1. 3/2003 Rev 1 II.3.4 & 13-15a – slide 1 of 31 Part IIQuantities and Measurements Module 3Principles of Radiation Detection and Measurement Session 4,Liquid Scintillation 13-15aResolution, Coincidence 13-15aResolution, Coincidence Session II.3.4 & 13-15a IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

2. 3/2003 Rev 1 II.3.4 & 13-15a – slide 2 of 31  We will describe the liquid scintillation process includes the meaning of:  solvent  solute  scintillation “cocktail”  We will also explain the purpose of a scintillation detector and  We will describe what is meant by coincidence counting and the significance and types of quenching Overview

3. 3/2003 Rev 1 II.3.4 & 13-15a – slide 3 of 31 Liquid Scintillation Overview  A small sample is taken using a small filter wipe of an area suspected of contamination  The sample is then put in a vial with a given amount of scintillation “cocktail” – a mixture of different agents  If the sample has radioactive contamination, it will cause a reaction with the scintillation cocktail and subsequently a flash of light will be emitted  The light signal is proportional to the activity in the sample

4. 3/2003 Rev 1 II.3.4 & 13-15a – slide 4 of 31 Liquid Scintillation  The light is not necessarily in a spectrum that is visible to the human eye  A scintillation detection system is used to measure the light produced  These flashes of light are used to measure the amount of activity the sample contains

5. 3/2003 Rev 1 II.3.4 & 13-15a – slide 5 of 31 Liquid Scintillation  Advantages of liquid scintillation counting are:  It can detect many different types of isotopes  It provides an means of measuring low energy beta particles

6. 3/2003 Rev 1 II.3.4 & 13-15a – slide 6 of 31 Liquid Scintillation  Disadvantages of this type of detector are:  Expense of equipment  It is not portable, and  It requires laboratory support and skilled professionals to operate

7. 3/2003 Rev 1 II.3.4 & 13-15a – slide 7 of 31 Liquid Scintillation  The process of liquid scintillation counting is relatively simple  The beta decay electron emitted by the radioactive isotope in the sample excites the solvent molecule, which in turn transfers the energy to the solute, or fluor  The energy emission of the solute (the light photon) is converted into an electrical signal by a photomultiplier tube

8. 3/2003 Rev 1 II.3.4 & 13-15a – slide 8 of 31 Process Overview

9. 3/2003 Rev 1 II.3.4 & 13-15a – slide 9 of 31 Liquid Scintillation Process  The process of liquid scintillation involves the detection of beta decay within a sample via capture of beta emissions in a system of organic solvents and solutes referred to as the scintillation cocktail  This mixture is designed to capture the beta emission and transform it into a photon emission which can be detected via a photomultiplier tube within a scintillation counter  The cocktail must also act as a solubilizing agent, keeping a uniform suspension of the sample

10. 3/2003 Rev 1 II.3.4 & 13-15a – slide 10 of 31 Liquid Scintillation Process  The scintillation counting system consists of three primary components:  the radioactive substance  the solvent, and  the solute (or fluor)

11. 3/2003 Rev 1 II.3.4 & 13-15a – slide 11 of 31 Solvent  The solvent is the first compound in the scintillation cocktail to capture the energy of the beta particle  The solvent molecule achieves an excited state, and the excess energy is transferred from solvent molecule to solvent molecule  The solvent tends to remain in an excited stated for an extended period of time, decaying into the ground state without the emission of light; thus, solvents tend to have a low quantum fluorescent yield

12. 3/2003 Rev 1 II.3.4 & 13-15a – slide 12 of 31 Solvent List of solvents and their characteristics

13. 3/2003 Rev 1 II.3.4 & 13-15a – slide 13 of 31 Solute  Solutes (or fluors) exhibit properties which in many respects are just the opposite of those of solvents  They tend to decay rapidly mainly through the emission of light photons, thus having a high quantum fluorescent yield  Solutes that directly absorb the excitation energy of the solvent are also known as primary solutes

14. 3/2003 Rev 1 II.3.4 & 13-15a – slide 14 of 31 Solute  Early scintillation counters were sometimes unable to detect the short wavelengths emitted by primary solutes; as a result, secondary solutes were added to ampilify the primary emissions  Secondary solutes were also complex organic compounds with the ability to absorb the decay energy of the primary solute and rapidly emitting it at a longer wavelength, shifting the overall signal to a wavelength more easily detectable by photomultiplier tubes

15. 3/2003 Rev 1 II.3.4 & 13-15a – slide 15 of 31 Solute  As more sensitive photomultiplier tubes were constructed, secondary solutes became unnecessary  However, they may still be used to improve counting efficiency, as both the shorter and longer wavelengths can be detected

16. 3/2003 Rev 1 II.3.4 & 13-15a – slide 16 of 31 Sample Preparation

17. 3/2003 Rev 1 II.3.4 & 13-15a – slide 17 of 31 Process  The solvent is the first compound in the scintillation cocktail to capture the energy of the beta particle  The solvent molecule achieves an excited state, and the excess energy is transferred from solvent molecule to solvent molecule  The solvent remains in the excited stated for an extended period of time, decaying into the ground state without the emission of light.

18. 3/2003 Rev 1 II.3.4 & 13-15a – slide 18 of 31 Process  The solute then absorbs the excitation energy of the solvent, and quickly returns to the ground state by emitting light  If a secondary solute is used, that solute absorbs the signal of the first solute and emits a second burst of light at a longer wavelength

19. 3/2003 Rev 1 II.3.4 & 13-15a – slide 19 of 31 Counting

20. 3/2003 Rev 1 II.3.4 & 13-15a – slide 20 of 31 Sample Analysis

21. 3/2003 Rev 1 II.3.4 & 13-15a – slide 21 of 31 Process Development  Soon after the discovery of the basic principles of liquid scintillation in 1950, instruments designed for counting began appearing, with the first commercial model becoming available in 1954

22. 3/2003 Rev 1 II.3.4 & 13-15a – slide 22 of 31 Process Schematic

23. 3/2003 Rev 1 II.3.4 & 13-15a – slide 23 of 31 Photomultiplier Tube

24. 3/2003 Rev 1 II.3.4 & 13-15a – slide 24 of 31 Coincidence Counting  Most commercial scintillation counters are coincidence systems utilizing PMTs in tandem to monitor for a photon event  A pulse is not registered unless both PMTs view the incident photons within the predetermined time interval usually 20-30 nsec

25. 3/2003 Rev 1 II.3.4 & 13-15a – slide 25 of 31 Coincidence Counting  If a pulse is recorded by the two PMTs within the 20-30 nanosecond window, a coincidence pulse is recorded that is a measure of the number of single events which occurred during the window. This is called coincidence counting  If an event occurs within only one of the PMTs, a coincidence pulse will not be recorded

26. 3/2003 Rev 1 II.3.4 & 13-15a – slide 26 of 31 Pulse Summation  Early liquid scintillation counter models only monitored the pulses from one PMT, using the other to detect for coincidence only  Current models uses a pulse summation circuit, which adds together all of the pulses from the output of the PMTs within a given time window  This technique is particularly helpful for dual isotope counting of 3 H and 14 C

27. 3/2003 Rev 1 II.3.4 & 13-15a – slide 27 of 31 Pulse Amplification  There are two types of pulse amplification:  linear  logarithmic  A linear amplifier generates a pulse proportional to the energy obtained from the scintillation solution  A logarithmic amplifier processes the summed output of the 2 PMTs into a single pulse equal to the log of the summed PMT pulses

28. 3/2003 Rev 1 II.3.4 & 13-15a – slide 28 of 31 Quenching  Quenching is the reduction in the efficiency in the energy transfer process in the scintillation solution  This presents a problem in liquid scintillation counting due to a reduced signal  Types of quenching are:  chemical  dilution  self-quenching  color  optical

29. 3/2003 Rev 1 II.3.4 & 13-15a – slide 29 of 31 Quenching  Chemical – scintillation solution components interact with excited molecules before they can emit a light photon  Dilution – dilution reduces the probability of scintillation events  Self-quenching – the concentration of the solute is too high which interact with excited molecules and dissipate the energy before a photon can be produced

30. 3/2003 Rev 1 II.3.4 & 13-15a – slide 30 of 31 Quenching  Color – color in the sample materials absorb some of the fluorescent photons before they can leave the counting vial  Optical – fogging on the outside of the scintillation sample vial impedes transmission of photons.

31. 3/2003 Rev 1 II.3.4 & 13-15a – slide 31 of 31 Where to Get More Information  Cember, H., Introduction to Health Physics, 3 rd Edition, McGraw-Hill, New York (2000)  Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8 th Edition, 1999 update), Wiley, New York (1999)  International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)