I have some questions about tritium measurement. in our laboratory there is a Quantulus 1220. I counted an empty 20 ml polyethylene vial and the result (30-170 channel) is 1.6 cpm.the result of 8 ml deplete water (1.5 ppm)+12 ml ultima gold LLT is 2.01 cpm.
Is the results normal?
how can I reduce blank count less than 1 cpm?
It is noteworthy that spectrum of sp21 and sp22 shift to low energy
Reduce the volume of the deplete water as well as cocktail. adjust different ratio of water to cocktail and see the result, as well as adjust the optimized window where FOM=E2/BG IS HIGH.
Read the following publication
Article Liquid Scintillation Counting Methodology for 99 Tc Analysis...
I have experience with measurement on Quantulus 1220. So I’m using 8:12 ml ratio for sample:cocktail, as liquid scintillation cocktail UltimaGold uLLT (it has lower background than UltimaGold LLT). The background for UltimaGold LLT was around 0.8-1 CPM. Depleted water that I use it is distilled again by my self in the laboratory. When I set the parameters measurement for program 6, using QM, I add to SP11, and SP12 (implicit settings) SP21 and SP22, to see what it is happened outside the measurement chamber. You must check also the counts numbers in SP11, the interference phenomenon that can appear in sample are recorded there. Usually I don’t have more 0.08 CPM (for a bkg. around 0.6-0.7 CPM).
If you are using calibration curve for efficiency determination and a fixed windows, you will have a higher background due to auto adjusting of the windows for external irradiation. If you use internal standard method for counting determination, you must use EasyView in order to establish the optimized windows at the best FM (in fact varying the window you find a region in the tritium spectrum where you have in the same time the lower background of the measurement, and the higher measurement efficiency).
There are some other features that you can use in setting different level for PAC and PSA adapted to your type of cocktail. Try with PAC=10 and PSA =256, in order to have a bkg. reduction. These parameter you must establish adapted to your cocktail.
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