Answer to Question #7546 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

I have a question regarding the measurement of 226Ra by gamma spectroscopy. I've seen that some labs use the 214Bi line, while some use the 214Pb line to measure 226Ra in equilibrium. Are there significant advantages to the use of either line?

A

As you know, both 214Bi and 214Pb emit measurable gamma rays, and both can be useful in the indirect determination of 226Ra, a precursor in the decay chain that produces the lead and bismuth progeny. The 214Pb emits lower-energy photons than does the 214Bi, the three most abundant gamma rays from the lead being at 242 keV (7.43 percent), 295 keV (19.3 percent), and 352 keV (37.6 percent). The dominant gamma rays from 214Bi are more in number and higher in energy than the lead gamma rays; the range of useful energies is from about 600 keV to about 2.5 MeV. The bismuth gamma ray of highest yield is at 609 keV (46.1 percent); there is a gamma ray at 1.12 MeV (15.1 percent) and one at 1.765 MeV (15.4 percent). The others have individual yields no higher than about 5 percent.

In general, among the features that should be considered in choosing a particular energy gamma ray for use in quantifying a particular radionuclide by gamma-ray spectrometry are its decay yield, the detector photopeak counting efficiency at that energy, the magnitude of the Compton continuum under the photopeak (the larger the Compton contribution the poorer will be the statistics of determination, all other factors being equal), and the degree of interference with other closely lying peaks in the pulse height distribution. The ability to discriminate among photopeaks that are relative closely spaced in energy depends heavily on the detector type used, and this may also influence the decision as to which gamma-ray energy is preferable. Such considerations apply especially when comparing a relatively low-energy resolution detector such as NaI(Tl) with a much higher resolution detector such as germanium.

An advantage of using the 214Pb lines in estimating 226Ra is that at the lower energies the detector efficiency is higher. The smaller the volume of the detector, the more important this consideration may be. If the 226Ra content of the sample is relatively low, the higher efficiency may be particularly desirable. A possible disadvantage of using the 214Pb is that the lower-energy portion of the pulse height distribution tends to suffer more influence from higher-energy photon interactions in the detector, thus increasing the Compton background somewhat, and the lower-energy portion may also tend to be rather more crowded with other gamma-induced photopeaks. Naturally, the radionuclide composition of the sample can have a large impact on the prevalence of photopeaks in various energy regions and the extent of interference that these might present.

The 609 keV gamma ray from 214Bi has a higher yield than any of the 214Pb photons and any of the other higher-energy photons from 214Bi and, with a reasonably large-volume detector, this peak may be the largest peak in the entire pulse height distribution; this may be a reason for selecting this peak. For a large-volume detector even the lower yield, higher-energy gamma rays of 214Bi may be advantageous because the higher-energy peaks tend to be in the lower Compton background region and in an area of the pulse height distribution where other gamma-induced photopeaks may be less abundant. As long as the 226Ra content is high enough to yield acceptable counts in the higher-energy photopeaks, despite some loss in efficiency and rather low yields at the higher energies, this makes for a clean isolation of the 214Bi photopeaks.

In closing we should note that some analysts use both 214Pb and 214Bi gamma rays to evaluate the 226Ra content. Independent analyses for 226Ra, using results from photopeaks for individual gamma rays, are averaged together to obtain the mean value estimate for the 226Ra content. The 295 keV and 352 keV gamma rays from 214Pb and the 609 keV gamma ray from 214Bi are probably the most commonly used in this approach. Since gamma-ray spectrometry yields the count data simultaneously for all the gamma rays, such an approach requires little extra work and tends to reduce certain systematic uncertainties that might affect a result for a single gamma energy.

I hope this answers your question.

George Chabot, PhD, CHP

 
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