Footprints of the Atom

The Cambridge physicist C. T. R. Wilson was studying the formation of fogs in 1898 when he started on a train of ideas and discoveries which led ultimately to the perfection of the Wilson Cloud Chamber as a marvellous aid to nuclear physics. This first fog-making apparatus was, however, very simple just two glass jars connected by a pipe with a tap in it. One jar contained moist (saturated) air and the other was pumped empty of air. When the tap was opened the air expanded quickly into the empty jar. When gases expand very quickly they cool. You may have noticed that the air rushing out of a bicycle tyre when it is suddenly let down is quite cold. As a result of such cooling, clouds form in the moist air, since only a smaller amount of water vapour can be held by the air at the lower temperature.

Fogs cannot form unless the cooling is very marked. This is because small drops tend to evaporate again more easily than the large drops. It is therefore very difficult for any drops to begin to form at all unless they form immediately into large drops, as they would if the cooling were pronounced. The dust particles in ordinary air act as a very convenient beginning for the drops, because they are already of sufficient size for the drops formed on them to avoid re-evaporation. This explains why fogs and mists are much more common and more persistent near large manufacturing towns, where there is a lot of smoke and dust, rather than in the clear air of the countryside.

One of Wilson's cloud chambers, designed in 1912, is shown in figure 1. The moist air is contained in a glass-topped cylinder B by a close-fitting piston. The air in flask A is pumped out by a vacuum pump, while the stopper C is kept closed. When you want to operate the apparatus this stopper is pulled out so that the air beneath the piston rushes out into A. As there is nothing to hold up the piston it falls, and thus allows the damp air to expand and cool, and form a cloud.

Wilson soon discovered that even if he used very clean air he still occasionally got clouds with only a moderate amount of cooling when there was some radio-active substance or a source of the then recently discovered X-rays near his cloud chamber. He soon showed that this was because the air in his chamber was being 'ionized' by their radio-activity or by the X-rays.

Let us see what happens when air is ionized. The atoms of the various gases that make up air are all built of a heavy positive nucleus surrounded by very light negative electrons. The amount of positive charge on the nucleus is exactly balanced by the number of negative electrons surrounding it, so that the atom as a whole is electrically neutral. Although it takes quite a hard knock by a particle from one of the powerful 'atom-smashing' machines to break off a bit of the nucleus, the negative electrons are held to the atom only by the electrostatic attraction of opposite charges between negative electron and positive nucleus. It is therefore much easier to remove an electron from the atom, either by strong electro-magnetic radiation (X-rays) or by collision with another atomic particle like those shot by radio-active substances.

Before impact the atom was neutral, but after it has lost a negative electron it becomes a positively charged 'ion'. The electron it has lost is a negative ion; it is free to move until it joins another positive ion to form a neutral atom. When an atom is split in this way into two ions, positive and negative, we say it is ionized.

If ions of this type are present in moist air their effect is twofold. First, they attract many more normal atoms to themselves, with the result that a cluster of atoms is formed. Secondly, the charge on the ion reduces the tendency of small drops to evaporate. All this means that the charged ions in moist cooled air act as suitable nuclei, like dust and smoke, on which fog and clouds can form.

To make this clear we will follow up what happens when an alpha-particle passes through a cloud chamber immediately after expansion has taken place, and when the moist air has been cooled so that it is ready to form clouds. An alpha-particle is a fragment shot out of the nucleus of a radio-active atom. It is about four times as heavy as a hydrogen atom. It has a double positive charge, and it will travel a few inches in normal air before it is brought to a stop by repeated collisions with the atoms in the air. While it is travelling very fast through the air of a cloud chamber, its positive charge attracts many of the outer negative electrons of the atoms in the air, which may be drawn i out of their original atoms, leaving them ionized.

The passage of one alpha-particle thus leaves a trail of ionized atoms behind it all along its track, each of which is capable of acting as a centre about which a tiny drop of water can form. If the alpha-particle is shot into the moist air of a cloud chamber just after a cooling expansion has taken place, so that the air is supersaturated, a trail of little water drops appears in its wake, looking just like the cloud trails left by a high-flying aircraft.

The first time that an atomic nucleus was split artificially was in 1919 when Lord Rutherford turned a nitrogen atom into an oxygen atom by bombardment with alpha-particles. This nucleus-splitting reaction has been studied by Professor Blackett with the aid of a cloud chamber. He obtained an actual photo graph of the famous event. In this photograph a beam of alpha-particles appears as white trails crossing the chamber. One of them stops half-way as it hits a nitrogen atom in the air and the lighter trail of a proton (a positively charged hydrogen atom), knocked out of the nitrogen nucleus, moves off to the left, while the resulting oxygen nucleus gives a thick short trail to the right. We have come as close as we can to actually seeing the invisible atom.

(From an article by R. R. Campbell in Adventures in Science edited by B. C. Brookes.)