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Fundamentals of Liquid Scintillation Counting

Liquid Scintillation Signal Interpretation

A beta particle, passing through a scintillation cocktail, leaves a trail of energized solvent molecules. These excited solvent molecules transfer their energy to scintillator molecules, which give off light. Each scintillator molecule gives off only one photon on activation, (and the wavelength of that photon is characteristic of the scintillator, not the ß-particle), but multiple scintillators are activated by the energized molecules generated by one β-particle.

The path of a β-particle in a cocktail is generally less than 0.1 cm.  The half life is correspondingly short, which means that the burst of photons from an emission event derives from a small space and reaches the PMT with sufficient simultaneity to be read as one pulse of light.

The number of photons generated is directly proportional to the path length of the β particle, which is in turn determined by its emission energy (the β particle rebounds from solvent molecule to solvent molecule, until its incident energy is exhausted). The intensity of each light pulse corresponds to the emission energy and the number of pulses per second corresponds to the number of radioactive emissions.

Particle passing through scintillation fluid.

Passing through scintillation fluid, a single beta particle gives rise to multiple, nearly simultaneous emissions of light. These photons are registered by the photomultiplier tube as one pulse of energy. The magnitude of this light pulse corresponds to the number of photons.

Pulse Analysis

The scintillation counter classifies each pulse of photons according to the number of photons in the pulse, which corresponds to the energy of the individual b emission event. Pulses are collated into channels, and the counts per minute (CPM) in each channel is recorded. Each channel corresponds to a specific range of β energies (channels are also known as counting windows) and counts with energies above or below set limits are excluded from a particular channel.

The usual practice is for three channels to be selected: one each for low, medium and high energy emissions. The lowest channel corresponds to the energy of 3H emissions, the highest to 32P. When the counts have all been collated the researcher knows the intensity of radiation—expressed in CPM—and its energy distribution (spectrum). The CPM is proportional to the amount of isotope in the sample, and the spectrum indicates the identity of the isotope.

Within a theoretically ideal cocktail, all of the energy from each β particle would be collected and converted into light. The spectrum of emitted β energy and the DPM values could then be taken directly from the data. The highest energy emissions would be compared with the Emax (maximum emission energies) for known radioisotopes to confirm the isotope identity. Real cocktails, however, are less than 100% efficient in energy collection and conversion, especially with lower energy β emissions. This makes data interpretation somewhat more complex.

H, C, P Emissions

A scintillation counter collating the energy spectrum of ß emissions into three channels would read the majority of 3H emissions in the low energy channel, 14C in the intermediate channel, and 32P in the high energy channel.

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