Liquid Scintillation Counting (LSC) or Spectrometry (LSS) is a measuring technique which is especially suitable for the determination of low energetic β-emitters. It is the method of choice for radionuclides like 3H (Eβmax = 18 keV), 14C (Eβmax = 156 keV), 35S (Eβmax = 167 keV) and 63Ni (Eβmax = 67 keV) for more than sixty years.
However, liquid scintillation counting technology is capable to detect all processes emitting light photons, both, directly and indirectly. Therefore, the method is also applicable for high energetic β-emitters, electron capture nuclides and α-emitters as present in recent decommissioning activities.
Due to the measurement as liquid solution under 4π-geometry, nearly without absorption and self-absorption, it is possible to determine also low energetic β-emitters with high counting efficiencies. In liquid scintillation counting, organic aromatic compounds are dissolved in a suitable solvent, known as scintillation cocktails, instead of solid crystals. Scintillators applied are based on electronic transitions from organic aromatic molecules with symmetric properties and π-electrons in resonance.
The energy of the in-coming radiation is preferably transferred to the solvent molecules by electronic excitation before being migrated free of radiation and trapped finally by the scintillator molecules. Commercially available scintillation cocktails additionally transfer the energy from the primary to a secondary scintillator (wave length shifter) emitting photons with a wave length of 400 to 420 nm (prompt fluorescence). They hit the photocathode (mostly two in opposite direction in coincidence) and liberate electrons effectively. Amplified, they create an electrical pulse with a pulse height being proportional to the energy of the decaying particle. Thus, in contrast to proportional counting, LSC presents a semi-spectrometric method. By applying upto three energy windows, the spectra can be unfolded comparable to γ-spectrometry.
The mechanism of energy transfer for the production of photons is presented below (fig. 1).
Figure 1: Mechanism of energy transfer in liquid scintillation counting
Due to the intimate contact between sample and scintillator molecules as homogeneous solution, the measuring efficiency in Liquid Scintillation is extremely high. α-particles as well as medium and high-energetic β-emitters are detected with 95 to 100 % efficiency practically quantitative. Electron capture nuclides as present in decommissioning activities (55Fe, 41Ca) are measured upto 40 % mainly by their Auger electron emissions, if higher than 1 keV.
A survey of typical values is given in table 1.
Table 1: Typical counting efficiencies for various radionuclides by LS
* Lower values for devices with single photomultiplier PM due to luminescence discrimination in the lower spectral part
If suitable calibration standards and TDCR are unavailable, a good approach for the efficiency is 100 % for α-emitters without α/β-discrimination and 95 % for medium and high energetic β-emitters for unquenched samples in an open energy channel. Lower counting efficiencies compared to the theoretical values arise from quenching, but can as well be caused by pulse height discrimination settings and pulse shape discrimination (PSD) when counting in an optimized α-channel.