What he saw stopped him in his tracks. Instead of the expected uniform glow around each grain, he saw a multitude of individual flashes of light through his microscope. These flashes were the individual impacts of alpha particles, a phenomenon never seen before.
Crookes, an elegant Victorian experimentalist and showman, designed a small portable device to demonstrate his discovery. A brass tube held a magnifying lens at one end. In focus at the other end, a zinc sulphide screen. A speck of radium bromide was held on a needle point, inside the tube, at an adjustable distance from the phosphor screen. To use this apparatus, which Crookes named a Spinthariscope, users first had to allow their eyes to become dark adapted. Then, looking through the lens, the individual flashes of light, or scintillations, could be observed. Moving the radium source nearer to the phosphor screen increased the scintillation rate per unit area - in Crookes` more descriptive words:
"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."
The first public demonstration, at a gathering of the British Royal Society, on the 15th of may 1903, caused a minor sensation. Radioactivity could actually be "seen" for the first time, and the "turbulent luminous sea" was an amazing sight to be behold. Soon, spinthariscopes were being produced commercially, as a scientific demonstration item. Gentlemen and Ladies of the time carried them, to appear up to date with the wonderful progress of science.
It used to be easy to see this phenomenon. I remember using my microscope in the dark, as a child, to view the surface of the luminous paint on a clock hand. Radium 226 based paints are no longer used for luminous timepieces, the phosphors in modern luminous paints merely store incident light. The paint on old timepieces has by now degraded - the phosphors properties destroyed by the radiation. However, the radium contained in the paint is still there, and should be considered a severe health hazard. Please don`t try to remove any of this, now crumbly, material from an old timepiece. The inhalation of a microscopic particle of this material could cause lung cancer.
A much safer radioactive source is used inside most smoke detectors. A small amount of americium dioxide is present within a bonding compound on a metal substrate within the housing. Scratching, bending or burning the part containing the americium would be extremely hazardous. Please don`t dismantle smoke detectors. Americium, element 95 on the periodic table, is an artificial element. Man-made within nuclear reactors, americium is not naturally found on earth (although huge amounts are created and thrown into inter-stellar space during supernovae).
The isotope of americium used within smoke detectors is Am241. It decays with a half life of 432 years into another artificial element, neptunium, Np237. For each americium atom, the transmutation into neptunium is a very energetic event. The nucleus spits out an alpha particle (helium nucleus) travelling at an appreciable fraction of the speed of light. If this alpha particle is stopped by a phosphor, it`s kinetic energy is transformed as a flash of light from the excited phosphor.
Phosphors suitable for detecting alpha particles can be found coating the insides of fluorescent light tubes and cathode ray tubes. Obtaining the phosphor from either of these sources is very hazardous: Cuts from broken glass shards & poisoning by mercury vapour inhalation and other heavy metals. I can`t recommend it.
Risking all, I ripped apart a smoke detector and smashed an oscilloscope CRT. Sprinkling a tiny amount of the green phosphor onto the americium "window" and observing at modest magnification, I was rewarded by a veritable ocean of scintillations.
An atomic bomb can be constructed from smoke detectors. Americium 241 is a fissile material, the critical mass being in the region of 80 kg. To amass such a mass would require billions of smoke detectors to be sacrificed. Also problematical is the heat generated in the sub-critical assembly of americium by it`s radioactivity - about 10kW!
Please don`t experiment with large quantities of fissile material in the home, and always keep pets and small children away from any just-sub-critical assembly