β-Particle Emitters, Determination of
Published Online: 15 SEP 2012
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Encyclopedia of Analytical Chemistry
How to Cite
Caprette, D. R. 2012. β-Particle Emitters, Determination of . Encyclopedia of Analytical Chemistry. .
- Published Online: 15 SEP 2012
Radioisotopes have unstable nuclei that decay in a statistically predictable manner with the release of radiation in the form of particles or electromagnetic waves. In β-decay ‘fast electrons’ called β-particles are released, which have a spectrum of kinetic energies ranging from near zero to a characteristic maximum (Emax). Emax can vary by orders of magnitude among different types of emitters. The low mass of β-particles causes them to lose energy quickly through molecular collisions, producing excitation of valence electrons, ionization of atoms, and in some cases specialized radiation (bremsstrahlung and Ĉerenkov radiation, and Compton scattering). Detection of β-particle emissions is complicated by the wide range of possible energies and the weakness of some emissions so that a single type of detector is inadequate for the analysis of radiation from all β-particle emitters.
Detectors are characterized by a sensitive volume in which particles interact with matter to produce a signal, an example of which is the emission of photons. The signal can produce a continuous current or a pulse. In pulse mode, it is desirable to choose a detector such that the pulse is proportional to the energy of the particle, giving the instrument the property of energy discrimination. This property permits spectral analysis and methods of quantitation that are based in part on energy spectra. Liquid scintillation counting (LSC) is the most accurate and widely used method for detecting low-energy β-particles. Usually, the information obtained by LSC must be corrected for signal loss due to absorption of incident particles or emitted photons. This signal loss is called quenching. For quantitation of low-energy β-radiation, quench corrections are usually made using the sample channels ratio (SCR) or an external standard method. All such calculations are subject to error and derived values are subject to the compounding of precision error during mathematical manipulation of the data.
Scintillation counting with organic crystal detectors is the method of choice for high-energy β-particle emitters. Gas-filled detectors are of limited value for β-particle detection, although windowless proportional counters are appropriate for detection of low-energy particles, especially in gaseous form. When neither energy discrimination nor precise quantitation is required, Geiger–Müller counters are acceptable for high-energy β-particle detection. They can detect all but the least energetic of low-energy emitters, although Geiger–Müller counters are not practical for the analysis of activity from such emitters because the signal is very weak, and the energies from incident particles are frequently absorbed before reaching the sensitive volume. Silicon-barrier semiconductors support energy discrimination and are of value for some applications. Visual imaging with photographic film is used for localization of β-particle emitter activity, as in labeling of biological tissues or macromolecules.