Practical Exercise For Liquid Scintillation

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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
  1. Determination of the counting efficiency of 3H and 14C
    a) Prepare one standard sample (unquenched) for each of the two radionuclides 3H and 14C 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 3H spectrum, and channel 2 as that part of the 14C spectrum which does not overlap with the 3H spectrum. For 14C record the counting rate in both channels.
    c) Determine the counting efficiency εCH1(3H), εH1(14C), εCH2(14C) and εCH1 + CH2(14C) from Eqn.2. in Interfering processes
    d) Record (plot) the scintillation spectra for the two radionuclides.
  2. Determination of unknown concentrations of 3H and 14C in mixture
    e) Obtain from the laboratory assistant an unknown and unquenched mixture of 3H and 14C. 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.
  3. Recording of a quench correction curve
    h) Produce a quench correction curve for 14C as follows: Obtain 10 scintillation vials and label them from 1-10. To each vial add 1.00 mL 14C-solution 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 CCl4.
    j) Count the samples, and record the counting rates in CH1 and CH2 for all samples.
    k) Calculate the counting efficiency εCH1 + CH2(14C) as a function of the ratio RCH2/RCH1 and plot the curve.
    l) Plot one of the quenched spectra and compare the shape of this to the shape of the non-quenched curve.
  4. Determination of unknown concentration of 14C in a quenched sample
    m) Obtain from the laboratory assistant an unknown amount of 14C 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 14C by the two methods Channel Ratio method and Internal Standard method. Compare the results.
Reporting Schemes ans results
Table 1 Bakground Counting
Number of counts Sb
                                                        
Counting time (min)

counting rate Rb (cpm

2 Disciminator Settings

CH1      
CH1      
Upper limit (keV)


Lower limit (keV)


Table 3 Determination of the counting efficiency of 3H and 14C
Description
                                                         3H   
                                                   14C
Applied Counting Program


Counting rate CH1: RCH1
(background-corrected cpm)


Counting rate CH2: RCH2
(background-corrected cpm)


Disintegration rate standard
(dpm)


Counting efficiency CH1: εC'H1



Counting efficiency CH2: εC'H2


Total counting efficiency in CH1+CH2: εC'H1 + C'''H2


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