Atom counting accelerator

Linked Systems Isotopic Enrichment. The power of advanced measurement techniques can often be extended by linking them with other techniques. Such is the case for the detection of trace quantities of natural radionuclides and isotope enrichment. We have already found this extremely valuable for gas proportional counting of 37Ar and accelerator atom counting of 14C [8,9]. The first nuclide was enriched by means of thermal diffusion (Ar) the... 

Principal characteristics of small sample liquid scintillation counting (lsc), gas proportional low-level counting (11c) and atom counting by accelerator mass spectrometry (AMS) are summarized in Table 1, and systems we have used are shown in figure 1. The most important differences (apart from cost and availability)... 

The first entry in Table 4 refers to the accelerator (AMS) experiments noted earlier in which we investigated (a) the feasibility of direct atom counting given only a few micrograms of carbon (using the new international radiocarbon dating... 

The ratio A/A0 is measured either by P" counting or with the tandem accelerator mass spectrometer (TAMS) in which direct atom counting is used. 

Accelerator mass spectrometry (AMS) extends the capabilities of atom-counting using conventional mass spectrometry, by removing whole-mass molecular interferences without the need for a mass resolution very much better than the mass difference between the atom and its molecular isobar. This technique has been used with great success for the routine measurement of C, Be, " Al, C1 and, recently, (see Table 5.15). Analysis of " C by AMS can, for example, generate dates with a precision that is at least equal to the best conventional beta-particle-counting facility. In many cases, where small sample analysis is required, the AMS method has proved superior (Benkens, 1990). A complete description of AMS can be found in review articles (Litherland et al., 1987 Elmore and Philips, 1978) or recent conference publications. The application of AMS to measurement has been discussed in detail in Kilins et al. (1992). 

Kutschera, W. 1994. Atom counting of long-lived radionuclides. Nuclear Instruments Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment 353(1-3), 562-562. 

Accelerator mass spectrometry (Fig. 21) is de-.signed for the most precise atom counting of cos-... 

In Secondary Ion Mass Spectrometry (SIMS), a solid specimen, placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, that are suffi-ciendy energedc to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions, called secondary ions. The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering dme (depth). 

In the technique developed by Willard Libby in Chicago in the late 1940s, the proportion of carbon-14 in a sample is determined by monitoring the (1 radiation from C02 obtained by burning the sample. This procedure is illustrated in Example 17.4. In the modern version of the technique, which requires only a few milligrams of sample, the carbon atoms are converted into C ions by bombardment of the sample with cesium atoms. The C ions are then accelerated with electric fields, and the carbon isotopes are separated and counted with a mass spectrometer (Fig. 17.19). 

Thinking Critically The K-Ar data for this experiment were obtained using a mass spectrometer. In this process, a small sample is heated with a laser until its constituent atoms vaporize and become ionized. A voltage is then applied that accelerates the charged ions towards a detector. The lightest ions reach the detector first, and the numbers of ions of each mass are identified and counted. There are a number of practical concerns that researchers must address in order to be confident that the measurements truly yield an accurate age for the object. List and explain a few possible concerns. 

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