The discovery of artificial disintegration

The serendipity that led to a better understanding of nuclear composition occurred in 1919. That year Rutherford realized that when he bombarded nitrogen gas with alpha particles from bismuth-214, fast particles were produced that could travel further. in gas than the alpha particles themselves. When these particles hit a scintillation screen they produced weaker flashes of light than those produced by alpha particles. Quick calculations indicated that this intensity was roughly the intensity that positive hydrogen ions would be expected to produce [1].

Measurements of the effect that a magnetic field had on the trajectories of these particles suggested that, in effect, they were protons. With the skepticism that characterizes all good scientific research, Rutherford ruled out, through careful experiments, the possibility that the protons came from the hydrogen present as an impurity in nitrogen.

Since the nitrogen atoms in the gas were the only possible source of protons, Rutherford concluded that an alpha particle, colliding with a nitrogen nucleus, can occasionally eject a smaller particle (a proton) from the nitrogen nucleus. In other words, Rutherford deduced that a particle can cause the artificial disintegration of a nitrogen nucleus, the proton being one of the products of this disintegration. But this process does not happen easily. Experimental results showed that only one proton was produced for about every million alpha particles that pass through the gas.

Between 1921 and 1924 Rutherford and his colleague James Chadwick extended the work on nitrogen to other elements and found evidence that it was possible to produce the artificial decay of all light elements, from boron to potassium, with the exception of carbon and oxygen [2].

The next step was to determine the nature of the nuclear process that leads to the emission of the proton. Two hypotheses were suggested for this process:

(a) The nucleus of the bombarded atom loses a proton, which is "dislodged" as a result of a collision with an especially fast alpha particle.

(b) The alpha particle is “Captured” by the nucleus of the atom it has hit, forming a new nucleus that, immediately afterwards, emits a proton.

How to experimentally distinguish between the two hypotheses? A new instrument would mark all nuclear research from then until today: the cloud chamber.

Notes:

[1] The name proton for the positive hydrogen ion would be adopted the following year , in 1920, at the suggestion of Rutherford himself.

[2] Years later and with better technology it was shown that these elements could also be disintegrated artificially.

About the author: César Tomé López is a scientific popularizer and editor of Mapping Ignorance [19659013] The article The discovery of artificial decay has been written in Cuaderno de Cultura Científica .

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