CINCH NucWik

Developed by
Center for Radiochemistry and Nuclear Materials
Department of Chemistry
Loughborough University

The volume effect and colour quenching in Čerenkov counting are demonstrated via channels ratio (or windows ratio) method, using ³²P standard. The students get familiar with general features of Čerenkov counting and know how to utilise LSC without using a scintillation cocktail solution.

Čerenkov radiation is electromagnetic radiation, where a charged particle (for example, an electron from beta decay) passes a dielectric medium (e.g. water) with a greater speed than the light has in that medium (see the figure below). This can be observed as a blue glow in the medium. Modern liquid scintillation counters are suitable counting instruments for Čerenkov radiation. Tritium yields, on average, about 28 photons per disintegration extending to a maximum of 90. This is in the Čerenkov range, at least for the higher energy emitters, and instruments developed for determining tritium at reasonably high efficiencies are suitable for measuring Čerenkov radiation. Therefore, it is possible to count β-emitters in aqueous solution with reasonably good efficiencies, although counting efficiency is clearly dependent on the ratio of the number of β particles emitted with energies higher than the Čerenkov threshold, to the total number emitted. An advantage of this type of counting over normal scintillation counting is that many problems of solubilisation are avoided. It also makes possible the counting of samples in strongly acid or alkaline solutions without any special sample preparation. Since Čerenkov light is highly directional, counting will obviously be influenced by the geometry of the system, and since the light is at the violet end of the spectrum extending into the UV, the photomultipliers used in the equipment should, preferably, have quartz windows together with a high quantum efficiency. The liquid scintillation counter has two such photomultipliers, the outlets of which are connected to a pulse summation circuit and logarithmically amplified. The signals are then fed at the same instant in time. Such a circuit has the effect of reducing background noise but is something of a disadvantage where the light emission is not isotopic since there is a loss of counting efficiency.

Colour quenching is also likely to occur, but because Čerenkov radiation arises from the coherent disturbance of many adjacent molecules chemical quenching is not a problem. As with scintillation counting the most important methods of correcting for colour quenching are the channels ratio and external standard methods. It is the purpose of this experiment to investigate the volume effect and colour quenching.

Part 1 - The Volume Effect

1. An aqueous solution of ³²P solution has been provided. Write down the activity of ³²P in the solution and the reference date for the value. Place 1 ml of this solution in a polythene 20 ml scintillation vial. Measure this sample in the scintillation counter in the ³H counting window (i.e. range of channels), which should be set so that the full tritium range (peak) is displayed in window one and a third of the tritium range (peak) in window two
2. Set up a further 14 scintillation vials each of with 1 ml of ³²P solution. Add 1 ml of H₂O to one vial, 2 ml to the next, 3 ml to the next, and so on until 14 ml has been added to the final vial
3. Measure them all on the scintillation counter using the same protocol as with the first measured sample
4. Plot a graph showing the counting efficiency as a percentage of the maximum activity
5. Deduce the optimum counting volume.

Part 2 - Colour Quenching

1. Put 1 ml of the ³²P solution into a scintillation vial and dilute it up to 12 ml with distilled water
2. Count the sample using the same measurement protocol as in Part 1
3. Add successively 0.1, 0.1, 0.2, 0.4, 0.4, 0.6 and 0.6 ml of the 0.25% v/v of methyl violet in alcohol solution provided to the vial of ³²P (instead of methyl violet in alcohol, some other violet or blue quenching chemical can be used)
4. Record the counts for ³²P after each addition in the both of the counting windows
5. Work out the channel ratio and plot a graph showing the activity of the solution as a percentage of the unquenched sample against the channel ratio
6. Repeat the experiment (steps 1-5) using the 0.25% v/v of methyl orange in alcohol solution provided
7. The quench curves obtained from using two different quenching reagents should be identical until a very high degree of quenching is reached, after which some variation in the two curves may occur

Can you figure out the reasons behind the optimal sample volume, why this volume gave the best counting efficiency? Why there are differences between the two quenching curves?

• All work with ³²P must be carried out in the spill tray. Lab coat, gloves and safety spectacles must be worn.
• MSDS (Material Safety Data Sheet) for each of the used reagent gives necessary information about working safely with that particular chemical, and its disposal after the work. The MSDSs are easily available from the websites of the chemical manufacturers and authorities.
• Dispose all liquid and solid waste in the provided waste buckets, not down sinks or in waste bins.

• Liquid scintillation counter
• 100-1000 µl autopipette

• Distilled H₂O
• 0.25% v/v methyl violet in alcohol solution
• 0.25% v/v methyl orange in alcohol solution

(or some other available quenching agents with violet/blue and orange colours)

• polythene LSC vials
• tray
• 100-1000 µl pipette tips
• permanent marker

• 20 ml of ³²P solution with activity level adequately high for obtaining representative results during short measurements

Acceptable working report should contain at least the following features:

• brief description of the sample preparation and measurement procedures
• used equations and calculation example for one sample and measurement
• efficiency/volume curve with used data values in a table
• two quench curves with used data values in a table
• discussion about the effects of sample volume and colour quenching on the counting efficiency of an LSC counter, by using Čerenkov counting mode

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