Nuclear Medicine Physics: The Basics, 8e by Ramesh Chandra PhD & Arman Rahmim PhD

Author:Ramesh Chandra, PhD & Arman Rahmim, PhD
Language: eng
Format: epub

Figure 9.5. Schematic presentation of a liquid scintillation detector. The electronics used in this case are identical to those used in an NaI(Tl) scintillation detector. However, in a liquid scintillation counter, the radioactive sample and the scintillator are mixed together with the help of a solvent in a sample detector vial (A).

The interaction of β particles in the solution with the scintillator, and with solvent molecules, results in the loss of energy, some of which is then converted into light by the scintillator molecules. The amount of light produced in this case, as in an NaI(Tl) scintillator, is directly proportional to the amount of energy lost. Because a β-ray has a short range in liquids and loses all its energy in the solution, the amount of light produced is proportional to the energy of the β-ray. The voltage of the pulse produced by a PM tube in turn is directly related to the amount of light incident on the PM tube. Consequently, the voltage of the pulse produced is directly proportional to the energy of the β-ray. Pulse-height analysis of the pulses produced then allows the simultaneous use of two or more radionuclides with differing β-energy spectra in the same sample (e.g., 3H and 14C).

The sample detector vial and the PM tube are enclosed in a light-tight compartment to exclude the room and other stray light from reaching the PM tube. In modem liquid scintillation counters, the sample-detector vial is viewed from opposite sides by two PM tubes rather than one. In this arrangement, using coincidence circuits, electronic noise can be reduced significantly, thus enhancing the sensitivity of low-energy β particles that otherwise will be lost in the electronic noise.

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