Liquid scintillation solutions share with other scintillating materials the property of converting into visible light the energy deposited in them by ionizing radiation. In theory, if a sample of a beta emitter is dissolved in a liquid scintillator solution, every beta particle emitted will be absorbed completely because the range of penetration of beta particles in liquids is quite short (ranging from 0.008 millimicron for ³H to 7.9 millimicrons for ³²P in a medium of unit density). The kinetic energy of the beta particles is largely used up in the ionization and excitation of the most abundant molecular species present, the solvent in which the scintillating material was dissolved. A fraction of the energy thus expended by each beta particle is transferred from excited solvent molecules to scintillator molecules; thus the electrons in the atoms of the scintillator molecules are raised to an excited (higher energy) state. When these electrons return to the ground, or unexcited, state, a fraction of them emit a photon of light. Thus each beta particle produces a burst of photons. [Illustration: Figure 18 _Technician placing a tray of samples in a liquid scintillation counter. The radioactivity of each sample is recorded as the trays revolve._] If a vessel containing the liquid scintillator and the radioactive sample is placed near a suitably sensitive instrument known as a photomultiplier tube, each burst of scintillator photons activates this device and causes it to release a burst of photoelectrons. Each burst of photoelectrons is multiplied successively in a series of electronic steps; as a result, there is a suitably large electrical-output pulse to be recorded. One of the principal advantages of the liquid scintillation method is the ease of sample preparation. We need only transfer a known volume of a liquid sample or weigh a given mass of a solid sample into a sample bottle, add a known amount of the liquid scintillator solution, and stir until there is a homogeneous solution. Samples thus prepared are placed in a refrigerated counting apparatus. After a short waiting period to allow time for the samples to cool and for a natural, short-lived phosphorescence (due to exposure to room light) to subside, the samples are ready to be measured. [Illustration: Figure 19 _Placing radioactive samples in a refrigerated unit for liquid scintillation counting._] One disadvantage of liquid scintillation counting is that different compounds show different degrees of quenching (loss of emitted photons), and the effect must be checked for each class of compounds in each concentration range. This checking is usually done with an internal standard technique, the sample being counted before and after a standard, or known, emitter is added.