Alpha counting spectrometry

Centrifugation alone is not sufficient to separate Np solid material from the solution (j) ). Thus, the Np solution concentrations and oxidation state analyses were determined from aliquots filtered through approximately 1.8 nm pore-size Centriflo membrane cones (Amicon Corp., Lexington, MA). Each filter cone was pretreated with an initial aliquot of the suspension to be filtered. Tests using successive filtration confirmed that no significant amounts of Np were being sorbed by the filter cones. The solutions were alpha counted by liquid scintillation techniques. Major cations in the solutions were determined by inductively coupled plasma spectrometry (ICP) analyses. 

U-series disequilibria are most naturally expressed in terms of activity ratios (e.g.. Section 3.14.2.2). Alpha counting measures activity directly, whereas mass-spectrometry yields atomic ratios which need to be converted into activities using activity constants. This introduces an additional component of uncertainty (l-8%o) to the absolute accuracy of mass-spectrometric activity measurements (e.g., Holden, 1989 Jaffey et al., 1971 Meadows et al., 1980). This uncertainty, however, is small compared to the uncertainties in particle counting measurements. Moreover, the high precision of mass-spectrometric measurements has allowed some activity constants to be refined using samples where secular equilibrium can be assumed (Cheng et al., 2000). 

Preparation of purified samples for alpha-particle spectrometry is usually by either electrodeposition or micro-coprecipitation. Either technique takes at least an hour. Many beta-particle emitters are precipitated with several milligrams of carrier and weighted for determining the chemical yield prior to counting, which also takes time. In contrast, a few millilitres of solution are satisfactory for ICP-MS. If the analyte concentration should exceed the linear part of the calibration curve, a simple dilution overcomes the problem. 

Cl284-94 listed in Appendix A-1. Other actinides that emit alpha particles are also prepared with this method. After pnrification, nranium isotopes usually are identified and measured by alpha-particle spectrometry with silicon diodes. Tracer can be added to the initial solution and counted with the uranium isotopes of interest. 

The preceding uncertainty equations presume that all pairs of input estimates are uncorrelated, which may or may not be true. One of the most common examples of correlated input estimates in radioanalytical chemistry occurs when the chemical yield Y is calculated from an equation involving the counting efficiency e. This happens in measurements by alpha-particle spectrometry with an isotopic tracer. In this case, the uncertainty equation can be simplified by treating the product e x T as a single variable. What happens in effect is that the efficiency cancels out of the activity equation and for this reason its uncertainty can be considered to be zero ... 

The problem of uncertainty evaluation for background corrections in alpha-particle spectrometry is that the background count rate may be extremely low. Counting for several days may yield only a few, or even zero, counts. At least two issues may arise. First, the Poisson uncertainty estimate is relatively imprecise when the mean count is so low. In the worst case, both the background count Cb and the gross count Cq are zero, so that the uncertainty equation will calculate zero uncertainty for the measured activity, which is inappropriate for a laboratory to report. In any situation where it is possible to observe zero counts (C = 0), the Poisson uncertainty estimator Vc should be replaced by VC -I- 1. Low-level alpha particle spectrometry is a situation where zero counts may indeed be observed. Second, it becomes important to consider possible variability in the background (e.g., from spurious counts), which may be relatively large in comparison to the Poisson uncertainty. 

The counting room also houses alpha-particle spectrometry systems, which require a vacuum line or pumps. These detectors can be placed anywhere within the counting room and operated without interference from any elevated external radiation background. 

Alpha counting and alpha spectrometry are the two most common analytical methods for measuring plutonium concentrations in environmental samples. Other measurement techniques available are liquid scintillation, mass spectrometry, and gamma spectrometry. 

Th and U isotopes activity concentrations were determined in the samples by alpha-particle spectrometiy. For the detennination of these isotopes by alpha-particle spectrometry in the samples, a sequential well-established radiochemical method to the samples was applied [19]. In this method, the isotopes are electrodeposited onto stainless steel discs after its isolation from the bulk matrixes. The planchets were measured using an EG G Ortec alpha spectiometiy system with ion-implanted silicon detectors. Counting times ranged from 2 days to 4 days, depending on the activity concentrations and the recovery obtained in the chemical separation. 

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]...

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 ). 

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