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

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?

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%), 295 keV (19.3%), and 352 keV (37.6%). 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%); there is a gamma ray at 1.12 MeV (15.1%) and one at 1.765 MeV (15.4%). The others have individual yields no higher than about 5%.

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