Beta radioactive tracers

The most common beta radioactive tracers for interwell studies are labelled with tritium 3H, 14C or 35S. All of them are usually measured by means of a liquid scintillation counting technique. A small volume of a liquid sample is mixed with a special solution known as a ‘scintillation cocktail’, commonly in a 20 mL light transparent (glass, polypropylene, teflon) vial. Beta particles cause emission of light when passing through and slowing down in the scintillation cocktail. These light pulses are registered by photomultipliers (PMTs) suitable for that particular photon wavelength. The light output in a pulse (light intensity) is proportional to the energy of the beta particle. This process is termed scintillation, and since it happens in liquid media, it is known as liquid scintillation.

The vial is placed inside an instrument, a liquid scintillation counter, which normally has two PMTs operating coincidentally to reduce the background. The liquid scintillation counter analyses the pulses from the PMTs and provides information about the energy of the beta particles and the rate of beta emission (activity) in the sample.

Pulses are sent to an analogue-to-digital converter where they are digitized and stored in an address memory according to their amplitudes, which are proportional to their beta energies (energy spectrum in a multichannel analyser).

In order to reduce further the background coming from natural radiation, a lead shield usually surrounds the PMTs and the vial while the sample is in the measuring position. Modern low background detection equipment also has a so-called active shield. In most cases it consists of a liquid scintillation detector surrounding the PMTs and the counting sample. This shield detector is operated in anticoincidence with the PMTs, such that any event which is registered both in the two PMTs and in the shield (cosmic rays, environmental radiation) detector simultaneously is rejected. In the case of simple non-spectrometric detection equipment (single channel analyser), the contribution of the background to the sample count rate can be further reduced by setting a counting window over only the interesting energy portion of the energy distribution. This is achieved by selecting narrow upper and lower limits. In the case of tritium, the upper gate should, for instance, be set at 19 keV.

Various processes may perturb the beta spectrum obtained in a liquid scintillation process. The most important of these are:

• Chemiluminescence: When different chemicals are mixed in the sample vial together with the scintillation cocktail, chemical processes may start which have relatively slow kinetics and which result in the emission of low energy photons. These photons may contribute to the very low energy end of the beta spectrum. Chemiluminescence may be reduced or completely removed by gentle heating of the vial to 50-60°C for some minutes before counting in order to speed up the chemical process.

• Phospholuminescence: When a sample vial with the scintillation cocktail is exposed to white light (daylight or lamp light), the light energy may be temporarily ‘stored’ and slowly released during sample counting (phosphorescence). Also, this light will contribute to the very low energy end of the beta spectrum. Therefore, counting samples should always be stored in the dark for a few hours before counting starts.

• Colour quenching: A coloured sample liquid may absorb some of the light emitted by the scintillator. Yellowish or brown colours are the heaviest colour quenchers. Hence, attempts should be made to remove such colours during the sample preparation process and before counting.

• Chemical quenching: Some components in the sample may kill the energy transfer process that takes place in the scintillation cocktail and which eventually results in light emission. Such chemicals absorb the energy and release it in the form of heat. Heavy chemical quenchers include, for instance, organic compounds containing oxygen and in particular chlorine.

• Physical quenching: Solid particles or non-transparent emulsions in the sample may prevent light from being detected by the PMTs.

All of these forms of quenching result in a shift of the energy spectrum towards lower channel numbers because the number of photons detected by the PMTs per beta decay is reduced. Figure 21 shows in principle the effect of quenching. Quenching may change from one sample to another. Evaluation of the quenching effect is necessary in order to calculate counting efficiency.

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FIG. 21. The effect of quenching on a liquid scintillation beta spectrum.

In summary, liquid scintillation counting requires careful sample preparation. Chemical separations are most often involved and when these procedures are optimized, very low detection limits may be obtained, ranging from 2 Bq/L for HTO to <0.02 Bq/L for S14CN-.