Answer to Question #8597 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

In gamma spectroscopy, we can calculate the 238U using 214Pb and 214Bi. Can we estimate 232Th using 212Pb and 212Bi?

I am using an Ortec system and my analysis report sometimes gives a value of 228Ac which can be used for estimating 232Th assuming they are in equilibrium. Last report I printed has activity concentration for 208Tl, 212Pb, 212Bi, and 228Ra. Can any of this be used to estimate 232Th and, if yes, then how?

Usually I am using the following method to estimate 238U:

(214Pb + 214Bi)/2 +/- (SQRT((Error 214Pb/2)^2+(Error 214Bi/2)^2)

Is it okay to use this method?

A

It is possible to use the results for some of the progeny that you note to estimate the 232Th activity but not in all circumstances. As you know, the long-lived thorium decays initially to 228Ra, which has a 6.7 year half-life, the longest lived of all the radioactive progeny that follow. Thus, in a pure sample of 232Th, if no physical or chemical processes are ongoing that would disturb the 232Th-228Ra equilibrium, it would require about 35 years before the 228Ra achieved about the same activity as the 232Th. The 228Ra decay produces only an extremely low yield and low-energy gamma ray, which would not be reliable for making an estimate of 232Th. The decay product of 228Ra is 228Ac; the 228Ac has a 6.17 hour half-life so it grows into the 228Ra quite quickly and will achieve activity equilibrium with the 228Ra and the 232Th, both of which are solids under normal conditions. Therefore, assuming no processes have been ongoing that would disrupt the equilibrium, you should be able to use the 228Ac activity to determine the 232Th activity.

The progeny that follow 228Th in the decay chain would also achieve this equilibrium if none of the decay products were lost during the observation time. One of the progeny is 220Rn, a noble gas that might escape from the sample matrix under some circumstances. The 220Rn has only a 55-second half-life so its potential for escape before decaying to solid 216Po is not as great as it would be if it had a longer half-life but, depending on the sample chemical and physical characteristics, some radon might escape if the sample is not sealed against such losses. Naturally, any loss of radon will negate the approach to equilibrium of the subsequent progeny, including the 212Pb and 212Bi that you mention, and the activities of these may not be equivalent to the activity of the 232Th. You can judge whether the lack of equilibrium is significant by comparing your activity estimations for the 212Pb and 212Bi with the activity of the 228Ac. They should be pretty much the same if the equilibrium has been maintained. If the equilibrium has prevailed, you may also use the 208Tl results to determine the 232Th activity. In this instance, however you must recall that the 208Tl is produced in a 212Bi branching decay process so that its expected equilibrium activity would be only 36 percent of the 232Th activity.

If you find that the equilibrium has been maintained, you may also use the average of the activity estimations for the 212Pb and 212Bi as you have done when using the 214Pb and 214Bi. You will note, however, that the gamma ray yields from 214Bi are rather low, the highest being about 7 percent for the 727 keV gamma rays and, depending on your counting statistics, this can lead to a poorer activity estimate for the bismuth compared to the lead. I am assuming you are using a germanium detector that allows suitable energy resolution to discern the species of interest. Depending on the detector characteristics, you could experience some contribution of the 224Ra 241 keV gamma rays to the 239 keV gamma ray region of the 212Pb, although this should be fairly small.

Good luck in your continuing analyses.

George Chabot, PhD, CHP

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