On the discovery and characterization of x-rays (1896 style!)
Updated: Jan 15, 2020
History can be a fun thing to learn about. So here we will pick apart a paper that was published in 1896! Here's a link to the original paper: https://www.nature.com/articles/053274b0
“A discharge from a large induction coil is passed through a Hittorfs’s vacuum tube, or through a well-exhausted Crookes’ or Lenard’s tube. The tube is surrounded by a fairly close-fitting shield of black paper; it is then possible to see, in a completely darkened room, that paper covered on one side with barium platino-cyanide lights up with brilliant fluorescence when brought into the neighbourhood of the tube, whether the painted side or the other be turned towards the tube. The fluorescence is still visible at two meters distance. It is easy to show that the origin of the fluorescence lies within the vacuum tube”
This is the introductory paragraph to the 1896 Nature paper describing x-rays and their new use for imaging. X-rays were discovered in 1895 by Wilhelm Röntgen, and his findings were published a year after in a manuscript entitled, “On a new kind of rays” in Nature. This manuscript outlines the initial findings of this (at the time) unknown source of radiation. The term “X” was used to denote that it was an unknown source of radiation (as well as “for the sake of brevity”), and the name stuck throughout time.
In the process of piecing together how this radiation led to fluorescent signal on coated barium platinocyanide paper, a variety of materials were described based on their transparency to the radiation source. Röntgen found that paper and a “book of a thousand pages” were transparent material (rays could go through and cause fluorescence on the paper). Pieces of wood were transparent too. He also exposed different metals to the radiation source and the coated paper and found that think pieces of platinum, copper, or silver allows some rays to pass, but thicker pieces of lead were not penetrable to the rays (no fluorescence on the paper).
An initial hypothesis that had been made for the penetrability of the rays was that it was dependent on the density of the material. However, further experimentation showed that density did not seem to play a role in transparency because plates of Iceland spar, glass, aluminum, and quartz with the same thickness and relatively same density showed differences in ray transparency.
Röntgen then tested different metal thicknesses to determine which thicknesses led to the same level of transparency among the metals. He found the following:
Metal Thickness density
Platinum 0.018 mm 21.5 g/cm3
Lead 0.050 mm 11.3 g/cm3
Zinc 0.100 mm 7.1 g/cm3
Aluminum 3.5 mm 2.6 g/cm3
The hypothesis that density played a role in the transparency of materials to the x-rays was dropped.
He continues to describe his findings with respect to its effect on the body. He makes a statement of “The retina of the eye is quite insensitive to these rays; the eye placed close to the apparatus sees nothing”! This certainly would not be an experiment that would be performed nowadays!
In further characterizing x-rays, he continues to work with the metals mentioned above. He placed star shaped platinum, lead, zinc, and aluminum on top of glass plates and found that the coated paper had star shaped imprints for all of the materials except for aluminum. With this finding, he then hypothesizes that metals reflect x-rays.
Although all of the pieces were not worked out this early in the discovery of x-rays, by the end of the manuscript he hypothesizes that the rays could be ultraviolet light, which ended up being nearly correct. As we classify waves of light currently, x-rays have a wavelenghth of 10e-9 to 10e-11, while ultraviolet light have wavelengths of 10e-7 to 10e-8.
His work paved the way for utilizing x-rays for medical imaging. We have obviously leaved a lot since the discovery of x-rays!
There are five basic "transparencies" that we see in normal radiographs that are obtained these days:
Metal : bright white [not transparent]
Fluid/Soft tissue: gray
Fat: dark gray
Air: black [transparent]
The different characteristics of these materials allow us to visualize bones and tissues on radiographs! All thanks to the discoveries that were made in the 1800's!!