The Atomic Bomb Ring

The idea that matter is composed of vast quantities of atoms dates back all the way to the Ancient Greek philosophers Leucippus and Democritus in the fifth century BC. They proposed that the world around us is ultimately composed of indivisible entities they called atoms from the Greek ‘a’ meaning ‘not’ and ‘tomí’ meaning ‘to cut’. But atoms, as we know them today, definitely do have subcomponents, so it certainly is possible to split the atom. Indeed, in 1947 an American cereal company Kix offered anyone with fifteen cents and the top off a cereal box the chance to see atoms splitting right before their eyes.

Incredibly, the Atomic Bomb Ring really did allow the children of the atomic age to witness the decay of atomic nuclei as they munched their breakfast cereal. Attached to the ring was a bullet-shaped capsule that separated into two halves. As the advert says, one half contained a secret message compartment, but the other half ‘the warhead’ contained a hidden atom chamber. This silver chamber was a device invented four decades earlier by renowned British physicist Sir William Crookes.

Scintillations of Fire

Crookes is remembered for his development of an electrical discharge tube—the Crookes tube—which played a key role in the discovery of both X-rays and the electron. It would later evolve into a fundamental component of old CRT television sets and computer monitors.

In 1903 Crookes had obtained a tiny sample of radium, which at the time was the most valuable substance on Earth. The newly-discovered radium was highly radioactive and its mysteries were still being revealed by researchers. Crookes held a screen coated with zinc sulphide close to the sample and the rays emitted by the radium would cause it to glow. While studying this effect Crookes spilt some of his precious sample, so he searched for specks of radium with his zinc sulphide screen, holding a magnifying lens to see any faint glow on the screen. But rather than a faint continuous glow, he could see individual flashes of light. Crookes was astonished, each flash was produced by a single alpha particle emitted by an atom of radium.

Crookes turned his discovery into an amusing toy so that others could see the wonders of the atom. At one end of a brass tube he fitted a zinc sulfide screen with a speck of radium salt on a needle tip about one millimetre in front. Then the sparkling display on the screen could be viewed through a lens from the other end of the tube. By turning a screw the distance between screen and radium sample could be gradually altered. Crookes described the effect of such an adjustment in The Chemical News (Crookes 1903):

When the scintillating points are few there is no residual phosphorescence to be seen, and the sparks succeeding each other appear like stars on a black sky.
 
On bringing the radium nearer the screen the scintillations become more numerous and brighter, until when close together the flashes follow each other so quickly that the surface looks like a turbulent luminous sea.

Crookes gave his remarkable little instrument a suitably wondrous name. He called it a spinthariscope, taking inspiration from the Greek word for spark or scintillate (spítha) in these lines from the Homeric Hymn to Apollo:

Here from the ship leaped far-darting Apollo
Like a star at midday, while from him flitted scintillations of fire,
And the brilliancy reached to heaven.

Spinthariscopes are still available today, but they no longer contain radium. The image below is a spinthariscope manufactured by United Nuclear. It contains a tiny sample of radioactive thorium ore from a mine near Great Bear Lake in Canada.

The Structure of the Atom

Although Crookes thought of the spinthariscope simply as an amusing toy, it would play an important role in physics. The spinthariscope was the basis for an experiment proposed by Ernest Rutherford at the University of Manchester in 1908. Rutherford suggested that Hans Geiger and Ernest Marsden should investigate the effects of bombarding gold foil with high-energy alpha particles from a radioactive source. The tool they used to study the alpha particles was essentially a spinthariscope. 

Sitting in a darkened room Geiger and Marsden counted the flashes produced by alpha particles hitting a zinc sulphide screen following their encounters with gold atoms in the foil. This was one of the most important experiments in the history of physics as it enabled Rutherford to deduce that almost all the mass of an atom is located in a tiny nucleus that is surrounded by orbiting electrons. Crookes had given physicists the key to the Nuclear Age.

We now know that each flash seen in a spinthariscope follows the transmutation of an atomic nucleus as it undergoes radioactive decay by emitting a high-energy alpha particle. In the spinthariscope offered by United Nuclear each decay transforms a nucleus of element number 90 thorium into a nucleus of element number 88 radium. Furthermore, the emitted alpha particle is formed of two protons and two neutrons and is identical to the nucleus of a helium atom. So it wasn’t just the sight of splitting atoms that was on offer with a Kix cereal packet, but the splitting of the tiny atomic nucleus.

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