Spectrometry, alpha

FIGURE 1.28 (a) Schematic representation of alpha particle emission and (b) alpha spectrum of uranium (with a spike) after separation from other actinides in a soil sample (also 

A typical alpha particle spectrum of natural uranium and U) spiked 

Several types of detectors for alpha particles are available, but the solid state ion implanted silicon semiconductors are most commonly used. As the counting time required for measurement of samples with low activity could be quite long (several days) and the cost of an analytical system is not very high, many laboratories operate several alpha spectrometers in parallel. 

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Uranium and thorium are the first members of natural radioactive chain which makes their determination in natural materials interesting from geochemical and radioecological aspect. They are quantitatively determined as elements by spectrophotometric method and/or their radioisotopes by alpha spectrometry. It is necessary to develop inexpensive, rapid and sensitive methods for the routine researches because of continuous monitoring of the radioactivity level. 

Thorinm-232 is the only non-radiogenic thorium isotope of the U/Th decay series. Thorinm-232 enters the ocean by continental weathering and is mostly in the particulate form. Early measurements of Th were by alpha-spectrometry and required large volume samples ca. 1000 T). Not only did this make sample collection difficult, but the signal-to-noise ratio was often low and uncertain. With the development of a neutron activation analysis " and amass spectrometry method " the quality of the data greatly improved, and the required volume for mass spectrometry was reduced to less than a liter. Surface ocean waters typically have elevated concentrations of dissolved and particulate 17,3 7,62... 

Sources. It is well known that the optimal geometry for source preparation for alpha spectrometry is as an infinitely thin and uniform deposit on a flat surface. In practice. 

Figure 2. Alpha spectrum for a radium adsorbing manganese-oxide thin film exposed to a groundwater sample, after Surbeck (2000) and Eikenberg et al. (2001b). A 2x2 cm sheet is exposed to O.l-l.O L of sample for 2 days, capturing nearly all of the radium in the sample. These sample discs can be used directly for low-level alpha spectrometry without the need for further separation and preparation methods to produce planar sample sources. Energy resolution is nearly as good as for electroplated sources, and detection limits are typically 0.2 mBqA (6 fg Ra/L) for Ra and " Ra for a one-week counting period. These sensitivities are comparable to traditional methods of alpha spectrometry. [Used by permission of Elsevier Science, from Eikenberg et al. (2001), J Environ Radioact, Vol. 54, Fig. 4, p. 117]...

Morvan K, Andres Y, Mokili B, Abbe JC (2001) Determination of radium-226 in aqneons solutions by alpha-spectrometry. Anal Chem 73 4218-4224... 

Figure 12 continued, (d)-(g) Mineral isochrons for rhyolites from the Olkaria volcanic center (Kenya), (d) and (f) alpha spectrometry resnlts from Black et al. (1997). (e) and (g) TIMS resnlts from Henmaim and Davies (2002). All the rhyolites have eraption ages between 3.3 and 9.2 ka. Note that the same sample (570) analyzed in both stndies gives rather different ages (f and g). Same abbreviations of mineral names as in Fignre 10 + Qz qnartz KF alkali feldspar Amph amphibole Bt biotite. 

Table 1 lists MORE studies to date and Figures 2 and 3 present data from different areas of ridge. The first study of U-series disequilibria in MORE was the pioneering work of Condomines et al. (1981) (Fig. 2A). These workers analyzed samples having a relatively wide range in composition (Mg 72 to 57) from the FAMOUS region of the Mid-Atlantic Ridge (MAR 37°N) by combined alpha spectrometry (for U and Th isotopic ratios) and mass spectrometry (isotope dilution measurements for U and Th... 

Other early alpha spectrometry studies examined U-series disequilibria in East Pacific Rise (EPR) samples. Newman et al. (1983) measured samples from the RISE study area (21°N EPR) finding significant variations in Th/U and ( °Th)/( U) (Fig. 2B). Rubin and Macdougall (1988) measured ( °Th)/( U) and ( Ra)/( °Th) for samples from both the northern ( 10-12°isn and southern (-20-25°) EPR (Fig. 2C,D). This study was first to document large disequilibria in young MORB and interpreted the... 

Th, Th and Po, all decay by alpha emission and are thus measurable by isotope dilution and alpha spectrometry (Ivanovich and Murray 1992). However, " Th is produced by the alpha decay of and in turn decays by beta emission to via the short-lived intermediate " Pa (half-life 1.18 m) ... 

Sekine K, Imai T, Kasai A. 1987. Liquid-liquid extraction separation and sequential determination of plutonium and americium in environmental samples by alpha-spectrometry. Talanta 34(6) 567-570. 

Vandrish, G., K. Theriault and F. Ryan, The Pylon 190 Standard a Novel Filter Calibration Standard for Alpha-Spectrometry, in Proceedings International Conference on Occupational Radiation Safety in Mining (H. Stocker, ed) vol. 2, pp. 390-393, Canadian Nuclear Association, Toronto (1984). 

Each serie of measurements consisted of two parallel samples with counting during and after sampling, one with the screen diffusion battery and the second as the reference sample, so that the fractional free radon daughters could be calculated. The radon daughters are collected on a membrane filter (filter diameter 25 mm, pore diameter 1.2 ym) and the decays of Po-218 and Po-214 are counted by means of alpha spectrometry with a surface barrier detector (area 300 mn ). 

Lin [ 1 ] used coprecipitation with lead sulfate to separate 237-actinium from sea water samples. The 237-actinium was purified by extraction with HDEHP, and determined by alpha spectrometry via Si (Au) surface barrier detection. The method has a sensitivity of 10 3 pCig"1 of ashed sample. 

Some of the methods commonly used for the determination of thorium in biological materials are given in Table 6-1. The colorimetric methods are not capable of isotope-specific determination of thorium isotopes. Alpha spectrometric and neutron activation analysis are useful in the quantification of isotope-specific thorium and thorium-232, respectively, and have better sensitivities than colorimetric methods. Alpha spectrometry is the commonly used isotope-specific analysis for the determination of thorium-232 and the thorium-230 derived from the decay of uranium-238 (Wrenn et al. 1981). Standard reference materials (SRMs) containing thorium in human liver (SRM-4352) and human lung (SRM-4351) necessary for the determination of absolute recovery in a given sample are available from the National Institute of Standards and Technology (Inn 1987). 

Actinides were determined at the ultratrace level in moss samples collected from the eastern Italian Alps (1500 m a.s.l.). The frozen samples were cut into 1-2 cm sections and analyzed separately to obtain the distribution curves of the vertical concentrations. For plutonium and americium isotope analysis, 1-2 g of the samples were ashed, leached, separated with respect to analytes and analyzed by alpha spectrometry and LA-ICP-MS after the plutonium or americium had been electroplated on a stainless steel disk.23 Estimated limits of quantification of LA-ICP-MS for actinide radionuclides deposited on stainless steel plates after chemical separation are summarized in Table 9.45. For most of the long-lived radionuclides in moss samples, lower limits of determination were found at the 10 15gg 1 concentration level compared to those of a - spectrometry 23... 

Field Studies. We have attempted to compare the relative availability of actinides to small mammals living in contaminated environments near ORNL. Shrews, rats and mice have been collected from a 30 year old contaminated floodplain forest ecosystem ( ). Cotton rats (Sigmodon hispidus) have been collected from the banks of a former liquid radioactive waste pond which contains Pu, Am and Cu in sediments and shoreline vegetation. Analyses were performed by isotope dilution mass spectrometry (U, Th and Pu) or by alpha spectrometry (Pu, Am and Cm). 

Donivan S, Hollenbach M, Costello M. 1987. Rapid determination of thorium-230 in mill tailings by alpha spectrometry. Anal Chem 59 2256-2558. 

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