Measurement of the emanating power allows the investigation of transformation, decomposition or other reactions in solids, or of ageing processes in precipitates. In Fig. 18.7 the emanating power of CaCOs is plotted as a function of the temperature. The transformation of aragonite into calcite at 530 °C and the decomposition of CaCOa into CaO and CO2 at 920 °C are clearly discernible. Near 1200 °C the high mobility of the ions in CaO becomes noticeable it indicates the sintering of CaO. 

Figure 18.7. Emanating power of calcite and aragonite as a function of temperature. (According to K. E. Zimens, Z. Phys. Chem., B37, 231 (1937).)...

This reaction also runs parallel to the emanating power. However, quantitative evaluation of the emanating power with respect to the reactions involved is not simple, because of the complexity of the processes. 

The ageing of thorium hydroxide and iron hydroxide in the presence of water is obvious from Fig. 18.8. Whereas the emanating power of thorium hydroxide decreases very slowly, that of iron hydroxide falls olf relatively fast, indicating faster ageing. Emanation techniques have also been applied to obtain information about surface areas and densities of porous substances. 

If one makes the simplifying assumptions that transport of Rn is by molecular diffusion alone and ffiat soil has uniform porosity, Rn concentration, and emanating power, then the fraction of Rn that escapes to the atmosphere as a function of depth, Lf z), is given by (e.g., Clements and Wilkening, 1974)... 

Figure 1 Depth profile of radon loss fraction. Filled circles and error bars represent the mean and standard error of the radon loss fraction from all samples from the indicated depth for which the Cs/ Ra ratio is less than 0.05. The boxes represent 1 a deviation and the whiskers the extreme values of each population. The number of samples is given in the column on the right. The curve corresponds to Equation (1) with an emanating power of 0.28 and...

Emanating power is defined as the fraction of radioactive gas atoms which escapes and is measured by the ratio of rate of escape to rate of formation. 

Hahn (1931) applied the method to studies of the ageing of precipitates. The emanating power of freshly formed, dry iron(III) hydroxide is high and falls by only about 2% a year. Digestion in water quickly reduces the emanating power, indicating a rapid ageing. 

Structural changes in a solid can be detected by changes in emanating power. For instance, that of anhydrous BaCl2 increases with temperature to 540° and then falls a little, but again begins to rise and reaches a maximum at 925°. The first maximum is associated with the change from a porous form to the monoclinic a-form, the second with one from the monoclinic a-form to the cubic /3-form (Lieber, 1939). 

Emanation power, emanation efficiency, coefficient of emanation, percent emanation, and escape-to-production ratio all mean the same thing, namely the fraction of Rn atoms formed in a solid that escape from the solid. 

It is obvious from the data of Tables ll-VIII to I l-XII that the emanation power varies widely. As a rule, the finer the particle size the greater the emanation power. However, when samples are severely weathered or damaged, as in the case of radioactive ores, further comminution of a specimen has little effect on the emanation power. In both cases it is believed that Ra has migrated from the original site of its parent in the crystal lattice and has adsorbed on the surfaces of microfractures facilitating Rn escape. When certain samples are pulverised in the laboratory they may even emit less Rn than the whole lump. This can happen when U and its decay products are associated with minerals that do not pulverise as easily, i.e., are less friable. Starik and Melikova (1957) found that a whole lump of ore had an emanation power of 46%... 

Emanation power (%) of uranium ores in air at ambient temperature... 

It is somewhat surprising that so few Rn emanation power studies have been carried out on soils. The study by Delwiche (1958) on Rn release from "Great World Soil Groups" gives relative emanation powers for all but one soil. His results show clearly that the more highly weathered soils emanate more Rn and that the clay intervals emanate more than the silty and sandy intervals. Red podzolic, lateritic podzolic and solodic soils emanate the most and desert soils and western brown forest soils the least. Quantified emanation powers for soils and stream sediments are shown in Table 11-XII. 

Barretto et al. (1972) found that the emanation powers of 19 samples of various soil types were in the range 10-56%. The results of Delwiche (1958) and Bonotto and Andrews (1999) on other soil types lie at the lower end of this range. Komer and Rose (1977) report an average emanation efficiency of 42% for five stream-sediment samples. 

The following general conclusions about the Rn emanation power of natural... 

Increasing temperature and moisture content increases the emanation power of soils and rocks but may decrease the Rn content of pore gas under certain conditions. 

A clear direct dependence of Rn emanation rate, but not of emanation power, on U concentration is evident. 

The measurable range of emanation power for any one type of material is great. 

Starik, I.E. and Melikova, O.S., 1957. Emanating power of minerals. AEC-TR-4498 (translated from Trans. V.G. Khlopin Radium Inst, 5 206-226). 

An ETA curve is a plot of emanating power E as a fimction of time and the F/time relationship has been developed and defined by Balek and Tolgyseey. A major difficulty of ETA is that preparation and handling of samples require sophisticated radiochemical facilities coupled with all the associated precautions. However, the net amounts of radioactive gas incorporated into samples are so small that the evolved gas, after dilution with the carrier, does not pose a significant hazard. 

In this equation D (m2.s 1) represents the radon diffusivity, X the radioactive decay constant (s 1), C (Bq.m3) the radon concentration in the pore space, R (Bq.kg1) the radium concentration in the material, p (kg.m3) the bulk density of the dry material, E (dimensionless) the radon emanation power coefficient for the pore spaces, s (dimensionless) the total porosity and 0 (dimensionless) the moisture. The solution of the diffusion equation for an homogeneous medium represents the flux release from the waste material to the surface, Jt (Bq.m 2.s ). For a system without cover we obtain (Rogers, 1984) ... 

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