Answer to Question #9278 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

Q

What would be a mock phantom source for 123I AND 124I for a thyroid bioassay? I currently use a 129I source with a Ludlum 44-17 detector for 125I and a 133Ba source with a Ludlum 44-10 detector.

A

While mock sources for 131I and 125I have been used for many years, I do not believe that very useful mock sources have been available to simulate 123I and/or 124I in thyroid bioassay calibration procedures. I shall make some suggestions, but the approach will depend, in part, on how you are using the 2" x 2" NaI(Tl) probe that you specify. In particular, is the probe being used to do gross gamma counting or is it being used in conjunction with a multichannel analyzer that allows reasonable energy discrimination? The following discussion will include considerations of all the major photons being emitted and the latter part will touch upon the possibilities when specific energies are considered as when an energy analyzing system is used.

As you know, the major gamma ray from 123I decay has an energy of 159 keV with a yield of 83 percent. The radionuclide emits a number of higher energy gamma rays up to about 1,070 keV, but these are low yield, most much less than 1 percent, with the highest being 1.39 percent at 529 keV. There are numerous characteristic x rays emitted, with the most important ones ranging in energy from about 27 keV to 32 keV with a combined yield of about 87 percent.

The closest source that I am aware of that could be useful for 123I simulation is 123mTe. It emits the same 159 keV gamma ray with a yield of 84 percent. The distribution of x rays from 123mTe is similar to those from 123I, but with a somewhat lower yield in the 27 to 32 keV interval, being about 51 percent compared to the 87 percent for the 123I. The half-life for 123mTe is about 120 days, much longer than the 13.3 hour 123I, but still shorter than one would like for a simulation source that one might like to use over a period of several years. I believe 123mTe may be available from Eckert and Ziegler (Isotope Products).

I am not aware of a reasonable source to mimic the gamma emissions from 124I. This radionuclide emits photons of many energies, the most significant of which range from about 600 keV to 1,700 keV, with combined yields somewhat greater than 90 percent and with the greatest individual yield of 63 percent at 603 keV. There are additional higher (and lower) energy low-yield gamma rays emitted as well. The radionuclide also emits characteristic x rays in the interval from about 27 to 32 keV with a combined yield of about 57 percent. The weighted average gamma energy is about 850 keV for the 124I decay. I cannot identify a radionuclide that I think would provide a reliable simulation but considering the distribution and relative yields of the various photons, and considering the manner in which NaI detector efficiency changes with energy, I might be inclined to attempt to use a radionuclide such as 54Mn, which emits a single gamma ray of 835 keV energy with a yield of essentially 100 percent, to simulate the 124I in bioassay calibrations/source checks. This would require some experimentation to determine possible validity. The 54Mn has a half-life of 312 days, which is not super, but acceptable. This radionuclide may also be obtained from the above cited supplier and probably from others as well.

Since neither the 123mTe nor the 54Mn that are mentioned here are sold specifically as mock sources for the iodine isotopes of interest, if you were to try them you would have to do the work necessary to demonstrate their applicability and acceptability for the tasks at hand. If you have the specific interest in dealing with bioassay situations in which an individual may have been exposed to both 123I and 124I you would have to develop procedures for measuring the 159 keV photons from the 123I and the higher energy photons from the 124I. If you are using the Ludlum 44-10 (2" x 2" NaI(Tl)) probe with a multichannel analyzer it would be a relatively simple matter to identify energy regions of interest (ROI). In such a case the 159 keV photons would be easy to measure in a lower energy ROI. You might then set an upper level ROI that measured primarily the 603 keV photons from 124I. In such an instance you might consider using 137Cs as a mock source to simulate the 124I during calibration procedures. The 137Cs (137mBa) emits 662 keV photons which would be detected with slightly lower efficiency than the 603 keV photons from the 124I, which would result in slightly conservative estimations in field use. For such a calibration source you would also have to make a slight adjustment for the differences in yields of the photons from the 124I and the 137Cs. The 124I 603 keV photon yield is 63 percent and the yield of 662 keV photons from 137Cs is 85.1 percent. Thus, considering only these photons, 1 microcurie of 124I would have the same photon yield as 0.74 microcuries of 137Cs. You would also want to adjust the ROI to include most of the counts in the 662 keV photopeak. The 137Cs has the advantage that its half-life of 30 years makes decay corrections less significant.

I regret that I cannot identify sources that have already been established for your purposes. I wish you well in your pursuits.

George Chabot, PhD, CHP

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