Becquerel Discovers Radioactivity

  "Chance favors the prepared mind."

--Louis Pasteur

 In November 1895, Wilhelm Roentgen announced his chance discovery of a very penetrating radiation which he called X-rays. The X-rays were produced by high energy cathode rays colliding with the glass walls of the cathode ray tube. Maxwell had said that accelerated charges radiate energy; Roentgen's high energy electrons stopped by the glass walls after all are accelerating. In his report Roentgen noted that the action of the cathode rays on the glass caused the emmission of very penetrating rays he called X-rays. he also noted, almost as a throw-away line, thet they made the glass fluoresce. This fact prompted interest in Henri Becquerel, a physicist at the Sorbonne in Paris. The Becquerel story is a marvelous congruence of the scientific method and serendipity.

Becquerel was an authority on the phenomena of fluorescence (the property of a material that causes an immediate emission of light when bombarded with radiation) and phosphorescence (emission of light is delayed). Noting the emission of X-rays and the fluorescing glass tube, he speculated that if there were a connection, other fluorescent materials should also emit x-rays. In February 1896, he began a series of investigations of fluorescent materials. Roentgen had suggested that X-rays could expose photographic plates, so Becquerel used a plate as his detector. He wrapped the plate in black paper, put a fluorescent rock on top of the plate and placed this arrangement on the window sill to expose it to the sun's UV rays. When he developed the plate it was exposed and clearly showing the outline of the rock. Looking good.




 From the top

 Among the earliest medical X-rays.

 Becquerel, Roentgen
 M. Curie  

Becquerel was a good scientist. He knew that a positive result from a single trial was no sound basis for concluding that the hypothesis--a connection between fluorescence and the emission of X-rays-- was valid. He tried variations of the experiment in an attempt to rule out spurious results. For instance, he placed a glass plate between the flurescsent rock and the photographic plate. He wondered if the heat of the sun might boil off a gas from the rock. In the course of these experiments, he prepared a plate covered with black paper, put the fluorescent rock on the paper, and put the assembly in a closet for use the next day. The weather turned dreary for the next three days. Photo emulsions were not very durable in those days. The plate was of no use to him, but he decided to develop it anyway. To his dismay, the plate was exposed, in the outline of the rock and more intense than ever before. Clearly the sun was not a factor in this turn of events. At face value, emanating from the rock is a very penetrating radiation that occurs spontaneously. So what is so special about the rock? Chemical analysis revealed that the rock contained compound potassium uranyl-sulfate. Dissecting this compound revealed Uranium to be the culprit. Uranium was the heaviest of the elements but its exact place (box 92) on the periodic table was not immediately determined. Uranium was used in glass-making (today we add lead to crystal) to give it strength; one hundred years ago, it was uranium. Uranium was also used in bomb casings; it is both hard and heavy.

Remember, Becquerel was looking to simulate a property of x-rays. Instead he got an entirely new effect. Several new questions came to mind: 1) What was the nature of these rays? Are they particles? Or are they part of the electromagnetic spectrum? The same questions were being asked during this time about the nature of cathode rays. 2) Do other elements emit these strange rays? Or is this phenomenon confined to just uranium, the heaviest element? If only one element gave off these rays, then we are looking at a freak of nature. But if many elements gave off these rays, then we are looking at a new property of matter. 3) If the strange radiation is composed of one of several particles that come from several elements, then what does this phenomenon suggest about structure deep inside the atom?
The spontaneous emidssion of radiation should make us o all feel proud as scientists, knowing that we have pushed back that shroud of ignorance that exists. Trouble is, the trouble is, every time we answer an old question, we raise two or three new questions.


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Radiation Components Identified

in 1896 for the time being, strange emination produced by uranium was known as Bequerel rays. These rays were subsequently identified as three separate kinds of radiation. Go to

Alpha particles were identified by Ernest Rutherford to be doublly ionized helium nuclei when he did an experiment commonly known as Rutherford's mousetrap. In this procedure, Rutherford was able to collect in a sealed glass tube alpha particles emitted by radon. After a time had expired, he passed a current through the space occupied by the alphas and obtained light which, when passed through a diffraction grating, yielded the helium spectrum. It would appear that some atoms. in their need to reduce instability, jetison this large fragment. This reduces the atomic number (the number of protons) by two and the mass number (the number of protons PLUS the number of neutrons ) by four. See the box below.
see also

It was J. J. Thompson who identified the beta particle to have the same charge to mass ratio has the electron, a coincidence too close to not associate these two particles together. In some selection process, a neutron is chosen to decay into a proton which is retained and an electron which is cast off as a beta particle. In beta decay, the mass number stays the same while the atomic number increases by one.
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Gamma rays were subsequently determined to be part of the electromagnetic spectrum and having the shortest wavelength, highest frequency part of the spectrum. Gamma emission allows a nucleus to dump energy but does not change mass number or atomic number.

Atoms Are Changable

Once it was established that atoms had constituent parts (electrons and something positive to render the net atomic charge = zero), it was not so far-fetched to wonder if alphas and betas were the building blocks for atoms. Certainly, a case could be made that.

Physicists began to study Uranium, looking to see how it decayed. Today, we know that 99% of uranium is U-238 and has 92 protons and 146 neutrons. A sample of U-238 decays into thorium by giving off alpha particles. If that is the case, then atoms are changable and the 100-plus year prohibition to atoms changing established by John Dalton must be discarded. But thorium , Th-234, is also radioactive; it decays by beta emission into protactinium 234. It turns out that this element is also radioactive; it, too, is a beta emitter, and decays into TA-DAH!!! Uranium-234???

The Atoms of an Element Are Not Identical

Early in the discovery of radioactivity, some chemists were delighted that vacancies in the periodic table might be filled by newly discovered elements. This hope was quickly dashed when it became apparent that there were likely more new elements than there were vacancies for them. And while the periodic table could have been replaced, there were still powerful reasons for retaining it or something close to it. It was not until 1922 and 1923 that two british scientists, Frederick Soddy and John Aston won separate Nobel Prizes in chemistry for their discovery of isotopes ( from the Greek roots Isos meaning "same" and topos meaning "place". The full U-238 deccay series is shown in the box below. Rather than being assembled from end-to-end, this series, and three others like it, was assembled much like a jigsaw puzzle.

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The U-238 Decay Series

For an interesting look at life at Cavendish Laboratory during the Rutherdord years. go to

Click here to go to derivations

This applet will convey the idea of radioactive decay

This applet shows chart of the nuclides

Go to radioactive decay problems


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And what is the age of the Earth? Go to

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last edited 12/29/05


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