- #1
sgstudent
- 726
- 3
Hi, I am measuring Pb-210 and Sr-90 with the LSC and using the dual label counting method to determine the activity levels of Pb-210. Results are good, but now I am figuring out whether my method of determining the MDA of Pb-210 is correct.
For Sr-90 it is simple - I just do ROI gating to exclude Pb-210 and determine the efficiency and background in this region. Then I use the MDA formula to get it. Likewise, I calculate the Sr-90 activities by taking the Sr-90 CPS in this region and dividing it by my method's recovery*efficiency in this ROI region.
But for Pb-210, it is completely overlapped by Sr-90, so what I do is to have my background as a sum of the measured background in the Pb-210 region, and also the Sr-90 counts in that Pb-210 region. So I calculated the Sr-90 counts in the Pb-210 region by taking the Sr-90 activity and multiplying it by my method's recovery*Sr-90 efficiency in the Pb-210 region. This multiplication gives me the contribution of Sr-90 CPS in the Pb-210 region, which I add to the background counts. Since I have the efficiency of Pb-210 from a pure standard and the background CPS, I plug them into the MDA formula to get it.
I feel like this method is valid because treating the Sr-90 counts as background is perfectly fine to me since it feels the same having a very high background. Anyways this is the same as having a very accurate process blank to be where we can calculate more exactly the background count rate. And also I think that the efficiency I obtained from the pure standard of Pb-210 still holds for this MDA calculation, because empirically we do use that efficiency in the Dual-label counting method, and also scientifically, the efficiency should not change even if the background is higher since there is no quench effect happening (just a systematic increase in the count rates on the y-axis of the LSC spectrum).
And so the conclusion I have is that the MDA for Pb-210 is not going to be a fixed number, but depends on how much Sr-90 is in the sample.
Does anyone have any experience with such MDA calculations with the Dual-label counting method? I have not found many publications that explicitly shown their method of calculating the MDA.
For Sr-90 it is simple - I just do ROI gating to exclude Pb-210 and determine the efficiency and background in this region. Then I use the MDA formula to get it. Likewise, I calculate the Sr-90 activities by taking the Sr-90 CPS in this region and dividing it by my method's recovery*efficiency in this ROI region.
But for Pb-210, it is completely overlapped by Sr-90, so what I do is to have my background as a sum of the measured background in the Pb-210 region, and also the Sr-90 counts in that Pb-210 region. So I calculated the Sr-90 counts in the Pb-210 region by taking the Sr-90 activity and multiplying it by my method's recovery*Sr-90 efficiency in the Pb-210 region. This multiplication gives me the contribution of Sr-90 CPS in the Pb-210 region, which I add to the background counts. Since I have the efficiency of Pb-210 from a pure standard and the background CPS, I plug them into the MDA formula to get it.
I feel like this method is valid because treating the Sr-90 counts as background is perfectly fine to me since it feels the same having a very high background. Anyways this is the same as having a very accurate process blank to be where we can calculate more exactly the background count rate. And also I think that the efficiency I obtained from the pure standard of Pb-210 still holds for this MDA calculation, because empirically we do use that efficiency in the Dual-label counting method, and also scientifically, the efficiency should not change even if the background is higher since there is no quench effect happening (just a systematic increase in the count rates on the y-axis of the LSC spectrum).
And so the conclusion I have is that the MDA for Pb-210 is not going to be a fixed number, but depends on how much Sr-90 is in the sample.
Does anyone have any experience with such MDA calculations with the Dual-label counting method? I have not found many publications that explicitly shown their method of calculating the MDA.