Lab Exercise - Measurement of Isotopes Using Čerenkov Radiation

Developed by

Center for Radiochemistry and Nuclear Materials
Department of Chemistry
Loughborough University

Learning Goals

Here the learning goals and purpose of the exercise should be presented in a reasonable way.

Explanation and Exercise Guide

Theory

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.

Experimental Procedure

Part 1 - The Volume Effect

1. An aqueous solution of ³²P solution has been provided. Place 1 cm³ of this solution in a polythene 20 ml scintillation vial and count in the scintillation counter in the ³H channel, which should be set so that the full tritium range is displayed in channel one and a third of the tritium range in channel two (protocol 28)
2. Set up a further 14 scintillation vials with 1 cm³ of ³²P soln. Add 1 cm³ of H₂O to one vial, 2 cm³ to the next, 3 cm³ to the next, and so on until 14 cm³ has been added to the final vial
3. Count them all on the scintillation counter using protocol 28
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 cm³ of the ³²P solution into a scintillation vial and make up to 12 cm³ with distilled water
2. Count the sample using protocol 28
3. Add successively 0.1, 0.1, 0.2, 0.4, 0.4, 0.6 and 0.6 cm³ of the 0.25% v/v of methyl violet in alcohol solution provided to the vial of ³²P
4. Record the counts for after each addition in the both of the channels
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 expt using the 0.25% v/v of methyl orange in alcohol solution provided
7. The quench curves should be identical until a very high degree of quenching is obtained, after which some variation in the two curves may be obtained

Questions for the Students

OtherHow to do calculations or other important aspects for the theory that is not directly related to exercise.

Safety Aspects

• All work with ³²P must be carried out in the spill tray provided, and a lab coat, gloves and safety spectacles must be worn. Heating of solutions should be carried out using a dry bath housed in the fume-cupboard.
• Planchetted samples should be carried to the Geiger counter on a tray and should only be handled with tweezers.
• DO NOT dispose of any liquid or solid waste down sinks or in waste bins. Place in red waste buckets provided.

Equipment

Description of the equipment needed and used during the exercises.

Preparation for the lab Supervisor

The preparation that the lab Supervisor needs to do to ready the lab.

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