Practical Exercise For Liquid Scintillation
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Revision as of 09:46, 9 July 2012 by Hansvl@uio.no (talk  contribs)
Written and developed by Prof. Tor Bjørnstad (IFE/UiO)
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The students are first divided into two groups. Each group follow the procedures described below.
Laboratory Procedure
 Determination of the counting efficiency of ^{3}H and ^{14}C
a) Prepare one standard sample (unquenched) for each of the two radionuclides ^{3}H and ^{14}C in two separate liquid scintillation vials. This is done by extracting an aliquote of 1.00 mL from the respective mother solutions organized by the laboratory assistant into the two vials. Add 10 mL scintillation cocktail to each vial and shake to a homogeneous solution.
b) Count the standard samples on the Beckman LS counter in the MCA mode. Define channel 1 as the counting window covering the ^{3}H spectrum, and channel 2 as that part of the ^{14}C spectrum which does not overlap with the ^{3}H spectrum. For ^{14}C record the counting rate in both channels.
c) Determine the counting efficiency ε_{CH1}(^{3}H), ε_{H1}(^{14}C), ε_{CH2}(^{14}C) and ε_{CH1 + CH2}(^{14}C) from Eqn.2. in Interfering processes
d) Record (plot) the scintillation spectra for the two radionuclides.  Determination of unknown concentrations of ^{3}H and ^{14}C in mixture
e) Obtain from the laboratory assistant an unknown and unquenched mixture of ^{3}H and ^{14}C. Prepare a sample as above.
f) Count the sample and calculate the concentration (in Bq) of both components by using the counting efficiencies determined above.
g) Plot the composite spectrum and explain the shape.  Recording of a quench correction curve
h) Produce a quench correction curve for ^{14}C as follows: Obtain 10 scintillation vials and label them from 110. To each vial add 1.00 mL ^{14}Csolution and 10 mL scintillation cocktail.
i) To the 10 samples sequentially 10, 20, 30, 50, 70, 100, 140, 180, 230 and 300 μL of the chemical quencher CCl_{4}.
j) Count the samples, and record the counting rates in CH1 and CH2 for all samples.
k) Calculate the counting efficiency ε_{CH1 + CH2}(^{14}C) as a function of the ratio R_{CH2}/R_{CH1} and plot the curve.
l) Plot one of the quenched spectra and compare the shape of this to the shape of the nonquenched curve.  Determination of unknown concentration of ^{14}C in a quenched sample
m) Obtain from the laboratory assistant an unknown amount of ^{14}C in a 10 mL measuring flask. Dilute with the appropriate liquid to exactly 10 ml.
n) Prepare a counting sample as above and count.
o) Determine the concentration (Bq) of ^{14}C by the two methods Channel Ratio method and Internal Standard method. Compare the results.
αLSC
The procedure will be described later.
Reporting Schemes ans results
Number of counts S_{b} 

Counting time (min) 

counting rate R_{b} (cpm 


CH1 
CH1 
Upper limit (keV) 


Lower limit (keV) 


Description 
^{ 3}H 
^{ 14}C 
Applied Counting Program 


Counting rate CH1: R_{CH1} (backgroundcorrected cpm) 


Counting rate CH2: RCH2 (backgroundcorrected cpm) 


Disintegration rate standard (dpm) 


Counting efficiency CH1: ε_{C'H1} 


Counting efficiency CH2: ε_{C'H2} 


Total counting efficiency in CH1+CH2: ε_{C'H1 + C'''H2} 


On the spectra plots indicate upper and lower limit for CH1 and CH2.