Describe a measurement that can determine whether a proportional tube is operati
ID: 1775170 • Letter: D
Question
Describe a measurement that can determine whether a proportional tube is operating in the true proportional region. Be explicit about which quantities are measured and how the data are used to make the determination. Assume that you have available any necessary type of radioisotope source. Describe a measurement that can determine whether a proportional tube is operating in the true proportional region. Be explicit about which quantities are measured and how the data are used to make the determination. Assume that you have available any necessary type of radioisotope source.Explanation / Answer
The three methods are: (1) Methods Based Upon Gas Ionization (2) Methods Based Upon Excitation and (3) Methods Based Upon Exposure of Photographic Emulsions.
As a charged particle passes through a gas, its electrostatic field dislodges orbital electrons from atoms sufficiently close to its path and causes ionization.The ability to induce ionization decreases in the order
> > (10 000: 100: 1)
Accordingly, - and -particles may be detected by gas ionization methods, but these methods are poor for detecting -radiation. If ionization occurs between a pair of electrodes enclosed in a suitable chamber, a pulse (current) flows, the magnitude of which is elated to the applied potential and the number of radiation particles entering the chamber The various ‘regions’ shown in Fig. 13.10 will now be considered.
In the ionization chamber region of the curve, each radioactive particle produces only one ion- pair per collision. Hence the currents are low, and very sensitive measuring devices are necessary. This method is little used in quantitative work, but various types of electroscopes, which operate on this principle, are useful in demonstrating the properties of radioactivity.
At a higher voltage level than that of the simple ionization chambers, electrons resulting from ionization move towards the anode much more rapidly; consequently they cause secondary ionization of gas in the chamber, resulting in the production of secondary electrons, which cause further ionization and so on. Hence from the original event a whole torrent of electron reaches the anode. This is the principle of gas amplification and is known as the Towensend avalanche effect, after its discoverer. As a consequence of this gas amplification, current flow is much greater.
As can be seen in Fig. 13.10, in the proportional counter region the number of ion-pairs collected is directly proportional to the applied voltage until a certain voltage is reached, when a plateau occurs. Before the plateau is reached there is a region known as the limited proportion region, which is not often used in detection and quantification of radioactivity and hence will not be discussed.
The main drawback of counters that are manufactured to operate in the proportional region is that they require a very stable voltage supply because small fluctuations in voltage result in significant changes in amplification. Proportional counters are particularly useful for detection and quantification of -emitting isotopes, but it should be noted that relatively a few such isotopes are used in biological work.