Cesare Rossini (Perkin Elmer) Rome, October 24 2006

• "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”
• By determining the radiocarbon activity versus total carbon content we can tell the biological content of any organic product
• 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