Archived - Lutetium gamma spectrum
Posted: Tue Apr 13, 2004 9:22 pm
Archived due to the superb presentation and info on Gamma spec work. RH
FYI...just an example of a gamma spectrum that illustrates some tricky peaks.
This is a gamma energy spectrum taken from a 23 g lump of 0.9999 Lu metal, using a Bicron 1.5x3" NaI(Tl) detector. Lu-176 (natural abundance = 2.6%) has a half-life of 3.8 x 10^10 years and emits beta / gamma radiations. The activity is easily picked up with a Geiger counter.
The ENSDF database of nuclear data (ie.lbl.gov) lists four gamma energies for Lu-176:
88.36 keV (emission intensity of 15.6)
201.83 keV (83.3)
306.84 keV (100)
401.1 keV (0.4)
The first three of these can be discerned in the spectrum above (within a small bit of uncertainty of course). The peak at 510.4 keV is a "sum peak," recorded whenever 202 and a 307 keV gammas are simultaneously absorbed in the detector. The peak at 389.6 keV is ANOTHER sum peak, superimposed on the weak 401.1 keV line. This sum peak occurs when a 307 keV gamma and an 88 keV gamma enter the detector at the same time. Why so many sum peaks? The point here is that one has to know not just the decay energies but the decay scheme in order to know when sum peaks should be expected. It so happens that in Lu-176, the transitions in which gamma rays are emitted at 202, 307, and 88 keV are all sequential following a single beta decay, and hence these radiations are emitted almost simultaneously. The level diagrams for just about any radionuclide are accessible from ENSDF (link above).
Finally, the peak at 47.2 keV is from Hf and Lu K x-rays. The broad peaking from ~130 - 170 keV results from backscattered gammas, and the Compton edges for both the two big peaks. Where to look for the "backscatter peak" and the Compton edge are simple mathematical relations to the full energy peaks; I use a spreadsheet to automatically predict where these will be.
How do you know if you have a sum peak or backscatter peak without knowing the decay scheme or math formula, respectively? The answer is that the relative intensity of sum peaks and backscatter peaks is very strongly dependent on the physical configuration of the sample and detector. If you move the sample closer and note a peak that doubles in height relative to another, suspect that it might be a sum peak. If you put a lead sheet behind your sample and note the sudden appearance of a new peak, consider whether it makes sense for it to be a backscatter peak.
Anyway...gamma spectroscopy can be misleading sometimes and it helps to know the physical principles well.
-Carl
FYI...just an example of a gamma spectrum that illustrates some tricky peaks.
This is a gamma energy spectrum taken from a 23 g lump of 0.9999 Lu metal, using a Bicron 1.5x3" NaI(Tl) detector. Lu-176 (natural abundance = 2.6%) has a half-life of 3.8 x 10^10 years and emits beta / gamma radiations. The activity is easily picked up with a Geiger counter.
The ENSDF database of nuclear data (ie.lbl.gov) lists four gamma energies for Lu-176:
88.36 keV (emission intensity of 15.6)
201.83 keV (83.3)
306.84 keV (100)
401.1 keV (0.4)
The first three of these can be discerned in the spectrum above (within a small bit of uncertainty of course). The peak at 510.4 keV is a "sum peak," recorded whenever 202 and a 307 keV gammas are simultaneously absorbed in the detector. The peak at 389.6 keV is ANOTHER sum peak, superimposed on the weak 401.1 keV line. This sum peak occurs when a 307 keV gamma and an 88 keV gamma enter the detector at the same time. Why so many sum peaks? The point here is that one has to know not just the decay energies but the decay scheme in order to know when sum peaks should be expected. It so happens that in Lu-176, the transitions in which gamma rays are emitted at 202, 307, and 88 keV are all sequential following a single beta decay, and hence these radiations are emitted almost simultaneously. The level diagrams for just about any radionuclide are accessible from ENSDF (link above).
Finally, the peak at 47.2 keV is from Hf and Lu K x-rays. The broad peaking from ~130 - 170 keV results from backscattered gammas, and the Compton edges for both the two big peaks. Where to look for the "backscatter peak" and the Compton edge are simple mathematical relations to the full energy peaks; I use a spreadsheet to automatically predict where these will be.
How do you know if you have a sum peak or backscatter peak without knowing the decay scheme or math formula, respectively? The answer is that the relative intensity of sum peaks and backscatter peaks is very strongly dependent on the physical configuration of the sample and detector. If you move the sample closer and note a peak that doubles in height relative to another, suspect that it might be a sum peak. If you put a lead sheet behind your sample and note the sudden appearance of a new peak, consider whether it makes sense for it to be a backscatter peak.
Anyway...gamma spectroscopy can be misleading sometimes and it helps to know the physical principles well.
-Carl