Level: introductory
References:
In the paper on which this exercise is based, Rutherford detected the hydrogen atom fragment. He also knew that the alpha particle was an ordinary helium nucleus. The neutron had not yet been discovered, though, so nuclear composition in terms of protons and neutrons was not then known. In fact, Rutherford does not even tentatively identify the element produced other than hydrogen. To me, the discussion section of his paper suggests that he envisioned something like the reaction described in part (a) of the exercise. That is, it reads as if the alpha particle acted as a projectile that chipped a piece from the nitrogen nucleus.
The reaction really goes as in part (b), though, as elucidated by P. M. S. Blackett (1897-1974) in 1925. Blackett used the Wilson cloud chamber method to photograph the paths of the colliding particles before and after collision. As he noted, there were only two particles after the collision, one of which was the fast hydrogen nucleus (proton) and he correctly inferred identify of the other fragment.
Revision: Thanks to Steven Krivit for the inspiration to revise the background material on this exercise. He and I have discussed the relative merits of Rutherford and Blackett regarding the credit for the discovery of artificial transmutation. Krivit brought to my attention that Blackett was the person who identified the products of the collision of alpha particles with nitrogen atoms. Blackett received the Nobel Prize in Physics for 1948 for his development of the Wilson cloud chamber detection method and for discoveries made using it. (C. T. R. Wilson won the Prize in 1927 for inventing the method.)
Krivit believes that Blackett is widely overlooked for his discovery of the products of this nuclear transmutation, and that Rutherford is often incorrectly credited with identifying those products. (See this letter to the editor of Proc. Roy. Soc. A.) To this extent, we agree, including on the fact that Rutherford did not know what the products of the interaction were.
Where we disagree is in how much credit, if any, Rutherford should receive for bringing about this first example of nuclear transmutation. It seems clear to me that Rutherford was the first person to bring about artificial transmutation and that he believed that a transmutation in fact occurred. He identifies the observed fast hydrogen nuclei as coming from the interaction, and wrote of the disintegration of nitrogen. In my mind, doing the experiment and understanding that an element (nitrogen) was changed by the encounter, presumably to another element or elements, was enough to credit him with artificially transmuting nitrogen.
To me, the question of assigning credit for important discoveries is a fascinating exercise in the study of details in the practice of science. It is a question on which informed scholars possessing the same set of facts can and do disagree.
Last but not least, re-examining these exercises permitted me to see and correct a misstatement about the disintegration of nitrogen in this process. Previously I had written "He [Rutherford] inferred (correctly) that nitrogen nuclei were fragmented ..." The parenthetical statement should say incorrectly; after all, one of the main premises of the exercise is a contrast between what Rutherford believed and what really happened.
Solutions: To download solutions, go to:
http://web.lemoyne.edu/giunta/classicalcs/ruthblack.doc
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