Crookes Saw a New World

by Melissa Lovin

In 1903 Sir William Crookes was experimenting in his London laboratory with the fluorescence of alpha particles on a zinc sulfide screen when he accidentally spilled some of the radium he was using as his alpha source. Crookes, in trying to locate the radium on the screen, viewed it under a microscope. He was surprised to see that, instead of the expected uniform glow from the alpha particles, he observed distinct flashes of light from each individual alpha particle.

Eager to share his amazing discovery, Crookes set out to build a device that would allow others to view the magnificent view. The end result was a simple brass tube with a zinc sulfide screen at the end. A small speck of radium salt was put in front of the screen, and a simple lens to aid in viewing.

The radium was placed at the end of a needle which could be adjusted closer to and farther from the screen by simply turning a screw. When the radium was moved toward the screen the flashes became brighter and more numerous. When it neared the screen Crookes observed that the flashes looked like a "turbulent, luminous sea", being so close together and of such luminosity that he was unable to distinguish the separate particles.

Crookes called his new invention the Spinthariscope which he derived from the Greek work 'scintillation', taking it from Homer's Hymn to Apollo.

At a special party in honor of the British Royal Society on May 15, 1903, the highest ranking individuals from society as well as the scientific world first viewed Crookes' spinthariscope which was touted by some as the piece de resistance.

The spinthariscope soon become the the rage of London society as the must-have of the new century. Ladies and Gentlemen carried them in their purses and pockets, anxiously awaiting the perfect moment to produce their new toy and demonstrate it's brilliance.

It is also now hailed as the first radiation counter. While the electrometers, electroscopes and photographic plates were integrating instruments, the spinthariscope was detecting individual decay events.

In laboratory situations, a microscope was used instead of Crookes' simple lens. Scientists used the spinthariscope to count the scintillations, in what was a tedious, difficult task. However, it was the spinthariscope that permitted the first application of coincidence counting in the nuclear sciences. It was used in 1924 by Geiger and Werner to count the actual number of scintillations, revealing the number of alpha particles in the process.

In addition to that important application, Ernest Rutherford an Hans Geiger also used the spinthariscope to measure the charge on an alpha particle, which helped the two scientists gather evidence that the alpha particle was a helium nucleus. This, in turn, caused a refinement in Avogadro's values for the number and charge on an electron. A spinthariscope was also responsible for leading Rutherford to propose his nuclear model of the atom.

What started out as a chance encounter caused by an accident, William Crookes' spinthariscope has gone from mere plaything to a forerunner for some of the greatest, most basic atomic discoveries of the 20th century.