Stories About Chemistry
87. Activation Analysis
Natural arsenic is a very stable element. In any case, nobody has ever detected radioactivity in it.
Arsenic has another peculiar feature. It is what one might call a “lone” element. Many of the others are mixtures of two, three, or more isotopes. For example, tin consists of ten different kinds of atoms and they are all found in nature.
But arsenic is single. Its nucleus contains 33 protons and 42 neutrons, and this combination is a very stable one.
But if an extra neutron is in some way induced to enter this nucleus, its stability vanishes, and a radioactive isotope of arsenic forms, an isotope that can be detected without chemical methods. All that is needed is an instrument for registering radioactive emission. The larger the amount of active arsenic, the more intensive will be its radiation.
Such is the basic principle of the simple but very important method of activation analysis. It enables the determination of infinitesimal amounts of substances, fractions of a gram represented by numbers with 10 or 12 zeros after the decimal point. To do this the object under study had only to be irradiated with a beam of neutrons, and the intensity of the radiations emitted by the resulting radioactive isotopes measured.
That was how the historians discovered the circumstances of Napoleon Bonaparte’s death. Is it not a splendid example of aid profferred by the exact sciences!
To modern analysts activation analysis is an all-seeing eye. It easily discerns what hardly any other analytical method can detect.
Pure germanium is commonly known to be an excellent semiconductor. But if it contains only a few atoms of such an impurity as, say, antimony, as little as one antimony atom per million million germanium atoms, in fact, the semiconducting properties of germanium entirely disappear.
That is why germanium has to be checked very carefully for impurities, and this can be done only with the aid of activation analysis.
And so neutrons speed on their way to a plate of germanium. Chemists know that it contains some amount of antimony. Maybe so little that it can be neglected, but maybe so much that the “pure” germanium will have to be rejected. The atomic nuclei of germanium and antimony react differently to neutrons. The former allow them to pass-by quite indifferently, but the latter absorb them avidly. For this reason only radioactive isotopes of antimony are formed. The rest is done by radiation counters. Then we can tell for sure whether there is much or little antimony in the germanium.