Discover of Radioactivity

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Discover of Radioactivity

• Chemistry 332

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Where are we?

• This story starts in 1895 with the work of the
  German physicist Wilhelm Roentgen.
• He was working with a Cathode Ray tube and
  noticed something very interesting...

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Cathode Ray Tube?
(With a fluorescent screen the energy path glows
green)
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So What?
He was working on his experiment on this side of
the room and had a different experiment across
the room
Chem Rules!
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He then flipped the switch...
Chem Rules!
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So he turned it off...
And the Cathode ray tube went dead
Chem Rules!
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So he tried it again!
Chem Rules!
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Conclusion?

• The Cathode Ray tube was giving off some sort of
  energy in all directions!!!!
• Roentgen next thought, I wonder how powerful
  these energy rays are?
• So he took the fluorescent screen into the next
  room, came back and flipped the switch.

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The Set-Up
Fluorescent screen
Cathode ray tube
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Conclusion
Fluorescent screen
The tube must be giving off POWERFUL invisible
energy
Cathode ray tube
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What are these?

• The rays of energy were invisible
• They were strong enough to penetrate the wall
• They were unprecendented
• So he called them.
• X-Rays

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So Roentgen Discovered X-Rays

• X-Ray machines began popping up everywhere
• Even shoe stores had them so you could see your
  feet bones
• Some thought they were the Devils tool
• More importantly they led to further studies

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Science is moving faster now...

• Its now 1896, one year after Roentgens work
• Antoine Henri Becquerel has heard of the X-Ray
  and its link to fluroescence and decides to do
  his own experiments with phosphorescence

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His Hypothesis

• Some phosphorescent materials emit X-rays when
  exposed to high energy
• These x-rays can penetrate substances but are
  ultimately just like visible light (just stronger)

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His Set-Up
Sunlight
Photographic film
Would the film be exposed?
Yes
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But.
Sunlight
Photographic film
Would the film be exposed?
No
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But, but.
Sunlight
Photographic film wrapped in thick black paper
What happens to the phosphorescent material in
the sunlight?
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This glowing gives off energy

• If the energy was just visible light, would the
  photographic film be exposed?
• No, its wrapped in black paper the normal light
  cant penetrate
• If the energy also contained X-Rays, would the
  photographic film be exposed?
• Yes, X-rays are just like light but can easily
  penetrate the papter.

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What was the result?

• Sure enough, the photographic paper was exposed!
• This meant that phosphorescent materials, when
  glowing due to the energetic effects of the sun,
  give off X-rays!

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So what?

• Really nothing new happened there with that
  experiment
• But on Feb 26th, Becquerel decided to repeat the
  experiment but couldnt due to cloudy skies over
  Paris.

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Drats!

• Becquerel took the phosphorescent materials and
  the photofilm wrapped in black paper and stuck
  them in a drawer.
• They stayed there, undisturbed in the dark until
  March 1st four full days

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Lets Review

• Becquerels conclusion was that sunlight made the
  phosphorescent material glow
• This gave off penetrating x-rays
• This caused the photofilm to be exposed even
  though it was wrapped in black paper
• Should the photofilm, after sitting the dark,
  wrapped up in black paper be exposed?
• No, theres no light to cause the phosphorescent
  material to glow and thus give off x-rays

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Just out of curiosity

• Becquerel developed the photographic film anyway.
• What did he find?

• The photofilm was exposed just as much as his
  previous experiment!
• What does this mean?

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Sunlight doesnt matter

• The phosphorescent material was spontaneously
  giving off energy regardless of the sun
• This energy was invisible
• It was also strong enough to penetrate the black
  paper
• Were these X-rays or some other energy?

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How many substances do this?

• Marie Curie (a student of Becquerel by the way)
  and Pierre Curie spent years isolating substances
  that exhibit these properties
• But what are they?

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One last experiment

• Ernest Rutherford, using Becquerels ideas and
  material isolated by the Curies performed an
  experiment on the X-rays found by Roentgen as
  well as the energy found by Becquerel

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His Set-Up

Strong magnet
-
X-ray or other energy source
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The results!

Strong magnet
-
What charge are X-rays, then?
X-rays
Neutral, just like light
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The results!

Strong magnet
-
What charges are these?
The energy discovered by Becquerel and the Curies
Positive, negative, and neutral
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What is this?

• This mysterious energy given off by the
  fluorescent substances was of three different
  types
• It was given off spontaneously
• It was invisible
• It was of very high energy
• What is it?

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Ill give you a hint

• The fluorescent ore used by Becquerel was
• K2UO2(SO4)2
• Notice anything unusual about that formula?
• Check out the center atom.
• What do we know about that element?

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Radioactivity!

• So what did this have to do with X-rays or
  fluorescent materials?
• Nothing!
• Except that without those experiments,
  radioactivity would not have been discovered so
  early.