Answer to Question #5204 Submitted to "Ask the Experts"

Q

In trying to determine an accurate counting efficiency for a NaI well counting system, a ⁵⁷Co rod with activity of about 0.05 μCi is counted. In applying the formula for efficiency determination, the photon abundance of ⁵⁷Co is taken into the formula. The result for most well systems is 65 percent efficiency. However, we wish to determine the actual counting efficiency for ^99mTc, the material most often involved in fixture contaminations in nuclear medicine. ^99mTc has a different photon abundance than does ⁵⁷Co. Is it acceptable to count the cobalt rod and apply the ^99mTc photon abundance in the formula? If not, what approach should be taken?

A

Yes, your approach of counting the ⁵⁷Co and accounting for the actual photon abundance from ^99mTc is appropriate. While the photon energies from the two radionuclides are not exactly the same, they are sufficiently close that the error is reasonably small in assuming that the detector's intrinsic efficiency for the photons from cobalt is the same as that for the technetium photons. The calculation to determine the counting efficiency for the ^99mTc would go more or less as follows. I am assuming that you are using a multichannel gamma analyzer with the well detector.

Assume that a ⁵⁷Co source is counted in the well of the NaI detector and that we will determine the count rate in the photopeak region of interest (we shall assume that the region of interest includes both the major photons at 122 keV with an abundance of 0.856 gammas per disintegration and the 136.5 keV photons with an abundance of 0.106, the combined gamma abundance being 0.962 gammas per disintegration). Let us assume that the source activity on the day of calibration is 0.050 μCi, which is 1.110 x 105 disintegrations per minute (dpm) and that a one-minute count yields 8.753 x 104 counts in the region of interest. The gamma counting efficiency is then determined as:

Eγ =8.753 x 10⁴ cpm/[(1.110 x 10⁵ dpm)(0.962 γ/d)] = 0.820 c/γ.

The radionuclide ^99mTc emits gamma rays of energy 140.5 keV with an abundance of 0.889 gammas per disintegration. Assuming the same gamma counting efficiency for the photons from ^99mTc as for those from ⁵⁷Co, we can calculate the counting efficiency for ^99mTc activity as:

E_99mTc = (0.820 c/γ)(0.889 γ/d) = 0.729 c/d.

The actual value obtained will vary depending on the actual sizes of the detector and of the well and on the specification of the region of interest. I hope this answers your question.

George Chabot, PhD, CHP