Cesare Rossini (Perkin Elmer) Rome, October 24 2006



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  • "14-C measurement as a tool in the determination of biogenic fraction of SRF"

  • Cesare Rossini (Perkin Elmer)

  • Rome, October 24 2006


Terminology

  • Proportional Scintillation Method (PSM)

  • Liquid Scintillation Counting (LSC)

  • These 2 definitions are equivalent .

  • The term “Liquid” identifies a measurement of radioactive materials performed in a liquid phase, where special substances (primary and secondary scintillators) are present in an organic solvent.

  • The term “Scintillation” identifies a process where radioactive materials interact with the above mentioned substances able to convert radiations into photons.

  • The term “Counting ”is referred to photon counting process.



Terminology



Terminology



Terminology: the scintillation process



Terminology: the scintillation process





Terminology: the scintillation process

  • Coincidence circuit only accepts pulses impinging on both PMTs within a time window (coincidence gate), typically 18ns:

    • Benefit is lower background-higher signal to noise
  • Summation circuit combines pulse amplitudes from each PMT:

    • Benefit is higher signal to noise - PMT output proportional to total intensity of the scintillation; less effect of geometrical location of scintillation pulse in the vial




  • Copyright Buchholz, Dibble, Mueller and Cheng, LLNL 2004



Liquid scintillation analysis of radiocarbon (14C) can be used to determine biological content of any organic material (liquid or gas).

  • Liquid scintillation analysis of radiocarbon (14C) can be used to determine biological content of any organic material (liquid or gas).

    • All living biological material has a constant amount of radiocarbon per weight unit of total carbon
      • Radiocarbon is formed by cosmic radiation bombarding nitrogen molecules in the upper atmosphere
      • Biospheric equilibrium activity is about 14.6 decays per minute per gram of carbon
      • Radiocarbon will disappear from the biological material through radioactive decay process (half-life 5730 years)
    • Products made fully out of fossil carbohydrates do not include any radiocarbon any more since it has decayed out completely within the millions of years the product has been “underground”
    • The methodology for these measurements is extremely well established since radiocarbon dating leans on the same principles


LSC Analysis of CO2

  • CO2 absorbed into a trapping reagent, 98 % or more recovered.

  • Sample mixed with cocktail in a low K glass vial.

  • Sample counted by LSC at greater than 60 % efficiency.

  • Inactive sample as background reference

  • Carbon activities to be multiplied by 0.93 to correct for extra carbon from atmospheric atomic bomb tests

  • 0 % 14C means fossil based compound, 100 % neat biogenic compound.



Ultra low level LSC: : Quantulus

  • Quantulus ”hearth”



Ultra low level LSC: : Quantulus



Ultra low level LSC: : Quantulus

  • Optimal passive attenuation of environmental radiation by thick lead shield (630 kg.)

  • Active rejection of residual background by true event detection in an external guard detector, which is optically isolated from the sample

  • Further background rejection by adjustment of bias threshold, pulse amplitude comparison and pulse shape analysis.

  • 2 couples of programmable MCA (Multichannel Analyzer)



Ultra low level LSC: : Quantulus

  • Drinkable water analysis (Tritium, AlPha/Beta, Rn, Ra, U)

  • Food analysis ( Strontium/Ittrium)

  • Radioactive fall-out (Sr-90,Cs-137,Plutonium)

  • Hydrologic & Oceanology studies (Tritium, C-14)

  • Archaeological and artistic samples dating (C-14)

  • Wine analysis (food adulteration)(C-14)

  • Biofuel (C-14)



Ultra low level LSC: : Quantulus




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