Liquid Scintillation IRAD 2371

1. Introduction into LS theory and practice Liquid ScintillationIRAD 2371

2. Agenda • Energy deposition • Overview of the LSC process • Theory of operation • Quenching • Machine • Uses

3. Problems • Problems with counting changed particles • Don’t go through matter very readily • Means that they are easily shielded • So if a particle need to go through any barrier it will not do so effectively

4. What LSC is used to count • Low energy betas • Low energy x rays • Alphas

5. Why LSC • No barrier that charged particles need to go through • Close contact between the isotope atoms and the solvent • Close contact between solvent and phosphor • PMTs • Leads to high efficiencies and ability to detect low energy particles

6. Energy deposition • Amount energy that is deposited per unit track length by radiation is dependant on • Total energy imparted • Speed • Mass • change

7. Charge • Small ions deposit lower amounts of energy • As Z increases the energy deposited per unit track length increases • High Z ions deposit lots of energy in short distance • Higher energy deposition greater response from the scintillator

8. Energy • absorption and re-emission, cocktails contain two basic components, the solvent and the phosphor(s). • The solvent carries out the bulk of the energy absorption. • Dissolved in the solvent, molecules of phosphor convert the absorbed energy into light

9. Solvent Makes up to 90-99% of the total volume of scintillation fluid Solvent collects energy of stopping particles Aromatic hydrocarbons are best solvents Ring structures in the molecule captures energy from radiation Energy passes among solvent molecules until it hit a phosphor molecule

10. Phosphors • Makes up to 1-10% of volume of fluid • Primary and secondary phosphors • Primary -convert capture energy to light • Secondary- wavelength shifters used originally to shift the wavelength of the primary phosphors so the they would interact with the PMT better. Even thought he tubes now are better they are still included as they increases efficiency

11. Photons • The number of photons created is proportional to the energy deposited in the solution • Which is dependant on the length traveled in the solution • Which is dependant on the initial energy with which the particle was emitted

12. Energy Path • Beta decay creates free electron • E and solvent = energized solvent • energized solvent and flour= • energized flour and solvent • energized flour creates light • Light and sec flour = energized sec flour • energized sec flour creates light • Light enters PMT creates signal

13. Energy Path • Efficiencies vary depending on • Isotope • Sample composition • Specific flours • But usually low • Only about 4% of energy from particle is converted to light • But other parts of the LSC that helps with signal generation

14. Beta energies • Max energy is determined by radionuclide • Average energy is 1/3 that of max energy • So will get a variety of energies deposited in the scintillation fluid • Each of these energies creates different amounts of electrons • Each creates a different magnitude signal • Will get a variation of energies on the spectrum

15. Spectrum

16. PhotoMultiplier Tubes • Flours release the light which enters PMT • Each system has 2 PMTs • This cuts down on noise produced by random light • PMTs convert the light emitted by the flour into an electron which is sent to the first dynode • Dynodes multiply the electrons as they pass through the PMT • The Anode collects the multiplied number of electrons and generates a signal

17. Quenching • Quenching is anything that will reduce the energy transfer between the solvent and the flour • Can be • Physical • Chemical • Color

18. Physical Quenching • Physical quenching is easy to determine • Anything that will get in the way physically from the particle moving through the fluid • Anything that will get in the way of the light getting propagated through the fluid on its way to the PMTs • Smear, or any debris in the sample • Can take into account if you count your standard the same way

19. Chemical Quenching • Other chemicals in the sample may interfere with energy getting collected by the solvents • Chemical quenchers absorb the energy of the radiation before it is converted to photons • Reduce the number of photons that are generated but each charged particle

20. Color Quenching • Color quenchers absorb the light that is released by the flours • The number of the photons produced by the flour is not impaired the but number that gets to the PMTs are reduced • All three quenching reduces efficiency of the system

21. Interferences • Chemoluminescence- caused by the chemical reactions between the sample and the scintillator fluid. Reactions creates an excited molecules that emits light • This light then interferes with sample counting • Usually chemoluminescence decreases in several minutes to several hours • Can count sample twice in a time period and if counts have gone down dramatically , you may have had chemoluminescence

22. Interferences • Static electricity – in dry environments static electricity can build up on the container • If this static discharges in the instrument it will add a great error in counting • Plastic vials and latex gloves increase the static • Can eliminate or minimize by wiping down the vial with a moist cloth

23. Sample Preparation • Sample has to dissolve in fluid • Water based samples need water based fluid • Organic based samples need organic based fluids • Have some that can accommodate both types of samples

24. Sample Preparation • Sample have the be prepared the same way the standard is • If you are counting a solid (smears) then put smear in standard and count , will minimize errors • Anything that goes into your samples must be done to the standard to create similar bias

25. Sample Preperation • Ideally samples should be clear , pH neutral solution • Mix solvent and sample well • Let bubbles settle • Let sample stand for several minutes to minimizes effects of chemoluminescence

26. Signal processing • As single exits the vial it will interact with the PMTs (usually a pair) • From the PWTs the signal will enter a preamplifier • Preamp to a coincidence counter • Then to an amplifier • Then to single channel analyzer

28. Signal processing • Can use discriminators to separate signals • Can set up windows (same as other detection systems) to collect signal only in a certain energy range • Since energy is dependant on the radionuclide , one can separate radionuclides by energy

29. Signal processing • Most often used radionuclides in medicine are H-3, C-14, P-32 • Fortunately they have a great energy difference between their beta • H-3 18.6 keV max, ave 6.2 • C-14 156 keV max, ave 52 • P-32 1710 kev max, ave 695

30. Spectrum

31. Different energies

32. Uses • Medical • Research • Water sampling • Ground water flow measurements • Compliance • Nuclear power plants • Environmental sampling

33. Medical and Research • 90% of all drugs are tested with the use of radionuclide tracers or additions • Testing of fluids from the body to see where and how effective drugs are • Can use LSC to determine doses to people from low energy emitting radionuclides

34. Ground water • H-3 or C-14 tracers are used to determine direction and rate of ground water flow • Used to movement of water through formation for oil production • Add some T2O to ground water and them take sample s from a well down flow from area when radionulcide is detected then can determine how long it took water to travel from point A to point B

35. Nuclear power plants/Env Sampling • One of the largest produced radionulides in power plant is H-3 • Need to determine if it is being released off site • Can see if it the low energy emitting radionuclides are being biomagnified • Very good for evaluating water samples

36. Questions