Scintillation counter

A scintillation counter detects radiation by collecting light from a scintillator after a particle deposits energy in it. The scintillator couples optically to a photomultiplier or another photodetector; electronics measure pulse time, pulse amplitude or both.

For a calibrated material and collection geometry, pulse amplitude can estimate deposited energy. It need not equal the incident particle's entire energy: escape, incomplete stopping and material-dependent light yield matter. Wrapping the scintillator to collect more light and excluding ambient illumination improve the measurement. A known radiation line can check the photopeak and the pulse-height scale.

Liquid scintillation counting measures radiation through light produced in a liquid scintillator containing the radioactive sample. Energy deposited in the medium leads to optical emission, which photomultipliers detect as pulses. The apparatus is a scintillation counter, but the measured pulse rate still needs a model connecting it to radioactive activity.^[ref]

In an idealized one-count-per-decay model with background subtracted and other corrections negligible,where $A$ is radioactive activity and $\epsilon$ is detection efficiency. A hypothetical net rate of $100{\mathrm{s}}^{-1}$ with $\epsilon =0.40$ corresponds to $A=250\mathrm{Bq}$. At the same activity, an efficiency of $0.20$ would produce only $50{\mathrm{s}}^{-1}$. A smaller count rate need not mean that fewer decays occur.

A motivating measurement example is L. L. Lucas's comparison of tritium-containing water standards. Liquid scintillation counting supplied massic activity ratios that could be compared with independently recorded mass-based dilutions. Agreement within stated measurement uncertainties supports consistency of the standards; it does not turn the counting apparatus into an uncertainty-free activity meter.^[ref]