Decay counting

Measurement of specific activity. The half-life of a nuclide can be readily calculated if both the number of atoms and their rate of decay can be measured, i.e., if the activity A and the number of atoms of P can be measured, then X is known from A = XP. As instrumentation for both atom counting and decay counting has improved in recent decades, this approach has become the dominant method of assessing half-lives. Potential problems with this technique include the accurate and precise calibration of decay-counter efficiency and ensuring sufficient purity of the nuclide of interest. This technique provides the presently used half-lives for many nuclides, including those for the parents of the three decay chains, U, U (Jaffey et al. 1971), and Th. 

In this chapter we discuss improvements documented in the literature over the past decade in these areas and others. Chemical procedures, decay-counting spectroscopy, and mass spectrometric techniques published prior to 1992 were previously discussed by Lally (1992), Ivanovich and Murray (1992), and Chen et al. (1992). Because ICPMS methods were not discussed in preceding reviews and have become more commonly used in the past decade, we also include some theoretical discussion of ICPMS techniques and their variants. We also primarily focus our discussion of analytical developments on the longer-lived isotopes of uranium, thorium, protactinium, and radium in the uranium and thorium decay series, as these have been more widely applied in geochemistry and geochronology. 

Mass spectrometric techniques for analysis of Th- U disequilibria were first developed to date corals for paleoclimate research (Edwards et al. 1987). Soon thereafter, workers at Los Alamos National Laboratory (LANE) developed methods for silicate analysis by TIMS (Goldstein et al. 1989). Typical TIMS analysis of MORE requires 0.5 to 1 gram of material in order have an analyzable load of 100 ng of Th. TIMS analyses of U and Th last 2-3 hrs and produce a precision of 0.5-2% (2a). SIMS techniques for measuring Th isotopes have also been developed (England et al. 1992 Layne and Sims 2000). Analysis of Ra and Pa isotopes by TIMS was developed in the early 1990 s significantly increasing the sensitivity over decay counting analysis (Volpe et al. 1993 Cohen and Onions 1993 Pickett et al. 1994 Chabaux et al. 1994). 

A specific form of this equation was solved by Barnes et al. (1956) when he presented the first decay counting °Th data for corals. The equation of Barnes et al. (1956) did not include the second term on the right side... 

As discussed above, mass spectrometric techniques are the methods of choice for measurement of nuclides pertinent to this study. They supercede earlier decay-counting techniques because of their ability to detect a much larger fraction of the nuclides of... 

An older and long-established technique, radiocarbon decay counting, also known as the "conventional" method of radiocarbon dating, is based on detecting and counting the amount of beta radiation emitted in unit time by radiocarbon atoms in a sample of known weight. 

The conventional radiocarbon decay counting technique generally provides reliable results, but it has some limitations the following are worth mentioning ... 

Dating with Radiocarbon. The important information held in a sample to be dated by radiocarbon is its present radiocarbon concentration comparing this concentration to that of radiocarbon in the atmosphere, which is considered to be constant (however, see discussion below), yields the conventional radiocarbon date of the sample. All that is required to establish the age of a sample, therefore, is to determine the present-day relative amount of radiocarbon in the sample. Once this has been determined by either the conventional radiocarbon decay counting or by the AMS method (see Fig. 63), a number of internationally established conventions and assumptions are used to calculate the age of a material or object ... 

Why then, is such a complicated and expensive set up necessary AMS combines mass spectrometric features with efficient discrimination of isobaric and molecular interferences. Therefore, it can detect and quantify atomic species of very low abundance. In the case of 14C dating, before AMS was utilized, about 1 g of carbon was needed to date an archaeological item. One gram of fresh carbon contains about 6 x 1010 14C atoms, of which 14 decay per minute. To get 0.5% statistical precision using decay counting, a 48 h acquisition time is necessary. The same result can be obtained with AMS in about 10 min and with only 1 mg of carbon. 

The use of aluminum rabbits meant that data on the short-lived elements obtained from the 5-min decay count would be lost. Significant personnel radiation exposures were obtained from handling the aluminum rabbit (.—50 g) directly from the reactor core because of the 1780 keV gamma of 2.2-min 28A1 and a remote method used to open the rabbit took too much time (in excess of 20 min). 

Decay time was critical to the determination of elements from the 5- and 30-min decay counts, so we decided to use the rabbit irradiation facilities with the highest thermal neutron flux (1014 n/cm2/sec) to build up the specific activity of short-lived isotopes. The higher flux also provided a greater sensitivity. 

Element standards were grouped into four standard libraries, corresponding to the four decay counting times. Decay time boundaries for each standard library are shown in Table II. In each library, at least two elements were calculated from different standards. These two standards represented different concentrations, counting geometries, dead times, decay times, and sample matrices. Visual inspection of the computer listing provided a rapid spot check for computer program malfunctions. 

For both decay counting and AMS, it is critical to prepare standard materials of known 14C/12C content. These include the 0X1 standard described below and materials relatable to it, as well as materials that are radiocarbon-free. Standards allow assessment of the overall accuracy and the effects of sample pretreatment procedures, and radiocarbon-free samples provide a blank to determine the radiocarbon introduced to the sample during processing. 

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