Experiment VI-1


Is Radiation Another Reversible Phenomena?


Were it not for several unexpected discoveries, we still might not be aware of high energy radiation which is nearly always unnoticed by our senses.  As the 19th Century drew to a close, many investigators were studying the behavior of electric sparks passing through glass tubes from which air had been removed.  It had been found that some substances would glow when struck by such sparks.

November 8, 1895 Wilhelm Konrad Röntgen (1845-1923) noticed that outside but near such apparatus (up to 2 m away) a scrap of paper coated with one of these substances, barium platinocyanide, lit up with brilliant fluorescence.  This seemed strange because it was already well established that the electric current couldn't pass through insulators such as glass and, at this voltage, only short distances through ordinary air.  This made it impossible for the electricity to itself travel the distance required to cause this glow.  Röntgen realized the unexpected glow must be due to a much more penetrating unknown radiation which he therefore called X-rays..  Within the next few days Röntgen determined the properties of the X-rays (read more detail), identified them to be a form of electromagnetic radiation, then mailed his findings to other scientists on New Year's Day 1896.  (A translation of Röntgen's original German publication is among Carmen Guinta's classic papers.)Becquerel

Three weeks after Röntgen announced his discovery, Antoine Henri Becquerel (1852-1908, shown at right) heard Henri Poincaré report to the French Academy of Science in Paris.  Poincaré described the discovery that incident X-rays can cause fluorescence and showed photographs that Röntgen had taken with X-rays.  Poincaré wondered if X-rays were emitted by other luminescent bodies.  The next day Becquerel began to investigate the connection between X-rays and phosphorescence.  (We now know that for many phenomena the conservation of energy allows the time reversal of the process to also occur.  But the concept of conservation of energy was relatively new.  Poincaré and Becquerel may have been motivated more by a hunch of further connections rather than wondering whether a reverse process causes phosphorescing minerals to release X-rays.)  After several weeks with initial negative results with other materials, Becquerel attempted to determine if Uranium compounds might emit X-rays while sunlight caused them to luminesce (with <1/100th second delay).  (read more detail)  Becquerel hoped to detect X-rays using Röntgen's technique with photographic plates covered with black paper.  The paper would block exposure by sunlight but not stop exposure by the more penetrating X-rays.  Rain clouds scattered the sunlight needed to cause the phosphorescence, resulting in Becquerel leaving the covered photographic plates with a mineral called Pitchblende on top, in a dark drawer ready to resume the experiment when the sun returned.  When poor weather continued several days and Becquerel felt a need for preliminary results, he developed the photographic plates anticipating very little if any exposure.  Instead he discovered that the Pitchblende had emitted penetrating radiation similar to X-rays which greatly exposed the photographic plates.  Becquerel noted that the exposure could not be attributed to the luminous radiation emitted by the very brief phosphorescence.  Becquerel was cautious to avoid claiming that this radiation was actually X-rays.  It was clear that Pitchblende emits radiation even without being energized by the sunlight necessary to cause luminescence.  (Carmen Guinta translates Becquerel's original French papers.)


You could repeat Becquerel's experiment provided you have a radioactive source and photographic film.  If you don't have the needed materials the experiment has been repeated many times by the author and his students so that a couple of their photographs can be shared with you.


Exposure of photographic film by radioactive material:
  1. The film needs to be covered with a material to prevent visible light from causing exposure.  Heavy black paper or plastic may be suitable.powders on Polaroid
  2. Radioactive materials might include ceramic material with uranium glaze (such as orange, antique Fiesta dinner ware), an old Colman lantern mantle, an antique watch or clock dial that glows in the dark, or a smoke detector labelled as containing radioactive material.
  3. Place possible radioactive material on the covered photographic film and put in a location where it will not be disturbed or cause problems for three or more days.  If you don't have needed material, study the six small plastic boxes at right, each containing different powders that might be radioactive.  The covered photographic film is a product manufactured by the Polaroid Corporation for professional cameras. (product: sheet film on their web site)  To allow successive photographs using different kinds of Polaroid film, each light sensitive emulsion is separately wrapped in heavy, light stopping paper.
  4. Record the placement of each suspected radioactive material in relation to the photographic film.

picture being made!!! Development of photographic image:
  1. After three or more days, remove the materials that are possibly radioative and develop the film using the method appropriate for the photographic film used.  Polaroid designed their product to contain a paste of necessary chemicals in a sealed foil pouch.  When the film is removed from the camera back between two rollers, the pouch is ruptured and the chemicals spread evenly over the emulsion surface causing the development of the image.  We have developed the emulsion above by manually rolling over the covered film and pouch with a rubber roller.
  2. After the development, inspect the image looking for indications of exposure corresponding to the locations that were near the suspected radioactive materials.  Notice that the manual process is not without errors.  The ragged white corner corresponds to rolling too lightly, failing to spread the developing paste to that corner.  The smaller white band and dots, commonly found on pictures developed this way, probably are due to rolling too hard.
  3. If we place a second piece of covered photographic under this exposed film for three or more days then develop the second piece of film, there is no exposure on the second photograph.

Analysis of observed results:
  1. Determine which of the objects (or boxes) apparently contained radioactive material causing an image on the film.  Note that only one box was located in a corner (A) so that even if the photograph is flipped upside down, we can still determine which boxes contained radioactive material.
  2. Are the substances equally radioactive based on the intensity of the exposure?
  3. Can you determine which are radioactive by the visible appearance or colors of the substances?
  4. If the second piece of film placed with the first shows no exposure, can the radioactive material in the boxes make the photograph itself radioactive?

Additional References

Röntgen and Becquerel were awarded Nobel Prizes for Physics in 1901 and 1903 respectively:


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created 10/18/2003
revised 12/26/2003
by D Trapp
Mac made