By definition, alpha particles do not have any electrons. This, in turn, should mean they do not emit a spectrum because a light spectrum is caused by electrons jumping between orbitals. Yet it is by alpha particles emitting a spectrum that Rutherford determined them to be helium nuclei (see here for details https://history.aip.org/exhibits/rutherford/sections/alpha-particles-atom.html).

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    $\begingroup$ Because as they propagate they can acquire electrons via exchange with background gas. Such charge exchange is a well studied phenomenon. $\endgroup$
    – Jon Custer
    Jul 27, 2018 at 0:10
  • $\begingroup$ If there were other gases in the container Rutherford isolated the alpha particles in, extra light spectra would have shown up in Rutherford's experiment. These light spectrums should have made it very difficult for Rutherford to pick out helium spectrum. Yet according to Wikipedia, the helium spectrum was could be clearly seen ( see en.wikipedia.org/wiki/… for details). $\endgroup$ Jul 27, 2018 at 0:25
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    $\begingroup$ The helium spectrum is different from the othe gases, of course. If you start with no He and start seeing He, where did it come from? $\endgroup$
    – Jon Custer
    Jul 27, 2018 at 1:43
  • $\begingroup$ You are correct, it had to come from the Helium. But if the Helium gained electrons from a background gas, then Rutherford should not have been able to pick out the He spectrum from the garbled mess of various light spectrums. So it seems we have a conundrum: that Helium emitted its spectrum but there was no way that it could have had electrons at that time. $\endgroup$ Jul 27, 2018 at 1:49
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    $\begingroup$ Why do you think the spectra could not be distinguished? $\endgroup$
    – Jon Custer
    Jul 27, 2018 at 1:51

2 Answers 2


The paper usually referenced (e.g. by Wikipedia) as the identification of the $\alpha$ particle as the Helium nucleus is "XXIV. Spectrum of the Radium Emanation", E. Rutherford and T. Royds, Phil. Mag. Vol. 16. No. 92 (Aug. 1908 - DOI link). Details on how the $\alpha$ particles were isolated and purified are given in "XXIII. Experiments with the radium emanation. (1) The volume of the emanations", E. Rutherford, Phil. Mag. 16(92) 300-312 (1908, DOI link). Now, the possible linking of the $\alpha$ particle and He had existed for a while, such as in "XLI. The mass and velocity of the $\alpha$ particles expelled from radium and actinium", E. Rutherford, Phil. Mag. Vol. 12 No. 70 348-371 (Oct. 1906 - DOI link). However, such indirect proof was not considered sufficient.

The experiment is described in the 1908 paper pretty well. The "radium emanations" where captured in a glass tube. After various purifying steps (condensation with liquid air, reaction with potash to remove carbon dioxide, ...). What was left was sealed in a glass tube with platinum electrodes. The electrodes were used to excite the remaining gas. The spectrum of the discharge was observed visually and photographed as well.

To quote the 1908 paper:

In order to photograph the spectrum, a spectrograph with a glass prism of two inches base was used. The length of the spectrum on the plate between $\lambda$ 5000 and $\lambda$ 4000 was 1.5 cms. Arrangements were made so that visual observations of the wave-lengths could be made by the Hilger spectroscope while the plate was being exposed. Two photographs were taken before the emanation spectrum ran out. The first (photograph 1) showed about thirty of the more intense emanation lines. The second (photograph 2), which had a much longer exposure, showed over a hundred lines. A helium tube was used for comparison purposes, and its spectrum obtained above and below the emanation spectrum. The plates were measured up with the aid of a Kayser's measuring machine. The wave-lengths were deduced with the aid of the Hartmann dispersion formula

So, the emission spectrum of the $\alpha$ particles was not measured as they were emitted. Instead, the $\alpha$s were released from the radium into a glass tube, separated through purification, and then used in a discharge tube. The direct comparison with a He lamp seals the deal. (As for the 'emanation spectrum' running out, recall that He diffuses quite easily through glass so it leaks out quickly.)

  • $\begingroup$ According to Wikipedia, the tube was evacuated before the alpha particles were emitted. The way the alpha particles entered the evacuated tube was by penetrating a very thin window. This narrative of how the alpha particles were isolated seems to be in conflict with the narrative of the paper you describe. Could you please send me the link to where you got the information so I could make sure that I'm not being lead astray? $\endgroup$ Jul 27, 2018 at 15:26
  • $\begingroup$ @AnthonyDucharme - The DOI link and human-readable reference are given in the answer. I’ll review to be sure I understood the paper as well. $\endgroup$
    – Jon Custer
    Jul 27, 2018 at 20:44
  • $\begingroup$ Thank you for showing me where I could find the paper. Upon examination, I found it to be a narrative detailing Rutherford's endeavors with the radium spectrum rather than the alpha particle spectrum. This, in turn, means all the information described in this specifically related to Rutherford's studies with Radium. If it were not for the fact that Wikipedia describes a different isolation method than the one described in this paper on Radium I would say it can reasonably be inferred that Rutherford used the same purification method in both his experiments involving Radium and alpha particles. $\endgroup$ Jul 27, 2018 at 21:45
  • $\begingroup$ Wikipedia's source for Rutherford's experiment is this nobelprize.org/nobel_prizes/chemistry/laureates/1908/… . If you are interested in reading it yourself, I would suggest searching for the key phrase,"While the whole train of evidence we," and begin from there. $\endgroup$ Jul 27, 2018 at 22:11
  • $\begingroup$ @AnthonyDucharme - Given that the optical emission of the 'radium emanation' matches that of helium (and not radium), I think the 1908 papers are, in fact, what you are looking for. $\endgroup$
    – Jon Custer
    Jul 30, 2018 at 13:24

Protons or neutrons can also emit photons. You don’t necessarily need electrons. https://www.quora.com/Can-an-accelerated-proton-emit-electromagnetic-radiation

  • $\begingroup$ Yes, this is true, but if the light spectrum were produced by neutrons and protons emitting photons how exactly would you explain the discrete lines and frequencies of the light? $\endgroup$ Jul 30, 2018 at 16:54
  • $\begingroup$ This is the question that your response brings to mind. $\endgroup$ Jul 30, 2018 at 16:55
  • $\begingroup$ @AnthonyDucharme electrons are all identical and it’s there different arrangements and combinations that give them specific spectrums or energy levels. And just like electrons a nucleus and its arrangement of protons and neutrons will create different spectrums. $\endgroup$ Jul 31, 2018 at 15:03

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