One major issue in liquid scintillation spectrometry is quenching. Quenching is defined as the irreversible absorption of decay energy or photons during the energy transfer from the decaying particle to the photocathode. Quenching shifts the whole pulse height spectrum towards lower energies and consequently results in the reduction of the counting efficiency for radionuclides with continuous energy like β-emitters (fig. 2).
The main causes are:
- Chemical quench: The photoemission is reduced by substances taking over the excitation energy of solvent molecules without transferring them to the scintillator. Chemical quenching prevents the energy transfer from decay particle to the scintillator. It is in practice the most frequent kind of quenching.
- Color quench: The number of photons produced may be diminished by light absorption from dyestuffs or turbidity. Color quenching hinders the transfer of the pre-formed photons from the primary to the secondary scintillator and from the secondary scintillator to the photocathode.
Figure 2 Pulse height spectrum of samples with different degree of quenching
Quenching for β-emitters reduces the counting efficiency. As the degree of quenching cannot be recognized directly, suitable corrections must be applied for quantitative activity determination of the sample. More details on the quenching mechanism can be found in [Moebius and Moebius 2012].
Current methods for quench correction are:
- Internal standardization
- External standardization
- Channel ratio method
- Shift of the inflexion point of the Compton edge (H-number)
- Shift of the centre of gravity of the spectrum
- Comparison with standard samples
- “Instant DPM” method
- CIEMAT-NIST method
The channel ratio method represents a universal method for quench correction. It does not depend on external standardization and can therefore be used in mobile instrumentations. The recently commercially applied TDCR-method allows quench correction without standard and external source, but using devices with three PM tubes.
» Internal standardization
The ratio between the difference in the count rate of the sample before and after the addition of a known amount of standard of the same isotope and its activity is a measure for the efficiency ε. As sample and standard are affected equally by quenching, the activity of the original sample can be calculated through:
Internal standardization is a universal method which allows as well quench correction in heterogeneous and turbid samples.
» Channel Ratio Method
For the channel ratio method the sample is measured in two different energy regions of the pulse height spectrum (fig. 3).
While channel 1 only detects the high energetic pulses, channel 2 measures the whole spectrum. With increasing quenching, the number of pulses in the high energy channel 1 is much more reduced than the total count rate in channel 2. Thus, the ratio of the count rates varies and is a characteristic measure for the degree of quenching. A calibration curve for the channel ratio versus the corresponding efficiency is plotted by preparing a set of standard samples with different quenching. The detailed procedure for this universal correction method is described in 2.1.1.
Figure 3: Channel setting for the channel ratio method
The CIEMAT-NIST method is based on mathematical models, which apply the fact that the distribution of the produced photoelectrons from a secondary electron multiplier is known (Poisson distribution). For β-emitters e.g. the continuous β-spectrum has to be calculated. The unknown free parameter on geometry and counting conditions is determined by measurements of a tracer nuclide (mostly Tritium) under equal experimental conditions. It only needs a single calibration measurement and determines the efficiency relative to the standard.
The TDCR-method (Triple to Double Coincidence Ratio), as originally introduced by Broda [Broda et al. 1988], does not need an external source or standard. It is an absolute (primary) method, but requires three photomultipliers ABC for the determination of the TDCR as free parameter. The introduction of three photomultipliers in coincidence additionally allows a nearly coincidence free counting. Devices using this option are commercially available since recent years (HIDEX 300 SL and 600 SL). They have been successfully used for various applications (see LSC2017 Conference in Copenhagen) and present nowadays an established technique [Broda et al. 2007]. The method is therefore described more detailed in a separate chapter (1.5.).
Broda R., Pochwalski K. and Radoszewski T. 1988: Calculation of liquid-scintillation detector efficiency; Appl. Radiat. Isot. 39 (1988) pp159
Broda R., Cassette P. and Kossert K. 2007: Radionuclide metrology using liquid scintillation counting; Metrologia 44 (2007) 36-52
Moebius S. and Moebius T. L. 2012: Handbook of Liquid Scintillation Spectrometry, DGFS e.V. and Karlsruhe Institute of Technology, Karlsruhe 2012