Methods for the determination of radium isotopes in environmental solids by gamma spectrometry can be obtained by two means, the direct measurement of the principal photopeaks (186 keV in the case of 226Ra), and indirect measurement using the gamma radiation emitting progeny. The advantage of using the direct measurement method, particularly for 226Ra, is that the sample can be prepared and measured immediately without the need for progeny ingrowth. The disadvantages of this method relate primarily in that the emission probability of this gamma photopeak is quite weak (3.28%), and furthermore by the presence of the interfering primary gamma emission of 235U at 185.7 keV that has a higher photon emission probability of 57.24%. While 235U generally presents at a much lower activity concentration than 226Ra in environmental samples, it has an effective photon emission of ~ 2.6% at this energy (assuming a natural U isotopic ratio). The energy resolution of currently available HPGe spectrometers is generally considered insufficient to separate the two peaks. If no chemical separation is carried out initially, quantification of 226Ra via the 186 keV gamma line must be conducted by attempting to quantify the contribution of the 235U isotope to the doublet peak.
In order to do an interference correction for the 186 keV peak you need to somehow determine the contribution from the interfering nuclide to the peak (in this case U-235). So the main question is: How can we determine the U-235 activity in the sample? Once you know that the interference correction itself is quite straightforward.
Some ways of doing this are:
- Estimate U-235 activity from one of its other full energy peaks (at 144 keV or 163 keV) if those peaks are visiblein the spectrum. However for a lot of environmental samples this is not possible as the activity is generally low.
- Estimate U-238 activity from its daughter Th-234 which has a peak at 63 keV, then calculate U-235 activity from the natural activity ratio A(U-238)/A(U235) = 21.44. This ratio is true for "non-enriched" uranium. Note that the 63 keV line needs to be suitably corrected for self-absorption effects in the sample.
- Determine U-235 activity by alpha spectrometry.
- The last method is the easiest, but also based on the most assumptions. If the natural activity ratio for uranium can be assumed AND you have secular equilibrium between U-238 and Ra-226 (i.e. their activities are the same), then approximately 57 % of the 186 keV peak can be attributed to Ra-226 and 43 % can be attributed to U-235. This method is described in a little bit more detail in the book "Practical Gamma-ray Spectrometry" by Gordon Gilmore.
Hopefully this answer can point you in the right direction. Best of luck with the sample analysis and interference corrections!
There is an empirical equation can be used for the required correction based on the natural abundance of Uranium and the activity concentration of U-238. Practically, more details is represented in the following article:
Article Characterization and radiological impacts assessment of scal...
Hopefully, it will be helpful for you.
In complement to @ Alexander suggestions, as an alternative to the use of the 63 keV peak to estimate 238U activity, see this reference for using a small 1001 keV peak.
Uyttenhove, Lemmens, Zizi
Depleted Uranium in Kosovo: Results of a survey by gamma spectrometry on soil samples, November 2002, Health Physics 83(4):543
- DOI: 10.1097/00004032-200210000-00014
How to correct the self-absorption effects in the sample for 63 keV line Alexander Mauring sir
What is coincidence summing in gamma spectrometry and when it is required to do correction for it??
Could you please help Alexander Mauring M. F. Attallah François Tondeur
Use the 63 keV line to estimate the U content, but you should measure far away from the detector > 15 cm in order for the Coincidence correction factors to be approx = 1
You can also use the 242 keV & 352 keV lines from Pb-214 and the Bi-214 lines.
In evironmental samples, there is a valid methods to correct for 185.7 keV peak of 235 U contribution to 186.2 Kev peak of 226Ra by choosing spectral interference reference peaks, however all are based on assumption readioactive secular equilibrium. Maybe the best peak is 1001 peak to determine 238U in sample. However low counting statistics degrades the precision,that is, its activity measurement uncertainty is worser due tu weak gamma-ray peak. The analyst should consider pros and cons about choosing analytical peaks such as: 63 keV (234Th), 92 kev doublet (234Th) and 1001 keV (234mPa).
please consider my papers appeared in literature and also other scientist’s papers related to high resolution gamma ray spectrometry.
If you need to have some papers, some of them are uploaded in researchGate webinar page
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