How did Meitner calculated the amount of energy lost....

[Peter Ke]

It's my first time here, so I'm not sure if this is the right place to post it. If I post it in the wrong section, then I'm sorry.

Anyways, I just have a curious question. Based on this video I watched --> ""

at around 1:40:42 , I really want to know how she actually calculated the amount of energy lost during the nuclear fission of the uranium atom by using E=mc^2. I'm just curious that's all because the video doesn't show the actual calculation. So, if one of you guys can show the process of the mathematics, that would be great!

Thx!

 

[russ_watters]

Welcome to PF!

She says what calculation she's doing, but I can't understand it with her accent. Somehow she's calculating how much mass is lost in the fission process, perhaps by knowing the mass of the uranium and the products and simply subtracting. Then you just plug that number into e=mc^2. Here's an example:
http://www.bbc.co.uk/bitesize/higher/physics/radiation/nuclear_reactions/revision/2/

 

[Borek]

She says clearly that the two nuclei (produced in the fission) are lighter than the original nucleus of uranium by about 1/5 of the proton in mass. When you know this number it is just plug and chug

 

[russ_watters]

She says clearly that the two nuclei (produced in the fission) are lighter than the original nucleus of uranium by about 1/5 of the proton in mass.

Yeah - it was what she said right before that, before she calculated that result that I couldn't understand. Sounds like "let me do a packing friction calculation", but I don't think that's right...

 

[nasu]

Yes, that's what she says.
It seems to be sort of jargon of early nuclear physicists.
See here, for example:
https://en.wikipedia.org/wiki/Nuclear_binding_energy

At the end of the section "measuring binding energy"

 

[OmCheeto]

Yeah - it was what she said right before that, before she calculated that result that I couldn't understand. Sounds like "let me do a packing friction calculation", but I don't think that's right...

LISE MEITNER: "Wait, let me do a packing fraction calculation. The two nuclei are lighter than the original uranium nucleus by about one-fifth of a proton in mass".
[ref: Nova transcript]​

I can't find the original letter that sparked the conversation, sent on 19 December, 1938, but I did find the paper published a few months later:

Concerning the Existence of Alkaline Earth Metals Resulting from Neutron Irradiation of Uranium
published 6 January 1939

I'm afraid I can't follow it.

Wiki gives a clue:

Otto Hahn, Discovery of nuclear fission
Further refinements of the technique, leading to the decisive experiment on 16–17 December 1938 (the celebrated "radium-barium-mesothorium-fractionation"), produced puzzling results: the three isotopes consistently behaved not as radium, but as barium. Hahn, who did not inform the physicists in his Institute, described the results exclusively in a letter to Meitner on 19 December: "...we are more and more coming to the awful conclusion that our Ra isotopes behave not like Ra, but like Ba. ... Perhaps you can suggest some fantastic explanation. We ourselves realize that it can't really burst into Ba." In her reply, Meitner concurred that Hahn's conclusion of the bursting of the uranium nucleus was very difficult to accept, but considered it possible.
On 22 December 1938, Hahn sent a manuscript to Naturwissenschaften reporting their radiochemical results, which were published on 6 January 1939.

I knew the theory behind mass defect, but not the history of its discovery.
Very interesting.
But how they determined the details, is still beyond me.

 

[OmCheeto]

She says clearly that the two nuclei (produced in the fission) are lighter than the original nucleus of uranium by about 1/5 of the proton in mass. When you know this number it is just plug and chug

I plugged and chugged and came up with 169 mev, based on wiki's most convenient: U236 fission into Ba141 + Kr92 + 3 Neutrons image

Specifics available upon request.

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[edit]
And given that I'm prone to forgetting where everything is:
file: pf.random.homework.problems.numbers
tab: fission of U236