Safety Assessment

• 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.

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 for the Measurement of Isotopes using Cerenkov Radiation