The term radioluminescence also encompasses x-ray fluorescence, which is discussed below. It refers to both the fluorescence and phosphorescence produced by nuclear radiation (e.g., gamma rays). This definition can be expanded to include excitation by alpha particles, as in the spinthariscope. Alpha particle excitation can also be classified as anodoluminescence, since alpha particles are positively charged! Fluorescence is a type of luminescence that lasts about as long as the excitation is maintained.
Actually, by one definition, fluorescence vanishes 10-8 seconds after excitation ceases. Light emission that persists beyond is usually (although there is something called delayed fluorescence) referred to as phosphorescence. Though not arbitrary, this definition applies only to certain molecular species. Another definition relates fluorescence to the emission of photons with no net change in the spin quantum numbers. If there were a change, the luminescence would again be called phosphorescence.
Luminescence is the emission of light unrelated to the black-body radiation produced by heating an object to incandescence. Fluorescence, therefore, is cold light. It is produced as a result of an atom's first absorbing radiation energy at one wavelength, exciting electrons to a higher energy state. This is followed by a rapid emission of photons, usually at a longer wavelength (Stokes' Law), as the excited electrons begin their return to the ground state. Increasing the temperature of a substance generally reduces its fluorescence. Reducing its temperature generally increases both its fluorescence and the persistence time of any phosphorescence.
Sir William Crookes (1832-1919)
Experimental chemist who worked on the separation of rare earths. His earlier spectroscopic studies led to his discovery of element 81, thallium. Crookes' eclectic mind spanned the gamut from studies in water supplies and waste treatment, through the photographic arts, to solar physics, high vacua, and atomic radiation. While visiting a business associate to discuss Crookes' patented method of gold extraction by amalgamation, he confronted a cattle disease by recommending carbolic acid, then a new chemical just coming into use, as a disinfectant. In 1869, Crookes wrote an article, "On the Spectral Phenomena of Opals," describing the application of his binocular spectrum microscope to the study of transmitted and reflected light from opals. A few years later, Sir William invented the radiometer, which today is almost as commonly found as Silly Putty. Then, in 1903, using a small sample of a radium compound given him by a certain Mme. Curie, he invented the spinthariscope. (Double click to hear him speak.)
The apparently simple construction of the spinthariscope belies the potential within. A fragment of a radium compound mounted several centimeters from a zinc sulfide coated screen bombards the phosphor with alpha particles (beta and gamma radiation too). The particles excite the phosphor to glow in pinpoint locations at random. Since the phosphor phosphoresces quite well, the apparatus is kept in total darkness until viewed. Room light excites the screen and washes out the scintillations being sought. Enclosing the apparatus within a light-tight housing helps considerably. The alpha particles are emitted from the radium nucleus as it decays. Radium has a half-life of 1600 years and decays into radon, also an alpha emitter, and down the chain to a radioactive isotope of lead. Despite the long half-life of radium, because of the eventual oxidation of the zinc sulfide due to moisture, a spinthariscope will not last indefinitely. Though basic, this design presaged the scintillation counter and the x-ray image intensifier.
Note the similarity of this 20th century electronic image amplifier to the 19th century spinthariscope. To limit x-ray exposure for patients, electronic enhancement tubes were first introduced in the 1950s. Previously, radiologists would have to thoroughly accommodate to darkness and peer at glowing screens to discern motility under x-rays. Using the retina's rods reduces visual acuity. By amplifying the visible image thousands of times, the radiologist could make out finer details important for diagnosis. The input phosphor surface was composed of ZnS:CdS:Ag, which was used in the earlier x-ray screens. With 25-35 kV dc applied to the tube, electrons generated at the photocathode accelerated to the output phosphor and were viewed optically. The focusing adjustments were used to produce sharper and enlarged images as needed. The arrangement is simply to have the x-ray source at the extreme left with the patient sandwiched between source and intensifier. The input phosphor glow is due to x-ray fluorescence. The output phosphor glow is due to cathodoluminescence from the bombarding electrons.
With an x-ray image intensifier and an x-ray tube so close at hand, why not demonstrate how the devices can be combined to x-ray a hand! The x-ray tube was powered at less than 50 kV dc. At this relatively low voltage, soft x-rays are generated. This increases the absorbance of the radiation by soft tissue, reducing the contrast between bones and surrounding muscles, skin, etc. But where to find some hapless volunteer?
(Double click to activate the video. Click once more to stop the looping.)
This new video demonstrates the appearance of a spinthariscope in action. To record the low-light levels produced by radium's alpha particles bombarding the phosphor-coated screen required the image intensifier camera that I use for recording bioluminescence (see LED Fluorescence).
To acquire this video, I used a spinthariscope made by The Radium Company of Colorado. (They went out of business in 1924.) The spinthariscope's viewing lens was removed and the screen end was placed in a light-tight tube appended at the proper distance to an achromatic lens. The lens, in turn, was attached to the image intensifier. I found that microscopes absorbed too much light so I opted for a 25.4 mm diameter two-element, coated achromat with a 35mm focal length. The results are a fair representation of what you would see if you looked into the spinthariscope. Some of the scintillations were produced by electronic noise from the recording system, but the great preponderance is from alpha bombardment. There are at least two hot pixels as well.
Incidentally, Ernest Rutherford employed a device based on Crookes' spinthariscope to measure alpha particle trajectories for developing his theories on atomic structure. Some toy!
