Liquid Scintillation Counter (LSC)
Liquid scintillation involves the detection and counting of radioactive decay. The radioactive sample is combined with a liquid scintillation cocktail or solid scintillator. Decay of a radionuclide produces an ionizing particle. Part of the kinetic energy of this ionizing particle is transferred to the “scintillator” which converts the energy of the particle emitted during the radioactive decay process into light, which is then detected by the LS system. The number of photons produced from one ionizing particle is proportional to its kinetic energy. All photons produced by one ionizing particle are emitted isotropically over a nanosecond time scale.
The collecting optics of the LS system direct the photons emitted to either of two photomultiplier tubes (PMTs). If both PMTs are activated by one photon burst, then one nuclear decay event is registered and converted into a measurable electrical pulse. The voltage pulse produced by the PMTs is proportional to the number of photons. Therefore, the pulse height at the output of the tubes is proportional to the energy of the particle.
The pulses from the PMTs are analyzed, converted to digital form, and stored in the appropriate channel of a multi-channel analyzer, corresponding to the particle energy. The data accumulated in the multi-channel analyzer over the counting time of the sample is used to determine the energy of the particles in the sample and the rate (counts per minute, or cpm) of radioactive decay in the sample. The cpm is the total number of pulses in the channels of the multi-channel analyzer divided by the total time in minutes for obtaining the counts.
Liquid Scintillation Cocktail
3 main components that make up a scintillation cocktail:
1)Emulsifiers
2) Solvents
3) Fluors
Missouri S&T’s health physics department uses a cocktail call EcoLumeTM Liquid Scintillation Fluid from MP Biomedicals. Since your cocktail is essentially your detector in this case, what type of cocktail you use, can influence the results you get (i.e. efficiency for different isotopes). In this case EcoLume is a general purpose cocktail that accommodates a wide range of aqueous and non-aqueous samples.
Components of EcoLume:
p-bis-(o-METHYLSTYRYL): fluor
BENZENE: solvent
2,5-DIPHENYLOXAZOLE: fluor
Linear Alkylbenzene: solvent
Non-ionic Surfactants: emulsifiers
Phenylxylylethane: solvent
Samples
3 samples:
1) Missouri S&T nuclear reactor pool water (1mL)
2) Water from drinking fountain on campus (CSF) (1mL)
3) H-3 sample from Biological Sciences laboratory (1mL)
Note: background sample is 10mL of EcoLume liquid scintillation fluid in a 20mL vial.
Sample 1: Determine if the amount of Tritium (H-3) found in the Missouri S&T Reactor water is greater than the permissible limits set forth in 10CFR20 Appendix B, Table 2
Sample 2: Determine if there is any detectable radioactivity found in the drinking water from a water fountain on campus.
Sample 3: Calculate the total activity of H-3 in a container based off a 1mL sample of H-3
Calculations:
Sample 1: 10CFR20 Appendix B, Table 2 Limit for H-3 in water is 1.0 x 10-3 mCi/mL
Results of analysis: _________________ dpm/mL
Calculation: activity = ______________dpm/mL =
2.22 x 106 dpm/mCi
H-3 activity of sample: _________________mCi/mL
Sample 2: Convert counts per minute obtained with the detection instrument to mCi
Results of analysis: H-3 _______dpm/mL, C-14 _______dpm/mL
Total dpm/mL: ___________________ dpm/mL
Calculation: activity = _______________dpm/mL =
2.22 x 106 dpm/mCi
Activity of sample: ___________________mCi/mL
Sample 3: There is a 1 L container of H-3 located in our Dangerous Materials Storage Facility. We need to estimate the total amount of H-3 activity in this container by taking a 1mL sample.
Results of analysis: H-3___________ dpm/mLCalculation: activity = __________________dpm/mL =
2.22 x 106 dpm/mCi
Total activity in container: __________ mCi/mL x 1000 mL
Total activity in container: mCi
More Information:
LS cocktail components:
Solvent: A chemical component of the liquid scintillation cocktail that dissolves the sample, absorbs excitation energy and emits UV light which is absorb by the fluors.
Fluor: A chemical component of the liquid scintillation cocktail that absorbs UV light emitted by the solvent and emits a flash of blue light.
Emulsifier: Component that combines the solvent and fluor together into a homogenous mixture
Negative effects of LSC:
Chemiluminescence: Random single photon events which are generated as a result of the chemical interaction of the sample components.
Photoluminescence: Delayed and persistent emission of single photons of light following activation by radiation such as UV.
Quenching: Anything which interferes with the conversion of decay energy emitted from the sample vial into blue light photons.