Re: [Phys-l] Is the Gamow; Bohr-Weizsäcker model passé?

[Bernard Cleyet <bernardcleyet@redshift.com>]

> 1) I do not  see any Gamow included. But that is OK.


Thanks; the link wasn't.

Here's the text:  (First page only.)

MANY people who follow science will have heard of Ernest Rutherford's experiment in which he discovered the atomic nucleus. Not so many, however, will know that it was his colleague, the German physicist Hans Geiger and their student Ernest Marsden who actually measured the scattering of alpha particles from gold foil and discovered that while most alphas penetrate the foil, some are scattered back towards the source. The great Rutherford, a New Zealander by birth, didn't even have his name on the paper in which the results were published. But it was he who eventually interpreted the results in terms of a core of positive charge concentrated at the centre of the atom, and he who calculated what generations of physics students have come to know as "Rutherford scattering". So perhaps it is only natural that most people have assumed that it was Rutherford who did the original experiment.

I suspect that the main reason for such misunderstandings is that textbooks (and lecturers) have enough ground to cover without getting tangled up in historical detail. So the history is summarised as briefly as possible, and as references become increasingly remote from the original source, an inaccurate picture begins to take shape. It's rather like Chinese whispers, where the message is gradually altered from its original form to something quite unrelated by the time it reaches the end of the chain.

Another example of rewritten history came to my attention when I was preparing this week's Inside Science, "Heart of the atom". For some reason I decided to check when it was that the famous Danish physicist Niels Bohr began to think of the atomic nucleus as a liquid drop. To my surprise, I discovered that the idea did not originate with him, but with George Gamow, a Russian who is probably best known as one of the originators of the big bang theory of the early Universe and the author of the splendid Mr Tompkins stories. Yet most of us who have studied physics, link Bohr with the liquid drop model, just as we link Rutherford with the gold-foil experiment.

The book that set the record straight for me is Niels Bohr: A Centenary Volume, edited by A. P. French and P. J. Kennedy (Harvard University Press, 1985). In the chapter on "Niels Bohr and nuclear physics", Roger Stuewer, nuclear physicist and historian at the University of Minnesota, describes how the young Gamow left Leningrad in June 1928, and having impressed Bohr, was awarded a year-long fellowship at Bohr's Institute of Theoretical Physics in Copenhagen. It was near the end of 1928 that the liquid drop model first took shape in Gamow's fertile mind, and in February 1929, during a visit to Cambridge, Rutherford invited him to present his ideas at the Royal Society in London, during a "Discussion on the structure of atomic nuclei", opened by Rutherford himself. The following year Gamow - who by now was a visiting fellow at the University of Cambridge - published a more detailed quantitative analysis in the Proceedings of the Royal Society.

Gamow was ahead of his time. In 1930, the neutron - a basic constituent of nuclei - had yet to be discovered. Gamow's original model of the nucleus contained mainly alpha particles, plus some protons and electrons, which he assumed were bound together by forces rather like those at work in a drop of liquid. The basic idea was not unreasonable, since both alpha particles and electrons emerge from the nucleus in different forms of radioactivity. However, the discovery of the neutron in 1932 was to lead eventually to our present picture of the nucleus as being built from protons and neutrons, although for a year or so various hypotheses appeared based on different combinations of protons, neutrons, electrons and alpha particles.

In October 1933, at the Solvay Conference in Brussels - then a leading international scientific meeting - the German physicist Werner Heisenberg discussed the various hypotheses for the nucleus, beginning with Gamow's liquid drop model. A major difficulty with this model concerned the presence of electrons. Moreover, Bohr, who took part in the discussion following Heisenberg's talk, worried that even a heavy nucleus would not contain many alpha particles. Indeed, he did not seem very impressed. Three years later, however, the German physicist Carl Friedrich von Weizsäcker developed a formula for nuclear masses based on Gamow's work, which he acknowledged in his paper in the journal Zeitschrift für Physik.

So how is it that we now usually associate the liquid drop model with Bohr rather than Gamow? Stuewer points out that when the Strasbourg-born Hans Bethe published the second part of his authoritative review of nuclear physics in Reviews of Modern Physics in 1937, he based his discussion of the liquid drop model on a paper by Bohr and his colleague Fritz Kalckar, which made no reference to Gamow. Bethe's review became a "bible" of basic nuclear physics, and since then, asserts Stuewer, "virtually everyone has assumed, incorrectly, that Bohr conceived the liquid drop model".

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more later.

bc





On 2010, Feb 07, , at 14:45, ludwik kowalski wrote:

> On Feb 7, 2010, at 5:27 PM, Bernard Cleyet wrote:
> > I include Gamow because:
> >
> > Mr Tompkins and the Chinese whispers - 08 July 1995 - New Scientist
> >
> >
> > -------------------------------------
> >
> >
> > Ignoring the question, would charging a drop (water?) cause it to fission or would it alpha decay?  Also could one "shoot"  a small drop at a large one and induce fission?
> >
> > The reason I ask is, as those who have "looked" at my recent post know, a friend photographs drop collisions.  All are of ~ equal size and the result is "splat".  Perhaps a small drop would induce fission instead. (?)
> >
> > bc
>
> 1) I do not  see any Gamow included. But that is OK.
> 2) OK, I am ignoring the first question.
> 3) The second question refers to neutral drops of water. It consists of two sub-questions, for any particular kinetic energy of the small drop, and any particular impact parameter (initial angular momentum) of the two-body system.
>
> a) Would the two drops (large and small) fuse?
>
> b) What happens after that ?
>
> Right?
>
>
> Ludwik Kowalski
> = = = = = = = = = = = = =
>
> Ludwik's new book--"Tyranny to Freedom: Diary of a Former Stalinist"--is now available at www.amazon.com
>
> also see:
>
> http://pages.csam.montclair.edu/~kowalski/mybook2.html
>
>
>
>
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